Over intlated tires - how much do you do?

[Piwoslaw]

Originally Posted by slowmover

...This is funny because I acknowledge that over-inflating tires will cause longer braking distances, ...

Not necessarily. Check out the links in this thread:
"Inflation pressure does not affect grip": Autospeed article.
In particular, FEA chapter III. tire pressure survey and test results shows that increasing tire pressure decreases braking distance in most of the tested cases. I'm not sure how far this extrapolates, though.

[JacobAziza]

aw man, you beat me!
I was JUST about to post that link.

Yeah, well... I'm going to do it anyway!!

http://ecomodder.com/forum/showthrea...cle-11433.html

[slowmover]

Where's the evidence that overinflation is beneficial? In forty years of reading about cars (and nearly that in owning them) I've yet to come across that benefit quantified.

Debunking a Mileage Myth: Can You Really "Pump Up" Your Fuel Economy? - Popular Mechanics

Is a typical response. It "may" be that some slight percentage pressure increase results in better handling/braking, but on new cars/trucks this is doubtful compared to the vehicles of the 60's and 70's. CAFE standards would seem to make a pressure increase an easy way for the automakers to achieve better numbers.

The problem is tire design & construction: tire tread and tire casing. Long life and higher fuel mileage offset by ride quality, traction, etc. These mean far more to fuel mileage -- by itself -- than ultimate pressure numbers.

In this linked .pdf from Cummins about big truck fuel economy, check the table of contents for the pages on tires. You'll see that to achieve even a 2% increse that the tires are over-inflated by 20% from optimum. The companies I drove for (or the trucks I leased) had company-mandated tire pressures ranging from 100 to 105 psi, sometimes 110, depending on truck, load, position and other. We were advised to never exceed this. And fuel economy was constantly stressed. (And it takes a good half-hour or longer to inflate 18-tires, I can tell you . . and you NEVER have a spare half hour as a truck driver . . . .

http://cumminsengines.com/assets/pdf...whitepaper.pdf

Let's say that the 32-psi tires on a car are really a bit better with some more air. We're willing to give up ride quality. But we'll have to go to 20% over, or 40-psi to achieve it. A 2% gain? On a 22 mpg car that's barely .5 mpg. And it assumes that the vehicle will operate in a steady state environment that the tires warm to operating temperature after 1.5-hours of driving in order to achieve it. A quite narrow, non-realistic "result" since for most driving that 2% won't be achieved.

Meanwhile we accept degraded performance as tire tread / casing design & construction mean more.

All I see is increased risk at a higher cost (tire damage, braking/handling problems, decreased service life) for a gain that isn't quantified.

Driver skill (or other factors) are more likely explanations of "increased mpg via overinflation" for all miles of driving.

Just buy the best tires, observe the placarded numbers and get a tire gauge that can be calibrated. And use it. And work on skills. Trip planning. The day on which we are sick, non-attentive and it's dark outside, rainy and lot's of traffic is the day it all matters. Just once is what it takes.

.

[euromodder]

Originally Posted by slowmover

Just buy the best tires, observe the placarded numbers and get a tire gauge that can be calibrated. And use it.

I'm with you on the good tyres and pressure gauge, but the numbers for tyre pressure on the car manufacturer's placards are often way too low !

I originally started to increase the tyre pressure (set at placard numbers) to cure a handling and wear deficiency when I put on 17" wheels.
At the placard values, the car was wobbling on its tyres, while the outside threads were almost melting away like butter in the sun.

But I won't go over the tyre manufacturer's max pressure rating.
Surely they keep a healthy safety margin, and I'd like to use that

[JacobAziza]

Originally Posted by slowmover

Where's the evidence that overinflation is beneficial?
.

In the link posted just above your comment:
http://ecomodder.com/forum/showthrea...cle-11433.html

Tire pressure has no affect on grip in dry weather, higher pressure reduces hydroplaning it wet weather.

I don't overinflate thinking it will cause a jump in MPGs. I overinflate because then it takes time to get back down to max. If I start at max, and it is constantly losing a tiny amount of pressure, I am always below max.
And while going over won't help MPG or handling (very much) going under can hurt both (more significantly).
And I don't feel like checking it every single day or two.

[tim3058]

Good info there Slowmover, but is there a possibility that a stiff heavy-truck tire's mpg's would be less dependent on tire pressure than a passenger car tire? I agree if it were a significant boost you would think the auto mfr's would jump on it, maybe the harsher ride/susp. issues complicate it for them.

I didn't do an actual test, but when I pumped the 44psi max Civic tires up to 50 my butt-o-meter noticed a harsher ride, and reduced rolling resistance. So much so when the front left tire (which has a very slow leak) gets down to 35psi I can tell I need to put air in it by how poorly the car rolls and the slightly higher vacuum to hold highway speed.

[UFO]

Originally Posted by Sulfuric

I run about 65 on my Bridgestone Ep100, sidewall is 44. BTW tire burst pressure is well over 100psi for undamaged quality tires. The only difference is comfort and possibly higher chances for tire damage is you hit a nail or something bad.

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.

[UFO]

Originally Posted by JacobAziza

Most of my experience is from bicycles, so it may not be a direct comparison. It may not be relevant at all.

Running any less than rated sidewall pressure (on a bike) gets you a slight decrease in ride bumpiness (slight enough to be irrelevant if you have suspension) at the cost of not only more rolling resistance, but also noticeably worse handling.
This is because if the tire isn't stiff, it flexes a little in the corners.
Further, unless traveling on loose dirt or mud or snow, having low pressure does exactly zero for traction and braking.
I consider the number on the sidewall to be THE recommended pressure, not the maximum.

I don't remember who it was, or what the post topic was, but somebody on this forum once claimed that high tire pressure does not in fact adversely affect braking distance (on a smooth dry surface) as is commonly assumed.

AFAIK the biggest reason not to exceed the sidewall max is that the tire and/or rim is only so strong, and if you go over by enough it can literally explode. For the purpose of avoiding lawsuits, the manufacturer picks a number well under the actual maximum a rim/tire can handle.

Not that I ever drive at the limits of the tire's ability, but I haven't found running high to have any adverse affect on braking or handling or tire wear on my motorcycle or truck.
I run the truck at 85-90 (rated at 80)

Bicycle tires have a round profile and do not have any structure. Their sidewall pressure rating IS the recommended pressure; you are correct, lower pressures increase rolling resistance because you increase the contact patch due to no tire structure. Car and truck tires are completely different, and the sidewall pressure rating IS the MAXIMUM rating.

[some_other_dave]

Originally Posted by slowmover

Where's the evidence that overinflation is beneficial? In forty years of reading about cars (and nearly that in owning them) I've yet to come across that benefit quantified.

Shoot. I know I've seen some A/B/A testing around here somewhere, but a quick look failed to turn up the thread. I know I have also seen quite a number of posts (more than a dozen) where people noted longer glide distances (in some cases, instrumented) after inflating their tires to sidewall maximum.

In my own case, I have tested three different tires on one of my cars for best cornering grip with different tire pressures. Acceleration and braking were not important to me when I did those tests, but in all cases I found that the best grip was achieved with pressures higher than the placard pressures. (This is for hard-compound true street tires, not R-compound tires nor the not-quite-R compound "street tire class" tires that are now popular in some circles.)

Every tire will have a pressure range where it grips the best. In every case that I know of (for passenger car tires), this pressure range is above the placard pressure that the vehicle manufacturer recommends. It is often not the same as the max sidewall pressure, but the max pressure has always (for me so far!) given better cornering grip than the placard pressures.

For those in the "I won't give up any grip because it's a safety margin" camp: You already have. If you're not running racing slicks, you've already given up grip. You have compromised the grip for minor concerns like being legal to use on the street, actually having some grip in the cold or wet, having some puncture resistance, and so on. So the question becomes: Which set of compromises do you pick?

-soD

[cfg83]

UFO -

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.

(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". Here are 200+ posts on the subject :

http://ecomodder.com/forum/showthrea...ires-2582.html

The gist of the argument is that the manufacturer does not *test* the tire above Max rated PSI, so you are "on your own" in terms of whether or not the tire can handle the extra pressure.

I think the GM EV-1 had tires inflated to 50 PSI to aid in reducing rolling resistance :

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

TIRES
Tire Mfg: Michelin
Tire Model: Proxima RR Radial
Tire Size: P175/65R14
Tire Pressure F/R: 50/50 psi
Spare Installed: No; Self Sealing Tires

CarloSW2