Tyre pressure rule of thumb

[TerryG]

Those of you who are vexed by the question of the correct tyre pressure might be interested in this method of finding the optimum. It came from an Australian camper van website and is identical to the advice offered on a couple of American automobile forums:

1. Measure and record the cold start pressure of your tyres before you start out.
2. Drive for a distance of approximately 100Km.
3. Recheck the tyre pressures immediately after pulling over and compare them with the the cold start pressure you had at the start of your run. If the pressures are right, the hot readings should be 4 psi (28 kPa) higher than the cold start readings.
4. If there is a greater than 4psi (28 kPa) difference between these pressures, the tyre temperature is too high caused by the cold start pressure being too low so the pressure needs to be increased. If there is less than 4 psi (28 kPa) difference, the cold start pressure is too high and needs to be lowered.
5. Large 4WD tyres will have a differential of 6 psi (42 kPa).
6. Be sure to use the same accurate gauge for both readings (NOT fuel station gauges)

[g8dhe]

Does it explain why 4psi and not say 2 or 6 ?

[TerryG]

Does it explain why 4psi and not say 2 or 6 ?

Not that I've seen. I presume that it's the optimum temperature increase that the tyre manufacturers aim for to achieve the best all round performance
As far as I can remember, back when I used to race cars we would see an increase of 5 or 6psi so 4 would seem to make sense. I guess that in practice one could use the 4psi gain as a starting point then closely monitor wear across the width of the tyre over a period of time.

[briwy]

Presumably that assumes that the ambient air temperature is the same each time you measure the pressure?

If not, surely that has to be taken into account?

[TerryG]

Presumably that assumes that the ambient air temperature is the same each time you measure the pressure?

If not, surely that has to be taken into account?

I don't suppose the ambient temperature would vary much during the 50 or 60 miles you drove to get up to max tyre temperature, unless you started at dawn and the temperature was expected to rise dramatically during the day.
20 miles on a motorway would probably be enough anyhow; 100Ks sounds a bit excessive.

[mikeonb4c]

Phew, sounds like hard work. I tend to just kick the tyres and light the fires, and would love advice on how to assess ther pressure from the feel of the kick.

[mikexgough]

This has been raised many times before on various car forums..... rule of thumb for road use is after a run of 10 mile plus pressures raise by around 2 psi and ambient temperatures make nominal differences.
On my 17's I run 38 front 40 Rear... from cold at home ...

[TerryG]

T
On my 17's I run 38 front 40 Rear... from cold at home ...

Out of interest, have you checked the pressure increase when hot?

[briwy]

Many years ago when I was at college I did a essay on hysteresis in tyres. This is what makes tyres heat up when running.

Interesting article here which explains it well. Depends on many factors.

http://www.tyres-online.co.uk/technolog ... dblack.asp

Now what colour tyres would look best on a silver Bongo?

[helen&tony]

Hi
It's not as simple as that article. Apart from the fact the article mentions construction fairly roughly, it really fails to mention the massive variation in road surfacing, all of which give incredibly different wear and heating factors. A road tyre is a compromise at the very best, and aims to deal with concrete / cement, both smooth and rain surfaced (wearing your tyres out like nobody's business), and tarmac of varying qualities...PLUS, every car of the same make and model will vary in weight according to what's carried...thus , the Bongo varies from a bog-standard vehicle with one person, to a regular family usage. There are those who have conversions, those that carry a lot of tools/ spares/ full tank of petrol all the time, added accessories, etc. Add to this, the state of maintenance...namely an MOT pass with all joints and bushes able to pass MOT, but when small inadequacies throughout the range of running gear are added together, the whole setup performs poorly. For instance, a damper can pass a test, but give inadequate road performance....namely , most Bongos should have a new set of dampers at around 100,000 Km, as quoted by the damper makers , not just to get you to lash out on a new set, but to maintain the car manufacturers road holding specs. Instant failure to give good tyre mileage there, then...oh, and get wheels and tyres balanced and the tracking checked!!!!!
Now we come to the main factor ....the control unit of many cars is the primary fault in tyre wear...that's right....YOU...Style of driving is not just how fast or slow you travel, but the way you manage it, and the Mk 1 control unit has what's known as the human factor...the idiot behind the wheel. You can drive fast and be a good control unit, and drive slow and be a bad unit...In a chauffeur training, it used to be demonstrated as "Imagine you are driving with a bowl of water on the bonnet"...Every movement must be smooth and certain....after all, you don't want to have your diplomat in the back banging his head on the side window as you corner every movement must be certain to keep the bowl from spilling water. You can still manage acceleration rapidly and deceleration rapidly without spillage!...no silly gear changing, or brake stamping
Now compare an individual's driving style with that model...you then see what a tyre is up against...So....all those factors, and countless others, are there to stress the hell out of a tyre...plus the fact that the vast majority of people never run their tyres in...and EXACT tyre pressures?????....the least of your worry.
Everyone must find out from a rough starting point, and read the wear patterns to ascertain what is right for your Bongo.
I run mine at 32 PSI all round, and the tyres wear almost perfectly.
Cheers
Helen