Over intlated tires - how much do you do?

[UFO]

^^^
For me it's simple. I compromise right up to the point where the tire manufacturer is still liable.

[womprat]

I think it depends on the tyre. Tyres design for low RR will probably not gain from extra pressure.

I get an unscientific 6% with 32psi -> 40 psi (41 max sidewall) with Bridgestone sport tourers. These tires are grippy, and perhaps sacrifice rolling resistance for performance, so respond well to more pressure?

The 6% seems to be consistent average any way I do the numbers accross all tanks since I made the change to high pressure. I have a pretty consistent commute, and exclude tanks that include long road trips or highway driving. I ran the manurfacturers pressure for about 10 tanks first. I do lots of low speed coast downs where RR is going to be a big factor.

I find running higher pressure improves steering response and feel - which I prefer. I hate cars that don't feel like the steering is connected to anything, and positioning the car on the road is guesswork - pumping up the pressure will improve your fine control of the car and possibly save your bacon if you have to take evasive action. It's a bit harsh on rough surface and scary on gravel/dirt. :S

[endurance]

In re-reading my own writing I'm re-thinking my own logic... some. My last accident was in 2005. I was driving at 70 mph (65psl) in my 2002 Civic EX that had second season snow tires on. Some bonehead started merging without looking in his/her mirror and coming into my lane, so I started to swerve into the lane next to me, did a quick peek over my shoulder and saw someone coming at about 90mph in that lane only about 50 yards back, I corrected to hold my lane, the car lost traction, I started to spin and the next thing I know I'm sliding backward at 60+mph into the guardrail, bouncing off, into traffic and get broadsided at 50+mph.

In any case, besides the longest run-on sentence in Ecomodder history, I don't know what PSI those tires were at, what brand they were, or anything, but I know I don't want to ever have a car break free that easily again on perfectly dry roads. I'm going to start playing with tire pressure again to find a balance that makes the car handle better, if possible, on dry roads. Snow tires suck on dry roads, but summer tires suck on 3" of hard pack or 10" of powder. Everything has its trade offs and I need to re-evaluate my tire pressure to get the best handling I can get, whether that's 32, 35, 40 or 44psi. I'd suspect in a realistic CBA, two or three mpg is cheaper than my deductable...

[JacobAziza]

Originally Posted by UFO

There is safety margin from the sidewall pressure, but it varies with manufacturer, manufacturing process and tire to tire. I don't know where you get your confidence in that much pressure over the limit, but I do not share it; I think you are being reckless. Where did you get the burst data? Certainly NOT from the manufacturer, which would be the one source you might be able to trust.

"over-inflation almost never causes tire failure. The standard tire is inflated to about 30 to 35 pounds per square inch. Under hot weather and highway conditions, the temperature of the air inside the tire rises about 50 degrees. That increases the pressure inside the tire about 5 psi. The burst pressure of a tire is about 200 psi."

Car Talk

"Most new or undamaged medium radial truck tires can withstand three to four times the recommended pressure before bursting. In fact, the rim usually fails before the tire. ITRA has conducted several burst strength tests on new and used medium radial truck tires with special reinforced rims. The tires were pressurized with water. Most recently, three new tires and 13 used tires were "burst tested" with the lowest pressure burst recorded at 300psi and highest at 540psi for an average of 420psi"

At what point will a truck tire burst from overinflation?

I assumed the low "max" pressures were about liability, but it turns out manufacturers don't even set those numbers. They have nothing at all to do with tests of individual tires or what the manufacturer thinks they should be set to. The Tire and Rim association sets the numbers, universally, for a given size tire and weight range, independent of the individual tires construction

Tire Tech Information - Tire Specs Explained

[euromodder]

Originally Posted by womprat

I think it depends on the tyre. Tyres design for low RR will probably not gain from extra pressure.

They do.
When coasting I can feel the greater resistance when they are at 36psi rather than at 43 psi or above.

[ChazInMT]

I had my new Bridgestone Grid 019s inflated to 38 PSI from 30 Recommended on the door, when I went to get the tires balanced after 5000 miles, the yutz at Costco made a note on my paper work that the tires were "Overinflated by 8PSI" with no reference to whether it was based on the door sticker 30PSI or Sidewall 40PSI. I should sue!

[CapriRacer]

Originally Posted by cfg83

............(CapriRacer, sorry in advance if I am misquoting you)

I agree. CapriRacer commented on Max PSI. I believe that he said something akin to "as long as you drive the way we are driving, sidewall max PSI is ok, but inflation above Max tire PSI is not ok". ...........

Sorry, but that isn't quite right. I hope I am not contradicting myself (I didn't peruse the entire posting), but tire manufacturers do not test above what is known as the "rated" pressure, which for passenger car tires is 35 psi. What is written on the sidewall is the maximum allowable pressure - and the word "allowable" has some special meaning in this context. It assumes that the tires are going to be used in a high speed situation - like the German Autobahn - where the road is designed for such usage - a smooth, debris free, no pot holes, etc surface, where the additional pressure is used to reduced the heat buildup in the tire.

Also, ALL vehicle testing is done at the vehicle tire placard pressure.

Further, while it is true that NEW passenger car tires have bursting pressures over 200 psi, this is a one time event. If you read up on fatigue resistance, you'll find that if something is loaded repeatedly, like rolling the tire with a load, the fatigue resistance drops dramatically. In the range of value normal for tires - 106 cycles - the fatigue resistance is 20 to 25% of the one time value.

Further, I have seen some old tires that burst at 35 psi. Admittedly there were 20+ years old, but the issue is that they deteriorated over time.

But let's look at the specs for the EV1:

Originally Posted by cfg83

............GENERAL MOTORS EV1 VEHICLE SPECIFICATIONS
http://avt.inel.gov/pdf/fsev/eva/genmot.pdf

The tires are this vehicle were NOT standard passenger car tires. They were specially designed for this application knowing that the placard specification was for 50 psi. In other words, the tires were specially designed to be able to hold that kind of pressure.

[tim3058]

UFO et al, I doubt anything happens right at the tire max rating - as someone said, its just the point the mfr wishes to limit their liability. Overinflating a tire by 12% (44~50psi) would be well under the FS JacobAziza quotes in his post (3 to 4x rated pressure). If the mfr rated the tire for its actual bursting/unsafe pressure, then it'd be on the verge of a lawsuit with every tire. I definitely wouldn't be doubling the rated pressure though, as Capri said the tire gets weaker, does the fatigue resistance relate directly to pressure?? (would it be 25% of 200psi, or 75% or ???)

[brucey]

My tires since new have been ran at max sidewall pressure. Very even wear (actually none to speak of) and no blow outs of any kind. Very predictable handling too. A bit bumper, but nothing to complain about.

[JacobAziza]

Originally Posted by CapriRacer

Further, while it is true that NEW passenger car tires have bursting pressures over 200 psi, this is a one time event. If you read up on fatigue resistance, you'll find that if something is loaded repeatedly, like rolling the tire with a load, the fatigue resistance drops dramatically. In the range of value normal for tires - 106 cycles - the fatigue resistance is 20 to 25% of the one time value.

The The International Tire and Rubber Association burst tests I linked to that found a minimum burst pressure of over 300psi was (mostly) of used tires, not new ones.

I'd agree that running at, or even remotely close to that for extended periods would be inviting trouble, but, again, the "max" recommendations on the sidewall have literally nothing to do with the individual tires capacity.

The number is set not by the manufacturer or car maker, but by an independent organization that has permanent set standards based on tire size only.

The number really seems to be based more on tradition than any test or safety concern.
It was set once, by law, and never altered (as far as I can gather from the DOT website) since 1948.
Manufacturers have to run speed and durability tests on new tires, but the pressures they run them in are taken as a given.
I can't find any thing explaining what, if anything, the 'max' pressure ratings were originally based on - but it is clearly not anything to do with an individual tire's actual burst pressure, strength, or performance (since, as people here have pointed out, they aren't even tested at any higher pressure to see whether they preform better or last as long or are as strong)

As far as the car's "recommended" pressure on the door jam, as Ralph Nader pointed out back in 1965, that number is based on comfort for passengers, at the expense of safety, efficiency, handling, tire life, etc.

Can anyone find any concrete evidence that running at (or even above) the max rated pressure has any drawback other than a harsh ride and possibly less puncture resistance?