David Borden

@edfishjr , thanks for the correction and great examples! I got weight transfer mixed up with regards to shock adjustment. The symptom of a delayed time for a car to set / bite while braking is something I had experienced before with the C5 Z06 and it was due to too much rear rebound. Softening up the rear rebound fixed the issue. It may be that the front compression damping and or the low, med, high speed damping from front to rear was not right, not sure but I had the same phenomena happen on the GT3 and softening the shocks(assuming it was related to rear rebound but could have been wrong) improved it quite a bit.

I cant say for certain, but I don't think I was overloading the front tires as I would assume the ABS would kick in and you can normally feel that. I do brake very aggressively though, so it may not have shown up with a more delayed application of brakes. If I get a chance, I may do some testing to see if I can figure it out.

edfishjr

Something to consider, and once again not saying this is what is happening with your car: just as too much damping, in either direction, impairs lateral tire grip, too much rebound in the rear specifically binds up the rear suspension and prevents the rear tire from conforming to the surface and keeping good contact. This can impair rear braking performance upon brake application.

So, theoretically, too much rear rebound can impair the initial braking that we expect from the rear tires, which have lots of weight on them (50% in the case of a C6Z06, 60% or more in the case of a GT3) at the beginning of the braking event and we then may have to wait for the weight shift to further complete before we get full power from the front tire patches.

In this case, the delay is 1) actually the failure of the rears to do their full share of the braking at the beginning of the braking event and then we have to wait for the fronts to pick up the slack, and 2) we may be waiting for the rear springs to overcome the rear shocks (by pushing against the mass of the car as it stops tilting) and firmly set the rear tires back down to the ground where they can contribute more to braking.

Again, not saying that this is what is happening with your car.

Edit: the more I think about it the more I like 2), above, as the most likely explanation. The tilt + weight shift picks load off the rears and the springs, working against the rebound forces, take time to push them back down. This might be felt by the driver as a delay in achieving full braking.

David Borden

More than likely you are correct with your theory of partially lifting the rear tires, preventing them from doing their share of work until the shocks extend. The front tires are 245, the rears are 305 which I would think would amplify the problem given the rear weight bias. Whatever was happening appeared to be improved by softening the rear shocks. Given the time it would be interesting to play with front slow to mid speed compression to see if that affected it as well. Re would stiffening the front compression counter the rear rebound, making the time to slow quicker vs softer rear rebound and front compression? Good Discussion!

edfishjr

I was just thinking about your question that I bolded.

Increasing front compression 1) reduces the rate of pitch, so that it actually takes longer to fully compress the springs, and 2) speeds up weight transfer from rear to the front by making the front suspension stiffer, i.e. more kart-like. I think that effect 1) would help the rears maintain traction, by slowing pitch, but 2) increases the dynamic speed of weight transfer which probably does nothing to help the rear.

So, I think the best thing is what you've done: reduce rear rebound. This is what is mostly likely to have the greatest effect if it is, in fact, the direct source of the problem.

In general, Street class cars need more low-shaft speed compression, especially in the front, than any standard stock shock provides. This is to increase transient response when you can't do it the right way, which is stiffer springs. Many older shock designs are basically incapable of supplying the needed forces in compression and the industry substituted high levels of rebound to compensate, but this was a poor trade-off. As I understand it the DSC programming allows a significant increase in compression damping in the proper shaft speed range over what was OEM provided. This is good for both Street class and also good when stiffer springs are used and you really must increase damping to maintain control of the car and suspension.

David Borden

I agree with you. Years ago when we were heavily involved in autocross, many of the fast folks in our region, several were national champions, were running triples with slow speed compression adjustment. This was the primary tool they used to tune transient response. It does not appear you have the same issues with slow speed compression adjustment as you do with a shock with a bleed valve and a very digressive rebound curve. It works for sure, but seems to expose problems in braking. Despite this, even talking with Koni about their motorsports shocks at SEMA, they were trying to convince me it was all about rebound. My guess is this is more about the challenges of creating a reasonably priced shock that has slow speed compression.

edfishjr

That's interesting that Koni would say that. It was a Koni racing engineer (The Z in JRZ) who discovered that they were at the limit of what rebound could do in the late 1960's when Porsche 911s got more powerful with their big 2.4liter motor and wouldn't put power down well going up their mountain test road. He had to start decreasing the rebound and increasing the compression in order to get the cars to hook up.

I don't think they had digressive rebound curves back then, however, at least not on relatively cheap shocks suitable for M030 use. Pretty sure it was all linear.

And we always have to remember that autocross is a very different animal from road-racing and that Street class is a weird case unto itself.

Greg_STL

Back in post 88, I complained about my Cayman acting wonky with the nannies on and trying to use launch control at AutoX. This last weekend I ran in manual PDK mode and with short press PSM off (PSM Sport). Between that and the recent alignment, the car was much much better to drive. Even before the results were sent out, I was really happy because it was just much more fun to drive when I didn't feel like the car was fighting me. I had my best finish by PAX ever with an 8th place finish. I know the advice was to go all nannies off - but with only 265 width tires in the back, keeping traction control on was worth it. I know GT3/GT4s can independently select PSM and Traction control but on the lesser Caymans you cannot run PSM off with Traction control on. Maybe when I put on my 255/285 RE-71s I'll re-visit the PSM settings.

1st event - Long press PSM off, launch control, PDK auto, stock alignment - 24th out of 57
2nd event - PSM on, launch control, PDK auto, stock alignment - 15th out of 56
3rd event - Short press PSM (Sport PSM), manual PDK, no launch, -1.4F/-2.1R camber - 8th out of 51

I have also been letting air out of my tires to basically hold the stock 35 psi front and rear between runs. It was high 90s this last weekend, so I even used a cheap water sprayer ($5 from Home Depot) between my 3rd and 5th runs.





It is amusing that the satellite image used as a background for my TrackAddict app actually shows one of our AutoX events at the arena.





I definitely have more to go, but I feel like I have enough of the settings figured out that it is up to the nut behind the wheel now.

sgreer78

Good progress. I really had trouble finding time Sunday. Next event I'll talk with you about tire pressures and things I found with my GT4.

Abt12

Necroing this thread as I'm setting up my (new to me) 718 GTS 2.5 up for dual use AutoX/Track.

Couple questions for those who have been running for a while...
1. Consensus for "starting" alignment still Max Neg Front camber, -0.5deg additional rear (e.g., -1.2 front, -1.7 rear), 0 toe front, 1/16"-1/8" toe in rear?
2. Any recommendations for sway bar changes (potentially with an adjustable bar for more room to play)?
3. Any driving dynamics tips? I'm coming from a front-biased AWD Golf, so pretty big change.

Thanks!