Tire Pressure and Sag

[cafn8]

Originally Posted by brucepick

I spent a fair bit of time hanging out on the Subie message board
http://www.ultimatesubaru.org/forum/

Learned that 4wd Subies really need matching diameter tires. Gotta rotate them at every oil change, about 3-5K miles. Subie has specs for how much difference is allowed in treadwear between tires. If not tended to you wreck the "differential", or whatever it's called where the front/rear drive trains meet.

As for the different pressure spec'd for front and rear, my guess is that the ~10% difference is just what's needed to end up with matching diameter front + rear. Most of the weight is in the front, thus slightly higher pressure there.

I tend to believe the post reporting success at 40 psi. That's high enough to avoid lots of tire squish and reshaping as it rolls. Having the fronts just a bit higher psi makes sense to me. I would expect things to go well at 44/42 as you have it now.

Would I go higher, like 50/48 or higher? Dunno. My rwd car is 52/50 now but that's my car, not your 4wd Subie.

Sounds like we're on the same page here. My thought, however, is that it probably isn't a straight percentage issue. When I think about it, the tires sag less and less, the more pressure I put into them, right up to infinity PSI (in both front and rear) where the tire does not sag at all. My 44/42 is based on "half again as much pressure" being equal to "a 3rd less pressure difference". Intuitively this makes sense to me, but it could be complete BS (and I don't mean Bachelor of Science.) If anybody knows more about this I'd love to know how the actual numbers work out.

[dkjones96]

Tire pressure is not only for ride but tire wear and traction. If you have too much pressure it wears on the inner part of the tire the most, if you have too little the outer edges of the tire wear more.

The automaker takes the weight of the vehicle in the front and back tires, then takes the size tire they want on that particular vehicle and calculate what pressure the tire needs to run to have the largest pad for traction while maintaining a nice ride and adequate tire wear standards. The larger the tire the less pressure is needed within to keep the tire inflated properly. That's why low profile tires can run in excess of 50lbs at standard inflation. Typical cars run with 65-70-series tires about 32-35 and SUV's like mine with 75-series or greater usually run about 25-29.

A properly inflated tire, on a well aligned car, with a driver that doesn't take corners fast or spin the tires much, if at all, should suffer from dry rot way before the tire treads get low. That's how all my tires end up and i run at most 10-15% higher than rated pressure.

[brucepick]

With all due respect to dkjones96,

If we wanted to get approximately EPA mpg in our cars, we'd probably follow the vehicle manufacturer's tire pressure recommendation.

However we're interested in getting increased fuel economy, and we've learned that increased tire pressure is one of the most universally effective tools towards that end.

If you question the safety of this, or question the effects of increased pressure on tire life, please have a look at Driving Under Pressure.

[dkjones96]

I said nothing about fuel economy in my post. Everyone knows that running higher pressure gets you better mileage because less energy is expended because of flexing side walls.

[DarbyWalters]

Originally Posted by brucepick

Driving Under Pressure.

This article has quite a few errors in it. No tires will not balloon out...but they will ride the center ridge more with too much pressure (steel belts do flex) and ride the edges more when underinflated for the intended vehicle in a specific condition. The only way to get the correct pressure for maximum traction (dry) is to take tread temps Inner/Center/Outer and try to make them equal by adjusting pressures (alignment will also affect tire temps...especially camber-that is why race cars are set up with Zero Bumpsteer to allow for camber adjustments without throwing out toe in/toe out measurements). Without ripping the article up completely...since I am at work...there is always a "ying" to the "yang". If you pump up the tires with extra psi, they will tend to hydroplane less than a tire at a lower temp...HOW?...because the tire DOES change shape and a smaller tire section makes contact with the ground so the water escapes easrlier before it can lift off the surface. Car Makers put a pressure on the door that covers most situations on normal roads and tire makers put Max PSI on tires for liability reasons.

[brucepick]

Originally Posted by DarbyWalters

This article has quite a few errors in it. No tires will not balloon out...but they will ride the center ridge more with too much pressure (steel belts do flex) and ride the edges more when underinflated for the intended vehicle in a specific condition...

I bow to your greater wisdom. I had to google Zero Bumpsteer and bumpsteer because I hadn't heard of them before. All in all, what you wrote makes sense.

Right now my tires are about 10% over max sidewall, but that's only been maybe 1-2 thousand miles. I don't think I want to go over that. They seem to be doing well and I like the handling. I'll be checking tread depth before too long. Hopefully it's fairly even.

[DarbyWalters]

Hey, tires are actually very complicated. I tested tires (all types) for VTS (Vehicular Testing Services), Auto-Xed nationally with BFG, Road Raced for Mazda using BFG/Goodyear/Hoosier and also raced go-karts with many brands...and I am still learning. But what I did learn is how to get the most out of a set of tires/alignment/chassis setup. I don't think that going 10% more than the "MAX PSI" is going to make your tires explode...but you do give up some performance parameters when you do so. I would definetly take into account outside temps and adjust accordingly...hotter outside temps and going on a long trip-maybe adjust down a few pounds.

Tread depth is also a factor in pressures. Tires with less tread depth will not "heat up" as much as new tires at full tread depth. Race Cars like slicks or shaved tires because they are more linear in performance changes. Buying used tires in good shape might be another "SuperHypermiling" craze ;-)