Tyre pressures set above manufacturers recommendations

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BarryC

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Fellow Frogger
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Having just come back after a Tyre repair (nail), my ever so helpful Tyre repair man said that my tyres looked a bit under inflated, and were wearing unevenly as a consequence. I’ve always used the car recommendations as per door pillar figures. He recommended that I use 40 psi rather than 32. Any comments. I’m no Einstein, and I believe, neither is he.
 
Manufacturers tyre pressures are decided after many hundreds of thousands of miles of road testing on all kinds of surfaces, worldwide. All manufacturers specify higher pressures for constant high speed running but their standard suburban pressures are designed to give your tyres long life and your ride some comfort. The difference in ride on bitumen between under-inflated and correctly inflated tyres is really noticeable, and if under-inflated for long enough will shorten your tyres life by an expensive amount. If you drive like a demon, I'd be using the manufacturers high speed recommendations, they will probably stop your tyres howling too much and give you oversteer or understeer depending on your throttle position, but will wear the crowns of your tyres out first.........
 
A key consideration for most manufacturers is ride comfort, thus lowish pressures. This can mean that tyres tend to wear loaded outside edges more than they otherwise would (especially on front tyres & especially if the front/rear tyre balance is, as is usual, set up to promote safer-for-most-drivers understeer).

I would accept the 40 psi replacing the OE 32 unless you find it objectionably uncomfortable. It you do, adjust down in 2 psi increments. I'd also recommend changing the front/rear balance to lessen the understeer. Say, 40F & 38R replacing 40/40. I say this because many standard placard settings bias the car's balance towards understeer in a way that is counterproductive for any driver with some feel for the car's dynamic behaviour. But again, suck it & see; you can always change it back. A good test of things is lifting off on a wet roundabout & seeing if you are happy with the vehicle's behaviour. I can't imagine that a 2psi reduction at the rear will make the tail wayward in such a manoeuvre but, again, explore & adjust.

cheers! Peter
 
Thanks for your measured reply. In my driving of the car since the change, I really haven’t noticed any increased hardness. Handling changes will need a greater amount of time for me to quantify. I’ll keep them at 40 as you suggest, and then maybe run another set at 38 or so..

Cheers
 
In my experience, I find it easier to trigger the ABS during heavy braking on roads which are both cold and wet when I inflate the tyres significantly above the stated pressures on the tyre placard (with no corresponding increase in vehicle load), but I also feel that the car aquaplanes less when there is much standing water on the road with the higher tyre inflation pressures - go figure.

In terms of handling on dry roads though, I rather like how the car feels when cornering in a, er... brisk manner. Certainly aids in preserving the outer edges of the tyre tread, that's for sure.

As for ride comfort, I find it's a mixed bag. Some cars respond well to an increase (where there is an unexpected improvement in the secondary ride, which I find counter-intuitive) while in other cars the effect is as expected. I suppose it depends on how the tyre spring rate (inflation pressure), as well as the tyre itself, interacts with the suspension system as a whole. Not being a vehicle dynamics engineer, it's a black art to me.
 
Could anyone be harassed by our legal system for advocating tyre pressures different to those recommended by the manufacturer? Supposing a vehicle was involved in a fatal accident, a contributing factor being determined as incorrect tyre pressures, and subsequently the estate discovered that the tyre pressures had been consistently different to the car manufacturers specifications as had been recommended by a well known and established web forum and those values had been used by the deceased before their demise, where would that put the purveyors of such information?
 
BarryC said:
Having just come back after a Tyre repair (nail), my ever so helpful Tyre repair man said that my tyres looked a bit under inflated, and were wearing unevenly as a consequence. I’ve always used the car recommendations as per door pillar figures. He recommended that I use 40 psi rather than 32. Any comments. I’m no Einstein, and I believe, neither is he.
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My personal theory is:

- given that the inflation pressure in a tyre can naturally decrease by 1 psi per month, since the structure of a tyre is permeable (a process known as diffusion), and

- given that many motorists check their tyre pressures less than once per month, and

- given that underinflation causes tyre wear to increase exponentially, leading to consumer complaints that the tyre dealer would rather not deal with,

a tyre fitter would consider it prudent to overinflate the tyres, since a tyre with an initial P of 40 psi and a final P of 34 psi after 6 months is better than a tyre with an initial P of 32 psi and a final P of 26 psi after 6 months - at least, for those who do not check their tyre pressure once a month - which, as far as the tyre dealer is concerned, is pretty much everyone.

Kim Luck said:
Could anyone be harassed by our legal system for advocating tyre pressures different to those recommended by the manufacturer? Supposing a vehicle was involved in a fatal accident, a contributing factor being determined as incorrect tyre pressures, and subsequently the estate discovered that the tyre pressures had been consistently different to the car manufacturers specifications as had been recommended by a well known and established web forum and those values had been used by the deceased before their demise, where would that put the purveyors of such information?
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I'm not a lawyer, but in my opinion:

I would think responsibility would remain with the driver, as he or she is not compelled to follow the advice of strangers on the internet.

(As an aside, I wonder if forums can be covered by public liability insurance?)

So what constitutes a vehicle that is acceptably safe to operate in the eyes of the law? Perhaps one that meets the roadworthy regulations.

If at the time of accident the tyres had less than 1.5 mm of tread at any point, or had a load index not more than what is stated on the placard, or was fitted with tyres of a different construction on the same axle (e.g. mixing bias-ply and radial tyres), then these are clear breaches of the roadworthy regulations.

However, deviating from the tyre inflation pressure stated on the tyre placard does not constitute a breach of the roadworthy regulations, if you take a black letter (literal) approach to law.

Another example would be the fitment of non-matching tyres. Not ideal, but does not breach the roadworthy regulations.

So, I'd argue the vehicle was roadworthy at the time, but the driver did not drive to the conditions of the road, nor the vehicle itself, and if the driver made the decision to deviate from the specified tyre inflation pressure, the driver should have been aware that it would potentially affect the roadholding of the vehicle - not that the driver was obliged to follow advice from strangers on the internet in the first place - and should thus be held wholly responsible for his or her actions, rather than the forum.

Even if it is found that a deviation from the stated tyre inflation pressure was a contributing factor to the accident, again, I think the responsibility lies with the person who altered the inflation pressure (wonder what the tyre dealers would think of this). but again, I'm no lawyer.

Oh, and before I forget, the material on the website is provided for general information and educative purposes only. The content does not constitute legal advice or recommendations and should not be relied upon as such. Appropriate legal advice should be obtained in actual situations. While every care has been taken in the preparation of this post, I do not accept responsibility for any errors, including those caused by negligence. I make no statements, representations or warranties about the accuracy or completeness of the information and you should not rely on it. You are advised to make your own independent inquiries regarding the accuracy of any information provided in this post. :D
 
Diesel_vert said:
... (As an aside, I wonder if forums can be covered by public liability insurance?) ....
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I reckon most fora would just close down in a heartbeat before their owners would cough up many thousands for a policy that would probably have more holes than a swiss cheese.

If it came to it, you could always find an expert witness such as John Cadogan to run the overinflation is better argument.
 
David S said:
I reckon most fora would just close down in a heartbeat before their owners would cough up many thousands for a policy that would probably have more holes than a swiss cheese.

If it came to it, you could always find an expert witness such as John Cadogan to run the overinflation is better argument.
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Or several university professors or industry experts that might suggest that overinflation was bad practice...............Some of us might ask ourselves why car manufacturers actually placard tyre pressures? From my aviation experience recommended tyre pressures on your local Boeing or Airbus are fastidiously adhered to, but I guess a pilot could ask for 20% more pressure if he wanted it......:rolleyes:
 
Then it may be a question of who has the most credible experts!

Aircraft tyres are probably quite different because of the change in altitude and the fact that you aren't concerned about ride quality given by tyres on an aircraft, only whether they let the plane take off and land in all weathers.
 
Having seen a few aviation incidents first hand I can report that a tyre that deflates after takeoff or on a heavy landing is considered to be a serious problem. Likewise an overinflated tyre that goes pop on landing at a hot airport. Fortunately I only ever saw these instances from the sides of the runway, never onboard the aircraft involved, so the actual tyre pressures are in fact critical for the aircraft to be able to operate throughout their global flight envelope.

I have ridden as navigator in a rally car on dirt that had so much tyre pressure (over 42 PSI) that my teeth rattled and my glass eye nearly fell out. After several sections I suggested we reduce the tyre pressures to about the same as the recommended high speed pressures and all of a sudden our section times started to improve to the point where we were cleaning them. And I could see out of my glass eye again...........
 
All the advanced courses I've done recommended just what your tyre guy said.
It does make for a harder ride but makes your braking a lot more efficient.


Sent from my iPhone using aussiefrogs
 
87205gti said:
All the advanced courses I've done recommended just what your tyre guy said.
It does make for a harder ride but makes your braking a lot more efficient.


Sent from my iPhone using aussiefrogs
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That's fine, if you can quantify the supposed benefits with real world examples, like shorter braking distances. What tyres, specifically, are reputed to deliver better braking when over-inflated? What surfaces, and surface moisture combine with these magic over-inflation pressures (which are what?) to deliver such benefits? What level of humidity and ambient temperature was present in all the tests that were conducted to support this theory? I've had dealings with experts in all kinds of matters in my lifetime including those that make money from teaching people how to drive cars and most of them haven't lived as long or as dangerously as I!
 
Wife's Corolla manufacturer spec is 2.2 bar, which is almost 35 psi, so quite close to figures suggested here. On Bridgestones RE003 it handles quite well for a Corolla (auto) but you have to drive it.
 
2.2 bar is 32psi
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2.2 bar, which is almost 35 psi
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I'd suggest within 10% is reasonably classified as "almost" :wink2:

After all, what kind of accuracy would you expect from a servo tyre pressure gauge that's been chucked on the concrete all it's life.

Expecting 10% accuracy is likely dreaming.
 
My mistake, a little bit low. I will check the tyres again. I usually check the pressure with my own gauges (I have two different makes/specs), and servo pressures are pretty accurate against them. Now, I am not sure how good my gauges are, but if they all agree, I would say let's trust them.
 
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