Huh? Of course it changes the pressure Think of it this way... there are the same number of air molecules in a compressed and a non-compressed tire. However, the VOLUME of the tire is different. Therefore, the pressure is higher in the compressed tire. Not significantly, of course. But it changes

I have air suspension in my truck, with an inline pressure guage, and I can tell you that every bump compresses the suspension causing a momentary increase of pressure.

 

Anything around a 15 to 20 buck range that is rated for 110 to 120 PSI will work, you want it to be compact too - so it doesn't take lots of space to carry.

They will take a while to pump up a big or high pressure tire (shucks - I used to pump up the 33 inch 'Swampers on the F100 with one), but if you let the pump cool off a bit between tires it should be okay. I'd say that for every ten minutes it runs, give it five to cool back down. I have two or three of them that have been around for over six years.

I'm serious about installing plugs on the road without even jacking up the vehicle though - in the time it takes another man to wrastle a jack up under and bust the lugs, tighten it all back up, and jam everything back into wherever; I'll have popped in a plug, aired back up, and got back on the road five times over (and without breaking a sweat). Plus I don't have to schedule time to drop off a tire to have done to it what I can do my ownself...

The only time I mess with a jack is if the bead came off the rim, and a helpful tool there is a ratchetting tie down strap that can be put around the tire to squish the beads back out so they contact the rim again.

 

Good point. If you squeeze a balloon, what happens to the pressure inside?

I can think of a test:

Pressurize a wheel and tire OFF OF the truck, install it, then read the pressure with weight on the wheels again.

Alternately: Pressurise the tire and wheel while the truck is on its wheels, then jack up the wheel and read the pressure. The difference may only be slight - but there should be a difference.

 

ALWAYS check your tires religously. I always look at the tread and sidewalls to make sure the tires look good before taking a trip. An ounce of prevention is worth not having problems down the road.

 

> a pair of terminal clamps?

Terminal clamps for sure. Once when I couldn't get to the axles (my 16 foot leads were too short) on an equipment trailer, I stuck a piece of metal into the 7 way and clamped onto that. What I did (since the power terminal was not hooked up) was put the lights and headlamps on to get close to 12V and then put it on high idle with a CLUB. Took a while, but, "got her done".

 

It really doesn't take much to add wire to an electric "ROAD PUMP". But the flip side is, how much 'spaghetti' do you want to have to roll up?

Since I never street a machine without jumper cables anyway, I just make sure all my "OH CRAP!" equipment is compatible.

For a real fun time, try clipping battery jumper cables onto a cigarette lighter connector!

 

It's amazing how little that pressure changes though. Fill a tire to 50 lbs, mount it on your truck - with a good thousand pounds of weight on it now - and it will still read about 50 lbs. I wonder why it changes so little - anyone with a science background who could explain that better ?


I agree with Greywolf's method for plugging tires, when possible. As a carpenter, where people can drop nails and screws on a jobsite - I've fixed countless tires for guys that way. Same repair you'd get at many service stations; you're done without ever having to pull the wheel. Stay away from the cheap Chinese compresors though. I fried one last summer. Brand new, and it couldn't even fill a small inflatable to pull behind my boat before it burned out.

 

If you believe that try pumping up a tire on a vehicle and off with a hand or foot operated pump. I gaurantee the tire on the car is gonna work you harder. Pressure may not be significantly different but when the weight of that car is compressing that tire it is pushing air back moreso than when no weight is on the tire. Have you ever deflated a float? Did air not come out faster when you put your weight on it?

 

Think about how thick the tire is constructed - when properly inflated, it isn't going to show much pressure change due to the sidewalls and tread taking up some of the load. Even 120 PSI tires will reflect that, they are the thickest and toughest tire composition I can think of second only to aircraft wheels. The rubber and cords/plies do a lot of the work of maintaining support.

I'd LOVE to hear what Torque1st has to say on this topic...

 

I was never a fan of the canned tire repair. That stuff would void a tire manufacturer's warranty. Well, so would a nail, but at least there was a road hazard warranty, and pro-rated trade in. Not the case with the goop.

I'd get some customers who put that stuff in their tire, as a "limp home" option until they brought it to us to repair properly. It would have to be cleaned out thoroughly with stuff called "stoner" so it was dry in there. This way took twice (or sometimes three) times longer than if they hadn't put it in there.

Costco's tire warranty was a fair warranty, but you really had to be on the ball. Got a flat? Plug it, or put the spare on. Then get it to Costco to get it plugged and patched, and reinstalled for free. If the tire was driven on for any extended distance while flat, the technician would immediately find it, and void the warranty. It was real easy to find too, because while it may not be evident on the outside, the inside wore out faster. If it ever did get to the point that it was evident on the outside, we (the tire guys) just laughed. We'd get the occasional sob story about someone who got the flat on a bridge and couldn't stop. In those cases, we'd just give them the benefit of the doubt, as long as they had the spare on when they rolled up.

As for tire pressure on and off a truck, I'm not gonna get into it

 

Sorry Buddy,copying the link was kicking mu butt!
Here's a pic,just go to Cabela's.com, Automotive & ATV > Auto Towing & Performance > Trailering Accessories .

 

Sure, I've got a science background, that's why I said it in the first place. One equation:

PV = nRT

P is pressure, V is volume, n is the number of molecules, R is the gas constant (a number) and T is the temperature in Kelvin.

Since the tire is a sealed system, n doesn't change whether the tire is on or off the vehicle, T would remain the same as well for experimental purposes (eliminating a variable). That leaves PV = a constant. Now, some may dispute this part, but a tire's volume does not change on or off a vehicle, only its shape. The part on the bottom that flattens out (seemingly decreasing the volume) causes the sidewalls in the same area to flex outward (increasing the volume by the same amount). To decrease the volume of the tire, you would have to exert pressure on its entire surface area simultaneously.

Now, if a tire blows on a resting vehicle because you've overloaded it, it won't blow because of the pressure inside the tire, but because you've deformed the sidewall enough to pop the bead off the rim in one spot.

Jason

 

Okay, I'm with you that far; but I agree with what 70Blue posted above. That equation ignores the work you are doing in lifting the vehicle. When you pump up a tire on a vehicle, you are lifting maybe 1000 lbs of vehicle maybe half a foot higher. Isn't there also a law about conservation of energy ? The work to lift that mass to that height is going to require a certain amount of energy - and I believe that energy has to come from somewhere. I believe that creates extra load on the pump. If you try pumping a tire up by hand, both on and off the vehicle as he says - I think you'll notice the difference.

 

Nope. The work is in increasing the pressure in the tire. Atmostpheric pressure is about 14.7 psi. So to go from that to say, 50 psig (gauge psi, add 14.7 for actual pressure), you have to do quite a bit of work to add that 50 psi.

Think about what you have to do to jack up a vehicle a few inches. You trade large strokes at low pressure (handle) for very short ones at high pressure (lift). An air pump isn't much different.

Jason

 

[QUOTE=95van]It's amazing how little that pressure changes though. Fill a tire to 50 lbs, mount it on your truck - with a good thousand pounds of weight on it now - and it will still read about 50 lbs. I wonder why it changes so little - anyone with a science background who could explain that better ?


Basic Thermodynamics. P1V1 = P2V2. Where P is pressure and V is Volume. And 1 is condition 1 and 2 is...

When you air up a tire off the truck, it has a pressure(P1) which you read and a certain volume(V1). When you put weight on the tire, it will deform a bit, the bottom will be a bit flatter, but the side walls and perhaps the rest of the tread will expand a little. Maybe the expansion is not as much as the flattening but the volume(V2) will change only a little bit. You should be able to read the pressure(P2) difference with an accurate guage but unless other conditions changed you won't see a lot.

Only if the tire deforms a lot, like an under pressurized tire, will you see a significant volume change which leads to a pressure change. Properly aired tires will change only a little and this assumes temperature etc has not also changed, like sitting in the hot sun or going for a drive.

Just some long ago edumacation,

Jim Henderson