Why isn't tyre pressure an exact science?

Submitted: Friday, Mar 24, 2006 at 14:43
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Since reading Norm C's post (this is all your fault ;) ) about skinny tyres, my faith in my tyre pressures went out the window. After numerous searches here and elsewhere I just don't know where to turn (sob!). I've always used Pirelli's Rule; check when cold and if they are 4 psi higher when warm then this is the correct pressure.This is why I have always run 32psi on an empty vehicle. If Pirelli's rule is correct, what is it correct for? Comfort, even wear or tyre load?
Then there's the manufacturer's reccommended pressures which in other posts has been explained as comfort (which is 32 for 275/70/16).
In Norm's post plenty of people run 235/85's at 45psi or more but I presume this is loaded up.
Mine are BFG A/T's which seems to mean that the brand makes a difference as well (references to stiffer sidewalls etc)
I'm thinking I would like my tyres to wear evenly so what pressure = even wear?
The BFG website has loads of info but nothing about pressures.
The Beaurepairs website in its 'tyre tips' section states "Always keep tyre pressures at the levels recommended by car manufacturers". Then in the same paragraph states "Using the recommended high speed/full load pressures will provide improved safety and wear for day to day driving". ??? That's 80psi for my tyres!
So what's left? Experimentation. Run your $1000 tyres at whatever pressures you think are right then when they start to wear unevenly you adjust but will still have more wear on the already worn part.
So I'm running 40psi. Why? I think they need more than 32 but less than 50. How do I know that? I don't; it's guessametrics. It seems that for all the research into tyre construction/handling etc when it gets into the consumer's hands there is no difinitive information on what to do with them except put them on your wheels.
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Reply By: Member - Beatit (QLD) - Friday, Mar 24, 2006 at 14:56

Friday, Mar 24, 2006 at 14:56
G'day Bware,

The only wear issue not considered by your statement is that overinflated tyres (could be as little as normal road pressures) may well be too much on some gibber road. Reduced 2 relatively new tyres to junk in the space of 100K on our last trip. It pays to do what the loacals do when out there because they have already worked on the problem and solved it the best they can.

Kind regards
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Follow Up By: Member - Bware (Tweed Valley) - Friday, Mar 24, 2006 at 15:05

Friday, Mar 24, 2006 at 15:05
Agreed Beatit, I knew I would forget something. My post should be read in the context of normal/highway driving, as off-road/sand is another thing all together with with it's huge variations in opinion.
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Reply By: Darian (SA) - Friday, Mar 24, 2006 at 16:13

Friday, Mar 24, 2006 at 16:13
I use BFG 245/70-16 on the Jack - the first set were gone on the shoulders at 80k - but had some meat left in the middle. So I run the pressures up on this next set, while doing all of my bitumen travel.The (approx) PSI guide that I have setlled on:

Bitumen F36 R40 (bit boney but I can live with it)
Gravel F27 R30 (AND I take it quite easy !)
Firm sand F23 R26
Really soft powdery sand F15 R18 (can even tow the camper in the deep powdery stuff at these levels)

Whatever - works for me.
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Follow Up By: Gerhardp1 - Friday, Mar 24, 2006 at 16:41

Friday, Mar 24, 2006 at 16:41
I run my Jack at 40psi front and rear on the bitumen, the original tyres lasted 80000k odd with even wear. Run the current ATs also at 40psi, 40,000k so far with dead even wear.

I don't find it uncomfortable even with the beefier Torsion bars and rear springs.

Off road I drop to 28psi on gravel stuff. No sand around Melbourne but would set to 18psi if I found some.
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Follow Up By: Member - Bware (Tweed Valley) - Saturday, Mar 25, 2006 at 04:53

Saturday, Mar 25, 2006 at 04:53
Thanks guys. Sounds about right for me.
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Reply By: Wayne (NSW) - Friday, Mar 24, 2006 at 16:56

Friday, Mar 24, 2006 at 16:56
Bware,

I have come across a lot of views on tyre pressure.

I had a bloke that was told by the tyre fitter "expert" that 65 psi was the pressure to run 265/75/16 BFG A/T. The so called "expert" looked on the side of the tyre and saw "Max Pressure when cold". This must be the best tyre pressure. That is how simple it is to be a "expert".

I have been on the outback dirt roads and have been told that the tyre pressure that I am running is too high.
The "experts" say to run them at 26psi on the dirt. I then ask what vehicle are you driving?
The Lux ute over there, or something like that is the answer.
That pressure is fine for a ute that has no weight. For a fully ladened vehicle that is packed for a trip it is a bit low. The "expert"also say that you will get a puncture if you run the tyre pressure and higher than 26 psi.
So they will never get a flat at 26 psi.If they cold guarantee that I would not get a flat I would run the tyres at 26 psi.

So what pressure do I run as an "expert".
38 psi, expect on sand then it goes down between 20 psi/ 15 psi.

I can not guarantee that you will not get a flat, but this is the pressure that I run, for my vehicle and the speed that I drive at on dirt roads.

There can be a lot of debate about this, some run a higher pressure and some a lower, but it would be only 4 psi either way.

The vehicles that I run 38 psi on have been the 80 Series and the Troopie.The 80 Series had 32/11.5R/15 BFG Muddies and the Troopie 285/75/16 Cooper STT.

So your can make up your own mind about the tyre pressure you should run or listen to a "expert"

Wayne

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Follow Up By: Member - Bware (Tweed Valley) - Saturday, Mar 25, 2006 at 04:52

Saturday, Mar 25, 2006 at 04:52
Yeah, there's plenty of them around LOL.
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Follow Up By: Gerhardp1 - Saturday, Mar 25, 2006 at 09:34

Saturday, Mar 25, 2006 at 09:34
I was told that an expert was a drip under pressure.......

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Reply By: Member - Blue (VIC) - Friday, Mar 24, 2006 at 17:03

Friday, Mar 24, 2006 at 17:03
I just hit 50,000km on my BFG AT's, still have an average of 9mm tread depth. Shoulders are showing some signs of abuse as I live in a court which has a course aggregate bitumen, even with regular rotation I can't seem to avoid this. I run 40psi all round on a Mitsu Triton.
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Follow Up By: Member - Bware (Tweed Valley) - Saturday, Mar 25, 2006 at 05:22

Saturday, Mar 25, 2006 at 05:22
Thankyou Blue
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Reply By: Exploder - Friday, Mar 24, 2006 at 18:52

Friday, Mar 24, 2006 at 18:52
My tires MAX Cold Inflation temp is 50PSI I run them at 40-44PSI on road.

Off road- well every body is different, but ballpark figures have been discussed many times.
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Follow Up By: Member - Bware (Tweed Valley) - Saturday, Mar 25, 2006 at 05:23

Saturday, Mar 25, 2006 at 05:23
Thanks Exploder
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Reply By: Ruth from Birdsville Caravan Park - Friday, Mar 24, 2006 at 19:53

Friday, Mar 24, 2006 at 19:53
I run skinny tyres on split rims - on highway about 45 psi - in the Desert about 20 psi (mostly less), definitely lower than 45 on the gibber.
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Follow Up By: Member - Bware (Tweed Valley) - Saturday, Mar 25, 2006 at 05:24

Saturday, Mar 25, 2006 at 05:24
Thanks Ruth
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Reply By: Willem - Friday, Mar 24, 2006 at 19:56

Friday, Mar 24, 2006 at 19:56
Because tyres and their use of isn't exact. QED
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Follow Up By: Wayne (NSW) - Friday, Mar 24, 2006 at 21:13

Friday, Mar 24, 2006 at 21:13
Willem,

I think that you have said it all.

Wayne
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Follow Up By: Alloy c/t - Friday, Mar 24, 2006 at 22:42

Friday, Mar 24, 2006 at 22:42
I could claim that I was waiting for someone with some to pickup the mistake !!!! lol.
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Follow Up By: Member - Bware (Tweed Valley) - Saturday, Mar 25, 2006 at 04:50

Saturday, Mar 25, 2006 at 04:50
Well maybe 'exact science' isn't the most appropriate phrase, but there isn't even a rough guide except for the standard 'inflate to manufacturers recommended pressures' which everyone here thinks isn't the best. After all the money on producing amazing tyres you can't get a rough guide like 'these tyres will wear evenly on a large 4wd at around 40psi'. I think it's a bit incongruous with the amount of technical detail that goes into making the tyres. Forget 4wd's and eliminate all the variables they introduce; what about standard tyres on lets say a Corolla. It's like designing the space shuttle then sending the astronauts off and saying "We haven't programmed it to return to Earth, you guys have to work it out yourselves"
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Follow Up By: Willem - Saturday, Mar 25, 2006 at 20:55

Saturday, Mar 25, 2006 at 20:55
Bware

You may be looking for answers which are non existent and which may be improbable. There are too many variables.

My philosophy on tyre usage is this:

I buy whichever brand of tyres. I discuss with the tyre reseller what the best probable pressure is to run on sealed roads on a daily basis. I now run Coopers STT's at 32psi. I drop my pressures by 4psi when driving on gravel roads for any distance. On roughish bush tracks I drop the pressure another 4psi. In sand I opt for 15psi or lower if necessary. I guage that I will get 50,000km use out of the tyres by todays standards. If I get more then I am happy. If I get less, well, I will probably go for another brand next time.

When running splits I run 40psi on sealed roads on the steels and about the same in the rags. I would run 30 psi for gravel roads and rough bush tracks and down to 12 in sand. I would probably run around 15psi on OFFROAD situations and maybe up to 20psi, depending on the terrain

Now there are people on this forum who claim distances travelled on tyres to be in excess of 150,000km. This I find hard to believe but they are adamant that their figures are correct. Many say they get 80-100,000km from their tyres. This may be achievable. It all depends where you drive and how you drive. It also depends how your vehicle performs and how the suspension is set up.

Like I have said and like you have admitted.....there are too many variables....lol

Cheers

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Follow Up By: Member - Bware (Tweed Valley) - Sunday, Mar 26, 2006 at 04:16

Sunday, Mar 26, 2006 at 04:16
Thanks for your answer Willem.
I guess my question is philosophical more than the need for an answer. Let me put it another way; after purchasing new tyres, Joe Average asks the salesman what pressures to run them at and is told 32. Joe looks up his owner's manual and it says 32. Joe has discovered the 4psi rule and tried it and gets 32. After 6 months he notices uneven wear on the shoulder and seeks advice. The owner's manual says wear on the shoulder is caused by under-inflation. The tyre shop says under- inflation. To say that uneven wear is caused by under- inflation is to say that they haven't been run at the correct pressures, but Joe was running them at the pressures 'they' recommended. So we are told to under-inflate our tyres and that it is the correct pressure up until the day we query our worn shoulders, then they let us know that the correct pressures are incorrect! My wife's Kia Rio (don't ever buy one!) has done about 15k and running at the 'recommended' pressures we can already see the wear in the shoulder. The trouble with shoulder wear is that they are dodgey when cornering in the wet. So when you've still got 5mm tread left in the centre, if the shoulders are basically bald, the tyres have to go; perhaps 10-20k sooner than if they wear evenly. But this is recommended. Logically, wouldn't the recommended pressure be roughly half-way between under-inflated and over-inflated, which equals even wearing, rather than slightly uder-inflated for comfort?
This post has nothing to do with the fact that I've just spent $1040 on four rubber circles with holes in the middle LOL
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Reply By: Alloy c/t - Friday, Mar 24, 2006 at 21:07

Friday, Mar 24, 2006 at 21:07
Too many variables ! load/vehicle weight/ road surface /heat/cold ect ect ,get 100 so called tire technicians together and no 2 will agree on correct pressure on any vehicle at any given time or place ,,the 4 psi "rule" is a good starting point for all the points you ponder ,load/wear/comfort ,,all become a trade off ,comfort =more wear , higher load =less comfort can = better wear ,round and round infinitum.
Tires! big black round things that cost heaps and whose inbuilt design factors in constant wear and replacement.
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Follow Up By: Willem - Friday, Mar 24, 2006 at 21:22

Friday, Mar 24, 2006 at 21:22
Intelligence
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Follow Up By: Willem - Friday, Mar 24, 2006 at 22:39

Friday, Mar 24, 2006 at 22:39
LOL
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Follow Up By: Member - Bware (Tweed Valley) - Saturday, Mar 25, 2006 at 05:19

Saturday, Mar 25, 2006 at 05:19
I get the variables and maybe you guys are sick of posts like this, but, I'm saying the 4psi rule maybe isn't a good starting point. It seems to be the same as 'manufacturers recommended pressures' which is designed for comfort not wear. What about the tyre manufacturers? It really couldn't be that hard to give some sort of guide for maximum wear of their tyres based on weight for instance? For example, 'This tyre wears best at approximately 40psi on a 2.5 tonne vehicle, for every .5 tonne above this add 5 psi'. Or even just state that 'running tyres at car manufacturers recommended pressures is designed for comfort, not tyre life and it may cause uneven wear especially on the shoulders'
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Reply By: Wayne (NSW) - Saturday, Mar 25, 2006 at 07:50

Saturday, Mar 25, 2006 at 07:50
Bware,

Tyre manufactures don't do long term test on vehicles in Australia. 99% of tyres are imported and they are made to the country of origin conditions.

Some, after the tyre has been made then bring them into Australia, for further testing. I was given a set of tyres to test and have been along with other drivers reporting back on how the tyres are performing.

The other thing with tyre pressure is that if it was stated by a tyre manufacture that you should run the tyres at Xpsi and something happened. They would be sued at the drop of a hat. That is why they only recommend.

The other thing is how much pressure do you really have in the tyres. How good is your tyre pressure gauge. Unless you pay $300 for a oil filled top of the line unit that has been calibrated then the science of tyre pressure is never going to be exact.

IMHO if you were to run the tyres any where between 36 and 40 psi you will be very close to getting the most out of your tyres.

Wayne

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Follow Up By: Member - Bware (Tweed Valley) - Saturday, Mar 25, 2006 at 12:15

Saturday, Mar 25, 2006 at 12:15
All good points Wayne. I knew litigation had to come into it somewhere LOL. You can't even get a straight answer from doctors at the hospital anymore because they're afraid of litigation through mis-diagnosis. Political correctness and litigation: the gods of modern society.
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