4PSI RULE - Tyre Pressures

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Jeeps
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Unread post by Jeeps » June 7th, 2010, 9:22 pm

I have been running 26psi onroad in all 4 tyres lately and after a 2 hour non-stop drive at between 80-100klm/hr on the hwy on sunday i pulled over and checked my pressures and they'd gone up to 29psi. That's 3psi difference after 2 hours at 100klm which isn't too bad and obviously i could go lower if i want to get the 4psi rule spot on.

Running the lower pressures seems to give me better grip onroad, a more even wear pattern and the tyres soak up bumps a lot more! Previously i ran 38 psi so i've actually dropped 12psi for my onroad use!

I run 14-16psi offroad. I'm only running 32's so they're not monster tyres.

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RDM
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Unread post by RDM » June 10th, 2010, 12:57 pm

I use the 4psi rule and have done since I first came across Pete's post a few years ago, it works for me.
On a recent outback trip I was amazed and a bit saddened at some in our group who were ignorant and sometimes hostile to changing tyre pressures.
People would run 50psi and explain it as "they're bigger tyres than standard, It's loaded, the tyre fitter told me to run them at 40psi and more when loaded", on corregations they would crawl along at 10-15k/h and complain it's uncomfortable to go any faster.
As much as I like some of these people we would think twice before travelling with them again, sad but true.

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Unread post by cac » June 14th, 2010, 9:23 pm

i have come up with my own variation of the 4psi rule, because as someone stated above, i'd rather be wheeling than playing with tyre pressures :)....

i seem to have a knack of getting the tyres pretty close with just an educated guess, as i've confirmed this on a number of occasions by using the 4psi rule....

my theory is after you have found your starting pressure for normal bitumen driving, you change the tyres in 4psi increments (or 5psi if thats what works for you)...

so i run 28psi for on road, 24psi for poor bitumen/dirt roads, 20psi for low range fire trails etc, and 16psi for the sand and other more difficult stuff....if the conditions warrant it, i'll go lower, and if i'm heavily laden, i'll add 4psi to each of my above figures...

while i see the merits in using the 4psi rule as peter explains it, i can accept that by doing it as i've described, i can spend less time setting tyre pressures, and more time wheeling, even if it does come at the expense of tyre wear...though i'm still getting most of the benefits as those using the 4psi rule....

earlier in this thread i did suggest footprint lengths, and one could do the same using 3-4 different lengths, once again, not a perfect, by the book solution, but still one that would be relatively effective....
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TOPNDR
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Unread post by TOPNDR » July 12th, 2010, 2:19 am

I've found the 4PSI rule easy to come to terms with, and mostly I can guess very accurately the correct starting pressure.

Having just finished a 10 week, 17,000km trip, which included the GRR & Mitchell Falls, I was quite surprised at how many people told me my tyres (Yokohama G012 285/65R18) were going flat. Towing a 2.1 tonne camper trailer, I was running 22/25/24 psi, which on the gravel up to 80 kph, conformed to the 4 psi rule, and gave me a reasonably comfortable ride over the corrugations.

Turning onto the Kalumbaru Rd, the pressures were about 3 psi higher, and the ride, shocking. Next morning, lowering the cold pressure just 3 psi, made a huge difference to the ride.
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workhorse
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Are there instances where the 4psi rule doesn't work?

Unread post by workhorse » July 20th, 2010, 5:03 pm

I am an advocate of the 4psi rule and have been using it for over 20 years due to some advice given to as a lad.
I have just put new tyres (Toyo M55 - 225/75/16) on the ute (2000 Ford courier single cab steel tray 100kg toolbox/tools).
I have not been able to get a tyre pressure that complies with the 4 psi rule that feels good.
Highway 1- ran 36psi cold got 39psi hot (just less than 4psi) felt like I was on flat tyres, understeered, I din't know what the car was going to do. Highway 2 - Tried 39 front, 38 rear (unladen) got no change when hot. Car felt much better on the road more secure.
Dirt - ran 30 psi cold got 32psi hot. Had the 2 worst moments on my home road in 5 years. Understeered, felt like I was on flat tyres and the sidewalls were not bulging but pivoting on the rims so I would be thrown left or right with the corners.

I had been told (amidst the 1000s of comments when looking for new tyres) that these tyres may have too heavy a sidewall for my vehicle and use. Have been running Cooper ST with utmost confidence before the change.
I would appreciate some comments especially from those who use the 4psi rule.
Thanks

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Unread post by Peter Aawen » July 20th, 2010, 7:41 pm

Are there any instances where the 4psi rule doesn't work - short answer = NO!!

But there may be instances where either the operator can't be bothered, or the 4psi rule results that are guiding you towards the optimum balance between traction, handling, ride, wear, longevity etc don't suit your particular desires or maybe even needs from your chosen tyres. You might be prepared to accept somewhat lesser longevity for better high speed handling; maybe you are prepared to risk driving faster DESPITE it meaning that your chosen tyre won't give you the best traction at that speed; or then again maybe your chosen tyre isn't suited to carrying the load of your vehicle at the speed you wish to drive it at?!

A number of things to consider here. Do remember that the Look and Feel of tyres is very deceptive with regards to how the tyres physically perform. Those figures that gave you 'almost' 4psi are probly pretty reasonable to use to get close to that 'optimum' balance between all those things, but if you don't like the FEEL of the ride & want to run them higher on the highway, then certainly you can choose to do that, but just be aware that if you do, then you'll be getting something LESS than optimum traction (even tho you might think the traction feels better or they feel more secure - the 4psi results are telling you that you will be getting less traction than you would with lower pressures!) AND you'll be wearing the tyres out quicker than you would if you ran them at the lower pressures (front tyres will probly wear the edges quicker, rear tyres probly the centre of the tread quicker, but that too is a little dependent upon the tyres themselves, their load ratings & construction, your vehicle, driving style etc). And as every tyre placard, tyre manufacturer, or car manufacturer tells you, if you travel faster than 'normal' you generally need to add more air to avoid overheating your tyres & causing blow-outs.

That's what learning to USE the 4psi rule is all about, it really only takes moments to do once you are up to speed on it, the 'rule of thumb' will tell you what the ideal pressure for the tyres is to give you that optimum balance, but if you don't like that for whatever reason, you can vary your starting pressures & use something else; but the 4psi results will STILL be guiding you in working out what's likely to suffer as a result of that choice! From what you say about the tyres & the advice you got when you were making up your mind what to buy, you were probly already aware that some things would suffer if you chose those tyres, but you went ahead & made an informed choice about it!! And consider that while you may have scared yourself running them lower on the dirt, it wasn't necessarily the pressures that were wrong, it was more likely the speed you were driving at, given THOSE particular (heavy cased?) tyres under your (relatively light?) car! Less than optimum traction & traveling too fast for the pressure/tyre construction/compound usually combines to give you under steer & tyre squirm; you can remedy that by increasing your pressures (at a cost in puncture resistance, traction, & tyre longevity, stresses & damage to the vehicle & yourself thru vibrations etc, and you've gotta hope that you avoid damaging anyone else too!!) or you can decrease your speed without those costs but you'll take longer to get there & do less damage to the surface, the vehicle, & ultimately yourself & anyone else! BTW, I wouldn't mind guessing that running the pressures you feel more comfortable with also makes the tyres a little bit skaty on the wet bitumen too - pressures that are too high will reduce your traction while feeling firmer, but lower pressures will improve your traction and may feel squirmy by comparison at a given speed; but then again if you drive fast enough on them to bring the tyre squirm issue into play you could always slow down instead of adding more pressure, couldn't you?

Now I'm really not being judgemental here, all I'm trying to do is to point out (again) that your cold start tyre pressures are just one of the things that YOU control that have an impact on the 4psi increase, and another is the tyre choice, and yet another the speed you chose to drive at, or the way you drive! Your vehicle manufacturer put a lot of effort into working out what tyre compromise (& it surely was a compromise) will suit their target market in the widest range of circumstances. If you choose some other size/type/tread pattern in tyres you would generally try to do that to make sure that your choice suits what YOU want to do with the vehicle better, and if you want high speed stability AND on-road traction there SHOULD be a little difference in the tyre choice than say..... if you wanted off road traction & performance in the dirt. But we as 4WDers tend to want a tyre that will do everything under all circumstances, and there really isn't anything that will do that..... wellll, not yet anyway! ;)

So maybe you are already aware that your tyres may not be the BEST choice for everything you want to do, but you still chose them for some reason, and because of that you hafta decide what you are prepared to sacrifice in order to get them to work acceptably under all the circumstances you will encounter. And it's perfectly valid to chose to run higher pressures to give you a better Feel at the speed you wish to travel at, just do be aware that you will be sacrificing something else to achieve that!! I hope that you'd have enough responsibility as a driver who shares the road with a whole range of other users to make a choice or choices that doesn't unnecessarily compromise the SAFETY of your vehicle, the way you drive, and the way you interact with all those other users! Make sense??
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workhorse
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Unread post by workhorse » July 21st, 2010, 8:13 am

Thanks Peter.
I think you have confirmed what I thought. I have chosen an inappropriate tyre for my vehicle type and weight.
I am a believer in the 4 psi rule of thumb, but also recognise the effects of different tyres on vehicles. In this instance speed wasn't a factor in the vehicle's handling, in fact less speed was used because of the 'bad feel' in the handling.
Seems I'm now stuck with a new set of tyres (550km) that are not the best match for my vehicle.
Ah well, mistakes are still made despite age and experience.

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Unread post by Peter Aawen » July 21st, 2010, 2:26 pm

Speed would certainly have been a factor workhorse, but it would've been that speed (no matter how fast or slow) on THOSE tyres under THAT car/weight that was a factor; not necessarily that you were driving too fast - as you say, the tyres are not quite the right choice for that vehicle weight & body type, or the way you use that vehicle!

You could try selling the tyres (maybe even here on the Forum, thru the Classifieds - just read the Layout sticky first!) Sure, you'll probly lose some dough on the deal, but then you'll certainly be losing some dough in the long run (& probly safety & driveability etc) on keeping them under the car as it is!! Sell them & you'll be able to get a set of tyres that are better suited to what you want from the vehicle, & how you want to use it, & have the tyres perform etc.

BTW, have you spoken to your tyre dealer?? Some will have an arrangement where they can 'trade in' the unsuitable tyres & get a set better suited to your needs - not all, and most aren't allowed to sell second hand tyres at all, but some have arrangements that mean you don't necessarily do your dough completely if you do happen to get your tyre selection wrong! Worth an ask, & if they can't help, maybe your local wrecker - failing that lot, an advert in the Tyres & Rims section might be a good way to go! ;)
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Re: 4PSI RULE - Tyre Pressures

Unread post by Boxhead 71 » July 30th, 2011, 11:16 pm

Umm... Question for Peter... You say the difference between the sunny and shaded sides of the vehicle will be negated after 10 minutes of driving. When checking bearing temps with a temp gun on semi trailers there can be a difference of 20 degrees or more along the sunny side of the trailer[s], and the tyre temps have so much difference that just with your hand they feel twice as hot. I'm not saying any of this topic is applicable to trucks at all, but the tyre temps in the sun are gunna happen regardless of car, 4by, trailer, or truck. This is just an interesting point i'm raising; i'm still interested in trying the 4psi rule when i go to Perth next week. Anyone got a good starting hwy pressure for me?

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Re: 4PSI RULE - Tyre Pressures

Unread post by whizzo » July 31st, 2011, 10:42 am

Just for good measure a passsing shot at flotation tyres..that is any radial specified like 31x10.5 or 32x11.5 or such.

They are not a good load carrying tyre, they are by design a soft weak tyre with very poor load carrying capaity for their size.

In fact on load rating alone they will be found to be illegal on a great many vehicles they are fitted to.

Considering that many of our touring vehicles are very heavily loaded and heavy to start with, I do not think they are a good choice for heavily loaded high speed touring either on rough or sealed surfaces

In this application the same or similar tyre in a "metric light truck radial" would be a better choice, as there is a considerable difference in construction and load capaity.

Remember that a tyre is a preasure vessel and its ability to work and carry load is totally dependent on its ability to contain air preasure.

cheers

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Re: 4PSI RULE - Tyre Pressures

Unread post by yamaholic » August 24th, 2011, 3:26 pm

Hi all,

Great discussion here.

I have a question here about this 4 psi rule.

Would it work same for lower profile tyres? Lower profile tyres would have less sidewall flex and I was wondering if the 4 psi rule should be adjusted to become 3 psi.

Also, for ultra low rolling resistance tyres, because of less sidewall flex, would the rule best be changed or just remaining at 4 psi?

Lastly, I was also wondering if anyone has used the 4 psi rule for passenger cars with good success?

Thanks!

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Re: 4PSI RULE - Tyre Pressures

Unread post by Peter Aawen » August 24th, 2011, 6:31 pm

The 4psi rule of thumb has been used across the range of things that use tyres pretty much since pneumatic tyres got invented & so far it's worked well for everything! It is one of the few ways a driver can actually respond to the differences all the potential variations in temperature, road surface, driving style etc that can impact, and cos it reflects the driving you've just been doing, it caters for all those variances, altho you do need to make a bit of a judgement call about what you are going to be doing next. And while some people suggest that off road tyres might be better using 6 psi instead of 4, or that different profile tyres warrant some variations, the tyre industry calculations that this 'rule of thumb' is based upon don't do that, they still stick with aiming for a 4psi increase after an hours driving. After all, it's all reflecting what happens to the volume of air inside the tyre & how it is heated up (or otherwise) due to the flexing of the tyre carcass - so in reality the lower profile is already being considered.

So I'd suggest that you try it & see what it suggests - if you don't like it, you don't hafta use it, but it is one of the few ways that many drivers across the range of vehicles that use tyres have been able to optimise their tyre life, handling, performance, & ride with any degree of consistency, it's easy to do, & most people find that it takes less than a few weeks of practice to work out reasonably well how to estimate what sort of impact the various loads & weather conditions etc will have on their 'baseline' tyre pressures, & from there it's simply a matter of confirming that estimate once or maybe twice after you've been driving for a while. And don't forget that it isn't always necessary to change the tyre pressure if you get something other than the 4psi increase - you can vary your speed up or down as indicated, you can load up more or drop some load (bit hard if you are on a trip tho) or you can even make an effort to drive a little smoother & do less hard braking - all stuff that can impact, & tyre pressure is only one of the possible variables that the driver has control over. ;)
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yamaholic
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Re: 4PSI RULE - Tyre Pressures

Unread post by yamaholic » September 27th, 2011, 9:28 am

I have tried this 4psi rule on my passenger car and would to get your opinions.

On the door sticker, the recommended pressure is 29psi.
But using 4 psi, I have gradually reduced my front pressures to 25psi and my rear psi to 23 psi. According to the discussions, 23psi would not be enough and would need to go lower.
Am I doing something wrong? Less than 23 psi for rear tyres look difficult to me.

FYI, I take the cold pressure before I start, and the hot pressure after about an hours' drive. I've also noticed that the difference between 35minutes to 1 hour drive is not that much (in terms of difference of pressure).

THanks.

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Re: 4PSI RULE - Tyre Pressures

Unread post by Peter Aawen » October 5th, 2011, 3:03 pm

Are you still using the original tyres, or have you changed brand or size/tread etc? Generally going to a bigger tyre means you'll need less volume of air to carry the same weight.

You can always get your vehicle weighed & then go to the tyre dealer & ask what is the recommended tyre pressure for that tyre under that weight as shown in their documentation, they should have an Aust tyre manual that details that sort of stuff - then use that pressure as a cold starting point.
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Re: 4PSI RULE - Tyre Pressures

Unread post by yamaholic » October 7th, 2011, 3:11 pm

Thanks for the reply.

I am currently using stock tyres with about 50% thread life remaining.

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