1966 Dart GT IRS Project Journal Part 2

By David Belau

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Last week I wrote about how I picked a donor car for my independent rear suspension (IRS) project. I scored an IRS out of a 1993 Thunderbird Supercoupe, which included an 8.8 differential and disc brakes. This rear end is strong and has plenty of after-market support. So this week I’m going to talk about how I built my subframe. Continue reading…

Before I get into the meat of this post, I’d like to talk briefly about fear. I’m a perfectionist and if I’m going to do something, I want it to turn out well. This can be a very good thing, but can also be crippling. If I don’t have experience in an area, I fear that I can’t do a good job. This keeps me from even trying. If you’re the same as me, don’t let lack of experience stop you from trying something. There’s plenty of people who are smart and can lend a hand. My senior project in college was this contraption.  After that project, I realized that there isn’t much that can’t be accomplished.

Do you feel motivated? All right, let’s go! (I live in a van, down by the river…)

Before I even turned a wrench on the Dart, I knew that I needed to make some measurements. First, I measured ride height at the rocker panel, since I was happy with the ride height. Next, I marked the center of the rear wheel on the quarter panel.  This helps later when figuring your lower control arms, which dictate where the wheel will be.

After I took these notes, it was time to rip the old axle out. After this was accomplished, I needed to raise the car and level it. I used four RV stands placed under the frame of the car. These stands allowed me to get the car perfectly level. A word of caution: these type of stands are not sturdy. When I worked under the car, I had a second set of regular jack stands to back them up. Again, if you do something dumb and die, it’s not my fault. Don’t be dumb.

It’s good to start with a plan. There are a couple ways to start a project like this. One way would be to completely model the frame in a solid modeling program. This is a great way to go, but it is very time consuming. Since I didn’t feel like modeling the underside of my car, I didn’t do this. I decided to design and build as I went. So, with a sketch on a napkin, I was ready to go.

One of my goals with this project was that it be a bolt-in, without major changes to the car. I knew that the best way would be to mount my subframe to the bulkheads where the leaf springs were attached. The first thing I did was build mounts for the new differential and tack them onto the beginnings of my frame. In this picture, you’ll see that I left the rear tube long. That’s because I wasn’t sure how I would attach to the rear of the car. Material is 2.00 x 2.00 x .125 wall mild steel tubing. I selected this because it’s cheap and easy to work with.

In case you’re wondering, this is in my single car garage, before the Dart was taken apart. It got pretty tight at times, but I made it work.

The next thing I did was make a wood stand to put the frame/diff approximately where it would go under the car. This allowed me to see potential clearance problems with the floor pans. As it turned out, I needed to lower the differential a bit. Notice how close it is to the gas tank. Most muscle cars I’ve seen with an IRS have removed the stock tank and used a fuel cell. That’s too easy.

Next, I made frame members that attached to the leaf spring brackets. These come back and attach to the other part of the frame. I had a friend with a lathe bore the round stock out for me. I then used some nylon tubing to line the holes, to help with vibration. I also drilled out my leaf spring brackets to use the stronger 5/8″ bolt that came on the V8 cars. My stock 6cyl leafs used a 9/16″ bolt.

I left these long, because I wasn’t sure where they’d come into the other part of the frame. This picture shows how they attach to the car. Also visible is one of the RV jack stands that I used to level the car.

The next picture shows me measuring and figuring how to tie these two pieces together. It turns out that they were close enough to put into the same plane horizontally. Score!

This next picture shows these pieces tacked together. Again, that rear tube is left long because I wasn’t sure how I was going to attach to the rear.

This next picture shows the frame attached to the car and in place. I was attempting to see how to attach to the rear of the car (where the leafs were originally). I noticed two things: the tubing to the back would be very long and would probably interfere with the tires. Second, there wasn’t a good way to triangulate. So, I made the decision to go up and attach to the frame of the Dart with some saddles. The saddles, I figured, should spread the load on the sheet metal and not stress it too much.

Here’s a picture of the saddles installed. I had a friend at work use the waterjet to cut them out. The tube that runs between them was positioned to line up with the upper control arm mount. I’m skipping some steps here, but I built the upper and lower arms at this time. I’ll discuss them in a later post. Also visible is a notch I made to clear the top of the differential.

Here’s a teaser picture of the lower control arms. I added this to show the tabs that I used. I found them, along with a lot of other good stuff at: A&A Manufacturing. They’re a race car fabrication company. They had the best prices I found and their service was quick. Also note that the rear tube is now cut down.

I promised blood, so here it is! I was using an angle grinder that didn’t have a handle on the side. It decided to feast on my knuckle. After this I bought a new angle grinder with a side handle. Make sure to wear safety equipment!

This picture shows my upper control arm mount welded to the frame. I had these cut on the water jet.They’re made from .25 thick steel. Also visible are the tubes I used to connect the lower frame to the upper tubing. These aren’t the best configuration, since they don’t provide any lateral support. Square tubing gets real funny in compound angle joints.  So, these were the best I could do.

This picture shows the coilover mounts that I made. I added another tube between the saddles to mount these. Notice how I tied them into the tubing and the saddle. This is probably the most stressed part of the frame. The weight of the car and any bumps are supported here. I used .25 thick steel since the Thunderbird used .25 steel.

This is my bump stop. The upper control arm hits this when bottoming. It’s just the stock stop from the old axle. Don’t make a suspension without bump stops. Like Rosie O’Donnell on the cover of Maxim Magazine, it just doesn’t make sense.

I triangulated the frame here to resist deflection from cornering. Visible here is the second tube that I added between the saddles. I’m not happy with this, because it adds weight and isn’t sexy. It works for now. Also, notice that there are extra tubes down where the differential mounts. I had to move the differential forward to help keep the CV boots from hitting the upper control arm mounts. Again, I’m not happy with this, but it works for now.

Below is a picture of the completed, but not painted, subframe.

There’s a lot of thought that went into this project that I can’t really write here. If I did, it would be really long and boring. My take on the subframe is this: it’s a good prototype. There are a lot of things I’d change. First would be a switch to round tubing, using as many bends as I could (Not welded joints). I’d like to better triangulate it and remove some of the unnecessary tubes to lighten it up.

Remember in the beginning of the post when I talked about modeling the frame in the computer? I can see the value now. And now that I have a prototype, I can pull dimensions off of it. Below are a couple pictures of the next generation I modeled up in Solidworks. It’s just a quick and dirty rendering but shows what I’d like to do next time. An advantage to the computer is being able to make easy changes and see their effects. For example, the coilover mounts could be moved around to affect ride height based on different spring rates. (My adventures in spring rates are coming soon!)

That’s it for this post. Join me next time for a math lesson or how to get the right suspension geometry!