1966 Dart GT IRS Project Journal Part 4

By David Belau

dartslantsix

Last week I bored you with a math lesson and preached about proper suspension geometry. I also showed pictures of my sexy control arms. You can’t have one without the other, unfortunately. This week, I’m going to talk about all the little things that make this whole system work. Continue reading…

First, I’m going to answer a question that was asked about last week’s post. Bob Balderston asked:

“my question, as i have owned many vehicles with irs, does this unequal arm set up control the squat/offset that so often plagues this kind of set up? i dislike the look as well as the wear caused by the tires sitting out on odd angles; i am hoping that no matter what the ride height is, the arms remain parallel with the chassis at all times.”

Bob, the tires will have more negative camber as they compress. So, if there’s a load in the trunk that compresses the suspension, the tires will go more negative. This is a good thing in corners because of the body roll. This will keep the tire perpendicular to the road as the body rolls. If the tires stayed parallel with the body as they compressed, they would also roll in a corner resulting in loss of traction. The static camber as it sits at ride height can be adjusted for the type of driving. For instance,  a car that is driven mostly on the highway will have less static negative camber because it won’t be cornering hard. A car used for autocross will have more static negative camber because it’s driven hard and the body will roll more. This is set by running the car and then taking a temperature reading across the width of the tire. My car is set at 2 degrees negative camber at ride height. It doesn’t look strange at all.

Bob’s question brings up another topic in suspension geometry that I did not address last time. That topic is anti-squat geometry. This means as a car is accelerating, the suspension will tend to resist compressing. This pushes the tires into the pavement and increases traction. Funny cars are set up so that they will actually raise up when they launch. I considered adding this to my design, but after doing some reading, I decided against it. Here’s why: in a corner, if I got on the gas, I didn’t want the car to change how it was handling. This could be argued I’m sure, but it’s what I decided. I’m open to any arguments.

Okay, now on to all the little things that make this work. I’ll start with the half-shafts. Since I had to narrow the whole system, the half-shafts also needed to be shortened. There’s a few ways to do this: machine new shafts, cut and sleeve the shafts or cut and weld the shafts. I was on a tight budget, so new shafts were out of the question. I knew that sleeving would be the strongest option so I ordered some tubing that would fit. I also figured that this would allow me to do all the welding. The sleeve would keep the shaft straight and somewhat balanced. However, when I got the tubing, it wasn’t as tight a fit as I would have liked. So, I ended up handing this part over to a friend with a lathe. He beveled the cut ends and filled the void with weld material, then turned them down. I know that this isn’t as strong, but what could a Slant Six do to them? Turning them in a lathe also ensures that they’re straight and balanced. Here’s a picture of the shaft tacked together, before it was fully welded, when I was checking length.

Length is important because the joint has to plunge as the suspension compresses. If the length is wrong, it could cause the joint to fall out of the housing or hit the inside of the housing and self-destruct. The following are pictures of me checking at full bump and full droop. They were pretty much at the maximum angle that would still work.

The finished half-shafts are shown below, put back together after I was satisfied that the length was correct.

I had to get a new drive shaft made that would connect the Mopar slip-yoke to the Ford 8.8 differential. A local shop did this for me.

I rigged up this temporary exhaust that exits just in front of the rear wheel. There wasn’t room to run it all the way back. It’s just a generic turbo muffler. It looked good when the car was up on stands, but was too low once the car was on its wheels. It looked pretty funny.

For coil-overs, I went with Aldan Eagle. I could have gone cheaper with a non-adjustable shock, but I wanted to tune these to my set up. Aldan offered the best deal I could find, plus they’re made in the USA. I went with a single adjustable shock because I couldn’t afford the double adjustable. I also went with spherical bearings on the pickup points instead of bushings for less binding. They recommended 400 lb/in springs based on a Jaguar type system. This turned out to be too soft, but we’ll talk about that next time! Below is a picture of my first set of Aldan Eagles, I ended up returning them because they were too long. Aldan was cool and exchanged them for shorter ones. Gorgeous pieces!

The hubs for the Thunderbird are a strange 5.00 x 4.25 bolt pattern. I had these redrilled for 5.00 x 4.50 pattern to match the front hubs. I had a local drag racer do this for me. He did a great job.

The brakes are standard Thunderbird Supercoupe pieces. The car previously had drums in the rear, so a few things needed to be done for these to work. I kept my standard disc/drum master cylinder after verifying that it did not have a residual pressure valve for drum wheel cylinders. Next, I gutted my standard proportioning valve so that it acted as a distribution block only. The innards are shown below.

The front brakes do most of the braking on a car, so more pressure needs to go to them. To accomplish this, I installed an adjustable proportioning valve in the rear line. I got mine from Summit Racing.

For the brake lines in the rear, I used my stock rubber flex line up to the subframe. I then ran hardlines out to where the flexlines would attach. Double flares are the bane of my existence, so these lines in the picture got replaced because they leaked.

One thing I’m lacking are mounts for the flex lines to attach to. The next build will include these. I’m concerned about the hard lines fatiguing and breaking if they’re not supported. I eventually added a loop to the hard line to help keep them from fatiguing.

The flex lines I used are stainless braided lines from Oil Filter Service Co. out of Portland, Oregon. The guy there was super friendly and had what I needed on the shelf. They can also make rubber hoses based on your needs.

Another aspect of the brakes is the parking brake. These calipers have an integral parking brake. It would take a custom length cable to operate these, but I haven’t coughed up the money yet. The car is an automatic, so I’m not worried too much about not having a parking brake.

When I got all of this put together, I tried putting my stock steel wheels on the hubs. Wouldn’t you know it, the caliper hit on the wheel! At this point, I had maybe $30 left in my budget. So, I went to the junkyard. After some measuring, I found these Ford Explorer wheels that flare out and away from the caliper.

At this point, I had 0.24 cents and a piece of lint left in my budget. The new wheels were 15″ and my old wheels were 14″, so my old tires would not fit. I got out my bartering tools: a black ski mask and my .38 special. The first tire place I went to immediately called the cops. Obviously, they didn’t know the rules of bartering. The next place said they’d trade me straight across.  In exchange for my old tires and rims, I’d get a set of used tires mounted on the new rims. I was glad that we worked out a deal that didn’t involve pistol whipping.

When the tire guy rolled out my new (used) tires on my new rims, I felt my blood run cold. He’d mounted up a set of snow tires! My left eyelid started to twitch and I reached for one of my bartering tools. Lucky for him, my wife (wearing a pink ski mask, so cute!) said we should get out of there.

There’s a point in some projects where you don’t really care anymore. You just want the bloody thing finished. I was at this point. I knew that eventually I’d go to some nice 17′s with sticky tires, so I let it go. (It sure raised eyebrows at my first autocross with the new rear end!)

Below are a couple pictures of everything installed. Note the brake lines are looped.

The only thing that gives it away is the huge nut that connects the half-shaft to the hub. That’s just the way I wanted it.

That’s all for this week, next week I’ll talk about its maiden voyage and first trip to the autocross track. As usual, if you have questions, leave them as comments and I’ll answer them.