First competitive event in Silver Ghost and I really stunk up the place. Not completely sure why, but I’m trying to remain positive. I hope this isn’t a new normal. Here’s a positive image:
On Saturday I got five practice starts that led into a short autocross course. (I really like the idea of having some sort of practice course after the practice starts. We should have that at Two-Course Pro-Solos as well.) It was a race-track autocross. Drag-start, right turn, left turn, long straight filled with slalom cones down the center. Accelerate to a right turn, brake, accelerate into another slalom down the center. Accelerate then brake for a hairpin turn. Accelerate into another slalom and then a short finish straight. The total time was about 35 seconds.
I was told that my times were about 0.2s off the fastest person in BS and eventual winner. I was told I was 0.2s faster than the next guy, who eventually got 2nd place. I finished a poor 4th.
So, during practice I was doing okay. What happened on Sunday, competition day?
I’m just going to say that neither the driver nor the car liked the actual, extended 55-second course we were faced with on Sunday. I know that I have a problem seeing the gaps in a long set of cones all in a row stretching out in a perfect line front of me. Maybe a depth perception thing, I don’t know. It takes me too many runs to figure it out. (Must get it done in three, as we say in the trade.) I’ve never driven those types of courses well and I kept that lackluster record intact.
It’s not a problem hitting cones. I rarely hit cones, even when the course designer does his best to make me, and this event was no exception. The one run of six I did cone was because I dropped a rear wheel off the paved surface into the dirt and oversteered across the track right into a wall of cones. I got five in a single whack and probably deserved to be given an Off-Course. (Not that it would make any difference.)
There was another issue, which I don’t want to make too much of, but I’ll just leave this here: After the three runs in the morning session, the top two people in ES had both raw-timed all of BS, even though ES was first off and BS ran a little bit later. (It was warming up rapidly and the course always improves the more cars go over it.) The top guy in ES was the course designer driving a Toyota MR2. In 12 years of autocross how many times have I seen ES raw-time BS? How many times have I seen the 140hp sports cars in Street-class negotiate a course faster than 300hp to 400hp (and more) sports cars in Street-class with good drivers in both types?
And when it’s the course designer that does it, well, it’s not a good look.
And when 15% of all runs in the competition ended in an Off-Course, and even more with cone penalties, well, that’s not good either. (I counted.) I don’t think I’ve ever seen anyone go off-course on six of six runs and have no time at all for either session or the event as a whole. Two people managed that. Four others went off-course in three of three tries in one or the other session and also had no time at all for the event since the best times from each session are added.
Overall the car was good. I could have really used some compression damping in the front shocks to better negotiate the heavy dose of transients and thread-the-needle-and-turn-quick elements. The Penskes can’t get here soon enough!
Travelog section: Just south of Montgomery on Hwy 231 we stopped in at a sort of farmer’s market, restaurant and coffee shop establishment. Assorted goats and various farmyard fowl wonder around. We like the massive baked potatoes smothered with hot vegetables and BBQ sauce.
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.
The Falken 660 tires I’m running have a nominal as-molded tread depth of 8/32 inch. After the Moultrie Pro-Solo test event, where I got 60+ short runs and another 40 or so practice starts, both front and rear tires had lost 1/32″ in the center, 2/32″ on the outer edges and 3/32″ on the inner edges. This seems to me a reasonable wear rate on concrete. I was pleased that front and rear were very similar. The rear probably would have been worse except that those 40 extra practice starts were done in either wet or damp conditions which limited the wear rate even though I was having trouble controlling wheel-spin. Below are annotated pictures. Let me know what you think about the wear.
As I mentioned the inside of the front tires wore faster than the outside. In warmer temperatures more grip will be created, probably pushing wear toward the outside. At regular autocross courses there will be more long, high-G corners, also pushing wear toward the outside. However, as I will probably be adding air pressure and the Penske shocks will have more compression force the wear rate on the inside may very well stay a little higher. (Toe is not a factor here as it will always be set to zero or near-zero.) This wear pattern, inside faster than outside, is not necessarily an issue.
In a standard corner we theoretically only reach maximum lateral acceleration at one point- the apex, though we should be at the total limit of longitudinal plus lateral from start to finish of the corner. The apex is when wear on the outside edge will be the fastest, but it’s not for a very high percentage of the total time during a run. The apex is the point where we want to maximize the lateral grip from the tire and where our data system can most easily measure it.
Some might advise me to decrease the static negative camber in order to balance the tire wear. While that might work, I think it would be a mistake if you’re trying to win and not trying to maximize tire life. Reducing static negative camber would diminish the peak capability of the tire to produce lateral grip. In this Street-class car with relatively soft springs and rubber suspension bushings, which equates to too much roll and thus camber loss in the corners, we can never achieve enough static negative camber to keep the tire at its best angle (still slightly negative…not perfectly vertical) at the apex of the corner.
This is also why using a tire pyrometer in grid to measure the temperature across the tire and then adjusting camber to even the temperatures is not smart, no matter how many people tell you to do it. (I hope that includes all my competition!) We are interested with what happens mostly at those corner apex points out there on the course. The tire temps back in the pits are of little use to us. I really don’t care what the average heat generated across the tire width is across a run, which is what that tire pyrometer will tell you back in grid.
If you want to measure the surface temperatures in real time during a run with a set of thermal imagers so you can see what happens at those corner apex points, well, now you’re talking about something that could be useful. I think I know what it would tell me: the outside edge heats up really high at the apex because it’s lost too much camber and the edge is really highly loaded and wanting to roll under. The tire is not happy. The tire is screaming, “I need more camber to even out the loading and these temperatures so I can produce more lateral-G at the apex!” The rest of the time, the inside edge is warmer.
Using a tire pyrometer is no doubt useful for road-racing a real race car (with properly stiff springs, sway bars, and without soft rubber bushings) but it makes no sense to me for Street-class autocross where we are trying to achieve peak, momentary capability with tires that we already know are not at the correct angle to the pavement.
What I’m saying applies only to Street-class cars. Once up one preparation level to Street Touring, where you can install properly sized springs and bars, replace the soft rubber bushings with sphericals and even install camber kits then you need to worry about how much camber is too much. Even then a real-time look at the temps across the tire surface in the middle of a corner is preferred. With a 200′ skid-pad you could use a tire pyrometer to quickly check temperature differentials across the face before they cool too much.
The rear tires also wore more on the inside than the outside at the Pro-Solo test event, though rear camber is much less than in the front. Especially after this particular test event this makes sense. During launch the inside edges have more load (due to negative camber) so they wear more during wheel-spin. The only time the outside edge is highly loaded is on the outside tire in the middle of a corner when the car is rolled over and pulling maximum lateral acceleration. This is when the outside edge (of only the outside tire) wears fast. Given this it would be very strange if the outside edge had worn faster than the inside unless the course included a lot of high-G corners, which the ones last weekend did not. At the local test and tune in a few days there will be a lot more long sweepers and outside edge wear should be accelerated.
Autocrossers often use the triangles molded into the top of the sidewall to gauge how much the tire is rolling over. The triangles aren’t there for that purpose and they vary in location and size from brand to brand. They just make good reference points.
In Figure 5 you can see where the scuffed surface extends well down from the tip of the triangle. It looks to me like this is too far, meaning that the tire edge was rolling under too much when maximally loaded, even in the cold temperatures. I was only running about 31 psi in the fronts (28 psi in the rear) so almost certainly that will not be enough, especially in warmer weather on high-grip surfaces. These 275mm (10.8″) treadwidth tires are crammed onto only an 8.5″ wide rim, so there is, by definition, a lack of proper sidewall support. (According to the “standard” they need a minimum of 9″ of rim. This is autocross. We don’t need no stinkin’ standards.) More air pressure is probably needed. A friend confirmed this by informing me of the significantly higher pressure he runs for the same tire on a 9.5″ rim on a similar car.
The handling of the car was good, but not great. Overall the grip level was good, but it’s hard to make conclusions given the very cool temperatures. Also, I have Penske shocks on the way so I don’t want to go too crazy with conclusions or changes until they get installed.
I feel like turn-in could have been crisper. That, plus the indication of too much front tire roll-over, points toward needing more air pressure in the front tires to sharpen the response. The Penske shocks should also improve things because they will produce more compression forces on turn-in than the Konis do.
It was always possible to overload the front tires and create understeer by turning the steering wheel too far, too fast while trail-braking, but when I hit the right balance between speed, braking and steering angle the car turned really well. (I was finding the proper spiral entry path.) It took me until Sunday morning to finally get a feel for that point of balance. I’m not what you’d call a quick study. Thank God for all those runs.
The rear was quite stable. Maybe too stable. I had set the rear toe-in to 1/4″ total. You can always provoke oversteer with enough throttle coming out of a corner, of course (gotta love that LS3 and Z51 gearing!) but in general it did not exhibit any bad habits. No snap oversteer ever occurred. (That doesn’t mean I never got it sideways!) I will reduce the toe-in in the rear very slightly to assist rotation on turn-in and see how that feels.
Up Next: We have a local Test & Tune in three days on our local asphalt parking lot. Next post will probably be after that.
I was invited to the Pro-Solo Test down in Moultrie this past weekend. Moultrie is quite close to the site of this year’s Red Hills Tour, way down there in the South of Georgia.
After the problems with the Pro-Solo system early last year the SCCA began a crash effort to fix the issues. A few events got cancelled, but an improved system was then fielded mid-year.
What I didn’t know previously what that a significant effort has continued since then to further improve the system to make it even more reliable and easier to operate. So last weekend the SCCA ran a test in Moultrie on the concrete pad at Spence Field to shakedown the revised software and hardware implemented since the end of last year and prior to the first points Pro-Solo event in a few weeks in Florida. We did a lot of starts and runs over 3 days, testing single-course, dual-course and challenge modes multiple times.
The system worked really well as far as I could tell. I got something like 60 full runs and an additional 40 or so practice starts on Friday which began wet and slowly dried a bit as we went along. (This went a long way to minimize tire degradation as I learned to launch the car.) The Corvette was awesome the entire time. The clutch never overheated and stayed consistant for every run the entire weekend.
Just before I left I received a set of upper shock bushings made from polyurethane which will be installed with the Penskes when they get here. Some people may not realize that these are legal in Street class. They should make a slight improvement in shock response. The Ohlins I had on the C5 had no bushings at all… they fitted metal to metal at the upper end and transmitted a lot of additional noise into the cabin. The Konis mounted now re-use the stock rubber bushings. I have them cranked down pretty tight to limit lost motion.
The two very short courses were not quite identical and that was a good thing. In fact, I wish they had differed even more than they did for additional variety. Both had a tight section that rewarded excellent braking, car control and a good knowledge of the braking understeer point for the front end. These sections also rewarded narrow cars and better transient response.
Of course the drag start rewards power-to-weight and forward grip. I think the rear 305 Falkens did a pretty good job of putting the power down once I really tuned into the feeling and sound of incipient wheel spin and could control the application of torque. The Sunday morning session contained my fastest times on both sides and is shown below.
In the picture above you have, from left to right on the top row, the clock time of the first run which happened to be from the left side, its sequential number in that session and that side, the total time of the run, the Reaction Time, and the time to travel the first 60 feet. Then you have the same data again for the first run on the right side. For those not familiar, the quickest left side time is added to the quickest right side for your total at a competitive event.
The sharp-eyed and clued-in may notice that the clock times look a little strange, i.e. they don’t all swap from side to side like normal as we cross-over behind the start, alternating side to side. This is true. After the second run, which was on the right, I pulled back into the right lane again to even up the lines of cars on both sides and ran my third run from the right side again. Later in the session I did the same thing but on the left side. This could never happen at a real event, but at this informal test things were laid back.
The red, crossed-thru time was a red light run, i.e. I left too early and crossed the start beam before it had turned green, which is 0.500 seconds after the last yellow light. I had been playing around a lot with my launch technique, but by this time I was getting reasonably consistent in the 0.6’s and 0.5’s.
The 60-foot times are a little slow with the best being in the mid 2.2’s. I attribute this partly to the cold conditions which limited grip. In warmer weather I’d hope to be running consistently low 2.1’s, but I’ll have to improve my launches some more to get there. Barry K. cut a few 2.0’s in his mid-engined C8. Donour S., owner of a Lotus Evora he’s taking to STU this year, was consistently in the 2.1’s and sometimes dipped into the 2.0’s. I expect he’ll cut some 1.9s this year in that mid-engined car that has over 60% of its weight on the rear tires similar to the C8. It’s not horsepower that nets good 60-foot times. The car has most of the energy it needs to get started well already present in the engine’s rotating mass. It’s grip and technique that make for good 60-foot times.
You can find a video of the last 8 of these 10 runs on Youtube here.
This concrete site is not big enough to have much in the way of fast sweepers and probably couldn’t hold a real Pro-Solo event, though it could host a single-course event nicely.