Tyre Pressures . . the 4psi rule. I kinda like this

Not as simple as you would think. I've measured tyres on the sunny side of the vehicle as much as 20 degrees hotter than the shaded side. That blows the 4psi rule away.

Do it properly. Go to a weighbridge and consult load pressure tables for the tyres in use.

The old 4psi rule...I think Noah used it when transporting material for his Ark.

Cooper Tyres has a little handbook stating that for offroad use then 6psi (or thereabouts) is a better measure. I've got a copy of it somewhere.

Wish someone would put the lid back on this can of worms!

My consensus on tyre pressure are run whatever you feel comfortable with, most times I don't see the point in running low pressures on good dirt or hard compacted clay capped roads....... unless there are rocky or it is very sandy........ why don't people start adjusting tyre pressures on some sealed road as the road condition can be be pretty poor at the best of times.

As you said all tyres and all vehicles are different and what may suit one may not suit the other.

Plus if bagged out to much the side wall came become very vulnerable to damage.

And as for tyre placards..... our 200 series landcruiser weighs 3700Kg and the tyres are "E" rated meaning at maximum load you can run 80psi...... when we ran Mickey Thompson MTZ's at 25psi they were bagged out a lot but with the new tyres and stronger sidewalls at 15psi they are about the same.

It's funny..... years ago when I had rally cars we never worried about tyre pressures to much for different surfaces and with speeds nudging over 200Kph we never had a problem..... sure we use to destroy tires and in some case within 20 kilometers.

The 4 PSI rule works well for LT and car road tyres at an ambient temperature of 20c.
Hotter days and therefore hotter roads usually allow for an increase of up to 6 PSI.
OJ

I was told that when to use the following as a guide. Deflating your tyres, regardless of weight, you should deflate them to roughly one third of the sidewall.

Hi

More discussion on same subject in some previous threads for those interested..

Thread 91867 - Feb 2012

Thread 14146..way back in 2004..

Cheers
Greg

The first and most important thing we must understand is...when a tyre is inflated to the correct cold inflation pressure for its load it is a self equalising system....a hotter tyre requires more pressure to support the same weight.

I was reading a research document I linked in one of the other discussions.

One of the tyre manufacturers did some ( quite a lot actally) testing and they consider that a hot tyre may require as much as 15% to 20% more than its cold inflation pressure when running hot....peer comentators in the paper say that may be conservative.....so 20% of 40 psi is 8psi...so is it an 8 psi rule.

This extra pressure is provided by the air heating up inside the tyre...remember the self equalising system.

the reason for this was, trucks fitted with tyre pressure managment systems had the upper limits set too low .....and the systems where deflating the tyre unnecessarily resulting in pressures too low to correcty bare the load..

The use of these tyre pressure managment systems in heavy transport has seen CONsiderable reseach into tyre pressures and the effects.....there are several extensive papers published on the matter.




NOW my problem with the 2,4,6, whatever "rule" is that it is so ill defined and vaguely discribed.....AND....there are so many uncontrolled variables.

tell me.
1/What influence does ambient temperature this 4psi rule.....does it work below freezing, does it work in situations of very high air and road temperature..

People in very cold climates actually have a problem with pressure decrease due to heat being drawn from the tyre by the road surface.
In very hot areas, the tyre pressure will go up just sitting there...inflate the tyre in the shade then park in the sun where the tar in melting

2/ How long do you have to drive to get the tyres up to temperature, some say 15 minutes, others say 2 hours....does 1 influence this.

3/ how fast do you have to be driving for this to work and how does this influence 2 and 1

4/ What influence does the type of driving have on this so called rule....driving along a smooth straight road at a constant speed, puts less stress on a tyre, than hard acceleration and constant hard cornering..

5/What influence does rim type and and heat disipated by that rim or conducted by that rim from the brakes have on this so called rule.

6/ What influence does the size, type and construction of the tyre and how heavily loaded that tyre is in comparison to is loading capacity have on this rule.

7/ how does the proportion of the inflated pressure that the 4 psi represents effect the rule. Is it the same for a tyre that is load v pressure correct at 100 psi as it is for a tyre correct at 25psi.

8/ given a change in speed, how does being away from the pressure from the load v pressure effect how this rule works

The answer to all the above questions is that will all effect the so called rule wildly, so much so that it will be so massivly erronious that it can not be relied upon.

On my vehicle with the tyres correctly inflated,( weighed and inflated to table) in moderate weather, I can drive all day on the highway at 100 kmh and not get 2psi pressure increase....and still people look at my tyres and say they look under inflated.

Consider my mate Mikes Dihatsu...he baught it fitted with big 8 ply tyres....he checked his tyres once and found that one of them had almost no air in it...but looked no different and drove no different to the others and the tyre was stone cold like the others...does 4psi rlule work for him.


Consider too that the some of the pressure tables will show around a 50KG difference in load for 4psi change in pressure...ya recon the 4psi rule is that accurate.

Consider also, many of the load V pressure tables show that load V pressure is far from linear, and different tyre types have different curves.

So If the 4psi rule was to work in the linear portion of the load v prerssure curve, it can not work in the non linear parts of the curve.


There is a very good reason the accepted and approved practice revolves around correct load V pressure measured cold.

OH and remember this rule does not work in aired down situations...Copper say it does not.


If this so called "rule' does not work in every case and under all conditions it is not a "rule"

cheers