An Autocross Season- Part 14: A Shocking Mistake!

In planning a test program for Silver Ghost’s next event (which was today) I discovered a problem. I made a mistake when specifying force values for the Penske shocks.

It’s a little hard to explain, but I’ll try. I did the force calculations for 3in/s shaft velocity. Then I instead told Penske to put the knees at 2.5in/s +/- 0.5 in/s. Well, Penske put the knees more or less right where I said, at 2.5in/s, and they hit the force targets for 3in/s accurately with the adjustments at a particular setting. This means I have 20% too much force at 2.5in/s at the nominal adjustment settings. (3/2.5 = 1.2)

So, instead of having 90%Critical damping at 2.5 in/s I actually have 108%Critical. The shocks are stiffer at the nominal setting than anticipated. This is what has caused no end of issues with grip and balance as it turns out that for my car, at least, 108% of Critical is definitely too much damping for best grip on a bumpy site.

Mistake Explanation

Here’s how I screwed up, explained with the chart above.

I calculated a force at 3 in/s, let’s say it was 100lbf. (100lbf is not far from the real number on my front shocks in bump.) That’s point 1 in the chart. (These numbered point are called “knees” in the shock dyno curve.) I asked for as linear an increase as possible from zero to that point. In theory, that produces the line I’ve labeled “90%critical damping.” The force at every velocity along that line represents 90%critical damping. I did exactly the same thing for rebound.

But, when I wrote the spec, I changed that point to be at 2.5in/s, that’s point 2, and I forgot to recalculate the force for 2.5in/s. So, Penske gave me point 2, which is 100lbf at 2.5 in/s. See how the line from zero to point 2 has a steeper slope? Every point on that line turned out to represent about 108%critical damping.

What I really wanted was the line that leads from zero to point 3, which is a lesser amount of force at 2.5in/s and would lie on the original 90%critical damping line.

Also, you may notice that to the right of point 1 the almost horizontal line is significantly above the sloping line to the right of point 3. This means that for sharp bumps, which produce higher shaft velocities, the forces are also higher than they need to be. At those higher shaft velocities we want less force, not more, to give us grip over bumps. That’s why we want highly digressive shock pistons. Beyond the knee the slopes “digress” to lesser angles. A horizontal line would be best, but is not achievable.

Before today’s local event I took the dyno charts and figured out what settings, front and rear, would get me back onto the 90% line and also knock down the slope of the line after the knee. I had to put bump at full soft and then reduce rebound until the total got me back on the 90% line. Then I dropped the rebound even more, down until it was equal in magnitude to bump (I didn’t want rebound to go under bump) and found that at that point I was at 83%Critical. All these numbers are approximate, by the way. What I’ve drawn in the figure above is a very simplified, idealized representation of what are really much more complex curves. To get the shocks to be really what I wanted, with some adjustment in either direction, I’ll have to have them revalved. (Maybe next year.)

That’s where we started today, at 83% with bump forces roughly equal to rebound forces, i.e. a 1 to 1 ratio. The car was soooo much better! Grip was high and it didn’t push at all, using exactly the same tire pressures on exactly the same surface that was so difficult to run on two weeks ago and again one week ago at Peru. I was near the top of Pro class, much closer to the two or three really fast guys than I was 2 weeks ago.

During the lunch break I adjusted the shocks to the new 90%critical settings. I got a little push from the front tires and little more bounciness over the bumps. In the end my co-driver and I decided we liked the 83% setting on the front with the 90% setting on the rear, along with some tire pressure changes that we also tested today. That’s what we’ll use at Bristol, which is a bumpy asphalt site like our local site.

An Autocross Season-Part 13: 2nd In Peru

Silver Ghost got a good 2-day shakedown with myself and a co-driver on the new set of tires at our local Tour de Frank event (Milton Frank Stadium) the weekend before Peru. It didn’t start well.

The new tires consisted of 265mm Yokohama A052s on the front in place of the original 275mm Falken 660’s. In the rear I had a new pair of 305mm Falken 660s, just like before. I had previously driven the car around in circles to give the tires an initial heat cycle and then let them sit for 24 hrs before using them again.

There was a shocking lack of grip from the new front tires, causing us to miss all our braking points, plowing through walls of cones, and to understeer terribly and miss our apexes.

At the time I bought this second set of tires for the year the 275mm Falken was not available. Lots of tire shortages going around.

My co-driver and I found we had to massively lower the tire pressures. This, plus several runs, seemed to bring the Yoks into frame. By the time we had 6 or 8 runs on them and 4 less psi all around the car was much better, though it still understeered. I set it down to the supposedly common knowledge that Yoks do not like to be pinched while Falkens are much more tolerant to over-tiring the rim. (However, I had used this same tire on the exact same size rim on the C5 and it was fine.) I was able to place 6th of 19 in Pro class both days (7th of 98 overall) while still having extreme difficulty with understeer and poor braking into corners on the bumpy entrances to several key corners. Ryan T. from down the road in Birmingham was in his new B-Street Supra and placed ahead of me in 5th in Pro each day. I would be against Ryan at Peru and I now put him down as the tentative favorite, though I had little knowledge of the competitors we would face.

Loaded Up For Peru, Indiana and Grissom Airport

Fearing more understeer at Peru I softened the front bar to the middle setting from full stiff. On the practice course on Friday I ran 2 psi more pressure than at Tour de Frank, trying to prevent too much tire rollover on the higher grip surface. The car understeered but it did brake well on the smooth concrete, confirming that the car is just too stiffly damped at 90% critical* and high gas pressure for the bumpy Milton Frank surface.

I then reset the front bar to full soft and added 3 sweeps of compression to the rear shocks in an attempt to move the corner entry balance more toward oversteer and went back out for a few more practice runs. The car seemed better. Boy, did I have it wrong.

When it came to the higher speeds on the real course the car understeered badly into the corners and now power-oversteered out of the corners given any significant throttle, something it had never done before. I had to drive very timidly. When I looked at the standings for the first time after the first two runs I was in 4th of 8 drivers, a tenth away from 3rd, the final trophy position, and a half-second out of 2nd. (2nd and 3rd places were held by co-drivers of another Supra.) Ryan was in the lead in his own Supra.

In desperation I dropped the tire pressures down to exactly what we had used at low-grip Milton Frank and took the 3 sweeps of compression out of the rear shocks. I was able to push more on the final run. I went considerably faster and ended the day very slightly out of third and two tenths back from second. Ryan had a screaming 3rd run and left the rest of the class in his rear-view.

On day 2 I made no further changes and gained confidence and speed on each run. Even so, after two runs I looked at the standings and the driver in second place had put another two tenths on me, so I was now down by 4 tenths, but at least I’d taken over 3rd.

On my last run I got the clean four tenths I needed and edged into 2nd place by a few hundredths. The Supra driver had his last run remaining and only needed a very slight improvement to retake 2nd, but I was hoping I had put a little pressure on him, assuming he even looked at the standings or cared. I watched his first section. It looked faster to me than his previous run. I watched his second section. It was definitely faster. When he crossed the line the display showed a time plenty fast to beat me. Then it was announced that he had coned the run in what I’m calling the second section. I took the trophy for 2nd place.

After banking a couple of conservative, clean runs Ryan blitzed his 3rd and absolutely smoked the class, a well-deserved first Tour win for him. I guarantee it will not be his last. He used to race dirt tracks. That boy can boogie.

Peru B-Street Finishing Positions

My co-driver and I have one local event before going to Bristol for the back-to-back Pro and Tour events in July. Bristol has bumps and weird slants and dips and cambers. Too stiff and you’re likely to fly off into the creek that snakes through the property. (There’s a nice, cool swimming hole, too.) We have a testing strategy planned for the local. Hopefully, we can figure some things out and get this car (and the drivers) dialed in.

*In Part 14 I explain that I discovered the shocks were actually at about 108%critical, not 90%, as I thought.

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 100%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 more damping above 100% Critical the longer it takes to get back to the static position after being disturbed, like a tire being smacked by a bump.

**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. Motions ratios (mechanical advantages of springs and shocks given the specific geometry) must be accurately known. Damping has an effect on natural frequency as well, especially at the high damping values 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.