Thanks! I'm googling away!

edit: trying to wrap my head around what a rebound spring really does.

edit 2: so, a rebound spring produces a force in the same direction as the rebound damping force during rebound motion of the piston. By designing it to be able to reach a solid height without damage it can reduce the maximum droop travel and thus shorten the maximum extended length. The existence of the spring, when partially compressed by the force of the gas pressure, also limits the "apparent" at rest extended length, which doesn't have to be the actual maximum extended length, depending on how it is designed. This can complicate strut assy/disassy, it would seem to me.

The force from the rebound spring will be proportional to position, not to shaft velocity, like the rebound damping force. Therefore, that force is always there during movement of the shock in the extension direction from the "middle" point. That force is picking up the tire in the same way that an anti-roll bar picks up the inside tire when cornering.

Right?

 

That’s essentially it. Increasing rebound spring on one end of the vehicle is essentially similar to adding sway bar on that end. I made a set for GR86 a couple years ago that helped the rear axle when the front had a “street style” stiff front bar. Lots of variables on rebound springs, and you need a bunch of prototypes to tune effectively. The gap to the spring a 1G vehicle resting height is probably the most important consideration. The rest becomes a packaging problem, you can only cut 1” of stroke from stock, and the coil count and wire diameter all play into the block height.

you can also just use a rubber rebound stopper to limit the travel. Much more simple and not really a tuning tool, so less fiddling

 

This thread right here is why it's crazy to allow strut/shock and bumpstop replacements in street class but not spring replacement or (god forbid) electronic shock tuning outside of SS. So much can be done inside a shock with enough know-how and money! Super cool seeing y'all work on optimizing this though!

​​

 

I agree. I think it’s not reasonable to allow these mods in the “entry level class”

 

A Cayman In CS: Events 5 and 6

I put a pair of 295 Yoks on the rear as a fit check and went to my local TAC 6 event with no other changes, including the same tire pressures. It was a disaster, but at least the tires fit!

Way too much oversteer tendency meant that I could not aggressively trail-brake into corners without losing the rear end, which I proceeded to do time after time. If I beat people that's usually where I do it: on corner entry. I placed a disappointing 4th in CS and 22nd overall. I learned a lot by comparing my data to the winner of CS. Possible interesting bits: The data showed that the Cayman has a very slight acceleration advantage in both 1st and 2nd. Meantime, the ND seemed to have a slight grip advantage, but that may be due to the limitations of the Cayman oversteer situation. At least I hope so, because this entire enterprise is predicated on the Cayman having a grip advantage inspite of its weight. It may be that I'm running into the reality that lack of camber up front is always going to limit peak cornering grip to less than what the ND can produce.

Next event was with ALSCCA at Barber outside of Birmingham. Prior to the event I dialed in a healthy amount of toe-in in the rear. The rear was now super-stable and I won CS, beating two ND drivers by over half a second (on a 43 second course) that beat me the weekend before at TAC 6. It was too much toe, in reality, (5/16" total) because the car would now push powering out of a corner. It was really hot and I had a co-driver (who will drive with me at Bristol) and we confirmed again that you can overheat Bridgestones on a hot afternoon with 6 or 8 fast runs if you don't water early.

The Barber Proving Grounds surface is very smooth with consistently high grip. This meant that the present lack of damping wasn't so critical, i.e. the car wasn't bouncing badly like is likely to happen at Bristol.

Once back home I fabbed a string setup in order to produce an accurate alignment. (I didn't feel like leaving the car all day with the kids that work at my local alignment shop like they insisted. That's two shops that can't figure out how to make an appointment system work, so I decided to do it myself.) Initial results were that the front tires are both pointed to the right!

Edit: It only took one iteration on the strings to get what I wanted. The car now is at zero front toe straight on the axis, and 7/32nds (measured across 19.5") total toe-in in the rear, split equally to each side and straight on the axis. (7/32nds = .32 degrees per wheel) I know that this is more toe-in than most people use. It may get reduced later. But, I like to be very aggressive on trail-braking into corners so rear stability when the rear gets light is needed, as least for my confidence. This is what I plan to run at Bristol later this month.

 

A Cayman In CS: Bristol Pro and Tour

Bouncy.

Other than bouncy, the car was great. Good grip, no handling vices, ran perfectly all 4 days. My co-driver was frankly amazed at the grip it exhibited at some locations. But, we were slow, even compared to some of our local competition that I usually match up with that were there. So, trying to figure out what it all means.

First thing: initially we were definitely under-driving the car. Once I began to really push the car began to bounce. (Bristol is a bumpy site.) This limited speed, but only in certain places.
Second, it took a lot of runs before we began to figure out how to launch the car. Early on were in the 2.3 60ft range while the faster competition was in the high 2.1 range. Both of us finally got below 2.2, but just barely. And my reaction times were really, really bad. I had a flyer that was .25s faster to the shift point than all other runs, but it would probably take many events before I could reproduce that with regularity. But all that was just a small part of the overall deficit.

I know that the CS depth of talent is very deep, there were 4 CS drivers in the top 10 pax and 9 in the top 25, but I've raced against some of them before in different machinery and been pretty close. I can kind of place myself in the list w/r/t driving talent and I think I'm higher than 25th of 31. But even a 1s improvement on each side wouldn't put me in the trophies. Data comparison to a well-setup ND2 indicates we are on parity grip wise, but I was hoping to have an advantage there. I do not. This may be a hard limitation the result of the limited front camber. I've done considerable set-up variation and I think it's the best it can be right now grip-wise.

The Tour was really fun for me, as long as I didn't mind being 24th of 34. Again, a 1s improvement on each side wouldn't put me anywhere near the trophies. This year's courses (by Strano) were not fast like those in previous years, so we never had the top speed in 2nd advantage I was hoping for.

I've figured out a simple way to shorten the B6 front struts to make them legal and I have a bumpstop solution figured out and new ones ordered. The B6 higher damping should stop the bouncing, but I'm dubious how much that will really help. Half a second a side? That won't be enough to make the car competitive.

In summary, I definitely have a lot to learn/re-learn about driving a low-power car. I also need to drive this wider car (compared to the ND2) closer to the cones as I learn where the edges of the tires are. (I didn't hit one cone in 18 runs.) The new struts may help, but probably not enough to instantly put the car into contention for a trophy at Nats.

 

Seems like a perfect storm of negatives:
- Inadequate damping
- Course didn't let the car stretch any legs or even penalized the gearing
- Learning how hard to push it/launch it

Did you run yokohamas or bridgestones? On Strano's courses, I could see the 'stones being the better tire. They have a small lateral grip advantage over the yoks and give up a little longitudinal.

 

Great write-up! I watched you guys from afar, but didnt want to disrupt you in-between runs (plus, we were also swamped with our own conundrums regarding pace, and running around talking to various people, lol!)

The CS class was indeed PACKED with talent. Browsing through the list of names in various classes made this feel like a mid-summer nats! We had 1st, 2nd, and 5th place from Lincoln in AS. Not the time to suddenly discovery deficiencies . I agree with your assessment about damping. I think the vettes running MCS shocks had an advantage with dampening adjustments/capabilities, and the bumps continuously made the car unsteady enough that I was reluctant to get on power. On my last run on the first day (tighter course), I threw the car in full soft mode, but lost .6 seconds (ouch!) so that didn't seem to be the move.

 

I would certainly agree that this year's Bristol course was quite different than the past couple of years where it was all about going full send and sticking it out in 3rd as long as possible. This year's technical course, especially on day one, collected a lot of cone calls, and kept me out of the trophies in STX. Though I'm not the only one that can say that. I can also confirm that beating an ND, especially on a bumpy course, is a tall task. I drove one in CS last year at nats and the way they just sort of keep sticking when you think there's nothing left is quite impressive.

As far as shocks/damping, nice shocks make a huge difference. The first year I went to Bristol was in a 2nd gen twin in DS, and the bumps really upset the car to the point I was getting thrown out of the seat and thought I was going to spin out in 3rd. The next year I put some nice JRi coilovers on the car and jumped to STR (and got whooped by the NDs) and while I could feel the bumps, the car did not care about them. I could keep the throttle pinned through the bumpy sections at speed and it just stuck. This year I put the same setup (with softer springs) on a 1st gen twin for STX, and I truthfully never even thought about the bumps. My setup with those shocks is to put the high speed rebound on full soft to help out with the bumps, and generally make the ride better, and adjust the low speed compression/rebound to find the balance that I like, and make some final adjustments to rear tire pressure for small adjustments to rotation.

 

Stones.

 

I enjoyed watching your car while waiting in shade of the trees where the benches were before going out to work.

In the previous 2 years I was at Bristol in a C6 Corvette in BS with Penske shocks valved to my spec and a C5 before that. What bumps? The only bumps I ever noticed were the big double-humps in the middle, but those have been ground down now where they aren't the crazy throw-the-GT3-rear-ends-in-the-air type they used to be.

The shock spec I use is one specifically designed for use on bumpy lots.

 

I dynoed the B6's I'm about to install. They are not valved within a mile of what I would want but they have enough damping they should stop the bouncing. Since I figured out a way to make them legal without complete disassembly (and reassembly in a pressure chamber) I won't revalve them right away unless they really don't work.

 

Crazy!

 

Here's a long post about the car's evolution over the 4 national events it's run leading up to Nationals.

https://edfishjr.com/2024/08/15/987-...tional-events/

 

I thought you looked good at Peru/Grissom. I didn't get a chance to come chat but was keeping an eye on the CS action and was impressed with how close you were to fast company right in front of you. Good work!