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.

 

[GregLocock]

I think I'd have increased the tire pressure further in that case!

Cheers

Greg Locock

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

 

[MikeHalloran]

The dealer replaced it under warranty, and cautioned me specically about the pressure. I.e., they knew there was a problem. I had no further problems at 31 psi.

When they started making radials in the USA, the tread, aspect ratio, and cord material were different, and the sidewall pressure rating went up a lot.




Mike Halloran
Pembroke Pines, FL, USA

 

[CapriRacer]

Mike / John,

I hope you guys won't take offense if I point something out. I'm merely doing this to illustrate the problem that I am experiencing in trying to discuss this issue with folks who are not technically minded.

Neither of you referenced the placard pressure. (perhaps John did but it was unclear!) Not to mention the vehicle or even the vehicle type.

For example, I have been having a running arguement with a guy who has a pickup about his inflation pressure. The pickup is supposed to use LT metric tires at 60 psi. He has a set of P metric tires (same numbers in the size) that have a sidewall max of 44 psi. He insists that he should be running what is on the sidewall. This conversation gets complicated by the fact that other folks are posting within the conversation about their experience with their passenger car tires - some with 35 psi on the sidewall, some with 51 psi on the sidewall. Not to mention the few folks who are inflating passenger car tires to 60 psi, and suggesting that the pickup guy do the same.

While trying to discuss the proper inflation pressure for passenger cars, the conversation frequently gets tangled up with pickup truck tires.

Identifying the working parameters is important, not only for tires, but for other things as well.

Oh, and Mike: What you have described sounds like a "Run Flat" to me. That is: There was a puncture in the tire and the tire ran without pressure. The result is ultimately 3 pieces - 2 sidewalls and the tread. So apparently you stopped just before that could happen.

The inflation pressures you mentioned not only are so close that for practical purposes they are the same, but that "pressure" had nothing to do with the failure as described - other than to say the cause was lack of pressure.

 

[MikeHalloran]

The sidewall pressure is a limit, set by the tire manufacturer, and their lawyers. I would not recommend exceeding that, in public or in private.

The placard pressure is a recommendation, set by the vehicle manufacturer, based on their own criteria.

Extreme example: Placard pressure for the front tires on a Corvair was 19 psi, which made the car understeer, in a boat-like, run-flat sort of way, with the rear tires at a placard pressure much closer to the sidewall pressure, neither of which I remember right now. More nearly balanced pressures made the car handle much better... and oversteer when provoked. I'm guessing some idiot GM weenie provoked and spun a Corvair.

I have been under the impression that Detroit's placard pressures were traditionally based on providing just enough load capacity to support the vehicle for a reasonable tire life, so as to provide the boat-like ride that seemed to be a design goal. IOW, tire pressure was their preferred tool for controlling NVH. In recent decades, they have adopted NVH tactics developed where the roads are not flat and straight, and raised placard pressures to reduce rolling resistance and provide more precise handling.

( In my particular single data point, the dealer and I examined the tire very closely, and could find no evidence of a puncture in the tread, which was intact and barely worn. There may well have been a puncture in the sidewall, the remains of which comprised a ring of rubber flaps attached only to the bead, and a ring of radial strings, nearly all still attached to bead and tread. I haven't seen a similar failure before or since. )







Mike Halloran
Pembroke Pines, FL, USA

 

[JohnLear]

How could I take offence?
I didn't notice any reference to 'pickup' tyres only, and all I could offeer was my personal experience.

The car is a Honda Accord (double wishbone suspension), placard max recommended psi = 39psi, tyre sidewall max psi with various tyres has been 51psi, 44psi, or 38psi (this last on the current tyres, though it states; "Canada only", so since I'm in Australia I have no qualms about exceeding it...).

My main purpose in using higher psi is to increase steering and handling response with tyres that have very soft sidewalls (very hard to find tyres in this size that don't have very floppy sidewalls). I've found the higher psi makes a huge improvement on sealed surfaces, and some improvement on un-sealed surfaces.

I kept reading and being told that as psi went up I'd gain response at the expense of grip ("what, 45psi, are you mad, you'll have no grip and get punctures all the time...") but I haven't found this to be the case, i.e. both response and grip improved at higher psi, and tyre damage hasn't been an issue.