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 much lower values. 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.