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Stupid question about tires and air pressure.(8 posts)

Stupid question about tires and air pressure.Mazinger
Jun 16, 2003 5:51 AM
So when you're sitting on the bike, is the rear tire suppose to look completely inflated (completely stiff) or is it supposed to have some "give" like a car tire?
I'm just afraid of overinflating and blowing out the tube.

I hope I explained this clearly. Thanks!
There's always some give.Spoke Wrench
Jun 16, 2003 7:54 AM
At higher pressures, the flat spot at the bottom of the tire just gets progressively smaller. Bicycle tires will have an inflation range molded into the sidewall of the tire. Up to 120psi or so, I just go by that. Over 120psi, I'd check with the rim manufacturer to see how much pressure they recommend for their rims.
Harder isn't always better, and other secondhand tales...cory
Jun 16, 2003 8:29 AM
My brother-in-law used to live next door to a Specialized tire guy in San Jose, and I got a secondhand report on tire pressure from him. One thing he said was that it's common to set the max pressure by inflating a sampling of tires until they blow off the rims, then use HALF that pressure as the maximum. No idea if it's still true, or ever was, but that's what he said.
Ideal pressure will vary with conditions, too--you can run your tires harder on smooth asphalt than on cobbles or chip seal. And it's not just a comfort issue, because overinflated tires will bounce around, lose traction and increase fatigue on long rides. One old guide I've heard for years, can't remember where it came from, is to measure the height of the tire from the ground to the edge of the rim with the bike unloaded, then when you're sitting on it. Start with a pressure that compresses the tire 15 percent of that height when you're in the saddle, and adjust from there.
Max pressure is marked on the tireKerry
Jun 16, 2003 5:10 PM
If you don't exceed the maximum pressure, you have no worries about blowouts - there's a solid safety margin built in. If you need more than 110-120 psi (7-8 bar) to prevent pinch flats, then you should switch to larger cross section tires. Go by the pressure gauge, not by how it looks.
Max pressure is marked on the tireJimboTero
Jun 18, 2003 11:56 PM
I always ran the absolute max listed on my tire,(smaller contact area, less resistance, right ?) except sometimes letting a bit out before a rainy technical descent-untill today ! Pedaling through a corner I hit the ground with my new SPD/SL (Darn,darn darn!)and of course the rear wheel jumped up, but then landed with a resounding BLAM !! A blowout !! I'm 5'11", 168 lbs. I use a light tube, so from now on I'm gonna run about 5 lbs. under the max. I don't often hit my pedal but I thought about unexpected potholes, ect. I didn't enjoy the hour long, barefoot walk to the pay phone!
The tire didn't blow out, the tube did...TFerguson
Jun 19, 2003 4:11 AM
The pressure rating is for the tire, not the tube.

If the tube blows because of an impact, the most likely cause is a pinch flat where the tube is holed because the tire is compressed enough to hit the rim. Less pressure will make this more likely to happen. But in this case, I would think the tube was suspect if it blew from just a little bounce.

TF
You put air in the tube, NOT the tire !!JimboTero
Jun 19, 2003 6:17 PM
I've never heard of a tire that holds air without a tube. The number on the side of the the tire indicates how much air you can pump into the tube in the tire. The more air you pump into it, the thinner the rubber of the tube is stretched.
Try puting 100 psi in a tube without the tire. (nm)TFerguson
Jun 23, 2003 5:53 AM