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Friday, June 28, 2019

Critical tire temperature ?

Received another question
Hi Roger, I just read your article on sidewall strength and “blowouts”. You mentioned that if the temperature rises too high the sidewall cords may melt. I have a 2017 Winnebago Grand Tour with a built in TPMS system with pressure and temperature sensors. Most RVers know (or should know) to keep their tires at the proper inflation level. The one thing I haven’t seen addressed is tire/air pressure temperature. As I drive down the road I can watch the pressure rise with a corresponding temperature increase. Is there a critical temperature to watch out for exclusive of tire pressure? If so, I’m sure it would be different for each make/model and tire size. Do manufacturers publish any specific data for their tires (i.e. a critical do not exceed temperature)?
Thanks for all you do to keep us in the loop.

Ken

Ken, If you check out the posts on my blog you will find numerous posts on tire temperature and pressure.

RE Critical temp.  For short term (MInutes or seconds) I would consider structural temperatures in the 205 - 230F to be critical. Size, brand or design, doesn't make much difference. The problem is there is no way to measure the temperature as the location of the hottest location is at the edges of the radial belts. In racing, like at Indy we use a needle probe and stick this into the tire about 1/4 to 3/8" deep. External surface temperatures are much cooler because of external air moving over the surface. With rubber being such a good insulator the heat does not travel to the surface fast enough (minutes) relative to the ability of the temperature to rise (seconds) at the critical location we see it possible to have internal rubber failure before the surface temperature gets to a critical point.
I think you can understand that the rubber can get softer with an increase in temperature. As it gets softer, it stretches more, as it stretches more it gets hotter, etc. Up to about 210F, this increased softness can be managed with construction and thickness design changes but each of those changes can result in some other property getting worse. Also, that 210F point, changes with time and temperature history. Tire design is a balancing act and a series of trade-offs. One thing that happens with a loss of strength and increase bending is the potential for tearing of the rubber at the molecular level which can result ultimately with the tearing to grow to the width of the belts which in turn results in a belt separation.

BUT

The key thing to understand is that temperature damage is cumulative and the chemical reaction that occurs can eventually result in a loss of elasticity at higher temperatures so the rubber tears which leads to a separation. The rate of these changes can itself change over time and the rate is also affected by the temperature.
Even for a single rubber compound, there is no exact temperature for it to "fail". The physical properties of rubber changes with temperature but here there is no exact point at which the strength goes from 100% to zero.

TPMS gives an approximate temperature of the air inside a tire which is also lower than the critical temperature. Most TPMS have a high-temperature warning set to 158F and if you see that reading then you do not have enough pressure for the load you have or are driving too fast. This does not mean you can lower pressure till you see a reading of 158 as you are still doing damage and "consuming" the finite life of the rubber in a tire.

Read my blog, Learn the actual load (when the RV is its heaviest) for each tire position (4 corner weights) Consult the tables, Learn the MINIMUM inflation pressure, add a +10% inflation margin to learn your "cold set pressure" for each axle based on the heaviest axle end, Adjust your TPMS low pressure level to never be lower than the minimum. The go and enjoy your RV

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