CranesInTexas

Hey all. Just got back from a nice 50 mile charity ride down here in Texas. The morning started in the 40's and its now 2:30 and 85.

My question is regarding inflation levels for cold morning starts. I ride an aluminum trek (1200) with 25mm Gatorskins. Under normal weather i pump to 120 psi and since i'm 6'2" and 170 lbs i get a slight sag in the rear.

What psi should i be pumping to at 40 degrees knowing that later in the ride the temp will climb? my fear is that i pump to 120 psi cold and then the psi will climb while riding and with temp increases.

thoughts?

thanks

Charles Ramsey

PV=nRT measure the tempature on the kelvin scale. 40 degrees fahrenheit= 500 kelvin 100 degrees fahrenheit= 560 kelvin devide 500 by 560 and you get .89 multiply .89 by 120 psi and you get 107 psi gatorskins are used by santana or their tandems they generaly over inflate their tires by 20 percent so I wouldn't worry about it too much.

cooleric1234

Assuming the volume remains constant :-)

deraltekluge

40° F is about 278 K. What you meant is that 40° F is about 500 fahrenheit degrees above absolute zero, and 100° F is about 560 fahrenheit degrees above absolute zero. Your ratio is correct...the difference in pressure between 40° F and 100° F (assuming no leakage) would be about 11%.

Dewey Oxberger

This cracks me up. Years ago I was riding down a long, steep canyon road. I worked the brakes hard to keep my speed under 50 mph. I was doing the math in my head wondering if I'd heat the tires, cause the pressure to rise, and have a blow-out. PV=nRT said no but the temperature will soften the rubber of the tire and... AAAAAAAHHHHHH!

I'm still scared when I think about it. LOL.

StephenH

That's 500 on the Rankine scale.

youcoming

At 170lbs what are you worrying about. I live in Ontario , Canada and I pump my tire to the same 118 back 110 front all season long, never had an issue. And I weigh 223lbs. Stop thinking and just ride.

Charles Ramsey

Woops my bad I meant rankine still the calculation is corrent. The tire will of course expand a little making the pressure difference less than I stated.

CranesInTexas

oops, i definately meant 270 lbs. i havent seen 170 since i was perhaps 13/14 years old. Trust me, if i was sub 200, i wouldnt have posted.

Thanks everyone else for your information. Now that the max ratio of 0.89 can be applied in cold weather extremes, it poses another question.

What is the true max PSI a 25 Gatorskin can take. Sidewall says 120, but someone mentioned that they know above that is done regularly.

Thanks.

Road Fan

The tire will expand only slightly, way less than a millimeter across. Tire carcasses are not very elastic at all. The contant volume assumption is a reasonable approximation, IMO. There IS expansion, it's just really small.

Road Fan

I'd think the final limit has as much to do (if not more) with the rim/bead interface as with the carcass strength.

Plus, I don't think any manufacturer will publish the test-to-failure results. Before they know it, people will be experimenting with the "true capability," and eliminating the safety margins that are present.

And if you get a maximum value that does not come from the tire manuf, i.e. Continental ("I red itt on da Intarwebs"), how can you know the testing was extensive enough to find the limit? if it failed at say 165 psi, how will you be able to say it will not fail at 160 psi? And is probability of failure a significant part of the "true capability" spec?

Road Fan

For cars and motorcycles, inflation levels recommended by the vehicle manufacturer (not the maximums printed on the sidewall) are intended to be used when the tire is cold, that is, early in the usage day. Vehicle manufacturers are well aware of the rise in static pressure that will occur through the day, and have considered that in spec'ing out their recommended pressure settings.

MikeWinVA

As an engineer, my guess would be the working pressure is roughly 50% of the blowout pressure. This would assume a perfect condition and spec'd rim and tire, unloaded, and perfectly mounted. A basic rule of thumb designing a load bearing system is to give yourself 100% or more headroom (helps to keep the lawyers away).

This does not mean go out and try it! High pressure tire blowouts (not tube blow outs) where the carcass fails can be dangerous and hurt you.

CranesInTexas

Thanks for the engineering tips and warnings. I personally dont plan on pumping my tires to 200 psi, but its good to know that i can pump to 120 psi cold and not fear overinflation later in the ride.