This has been a bit of a puzzle of rme for years. My grandfather read the instructions for a spare tire one day. It said in the instructions to inflate the tire to 35psi while off the vehicle. Grandpa said, "Why does it matter? 35 psi is the same on the vehicle or off."

Since I just finished high school physics, I told him that when you pump up the tire on the vehicle, it will raise the vehicle up a few inches. The energy needed to raises the vehicle has to come from somewhere. That was not a very satisfactory answer for him nor me. Grandpa just said it does not matter since 35 psi is still going to be the same on the vehicle or off.

The question came up again, now many years later. So all of you engineers and physics buffs, can you explain why it matter whether a tire is inflated on a vehicle or off?

I've wondered this too. Seems like pressure should go up in the tire as weight is put on the tire, but only if the space in side the tire gets smaller.

[This message has been edited by 2.5 (edited 01-09-2013).]

When you put the tire on the car the bottom of the tire is flattened a bit thus changing the interior volume of the tire. Volume goes down pressure goes up (assuming no temp change). It's an answer be it right or wrong.

quote Originally posted by Phil: When you put the tire on the car the bottom of the tire is flattened a bit thus changing the interior volume of the tire. Volume goes down pressure goes up (assuming no temp change). It's an answer be it right or wrong.

Yeah I would think so, but the tire also bulges out on the sides.

Load on tire can affect PSI inside but not much in most cases... Vehicle Load will compress Tire and thus air inside Resulting in the PSI goes up. But Real reason likely this, Spare leaks air over time and Normal tire as spare more so... Normal tires will loss air ~1PSI a month. 35 now but months later then likely low, even flat, to use.

(Change air pressure affect total spring rate. One reason why NASCAR/etc pit techs are adding/removing a 1-2 PSI in one or all tires.)

Temperature will too...
10*F = 1 PSI. Tires w/ 30PSI at 90*f then need more air added to tire in 60-50* or lower.

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[This message has been edited by theogre (edited 01-09-2013).]

quote Originally posted by Phil: When you put the tire on the car the bottom of the tire is flattened a bit thus [reducing] the interior volume of the tire. Volume goes down pressure goes up (assuming no temp change).

Correct. And since compact spares have a smaller volume to start with than the standard tire, the effect is magnified somewhat. Setting the pressure with the tire on the ground and loaded may result in slight underinflation, reduced load carrying capacity, and increased sidewall heating ... not a good idea, since most spares are already in pretty bad shape due to age and weathering.

[This message has been edited by Marvin McInnis (edited 01-10-2013).]

Is it possible to load a vehicle so heavy it blows the tires?
That would suggest the heavier the load on the tire, the higher the PSI in the tire - for a given amount of air.
i.e. 35 psi suspended will require more air than 35 psi on the ground. The tire pressure would be slightly more than 35psi when you put the car back on the ground. Filling the tire in the air takes the weight on the tire out of the equation so you get the same volume of air at 35psi. (not accounting for temperature differences)

quote Originally posted by Formula88: Is it possible to load a vehicle so heavy it blows the tires?

I would assume that the suspension would bottom out wrecking something else before the tires blew out...

quote Originally posted by 1988holleyformula: I would assume that the suspension would bottom out wrecking something else before the tires blew out...

Probably. I've seen tires pressurized to 80psi and more and not blow up. It would take a lot of weight to drive the pressure up high enough to cause bursting. Under driving conditions it might be possible that excess pressure (from an overweight vehicle) could cause the tire to delaminate which would eventually lead to it blowing out.

I set my tires pressure to 40 pnds cold in all my cars. Reduces sidewall flex on curves and my tires last MUCH longer. Ive rarely worn out a set of tires in under 100,000 miles. Seems most cars these days wear out the outter edges of tires quick whether its a geometry thing or not, i have no idea. Chrysler LX platform is really bad about this. Higher than recommended pressure, at least for me bulges out the center more so my tires wear flat across the tread. Ive got 2 cars right now with 100,000 mile tires that are very good from edge to edge to prove it.

http://ask.metafilter.com/1...he-weight-of-the-car

http://ask.metafilter.com/1...vehicle-payload-much

Little discussion on it.

[This message has been edited by Dodgerunner (edited 01-10-2013).]

inflation of a flat tyre when on the vehicle
tends NOT to work on tubeless tyres
the air just leaks out at the bead

one trick is too blow the tyre on
with gas or ether [starting fluid]
but that is dangerous as it can burn the tyre or worse case the car
and even that trick ''works'' better if you jack up that tyre

Only if the bead is off. I inflate flat tubleless tires all the time on wrecks. If it is off the bead, ive inflated them by just jacking it up and pulling the tire toward the bead enough for it to start filling up.

quote Originally posted by rogergarrison: Only if the bead is off. I inflate flat tubleless tires all the time on wrecks. If it is off the bead, ive inflated them by just jacking it up and pulling the tire toward the bead enough for it to start filling up.

Me too.

Would the difference in tire pressure on a vehicle and off be measurable by a regular tire guage?

If so, I will give it a try the next time I take a tire off of one of my vehicle. That would be interesting to meassure a difference.

quote Originally posted by Doug85GT: Would the difference in tire pressure on a vehicle and off be measurable by a regular tire guage? If so, I will give it a try the next time I take a tire off of one of my vehicle. That would be interesting to meassure a difference.

If you read the bottom of the 1st post of mine above a guy did that with a digital tire gauge and found the difference to be VERY small if measurable at all.

[This message has been edited by Dodgerunner (edited 01-11-2013).]

quote Originally posted by Dodgerunner: If you read the bottom of the 1st post of mine above a guy did that with a digital tire gauge and found the difference to be VERY small if measurable at all.

Shouldn't it be more than that? On my truck, one front tire is holding about 1300 pounds of weight. If I were to inflate a flat tire on that wheel, then I would be lifting 1300 pounds aboout 3 inches.

The other way to look at it is the tire contact area to the ground. Just guessing 7" wide tire and 5" flat contact area is 35 sq inches. Take that times 35 psi and you have 1225 lbs of force you can support. As you increase weight the tire flattens more and increases contact area which also increase supported load.
A truck tire on a large truck would even have more contact area and then there are dual tires. Maybe why large trucks always use larger tires.

One way of looking at it.

[This message has been edited by Dodgerunner (edited 01-11-2013).]

quote Originally posted by Formula88: Is it possible to load a vehicle so heavy it blows the tires?

It would be, but I've only seen it when someone wasnt paying enough attention. On a haywagon on a farm for example.

We had little extra cash. The old pickup tires after bald, went on the haywagons. The wagons you pull across a hayfiled and fill with haybales off the baler. Sometimes that hay is heavy bales, sometimes light. The tires probably had 40-50 lbs in them. But the haywagon full of heavy bales weighed way more than that tire was rated for.

That combined with a few bumpy conditions in the field as we crept across it loaded to the hilt, tures bulging badly, did cause a tire or two to burst. I do think if we took the time to inspect the tires as we went, and had a portable compressor with us that could sir them up harder they would not have popped though. But they would have likley been inflated well beyond their capacity printed on the side as well.

[This message has been edited by 2.5 (edited 01-11-2013).]

Overloaded tires bulge the same way as low tires do. The interior volume of the tire really doesn't change much either way because of the bulging sidewalls. However, the more flex in the sidewall, the more heat is generated as the tire rotates. It's the heat that usually causes the tire to come apart and blow.