tire pressure Q

[SadieKate]

Huh? Does he does this to his car, the basketball, the football, etc., etc., etc.?

[madisongrrl]

I'm not sure what your friend is talking about, but it is a good idea to release the tire pressure between rides. I left my road bike in the back of my truck on a hot summer day. As the day wore on, the temp increased and my tubes popped. 5-10 dollars down the drain. If the temp goes up, then the pressure goes up.


Take it from a chemist PV=nRT (Ideal Gas Law)

[RoadRaven]

The only time I deliberately let air outta my tires is on really hot days (like MadisonGrrl) when the bike is stored in the car - the heated air in the tires can expand so much it can burst the tube... yes, its happened to us...

The air naturally "seeps" out of the tube anyways between rides - and a general rule of thumb should be to check tire pressure / pump your tires up at least every 3 days...

I find your friends advice interesting and wouldn't hold to much time in your thoughts for it.

[bcipam]

I know of no one who does this and alot of my friends are real bike geeks. I'll ask around.

[DebW]

There are butyl tubes and latex tubes - one leaks more slowly than the other, I don't remember which. Checking your tire pressure every 3 days or at least once per week is recommended. If you let out pressure between rides, well, you get more arm exercise before each ride. But it's really unnecessary. Even leaving your bike in a hot car is unlikely to blow the tubes unless you normally run your tires within 10% of the maximum rating for your tire. In the ideal gas law, P is linear with T, so a temperature change from 70F to 120F is only a 10% increase (convert T to Kelvin).

[madisongrrl]

In the ideal gas law, P is linear with T, so a temperature change from 70F to 120F is only a 10% increase (convert T to Kelvin).

10% can be very significant...especially if you are racing at multisport events which generally start in the wee hours of the morning.

A perfect example would be the Green Bay Duathlon that is held in Wisconsin in early October. You set up in the transition area between 5-6:30 in the morning. It's sucks, it's cold, there is stinking frost on the ground (I absolutely hate fall!) and the temps in the low 30's. You pump up your tires to the pressure that is correct for you....based upon your weight, bike, tires, course etc. The race starts at 7pm, but your run wave doesn't go off until 7:30. (I generally finish up the couse before 10, though there are plenty of people who finish up at 11:30-noon+...you really have to applaud them for being out on the couse so long). Now the temp is topping out in the low 70's (this happend in 2004).

If you run your tires at a 115 psi, even a 5% increase puts them up at 121 psi. Your bike will ride differently (harsher), you might be more prone to flatting and your traction will be reduced. I have noticed that my Cervelo rides differently even when I change the tire pressure 5 psi. The first year I had that bike I ran the pressure way higher than it should have been and gave myself a harser ride than was unnecessary (and when riding in the aero position.....boy does that hurt the bits and pieces). Lessons learned...

I don't think we need to obsess over tire pressure, but just the same its an important consideration on race day. I'm generally not worried about pressure and temp when I do shorter mountain bike races. But I still can't figure out which tires and pressure I should be running based upon my bike, my weight and course conditions. It seems like the longer I'm at this sport, the more I realize that I have to figure out.

Best,

[madisongrrl]

Even leaving your bike in a hot car is unlikely to blow the tubes unless you normally run your tires within 10% of the maximum rating for your tire.

I had my rode bike in the back of my truck. We were mountain bike camping at the Kettle Moraine, WI in late September. It was very cold at night (jackets, pants etc.) and very hot during the day (shorts and tanks). I had pumped the tires up to the max (120psi), but I did it when it was cooler. We spent the day mountain biking and the rode bike was locked in the back of my enclosed truck, baking in the parking lot. Both tires were flat as a pancake (we actually heard one of them go).

My two questions are: Do you think the heat caused this or was it something else? How many psi's would it take to pop a tube in an average 23mm, 120psi max tire?

[DebW]

I had my rode bike in the back of my truck. We were mountain bike camping at the Kettle Moraine, WI in late September. It was very cold at night (jackets, pants etc.) and very hot during the day (shorts and tanks). I had pumped the tires up to the max (120psi), but I did it when it was cooler. We spent the day mountain biking and the rode bike was locked in the back of my enclosed truck, baking in the parking lot. Both tires were flat as a pancake (we actually heard one of them go).

My two questions are: Do you think the heat caused this or was it something else? How many psi's would it take to pop a tube in an average 23mm, 120psi max tire?

If you pumped to 120psi at 32 degrees and your truck went up to 120F sitting in the sun, that's an 18% pressure rise, or up to 142 psi. Not surprising that would blow tires. I believe that the pressure printed on the tire is the max you should ride at, though you don't necessarily need that much. There must be some margin above the printed max, otherwise tires would blow when you hit a bump while riding, but not sure how much. BTW, it's not the tube that's critical, it's the tire bead blowing off the rim, which will of course blow the tube because the tube then expands very suddenly. With tubulars, it's the strength of the stitching.

[Nanci]

I don't have anything to contribute, but this is a really interesting thread!

Didn't the guy in the Tire Pressure Article a couple weeks ago say that max pressures were based on a 135 pound rider? (They were recommending riding at 90psi) I've been riding at 95psi since then, and it seems fine to me.

I've never heard of releasing tire pressure. Not saying you should/shouldn't, just never heard of it.

Nanci

[Melody]

There are butyl tubes and latex tubes - one leaks more slowly than the other, I don't remember which.

Latex leaks considerably faster than butyl. Typically with latex my understanding is you want to pump up your tire each time. To offset that, however, latex is (supposedly) more puncture resistant, stretching instead of being penetrated by sharp objects.

It can be difficult finding latex tubes now though. I wanted to play with latex tubes but I went to 4 LBS' in the area and none had latex but I could find low weight butyl racing tubes.

Mel