majormojo

This is my first post here; I recently purchased a CPO 2011 Cayman S. I'm working on alignment and tire pressures, setting up for autox (SCCA SS class) and track days. Priority is to get max benefit with lowest wear from the tires (RE-11) and stay within the SS class rules (for now - longer term, optimizing for track use will probably push me out of SS autox class...).

Background info:
Car: 2011 Cayman S - PDK + Sport Chrono. H&R front stab bar, OEM rear.
Tires: Bridgestone RE-11 F:235/40/18 R:265/40/18 (OEM size on OEM wheels)
Alignment:
Front:
camber -0.7 L/R
toe 0.00 L/R

Rear:
camber -1.8 L/R
toe +0.08R +0.08L (0.16deg total toe-in)

Those camber settings are the max possible on the car. The front sucks, but without adding adjustable control arms that's the best I can get.

So now I've run a couple autox events and a few track sessions and I logged tire pressures and temps after each session. See images. Data layout: four boxes for each wheel. Top box is temp, bottom three are tire temps of tire looking down from top of vehicle. [outside] [middle] [inside] on driver side, [inside] [middle] [outside] on passenger. Pressures taken with Longacre analog gauge, temps with Longacre probe-type pyrometer.

Autox 1:


Autox 2:


Track 1:


Track 2:



The conclusions I've come to are:

- Target hot pressures should be around 44-46F/46-48R. This is congruent with what others have found for the RE-11. Seems to do a reasonable job of keeping even tire temps up front, despite the limited camber.

- Rear camber is a bit too much, thus higher inside temps. I'm thinking to reduce the neg camber in back to about -1.0 deg L/R.

- Tire temps over ~155F, I could really feel the tires get greasy. Seemed grippy and consistent up to that. I'm thinking that's just a limitation of street tires and something to manage.

- Rear toe I'm not really sure. My instructions were misunderstood, what I'd asked for is 0.08 deg total toe in and got 0.08 each side instead. I don't know if I can/will really feel the difference, but I'm thinking that a bit less will allow the rear to rotate a bit easier, which I would like in autox especially.

Any other observations or insight? What do you think this data says?

Thx,

sjfehr

Welcome! I just bought a 2010 Boxster S, PDK w/Sport Chrono last week and am about to start testing myself. I think 44+psi may be too high; you're not going to be able to get any worthwhile data from your pyrometer without being able to adjust the camber. Don't put too much emphasis on that, as your best grip may not come with the most equalized temperatures- you really need to hit a skidpad and do testing. I did a bunch of skidpad testing on 986S two months ago and determined 37/37psi (hot) and spraying to 90-100F was optimal. Now, I was running 225/45-17 & 255/40-17 ZIIs and RE-11 are going to be different (as are the 255/35-18 & 275/35-18 ZIIs I've got on my 987.S), but you may want to try lower pressures and see how your times look.

You also may want to pick up a sprayer, as 160F is obviously beyond the peak grip of those tires...

Are you using any data capture, like on your cellphone? You may be able to push it harder; with so little camber on these cars, you should be really cooking the outside corner of your tires a lot more than you're showing in your #s. For reference, here's what my 986S looked like during the test & tune when I was testing different pressures and temperatures:

edfishjr

Given that the car is rear-weighted, I'm not sure there is anything excessive about 1.8 degrees rear camber. You say it is too much, but don't say why. Do you feel it is hurting getting power down off the corners? About 10 degrees warmer on the inside of a well cambered tire is optimal, so that alone does not mean you have too much camber. Even tire temps across the face usually mean too little camber for peak grip, I think. (As illustrated by your front tires!)

allen_skillicorn

What was the handling balance like?

Your front pressure does sound a little high for peak grip. I personally run the rears excessively high to reduce understeer.

Your results do look good, so I wonder if sjf is right that you need to push harder.

majormojo

Thanks for the responses, I appreciate the input. Excellent food for thought. For a bit more background, this is my first Porsche; I'm coming from a Miata that I autocrossed for the past 7 years, going from ES through ST* and CSP classes. So that's what I'm used to, and I'm still very much learning this new car.

sjfehr: My thinking with using the pyrometer was to aid in adjusting pressures, looking for an even spread - specifically, adjusting so the mid-tire temps are also the mid temp across the tire - theory being even heat spread means most consistent tire patch given the camber limits. Is that incorrect?

I think my natural (learned) tendency is to immediately back off when I feel the front start to push. I think what you're saying is I should press on and 'push through the push' a bit more, in the process ultimately getting more out of the tires?

Would love to do skidpad testing but I can't see how I'd ever get access to a skidpad to do that.

For data capture, I do have CMS laptimer on iPhone and a Dual 5Hz GPS. That would be useful for skidpad testing I think, but without that I'm limited to what I can capture on autox runs and there's so much variability (driver learning his car...) and so few runs to measure, that will take a long time. I do have some data caps from those runs above, I'll see if I can share them in a useful format.

edfishjr: It seemed to me that the amount of rear camber was causing increased tire temps (and therefore load) on the insides of the rear tires - opposite to what I'd expect in a car with limited static camber and "wrong" camber changes under lean/load.

Why do you say 10deg warmer on inside is optimal? What does that mean with respect to how the tire is working?

Mostly, I'm using the tire temps as a kind of proxy for lateral grip since I can't measure that consistently (and honestly I'm not such a sensitive driver and familar with this car that I can sense really small handling changes yet). So I'm relying on the pyrometer to tell me how the tires are reacting. Perhaps that is not a valid approach.

The 160 temp was after a 20 min track session, not something I would see on autox. I do have a sprayer I can use though. I've been unable to find any data on optimal temps for the RE-11.

peallens: Overall the handling was more understeery than I'd like and the rear doesn't feel quite as loose or tossable as I'd like it, especially through a slalom or set of offsets running at a consistent speed. Perhaps that is my unsophisticated description of simple understeer. The increased tire pressures (40+) on the last few track sessions did coincide with a reduction in push in mid-corner sweepers and corner exits, till things got hot and greasy.

btwyx

I've used RE-11 on track (not on Porsches), and they seem happiest under about 160. You data would seem to bear that out. Your data was looking pretty good until that last set.

Personally I wouldn't be using RE-11 for autocross, Dunlop ZII have worked much better for me, and they seem to like working in the 120-140°F range. The ZII also seem to do about as well as RE-11 on track, if not better.

sjfehr

Even temperatures across the face make sense when adjusting camber (like for CSP), but just isn't possible when we can't get the right camber. And is rather course dependent whether you measure inside heat or outside heat after pulling into grid. You don't need a pro skidpad to test, just set a bunch of cones in a circle with a tape measure and go at it for a couple hours; that will give you the best empirical data you can get. Temperatures are indirect at best, and cell phone g forces are great as a driving aid, but I've found too noisy to help on the skidpad- I recorded mph when I did the skidpad testing.

You may just need to get used to the Cayman, as it will take a different driving style than you're likely used to. If you're not pulling better than 1g through every sweeper, you could have driven harder; I don't mean intentionally plowing, but from your temp data, it just looks like you could be driving harder (and faster) than you are.

edfishjr

I think of it like this: If we want max lateral G in a turn we need to have (for the sake of simplicity) no camber on the outside tire when car is at full roll. The outside tire is most important because it has the highest load due to lateral weight shift and producing max lateral G is typically the most important single thing in autocross.

But, that means that when driving anything less than full cornering we are running more on the inside edges of all the tires.

And it also means that when fully cornering the inside tire is really on its inside edge, though with less load proportional to lateral weight shift.

So, over the total time of an autocross run, given even left and right turns, any particular tire is working its inside edge more than the outside if the camber is such to produce maximum lateral G in a corner. The overall effect is that the inside edge should run a little warmer.

majormojo

Ok, I'll examine the data logs a bit more carefully. I know I'm peaking at a touch over 1.0 g in some turns, but perhaps not sustaining it as I should. I have another autox event coming this weekend and I'll focus more on the G force data and less on temps.

Thanks edfishjr for the explanation, I understand now what you mean. I was expecting that the extra heat generated on outsides in hard turns would be much more than the heat from the overall effect of running on the insides due to static camber. My experience with the Miata was running about -2.5 neg camber in the rear and I consistently saw hotter outside temps. But that was on 275 Hoosier A6 that generated a lot more total lateral grip. (1.25 - 1.3G if I recall).

The data logger (CMS laptimer) has the option to use GPS or phone accelerometer data. I'm guessing the GPS data to be more accurate, but I don't know.

The RE-11 tires were on the car with lots of tread left when I bought it a couple months ago. For this year, I wanted to run with minimal changes including stock wheels. For next season I'll pick up another set of wheels and can experiment with other tires. How do the ZII's work with cooler ambient temps? I live in Canada and the outdoor temperatures on our shoulder season events can be very cool (50-60 F) and even on the hottest days overheating of tires is rarely an issue at autox, without a co-driver. Said differently, an autox tire that works well cold is more valuable to me than one that works well at high temp.

majormojo

This is a video clip w/ data from the fourth autox run above.

btwyx

They work pretty well, we have 50-60's down here out at the coast in summer (Marina,CA). They only need about 120°F to be happy, but work when they're cooler than that. You might be interested in the ZII Star Spec which are due to arrive later this year, they're supposed to work better cooler.

DOUGLAP1

OP, thanks for posting all that tire temperature data. It is pretty interesting. One great source that I found for tire tuning is this guide from Michelin.:

http://www.tirerack.com/tires/tirete...nd_Feeding.pdf

(Note: Something on the Tire Rack website apparently blocks this link from going directly to the Michelin PDF document. When the general tirerack.com site comes up, if you type in "care and feeding" in the Tire rack search window, it goes right to it)

This guide is written around the MPSC tire, but seems to be applicable to any of the current extreme performance, max performance, and even DOT R-comps. This guide teaches that the optimum tire temperatures are not necessarily perfectly even across the tread, which is quite a bit different than the conventional wisdom that you see from some sources.

Like you, I frequently take tire temperatures and pressures , and have noted a few things that may be of interest to you:

1- In my experience the RE-11's on my daughter's M3 work about like the ZII's on my 986S, with optimum tire pressures at about 35 - 38 psig. Anything over this seems to compromise grip. On both race tracks and auto-x, my ZII's fall off rapidly if I let the hot pressures go over about 42 psig.

2- The current ZII's seem to like cooler temperatures better than the previous ZI design. With the ZII's, if the tread temperature gets over 140 F, they fall off. This used to be when the ZI's were just starting to grip good. I hate to think what the ZII Star Spec ideal temperature may be. I never thought I would see the need to spray street tires between runs, but it is here. The ZII would probably be a good tire for you given the cool weather you normally have. I do not have any experience with BFG Rivals, so can't comment on those.

3- The front tire temperatures on my 986S are pretty consistent across the tire from inside to out within 5 F or so when running either ZII's, or A6's (I have a lot of wheels & tires). But that is with close to -3 deg front camber. NT-01's don't seem to need as much negative camber, and like heat much more than the ZII's and A6's.

4- Rear tire temperatures on my 986S are always about 10 - 15 F hotter on the inside than outside when running auto-x, but are dead even when coming right off a race track with temperatures taken immediately by a buddy in the hot pits. I haven't completely sorted out this phenomenon, but I believe that it is due to chassis squat under hard acceleration in low gears in an auto-x environment. My theory is that the chassis squat causes more rear negative camber than the static camber we all measure, which heats up the inside rear tire under hard acceleration. In a race track environment, there is not enough torque in the upper gears at over 100 mph to get similar chassis squat. I would like to get access to a skid pad test to this theory out by taking tire temperatures during steady state cornering vs hard acceleration and cornering, but have not had the opportunity.

You obviously cannot fix the front camber issue on your car and stay in street class. When I faced a similar situation prior to modifying my front camber, I used a wider front tire, with almost a square set up front to rear (255 F / 265R). This is one band-aid that works for controlling understeer if you can also increase the front wheel width, but in SCCA street class, this is also forbidden. Perhaps you can get an extra set of stock rear wheels to run on the front unless SCCA closed that loophole too - read your rules carefully.

Running a slightly stiffer front bar can also help by reducing roll, which causes all McStrut design cars to go toward positive camber. However, if you go too stiff, it will actually make understeer worse. It is best to get an adjustable GT3 bar and experiment.

Other band-aids include running an overly stiff rear bar, or jacking up tire pressures as you seem to have found. However, I believe that all of these fall under the heading of unsticking the end that is working. In the end if you want to make the car handle well, you will need to do modifications that will throw you into ASP. I don't believe there is any way around that.

Good luck.

mopar bob

I'm not sure going to a GT3 rear bar is the way to go with a 265 rear tire at least with my 997. I put one on for the last race and driving school. I went from under steer to over steer. I started on the middle hole for the driving school and started to put the car sideways all the time and the car wouldn't come out of the turns as hard. The next day was the race so I set the bar on full soft but the car was still to loose. I'm going back to the stock rear bar for the next race. Go a little deeper and hard on the brakes get it turned and hard on the gas out of the turn. That is what the instructor keep trying to beat into me. On the sweepers I was cutting in to soon then had a hard time turning the car, by the way this was the line fast front engine cars are taking so I need to fight the myself not to use that line.
My RE11's like 42-45 in the rear and 38-40 in the front but this is a 911 so it may like higher pressures in the rear.

sjfehr

Conventional wisdom is that stiffening a bar transfers grip from the tires the bar is on to the others. So a stiff rear bar would transfer rear grip to the front. Conventional wisdom would have you fix an understeering car with a bigger rear bar. Too much of a good thing isn't necessarily all that great, though, as too much oversteer is even worse. It's counter-intuitive, but a stiffer front bar can, in some cases, decrease understeer. This is because the added stiffness reduces dynamic camber loss and causes the front tires to sit more flat while cornering, thereby increasing grip (while at the same time transferring some of that grip to the rear, increasing overall grip). Stiffer front sway bars are standard practice on S2000s for autocross, and I ran a GT3 front sway bar in the 2nd softest position on my 986 Boxster to very good effect.

ZII are fantastic cool weather tires, probably the best out there right now. They'll give great grip down into the 40s, well cooler than Rivals or A6 will. I daily drive mine, and have commuted in temps down into the mid teens without any issue- they warm up pretty quickly when driving anyhow, so it's really just the first few minutes. They are most definitely NOT snow tires, though.

mopar bob

You are correct that a loose car is not good as my times slowed down. That rear bar maybe good with the 305 x 19 tire, but still maybe to loose. I will be going back to the 19's on the next set of tires as my car came with that option. I thought that the 18's are 6 lbs lighter than the 19's that it would work better, OH what a rookie mistake.
At 36 lbs in the front and 38 in the back the tires had to much flex and my times slowed down, but that could have been me. At 38 front and 44 in the back the car feels crisp and I'm faster. As I'm a first year rookie I'm not a guru on this.