Tire pressues for 32 x 11.50 R15

[grimbo]

You'd have a tyre on the car with an inadequate load rating if that was the case so it's a silly example.

Steve.

i disagree, not all tyres are created equal, and that's the point i'm trying to make.

tyre pressure is not proportional to load capacity, after all my mountain bike takes 60 psi, and carrys alot less weight than my 4b does.

which is where the 4psi rule comes into play. i t allows for different tyres, different loads, different terrain etc

The point you are trying to make still doesn't hold up. As per my example the sidewall recommended pressures wouldn't e right for a Sierra and a F350

[-Scott-]

You'd have a tyre on the car with an inadequate load rating if that was the case so it's a silly example.

Steve.

i disagree, not all tyres are created equal, and that's the point i'm trying to make.

tyre pressure is not proportional to load capacity, after all my mountain bike takes 60 psi, and carrys alot less weight than my 4b does.

which is where the 4psi rule comes into play. i t allows for different tyres, different loads, different terrain etc

The point you are trying to make still doesn't hold up. As per my example the sidewall recommended pressures wouldn't e right for a Sierra and a F350

Grimbo is right. The pressure moulded into the sidewall is the maximum pressure to which the tyre should be inflated, when carrying its maximum rated load (and probably at its maximum speed.)

The tyre manufacturer has no idea of the size and weight of the vehicle to which the tyre is fitted, so the single figure on the sidewall cannot be a recommended pressure.

Use the 4psi rule. If pressure rises more than 4psi the sidewalls are flexing too much (which generates heat, which increases the pressure.) If pressure rises less than 4psi the sidewalls aren't flexing as much, which typically indicates the tyres are over-inflated, and will probably be wearing in the centre of the tread.

It's not like it's a hard thing to do. When you're on a trip, you should be checking your pressures every morning, and it doesn't hurt to check them once or twice during the day. Or buy one of those wireless monitoring systems, and watch the numbers on your dash.

[mike_nofx]

IMHO the 4psi rule is garbage. Google it and do your own research tho.

I'd also say to check the tyre plac. (maybe someone elses whose is still intact) but not sure if that would refer to radial or cross ply tyres, maybe need to find out if there is a difference.

Or, just try 35psi for a week. If you find its too soft/too hard bump pressure up/down 2 psi or so. You will soon find what suits you best.

Mike

'why do you think it is garbage?
If your tyre is over or under inflated it will change pressure by more than 4psi indicating you have the wrong pressure in your tyre. So where is the garbage part?

Why do i think its garbage?
I drive a Sierra, I air down to 8psi. Should i expect my tyres to reach 12psi after an hour? an increase of 50% in pressure?

In my club we have a bloke with a 101 rover, who runs 35+psi off-road (up to 40 maybe? you on here Ron?) which is an increase of only 12%

I would think the pressure would raise by a percentage, rather than a set 4 psi.

If running 1psi in a beadlocked zook, should you see a 400% pressure increase?


Anyway, with regards to tyre fitters. I believe they put extra pressure in the tyres to seat them. At least thats what i was told last time i had tyres fitted. They put 35psi in my Sierra, about 15psi above reccomended.

Mike

[-Scott-]

Why do i think its garbage?
I drive a Sierra, I air down to 8psi. Should i expect my tyres to reach 12psi after an hour? an increase of 50% in pressure?

In my club we have a bloke with a 101 rover, who runs 35+psi off-road (up to 40 maybe? you on here Ron?) which is an increase of only 12%

I would think the pressure would raise by a percentage, rather than a set 4 psi.

If running 1psi in a beadlocked zook, should you see a 400% pressure increase?

We're discussing the selection of optimum tyre pressures for daily driving - not the correct pressure for low-speed off-road work, which involves more variables. You believe this rule is garbage because you don't understand its application.

If you air down to 8psi, then hit the highway for an hour (if your tyres last that long) see what your pressures do then. Get your mate with his beadlocks to try the same.

I have pumped my tyres up to 30 psi cold, then found them at 38psi after an hour at 90km/h on the highway (I was aware my tyres were a little low, and didn't want to drive too fast.) This indicates my tyre pressures were too low for the driving conditions.

Anyway, with regards to tyre fitters. I believe they put extra pressure in the tyres to seat them. At least thats what i was told last time i had tyres fitted. They put 35psi in my Sierra, about 15psi above reccomended.

You're right - that is exactly what happens. They use high pressure to seat the beads, then they should reduce the pressure to something more appropriate. Too many fitters omit this last step (either too busy or too lazy - who knows), which is why you should never just check the pressures they have left you with, and maintain them. A bloke I used to wheel with couldn't work out why his newly fitted BFG muds (235/75 on a GV) had such poor grip - until he found they were running 50psi.

[mike_nofx]

We're discussing the selection of optimum tyre pressures for daily driving - not the correct pressure for low-speed off-road work, which involves more variables. You believe this rule is garbage because you don't understand its application.

Yes you're right, we're talking about road pressures here.
But apparently the 4psi rule is used for all terrains, on AND off road, for vehicles of any size, running any tyres!

Mike

[Burdy]

After a bit of research this is what i have found and might clear up a few issues


Tyre pressures - do-it-yourself check
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.

First inflate the tyres to the pressure recommended by the manufacturer of the trailer or the tyre you are using. Secondly, tow your trailer for a distance of 100 km, preferably on a highway.
Recheck the tyre pressures immediately after pulling over and compare them with the pressures 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 readings.
If there is a greater than 4psi (28 kPa) difference between these pressures, the tyre temperature is too high and the pressure needs to be increased. If there is less than 4 psi (28 kPa) difference, the pressure needs to be lowered.
Large 4WD tyres will have a differential of 6 psi (42 kPa).
Be sure to use the same accurate gauge for both readings.

[-Scott-]

Well, this thread is the first I've heard of 6psi.

Just to muddy the waters I've noticed that, regardless of whether I start at 30 psi or 38 psi, on a hot day, cruising at highway speeds, my tyre pressures seem to end up around 40psi.

As the stuff quoted by Burdy points out, the pressure increase is representative of tyre temperature - and the source seems to think that "large 4wd tyres" (whatever they are) should run at a higher temperatures than other tyres.

I'm not sure I agree with that.

[Shadow]

30PSI is what you'd put into normal car tires, 4wd tires should be running a bit more PSI at least... after all we're talking about a tire twice the size of a car tire.

its actually the opposite, the bigger the tyre the less pressure it needs.

If you have a 12" tyre and a 6" tyre (wide) then the 12" tyre has twice as much contact with the road, which means the pressure inside the tyre is acting on double the surface area (pounds per square inch). You do also need to factor in side wall stiffness aswell, but that is more applicable in low pressures.