Re:What tyre psi you running???

Car manufacturers talk to their tyre manufacturers to come up with a deal for X numbers of tyres at Y specification for Z cost per unit. Having established numbers and cost, the basic specification of the tyre in size speed and load index is then determined to the nearest industry standard with a factor of safety applied. The vehicle manufacturers then provide a pressure chart based on the pro rata maximum safe limits of the tyre when used with the specific model of vehicle. Hence when I used the maximum weight and pressure information on the side of my standard specification road tyres along with the rated load of the axles on my car the numbers correlated with the specification on the door plate. When you alter the specification of the tyre you then have to recalculate your pressure accordingly. Going from a 96 or 97 load index to a 100 or 102 or more is going to have an effect otherwise the whole concept of load and speed index rating of tyres is meaningless.

Don't forget that with alterations in pressure you not only affect the width of the tyres contact patch but also and more significantly you are altering the length of the contact patch. This is why true 4x4 vehicles have high profile tyres that are relatively narrow compared to their overall height. It means that as you lower the pressure you get a very significant increase in surface area touching the ground compared to what is possible for something like an SUV with big wheels and low profile tyres. This in turn has led to the evolution of systems like Terrain Response where computers have to try to manipulate the available grip to best effect instead of just mechanically increasing it. That's one of the reasons things like unimogs and humvees with on the fly ctis systems are so very much more capable than a Range Rover in technical off road conditions.

That's what I said. Terrain Response etc monitor wheel speed at the most basic level and use computer control over the drive line to attempt to maintain forward progress. Ctis affords control of the tyre pressure to maintain traction in a mechanical fashion. These are both off road systems designed to maintain control in situations of limited availability of grip. Running super low tyre pressure on a high traction sealed surface will overheat tyres i very short order. Hence we are having a discussion about the correct and safe pressure to set the tyre. The operating pressure that a tyre will sustain without failure is a function of the load and speed it is subject to. Too much in any given parameter will have some combination of negative effects be that uneven wear, erratic handling, undue heat build up, lack of traction or whatever its a long list. Because of this the tyre manufacturers are obliged under law to provide the necessary information to determine whether the choice of tyre is suitable for the proposed application. Because it would be impractical to have a specific tyre for every different vehicle tyres have reasonable operating ranges so that with some simple calculations of pressure relative to vehicle weight it is possible to use the same size and spec of tyre on significantly different vehicles for example a Jimny and a VW transporter.

I'm sorry if I'm labouring the point but there isn't one single pressure for one type and size of tyre to use in all situations and vehicle types. The weight of the vehicle, the type of ground it is covering and the speed at which it is doing so are all meaningful parameters for calculating the most suitable pressure for the conditions at hand.

You mean my preferred method of pressure assessment? Dunno.

Wow! I've just read this thread. Now I'm tired! Get it? Tired?

No pressure or anything but it's an easy thing to understand!

Kirkynut

Nice!