Ultra High Tire Pressures

[CapriRacer]

I have been conversing (discussing, arguing, what ever you want to call it) about using tire pressures above the maximum written in the sidewall. This is a technique that is being advocated by "Hypermilers" to gain fuel economy.

As a tire engineer, my gut goes all out of wack just thinking about it, but there is very little hard data. I can find studies on rolling resisiance - more is better, but the effect is diminishing - and an old SAE paper from 1980, that shows:

1) Wear rate on radial tires is better

2) Evenness of wear on radial tires is largely unaffected

3) Braking traction is largely unaffected

4) Cornering Coefficient goes up

5) Aligning torque goes down

5) Impact resistance goes down greatly

The only really "bad" thing in this list is the impact resistance - and since impact failures are fairly rare, it's hard to get any good numbers to make the case that inflating tires over the max is dangerous.

What I am looking for is good data on how a vehicle would handle - particularly on a bumpy road - at elevated inflation pressures Anecdotes from racing street tires (road racing and solo) is that more pressure is better. But I have a feeling that because a racetrack is involved, it is fairly smooth and that you might get a different answer if the road surface was bumpy.

Is anyone in a position to do some computer modeling?

Any other thoughts would be appreciated.

 

[patprimmer]

It depends on the cars suspension.

I had a medium soft sprung car. It responded will to tyre pressures around 45 psi for 205 60s on 7" rims, 1989 Aussie Ford Falcon 6 cylinder. The 45 PSI took the roll and squirm out of it. What my girl friend called getting rid of the tall ship syndrome.

I tried the same on my next car. Same class as the Falcon, but an Aussie Holden Commodore V6. It had the Police Special rock hard short travel suspension. I needed to run around 35 psi same tyres and rim size to stop it continually rebounding off the bump stops or to overcome what I called the pogo stick syndrome.

I don't have data, only driving impressions.

Regards

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[GregLocock]

"5) Impact resistance goes down greatly"

Are you sure they didn't mean impact harshness increased gretaly? I'd have thought that square edge pothole etc would be easier to survive (at least for the tire) if it were over inflated.

War story - at Lotus we used to ship the cars at 50 psi to keep them round. I used to drive a fair number of these cars before they were shipped, and never noticed any problems.

I wasn't going for hard laps, but would have been driving far more aggressively than a hypermiler.

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[CapriRacer]

Greg,

No, I meant impact resistance - that is the ability to roll over objects without failure.

BTW, here a link to my synopsis of the paper:

http://www.geocities.com/barrystiretech/sae800087synopsis.html

BTW, when you said you drove some of the Loti (plural of Lotus, ha, ha!), did you drive them on a handling course or what? This is an area where I have a large void of information that is applicable. A bit more detail would help a lot with regard to whether ultra high pressures are as risky as is being portrayed.

 

[GregLocock]

I would only have been driving at 0.3-0.5g at a guess, bending customer cars was not an option, and my reason for driving them was looking for assembly faults and so on, not handling.

However it was a test track that I knew very well and regularly drove rather fast on it, so would not have been hanging about.

That 'dynamic bruising test' is a bit weird. I am not astonished to learn that a 3 inch spike will penetrate a tire's belt. I guess I'm preconditioned by kerb impact and square edge pothole tests, where sidewall is important.


Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[patprimmer]

My experience in off road racing is that higher pressures resist real world impacts (like hitting big rocks). The tyre techs recommended 25 psi. We raced on 45 and got better traction, handling and less tyre damage.

Regards

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[NormPeterson]

It might be useful to know what the maximum sidewall pressures for the tires used in the SAE paper were, as they might not have been as high as what is now commonly available. IOW, running 5 psi above a tire marking of 35 psi MAX on a car where the tire placard lists 28 isn't quite the same situation as being 5 psi above 51 with a 35 psi placard.

A number of autocrossers do use rather high pressures for various reasons. But the practice is far from universal; last year's F-Stock National Champion (a multi-time champ running on DOT "R-compounds") does not.

As far as handling on rough roads at elevated pressures goes - doesn't that make for more stiffness between the road and the unsprung masses without adding commeasurate amounts of damping there?


Norm

 

[CapriRacer]

Norm,

If you read the synopsis, I point out that the rated pressure for the tires in the test was 32 psi. Today's passenger car tires are rated at 35 psi. I do not think what is written on the sidewall of a tire as a maximum is applicable to any discussion in this area for a couple of reasons:

1) What is imprinted on the sidewall is based on a government regulation. There seem to be different interpretations of this regulation - and needless to say, there are different things imprinted on the sidewall.

You said:

".....As far as handling on rough roads at elevated pressures goes - doesn't that make for more stiffness between the road and the unsprung masses without adding commeasurate amounts of damping there?....."

and that was exactly where I wanted to go. I would think this could cause the tire to hop uncontrollably in certain situations. I was hoping that someone could help me prove this to be true - or not. It doesn't matter which, just so long as we get an answer we can rely on.



2) If I remember correctly, it was common practice - based on the tire standards applicable at the time - that the pressure imprinted on the sidewall WAS the rated pressure. (I will have to look up the tire standard to be sure that it reads differently than today's standard.)

 

[patprimmer]

I should add:-

See my post 5th July re pressure, suspension and squirm vs hop on two road cars.

On our race car, we tried the tyre techs recommended pressures, but kept damaging rims and tyres. We increased pressure until the damage pretty well stopped. We got no noticeable adverse side effects. some hop or uncontrolled rebound might not be noticeable in an off road racing situation as there is so much rebound and bouncing going on anyway. We won more races at the higher pressure appart from the influence of much higher finish rate.

Regards

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[NormPeterson]

There is an individual on an enthusiast forum who is complaining of the car "pogo-ing" badly enough during moderately hard throttle over some sort of bumps (not described) such that he cannot seem to maintain a steady throttle, which aggravates the situation. Damper adjustments help, but don't eliminate the problem. On the chance that he's using sidewall numbers for inflation pressure, I'm trying to find out more information without giving away the real reason for asking. Stick axle car, if that matters.


Norm.

 

[patprimmer]

Travelling on corrugated roads or washboard roads can hit the natural frequency of the springs or whatever and cause a pogo stick action.

If it's a leaf spring, they can wind up due to torque reaction from the wheels and due to longitudinal forces transferred from the axle to the chassis via the spring.

Regards

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[carnage1]

If you feel the need to run a higher pressure tire why not get one that is rated to handle the inflation pressure you are after? the passenger car tires on my car are rated for 45 psi, and the tires on my pickup are rated at 60 psi.
I run my car at 35 and my truck from 18 to 60 depending on what im doing.

 

[GregLocock]

I have a strong suspicion that a truck tire at 60 psi has much less grip than the same size passenger car tire at the same pressure

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[patprimmer]

Also probably makes more noise and is higher profile, thus reducing some of the gains from increased pressure and changing effective gear ratio.

Regards

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Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[patprimmer]

Also, I am normally working with the tyres supplied with the car.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[NormPeterson]

carnage - in both the cases of the hypermiler and the autocrosser/open-track driver who is running on pure street tires, the intent is to push the compromises in the direction most favorable to that activity's performance goals. Other considerations become secondary at best (and some of the other hypermiling tricks used by some range between unsafe and downright scary).

As Pat has just noted, it's frequently a consequence of being "stuck with" the tires either originally supplied with the vehicle or installed as replacements prior to engaging in the activity in question. Many of the cars favored by hypermilers are already fitted with low rolling resistance tires, and most autocrossers are neither serious enough at first nor experienced enough to take full and proper advantage of competition rubber beyond about the level of so-called "Street Touring" category 140-treadwear rated tires.


Norm

 

[CapriRacer]

Let me clarify a bit:

My real concern is the fact that many hypermilers are ignoring the vehicle placard - to the point of pretending it doesn't exist - and using the sidewall max pressure - and sometimes well beyond.

I mention this because many vehicles come with 26 psi as the placard pressure and some tires have a max pressure of 51 psi. Doubling the placard pressure doesn't sound like a good idea.

But more importantly, these guys are so single minded about the idea of achieving max MPG, that their sense of reality has become distorted. I have watched as they picked apart a set of color tire footprints that show pressure distribution and incorrectly conclude that 2 of them were the same size - they weren't.

But I would like to be able to show something concrete about what happens when you inflate a tire uop to the max and beyond.

Thanks in advance.

 

[carnage1]

I can tell you from offroading that one thing can happen is that they explode. It is not uncommon to air down your tires depending on the type of mud your in. you may then wish to reinflate them. I have known people who overinflate there spare tire to 90 to 120 psi, then use a tube to transfer air from it to their other tires to reinflate them to a reasonable pressure. This is 1.5 to 3x the rated pressure and they have popped. (yes I realize this practice is pretty dumb with how cheap little 12v air compressors are now, and I also realize my redneck is showing.)
As for using the sidewall max pressure instead of the placard, it would change the shape of your contact patch most likely reducing traction and increasing tire wear a bit.
Also while researching tires this year I discovered references to too high of an air pressure increasing rolling resistance. the effect was caused by the vehicle having to absorb more up and down motion because the tires were unable to simply flex over the surface irregularities. As I recall this pressure seemed to be much higher than what our range would be so we did not concern ourselves with it.

 

[JohnLear]

I have no hard evidence to offer, but from personal experience I haven't (yet) encountered any problems with tyre damage / longevity running my tyres on quite rough dirt roads at around 45psi (which is both below and above the sidewall rating of the various tyres I've used). Sure there's an occasional puncture, but no more often (about 1 per annum on average) than when I was using much lower psi (around 32psi). Harshness is of course greater with the higher psi.

I don't drive particularly quickly on corrogated roads out of respect for the car's longevity (no rally 'practice'), but handling seems at least a bit better on rough roads with the higher psi, though loss of grip is somewhat more abrupt with less warning. The suspension is more or less stock save for Koni dampers set fairly stiffly, which might account for the lack of significant wheel hop with the higher psi?

In response to something patprimmer said about stiff springs seeming to work better with lower tyre psi and softer springs working better with higher psi; just an observation but:

In karting the stickier / softer the tyres are the stiffer the chassis typically needs to be, and stiffer chassis seem to work a lot better with lower tyre psi, whereas harder tyes require more flexible chassis, and more flexible chassis seem to work better with higher psi. Not sure how informative that is, but it probably suggests something...

My first post here, a little nervous daring to post in such obviously knowledgable company...

 

[MikeHalloran]

Back when there was only one brand of radial tire, I put a set on a big ol' heavy 'Merican car, and used my accustomed tire pressure of 35 psi. Unfortunately, the sidewall pressure of those tires was 32 psi, and within a month, one of them protested by blowing out. One entire sidewall, all at once. Quite spectacular. Luckily, it happened on a straight and level road, not at great speed.



Mike Halloran
Pembroke Pines, FL, USA