CapriRacer wrote:


OK. End of first tid bit. Next up: The speed restriction of ST tires and how that affects things.

ExRocketScientist wrote:

Me thinks I am seeing where the "stiffer sidewall" thing comes in. The ST sidewall is allowed to flex more, thus the higher load rating for the same physical dimension and pressure. This may give the impression that the LT has a stiffer sidewall because it is not flexing as much. They could actually both have the same inherent flexibility but the LT would crack easier when flexed the same amount as the ST.

Nothing about this in the 700+ page tire science document (they do have the load formulas and the K factor thing, but not the issue of the carbon black and the cracking). This is something new brought to the table.

Francesca Knowles wrote:

......I'd like to put in a request for chapter three:

My truck driver friends tell me that the tires on the tractor have a different tread pattern than those on their trailers.
As explained to me, the trailer is expected mostly to follow the cab, and for that reason the tread pattern is "straighter" than the pattern on the drive unit.
They also maintain that the trailer tires are "stiffer" than those on the tractor for the same reason.

Am I getting this right, and if so, does/should the same design logic govern tire design/selection for smaller trailers like ours?

Thanks!

CapriRacer wrote:

2nd installment:

I am sure most people here are aware that ST tires are speed restricted to 65 mph. There are lots of people who only discover that after a tire failure. LT tires are also speed restricted to 65 mph unless a speed rating is indicated. – which nowadays is the norm.

If one were to back calculate from the common Q speed rating in sizes that common in both types (There is barely an overlap), to a 65 mph speed rating (BTW, that’s a K speed rating), you’d find that the loading is essentially the same.

Editorial Comment: I am very unhappy that this speed restriction thing isn’t plain and obvious – but it isn’t. This but one of many things I think need to be fixed – both in the tire end of things as well as the vehicle end of things.

So why are ST type tires allowed a larger amount of deflection? Partially because the speed, but there is a difference in the usage – an ST is on a towed vehicle so it is not considered as hazardous if there is a tire failure – compared to a motor vehicle.

So what would tire engineers do when designing tires for these 2 types? The fundamental formulae for calculating the strength needed is based on the inflation pressure and the inside perimeter length of the cross section of the tire - and since it would be the same for both types (of the same size), the tires would basically have the same construction.

Next installment: If the tires are essentially the same, why are there differences?