Incorrect Information re: Tire Pressure in Efficiency Mods

[eco30BMW]

I'd asked the webmaster twice to take down this misleading information from the "65+ efficiency mods" section of the site and nothing has been done...

Increase tire pressure to max sidewall

Reduces rolling resistance which decreases the amount of power your car needs to move. It also increase steering response, increases wet traction, and decreases tire wear.

Increased tire pressure DOES NOT help wet traction, as a matter of fact it harms it. The fact is that you are increasing tire pressure to decrease your contact patch to decrease friction. Decreased friction equals less traction all around and would be particularly noticeable in the wet.

Your information is misleading and since the site also encourages people to maintain as much momentum as possible through turns, this can be dangerous to boot, making people think they have more grip than they really do.

Also, increased pressure will make the center of the tire wear faster, since this area is put in increased contact with the road. The center of most all-weather tires is critical in pumping water out from under the wheels and decreasing the tendency to hydroplane.

Please correct this. The information on this site should be based on science, not counterintuitive and fantastic claims.

[RobertSmalls]

I think we do a surprisingly good job of being scientific and accurate here. Part of being scientific is taking comments like yours seriously.

A smaller contact patch would improve contact with the road in standing water or snow, hence the "improved wet traction" claim. Perhaps it's just hydroplaning resistance and not wet traction? Dunno.

At first, I was apprehensive about inflating past the pressure recommended by the automaker. Now I inflate to sidewall max without a worry. I haven't had any issues with irregular wear. Here's a post from a member who carefully measured his tire wear and found they were wearing normally at sidewall max: http://ecomodder.com/forum/showthrea...les-12438.html

[Daox]

Where is your science to back up your argument?

I distinctly remember reading a paper that stated that increased pressure does in fact increase wet traction. I do not have the link handy, but I will look for it.

On some older tire designs center wear may be a problem. I personally have never had a problem with any tires I've used and I run over max sidewall pressure.

Maintaining momentum through turns is a good idea for saving gas. I see no reason to take that tip it down. Obviously taking turns at dangerous speeds is a hazzard. I think you better go post your message on every performance forum on the internet as they're much more likely to be taking turns at dangerous speeds than we are.

[Clev]

When a tire is under-inflated, the shape of its footprint and the pressure it exerts on the road surface are both altered, especially on wet surfaces. An under-inflated tire has a larger footprint than a properly inflated tire. Although the larger footprint results in an increase in rolling resistance on dry road surfaces due to increased friction between the tire and the road surface, it also reduces the tire load per unit area. On dry road surfaces, the countervailing effects of a larger footprint and reduced load per unit of area nearly offset each other, with the result that the vehicle's stopping distance performance is only mildly affected by under-inflation.

On wet surfaces, however, under-inflation typically increases stopping distance for several reasons. First, as noted above, the larger tire footprint provides less tire load per area than a smaller footprint. Second, since the limits of adhesion are lower and achieved earlier on a wet surface than on a dry surface, a tire with a larger footprint, given the same load, is likely to slide earlier than the same tire with a smaller footprint because of the lower load per footprint area. The rolling resistance of an under-inflated tire on a wet surface is greater than the rolling resistance of the same tire properly-inflated on the same wet surface. This is because the slightly larger tire footprint on the under-inflated tire results in more rubber on the road and hence more friction to overcome. However, the rolling resistance of an under-inflated tire on a wet surface is less than the rolling resistance of the same under-inflated tire on a dry surface because of the reduced friction caused by the thin film of water between the tire and the road surface. The less tire load per area and lower limits of adhesion of an under-inflated tire on a wet surface are enough to overcome the increased friction caused by the larger footprint of the under-inflated tire. Hence, under-inflated tires cause longer stopping distance on wet surfaces than properly-inflated tires.

Quoted from NHTSA (Tire Pressure Monitoring Final Rule(Part III)), bolding is mine.

Taking the above to its logical conclusion, inflating tires higher than placard makes the contact patch even a bit smaller, which would not increase stopping distances.

[user removed]

24,000 miles rotated once.

This car was totalled and I had it repaired. The alignement has never been checked or adjusted.

I go around corners fast enough to discourage most tailgaters.

Original 8-93 front brake pads have 50% left.

No problem in wet weather, and I might drop the pressure in snow.

Contact patch size is reduced with higher pressures which places more weight per square inch on each square inch of contact patch.

If that is misinformation then show us we are wrong.

The 3 grooves in the attached photo measure .230 average with a variation within .005 inch!

regards
Mech

[user removed]

Tread measured .250 when installed.

regards
Mech

[autoteach]

Yeah, while I agree that overinflation is not a good method of fuel savings, you wont win that argument here. Those that believe that running max sidewall is worthwhile outnumber those that believe otherwise.

[NachtRitter]

Originally Posted by autoteach

Yeah, while I agree that overinflation is not a good method of fuel savings, you wont win that argument here. Those that believe that running max sidewall is worthwhile outnumber those that believe otherwise.

I'm not clear if you're saying overinflation is the same as running max sidewall pressure. Would be interested to know the reasons why you believe running max sidewall ("overinflation"?) is not a good method of fuel savings.

[autoteach]

Overinflation would be over the car's recommended value. It may be a good way of saving fuel, but it does pre-maturely wear tires (but I am sure that someone will say that it doesn't, as I said above this is a very tough argument.) It also causes suspension to wear prematurely as well as drivetrain, but once again someone will dispute that. I also believe (this is something that I know will be questioned, read previous post) that it does provide significant safety hazards under certain driving conditions. Argue away, like it has been in other posts and for the same reason that I told the OP that this may be the biggest waste of his time. I will keep my inflations within reason.

[Weather Spotter]

I run my tires higher than MFG recommends at 44PSI instead of 32 (max sidewall is 50PSI) . Last winter I did some tests on my weekly drives home from school. Higher pressures increases my wet and snow traction by a good margin (over 15% better traction). I also tested stopping distance on wet and icy roads. I found that higher PSI was about the same to slightly better stopping distances.

My tire wear is better than when I was at OEM PSI. at 15K miles on these tires and they still look new, wear is very even (better than at OEM).

When I worked at a major auto maker I liked to put this topic up for discussion. The people who did track testing and noise testing came up with the same resign for the OEM specks on tire pressure: ride comfort and noise.

The idea is that it is a balance between noise + comfort + wear + MPG + cornering + users not checking pressure = the OEM recommended pressure.

From what I have personally found and what these people talked about, tire noise increases quickly above about 35 PSI on most tires. Also, most car MFG's have noise limits for their cars at certain speeds. keeping tire pressure lower is a cheaper way of reducing noise so that other components do not need to be as well noise insulated (which is expensive).

When I look at the US DOT, they warn about under inflating tires but do not have any complaints about over inflation (until you pass max side wall).

It comes down to personally choice between ride comfort (both noise and bumps) and increased traction & MPG. I found the sweet spot for my car to be between 42 and 46PSI, any more and the ride gets worse and the MPG does not improve.