HotMod build project
Part 1: The donor car.
In production years 1978 – 1987, the General Motors (GM) corporation adapted all of their 108” wheelbase midsize cars to a common frame. Whether it was an Oldsmobile, Buick, Pontiac or the popular Monte Carlo bodies and nameplate, if it was the midsize GM passenger car built these years, they all rested on a 108” GM metric frame.
At that time being the largest selling line of domestic passenger cars, these cars could be found everywhere, and were the last of the rear wheel drive midsize passenger cars that GM would build in mass quantity.
Many of these cars are still on the road today, and many more still can be found in wrecking yards. Parts are still available, and they have been adapted for use by racers everywhere.
These cars were a natural choice to use as the ‘HotMod’ for all of the above reasons.
‘HotMod’ rules dictate the use of OEM parts for the suspension. The rules are written so that everything (with the optional exceptions of the upper control arms & rear end housing) is required to be the stock parts that come standard on the midsize GM metric frame.
When locating a donor car for your build, and your starting from scratch, make sure all the suspension components are there and in usable condition. If this is the case, you have everything necessary to turn your donor car into your future racecar.
The more that is usable when you buy the car, the farther along in the process you will be. If you intend to run a 350 GM engine and automatic transmission, look for a car, possibly still in service, that has all of this. Many small car lots, or the local Bulletin Board type magazines may have the jewel your looking for.
If you aren’t as picky, or need less, scour the local wrecking yards. If none are available at the time, tell the yard operator what you want, and why you want it. Give him a card with your phone number on it, and ask him to call when he gets a unit in. A car that has been wrecked may be suitable provided any possible frame damage is limited to areas that can be removed. Stay away from a car with a damaged front frame horn unless you are willing to put it on a frame machine prior to starting the actual build.
There has been concern that these cars are impossible to find. In my efforts, I haven’t found that to be the case. It’s true that salvage operators are crushing cars, but are just as happy to sell them as they are to crush them. Don’t expect them to pull the body for nothing, though. That is the reason some are finding it harder to find these cars. There out there, but salvage operators expect to make their money off of them, and no longer disassemble them as they once did in the past. Either make arrangements for the salvage operator to pull the body if you wish, or be prepared to take it home and do it yourself. You can then haul the remnants off to the crusher to recoup some of your money.
Remove the front and rear suspension, and set off to the side for future use.
Remove the exhaust hangers, old brake and fuel lines and bumper braces. You won’t need or will be replacing all of these. Leave the rear crossmember to frame rail braces in place for the time being. If nothing else, they will keep the frame braced as your wrestle it around in the following steps.
Once you have the frame stripped bare, you need to either sandblast or spend some up close and personal time with your future pride and joy by applying a wire brush to remove years of surface rust. This is a time consuming and physical part of the process, but it will pay big dividends as we advance through the build process. Remember, rust makes for an extremely poor weld, so time well spent here will save future time.
The GM metric cars have 2 unique features.
They were the first frames that were totally robotically welded, and they made of 100% recycled steel.
While neither of these facts mean much other than you now have the ability to impress your friends in a friendly game of automotive trivia, it does create a couple necessary steps before proceeding past the bare frame stage.
First, these car frames were more prone to rusting out than before due to the quality of steel. The rear frame rails behind the rear shock mounts can be replaced, and this is one of the reasons why. The frames were notorious for rusting out in that area. Check where the sway bar bolted into the frame, and around the areas of the body mount bolts. These were also areas for extreme rust. Take a welders “slag pick” and go around the frame, sounding it for weak spots. Don’t be afraid to strike it hard enough to make sure that the steel is sound. Now is a good time to remind anyone involved to be sure to wear safety glasses or goggles when working with any striking tool, or grinding on any weak spots. You can fix a frame, but your eyes are invaluable. Same thing goes for your hands. Learn to wear work gloves during these projects. Rough edges can lead to researching the date of your last tetanus shot, otherwise.
Fix any bad spots by welding in a steel patch. Since you have the welder out already, carefully look over the frame seams and weld areas. Because the automated welding procedure worked on an average speed and heat range for the metal being welded, these frames sometimes exhibited less than textbook welds. Careful examination may show you cold welds, burn throughs, or areas where the robotic welder missed the joint area altogether. Take your time, look the frame over thoroughly and grind and weld anything that looks suspicious to you. Once you have finished with this process, you have completed the most labor intensive of the work to get your HotMod on the track. The frame has been readied, and it is the foundation of the entire build process.
Next, we lay out the frame supports and roll cage.
HotMod build project
Part 2: Laying out the frame & cage
After some serious investment of sweat equity, you now have a stripped frame that has the surface rust removed, and any suspect areas repaired.
It is now time to lay out the frame work that will become the roll cage.
For more veteran racers who have built their own cars in the past, this may seem a simple step, but for the novice this can seem overwhelming at first glance.
One very important thing to remember before we dive into this part of the project: If you are not a competent welder, find someone who is to weld the cage. This is not the time to begin to learn the art of welding. The cage, seat and safety harness mounts need to be installed by someone who is an experienced and skilled welder. Your future health may depend upon this! Do not shortcut this step. Practice your welding skills on non-critical pieces like frame tabs and bumpers, but leave the welding that directly relates to driver safety to someone who understands and knows how to weld correctly.
In regards to a roll cage, 3 possibilities exist.
You are installing a previously used cage from another race car.
You are installing a pre-manufactured “kit” cage.
You are going to fabricate your own cage “from scratch”.
If you are having the roll cage installed by a “professional” race car builder, you can ignore the following, but for most of the rest us, the following may be useful.
Before starting this process, take the time to do a little investigative research of your own. Look at some of the professionally built modified class cars that presently run at your home track. Talk to the driver and crew first, though. Most guys are willing to offer advice or show you their cars provided you introduce yourself prior to snooping around the car.
Make notes if you need to remind yourself of things you want to incorporate into your build. You will notice that nearly all the cars have the right (passenger) side of the roll cage tied to what is designed as a frame stiffener in the HotMod rule book. This basically runs in a straight line from the front to rear clip. Be mindful of this when you install your roll cage. This alone will save you time and materials, and make the fabrication process much simpler. Steve Smith offers an excellent line of “how to” books dedicated to auto racing. Access to the internet may also provide you a quick study on roll cage and frame design. Many chassis manufacturers have websites which include photos of a bare chassis. While this doesn’t allow you to actually measure anything, you can get an idea of how the “pros” go about designing an economical frame and cage.
Mating a previously used cage to the GM metric frame can be a lesson in compromise, or as Mick Jagger sang, “You can’t always get what you want”.
Unless the cage you are using also came from a like frame, you will need to be creative in mounting the cage to the frame. Install the driver’s side lower frame work (door bar base) either to the left frame rail, or to the optional external frame stiffener.
(See rule 1B, the HotMod allows for frame stiffeners on both sides of the frame. By incorporating this on the right side between the front & rear frame turn outs, you can narrow your cage, thereby decreasing the right side weight percentage. This will also provide additional driver room, and keep the seat further inside the door bars.)
Install the cage back as far as possible on the passenger car frame. A word of advice here: It may be a good idea to “mock up” the rear end in the chassis to be positive that the left rear tire won’t contact the back of the cage. This is the case with anything you feel could become a future fit problem through the entire build process. Mock fitting anything prior to final installation, when it concerns welding, is always a smart move. Incorporating an “X” in the frame, and the use of “outriggers” to tie into the right side frame rail will add rigidity to the chassis (make sure the “X” won’t interfere with the transmission tail shaft or driveshaft!).
If the old cage has right side door bars, you’ll probably need to remove them. An “X” in place of these door bars will add strength, save right side weight, and allow you to correctly install the body later on.
Many of the present HotMod cars in competition incorporated the use of cages that originally were Late Model or Open Wheel Modified chassis’s. This is a good option, but be mindful of a couple things when adopting these to your build. Pay close attention to the cage location on the donor chassis, adopting that to your build, and be mindful of the dimension from the mid-plate (rear motor mount) to the center line of rear end. Using a saw rather than a torch to cut the cage from the donor chassis will save you some future work, as well.
If you decide to go with a pre-manufactured “kit” cage, make sure that the main components are made from 0.95 wall thickness D.O.M. (drawn over mandrel) tubing.
These should include the driver door bars, main hoop, halo and upright bars. Electric weld tubing is satisfactory for many places in a race car, but the main roll cage and door bars should always be DOM tubing. For best results (both during construction and in competition) the tubing should be low carbon mild steel. Stay away from chrome moly tubing. It’s physical characteristics make it brittle if incorrectly welded, and it is very pricey.
Buying a kit cage is a lot like buying a suit.
Different material, styles, and sizes, but if you buy off the rack you get it cheaper than going to a tailor who will give you the fit you desire.
There are many fine cage kits and applications available, most of which are reasonably priced because they are built in volume. Take the time to decide what you need before purchasing, and make some phone calls to these companies if the cage width, height or length you desire isn’t a ‘stock’ item. The possibility exists that a cage built to your specs can be purchased for only a marginal difference in cost.
Make sure before purchasing any kit cage that all the tubing is notched to fit. This is a time consuming process to do on your own if that isn’t the case. Irregardless, you need to be prepared to do some of this with any kit cage. The joints should fit as close as possible before final welding begins. Always tack weld bars in place before finish welding. This will save you major headaches should you encounter a problem later.
Remember, an ‘x’ or diagonal bar is all you need in the right side door area, so make sure they are aware of this when ordering a cage. You can save some money on the cage cost.
These kits also come with straight sections of tubing to use as you see fit.
An “X’ in the cage behind the driver is always a good idea, and some variation of it should be incorporated into your roll cage. Saving weight here (the roll cage) is never a good idea. If you’ve never raced, or raced in a class that requires a minimum weight, it is hard to gauge what your car may weigh “dry”. This refers to the car without any ballast, fuel or fluids. The HotMod minimum weight rule is 2400 lbs., the same as current TSMA/AMRA style modifieds. The HotMod rear suspension is much simpler than what the TSMA/AMRA modified rules permit, and thus is much lighter. Most of the TSMA/AMRA modifieds still weigh in very near the minimum required. Understanding that, you need to realize that when building your HotMod it isn’t necessary to look for ways to save weight, by installing an ultra-light roll cage.
A race car’s overall weight is actually divided into 2 different “types” of weight. There is ‘sprung’ weight, and ‘unsprung’ weight.
‘Sprung’ weight refers to the weight the cars springs see. The springs support and react to the weight of the car.
‘Unsprung’ weight is weight that actually is out of the springs control. This is weight that is below the spring level. This is the rear end, it’s components and the front lower control arms, for example.
Any knowledgable racer will be able to tell you a heavy car with light components is better than a light car with heavy components. The reason for this is that you can adjust for ‘sprung’ weight with springs and set-up options. ‘Unsprung’ weight cannot be compensated with anything other than a lighter part replacement. That option isn’t available in the HotMod, and quite honestly, the ‘unsprung’ weight of these cars is already minimal. By keeping the majority of weight between the axles, as HotMod rules allow, the car has better weight distribution. Long story made short, don’t attempt to sacrifice safety for weight in the main cage area. A few extra pounds here will have little affect on the overall weight or weight distribution.
Consider a couple of words of advice.
More isn’t necessarily better. I’ve seen cars that look like tanks when finished just because the guy building it decided everything from the headlight covers to the ash tray needed roll bars. They installed roll bars in areas that more endangered, rather than protected the driver. Never add a bar in an area where the drivers head could come in contact in an accident. A properly installed main cage, using correctly sized tubing should do the job without adding a bar “just because we had tubing left over”. If a bar doesn’t serve a needed purpose, chances are it doesn’t need to be installed.
Secondly, triangulation is the key to strength. Each leg of the triangle transfers the load the next member. Providing the car is properly triangulated, the resultant loads transfer to the springs and shocks. They then do their job in a predictable manner. Radical handling problems can sometimes be traced back to chassis flex. This problem is nearly impossible to detect, and can be baffling when in the garage doing chassis set-up.
The required O.E.M. suspension on these cars isn’t designed to react like the current 4 bar cars in the Late Model & modified divisions. Many of these are designed to flex, making the entire frame a sort of torsion bar. Some of these chassis’s also need to be replaced fairly often, as the flex fatigues the steel so quickly that soon cracks develop, and the car no longer “works” as it did originally. Build your car so the springs do the work the high priced Detroit engineers intended, and you’ll be much more satisfied with the end results.
The final option is to build a cage from scratch.
If this is your choice, congratulations! You are a “do-it-yourselfer” of the first order with the patience of Job, and a garage full of neat tools.
This will require (in no particular order) a band saw, tubing bender, tubing notcher, angle finder, and a 20 cup coffee maker since you will be spending a tremendous amount of time in the garage.
Unless you have experience with figuring the radius of bends, be prepared to haul off a large pile of scrap tubing bent in variations of the letter L.
All joking aside, building a cage from scratch is a time intensive project, requiring a great deal of measuring and lay out work. The payoff to this is getting exactly what you want, with the satisfaction of knowing it’s your own craftsmanship and design.
The main cage is now in place. Next time we’ll install bars to the front & rear clips.
HotMod build project
Part 3: Completing the roll cage
Our project car is beginning to take shape, as we have completed the installation of the main cage.
We’ve tried test fitting everything we felt could be in conflict before the expert welder of the group completed the welding of the roll cage.
We now will go about bracing the rear and front of the car frame, tying them to the roll cage itself. We’ll refer to these ends of the frame as the front & rear clips.
A “professional” race car builder must first take into account his suspension mounting points. These are critical to insure that the springs and shocks don’t run out of travel, bottoming out in either direction.
You won’t need to be concerned with these dilemmas, as the HotMod rules require the suspension and shocks mount in the original O.E.M. stock locations.
That is specifically the reason for the rule, as a matter of fact.
The front bay bars extending from the cage to the front clip will serve 2 basic purposes.
First, they brace each individual frame horn.
Secondly, they provide mounting points for the radiator support and the hood pins.
The height of these bars (distance from the top of the frame to the bottom of the bar) on nearly all modifieds is predicated on the height required to properly mount the front shocks. Since this isn’t a concern for you (starting to appreciate that stock mount suspension rule, aren’t you?) the front bay bars become less critical, and more of a choice of preference. You will need to tie them together after determining the radiator location, so try to keep the bars parallel.
The tail of the car, or rear clip, will require a little more figuring, but again, since the suspension isn’t involved, it becomes a matter of preference. (That stock suspension mounting rule was a stroke of brilliance, eh?)
The factors we need to consider are the size and proposed location of the fuel cell and battery box. If you wish to remove the stock rear frame rails and fabricate the tail section you will need to incorporate that step at this time. The stock rear frame rails can be removed to the rear of the rear shock mounts only (see rule 1A). The rear shocks mount in a steel plate that also serves as the upper rear spring bucket. If you have decided to fabricate the tail section, take the time to get an approximate height of the bumper end of the frame rails. This will get you in the ballpark for your tail section height. The length of the replacement frame rails is dictated by the dimensions of the fuel cell you intend to use. Chances are you won’t need the frame rails to be any longer than the present stock rails, and possibly not as long. A thought to remember when calculating the rear clip length: This area is primarily for mounting the fuel cell, rear bumper and the rear body and interior brace. The longer it is, the more material it requires for the interior and quarter panels. Consult the rulebook for minimum & maximum dimensions (pages 11 & 12). If you calculate a little now, you may save a few dollars later on.
If you remove the stock rear frame rails, cut both frame rails in the same location, making the cut as close to perpendicular to the floor as possible, and at the back edge of the shock mounts. Next, tie the 2 OEM frame rails together with a length of box tubing that is capped at both ends, and cap off the exposed ends of the stock frame. This serves a couple purposes, bracing the rear suspension and clip, in addition to providing a foundation point should the rear clip need replaced due to damage. Should you take a bad hit to the rear of the car, it’s better to let the frame to the rear of the suspension absorb the damage. If bent beyond straightening, replacing it becomes a much easier process.
You may want to narrow the tail of the car up by moving both fabricated rear frame rails inboard a bit. Remember to verify that the fuel cell will fit (but this shouldn’t be a problem).
It is important to brace the rear cross member and spring/shock mounts to the cage.
Install a diagonal down brace from the top of the cage to the tail of the frame. On the drivers side of the car, this bar should extend from basically behind the driver’s head to the rear, and be close to parallel to the rear frame rail. If you are able to line this diagonal bar up with the frame rail, it will make the interior deck installation a little easier.
Off the diagonal down bars, install braces to the spring / shock mounts, taking care to avoid any future conflict with spring buckets. If you are planning to install weight jack bolts on the rear, or modify the upper spring bucket to accept a different height spring, install this brace bar after the spring bucket fabrication is complete.
Install a brace bar from each side of the main cage to the rear cross member, near the rear suspension arm mount. This brace bar should be kept as short as possible, and angle downward from the roll cage to the cross member plate. This will brace a high stress area that will see much higher forces placed on it than what Detroit engineers originally intended.
Additional bars are needed to protect the driver’s feet, and possibly triangulate the front clip if desired, but for this step it’s best to have the motor in place and determine the amount of room needed for the headers. These bars, while important, can be fit as a choice of preference after the necessities are installed.
If you still have the rear end under the car, and you intend to take advantage of the motor set back allowed in HotMod competition, this is the time to lay out your motor mounts.
This step isn’t as intimidating as it sounds, but will require patience.
A bare engine block with a transmission bolted to it will make the process easier. If you have your rear motor mount (or midplate) bolt them between the block and tranny as they would be mounted in the car.
If the frame is on jackstands, try to level the car as much as possible.
Measure from the center line of the rear end (from both sides of the housing) forward 70 inches (see rule 8b), and make a distinguishable mark on the frame. Remember that the rear should be as close to the correct ride height as possible when determining this dimension.
The motor can set farther forward, but cannot set any further to the rear than the 70” dimension.
Now that we have the distance we desire from the rear end, we need to determine the height the engine will set in the frame. If you have a racing style oil pan, a good rule of thumb is the bottom of the pan should be no lower than the bottom of the stock front crossmember. Setting the engine with the pan installed on a piece of box tubing or 2 x 4’s, and using a floor jack to manipulate it into position will make this easier. Making sure the 2 x 4’s hit under the frame rails when jacking the motor up will simplify this. Use caution that the motor and tranny are secure. This can be a hazardous step.
Without getting into some complicated math that I am certainly unqualified to explain, the engine height has a great deal to do with the cars roll center and center of gravity height. With asphalt race cars, the general rule of thumb is “low and to the left”.
Dirt cars aren’t quite as simple, and generally react better to a higher center of gravity, and a more balanced loading for the suspension. The roll center determines how weight is transferred through the forces of cornering, acting as an invisible axis for the weight mass to rotate about. Too high is just as detrimental as too low. What we’re shooting for is a happy medium.
Once the motor mounts are installed, the engine can be raised by use of spacers or shims, but lowering it will be next to impossible without refabbing the motor mounts.
For this reason, I suggest using the bottom of the cross member = bottom of oil pan height as the baseline starting point. I think you’ll find this baseline to work, and besides, any lower and the bottom of the oil pan will be caved in or drag. If you desire to initially raise the engine, and have positive results, you may have found the happy medium we were searching for!
At your 70” marks (or forward), weld in a 0.95 wall vertical roll bar between (and perpendicular to) the lower frame member and upper bay bar on each side of the chassis. Square the back of the engine block to the rear end housing, making sure that the engine is in the centerline of the car frame, and keeping the driveline relatively level.
This plays an important part in the life of the driveshaft’s universal joints.
You can now fabricate the mounting brackets to support the rear motor mounts from the previously installed roll bar uprights.
While you have the motor in position, you have the opportunity to also fabricate your driveshaft hoop (see rule 6B & it’s illustration), radiator support, and front motor mounts. Again, check you headers for fit before installing any front end bracing.
The front motor mounts are can be fabricated in the same fashion as the rear mount.
With the engine in place, build to the mounting plates. Some chassis builders weld a diagonal bar from the side of the front frame horns to the front cross member, and then install an upright from that to the bracket. Use 0.95 wall tubing for this, and install a couple of diagonal braces from the top of this upright to the frame. A lot of torque, weight and lateral stress will be placed on the motor mounts, so don’t be afraid to use gussets to help spread the loads.
(When building the ‘Project HotMod’ car, we mounted the engine slightly higher in the chassis with good results. One noticeable concern was the considerable driveshaft angle. After a full season of racing, no universal joint failure or driveshaft problems resulted, but this is something to keep in mind on your build.)
The radiator should fit in between the front cross member and fan. If this is the case, the supports can be welded of the back of the cross member. If needed, the cross member can be notched to allow the radiator to fit (see rule 1B). Check out the mounts on a variety of different modifieds at the track (they are all basically the same) and adapt this to your car. Install a cross bar running horizontally in front (and below the top of) the radiator, tying each of the front engine bay bars together. Install a pair of forward facing tabs that are slightly inside of the radiator’s core width. These tabe should angle downward slightly, lining them up with the top of the radiator. We will use this later as the top radiator “hold down”. You also need to weld in a cross bar in the area of the now vacant front cross member that will absorb the frame loads that the engine used to brace. Weld this bar in at the approximate height where the front motor mounts were previously located. This bar plays an important part in eliminating front chassis flex.
I strongly recommend bracing the front frame horns (see rule book page 13) in some fashion. Make sure that whatever you install doesn’t interfere with any of the steering members. At the very least, install a crossbar between the 2 frame horn, and weld a couple body tabs (angled towards the top of the radiator) to anchor the nose piece.
There is no right or wrong way to do much of this work. It may involve a great deal of trial and error before you are satisfied. Assisting a friend in a race car build several years ago, we installed and cut the front clip off a car 3 times before we were finally satisfied. That car went on to win opening night of the season, and recorded multiple feature wins and a couple of track point titles before being sold.
The moral of the story is the time and work you invest now can pay big dividends later on the track.
Next, we’ll tackle the body & interior supports, and install bumpers and rub rails.
HotMod build project
Part 4: Bumpers, rub rails and interior mounts
If you've followed this project from the start, you are well on the way to getting the steel fabrication portion on the project completed.
You also have come to realize that much time can be spent with little visual progress made.
Don't despair, Bunky. This is typical for anyone in these steps of the project, irregardless of experience. If you've went to the trouble of correctly fitting the tubing before welding, rather than just stuffing a bolt into the gap and welding (never a good practice) your time has been well spent. Learning to properly fit things in place prior to welding may take a bit longer, but it will eliminate the need to cut out and refabricate mistakes, and allow you to be proud of your craftsmanship.
As with any build project, there are times that show lots of progress with little work. Unfortunately, for the impatient among us, some of these steps will come later.
It's been my subtle suggestion all along to build a durable car. I hope you've picked up on these hints. The HotMod wasn't intended to be a throwaway car. It should last, if properly maintained and avoiding a catastrophic wreck, for several seasons. The rules won't obsolete it with new technology yearly, so building a safe car (that must weigh 2400 lbs. anyway) is our primary goal.
An overlooked but important part of any race car is driver comfort. Installing the seat, pedals and steering to fit the driver's frame is a step that many take for granted. It is also a step that requires more than one person. If the driver is particularly tall, sacrifices may have to be made in order to fit everything up, but take the time now to maximize the driver's comfort in the car.
If you have installed an X in the frame, you're next step would be to install your seat and safety belt mounts in the car.
Your racing seat and belts should always mount to the frame and cage structure. There is never an viable excuse for doing this another way. In the event of a serious wreck, the seat, safety belts, and subsequently the driver need to stay with the roll cage. Michael Waltrip survived a horrendous crash at Bristol in 1990, in part due to properly mounted belts and seat. If you learn anything from this project, it should be that properly installed and maintained safety equipment holds priority over anything else you'll do. If you can't afford the required and adequate safety equipment, you need to put the project on hold until you can afford it. Do it right, or don't race.
There should be a frame member (part of the cage) that runs under the bottom of the seat. Place the seat in the car and have the driver sit in the seat, adjusting it to the angle he prefers. Using this information, weld tabs to the frame member to anchor the seat base. An outrigger from the cage to the shoulder height area of the seat back can now be installed. Take care not to make this a potential spear or future hazard for the driver. Remember to take the driveshaft into account when installing the seat. The lap belt mounts should route the belts across the pelvic girdle. Most of the present day fabricated aluminum racing seats have a hole on each of the sides to train the lap belts through. These holes generally provide an accurate indication of the needed location for the lapbelt mount brackets. The shoulder harnesses should mount only slightly lower than the drivers shoulder height, primarily pulling back, rather than down, on the driver. Severe internal injuries can result from improperly mounted lap belts, and unnecessary pressure can be placed on a drivers back due to shoulder harnesses that "drapes over" the driver. The back of racing seats also has a hole to train the shoulder harness belts. NEVER drape the belts over the seat.
In the event of a hard frontal collision, the driver can slip through the shoulder harnesses, and suffer unnecessary injuries to the chest (heart & lungs), face and neck areas because of this oversight or shortcut.
Use adequate and properly welded brackets, good quality and proper length bolts with lock nuts when installing your safety belts. This is definitely not the area to cut corners.
The anti-submarine belt should also be installed. This little belt, also know as a "crotch strap" has gotten a bad reputation as an unneccessary item. Nothing could be further from the truth. Driver's who are hesistant to use this belt have no doubt done so because of the dreadful fear that their future "social life" could be endangered should this belt come into service. While it is true that the belt could keep a driver from submarining under his lap & shoulder belts, the real practical service this belt provides is keeping the lap belt from "creeping" up the drivers torso. Any driver with experience generally tightens his belts when opportunities arise. The anti-submarine belt keeps the belts in place and properly aligned. Broken ribs and internal injuries could result in hard frontal collisions if the belts ride up the drivers stomach area. I always used the anti-sub belt, and know many others who do as well. None of us presently sing soprano or ever had injuries that resulted from use of the belt. Make sure you employ it's use if you don't already do so.
Prior to installing the rub rails and rear bumper, we need to determine the body & interior deck dimensions. For some this may be simple, but those who are building their first car from the ground up may find some of the following helpful.
It helps to have the car frame at ride height, but isn't absolutely necessary.
HotMod rules allow for some body/interior deck rake. Don't go nuts trying to make your car look like a doorstop, and you probably won't have any problems.
First consider the window opening desired. Rules allow for 12' minimum to 18" maximum opening.
Unless your driver’s build runs on the slight side, I suggest a window opening larger than the minimum required. Today's race cars are generally built so tightly in regards to the window opening, it would appear to the casual observer that a degree in limbo dancing is a prerequisite to being able to drive. Watching some of our healthier sized drivers snake their way in & out of the window area of the LM & Modified cars cause concern in their ability to exit the car in an emergency.
When satisfied with the opening, wrap some masking tape around the rear roll bar upright, indicating the door heigth desired. From this point, run a straight edge across to the passenger side bar, level, and tape that bar for height.
1" box tubing can make a frame work to run from the car frame to support the rear deck. I advise angling this from its mounting point at the frame back toward the rear of the car, making the interior mount slightly rearward of the frame mount. A couple of inches or so will do the trick. Use a string or chalkline to get an idea of the run from proposed front firewall to back of the interior decking. The interior can have a 4" maximum overall rake (see rulebook page 12, line item 10) so let this be your guide.
For the car to look correct when finished, and ease the interior and body installation, use a this stringline as a guide for your interior support mount locations. Build your front firewall uprights off the the front bay bars at the rear motor mount location, or no farther than 72" foward of the centerline of the rear axle (see page 12, line item 11). Some adjustment on front and rear mount heights may be required before you are satisfied. Adjust the mount that will run across the rear of the main hoop (behind drivers shoulder area), again using a stringline. For width, the body can extend no further than 2" outside the right frame rail, and needs to be no wider than required to cover the driver side door bars. Maximum interior/body width is 66 inches, so keep this dimension in mind.
Some folks frame these deck support mounts by running a length of tubing from front to rear on the drivers side. They are then able to tie this bar into the top door bar, providing a support for the top of the drivers door. While this isn't necessary or required, it may be a something you may find useful in your build.
You will need a minimum of 6 tabs per side of the car to install body bracing. A couple on each side the rear clip, welded horizontally to the exterior side, should serve to brace the quarters. A couple on the lower frame rail between the front and rear tires, along with a tab on the front & rear cage upright halfway up the door should suffice for the side bracing. Weld a couple of tabs on the front and rear of cage to bolt your roof down.
With the front firewall location determined, locate the mounts for your brake, and if needed, clutch pedal. If your master cylinders are firewall mount style, mock up your firewall and clamp the pedal to it at the location desired by the driver. Build your mounting plate and bolt to pedal. Next build the frame work to support the pedal, remembering the prior geometry lesson on triangulation. Brake pedals can see a surprising amount of force applied by the driver, and need to be mounted solidly. Flex in the brake pedal must be eliminated. Pedal mounting kits are available, and may be a good investment. You may need to do some additional fabrication to suit your chassis, but this can be a real help for minimal investment.
The same can be said for the steering column bracket. Use a straight piece of tubing along wth the steering wheel to determine the location your driver prefers for the steering. Tack the mount to the cage's dash bar, and determine the location for the steering quickener mount if you intend to use one. Installing the quickener near the firewall make the steering column installation less aggravating. Otherwise, it’s a shot in the dark trying to determine where the firewall should be hole sawed for the column to pass through. This is also the time to install the driver protection windshield bars. The rules require a minimum of (3) 5/16" steel bars. You can install these bars directly from the cage's upper halo to the dash crossbar, but I suggest another step. A few chassis builders install a crossbar above the interior level, welding the vertical bars to it. This additional step will make the interior installation much simpler and easier.
Add some chassis tabs to the dashbar area to anchor your guage panel. These will come in handy when later doing the sheet metal work.
Bumpers and rub rail don't need to be fabricated out of heavy wall tubing. To correctly do their job, it's best if they are made of lighter material. The intent of all of these is to absorb any punishment, rather than passing on the force of the blow to the main frame. Better for them to bend than the frame itself. Properly installed, they can be unbolted and straightened or replaced.
Again, take time to look at different option for these on present modified cars. Different chassis builders install all of these in various but similar ways. All have went to the trouble of determining the most economical design for their chassis. If you find something you like, adapt it to your build.
Don't feel too badly about using someone elses ideas on your project.
Professional chassis builders do this as well, and besides, it's been said that "Imitation is the sincerest form of flattery".
Consult the HotMod rulebook (see rule 12A-H and diagrams on pages 12 & 13) for specifics.
External rub rails are required between the wheels (door area) but aren't required around the quarter panels. The rear rub rails are strictly optional, and can be run either inside or outside of the body. Rub rails can't extend further than the outside plane of the tires. Any exposed rub rail ends must turn in toward the body and be capped. Soft plugs work great for capping tubing and are inexpensive. Rear bumpers must either be cut flush and capped at the chassis mount, or make a 90 degree bend and be parallel to the frame rail. An exposed rear bumper end can make for a deadly spear in an accident, thus the reason for the rule. Be sure the rear bumper incorporates a fuel cell protector (rule 12 G) where the cell bottom is lower (or higher) than the bumper.
For the rear bumper and rub rail mounts, an easy and effective practice is to weld a short section of larger tubing to the frame, allowing the bumper or rub rail to frame member to slip inside. Drill and bolt through the entire deal, and you have a neat and removable installation. Be mindful that the rear rub rail mount on the drivers side of the car not be pointed at the driver. This should mount behind the driver, and not be long enough to get into the driver should it take a hard shot. Any rear rub rail must mount directly into the sides of the frame to avoid puncturing the fuel cell. Again, rear rub rails are optional.
Front bumpers are nearly all designed the same, with how they mount to the chassis as the only noticable difference. A nearby muffler shop with a tubing bender can be a great source for bending bumpers. Exhaust tubing is sufficient for bumpers, and unless your driving style includes liberal usage of the “chrome horn”, can be pretty durable. Likely more durable than the former friendships you had with guys you booted out of the way.
If you have determined your fuel cell and battery locations and mounts, and have welded them in (remember to weld a bolt to the frame near the battery for the ground connection) you should be close to being ready to paint the frame and cage. Take the time to roll the car around on it's sides to weld anything underneath. Ask someone else to check all the welds to determine that everything has been completely welded. Another set of eyes is always helpful when looking the car over at any time. When satisifed that the welding is complete, you are ready to squirt some paint.
We’ll talk sheet metal fabrication after you have the frame painted.
HotMod build project
Part 5 : Sheet metal fabrication
If you now have your project car frame & cage painted, you can begin the interior installation.
As we’ve mentioned previously, you may be experienced in this work, and could have a better system of doing much of this. As always, there are multiple ways of getting the same results, and what I pass on is nothing more than my personal experience and observations.
If you’re planning to tackle this work, and it’s your first time attempting this, I’ve found it much easier to have a lay out table for the sheet metal rather than to crawl around on the floor. This usually consists of nothing more than a couple of 55 gallon drums and 4 x 8 sheet of plywood.
Some needed tools include shears (electric, air or good old hand shears), a carpenters square, tape measure, sharpie marker, a long straight edge (aluminum angle works great!), tin snips and some vice grips or clamps. Sheet metal worker’s hand tongs or a pair of “duck bill” vise grips will also come in extremely handy.
Hopefully, you have access to a sheet metal break. If this isn’t the case, you may need to locate one you can use, or be prepared to improvise.
You need the capability to be able to make 90 degree bends for the panel seams. I’ve seen guys clamp the panels to work benches and hand bend, use pliers to make the bend after marking a bend line the length of panel, and even knew of a racer who used a door and it’s jamb to get his 90 degree bends. The latter method resulted in replacing the door and it’s hinges after ruining both.
Where there’s a will, there’s a way.
For simplicity sake, we are going to assume you have access to a break, and will be working with prepainted 4’ x 10’ aluminum sheets. While it hasn’t been my practice to specifically name brands or manufacturers up to this point, I need to do so in this case.
I have worked with aluminum sheets of differing grades and quality. Some have been adequate, some poor, but I have never been anything but wholly satisfied when working with ‘WRISCO’ aluminum products. Their 4 x 10 sheets come pre-painted and the finish side is covered with a tough layer of plastic sheeting. They have a fairly wide choice of colors, which is great for a guy like me who hates to paint. Past painting experience created an overspray mess in the garage, and generally spending more than I intended to get the job done correctly, so it also saves money versus trying to paint the body & interior panels. The grade of aluminum used is of good quality, and can be bar folded (doubled over) without splitting. I have saved a dollar or 2 per sheet in the past by purchasing an unknown quality of aluminum, then kicked myself because of the results. If you are going to fabricate any aluminum, my advice is to seek out and purchase aluminum sheeting produced by ‘WRISCO’.
There is a wide selection of choices for rivets. I use mill finished rather than painted heads. If you change body colors, you don’t have to paint the rivets is my thought.
“Exploding” rivets (the mandrel end opens and expands against the button head) are a little more expensive, but work great, especially when replacing the rivet in a crashed panel that has the original rivet hole “wallowed out”. A word of caution when installing these with an air riveter: don’t get your finger close to the back side of the rivet! These can pinch down unmercifully on your fingers.
Take the time to measure your interior deck panel lengths, and calculate how to optimize the use of the aluminum sheeting.
An old carpenter’s saying definitely applies here, as you should “measure twice & cut once” to avoid costly mistakes. If you do make a mistake, set the piece aside. Chances are good that you’ll be able to use it later on for something else.
Speaking of carpentry, approach this part of the project with a carpenter’s mindset. Just like an old house that is being remodeled, your car probably is somewhat less than square. Remember this when installing the interior and use the factory edges on the aluminum sheeting where possible to make things square. No matter how you do it, there will be plenty of trimming to fit involved.
Just like building a house, we first must have a good foundation. For the interior, this is the floor pan. The floor pan should run from front to rear firewall, and fit snug between your seat belt mounts (or the inside car drivers frame rail). The front right edge needs to be angled inward to accommodate the transmission. The outside (along the frame rail) should be straight. I recommend bending a 90 degree 1” (minimum) lip along these sides if possible. Bolting the seat in through the floor pan anchors it in place. Do this before advancing to any further sheet metal work. If the floor moves at all, it messes up future sheet metal work tremendously. I strongly suggest building the driveshaft tunnel out of the same material as you use for the floor pan (see rule 2 J). If a whirling driveshaft ever gets loose, you’ll thank yourself for this. The tunnel should be tall enough to clear the driveshaft safety loop, and can bolt or rivet to the inside floor lip.
So much of the cockpit area is dependent on how the cage is constructed that we won’t even attempt to go about the steps here, but instead recommend to again investigating how some of the cars in your area are constructed. Also consider that it’s sometimes better to build panels in a multiple pieces, rather than going through the aggravation of prying, twisting and cursing in the attempt to fit the single panel. The front firewall will be a perfect example of this for many attempting this project step for the first time..
Because you don’t need to consider things such as lift or pull bar mounts, the passenger side of the interior can be slanted towards the passenger side window opening. This could be an asset, should the driver encounter difficulty making an emergency exit.
The cockpit area should be functional, but is a matter of personal taste in regards to its design, and should be completed prior to installation of the interior decking.
When you are ready to start the interior deck, get out the string line, carpenters square and tape measure.
I’ve seen some excellent craftsmen who have lined up every interior seam front to rear, going to the trouble of hiding every rivet head from view. While this is impressive, we aren’t about to go to that degree of difficulty for this build.
I have always preferred to build the rear deck panels (from the main hoop back to the tail of car) before building around the driver’s compartment.
The reasons are these panels are generally bigger, require less notching around the roll bars, and should be fastened into place prior to installing the front panels.
Always overlap any front panels over the rear panels (ie: doors over quarter panels).
If you were a high school scholar in basic geometry, or even drafting, the following steps will be clear.
We will make the rear outside panels first. Tie your string line at the front deck support, and stretch it to the rear. Adjust the string to just touch the outside most roll bar, making the dimensional width measurement the same at front and back. Use your carpenters square to verify that your string line is perpendicular to either the front or rear support. Remember to use this support as your baseline starting point for the remainder of the interior fabrication and installation process. Hopefully, your panel dimensions should be a true rectangle, rather than some strange trapezoid shape.
A ‘work around’ for this is to measure the overall panel length and width, remembering to add for 90 degree bends for the seams.
If using a decent break, it will stiffen the panels to install a ½” bar fold (hem), and then install a (1”) 90 degree bend. Do this on the factory edge, and fit the panel in place with the bent edge against the roll bars, lining up the front (or back) edge with the deck support you plan to use as the baseline. Make a mark underneath the panel at the outside end of your front & rear supports. You should now be able to make the outside bend, and have a good fit. Repeat this step for the other side of the interior.
Anchor these panels in place with a couple of rivets or a ‘cleco’. These little boogers act as a “temporary rivet”, and are great for this kind of fabrication work.
I don’t recommend notching the outside panels around the roll bars. These panels are the ones that may need repair or replacement if damaged. Weakening them with notching makes them more difficult to fix or fabricate if needed. Never notch both panels around a roll bar. This creates a weak point where the interior will droop and eventually crack.
Next, build the panels that will fit around your cage’s rear roll bar(s). Take into account where your fuel cell filler cap is located, and avoid installing a panel seam in this area.
Three (3) panels in the interior area work well. They’re width is entirely up to you.
If you’ve squared the outside panels relatively close to the true rectangle shape, go ahead and bend up the panels that will fit around the roll bars. Place them on the supports, and measure at the front and rear to make sure they are equally distant from the panel they will anchor against. Mark the area around the roll bar on the interior panel, using the same dimension for your depth of cut around the roll bar as the distance between panels.
If you desire a tight fit, a cardboard template will make for a closer fitting panel.
If you use hand snips to make cuts, take the time to use a file & clean up those jagged edges. Kids seem to love to run their hands over everything (in fact I believe it’s a requirement for the “rights of passage” from childhood). Save them, the track inspector, your crew or yourself the pain of a cut caused from jagged edges anywhere on the car.
For the center filler panel, use the same steps as we did previously on the outside deck panels. When you bend your final 90 degree seam, make the panel just a bit narrower than your mark indicates. The panel will go in place without force fitting, and the rivets will draw everything up nicely. If you plan to install an fuel cell cap access panel, piano hinge works well for the door. A simple round opening that will accept whatever you use to fill the cell (fuel jug or funnel) works just as well. Taking the time to cut this opening before final installation works best.
If you are satisfied with the rear deck, anchor down the panels with rivets on the rear supports, and then crawl under the car to rivet the panels down the seam. The front of these panel will be riveted after installing the front decking.
Before doing this, if you haven’t installed the fuel cell you may want to do so now, rather than later removing the rear deck when you find it won’t fit when attempting to install from below. If you use plastic coated aluminum, don’t forget to peel back the protective sheeting at the seam areas and cross braces before riveting.
Take care to make sure the panels are kept level, and use vice grips rather than your (or an unwitting assistant’s) fingers to hold the panels together. There are few things that can ruin an otherwise pleasurable day in the garage like reversing a drill bit buried in a finger.
When riveting the interior and body into place, try to remember that you’re building a race car and not a skyscraper. While we certainly want the panels secure, try to suppress the overcoming urge to go crazy riveting panels every couple of inches. Deck panels generally need only the corners and a rivet or 2 in the middle fastened to the cross supports. Rivet the ends of both seams underneath and at each side of the roll bar openings, then space the rest of the rivets. With your hand spread open, the distance between the tips of your thumb and little finger (about 8” for me) is plenty close enough between these rivets.
Installing the front panels (passenger side) can usually be done with 2 panels, the outside one fitting over the roll bars, while the inner panel makes a filler to fit to the drivers’ compartment. Usually the outside panel will slide back over the main hoop, and inwardly over the right side ‘windshield’ bar. Use the string line again to determine the outside panel width after squaring against the inside edge of the ‘windshield’ upright roll bar. Use the same steps as we did for the center panel in the rear for the filler panel to the driver’s area.
Finish the area out around the footbox, driver’s door and dash panel, and you have the decking completed. Hopefully, the scrap left over is contained in a pickle bucket (seriously!) rather than piled in the back of a truck.
It’s been a full day, so stand back and admire your craftsmanship before taking your coffee thermos and going to the house. Don’t forget to turn out the light!
We’ll make the car a roller in the next edition of ‘Project HotMod’.
