Tires are out-of-round

The sidewall does not take into account the current load of the car on it. It is the absolute max pressure of the tire. Obviously, the pressure you want to inflate your tire to depends largely on the weight you have on it.

The more load that the vehicle is expected to carry, the more pressure that the tire is expected to be inflated to. At the maximum loading limit, you will also find the maximum tire pressure limit. But if you take the tire to its maximum pressure limit without taking it to its maximum load limit, it's not like as if the tire is going to explode due to not being put under enough load.

It will just have its outside tread edges a little off of the ground at worst, resulting in overinflation wearing patterns on the tread. But that's actually what we're after here. We want the tire to pop out a little bit, before we return it to its normal pressures.

Yousuf Khan

Underinflation is considered "pound for pound" more hazardous than overinflation. A bit too low is much more likely to result in catastrophic tire failure, than a bit too high.

"Too low" pressure results in excessive flex, which generates increased temps, that delaminate plies/belts and tire poppage.

do you work on your own car?

have you checked the tire pressure while the wheel is jacked up off the ground vs. when the car's weight is on it?

if not, go do so and come back and tell us if the pressure reading is different.

i'm going to say this one more time: the doorsill pressures can be realistically taken as the minimum recommended pressures for the tires. below these pressures you can dangerously overheat your tires and cause a subsequent catastrophic failure. however, for very short term use (on soft surfaces, on a drag strip, etc) dipping below these pressures may be tolerated.

the sidewall pressures may be taken as the maximum recommended pressures.

it is _extraordinarily_ likely when using the doorsill pressures that during hard cornering manouvers the shoulder of the front tires (which see the most extreme sideloads and weight transfer) will be rolled over and the contact patch of the tire distorted. this is immediately identified by wear on the shoulder past the tread. without **any question whatsoever,** increasing the tire pressure, up to the max sidewall pressure, will decrease this effect and net an increase in lateral accleration performance. the tires will hold their shape better and much more vehicular control will be available to the driver.

though this condition will generally result in greater center tread wear, it will also result in less shoulder wear. these two wear conditions must be balanced on a case-by-case basis. as i said before, if you don't push the car through corners, then you won't be rolling on the shoulders and you won't need to increase tire pressures.

americans drive lincoln town cars and like a cushy, mushy ride. tire pressures are ALWAYS a tradeoff between ride quality, tread life, economy (rolling resistance), and traction.

contrary to what some armchair drivers would have you believe, increasing the pressure of the tire up to the sidewall pressure does not result in miserable braking and accelerative performance. this is readily verified by GOING OUT AND TRYING IT YOURSELF.

for more information, see page 10 of

Wow, that is high. I wonder what the peak pressure in a tire is when I hit a pothole while braking on a hot day. No doubt it's less than 200, but tires do blow from this sort of thing, don't they? Is that because of rim deformation, or getting cut, or something in addition to the excessive pressure?

-John O

of course it will be the same. You've got the same amount of air in the same amount of space.

That tirerack document isn't a bad one. If I may quote it:

(wrt overinflation) "If your tires are overinflated by 6 psi, they could be damaged more easily when running over potholes or debris in the road."

(wrt to raising pressure for competition) "Tires should be reset to normal inflation pressures when returned to the street."

Autocross and most other competitions with increased tire pressure are usually held on surfaces free of debris and potholes. American streets certainly don't generally fall into this category.

Yousuf,

Setting aside all the barstool engineering that's been going on here, how many miles have you put on the tires since you posted your initial question, and what noticeably positive change have you experienced?

Rick

I've never seen a tire blow from hitting a pothole, under any pressure. They'll blow if the pothole contains nails or metal spikes of some kind, but not just a simple pothole.

Yousuf Khan

So far, I've probably only gotten 500km additional driving (no need to get on the highway for too long yet). And so far, the additional pressure has delayed the initial onset of the vibrations from 80 km/h to 95 km/h and the onset of really severe vibrations from 90 km/h to 105 km/h. I do believe the onset of vibrations are starting to creep up the speed scale very slowly (just a gut feeling at this point really, it's hard to tell the difference day to day). I haven't yet inflated the tire all of the way upto the sidewall maximum, I'm still a four psi below the maximum on the fronts; I've hesitated due to the debate in here which made me chicken out initially.

Yousuf Khan

Who said simple pothole? A relative lost a pair of tires--both right side tires--when she ran over a hole on I-275. Broke the wheels too.

I have personally see potholes deeper than eight inches, and many years ago I was in a car where we hit one with the right-rear wheel and the driveshaft was pulled out of the transmission. Didn't lose a tire though, now that I think about it. We get big ones up here...

Anyway.

-John O

i drive like an autocross a good percentage of the time i'm in the car... enough to warrant a quick response over a soft ass.

when i have a blow out i'll tell you. it ain't gonna happen.

ken

Well, in any of those circumstances I don't think anything you do with the tire pressures will help all that much.

Yousuf Khan

Last fall and about 3 months after installing new $170-a-tire Dunlop SP Sport 5000's on my Oldsmobile, I was run off the road by a teenager driving his mom's minivan. I swerved to the right to avoid hitting him, and hit a curb at about 50 mph. The curb was about 6 inches high and had masonry bricks stacked on top of it (retaining a little flower bed) for a total height of about 10 inches. It was at a business entrace, so that curb was basically perpendicular to my direction of travel. The right side of the car came far enough off the ground that the flowers in the aforementioned flower bed were intact for several feet from the impact site. When I inspected the damage, both right side tires had big bulges in the sidewall (the front worse than the rear), but neither had burst. I was able to drive about 2 miles to the tire shop, where they fixed me up with temporary replacements and ordered new Dunlops for me.

Neither (stock aluminum) wheel was damaged, though the the wheel alignment did need to be brought back into spec. I would imagine that much more damage would have occurred at the highway speeds that your relative was likely traveling on I-275, or with lower-profile tires. (The Olds wore

- Greg

Your description about vibration starting at about 80kmh (50 mph) sounds like the problem I've had with two different tires that hit potholes. Once examined by a competent tire shop, both tires (different time frame for each) were found to have broken cords in the tread belt. The tires needed to be replaced, but not before I got suckered into buying a new wheel.

Since the beginning of the thread has long since disappeared. my comments may or may not be germane to the discussion.

Yes, it's possible that's what the issue is here too. But I'm going to try to see if the solutions I'm trying now will fix it up. If it doesn't go away in a few weeks, then I'll look into replacing the tire(s).

Yousuf Khan

Well, of course I've already had it checked out! What do you think prompted me to ask this question in this newsgroup in the first place? The Subaru dealer (the guys who checked it out) suggested that I buy new tires, preferrably *four* new tires! But my feeling is that that's just their default suggestion. They never said that any belts were broken or anything, just that the tires were flatspotted (i.e. "out of round"). But flatspotting is very vague, and not all of them are permanent. I doubt that the dealer did much of a thorough job of inspecting the tires, otherwise they'd have more information for me. Right now it just looks like a cash grab.

Anyways, as for your worry about the thing blowing up at speed, it's already been three weeks since the inspection and they haven't blown up at speed yet. Nor have they blown up when I filled them to their maximum rated pressure either.

Yousuf Khan