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KrazyDawg 03-20-2008 08:39 AM

Tire Pressure Theory
 
I've been doing some reading on using a higher PSI than the manufacturer recommends. It seems the results are the opposite of what people are experiencing. Instead of wearing out the middle and experiencing "irregular" tread wear, people have reported no unusual wear and even wearing of the tires. Also, blowouts appear to occur for under inflated tires rather than overinflated ones when tire rack and other sites state that overinflated tires were more prone to damage from potholes and other debris.

My theory is that overinflated tires are not experiencing irregular wear due to the driver's hypermiling techniques e.g. driving without brakes and coasting in neutral. If an "aggressive" driver were to drive up to a red light and brake with properly inflated tires and drove in the same manner on overinflated tires, wouldn't he be experiencing irregular wear.

Gary Palmer 03-20-2008 08:57 AM

I agree with your primary assessments. On blowouts, I have not had any issues from running at 55 psi. On wear, I've seen minor wear on the middle of the tires, but the wear tends to compensate for the wear from turning corners with the front wheel geometry.

My position is that higher inflation seems to be helping a lot more than it's hurting, for me. If someone wants to try it, they are welcome to, and I don't think they will experience a problem.

But, if someone wants me to guarantee they can do something, forget it, they can do their own exeriment, or not gain the benefits.

GasSavers_RoadWarrior 03-20-2008 08:57 AM

IMO sidewall pressure is maximum recommended cold pressure for tires to survive being driven at 110% of rated speed for 4 hours straight on a 40C day on a harsh road. So you can push it a bit if you intend driving at the speed limit and your temperatures aren't so extreme.

On word of warning though, tires do stretch a bit when you go overpressure on them, it happens gradually though. So you'll put 50 psi in a 35 psi tire and it will be only 45 a couple of days later... then you top it up to 50 again... then maybe it's 47 next time... eventually it will hold 50psi fairly constantly, BUT the tire is now stretched and may not run right at standard pressures again. Also there is potential that you are nearing the elastic limit of the tire and that trying for 60 might cause it to burst catastrophically.

Soooooo... sidewall pressure is the safe cold pressure under all conditions, but messing with that makes you responsible for evaluating conditions.

jcp123 03-20-2008 09:31 AM

Especially with low-profile tires, a lot of the guys in the Mustang forums complain that they get very fast and uneven tire wear when running the factory pressure, so some of them are even upping their pressures to 40+psi for daily driving, too...

bobc455 03-20-2008 09:37 AM

Quote:

Originally Posted by KrazyDawg (Post 94114)
Instead of wearing out the middle and experiencing "irregular" tread wear, people have reported no unusual wear and even wearing of the tires.

For about the past 1-1/2 years, I work Saturdays at a tire shop. Mostly for fun & stress relief, the money isn't all that.

Anyhow, in the entire time, I have never noticed a set of tires that wore more in the middle than on the edges. Every tire wear pattern favors the edges.

As far as I can tell, the only downside of over-inflation is a harsher ride. I'm sure that "racier" drivers might notice a bit of traction loss also, but I don't roll like that.

-Bob C.

GasSavers_ALS 03-20-2008 10:33 AM

A simple formula that seems to work for wear and gas mileage is run the tires at 85% to 90% of the max pressure on the tire sidewall.
If your tire sidewall says max pressure 44 psi as mine do you can safely run 37to 40 psi.
Around town I run 37-38 and when I take long highway trips I up the pressure to 40 psi. I have seen almost no wear problems.
Factory sticker says 36 psi all around.

GasSavers_Erik 03-20-2008 12:42 PM

Bob C. You could do an experiment- when a customer comes in with worn out tires, put one in some sort of metal cage (do you all still have those for doing split rim truck tires?) and hook it up to a compressor. Get a safe distance back, turn up the pressure regulator a little at a time and let us know at what pressure it explodes.

That will give us an idea of a tire's no-load "burst" pressure.

But I'm guessing your boss will probably not be too happy with the big BOOM! coming from the shop area.

fowljesse 03-20-2008 01:40 PM

I have almost always run at the max sidewall pressure, if not more. My cars are made for cornering, and one is modified for racing. I take corners pretty fast, usually at cruising speed, and rarely use my brakes compared to most drivers. I've changed 8 tires so far, mostly due to blowouts, and damage from nails, etc... The tires that I've changed because of wear were slightly more worn in the middle, but not significantly. I've seen no loss in traction, but have never seen the skidpad limits of these cars. A couple of times, I had to manuever to not get sideswiped by fools not paying attention, and gripped fine. Braking is fine, too.
I recommend filling your tires at max sidewall, and going to a huge parking lot to test the limits. Maybe put up cones, or something, and testing the car max, and sidewall max.

GasSavers_RoadWarrior 03-20-2008 01:53 PM

Everything I've driven in the last few years, actually handle better on max sidewall than manufacturer recommended pressure. Done a few emergency lane hops, avoiding ppl trying to pull out into me on 'em and been glad I had them at higher pressure, the quarter second extra response time on a stodgy tire is 22 feet at 60mph, and I've missed the idiots by inches.

tweakmenow 03-20-2008 02:28 PM

Even wear at 45 psi.
 
Here are two pics of Kumho KR21 tires with 2k miles on them. They are on the rear of my Honda. I have run them at 45 psi since day one. The molding bumps are a "superb" wear indicator. :thumbup: https://mysite.verizon.net/tweakmenow/inside.jpg
https://mysite.verizon.net/tweakmenow/outside.jpg

1993CivicVX 03-20-2008 03:30 PM

Quote:

Originally Posted by RoadWarrior (Post 94118)
IMO sidewall pressure is maximum recommended cold pressure for tires to survive being driven at 110% of rated speed for 4 hours straight on a 40C day on a harsh road. So you can push it a bit if you intend driving at the speed limit and your temperatures aren't so extreme.

On word of warning though, tires do stretch a bit when you go overpressure on them, it happens gradually though. So you'll put 50 psi in a 35 psi tire and it will be only 45 a couple of days later... then you top it up to 50 again... then maybe it's 47 next time... eventually it will hold 50psi fairly constantly, BUT the tire is now stretched and may not run right at standard pressures again. Also there is potential that you are nearing the elastic limit of the tire and that trying for 60 might cause it to burst catastrophically.

Soooooo... sidewall pressure is the safe cold pressure under all conditions, but messing with that makes you responsible for evaluating conditions.

When I stopped by a tire shop I mentioned in another thread, the guy said you don't have to worry about tire bursting until 160 or 170PSI -- Tom and Ray said it won't burst until something like 200PSI. The other thing the tire shop guy's partner said was that the max sidewall PSI number on tires is NOT the PSI for the tire, but rather information pertaining to something else involved in mounting the tires, and that the actual Max sidewall pressure for the tire is less than whatever the number is on the side of the tire. She said usually it's around 30-32PSI! I have no idea if these people know what they are talking about, but they spoke as if they knew very well, for what that's worth. Nevertheless, I so far do not feel riding at 55PSI has any averse effects except maybe worse traction in wet conditions, but that may have just been an isolated incident of slipping on wet leaves and nothing to do with overinflated tires. The tire shop guy said my tires were in good shape and would replace just one tire with a new one (indicating the tires were not worn enough to justify replacing both) and they have about 10,000 miles or so of ~50+ PSI.

JanGeo 03-21-2008 07:56 AM

Wait a second - the tire print says "maximum cold inflation pressure 44 psi" (or words to that effect). Now they wouldn't say inflation and cold if it was not ment to indicate while operating on the vehicle just like the maximum load weight (at the max pressure).

1993CivicVX 03-21-2008 08:28 AM

Quote:

Originally Posted by JanGeo (Post 94211)
Wait a second - the tire print says "maximum cold inflation pressure 44 psi" (or words to that effect). Now they wouldn't say inflation and cold if it was not ment to indicate while operating on the vehicle just like the maximum load weight (at the max pressure).

I have no idea, just passing along what the tire folk were saying.

GasSavers_RoadWarrior 03-21-2008 11:43 AM

well I'd imagine they're right about them taking 100 or so PSI... because they have to take 30-40ish plus survive knocks. I don't know what the hell they were on about for the max sidewall pressure though.

by the way, was on a long summer highway trip once, pulled off for food, checked the tire pressures, read 50 psi, when they were inflated to 36 psi cold, nearly went and let pressure out..doh!.. after we had eaten .. checked them again and they were down to 40..

GasSavers_Ryland 03-21-2008 12:18 PM

My car owners manual, and my motorcycle manual both recommend increasing the tire pressure for any kind of high speed, or extended distance travel, a 4-5psi increase seems to be most common.
I have never had any issue at all with running 50psi, dead even tire wear, awesome traction in snow and rain, I replaced a pair last fall after running over a bunch of hard ware that was on the road poking a 3 holes, two nail holes and a good quarter inch hole thru my tire at 50psi, plugged it and added some sealant and drove another 2,000 miles with it again at 50psi before getting the tire replaced.

GasSavers_Otto 03-21-2008 01:10 PM

Every time you hit a bump or pothole, the tire pressure spikes. Overinflation may bring the tire somewhat closer to its bursting pressure, but if that were really a problem, we'd surely have heard about that here.

That said, tires can burst from overheating, in extreme cases. Hard braking makes heat that is transferred to the tires. Surely more a problem for jet aircraft and racing cars than economy cars.

fowljesse 03-21-2008 08:15 PM

The only problem I ever had with this is when my girlfriend gave me a Mazda 323 when I lived in Hawai'i. The car was in okay shape, and ran great. I have no idea how old the tires were, but within weeks of overinflating the tires to less than sidewall max, 3 of them seperated (I mean the tread form the rest). You may ask why I didn't deflate them to norm... I wanted to replace them, anyway, and did experiments with different pressure.

VetteOwner 03-22-2008 12:20 AM

Quote:

Originally Posted by Otto (Post 94249)
Every time you hit a bump or pothole, the tire pressure spikes. Overinflation may bring the tire somewhat closer to its bursting pressure, but if that were really a problem, we'd surely have heard about that here.

That said, tires can burst from overheating, in extreme cases. Hard braking makes heat that is transferred to the tires. Surely more a problem for jet aircraft and racing cars than economy cars.

lol well yes i can pertain to that, you know you have a dragging brake when your able to get a 3rd degre burn from just touching the metal hubcap....

but yes, tires are the things that absorb most of the bumps in the roads, the shocks just keep the tires on the ground and to keep the car from boucing up and down after a bump.

now if you think about it for about 12 micro seconds if thje tires are stiffer than factory now, then the energy absorbtion is now beign doen by the ball joints and cv axels and whatnot, which isnt what their designed to do and COULD wear out prematurely if the car has cappy ball joints, etc. but yes usualyl factory specs are low to make the ride seem comfy so id bump em up a bit but not too crazy


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