Update: I’ve added a better description of what was happening where we were losing the back end of the car entering the slalom, plus an additional close-up figure of that feature.
This past Saturday I attended a local Test and Tune (TnT) and got my first “real” autocross runs in Silver Ghost. It was a short, 34-second course, but had offsets, one of which had a negative slope to it, a 3-cone slalom, a 5-cone slalom, a more-than-90 degree standard corner and a big 180-degree turnaround. We also had a separate long slalom that fed into a skidpad which was great for warming up the tires on this cold morning. I was able to verify that, at the limit, the car still tended toward push but was quite controllable and drivable with the right foot. (You must try hard to really screw up a Corvette in Street class… good Chevrolet engineering.) The asphalt surface was bumpy in spots and was dusty and dirty at first until the line got swept clean. Silver Ghost was good, but not great.
I made a few changes prior to the event. After the Moultrie test I felt like the front tires were rolling over too much (see Part 8) and the car turned in somewhat lethargically, so I added a bunch of pressure to the front tires. I also added 2 psi to the rear tires and cranked the shock rebound up two sweeps at each corner. (These Koni shocks are only rebound-adjustable.)
The other thing I did was reduce the rear toe-in. In Moultrie the car had 4/16″ of total rear toe-in. It felt like the rear was too tight, meaning that the car didn’t rotate quite as easily as maybe it should. Toe-in produces stability. I reduced the toe-in to only 3/32nds of an inch, less than I ever ran my C5. The C6 is designed to be a more stable car at high speeds. It has an extra inch of wheelbase, even though the car is 4″ shorter overall. (I love the shorter, more compact form and I don’t miss the pop-up headlights one bit!) Geometry changes in the suspension also seem to produce more stability in addition to the longer wheelbase. We don’t want excessive stability in autocross.
After an initial set of runs my co-driver and I agreed that we had a problem. The 5-cone slalom was entered by by-passing the first cone and considering the entrance to be from the hard side of the second cone, meaning you had to brake hard down from toooo-fast and then turn in all the way around that second cone. (See the closeup of this section in the figure below.) In the figure the dashed green path is an area of full acceleration. The first cone of the slalom served only to pinch the entry, preventing getting much of any angle on the second cone. Right about there we had to threshold brake and immediately turn around the second cone, still decelerating.
This is one of the hardest tests for an autocross car and its driver. Can the pinched turn-in be done aggressively with so much weight shifted forward without the rear end sliding out? The front tires are no problem because they have so much load shifted to them. They’ll turn the car like crazy, whipping the back end around. An aggressive beginner will immediately spin just as the 2nd cone is passed and end up pointing at the word “slalom” right about at the end of the red line. A non-aggressive driver will lose time. Such a driver will brake early and release early in order to get more weight shifted back to the rear prior to the turn-in. Mucho stability with more rear grip, but slower.
This is one area where a mid-engined or rear-engined car design has an advantage. They keep more weight on those big rear tires than a front-engined car but still have plenty of weight on the front tires to prevent push. (Unless the brakes are released too early!) In our case the back end was too easily lost when braking and turning in to that 2nd cone and even, in fact, during each subsequent turn of the slalom. Neither of us actually spun, but this was only because we both have considerable experience in Corvettes. We could feel it happening early and correct, though not without hitting a cone or two.
The good news was that the car turned in much better than before and I liked the liveliness of the rear of the car, but it was just too much. My co-driver recommended adding back some rear toe. That would have meant going home (only 2 miles!) to my garage. Before doing that I tried lowering the rear pressure back to where it had been in Moultrie. Just 2 psi less… and that’s all it took! Now I could really crank on it entering and negotiating the slalom. The car rotated controllably and the rear no longer threatened mutiny. It’s actually kind of amazing when a car can do that. I’m really excited now with its slalom performance. People were coming up to me and saying, “Man, your car looks fast!” The clock agreed.
As for the shock change, I think it was mostly good, with one caveat. The start was a straight drag race. I felt like I had to be too careful releasing the clutch and ramping up the torque to avoid rear tire spin and/or wheel hop. I had no trouble avoiding wheelspin after all those starts in sketchy conditions down in Moultrie. I just had to be overly careful, which is the same as being too slow. A bunch of people took runs in the car but only one other that I remember (last year’s AS national champion) avoided excessive wheelspin. (Maybe he was just being super kind to my tires!) I plan to take one sweep of rebound damping out of the rears to make sure I get the best possible launch at the Pro-Solo in Florida in a few days, the first competitive event for Silver Ghost. I’m leaving the fronts where they are. The rebound damping builds very fast with shaft velocity in these Konis at the stiffer settings. I learned years ago that too much rebound can cause a loss of grip on certain (many? all?) surfaces.
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