Operating Tyres Beyond Design Limits

Hullo Rob

This is a very useful and timely article.

I only recently became aware of this issue when looking at getting some MTs and reading the Cooper's 4WD driver's guide. I guess the bells started to ring when I realised that the quoted max load was only applicable for a fully inflated tyre - something I have never done and am never likely to do (not wanting my vehicle to self destruct!)

I guess the only niggling thought I had with your chart was the assumption of a linear extrapolation to low pressures (20psi). My concern? A gut feeling that the rate of flexing and hence heating of the side wall may in fact be greater than that indicated by a linear functional model and hence a greater than indicated loss of load carrying capacity or need for a reduction in speed.

Thanks for doing this work and sharing it.

Cheers
Andrew

Hi Andrew,

I too appreciate this. I have been thinking at what tyres I should run my camper at, they are the same tyres all round but the cruiser has ~750kg per tyre while the camper has ~1100kgs per tyre. Most people recommend the camper pressure to be the same pressure as the vehicle, but as you rightly know, load rating is related to pressure. Will use this to work out how much higher my camper tyre presures should be.

Cheers

Captain

That's great work :-)

Must say though that a will most likely still load my truck up to the max, drive on 100k on 36psi front and 45 rear until I hit the dirt, then I will put them to 36psi all round until I hit the sand and then I will run them at 18psi all round for the duration.

So far thew Mazxxi Bravo 753s 245/76/R16S have been up to the impossible task we give them

Cheers Tony

Quote: "I find it most useful when changing terrain"
Rob where does your chart show pressure for different terrain?
Cheers Craig................

Great effort Rob.

Your attachments answer a question I had on ThreadID: 79764 which wasn't really answered satisfactorily.The correct answer from your page is:



: http://www.bridgestonetrucktires.com/us_eng/answers/doctor_facts.asp
In general, operating temperatures of radial tires can be anywhere between 65C-82ºC Under severe conditions the operating temperatures will range in the 93ºC+ area.
We know that when heat is applied gradually to a tire, the tyre will go through several stages. In general, as a compound is heated, molecular activity within the compound increases. At temperatures in the 120ºC range the tyre starts to lose strength and the influence of other stresses becomes greater.
Sometimes, with time as a factor, these other stresses will cause a tyre to be damaged.
During the manufacturing process, tyre vulcanization occurs at 120 - 160ºC. Depending on the factors mentioned above, tyre reversion also begins around 120 ºC. Of course, during the manufacturing process, these factors are highly controlled so we can produce a high quality product. As heat is, again with time as a factor, the molecular bonds disintegrate and the tyre starts to physically come apart (reversion). It is this physical destruction that leads to the "two halves" of the tyre that we sometimes see on vehicles.


Many thanks.

Cheers.

Hi Rob.

Nice read.
I have the same vehicle set-up as you.
I have been using "smart tyre" monitors for some time now. This allows me to see the relationship between Load, Speed, pressure and temp.
I can confirm your findings in relation to 285/75R16 LT's.

regards