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Cayman Tires & WheelsDiscussion of Tires, Wheels, Suspension, etc.
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Maybe it's the larger diameter rears combined with 50% worn fronts.
In my feeble little mind equal diameter front and rear would be ideal.
Porsche specs the cars with .94" taller rears for the raked look that people seem to like.
Perhaps 1.25" taller rears crosses a line that makes the car unstable?
If car is still squirrelly with new tires, get your alignment checked, both front and rear. As I read this thread, it sounds to me like an alignment problem versus a tire problem. New tires could reveal alignment issue that is masked by wear in older tires. Just my $.02.
FWIW, I too have the 275-235 combo on my car and they worked really well. I made sure to put 500 or so miles on them before my DE event last weekend, could have benefited from a tad more break in miles as they didn't provide much grip for the first few sessions. Overall I am very happy with the balance of this combo and would do it again. BTW my 275's aren't N spec tires but the 235's are, I've had zero problems with PSM to date. I did play with the PSI and found 33/35 hot, worked well on my 2.7l cayman.
PSM - Porsche Stability Management
While it can’t overcome the laws of physics, the revolutionary Porsche Stability Management (PSM) system does lend an added degree of balance and control to the Cayman’s mid-engine driving dynamics, inspiring surefooted confidence in corners and extreme situations.
A standard feature on the Cayman and Cayman S, PSM continuously monitors steering input, road speed, yaw velocity and lateral acceleration to calculate the actual direction of travel. If the car begins to steer off line, PSM instantly intervenes with precision brake inputs on individual wheels to help bring the car back onto the driver’s intended path.
If braking alone isn’t enough to correct the vehicle’s cornering line, PSM then calls on the Cayman’s engine management system, adjusting engine output as needed to help stabilize handling. PSM can also compensate in an instant for mid-corner changes in load resulting from deceleration or braking. When Sport mode is selected with the optional Sport Chrono Package, PSM’s threshold for intervention is raised, allowing for greater driver involvement. If you prefer driving without automatic PSM assistance, the system can be set to standby at any time. In this case, it will only intervene under heavy braking, where both front wheels exceed the ABS threshold.
For all of its technical ability, PSM goes virtually unnoticed in everyday driving situations, preserving the Cayman’s natural agility.
FWIW, a number of us here (including me) run 275/40-18 R compound tires on stock 9" wheels (an approved width per the tire manufacturer) on the track without any issues at all. Fronts are 245/40.
thunderball: I'd run 265s on 9" rims, but I'm more conservative than many when it comes to tire sizing. I suspect that a 275 really wants a 10" rim in order to work optimally. That doesn't mean that it won't work okay on a 9" rim. After all, it's only 10mm wider than what Porsche installed, but even 265s should work better on a 10" rim than on a 9" rim.
One of the top autocrossers in my SCCA region wears 295 Hoosiers on his Saturn Sky's 9" rims. I consider that very aggressive tire fitment, but it sure seems to work well for him. His tire mounter probably hates to see him coming.
Who installed an balanced the 275 tires? Could it be a broken tire with excessive radial pull. Proper wheel balancing machine should be able to detect that. Did the installer use a high end machine?
How wide are your rear wheels? If they are 10"... then this is odd.
And finally, are the new 265s working out for you?
__________________
Tomasz
Apologies for spelling mistakes, most of them are keyboarding errors. To enjoy this forum I run out of time to proof read.
Thanx Jim and stlpca. Tires have been ordered and knowing what little I know I thought that was correect. I went 245f and 275r. I wanted the r6 but don't come in those sizes.
That's probably contributing more to the poor handling/sense of poor stability. If you go to a wider tire on the stock widths (9" - 9.5") you run the risk of putting slack into the side wall allowing more lateral flex. The opposite is true, for example mounting 265 on 10" wide will harder the ride, pickup the slack by stretching out the side wall.
Another factor is the fact you had increased the sidewall profile by going up to 275, as was noted, your overall diameter increased, keeping the pressure at, with more flex in the sidewall just magnified the wobbly feeling. Probably going to 40psi or a bit more would have helped pick up the slack you introduced by going wide, increasing profile and mounting on narrow wheels.
Well got my new set of P-approved 265s and now the car is back to normal. Good ol sharp steering and balance is back !!
Conclusion, the 275s different size was the problem, just as 987Berto pointed out earlier.
I caution anyone who is thinking of doing what I did earlier. What a mistake. Besides, it was very dangerous. 265s and 275 huge difference. At least in the Michs PS2s.
__________________ The first South American modded reptile ~ 350HP - 4.4 sec 0-60 and 12.7 sec 1/4 mile. But looking for extra 100 HP!
Well got my new set of P-approved 265s and now the car is back to normal. Good ol sharp steering and balance is back !!
Conclusion, the 275s different size was the problem, just as 987Berto pointed out earlier.
I caution anyone who is thinking of doing what I did earlier. What a mistake. Besides, it was very dangerous. 265s and 275 huge difference. At least in the Michs PS2s.
I disagree. I switched from 265/35 to 275/30, added spacers to make the offset 36mm. It's perfect after doing a wheel alignment.
It's beginning to sound like there was something wrong with that particular set of 275's. Lots of others run 275's with no problems, and as soon as they were removed from Cocodrilo's car everything was fine.
keep in mind that I made the switch from 265/40/18 to 275/40/18 keeping the same ratio (40 profile). Prolly would have been better to go 275/35/18 or 285/30/18.
Car is fine, and responds just as new.
__________________ The first South American modded reptile ~ 350HP - 4.4 sec 0-60 and 12.7 sec 1/4 mile. But looking for extra 100 HP!
