An Autocross Season- Part 12: Back In The Mix

I got Silver Ghost back from the body shop on Friday, May 6th, installed the Penske 8300DA shocks and poly upper mounts on Saturday and drove to Lebanon, TN for the TRSCCA autocross on Sunday at the Nashville Superspeedway.

Silver Ghost Repaired And In Grid At Nashville Superspeedway
New Penske 8300 Double-Adjustable/Remote Reservoir Shocks
Poly Upper Shock Mounts

The car was great. I really missed it! I felt like I drove pretty well and paxed 4th of 118 drivers on a short, tight, low-speed course in the newly-paved infield lot. It was especially cool because the One Lap Of America crowd were racing around parts of the tri-oval and the infield road-course track simultaneously.

I set the knobs to match the targets I’d given Penske. Big thanks to Penske’s Steve Horn for guiding me through the process and helping me make the many decisions necessary.

And big thanks to Dennis Grant (DG) and his Dynamics Calculator that allowed me to figure out the shock forces to specify.

I’m going to explain a little about how I went about this whole shock specification thing for those who may be interested. (Some of this may have been discussed in previous posts.) I’ve now done this for several friend’s cars as well as three of my own. I wrote Penske a spec describing what I wanted with targets (force at velocity) and piston type (double digressive) and tolerances and gas pressure. Also required was shock body length (to be legal in Street) and reservoir hose length. Steve and I discussed my spec and he declared it doable as long as I was realistic, which I took to mean not to be too much of a corporate engineer who would throw a tantrum if the numbers were not exact. The impression I got was that the forces I was asking for were low relative to the design range of that shock and piston combo.

Those familiar with DG’s on-line “book” will know that he recommends a starting point for shock forces at the digressive knee of the curve to be those that produce a total of 65% Critical Damping*. His calculator allows you to figure out how much force that would be.

Except that DG isn’t talking about Street class autocross cars. He’s talking about cars that have optimum, autocross-racecar-stiff springs, which he defines as ~2.2Hz** in the front and 10% higher in the rear. My Corvette is relative stiff for a production sports car but is only ~1.7Hz. Should we still damp at 65% Critical?

In Street class we are stuck with two key things: 1) the stock weight, and 2) the stock springs.

Way back in autocross pre-history, like maybe the 1980s(?), someone figured out that we can do a couple of legal things with shocks in Street class to partially make up for the deficiency in spring rate. Those two things are 1) use additional shock damping to mimic stiffer springs in dynamic conditions, and 2) use high gas pressure to mimic a statically stiffer spring.

DG states that he’s not sure of the practical limit for shock damping of a Street-class car in order to mimic stiffer springs but thinks that it may be somewhere above Critical. In my old C5 I tried readjusting my shocks for 90%Critical as a guess. This required significant reductions in forces compared to what I’d been running, so I’d probably been at 100% or maybe even more. In fact, I couldn’t even get down to 90% in all cases they were valved so stiff. (I had 4 numbers to set: Front and rear bump, front and rear rebound.) The car gained grip, was easier to drive and got faster. (Just in time for it to be obsoleted in B-Street by the M2 and Supra.) So, that’s what I specified for the Penskes: 90%, with, hopefully, some adjustability above and below.

DG’s calculator seems to calculate the total damping and then allocate the damping at about a 2 to 1 ratio between Rebound and Bump. I decided I wanted a 60/40 split, so I did a little trick. I totaled bump and rebound forces from the Calculator and reallocated the total to produce that split. (Remind me to ask DG if he thinks that makes sense.)

I’ve discussed the allocation of damping between the rebound and bump directions in a previous post here where I reviewed the historical 4 to 1 ratio vs. more modern ideas of how to split up the damping. I don’t have a scientific reason for choosing a 60/40 split instead of DG’s 2/1 split, just experience that tells me that more compression damping is better for transient response, though possibly not good for grip, especially on a bumpy site. I also had Penske put in all the gas pressure these shocks can take and still be long-term reliable, i.e. not stress the seals too much.

The weekend after Nashville I trailered Silver Ghost to Charlotte for an SCCA Pro-Solo event. So far I’ve never done particularly well at Pros, I have a single trophy and no class wins, but I needed the seat time with the new setup and Pro-Solos are lots of fun. The one other car registered in B-Street was a no-show. I was put into Bump class against a Tesla Model 3, also by himself in the new Electric Vehicle Experimental (EVX) class. I was able to prevail on corrected time, and I’ll take the win, but was not proud of how I drove. I don’t have any idea how you can come up with an accurate index for a new class like EVX. Electric cars with Street Touring suspension allowances and no tire/rim limitations except whatever you can get under rolled fenders. Who knows how fast those cars should be?

Charlotte Pro-Solo Starting Line- the white stuff is waves of pollen!

I drove the course okay, but my starts were terrible. Reaction times were slow and 60 foot times worse. In the total of 12 starts over two days I managed to massively and excessively spin the rear tires 12 times. Giving away 0.4s in the first 60 feet made it interesting to watch the Tesla rocket away into the distance beyond the light tree while my tires churned away!

Drive Slower But Win- The Vagaries of Calculator Racing

I started the event with 97 runs on the tires and about 4/32nds of tread. Grip was only okay on this surface that’s known for high grip. I suspect the tires were heat-cycled out. The next set has arrived and will go on before a big event next month, the Peru National Tour, where we have seven drivers registered in three Supras, an M2, an RS3 and Silver Ghost. That will be a good test.

Next Set Of Tires: 2 Front 265-Yokohamas and 2 Rear 305-Falkens (Supply Chain Shortages Everywhere!)

*A spring/mass/damper system, like the ones each corner of your car (if you divide the total sprung weight into components at each corner) is critically damped if, when disturbed, the system returns to the static position in the least amount of time and with no overshoot, i.e. no bounce. So, %Critical is a measure of the rate the damper takes energy out of the system as it resists the motion. No real-world system is 0%Critical because there are always energy losses with any mechanical motion. Shock absorbers on cars can be configured to produce a wide range of %Critical, even more than 100%.

**The natural frequency in Hertz, or cycles per second, of a spring-mass-damper system is the correct way to talk about suspension stiffness. The spring rate is one input. Another is the sprung mass on that corner of the car. Damping has an effect on natural frequency as well, especially at the high damping values we sometimes run with racing shocks. The natural frequencies for front and rear are also produced by DG’s Calculator. The stiffness of the suspension largely controls the transition rate capability of the car, that is, how fast it can change direction.

An Autocross Season- Part 11: A Setback

I had to cancel the trip to the Red Hills National Tour event.

At our very wet Test & Tune 2 event on 2/27 I managed to slide off the pavement and damage the car. No excuse for it, really. It was my 10th run or so on the same, standard TnT course we did a couple of weeks earlier. And I love practicing in the wet… it really tunes up your sensitivity and reactions. First time in 12 years of doing, what, 25 to 30 days a year(?) that I’ve ever damaged a car.

The good news is that I bought Lockton autocross insurance this year. The adjuster has been to see the car. My body shop appointment for their estimate is not for a week. (Everything seems to be backed up.) Only then will I find out when they’ll schedule the work.

I don’t expect it to be fixed until April. In the meantime we have local events upcoming in which I will drive the 944 in CSP. My co-driver and I drove it at our Performance Driving School practice autocross last weekend and it was fun. We’ve now decided to split the cost of some Hoosiers. We won’t be competitive, the car is only slightly out of Street class, but it will keep us sharp.

An Autocross Season-Part 10: Florida Pro-Solo

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:

It Was A Cool Morning In Northern Florida

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.

First Practice Start With 3rd Yellow Lit: I must be going now.

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.)

First Competition Run Had A 0.555s Reaction Time (Thanks, Moultrie!)

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?

Zero times.

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!

Alabama Goat That Wants My Baked Potato

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.

An Autocross Season- Part 9: Test and Tune

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.

Twickenham Automobile Club Standard Test and Tune Course– Design by Charles Krampert

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.

Slalom Close-Up Where We Were Losing The Back End

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.

An Autocross Season-Part 8: Moultrie Tire Wear and Handling Impressions

Tire Wear

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.

Figure 1- Outside of Front Tire
Figure 2- Inside of Front Tire

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.

Figure 3- Outside of Rear Tire
Figure 4- Inside of Rear Tire

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.

Figure 5- Outer Edge of Front tire

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.

Handling

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.