Thread: Tyre pressures?

Originally Posted by jik22

Next question for me, as the tyres aren't standard size, is what rpessures do you run?

Hi Jeff,

I run 34psi all around on BFG AT/MT. I found that more than that (~40-44) gives more rattles, unstability on wet highway. Lower (~28-32) - you driving like with hand brake applied, fuel goes up, rubber goes hot a little bit. Of course, dirt roads and offroad different story.

If you can check how heavy front and rear axels and go from there. I'd ask in tyre workshop what's the best pressure for a brand you have.

Cheers,
Ruslan

Tyre pressures are a bit of a vexed question - problem is there are several different factors influencing what you use!
From a theoretical point of view the size of the tyres should not influence the pressure, but the problem is, the original recommended pressure is a major compromise anyway.

1. From the tyre manufacturer's point of view, pretty much the higher the better - I have had one tyre company seriously tell me that LT (light truck) tyres should NEVER be run below 60psi. Higher pressures keep the tyre cool and hence increase tyre life, but how critical this is varies with the tyre - in general the thicker the tyre walls the more important it becomes, as it does with increasing speed and ambient temperature. Since the heat buildup comes from energy straight out of your fuel tank, this concern is directly aligned with minimising fuel usage.

2. On the highway, the driver is more concerned about the handling than anything else, and generally this requires the pressure to be reasonably high, but importantly, in proportion to the weight on each axle, although some vehicles may require a bias front or back to counteract design problems.

3. On less than perfect surfaces, ride becomes a major concern, and particularly with load carrying vehicles such as Landrovers, when running empty, pressures lower than those suggested by 1 & 2 become very attractive.

4. On soft surfaces, flotation becomes critical, and the lowest possible pressure is needed. "possible" is usually dictated by when the tyres can be expected to start moving on the wheels, and this pressure ignores concerns in 1 & 2, and if the alternative is walking (often a long way), most drivers will accept the risk of tyre damage and poor handling.

The manufacturer, trying to sell a vehicle that handles well, rides well and doesn't have recalls for tyre failures, tries to steer between 1,2 and 3, usually with a further figure for soft offroad conditions between 1 and 4.

I would suggest starting with the book figures, and look for improvements on handling and ride with small changes in pressure, watching out for tyres getting hot - they should never get much hotter than ambient temperature, although they will always be a little warmer.

It is worth noting that the Australian supplement to my 110 owners manual increases the pressures (same tyres) from 25/30 (35/40 loaded) to 35/35 (60/65 loaded above 120kph), probably because of the need to allow for higher ambient temperatures - and the Australian reputation for driving flat out over rough roads. This difference shows how much slop there is in manufacturer's recommendations.

John

Keep them in the 30s seems to be the best compromise I've found.

Keep the rear a bit higher on the open road, even when empty, it improves the stability. Say 33 & 38.

But then its a bit understeery around town. If empty, level the pressures out say 33 or 36 all round.

Just depends on your driving style & ride preference.

Load the car up though & its a different story.

Regards
Max P

I run 35 all round (235/85/16's) in Defender, even with most of the camping gear in. The front has engine to weigh it down, so some gear in the back would balance it out I feel, so therefore no increase in pressure. Different when travelling long distances and at higher speeds, or when towing (a few extra PSI in rear) then I may increase to 38 to 40 all round. Gravel roads mean reduced speed (for me anyway) and reduced pressures. Just for interest, multiply by 7 for Kpa.

i run 40 psi front and rear in 750x16's gives a bit harder ride but reduces tyre wear and increases fuel economy

If I remember right there is a forumla to get the correct tyre pressures for your tyres and load its something like...check tyre pressures when cold then again after a run, the pressure incresse should not exceed ????if it does then the pressures were to low to start with so start again with a bit higher pressure when cold

T

Actually from a theoretical point of view tyre sizes have everything to do with required pressure.

A tyre's air capacity or volumeis what dictates it's load capacity, the more air you cram into it the higher the pressure becomes of course. The size and construction of the carcass dictates how much air you can stuff into it.
So a larger tyre will hold the same volume of air at less pressure to bear the same load.
Do a search of this site and there have been a couple of discussions on pressures, like this http://www.aulro.com/afvb/showthread.php?p=481428 where Dave's gone into some detail.

edit. take note that over inflation is just as bad as underinflation, from handling issues to damage to the tyres and the vehicle for that matter.

Originally Posted by tony

If I remember right there is a forumla to get the correct tyre pressures for your tyres and load its something like...check tyre pressures when cold then again after a run, the pressure incresse should not exceed ????if it does then the pressures were to low to start with so start again with a bit higher pressure when cold

T

That 4 psi sounds like the Pirelli formula, which is ok for cars, not sure about us.

Its trial & error. Cruising speed has a lot to do with it too.

On a trip I just check the tyre treads almost every stop with the palm of the hand. You'll soon get to know whether the rears are underinflated, or if any are going down.

With trailer tyres, its definitely every coffee/lunch stop.

Regards
Max P

Originally Posted by oldzook

Actually from a theoretical point of view tyre sizes have everything to do with required pressure.

A tyre's air capacity or volumeis what dictates it's load capacity, the more air you cram into it the higher the pressure becomes of course. The size and construction of the carcass dictates how much air you can stuff into it.
So a larger tyre will hold the same volume of air at less pressure to bear the same load.
Do a search of this site and there have been a couple of discussions on pressures, like this http://www.aulro.com/afvb/showthread.php?p=481428 where Dave's gone into some detail.

edit. take note that over inflation is just as bad as underinflation, from handling issues to damage to the tyres and the vehicle for that matter.

Dave's discussion is worth noting - however, this method of determining the correct pressure is the optimum pressure from the point of view of adhesion, as I note above, there are other considerations.
However, I don't believe your comments about tyre size make sense.
Assuming we take Dave's method and adjust pressure to make the tyre contact area the ideal shape, the only part of the tyre size that makes any difference is the tread width and the overall tread diameter. The actual volume of air is totally irrelevant, as is the height of the sidewalls. In practice, when you get to light truck type tyres, the stiffness of the sidewalls makes a significant contribution to support, so different ply ratings will require different pressures to give the same contact pattern from tyres the same size.

Generally speaking the larger the diameter and the wider the tread, the lower the necessary pressure, provided everything else stays the same, but for example, the wider the tyre, in general the less stiff the tread, so that it relies more on pressure to maintain the contact shape, so the difference is less. And as I commented - there are other things to take into account. There is no "perfect" pressure!

John