4PSI RULE - Tyre Pressures

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4PSI RULE - Tyre Pressures

Unread post by Pog » September 28th, 2009, 9:15 pm

After just seeing this thread:

http://www.4wdmonthly.com.au/forum/show ... hp?t=77696

Can someone PLEASE explain the 4psi rule PROPERLY. As in the correct procedure to calculate pressures according to the change in pressure. I myself have searched these forums many times looking for the answer to this with no conclusive answers whatsoever. Every time the question is asked, some smartass always tells the poster to search. But the answers just aren't out there!

I asked Peter @ Aawen4x4 this question a few months back via PM, and got a very comprehensive answer. But unfortunately my inbox filled up and I had to delete his message.

Peter, could you please explain the 4psi rule to the best your ability for everyones sake?

Mods, please sticky this thread!


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Unread post by cac » September 28th, 2009, 9:32 pm

Mods, please sticky this thread!
x 2!!

Pog...essentially you choose a starting pressure, try what's on the placard if you have standard tyres, and if not, then pump them up to a point where they are only just bagging out....

then go for a drive for an hour or so, and measure the increase in pressure from cold to warm....if its is less than 4psi, then your starting pressure was too high, if its more than 4psi, your starting pressure was too low, so wait for your tyres to cool off, then drop/raise them 1-2psi from your original starting pressure, and rinse and repeat....

i know when peter has explained it in the past he has given us a calculation to make it easier to work out how much to increase/decrease the tyre pressures, but tbh i think you'll get pretty close by using your common sense....if you get an 8psi increase from cold to warm, obviously you are way to low, so try 4-5psi higher, if you get a 1psi increase, then drop 3-4psi out....

if we are making this a sticky as well....can i suggest we talk standard footprint lengths, as this makes it easier to work out a good starting pressure to work with....

i've seen one member (nqgu) who is a tour operator and driver trainer who suggested this theory, and works on the following footprint lengths....

road 180mm
gravel 220 to 240 mm
sand 300 to 350 mm
Oh bugger 400 mm

i have tried setting mine at 180mm long for the road, and it gives a nice ride, without giving the impression the car is rolling around on its tyres, so i don't think i'm too far off the ideal pressure for the tyre size/vehicle weight, though haven't checked via the 4psi rule yet....

i also intend on writing down the pressure's i am running at the footprint length indicated, to make it a bit easier when airing down :)

anyone else have experience with setting footprint lengths rather than going with pressures?? be interested to hear how well the theory works....though i do plan to test it further....
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Unread post by Sim79 » September 28th, 2009, 9:33 pm

Do a search, its has been explained a few 100 times!!
Peter @ Aawen4x4 wrote:I use that 4psi rule everywhere, and for everything!! It works very well in ALL circumstances and across all variations that you can think of and a whole heap of others you never dreamed of! Use the tyre placard as a start point, drive for an hour, stop and check the pressure. If it's gone up by MORE than 4psi, your start pressure was too LOW, add pressure now! If it has gone up by LESS than 4psi, then your start pressure was too HIGH and youn need to drop 2psi NOW! Those new pressures should be used as your start points for the next cold start!

And when you are off-roading, then use it the same way. Start with the guidelines that come from places like the Cooper Tyre Book, the Mickey Thompson guide to tyre pressure, even the prado4x4 guides above. Apply the 4psi rule, and adjust your pressures as you go. You will take only a sort while to work out what pressure is a good start for you, and by using the 4psi rule you can adjust the pressures to what is the optimum for YOU and your driving style, load, vehicle, conditions, temp, etc. AND remember that when you drop your tyre pressure by say 20%, you should also drop your top speed by 20%!! Tyre pressures lower, speed lower!

BTW, Cooper tyres tend to run a lower pressures than most people generally think, probably because the extra layer of belting in the carcass reduces the flex in the sidewall a little and reduces the induced temp increase as you drive. Enjoy your driving, and don't get too worried by it all, do what you can without stressing, and live with it, it's the best you can do at the time! Shouldn't let a little thing like tyre pressures take away too much from the enjoyment of getting out there in your 4By!!
Peter @ Aawen4x4 wrote:It came (some years back) from a tyre guru who had spent his working life in the back rooms of the tyre companies that spent mega bucks supporting some of the race teams. He had a wealth of knowledge, and had a career that included time with a couple of the large tyre companies, and some of the big names in international racing, as well as successfully running one of the more respected tyre development and testing departments in the world! I was very impressed with his down to earth approach and knowledge, and he spent some time explaining that despite all the computing power that had been bought to bear on the subject of determining the 'optimum' tyre pressure for a given tyre for ALL situations, they still hadn't been able to get anything better than the '4psi increase after an hours worth of driving' and it worked wherever they applied it!

Since then, I've seen it appear in lots of places, the latest being the Cooper and Mickey Thompson 4WD Driver's Guides, where they have a section on tyre pressure, and they lay out the '4psi Rule'! They do suggest to use it only for bitumen road use, and despite being told by 'the man' that it worked anywhere, I too was a little slow to be convinced that it STILL works fantastically for Off-road use, surfaces, and conditions; it's just that the pressures hafta be significantly lower to start with! I suspect that is the reason for the rider in the Cooper and Mickey T documents; either that or they are protecting their butt from some percieved potential lawsuit. But it's worked for me driving trucks & buses, cars, 4WD's, and even tractors of various sorts, in all sorts of conditions, heat, snow, rocks, sand, et al, so don't be worried about applying it to whatever you are doing with tyres, it'll help get the pressures right for whatever you are doing!

So try it yourself! If you seriously give it a proper try, and adjust your pressures in the manner it suggests, you'll get longer life from your tyres, they'll be less prone to puncturing than otherwise, and they'll give you traction, road manners, and ride characteristics that not only enhance the life of the tyre, but make it easier on the vehicle and the occupants! Like they say, use the tyre placard to get a good starting point, then fine tune it using the 4psi rule from there. Use the guides suggested pressures for off-road pressures as a start point too, and then do the fine tuning with the 4psi rule. Again, it'll enhance your driving in more ways than you expect.

One of the first things that I noticed when I started using this rule off road was that when I went driving with others who weren't aware of the 4psi rule 2 things happened. 1 - they bagged me for stopping and playing with tyre pressures; and 2 - they thought I was a much better driver than the norm because I could go places that they couldn't without anywhere near the wheelspin or even at all! The only thing that I could see that was different was that I was using the 4psi rule and they were generally not changing pressures much at all! Now that was a long time ago, and it was when people generally worked on HIGH pressures only for off roading, to allow the tyre to cut thru the goop and get down to the firm stuff etc, and since then the whole ballgame has changed and people are much more aware of using lower pressures to enhance traction, but still, it was a telling point at the time. My tyre guru had given me a handle on something that REALLY DID WORK!

Try it, you'll see for yourself!

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Unread post by BLU-125 » September 28th, 2009, 9:51 pm

Any Cooper dealer will be able to provide a booklet about tyre care which includes detail about the pressure change rule.


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Unread post by DoctorI » September 29th, 2009, 11:10 am

cac, I tried setting my footprint lengths after a local (very well respected) 4WD instructor spoke about it at our 4WD club meeting. With my 33" tyres I found I was having to use ridiculously high pressures to achieve the recommended footprint lengths. So I concluded that a different set of lengths is required for different diameter tyres.
The other problem with setting footprint lengths is that you have to do them on a smooth level surface (i.e. your concrete driveway) not at the start of the off-road track. So you still need to build yourself a chart with the footprint length and the corresponding pressure for your vehicle for both front and back wheels. Once this is done, everything becomes easy.

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Unread post by cac » September 29th, 2009, 1:03 pm

So you still need to build yourself a chart with the footprint length and the corresponding pressure for your vehicle for both front and back wheels. Once this is done, everything becomes easy.
that's exactly what i am doing...i already drew up the chart with the footprint lengths and its in the vehicle, with the intention of adding the pressures as i went (and changing them as i add accessories and gear)....though i didn't realise about the flat level surface....

i too have wondered about different footprint lengths for different size tyres, and that's one thing i was hoping to get some more info on by posting the question...

as for getting the same footprint length on a taller tyre....it makes sense that you would need to run a few extra pounds in it to do so....especially when running a 35 on a zook or something.....maybe the measurements quoted above, and the ones you were quoted, work best for standard(ish) tyres....and you need to add a bit too them as you go up in tyre size....ie for every inch you go up from standard, add 10mm to your footprint length for the offroad sections, and leave the road measurement as is....after all, one of the reasons we fit bigger tyres in the first place is to gain a bigger footprint...might as well take advantage of it....

i think i've just answered my own question lol.....
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Unread post by fullmetaljacket » September 29th, 2009, 10:00 pm

I am a firm beliver of the 4psi rule. After driving 44,000km on my 33x12.5 GY MTRs and using the rule for 40,000 of them, they have worn a maximum of 2-3mm from new. At that rate I should get 120,000km before they need replacing (under normal circumstances). I do not know if the great wear pattern is due to the tyres I have may be a harder compound, frequant rotation shedule (tyre balancing and rotation on rim), or the 4psi rule, but its working for me.

If it wernt for Peter, I would still be inflating to excessive pressures and replacing prematurely worn tyres

So a big thanks goes to Peter, cheers mate
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Unread post by steveandviv » September 30th, 2009, 8:43 pm

Just remeber that the 4psi rule is 'mostly' correct. I'm sure Peter would aggree that in some conditions you have to evaluate the circumstances like deep sand etc. I know that obvious but you need to remember that. You will also learn from experinace. Don't blindly use the 4psi rule. learn what your truck works best at especially on corrigations where you may have to compermise the 4psi rule for safty and comfort.
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Unread post by M543 » September 30th, 2009, 11:31 pm

steveandviv wrote:Just remeber that the 4psi rule is 'mostly' correct.

I think you could be right there. The first time I tried it was about three years ago on my current car which has stock size tyres. I set them at factory pressures when cold (25 psi each end with car empty) and drove from home in Sydney to Newcastle on a warm late spring day and they went up 3 psi. This means my starting pressures were too high according to the 4 psi rule. I thought how much lower than 25 are you supposed to go if you want them to go up by 4?

I rang the tyre manufacturer and was told there were too many variables such as vehicle load, speed, road and air temperature for the rule to be reliable.

I noticed on a recent trip to Wilcannia, I set the pressures after dark the night before in Sydney then drove most of the next day to get there. Early the following morning, in much colder air, they were 3 psi lower. Three hours later with the sun up and without having moved the car, they were back up to Sydney pressures. This is probably some of the variables the tyre manufacturer was talking about.

I always use factory pressures on all surfaces except sand and they work. The car has been everywhere from the Snowy to Oodnadatta. The ride is good, the wear is even right across the tread and I am yet to get my first puncture with this car. I would say the tyres and suspension have been designed to work together on all surfaces at the recommended pressures with variations in atmospheric conditions taken into consideration.

I have noticed after years of working in the motor industry in rural areas, country people don't fiddle around with different pressures when driving from sealed to unsealed roads. With so many different surfaces in those areas, it would drive them crazy if the car manufacturers expected them to.

Of course this is with stock original specification tyres and would most likely not apply to non standard sizes. You would have to work out your own pressures then and the 4 psi rule would probably be a good starting point. The key to the whole thing though is the way the tyres are wearing. Too much air wears the centre faster while too little wears the outside edges. Cornering too fast, alignment problems, balance, shocks and steering linkage wear will cause other wear patterns that can usually be easily recongised.

If your favourite pressures are giving an even wear pattern then they are right regardless of how you worked them out.

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Unread post by Peter Aawen » October 1st, 2009, 5:19 pm

Sorry I didn't get onto this earlier, but I've been out driving (even some with the stubby lever engaged!!) The thing to remember about the 4psi 'rule of thumb' is that unless you've got tyres that are significantly over-sized for the weight they are carrying, then it is the ONLY method available that we have readily available to all drivers that ACTUALLY takes into account ALL the variables, cos it works off the driving you've just done in the last hour or so - ambient temp, air temp, road temp, road surface, driving style, load, tyre carcass, tread construction, etc, etc; BUT it IS retrospective - it's based on what you've just DONE - you hafta make a judgement call on what's facing you in the next hour, and adjust pressures accordingly!! (but the 4psi rule will let you fine tune that 'judgement call' into something quite accurate fairly quickly ;) ) So it is a system that you need to use pretty much all the time until you get a good idea of what variables make how much difference to your pressures, but I've noticed with all the people I've trained over the years it hasn't taken anyone all that long to get pretty good at getting their judgement calls pretty close within a few weeks of starting 'using' the 4psi rule. So try it, vary your pressures accordingly in response to things like different ambient temps, weather, road surfaces etc and modify your start pressures accordingly, you'll get better at judging what pressures to start at & then correcting them iaw the 'rule' as you go along.

The 'calculation' for working out how much to vary your pressures when they are out is just another 'rule of thumb', basically if you take 1/2 the difference between what your tyre pressure SHOULD have been and what it ACTUALLY is, and add/subtract accordingly, and you can keep doing that every 1 hour if you like, but it becomes a tiny adjustment eventually. The footprint lenght suggestion mentioned earlier works too, you just need to be consistent in what methodology you use rather than exact, same with your pressure gauge - the most critical factor is consistency rather than absolute accuracy.

Re the Tyre Dogs and their temperature/pressure variations, I've got a set of them, and I've run them for a long time now. It has become very obvious to me thru using them that the tyre temp reported by them is only very loosely acquainted with what's actually going on INSIDE the tyre, I believe it's cos they are externally mounted on the valves, and they reflect ambient & road temp a lot more than internal tyre temp, AND they are very much effected by external factors such as exposure to sunlight, wind, rain, snow etc rather than showing an accurate relationship between the 'real' internal tyre temp and air pressure. As such, I'm firmly convinced that while their pressure advice is pretty good, and even usually reasonably consistent (altho not necessarily all that accurate ;) ) their temperature readings are not something you want to rely on. And sometimes their pressure reporting can reflect only what's happening between the valve and the monitor, not what's going on inside the tyre, so if one Tyre Dog starts showing a higher or lower reading that's out of kilter with the others, it really IS a good time to stop and look for a reason!! But don't assume it's necessarily always a puncture or some other undesirable incident, it could be just that the rubber seal in the base of the monitor has split or it's jammed the valve stem in an odd position. I also run a fairly sophisticated (and expensive) internally monitored TPMS, and I've often run both systems on the same tyres over the same terrain - while the Tyre Dogs have reported useful pressure data, but the temperature info has often been worse than useless - at best it's confusing, at worst it's downright misleading, and sometimes you need to check the pressure readings against your reliable tyre pressure gauge too. So don't pay too much attention to the temp info from any externally mounted TPMS.

BTW, digging thru the coding for the sophisticated TPMS, I came across an 'increase factor' that it uses to calculate the variation between what your current pressure and the 'correct' pressure for that particular tyre is, and guess what??!! Yep, it too uses the 4psi rule to determine the optimum pressure for your tyres!!

Finally, (welllll, almost) your tyres (the rubber bits mounted on the rims) need a certain VOLUME of air to hold up the weight of your vehicle as you drive along. Skinny tyres that aren't all that large in dia need higher pressures to get the same total volume of air when compared with wider tyres or tyres with larger overall dia, so as you swap from the std (& generally relatively narrow) tyres that were fitted by the manufacturer, and fit wider/taller tyres to your vehicle, unless you've significantly increased the weight of your vehicle, you are not all that likely to need HIGHER pressures which would mean a greater volume of air - in fact, you are more likely to need LOWER pressures to give you the same total volume and therefore do the job. And that relates to why the footprint length system isn't quite as versatile as the 4psi rule - the optimum footprint length will vary with the load, tyre construction, tyre size etc, so it's not quite as easy to cater for all those variables in determining exactly what is optimum for YOU. The 4psi rule works on what the volume of air inside the tyre is doing, so it DOES take all those variables into account.

And now REALLY finally, some manufacturers have started recommending that for 4WD's we should be using a 6psi rule rather than the 4psi rule. No biggie, a lot of the reasoning behind that is simply that 4WD tyres are generally of a heavier construction than most passenger tyres, and MANY 4WDers have been running cold start pressures that are WAAYY too high, so looking for a 6psi increase means that you are starting with a pressure that's a little lower, but by using that rule you are still getting pretty close to the optimum pressure for your tyres, and if you use the rule (either 4psi or 6psi rule) as a matter of course, you are spending more of your driving time at a better pressure than previously - for almost all vehicles that weigh less than 3.5 tonnes. So if you are running more than 40psi in your tyres, check the sidewalls for the MAXIMUM cold pressure at full load - for most 15" 'flotation' type tyres, the MAXIMUM is about 35psi AT FULL LOAD (and unless you are off on a trip, you aren't likely to be fully loaded) and for most 16" tyres with a sidewall listed max of 55-65psi, MAXIMUM load is up around 2 tonnes per tyre or 8tonnes of vehicle!! And the bigger the overall dia, then the lower pressure you need to get the same total volume of air inside the tyre.

If it works for you, it's fairly simple altho it does take a little expenditure of effort to come to grips with initially, then it's relatively easy to work with as you go along & you could well get better ride, handling, traction, tyre life, & possibly even fuel economy out of your tyres; & if you don't want to bother with it, that's fine too, altho you probly shouldn't complain if you get less than optimum life, traction, or any of those other things from your tyres - Simple really! ;) Cheers
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Unread post by cac » October 1st, 2009, 6:39 pm

thanks for clearing that up peter....when running low pressures like on sand though, you really only want to drop your tyres so your footprint length gets longer and not wider don't you??
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Unread post by Peter Aawen » October 2nd, 2009, 12:20 am

That's pretty much what happens if you get the 4psi bit close to right, cac, altho there will always be some width increase as the footprint gets longer, simply because there is more sidewall flex possible (but not necessarily 'bagging'). As pressure goes down, the more recent design/construction of 4WD tyres will (almost) always mean that you get significantly greater length increase instead of greater width, and tyres like Coopers, Mickey T's, & even BFG's are made to optimise that feature.

But basically if you use the 'footprint length' method of determining optimum traction/ride/wear characteristics, there is still no hard and fast SET length for every (or even any) situation; the 'optimum length' varies depending upon the same 'whole heap' of variables that applies to the 4psi increase, so what works for YOU in your vehicle with your driving style and the load you carry isn't necessarily going to be correct for someone else in the same car on the same track with the same load, while applying the 4psi rule is always going to allow you (or anyone else using it) to be much closer to the 'optimum' because of its 'reaction' to all those variables, especially if you then continue to fine tune your ability to get it right. But again, the pressure that works for you after applying the 4psi rule might not be anywhere near the pressure that works for someone else applying the 4psi rule. Make sense??

Anything that gives you a set pressure (or length) for any given situation won't be taking the impact of all those variables into account, it will just be giving you a (sometimes) reasonable start point that may well be seriously wrong for today's specific combination of variables. On the other hand, the application of the 4psi rule still allows you to take the (same) reasonable start point, and then take all those variables into account and fine tune the pressure (and therefore the footprint length) as you go along to arrive at something much closer to the optimum for today. And as you use it more, you get better at 'estimating' what impact you'll get from today's conditions or what the track ahead looks like, etc, and you get better at picking what your theoretical start point should be and therefore what the resulting +4psi pressure should be.

That practice of taking into account the differences in variables each and every time you drive, and the fine tuning you do by using the 4psi rule and 'regular' checking of tyre pressures will allow you to develop your skills in deciding if you actually want to change (drop or increase) tyre pressures now to suit the expected conditions, or leave them as they are and vary one of the other 'variables' that you have control over, like speed or cornering/braking etc. For instance, the fella who drives a loaded work vehicle over a range of terrain every day might need to spend a little bit of time initially in developing the skills to estimate what changes he needs to make so that he's not always getting out of the vehicle and changing pressures, but once those skills are developed, he'll be able to vary other things like speed or cornering/braking techniques so that he can still get the best from his tyres across a range of surfaces & under a range of conditions given the pressures he's currently running, and without adversely effecting his tyres even tho he hasn't changed his pressures. If he doesn't develop those skills, it'll show up in the greater wear of his vehicles' tyres over those of other 'more skilled' drivers (greater wear as well as chipping & cutting, losing tread blocks etc), as well as showing up in the lesser overall traction obtained, the poorer ride, & the greater stresses on the vehicle; and therefore the higher running & maintenance costs for that vehicle. Just like a good race car or rally driver develops skills that allow them to get the best from their vehicle and actually finishing the race with a good placing rather than leading early but destroying tyres thru trying too hard and eventually getting overtaken and beaten, IF they even manage to finish. It just takes longer to show when it's a 4WD that should be able to get 40,000km plus out of a particular brand of tyre rather than a race or rally car that does its bit in less than 500km, but developing those skills are no less as important.
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Unread post by Trojan » October 2nd, 2009, 6:44 am

Peter @ Aawen4x4 wrote:The thing to remember about the 4psi rule is that unless you are running tyres that are significantly over-sized for the weight they are carrying, then it is the ONLY method available that we have readily available to all drivers that ACTUALLY takes into account ALL the variables, cos it works off the driving you've just done in the last hour or so - ambient temp, air temp, road temp, road surface, driving style, load, tyre carcass, tread construction, etc, etc;
Actually, i can't agree with you there, it may be a good rule of thumb for most situations, but it isn't a rule for ALL situations. It's a rule, not a law, so it's not always correct.

With the TSL's on my Suzuki, i can run 5psi and after a whole days driving the tyre pressure will still hardly change, and according to the 4psi rule, that means my starting pressue is to high. At 5psi, that's not even enough to keep a bead on a rim, let alone be to high to start off with.

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Unread post by cac » October 2nd, 2009, 7:53 am

Trojan...with the kind of speeds you'd be doing to safely run 5psi...would you be getting enough heat into the tyres to make the 4psi rule viable?? or are you doing a lot of stop start driving because you are getting out to check track conditions, or waiting for the rig in front to get through the next bit of track etc...

i think while peter is right in saying the 4psi rule works for a range of conditions, when you are off road and driving like i described, it does make it a lot harder to apply correctly....as stopping will allow the tyres to cool down again....

this is also the reason why i asked about footprint lengths....and why i prefer to use that method over the 4psi rule for certain conditions....
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Temps in different places

Unread post by hillbilly123 » October 2nd, 2009, 1:51 pm

I have tried this method on both my car and caravan and woud like an explanation of the following.

At the start of our trip in BRISBANE I applied the 4lb rule and it worked fine The ambient being about low 20s at the time (Feb)

I never adjusted anything and when we got into the wilds of Mexico checked the cold pressure and it was significantly lower.

On the road they were now coming up 6lb.

Later on and further south in colder weather they dropped further in the morning.

So should ambient temp be taken into account and adjustments made.

Another thing the two on the sunny side of the van run about 2lb - 3lb higher than those on the shady side proveable by when the direction changes they alternate pressures.

Also the temperature of them differs by a few degrees as well.

Can monitor this as I use tyre monitors which give pressure and temperature readouts.

Just a few thoughts on what is or isnt a hard and fast rule.

I dont subscribe to the footprint rule as different tyre constructions will give different footprints even on the same size tyre.

One thing I did notice was, one day, on undulating roads down the east coast my monitors were showing my rear tyre temps at about 65deg C.

The fronts were in the low 40's.

I got out and removed my rear rubber mudflap which was fullwidth with about 300mm cut out in the centre.

From then on tyres never got that hot again. They run about 5-8deg hotter than fronts.

This was when towing the van They are Ok when solo.

So what do the experts make of all this.

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