Tire Pressure

[Worldtraveller]

Suppose you have a tire rated to carry 5000 pounds when inflated with 50 psi.

If you inflated it to 100 psi, it would bulge in the middle (of the surface that is supposed to be flat on the road), causing reduced traction and uneven tread wear.

You could accomplish exactly the same thing by leaving the psi at 50 but reducing the load to 2500 pounds.

That said, let me make two points:

1. I'm no expert. I'm just parroting what I was told.

2. I have no way to weigh each wheel, so I just run at the pressure suggested by the door plate.

I'm thinking about tires on a typical passenger car, which I'm certain is basically the same as any pneumatic truck tire. The tires I'm running are 35psi max. That's where I run them. I think the door says to run them slightly lower at about 32. When I have my car serviced that's what they come back as, 32psi.

If I were to inflate them to 16psi I would be damaging the tire, the condition would be unsafe, and the monitoring system would give me an alarm.

Newer tires that have that low profile are inflated to higher pressures, but I don't think the relationship you heard about is valid.

With cars and small trucks, there’s only so much weight that can be carried and pulled, so the difference between optimal air pressure while being empty and pulling nothing vs maxed out and pulling at full capacity is negligible; however, a beast capable of moving mountains has a substantive need for extra air pressure that is harmful overkill if traveling light.

This pretty much nails the issue right here.

I am not an expert by any means, but I probably have more than the average amount of knowledge about how tire pressure 'works' and should be varied by load, and monitored. That's because 1) I'm an engineer that has to occasionally deal with this issue on aircraft; 2) I race motorcycles, and tire pressure (and how it corresponds to tire temperature and grip) can be a huge issue that has a significant impact on performance; 3) I've educated myself on the issue by reading, but also by talking with actual tire engineers from Dunlop, Pirelli, and Michelin.

If you really want to optimize your tire performance, remember that those "Max" numbers on the sidewall or on the door panel of your vehicle, are based on cold tire temps. If you have been driving for several hours, and your tire pressure is 3-7 psi higher, that's normal.

Figure your range of pressure, which is typically listed as only about 3-4 psi between the minimum recommended and the maximum, and figure your vehicles min and max loading, and that's basically a linear relationship. This is why fast is quite correct in saying that the differences between typical pressures, and optimal pressures are negligible.

To give an example where this isn't the case, my motorcycle trailer has a range of 25 - 45 psi. First, you'll note that this is a much wider range than a typical vehicle tire. The reason is that when towing the empty trailer, the weight on the wheels is ~400 lbs. When towing a maximally loaded trailer, the weight on the wheels is about 1900 lbs. That's a much larger range than the typical range of your passenger vehicle.

 

[Wiploc]

If you have been driving for several hours, and your tire pressure is 3-7 psi higher, that's normal.

Good information, thanks.

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I have a motor home with duallys.

I know that I'm supposed to keep those adjacent tires close in pressure, but I don't know how close. I've been adjusting them if they get as much as two pounds apart. Is that sufficient diligence, or should I be keeping them closer yet?

 

[Worldtraveller]

If you have been driving for several hours, and your tire pressure is 3-7 psi higher, that's normal.

Good information, thanks.

-

I have a motor home with duallys.

I know that I'm supposed to keep those adjacent tires close in pressure, but I don't know how close. I've been adjusting them if they get as much as two pounds apart. Is that sufficient diligence, or should I be keeping them closer yet?

That's probably within the error margin of about as good as you can do. There will always be some variation, and tire gauges aren't that accurate, generally.

The biggest thing for a dually setup is keeping the load as even as possible on all 4 tires, which is why you want the pressure to be close. If you're checking them that regularly, you're fine. The load per tire can be guesstimated by figuring the footprint times the pressure, so if you think about it, a couple psi difference will still be not more than 20-30 lbs difference between tires. That's well within what the tires can deal with.

 

[Wiploc]

Thanks.