Tyre pressure question

[fishaholic]

I have just got a new set of BFG AT's for the Rodeo on friday. I just did a quick pressure check yesterday and found them to be running on average at 38psi. To me that seems a bit high or is it due to the slightly larger rubber? I've gone up from stock 205's to 245/70/16's.

I thought low to mid 30's would be sufficent?

Andrew

[tom_286]

It depends on many things... such as how much weight are on the tyres (how much your car weights and what your carrying) and if its a hot day or you go for a long drive your tyre pressures will increase. Also a high pressure will give better tyre ware and better on road handling but you will lose compfort.

Just a few things to consider, hope it helps you a little...

Tom

ps. For the record I usualy run between 35 and 40 in my 285/75 r16s on my MWB landcruiser on road.

[HRT01]

Yep like he said!

[fishaholic]

Thanks for that. The ute has some tools in the back 90% of the time that i use for work, maybe around 70-80kg all up.(Apprentice Fitter).
Ill leave them where they are for a bit longer and then drop them a few PSI to see if it makes any difference to the on-road characteristics. But my main aim is basically to have them at the optimum pressure to get the best life out of them.

Andrew

[Humpy]

Bigger tyres usually mean you can run lower pressures. I would start with what you were using before

Tyre dealers put em up to what seats the bead plus a bit, not necessarily what suits your truck

[fishaholic]

Thanks for that Humpy. What would you suggest i run in the tyre's? 35psi?

Andrew

[Humpy]

im not an expert on tyre pressures and my zook would happily run around on 10psi, but Id say your first guess of somewhere between 32psi & 35psi would be good

[Peter @ Aawen4x4]

Set them at your best guess then drive for 1 hour, normal type driving. After 1 hour, the start pressure (cold) should have increased by 4psi. More than 4psi, your start pressure was TOO LOW, add more air now (about 1/2 the increase amount) and up your start (cold) pressure tomorrow by the same amount! Less than 4psi, your start pressure was TOO HIGH, drop ONLY 2psi now and revise your start (cold) pressure tomorrow by the same amount!

The aim is 4psi increase after 1 hours worth of driving, and the tyre manufacturers still haven't been able to come up with anything 'more accurate' than that to get the correct pressure for a tyre given any set of all the variables, inc driver, vehicle, ambient road, tyre, and air temp, speed, load, brakiing characteristics, cornering, etc, etc, etc!! This 'formula' is used by many as the 'infallable' method!! Try it, it can only help!

Have Fun!

[Humpy]

Set them at your best guess then drive for 1 hour, normal type driving. After 1 hour, the start pressure (cold) should have increased by 4psi. More than 4psi, your start pressure was TOO LOW, add more air now (about 1/2 the increase amount) and up your start (cold) pressure tomorrow by the same amount! Less than 4psi, your start pressure was TOO HIGH, drop ONLY 2psi now and revise your start (cold) pressure tomorrow by the same amount!

The aim is 4psi increase after 1 hours worth of driving, and the tyre manufacturers still haven't been able to come up with anything 'more accurate' than that to get the correct pressure for a tyre given any set of all the variables, inc driver, vehicle, ambient road, tyre, and air temp, speed, load, brakiing characteristics, cornering, etc, etc, etc!! This 'formula' is used by many as the 'infallable' method!! Try it, it can only help!

Have Fun!


Im only asking because im curious, not to start an argument ???

Which manufacturers have tested this?

Does this method optimize wear or handling?

Do racing teams use this method?

[Peter @ Aawen4x4]

I was originally told the 'formula' years ago by a fella in the racing game. Since then, I've spoken to a whole heap of tyre guru's from tyre manufacturing companies, race teams, tyre resellers, et al, and when asked about it, all comment on the fact that their company/org/business has various computer methods that they've developed to calculate the optimum pressure for their tyres and while initially they compared it to the '4psi rule of thumb' now in all circumstances the '4psi rule of thumb' is considered better than their 'best calculation!' The latest guy to confirm this was a Michelin (BFG) rep only a couple of months ago!

It is normally considered the 'optimum' pressure for the tyre, not optimised for ride, wear, or handling, but optimised for the longevity and traction of the tyre! You tell me?? Using this method, I have managed to maximise the life of many different brands of tyres, and usually manage to get better traction in any given circumstances than most using other methods of 'guesstimation!' I generally consider 80,000 km from muddies or STT's as normal, and 100,000 km as achievable with enough care for pressure! And this is while they perform as well or better than anyone else's in the traction department!!

Try it, you'll be pleasantly surprised I'm sure!! Everyone I know who has taken it up is getting better tyre life and better performance than they were before they started using the 4psi rule of thumb!!

BTW, I just put it out there! If you want to use it, try it!! If you don't want to, no skin off my nose!! I'll keep on using it and be happy!!

[Nemesis]

The 4psi rule has too many external variables on temperature to make it gospel.

You're most accurate bet is to find out the average gross weight of your vehicle (weighbridge, not rego papers) then contact a tyre professional (not a tyre fitter.) Using the Tyre and Rim Association - Standards Manual they can give you the optimum (cold) tyre pressure for your new tyre size.

Alternatively - Bridgestone Australia recommends that for original size tyres, 10% ABOVE what the placard states is the best average for Australian conditions.

[Humpy]

I was originally told the 'formula' years ago by a fella in the racing game. Since then, I've spoken to a whole heap of tyre guru's from tyre manufacturing companies, race teams, tyre resellers, et al, and when asked about it, all comment on the fact that their company/org/business has various computer methods that they've developed to calculate the optimum pressure for their tyres and while initially they compared it to the '4psi rule of thumb' now in all circumstances the '4psi rule of thumb' is considered better than their 'best calculation!' The latest guy to confirm this was a Michelin (BFG) rep only a couple of months ago!

It is normally considered the 'optimum' pressure for the tyre, not optimised for ride, wear, or handling, but optimised for the longevity and traction of the tyre! You tell me?? Using this method, I have managed to maximise the life of many different brands of tyres, and usually manage to get better traction in any given circumstances than most using other methods of 'guesstimation!' I generally consider 80,000 km from muddies or STT's as normal, and 100,000 km as achievable with enough care for pressure! And this is while they perform as well or better than anyone else's in the traction department!!

Try it, you'll be pleasantly surprised I'm sure!! Everyone I know who has taken it up is getting better tyre life and better performance than they were before they started using the 4psi rule of thumb!!

BTW, I just put it out there! If you want to use it, try it!! If you don't want to, no skin off my nose!! I'll keep on using it and be happy!!


Cool, ive heard this formula before and wondered if anyone had actually tested the theory or if it was a wives tale

Could you tell us what pressures this works out for your patrol for a few different conditions

Sorry to hijack your thread fisha

[Peter @ Aawen4x4]

Like Nemesis says, it changes a fair bit in response to variations in temperature, but generally, I use the following as a start point, then apply the 4psi rule!! This is for a reasonably heavy Patrol, running 33x12.50R15 Cooper STT's

low/mid 20's temp light/med load, bitumen road, local travel city: about 30 front 32 rear - add about 2 psi for every 5 degrees over 25 C

low/mid 20's temp med load, Hwy travel/speed up to 4hrs: 34 front 36 rear - same add for temp, plus 2psi for over 4 hrs travel and/or full load.

low/mid 20's temp med/heavy load, good formed dirt road, some loose enough for 4wd, travelling some distance: 26 psi front 28 rear max speed 80 kmh - lower top speed for temps over 25

low/mid 20's temp med/heavy load, looser/rockier dirt road, mainly 4wd, travelling some distance: 22 front 25 rear - add 2 psi for temps over 25 - drop pressures and max speed for sharper rocks/looser surface

General 4wd track work, variable surface, max speed about 40-60: 18 psi front, 22 psi rear - vary up/down depending upon temp/traction/load!

All of these I check after 1 hours driving, and vary depending upon the result. Generally, I can now pick the pressure given any day and it's temp, the surface we are on, the load I'm carrying etc and get within about 2 psi of what it needs to be, and either have to add 2psi or drop 2psi after the 1 hours driving!! If the surface deteriorates to require max traction, I'll keep dropping until I get the traction I need, and then its an experience/judgement call on getti ng it back up to the right temp once it's warm tyres!! The more you play, the better you get at it!

Have Fun!

[fishaholic]

Thanks Peter for those tips, very helpful, although my head is still spinning!! :P ;D ;D I will definately have to have a play with your formula and see what happens :D

Andrew

[Humpy]

Peter, does the formula work for sand?

[Peter @ Aawen4x4]

'Cos the formula works on how ALL the variables impact on the volume of air inside the tyre given the circumstances that you are driving in, the formula works IN ALL CASES!! The difficulty is that you will never get an identical set of variables on any two occasions, so it's going to give you a good starting point, and you'll then have to refine it as your driving day progresses!!

So for sand, if you start the morning driving on sand, choose your starting pressure and then drive for 1 hour. Check pressures then and see if they've gone up too much, or even not moved!! Vary as a result, and keep that start pressure in mind. It's a little bit harder if you start your day driving on bitumen BEFORE you get to the sand, but it still helps to watch the change in pressure after 1 hours driving. If you want to keep a record, I know that the guy who initially told me about it used to keep a log book with all his pressure settings in it, along with as many of the variables that he could get!! That made for a big log book, and it was always too hard for me!!

I just work on what today looks like compared with the last time I drove in these sort of conditions and temp, what did I use then? If it is hotter, the start pressure will be higher; colder, then it'll be lower, but only by a bit! Vary from that as a start according to the change after 1 hours driving and you'll very soon get to being able to pick a fairly accurate start pressure for any surface, load, temperature, track temp, driving style!! And once you're pretty close to the start pressure, you KNOW what the pressure SHOULD be hot, that is 4 psi higher!! So you CAN vary your pressures to what they should be EVEN IF you don't get the opportunity to START COLD!

For the sand example, if you have to drive on bitumen to get there, take the pressure at the end of bitumen, estimate what the START PRESSURE should have been for straight sand, add 4psi, and that's the pressure your tyres should be at now!! So set them to that, but check pressures again in a while to see what they have done, maybe even vary as a result of any changes then.

So the best thing you can do is to TRY IT! At the very least, it'll get your pressures to within a little bit of the optimum pressure from that 1 hour check, 'cos that's where you make the changes that are indicated. Remember, this formula is the REAL WORLD application of all the computer models that have been developed, 'cos it's reacting to the changes caused by ALL the variables impacting on the tyre and its' volume of air inside!! No-one is sure that the computer models get ALL the variables, while this 'Rule of Thumb' DOES, 'cos its taken from the real thing, you driving the car!! BTW, as they get better with the computer models, improving on the variables input, the computer generated answers are getting closer and closer to the 'Rule of Thumb' answer! And remember too, that MY driving your car on this bit of road/track, at this speed, etc, etc; is likely to produce a different answer to YOU driving your car in exactly the same circumstances, 'cos for the differences in driving style! So your pressure setting or my pressure setting can only ever be an indicator for what someone else should use!! And yes, there are significant differrences due to car, tyre size, driving style, and braking techniques!! You'll be surprised at how big the differences can be!

So just use it and learn from it!! I'm pretty sure that it'll improve the life and performance of your tyres over what you'd get with any static method of tyre pressure setting!! And it'll probably improve life and performance of your tyres over taking the 'optimum' cold pressure from the Tyre and Rim Associations Standards Manual, simply because it is varied depending upon your tyre and its' volume of air's reaction to ALL the variables, not just the aggregate weight and a set of 'standardised' variables!!

Have Fun, and learn to enjoy using your tyre pressure gauge!!

[sudso]

fishaholic I know your tyres are a bit smaller than mine (33x10.5x15 BFG Muddies) but I put in 30psi cold for on road and 18-25 for off road.
I could go lower for sand depending on it's softness.

I've had no problems with these pressures and with the bitumen use the tyre wear is nominal.
The grip in the wet is pretty good too but my muddys are tall for their width.
My Rodeo d/cab weighs in at 2.2T with me in it and half a tank of lpg and petrol. That's about 550kg over the unladen weight of a 98 d/cab.
Extra weight on the front is a winch and ARB steel bar,
on the back is a canopy, roof bars, LPG tank, long range petrol tank, tool box, spare wheel etc.
Count up all the other little additions here and there and it adds up quite quickly.

cheers

[fishaholic]

Thanks for that Sudso. I will try running them a bit lower than what they are at them moment (high 30's) I'll try with 34psi cold pressure and see how that goes. I used Peter's rule and had an increase of up to 4psi, but my cold pressure varied from tyre to tyre by about 2psi in the rear, with the front running close on 4psi higher than the rear ??? ::)

First thing to do is get them all the same. After that i can use peter's rule more effectively 8)

Andrew

[sudso]

The muddies feel the best at around 30-34 on mine but it depends if I got a load in the back in which case I put a bit more air in but I put the 31" 10R's (which are more of a highway tyre than an all terrain) on for longer trips on the bitumen and I run them between 35-40psi.

[luxa]

Usually its a compromise.

Tyre manufacturers will recommend high tyre pressures - as it gets the best milage out of the tyres and you'll be impressed with their product.

Vehicle manufactures want you to run lower pressures and it will give a softer ride and you'll think their suspension is plush!!! It would cost them big $$ to engineer a suspension system that can function well without the assistance of pneuamtic tyres!!

You need to consider that the tyres are actually part of the suspension - too high a tyre pressure not only wears the tyre unevenly and makes handling crap, but puts more strain on the vehicles suspension system as the tyre is no longer assisting in dampening.

Luxa

[Peter @ Aawen4x4]

Just got a copy of a Mickey Thompson pamphlet that details the 4psi rule in it for use 'on the road' and then details a series of pressure ranges for 'off road!' On of the reasons I've found that the 4psi rule works EVERYWHERE is that whe you do venture off the bitumen, you tend to drop your speed a little to match the terrain, at the same time as you need to drop tyre pressures! So the 4psi rule continues to work exactly as if you were 'on road' and really, when you consider that the tyre is actually just the container for the air that is holding the car up, you want it to heat up to the optimum temperature for it to do it's job, why should that be different for off road or on??!!

Anyhow, Mickey Thompson has a pamphlet on the streets advocating using the 4psi rule in their tyres!! If you try it and get to the stage of being able o use the rule fairly well, you'll find that it works well IN ALL CIRCUMSTANCES!

Have Fun, with your tyres at the right pressure (for YOU!)

[Humpy]

I haven’t properly tested this theory yet, but just from thinking about tyre pressures I have measured on my little zook. A 4psi variation doesn’t happen.

Before peter says I’m running my pressures too high, I have dropped my pressure to 9psi, driven around the bush all day and then driven on road for about half an hour to a servo and the pressure has read 10psi on the servo gauge (hardly reliable I know)

I have also driven on stockton beach with 4psi and the pressure barely even moves. I don’t think there’s physically enough air in there to increase 4psi even if I drove up the freeway at 120km/h

I guess what I’m suggesting is that maybe it works for a heavy vehicle, or maybe it works as a percentage of the pressure, ie 12.5% of 32psi is pretty close to 4psi and 12.5% of 9psi is 1.125psi which works for my example. I don’t think we can throw away any other ideas and just cling to this formula

My other concern is that this may only work out a good compromise for comfort and tyre carcass longevity. As it is the tyre flexing that heats up the air inside causing it to expand and increase the pressure. I don’t see how it would optimize treadwear, if your pressures are lower than required for the load (not necessarily speed) the shoulders will wear, to high and the centre will wear. Plus the tyres squish while cornering wearing the outer shoulder especially if the pressures are low, even at low speed

[Peter @ Aawen4x4]

Humpy, I'm surprised!! I can see why you have concerns if you aren't getting any increase in pressure at all, even with your tyres at 9psi!! What size are they??

I've got a couple of sets for the F'roza, and both manage a 4psi increase after an hour's driving, either off or on road!!! The smaller tyres are Cooper ST's in 245/75R15, and the biggies are Simex Centipede Mud Terrains in 31x10.50R15!!

Still, I can see your problem, and don't know what else you can do except guess!!

Have Fun!

[Humpy]

31x10.5x15 but remember the zook only weighs a tonne ;D


I havent driven the new one with 37s for more than 1/2 hour at a time yet ;)

[Nemesis]

The 4psi rule has too many external variables on temperature to make it gospel.

You're most accurate bet is to find out the average gross weight of your vehicle (weighbridge, not rego papers) then contact a tyre professional (not a tyre fitter.) Using the Tyre and Rim Association - Standards Manual they can give you the optimum (cold) tyre pressure for your new tyre size.

Alternatively - Bridgestone Australia recommends that for original size tyres, 10% ABOVE what the placard states is the best average for Australian conditions.


Listen to this guy, trust me he knows what he's on about ;) ;) ;)

Be warned about running too low a pressure at normal driving speeds (read: after bush bashing at low pressure) as the heat in the sidewall can damage the tyre from the inside out.

[brooksy]

I've found running between 33 - 36 psi has been great on tyre wear. I do go a bit higher in the rear if I am fully loaded on a trip about 38-40psi.


brooksy