Quote:
Originally Posted by The Atomic Ass
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The rated speed on the tire is related to heat, if I'm not mistaken, ........
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You are indeed mistaken.
The speed rating of a tire is more related to the construction - specifically, the number (or absence) of circumferential fabric overlays over the belt package. Typically these are made of nylon, but other materials have been used. These overlays restrict the centrifugal growth of the tire, thereby increasing the speed before the tread will come flying off (Ref: See Top Gear's Rich Hammond's Jet Car Crash)
You can tell if these overlays (also called cap plies) are present in your tires by looking on the sidewall for the construction materials. A typical construction would say: Tread plies: 2 polyester + 2 steel + 2 nylon (The "2 nylon" being the overlays)
But since I am doing a post:
Quote:
Originally Posted by theunchosen
The tires are going to be designed to withstand above and beyond by at least a factor of 1.2 of max sidewall, max temp and max speed(ambient air temp and then tire temp, because they will be drastically different). Given, they might not be able to get a safety factor of better than 1.2 in there so running it max sidewall on really scorcher days at high speed and you might end up upside down. I go to max sidewall and stay just under the speed limit. If you are at sidewall speeding is ok but you go over and the complicated equations for thin walled pressure members and tire elasticity with the thermal expansion of the tire belts. . .let's just say there exists the possibility of havoc. . .
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I'm going to start in a different place:
Standard Load Passenger Car tires (the type 90% of us have!) have a range of inflation values between 26 and 35 psi. The maximum load carrying capacity at these pressures are defined by tire standardizing bodies such as The Tire and Rim Association (commonly referred to in the industry as TRA). The relationship between inflation pressure and maximum load carrying capacity for a given size are published in books that become the starting point for pretty much everything else that is done concerning tires.
For example, tire design usually takes the maximum usage inflation pressure (35 psi), and uses that as basis for the design. In order to account for fatique, a factor of 4 to 5 is used as a multiplier - and then additional factors are applied for the imprecision of theoretical analysis. - and then the factory can only have certain combinations of ply fabric, so the resulting value becomes a minimum.
Put a different way: If 35 psi is used as the basis, then a factor of 4 or 5 (for fatigue resistance) results in 140 psi to 175 psi - and these would be the minimum burst pressure if no additional design factors were applied. So typically quoted burst pressure values of "over 200 psi" are right in line with this.
What this means is that even if the sidewall says "51 psi max", the design pressure is 35 psi - and what is written on the sidewall as a maximum is a "permitted pressure" as indicated in the TRA yearbook.
So I am concerned when folks reference the sidewall maximum when discussing inflation pressure. I think they ought to be referencing 35 psi - and I also think the placard pressure becomes an important item to reference - as this goes to the size of the contact patch.