Tire pressures

[Pu241]

http://ecomodder.com/forum/showthread.php/15-more-psi-7-better-mpg-26496.html

[Team Lightning]

John, that link is interesting reading!

[mikeyjd]

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.

http://ww2.nhtsa.com/cars/rules/rulings/TirePresFinal/safetypr.html

[mikeyjd]

Mike there are so many things a tire guy will tell you that's wrong with this statement. P metric tires aren't designed for these high pressures. 60 psi is LT Tire territory,and your tire is going to wear and handle like poop. With that much pressure your tire is going to crown and now you loose your contact patch. Resulting in prematurely wearing the center portion of your tire. If you want better mileage get skinny tall tires.

10,000 miles rotated once. No uneven wear to speak of thus far. No crowning to speak of whatsoever. Can point me toward any data/articles that back up the theory of crowning and premature center wear for over-inflating tires in every day driving conditions? It makes sense in theory but I haven't seen any literature backing this up. Thanks.

[mikeyjd]

Here is another related article from the perspective of law enforcement.

http://www.cleanmpg.com/forums/showthread.php?t=11652

[nitrofarm]

10,000 miles rotated once. No uneven wear to speak of thus far. No crowning to speak of whatsoever. Can point me toward any data/articles that back up the theory of crowning and premature center wear for over-inflating tires in every day driving conditions? It makes sense in theory but I haven't seen any literature backing this up. Thanks.

https://www.google.com/search?q=over+inflation+tire+wear&tbm=isch&tbo=u&source=univ&sa=X&ei=7XEMUrPLJ4OsyAHBwYDYDQ&ved=0CFAQsAQ&biw=1280&bih=929

[frankenfester]

https://www.google.com/search?q=over...w=1280&bih=929

I think this may be the "theory" stuff he mentioned prior. I've always ran higher than door placard because it made since to be close to the sidewall max. However, that law enforcement link is pretty mind opening.

-James

[mikeyjd]

https://www.google.com/search?q=over...w=1280&bih=929

Like I said before, I have no uneven wear thus far after 10k miles. I check my tires once a week to make sure there are no abnormalities and that the pressure hasn't changed. Any crowning will result in my backing off the psi for correction, but thus far this hasn't occurred at all. The tires are all wearing uniformly and slowly. The added pressure has given me better handling performance and efficiency, at the expense of ride comfort and tire noise.

[mikeyjd]

Curiously utility trailers still use bias-ply tires which can carry more load than radials and they are available in 13s and will take higher pressures.

"Trailer tires are done differently than car tires. Since trailer tires don't have to steer or drive the vehicle, the affects of side forces and torsional forces allow the tires to be rated at higher load carrying capacities than car tires. Put another way, if you compare the load rating of trailer tires with any tire designed for a powered vehicle, you'll notice the trailer tire is rated for a higher load carrying capacity.

Notice that trailer tires are designated differently than car tires. They either have a different size nomenclature (like 4.80-12, or 20.5X8.0-10) or they have letters in front of or behind the size to indicate the type of service the tire was designed for (like ST205/75R15).

Because of the service, trailer tires aren't concerned much about traction or treadwear. Many trailers don't have brakes. Also, trailer tires tend to get old before the tread is worn off. As a result, the tread compound in these tires isn't designed with those properties in mind.

What is the major design factor? Cost!! That means that the tread compound - the thing that is going to have the largest affect on rolling resistance - is going to be a low cost, low tech rubber. Rolling resistance is not even in the consideration.

So assuming you could properly navigate through tire size and load rating, you'd find that a trailer tire will have a major deficiency in either traction, treadwear or rolling resistance (or more than one.)

That's why trailer tires aren't used on cars and trucks.

- and since the demand for these types of tires is small compared to cars and trucks, trailer tires are made by what I would call 3rd tier tire manufacturers - not the major tire manufacturers (with one exception, which is an interesting study in itself). Needless to say, there are issues with these tires."

http://www.barrystiretech.com/

[getnpsi]

I alway overinflate tires that I myself purchased. I pumped my 12's up to at the sidewall pressure and almost died after owning my festiva three days. If its a tire that says 32, then i suggest not going much higher. the newer tech 44, 48, 51 psi more performance or touring tire, especially known brand names, id not be weary of breaking those rules...if i knew the age and history of tire...like being only mine.

[Zanzer]

^^ Your biggest problem there was the 12" tire, they're dangerous at any level of inflation LOL

[frankenfester]

^^ Your biggest problem there was the 12" tire, they're dangerous at any level of inflation LOL

And I think that's because they're basically trailer tires.

-James

[mikeyjd]

I run 60 psi all around (35max sidewall) They have been wearing noticeably better since I upped the psi from 45.

I wanted to make a correction here. My sidewalls are rated for 44psi not 35...