Quote:
Originally Posted by Walter
Quote:
Originally Posted by BillC
Going from 265 to 275 is such a small change that you shouldn't even be able to feel it
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I think it's more change than you think.
Stock the rears are about an inch taller than the fronts.
Barely a tenth of an inch from being beyond the 4% f/r that I think I read about here.
Add worn fronts and rears .32" taller and the computer might object, no?
The extra rake when there was already almost and inch,
plus worn and new tires would be noticable I would think. |
First, I said "in normal driving". I don't doubt that a good driver can tell a slight difference at race speeds, but at normal, street-driving speeds, it's just not that much of a difference. Going from 265/40 to 275/40 is only a 1.2% change in radius and circumference, not much at all.
Also, the OP said that the
PSM light would light up after he felt the car squirm. That indicates that
PSM is reacting to a loss of traction, not complaining about tire size differentials.
And, the OP added a post above mine that indicates the rear tires are at least 3-4 psi low. Running the tires low will affect traction and drivability far more than increasing the size 1.2%. In fact, if the rear tires really are low in pressure, their rolling radius may actually be
less then the original tires at proper pressure.
Finally, we're both guessing about the state of the front tires. The OP only said that his old rears were worn, nothing about the fronts. Since a common pattern for tire wear on Porsches is two sets of rears for every one set of fronts, it's quite likely that his front tires are in satisfactory condition and not significantly worn.
Until the OP posts back about the results of his playing with his tire pressures, we won't know if its pressures or sizes that are affecting his handling. The size differential might be affecting when
PSM kicks in, but
PSM wouldn't make the car feel scary-handling as described by the OP.
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PSM - Porsche Stability Management
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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. |
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