Given
51%
while driving a
Porsche 986 Boxster S
(225/40 R18 W) on
a combination of roads
for 15,000
spirited miles
I am currently on my fourth(!) set of these tires (225/40/18 fronts and 265/35/18 rears). Under normal street driving, always with the manufacturer's recommended pressures, the tires eventually develop what feels like flat spots but is more properly described as "radial force variation". The shake can be very intense and disconcerting - enough to make you worry about tire integrity and overall safety.
Severity is not constant - it depends on tire temperature (I believe) and it is much worse after having decelerated from high speed, e.g. from 120mph (I am in Europe). The shimmy is present when braking, rolling, or accelerating but it is felt only past 70 mph or so. It does not go away at higher speeds but it becomes less intense due to the higher frequency. It does not feel like coming always from the front wheels.
Hard cornering or repeated hard braking have, intermittently and to a varying degree, succeeded in alleviating the problem, only temporarily: It returns with a vengeance either the next day or after a bit of driving.
Testing (over months) with combinations of different rims and tires has excluded every other possible cause. Brake rotors were replaced, wheel bearings too, anything rubber in the suspension is good. Balancing also seems perfect. (Unfortunately no shop here has a balancer that exerts a force on the tire - like the Hunter in the USA). In short, putting different tires on my rims (even if they are used), without changing anything else, solves the problem immediately.
I have not moved to a different brand/model because the local Pirelli distributor have already replaced the tires thrice(!) for me - free of charge (do they know something is up? where there any "bad" batches that have found their way into the market?). This last set was fine for a couple of months and, for no apparent reason, it now developed the problem.
I should say that grip and response are very good in general so, if not for this problem, the tire would be excellent.