Been a bit since my last actual car update. I've raced the car a few times with the new tires installed and frankly...I'm just not happy with it's performance. At our last event I had to make a hero run on my last run to get back past Mat for the CAM T win by 2 tenths, but still placed like 37th in raw time out of 120 which is not up to par. I got to thinking about things, analyzing videos and data from the event, and decided it was time to get back to basics. It had been 3 full years since the car was on an alignment rack and tire temps taken after my runs at January event showed the inside edge of the front tires was 10 degrees hotter than the outside edge...a sign of too much camber (for this tire apparently). I am also having trouble putting power down without the rear stepping out if I have any steering angle in the car at all, probably related to the torque sensing rear diff. When I get back into the gas while still turned the inside wheel slips just a bit, the diff senses this and directs 2.5 times the torque to the outside wheel which then spins more and throws the car sideways. While I love the diff for corner entry and roll through portions of a turn, I hate it's corner exit traits and this is where this car makes it's bones...on corner exit. Something has to change.
First up, full physical evaluation to see where things are. I started by weighing the car.
This is the naked car, no driver weight.
Some how over the past 3 years the car has gained 80 pounds. The good thing is, it is almost all on the rear tires.
This is with simulated driver weight in the driver seat.
I'm actually pretty happy with this. Cross weight % and weight equals right side % and weight to the pound.
I also measured ride heights with the driver's weight in car and I'm happy with that as well, right where I like things.
I then rolled the car back, replaced the front scales with turn plates, rolled the car back forward and checked the alignment. This is where things went haywire.
This is the sheet from the last time I had the car aligned...on a computerized rack, with a trained tech, that I paid good money for.
Shame on me for not keeping a better eye on things. Now that I see where it was set with my own eyes, and realize there has been way too much camber in the RF for the past 3 years, a few things are starting to add up. One for instance is the RF has a tendency to lock up under hard braking when entering a right hand turn, the LF doesn't have this issue.
My goal was to take some static camber out to try to get the tire temps more evenly spread across the tire and I also wanted to see how taking a bit of caster out affected the rear tire weight...to see if "caster jacking" is somehow playing havoc with my diff causing my rear stepping out issue. I quickly realized that I had some work to do to get things back in shape. After a bunch of pulling shims, measuring, rolling the car back and forth, measuring again, etc etc...I came up with this.
LF Camber -1.25* RF Camber -1.00*
LF Caster +9.00* RF Caster -9.25*
Toe in 1/8"
That is close enough for now to see how the tires and the car react. We race again Feb 13 which will be next test.
Before I made any alignment changes, I measured the weight on the rear tires with the front tires on turn plates. Actually I took 3 measurements, 15* left turn, wheels straight, and 15* right turn. I'm not sure if this test was scientific or not as I'm certain there are other variables at play. The results were interesting though.
With the extra 1.50* camber in the RF and 10.00* LF and 10.75* caster RF, a 15* left turn lifted 66#s from the LR and put 20#s onto the right rear also resulting in 47#s moving to the LF I suppose while a 15* right turn lifted 41#s from the RR and put 41#s on the LR.
After I made all of the alignment changes, I did the same test again. This time a 15* left turn lifted 32#s from the LR and put 27#s onto the right rear resulting in 5#s moving to the LF I suppose while a 15* right turn lifted 44#s from the RR and put 44#s on the LR.
Basically basically the changes netted almost zero change in caster jacking on a right hand turn but really cleaned up and evened out the caster jacking on a left hand turn, the differences between the two probably have something to do with the car being so left side heavy. I can't wait to see if the changes help the rear end stick better at all, I kind of have my doubts. I am encouraged that I'll be able to dive into a corner harder and possibly carry more mid turn speed now with the front geometry evened out and better. Tire temps after a few runs will tell the real story though.
Moral of the whole story though...stop paying other people to do things you can do yourself... Every freaking time it seems like I pay someone else to do something, they screw it up. You'd think I'd know better by now.
Brian Hobaugh SCCA National Tour June 2014
First Hemi 'Cuda Convertible Ever Built
Short clips: Goodguys Pleasanton autocross and pit videos
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