Originally Posted By: friendly_jacek
Very good read and very convincing. However, how do you explain that I get a lot of shoulder wear in front tires running stock pressures (30-35 PSI, sample of 3 different cars) disappearing when I use higher pressures (38-42 PSI; max cold is 44).
Also, much higher pressures are recommended for high speed driving. I discovered that recently in an International service guide for corolla. For example, Australia 32 PSI under 140 km/h and 41 PSI over 140 km/h. Europe, 32/39 PSI for under/over 160 km/h: http://kre.unitra.sk/down/toyota/service_data_sheet/corolla_gasoline 2004.pdf
How does it mesh with your explanation?
As mentioned, there are various factors that can contribute to wear on the shoulders of the tire. The most common factor that is directly related to tire pressure is softer tire sidewalls. Under cornering, tires with lower air pressures tend to 'roll over' partially onto the sidewalls increasing wear on the shoulders. This happens less with tires that have stiffer sidewall construction. Increasing the tire pressure uses the pressure to 'hold up' the sidewall reducing the tire rollover. Common practice for autoX'ers to chalk the sidewalls to see how much/little rollover they're getting under maximal cornering.
As for why it's recommended to increase tire pressures for high speed travel:
All tires generate heat in the rubber from elastic hysteresis. The elasticity and integrity of the rubber is also dependent on the temperature range it is operated under. In the case of tire tread temperatures, cold tires have less ultimate grip, when they warm up to optimal temps, they exhibit maximum grip, if they exceed optimal temps, the rubber degrades faster and the intermolecular bonds in the rubber shear easier.
In the specific case of tires run at high speeds vs low speeds, the important hysteresis is occurring in the sidewalls. The continual flexing of the sidewall generates heat in the rubber. The faster the cyclic flexing occurs, the more heat is generated and the less time there is to remove/radiate this heat buildup. This is why running an under-pressured tire at highway speeds is dangerous. The sidewalls can heat up enough to cause the rubber to lose structural integrity causing a catastrophic blowout.
Have you ever seen tire carcasses on the side of the highway? If you examine the carcasses, you'll note that most of them delaminated and separated along the sidewall. The vast majority of them were operated at pressures too low for the load (higher loads obviously require higher pressures to compensate against the increased compression/flexing of the sidewall).
Conversely, you can see this issue of hysteresis is less prevalent when off-roading. Vehicles used in sand or mud can typically see ridiculously low tire pressures when compared to a street vehicle (done to increase the tire's footprint). On soft sand or on Arctic circle expeditions on ice for example, tire pressures are sometimes lowered to single digit psi. This can be done fairly safely because under those conditions, speeds tend to be restricted to single digits too. Elastic hysteresis occurs at a low enough rate that it doesn't severely compromise the tire's integrity (and of course, tire failures at single digit speeds aren't anywhere near as dangerous).
Unfortunately, on the street, the buildup of heat in the tire sidewall, while capable of increasing tire pressures a few psi, may not be capable of increasing the tire pressure enough to reduce the hysteresis to levels low enough so they won't compromise the tire's structure, so the simple solution is to add air to the tire before engaging in high speed driving.
Max
Very good read and very convincing. However, how do you explain that I get a lot of shoulder wear in front tires running stock pressures (30-35 PSI, sample of 3 different cars) disappearing when I use higher pressures (38-42 PSI; max cold is 44).
Also, much higher pressures are recommended for high speed driving. I discovered that recently in an International service guide for corolla. For example, Australia 32 PSI under 140 km/h and 41 PSI over 140 km/h. Europe, 32/39 PSI for under/over 160 km/h: http://kre.unitra.sk/down/toyota/service_data_sheet/corolla_gasoline 2004.pdf
How does it mesh with your explanation?
As mentioned, there are various factors that can contribute to wear on the shoulders of the tire. The most common factor that is directly related to tire pressure is softer tire sidewalls. Under cornering, tires with lower air pressures tend to 'roll over' partially onto the sidewalls increasing wear on the shoulders. This happens less with tires that have stiffer sidewall construction. Increasing the tire pressure uses the pressure to 'hold up' the sidewall reducing the tire rollover. Common practice for autoX'ers to chalk the sidewalls to see how much/little rollover they're getting under maximal cornering.
As for why it's recommended to increase tire pressures for high speed travel:
All tires generate heat in the rubber from elastic hysteresis. The elasticity and integrity of the rubber is also dependent on the temperature range it is operated under. In the case of tire tread temperatures, cold tires have less ultimate grip, when they warm up to optimal temps, they exhibit maximum grip, if they exceed optimal temps, the rubber degrades faster and the intermolecular bonds in the rubber shear easier.
In the specific case of tires run at high speeds vs low speeds, the important hysteresis is occurring in the sidewalls. The continual flexing of the sidewall generates heat in the rubber. The faster the cyclic flexing occurs, the more heat is generated and the less time there is to remove/radiate this heat buildup. This is why running an under-pressured tire at highway speeds is dangerous. The sidewalls can heat up enough to cause the rubber to lose structural integrity causing a catastrophic blowout.
Have you ever seen tire carcasses on the side of the highway? If you examine the carcasses, you'll note that most of them delaminated and separated along the sidewall. The vast majority of them were operated at pressures too low for the load (higher loads obviously require higher pressures to compensate against the increased compression/flexing of the sidewall).
Conversely, you can see this issue of hysteresis is less prevalent when off-roading. Vehicles used in sand or mud can typically see ridiculously low tire pressures when compared to a street vehicle (done to increase the tire's footprint). On soft sand or on Arctic circle expeditions on ice for example, tire pressures are sometimes lowered to single digit psi. This can be done fairly safely because under those conditions, speeds tend to be restricted to single digits too. Elastic hysteresis occurs at a low enough rate that it doesn't severely compromise the tire's integrity (and of course, tire failures at single digit speeds aren't anywhere near as dangerous).
Unfortunately, on the street, the buildup of heat in the tire sidewall, while capable of increasing tire pressures a few psi, may not be capable of increasing the tire pressure enough to reduce the hysteresis to levels low enough so they won't compromise the tire's structure, so the simple solution is to add air to the tire before engaging in high speed driving.
Max
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