Tyre pressures: Cold vs hot

Yes, but I was told 4psi was the rule....pretty close :)

Hi Mark,
I know it as the 4PSI rule. If you set your tyre pressure when cold, then go for an extended run, the pressure should increase by about 4PSI. If it increases more, the cold pressure was too low. If it increases less, the cold pressure was too high.

So yoou have the right concept, but AFAIK, it is 4PSI.


Norm C

Thanks Norm & Footloose. I had a guess at 5, glad I know the right figure now.

Cheers,

Mark.

Its called the Pirelli rule. Its a good guide, but don't take it too literally, otherwise you can end up with ridiculously low pressures on the highway, and too high pressures in the rough.

Mark

I fitted a new set of tyres late last year and decided to ring the tyre manufacturer and discuss pressures with them. These tyres were a type I had never used before. I was told to use the pressures recommended in the car owner's handbook and never exceed them by more than 4 psi. I mentioned the 4 psi rule and he said there are so many variables in it that every tyre testing program that he has been involved in showed that it was not reliable.

The most importand thing you have to worry about with tyres is how they are wearing. Underinflation will wear the outer edges of the thread faster than the centre. Overinflation will wear the centre faster. If the pressure is right then the wear will be even and the ride and handling will also be right.

For as long as I can remember I have always marked tyres so I could watch how they were wearing. I simply get a small nail, heat the head red hot and touch it lightly about five or six times across the surface of the thread. This will leave tiny dimples about half a millimetre deep. By watching these dimples you can easily see if the tyre is not weaing evenly long before you can detect uneven wear by any other method.

I am now up to 4,000 ks on these tyres with factory pressures and the wear is perfectly even. The ride and handling is also excellent. This distance included 900 ks in the Vic. High Country in dry conditions. I have noticed the rear wear faster than the front so I have started rotating them already to try and keep them even.

I suggest you mark your tyres and give their manufacturer a ring. It only takes a phone call and who knows what you will find out.

Brian

The 4 psi is a good starting point, and used in conjunction with a bit of experience and common sense, will see your tyres performing well.....

Say your tyre placard said 30psi....if you were going on a trip....then you would probably add 4psi for high speed highway driving (as it says in the handbook !! )
Then you would maybe add another 4psi to compensate for the crap you have in the back and maybe the CT you were also towing ???? Too late after your tyre blows out to work out your pressures were too low !!!!

Now if you were going to hit the dirt.....and still travel at say 80 to 100 k/hour, then leave the pressures the same, but a different story if you intend to go a lot slower !!!!

Now sand ????? temps will go up a fair bit in sand but you have no choice ( if you don't want to get bogged )

I'm struggling to understand the point of this "4psi rule". It seems like a lot of guess work and mucking around.

Why not simply check and adjust the tyre pressures whilst hot?

Barnesy

G’day MarkH

I threw the 5psi approach out the window for our last trip with the weather well over 40C each day. I did change the tyre pressure to suit the conditions. The tracks we travelled varied but mostly had corrugations, ruts and stones with the odd bit of sand encountered.

We went as low as 23psi (front cold) and 27psi (rear cold). As the days progressed these pressures would climb least 10 -15 psi. I would then stop during the day and reduce the pressure in the tyres to improve ride comfort and reduce tyre bounce and sideways hopping on corners. We didn’t travel to fast for the track and varied between 40 and 80kph depending of the conditions.

I don’t get too concerned about lower tyre pressure and side wall damage. IMHO I think side wall damage is just as likely to occur at all pressures and varies with the different types of stones on tracks across Australia. I believe that keeping a keen eye out for the large and/or sharp looking stones and being able to dodge these buggers is one of the answers to extending your tyre life.

cheers Anthony

I wouldn't be too perdantic about 4 or 5 psi...Most tyre gauges are a few psi inaccurate anyway..
4psi on your gauge may be 6psi on mine & 5 psi on someone elses..

the 4 psi rule explained

I think you should check out the link which will explain the 4 psi rule clearly (plus some more info too)

It states :
It is impossible to list the correct pressures for every caravan, due to variation in size, load, etc. This easy check will help you find the best pressure for your caravan tyres.

This is the info i have seen on many professional web sites and motoring companies. I have used it and it works well. good luck

Thanks everyone for your input. I'm getting lots of chances to experiment with this so I'll do some testing and report back on here in the future with how my set-up performed.

Cheers,

Mark.

I'm sure to get a few gasps here, but I tried running the 4psi rule with my split rims/tubes/Road Grippers last year on a trip across to WA from VIC, with fully loaded vehicle with fuel, water, camping gear, spares etc. To be able to limit pressure increase to 4psi at about 90 to 100kph on the bitumen, my pressures needed to be up around 70psi for the rears and a little less on the fronts. Funnily enough, with pressures less than this, the tyres 'looked' bagged out too much for the bitumen, but with 70psi, looked about right.

I did however accept a higher rise in pressures on dirt roads as I didn't want to run those pressures for ride reasons. Pressures for the whole trip were greater than I have ever used and funnily enough I didn't get one puncture in over 12k km. Tyre wear has been quite good since with a further 20k km on bitumen. These 'cheapo' tyres (bought as 'roaded' from the wreckers) look like they're going to last for about 60-65k km which is more than I have ever had from these tyres in the past. As a result, I feel that this particular combination of load, tyre, rim, car seems to suit higher pressures on 'hard' roads. It may not suit everyone, but I'm continuing with it for the moment. Around the city, I run about 50psi unloaded, and given the nice suspension, I don't feel the ride is overly harsh at all.

Cheers,

Mark