Just back from two days of testing & tuning down in Birmingham at the Hoover Met(ropolitan Stadium) parking lot. What a great site- big with very good, very grippy asphalt. As usual, the ALSCCA folks were very welcoming and did great job. The Iron City Match Tour scheduled for June 17-19 should be a fantastic event. Be sure to put it on your calendar.
But, before I get to how the car handled, let me catch you up. I borrowed a set of corner weight scales from a friend (Thanks Tom!) and got another friend (thanks Glenn!) to help me do the corner weighting. The empty car weighed 3043 lbs on 3/16 of a tank and the cross weights were way off somewhere near Saturn… around 1.5% out. We got it close to 50% but I’d only disconnected the rear roll bar, not the front. I felt like I was fighting the front bar, so I knew we’d have to do it again.
The next morning Clem Tire aligned the car. They found that last years’ home alignment was good, but the big thing I had them do was reduce the toe-in in the rear. Up until now I’d wanted a very stable car so I ran 0.28 degrees of rear toe-in per tire which is 1/4″ total. This year I had them reduce it to 0.22 degrees per tire, which is right at 3/16″ total. Not a lot of difference, I grant you, but along with some other changes I was hoping to get a little more slithering from the back end this year. When a well-driven Corvette slithers like a big lizard through a slalom it looks from the rear like the car is bending in the middle around each cone.
That night Glenn and I measured the corner weights with both roll bars disconnected. We had to crank up the left-rear corner even more to get close to 50%. Here are the results, with driver and helmet in the car and 3/16 of a tank of gas:
What does 50% cross weight mean, you may ask? For one thing, it doesn’t mean that there is equal weight on each tire. The values in the chart show that clearly and the % left and % rear numbers tell you what the static weight distribution is. Cross weight is something different.
What 50% cross actually means is that the differential left to right is equal front and back. (It’s calculated by adding the right-front weight to the left-rear weight and dividing by the total weight.) So, if the left-front has 51% of the front axle weight, then the left-rear will also have 51% of the rear axle weight when the cross weight is 50%. Per the numbers above the car actually has 51.5% on the left-front and 51.2% on the left-rear. So, the cross isn’t perfect (49.74%) but it’s less than 0.5% from 50, which is usually the target.
Why is cross weight percentage important? It helps make the handling of the car symmetrical. That is, it will have the same characteristics turning left as turning right. Equal cross can’t do this all by itself, however. Remember I said my car has more weight on the left side than the right with the driver in place? Most production cars are that way. And this condition will forever affect the handling. (Not to mention the 54% that’s on the front axle!)
How do you adjust the corner weights to get 50% cross? Well, in Street class, there’s only one way: adjust the ride height at a corner. (We can’t rearrange components.) Not all cars have such an adjustment from the factory. Luckily, the Corvette has an adjustment bolt at each corner. What I’ve done is set both fronts low and equal. Then, I adjust the rears. The right-rear was already as low as it would go and the left-rear about in the middle. So, I had to crank up the left-rear, physically lifting that corner of the car. Doing this also sends weight to the diagonally opposite corner, the right-front, and removes weight from the other two corners. Think of your car like a 4-legged table. If you shim up one leg, it teeters on that one and the diagonal. With a sprung suspension it’s not all or nothing like it is with the rigid legs of a table, so even a little air pressure difference in the tires has an effect. Set your pressures before you measure the corner weights!
Lifting the right front would have the same effect on the cross, and maybe I should have. I’ll have to think about that and what difference it might make. I had a specific reason for making all the adjustment at the rear. Raising the rear of a corvette increases the roll stiffness at the back, promoting more of that slithering talked about earlier.
So, now the car is aligned with healthy front toe-out set at the event, cross-weight balanced, shocks set stiff, more air pressure than last year, a narrower (better supported) front tire than last year… wow, it drove good!
I couldn’t believe the turn-in rate! I was early on every corner for the first three runs. The back end was wagging left and right way too much, but once I slowed the input to the steering wheel, it all came together for top PAX time for the day.
The car is definitely better in transition, slithering through the slalom the way a Corvette should. It is less stable but more fun to drive and more than one person commented to me how good the car looked on-course. I can’t tell if any peak lateral G has been lost… the site and courses didn’t allow me to figure that out, but I’m not worried. I’ve achieved what I set out to do, which was to improve transient response. If I can’t do well at Dixie Tour a month from now it won’t be the fault of the car.
With all the runs today (day 2 of the Test & Tune had very few cars) I was able to play with tire pressures and figure out the sweet spots. Minus 4 psi from baseline in the front didn’t seem to reduce peak grip, but it definitely slowed the transitions. Plus 4 psi in the front reduced front grip and induced understeer. Plus or minus 2 psi around the baseline in the front and I can’t tell the difference.
In the rear, minus 2.5 psi from the baseline was a disaster… totally uncontrollable at the 1-2 shift! Plus 2 psi and I could feel some loss of grip. So, I think I have a +/- 1 psi band figured out for the rear. Of course, this is all for one surface, on one particular day, without changing shock settings, but it gives confidence in the starting point to use in the next events leading up to Dixie Tour.