1966 Dart GT IRS Project Journal Part 5

By David Belau

dartslantsix

Last week I talked about all the little details that make this thing go and stop. If you’ve been waiting to see if all this nonsense actually performs, this is your week. I’ll show you the good, the bad and the ugly.

Before I get into this week’s post, a couple people have asked what the weight difference is between this set up and the live axle. I gained about 100 lbs, but I also upgraded from a wimpy 7.25 rear end to a stronger 8.8 Ford. If I redesigned the structure, I think I could lighten it up by at least 25 lbs.

My family has a long history of going to car shows. One in particular I have always enjoyed more than the rest. It’s a Father’s Day show at the Yreka, CA fairgrounds put on by the Siskiyou Cruisers. It’s in the shade, under the trees. Everyone drives down early in the morning and has breakfast. In short, it’s a great time. I had a goal to finish the car about a week before the show-to work out any bugs. (Murphy loves me for some reason.) I met that goal. Continue reading…

The first time I took it out, I just idled around the block. I was half expecting explosions or at least bad grinding sounds. There were none. Next, I ventured out into a higher traffic area and went to 35 mph. Immediately, I noticed that there was more road noise. I think this is because I solid mounted the differential and the subframe to the unibody. There were no problems, so I cautiously took it up to 55 mph. There were no vibrations, no explosions, no drama. I then got cocky, hit passing gear, and roared up to 75 mph. Guess what? Nothing happened. The car was stable and not scary.

One thing that wasn’t working well was the brakes. I checked the rotors and they still had surface rust on them. The rear brakes weren’t working at all! After some research, I found that the Thunderbird calipers have a ratchet mechanism for the parking brake. This adjusts for wear and the parking brake needs to be used for it to work. I didn’t hook up the parking brake, so I just crawled underneath and worked the lever on the caliper a few thousand times. This made the brakes work finally!

The week passed with no other problems coming from the car. That weekend I loaded up my wife and my 11 month old daughter and drove 100 miles to my parents’ house. It wasn’t a pleasant trip. My knuckles were white and my cheeks were clenched. The car was bottoming out a lot on the smooth highway. Every time it did so, there was a loud bang. Not cool. But, we made it okay and we drove with my family another 100 miles down to Yreka for the show that Sunday.

I was excited to show off all my hard work. The day wore on and not one person gave my Dart a second look. It could have been because it was parked next to my dad’s gorgeous ‘65 Dart with mile deep candy apple red paint. Maybe. I think I’m going to paint my Dart bright pink, so it will get noticed. (Take that, Dad!)

One thing I did take note of on the trip was that the hubs were getting hot. The bearings were new, so I wasn’t sure why they were getting hot. I could think of two possibilities: I didn’t get enough grease in the CV joint or the angle of the joint was causing too much friction. (Remember, I had to position the differential forward so the inner boot would not hit the UCA mount.) These were the original CV joints that came on the rear end; I have no idea how many miles are on them. Since they’re now operating at a different angle, they may have to wear in.

Once I got back, I decided I to needed to figure out exactly which springs I needed to stop the bottoming out. This was the beginning of my undoing. I searched and searched and couldn’t find a good way to figure this out. I didn’t have the money to guess and check. I looked into balancing the rear wheel rates with the front wheel rates for neutral handling. I also looked into balancing them based on harmonics.

I got a lot of great advice from “car guys“. I’ve come to realize most car guys are really engine guys, who don’t really know how to set up a suspension. You know the guys, they hike up their pants, puff out their chest and offer their superior wisdom on how to set up your car. I’d talk about wheel rate versus spring rate and I’d get a lot of blank stares. To save face, they’d usually proclaim that their springs were a spring rate equal to that of a one ton truck. Right. The only rule of thumb I found was from Herb Adams in Chassis Engineering. He says: “A one g bump is equal to the normal irregularities on a smooth road.” So I finally gave up on matching wheel rates and used this. Using my bump travel and the weight of the car (g=weight of car), I figured the wheel rate I needed. In my case it was 257.77 lbs/in. Now, I needed to convert this wheel rate to a spring rate. Using this site, I figured 600 lb/in springs would fit the bill. (My originals were 400 lb/in) The higher spring rate is due to linkage effects and the wicked angle my coil-overs are at. I got these ordered and installed them. With the higher rate springs, the rear of the car sat higher.

I took the car for a drive-it didn’t bottom and the ride wasn’t overly harsh. My suspicions were that the car would tend to over-steer on the track. My front wheel rate was 120 lbs/in compared to 258ish on the rear. Roll rates (Spring rates + sway bar effects) determine whether the car will under-steer or over-steer.

In Medford, OR, the Siskiyou Sports Car Club runs autocross on a kart track. I attended their test and tune day to really push the Dart. They only charge $15 for the entire day! (Seriously, why don’t more people do this? Get out there!) My brother in law, Aaron McKay, graciously took thousands of hi-res pictures of the event.

The results at the autocross were much better than at the car show. Within five minutes after pulling into tech, there were three guys under the car checking it out. After they saw the snow tires, I think the mystique was over. The Dart was a bit out of place in the presence of Porsches, Cobras and even a GT40.

After I was teched by a suspicious BMW driver, I hit the track and was immediately black flagged. I guess they didn’t like that gas was leaking out past my gas cap.

Once I fixed the cap, it was full steam ahead. Looks a bit out of place, eh?

Look at the camber of the inside tires below. The rear tire is still pretty much vertical, the front has gone negative. That means the car has too much body roll.

Just a bit of body roll.

This picture says a lot. That inside rear tire is about to come off the ground. I smoked it on a couple turns. To fix this, I either need to reduce the rear roll rate with softer springs (nope!) or the front roll rate needs to go up. You’ll see front wheel drive cars do this and that’s okay for them, since the drive wheels are still getting good traction. For a rear drive car, it’s better to see the front inside tire lifting.

So, how did the Dart do? I was really happy with how it did. There was no under-steer to speak of. There was no unpredictability and it stuck well.

The downside to racing any car is that it creates in you a desire to make it better. There are a few things I’d like to fix to make this set up better. First, the brakes didn’t work very well. I tried to get them to lock so I could adjust my proportioning valve, but they would not. I think I need to get a master cylinder with a smaller bore size to better match the caliper pistons. Next, I need to get stiffer front springs to keep the body roll at bay and better match the rear rates. Lastly, and the most obvious, is that I need better wheels and tires.

My friend Art Johnston filmed the car at the track and edited this video for me. (Thank you!) I normally don’t like music on car videos, but a Slant Six is practically silent, so we added music. I’m by no means a seasoned driver, so go ahead and tell me how much better you and your mother are at driving. Be sure to watch the slow motion S-curve and when I hit the nitrous on the front straight. Tight.

I hope you enjoyed this series as much as I did writing it. If you have further questions, leave them on my ride page. And if you missed any of the previous blogs, you can find them here:

Part 1

Part 2

Part 3

Part 4

What’s next for the Dart? I think it’s time for more power. The next project is turbo charging the Slant Six. If you have a stock Dodge Cummins turbo laying around gathering dust, I’ll gladly take it off your hands. If you were thinking about throwing away that wide-band O2 gauge, please think of me. Thanks for reading!