Building a Simple Hot Rod Chassis From Scratch

In a previous tech article, we showed you how to build a cheap, simple chassis table you could use to build or modify your frame on. With that project done, I decided to start building the chassis for a 1930 Model A Coupe project I’ve been gathering parts for. Knowing how to build a car chassis from scratch is a great skill for anyone interested in hot rods, classic cars and race cars or if you want to create a custom vehicle that’s a true original.

I’ve outlined my process below so others can learn the essentials of fabricating a basic chassis and frame to serve as their vehicle’s foundation. In addition to the article below, you can also watch my “How to Fabricate a Chassis” video that documents the process so fellow hot rod enthusiasts can follow along.

Hot Rod Chassis Building Tips To Get Started

Before I begin with this project overview, I want to provide some general tips about building a custom chassis. Regardless of what car you’re creating, there are some common keys to success — and common mistakes that lead to failure. Let’s look at a few dos and don’ts of chassis building:

• Make an outline of the chassis before you begin so at least you have a general idea of what the final product should look like.
• Always have a plan for your project that includes a timeline, budget, tools, etc., and be prepared to spend two or three times as much time and money as you originally wrote down.
• Be sure your shop is organized and don’t throw any old parts or materials away until the project is completed.
• Be realistic about your abilities. It’s better to farm out a step or two than to have your project end up in mothballs due to lack of experience.
• Don’t rush. Missing or skipping a step now means you’ll likely have to spend extra time later to fix a mistake or two.

Fabricating a Classic Hot Rod Chassis

The vision for this project was to build a traditional hot rod using a strong chassis that gives the car a nice stance, all while utilizing some old and original parts to give the build the “soul” of a car built back in the late 1940s and early 1950s. This means that, other than the raw material used and the replacement maintenance-type parts, we’ll be building it using old “stuff”.

I was going to use an original Model A frame, but by the time we straightened it (every “A” frame is tweaked or bent some) and cut it up to modify and box it, we were into more work than building a car chassis from scratch would take. I decided to build the box tubing chassis in such a manner that it’s done tastefully and doesn’t stick out like a sore thumb.

Step 1: Selecting the Chassis Frame Tubing

I started by ordering up some 3″ x 2″ x ¼” box tubing for the frame rails and rough-cut two pieces to length. For my build, the ¼-inch wall thickness was probably overkill, but I decided to use it for diversity in case I ever wanted to change the drivetrain to something larger and heavier. Also, the heavier tubing has a gentler curve on the outside of the tubing that I like better. If you’re building something lighter like a Model T, a little speedster or even an A with a V8-60 or the original 4-cylinder banger, you can get away with thinner wall tubing to save some weight. But you can’t go wrong with the heavier wall tubing, especially if you are building a truck frame from scratch that needs to be as strong as possible.

Step 2: Getting Started and the Crossmember

With my tubing laid on the table, I decided to start from the front of the chassis and work my way back. I began by putting the table on a nice flat spot on the shop floor and adjusting the table until it was level everywhere. I then printed out stock Model A and 1932 Ford V8 frame specs to use for guidance when laying out my chassis. Even though I’m going a custom route, it helps to have some factory specs to go off and keep you in check when getting started. I’m using a stock Model A front crossmember, so I marked and cut out the inside of the box tubing so that the cross member could slide in on each side. I left a little bit of material in the front to add a pair of factory A frame horns.

Now that my crossmember could slide in and out of the rails, we were ready to start setting the width of the frame and also get some measurements laid out that we’d be using throughout the build. Since I’m using a factory Model A crossmember, I could just push the rails together so they sandwiched the front crossmember, and we were on the money for the frame width at the cross member. From there, we needed to angle the rails outward, as the frame gets wider in the rear.

Step 3: The Frame Rails

The important part for me here was to hit the factory width of the rails at the cowl so it slipped down tightly over them when I channeled the body. A factory frame sort of “kinks” out even further after the firewall and again at the B-pillar. Since I’m channeling the car, I wanted my frame rails to sit inboard of the inner “rocker” area of the frame more than stock so I didn’t have to cut them out of the body. Because of this, I let the frame rails continue to naturally angle out from the factory frame width at the cowl. Once the rails were squared up and I was happy with the measurements, I clamped the rails down to the table and added bracing to keep the rails fixed at the width I wanted so I could fine-tune everything.

Step 4: The Chassis Engine Bay

I again checked that my frame rails were square with each other and the width was correct, then measured out the engine bay spacing. In this case, that is determined by the distance from the center of the front cross member to the firewall. Since I’m running a Flathead V8, I decided to make this area the same as a 1932 Ford V8, which is larger than a Model A. If you’re running a factory hood, you will be fixed to the factory dimensions. But since I’m only running a Deuce (’32 Ford) grille, that wasn’t an issue. I marked and scribed the center lines for the firewall measurements on both sides as these would be a data line we’d use throughout the build. As silly as it may sound, these first measurements are probably the most important part of building a car chassis from scratch. Just about everything you do until the car is built will be referenced off those lines. If they’re off, everything you do will be crooked or off. Double, triple and quadruple check that those suckers are dead-on!

Step 5: Welding the Crossmember

I then clamped the crossmember in place so that it was sitting with five degrees of caster. Ideally, you should put some initial caster in here, so you’re not trying to pull it all out with the wishbone angle (which also sets caster). I again checked my measurements by measuring diagonally from the corners of the same hole on the crossmember on each side and the firewall data lines we set already. Really make sure you dial this in before welding! I then welded the crossmember in with the MIG 175 and we had a base off which to build our front suspension.

While I was on a roll, I took a pair of factory Model A front frame horns and welded them onto to the extra material we left sticking in front of the crossmember. If you’re trying to stay to stock dimensions, you will want to cross measure here to the factory holes on the horns. I decided to just eyeball what looked “good” to me before squaring them up and welding to the frame rails.

Step 6: Lowering the Front End

With the front portion of the frame started, I was ready to make some modifications to get the front end of the car sitting how I wanted. There are a bunch of ways to get the front end sitting lower, like a “suicide” front spring perch, air suspension or cutting and putting a “Z” in the front of the frame. All these methods are pretty obvious and don’t really match the look I’m going for, so I decided to “sweep” the front frame rails. This method has become more popular over the years as it allows you to lower the front a little more and gives the frame a nice curvy appearance similar to the Ford frames that are a few years newer. This isn’t the only way to lower the front more; it’s just how I chose to do it on this build.

There are a couple ways to do this. One of them is to take a pie cut out of the top of the frame just in front of the firewall to bring the frame up all in one spot and weld. I wanted to accentuate the sweep, so I decided to space a number of thin slices across the top of the frame. I made these slices close together at first and spread the last few apart more. I used a reciprocating saw to make clean, thin cuts. The key is to cut three of the four sides of the frame so it can flex. Once I had all the cuts made, I pulled up on the frame to close up the gaps of the cuts. This resulted in sweeping the frame up about five degrees. I used an angle finder on the frame to set the upward sweep the same on each frame rail. I then beveled each of the seams for proper weld penetration and tack-welded each of the cuts to hold the frame in that position.

Step 7: Modifying the Front Axle

If the frame was left swept up just like this, it would throw the alignment of the car all out of whack, so we need to take that five-degree angle out at the front axle. In the first picture, you can see the five-degree sweep the frame currently has. I first added some upright braces to the front of the chassis table and tack-welded the side of the rails to the uprights, so when I was pushing down on the frame, it stayed true. I then took a pie cut out of the bottom of the frame rails just behind the front crossmember so the frame rails were level again at the crossmember. The crossmember itself still retains its five-degree caster as it’s already welded in place. For this portion of the project, always measure off the top of the frame rails, not the crossmember. I then beveled the seams and tack-welded them in place. Finally, I double-checked that all the angles were even and welded all the seams with the MIG 175. The result gives the frame a nice flow while sweeping up to lower the front ride height.

Step 8: Springs and Tires

With the frame sweep finished in the front, we were ready to pull the front half of the chassis off the table to start mocking up the rear ride height, wheelbase and body channel to set the stance. I decided to utilize a complete 1950’s “Dago” Dropped axle with “juice” (hydraulic) brakes and split wishbones I had stashed away. I removed the cobbled-together front spring and installed a nice reverse eye dropped “Super Slide” spring from the guys over at Posies Rods and Customs. They were nice enough to make me a set with the ends rounded and hidden super slide pockets so that once they were painted up, they could pass as an old, modified part with the performance of their high-end springs. I also mounted up the desired size wheels and tires I wanted to run on the car. These items must be decided ahead of time so that you can mock everything up correctly from your hot rod frame plans before making any major changes. On this build, I’m running 750-16 in the rear and 600-16 in the front.

Step 9: Body Height

With the back half of the chassis on jack stands and leveled out, I dropped the body over the top of the chassis and rolled the complete rear axle with the crossmember mounted to the spring under the body. This allowed me to fine-tune the amount I wanted to channel the body over the frame. I then temporarily mounted the drivetrain to weigh the front end down and get an idea of the final front ride height. This allowed me to set the rake on the frame and body without it changing too much once the car was assembled.

Now that the body height was set and the rear wheels were sitting where I wanted (in my case I didn’t need to block them up), I took measurements from the top of the frame rails to the top of the rear crossmember. This gave me a measurement of how tall my rear frame step was going to be. I also stood back and checked to make sure the wheelbase looked good and did some minor adjustments to get the wheelbase measuring what I wanted (just slightly longer than a stock ’32 Ford V8 wheelbase). I noted all these measurements and took everything back apart so I could mount the chassis back on the table.

Step 10: The Rear Step

After setting the chassis back on the table, we used mechanics wire to cross measure where we’d be cutting the back half of the frame for where the rear step would start. I chose to make it start just behind where the rear firewall would be so that, when the car was closed up, you wouldn’t see the large step (but we did lose most of our trunk space). I cut 45-degree angles into the rails at these measurements and tack welded the entire chassis to the table so it wouldn’t move during this process.

We then took our frame rail to rear crossmember measurements and built a fixture that put the top of the crossmember at the exact height we wanted and tack-welded the cross member to the fixture. Then we set the rear crossmember so that it matched our wheelbase and cross-measured from the front cross member to the rear cross member to make sure the wheels were straight. The fixture was then welded to the table to lock in the rear crossmember. This step is important to do two or three times to make sure the vehicle drives straight and doesn’t “crab” down the road!

At this point, it’s as simple as connecting the dots to make your rear frame step. I first measured out the shape of the end of the crossmember into a piece of tubing and notched out the bottom of the tube so it fit over the crossmember. I then took the drops from our rear 45-degree frame cuts and clamped them in place so I could lay another piece of tubing across the crossmember to see how long the leg of the step needed to be and where the pieces intersected. This allowed me to draw out the 45-degree cuts for these two pieces.

I then beveled all the joints and clamped the pieces in place. I adjusted them one final time until the steps were even and square on both sides. With everything measuring correctly, I fired up the MIG 175, welding the rear step up and finally to the rear crossmember. I will be adding wedges or braces to the corners of each joint when we tear the car down for final welding and paint prep.

Step 11: Suspension Assembly

With the chassis all welded up, we could cut it free from the chassis table and assemble the suspension. Of course, admiring the frame only lasted for a short time before we had to mock the body and engine back up to really get a good look at how the car was going to look! I’m really happy with how it sits, and with some small adjustments as we build the floor, I think it is going to have a really great stance!

With the chassis all welded up, we could cut it free from the chassis table and assemble the suspension. Of course, admiring the frame only lasted for a short time before we had to mock the body and engine back up to really get a good look at how the car was going to look! I’m really happy with how it sits, and with some small adjustments as we build the floor, I think it is going to have a really great stance! Now that the body and drivetrain are mocked up, I can start making crossmembers, engine/transmission mounts, and radius rod mounts for the frame.

Final Thoughts

This tech article is meant to inspire and give you a jumping point for building your own hot rod chassis. Every car is going to be a little different in measurements and execution, so be sure to think it all through before you start. Also, make sure that you’re confident in your welding skills before tackling a project of this magnitude. Remember, the aim is to build a strong and safe vehicle! Keep an eye out for future tech pieces as I build up this little A Coupe.