Wednesday, September 2, 2009

To Air or Not to Air

Since the Austin Tri is this weekend, we're re-posting an article from the June 2009 J&A Newsletter regarding air pressure in tires for overnight bike check-in.

To air or not to air?

For big events, bikes are sometimes required to be left overnight in the transition area. For some reason word spreads like wild fire to let the air out of tires so that they will not blow overnight. If you have been at an event like this and heard tires blowing right before the event, it has further increased your fear of this happening to you and made you a believer in this pre- race superstition. Below we will give the real reasons why tires blow in transition and a little info on why they will not.

Because people are nervous about getting flats, they tend to replace both tubes and tires for big events. This leads to tires and tubes not being seeded correctly and then blowing in transition overnight.

Because people are using bikes that are not very well maintained, they may have dry or weak spots on the tires. Mechanics / helpers / friends helping others getting ready in transition during bike check in will normally pump tires up to the max tire pressure. This exposes already weak, dry rotted or damaged areas of the tire. This is the main culprit of tires blowing.

A severe temperature change will only cause a very slight change in tire pressure overnight. A swing of 50 degrees will be a shift of less than 10 lbs. Most wheel manufactures' rims can withstand over twice the recommended tire pressure before the tire would pop off of the rim. So if your tire is to be aired up to 120 lbs, chances are your rim can hold twice that amount of force or more.

Rubber is also a porous material. Tubes and tires will lose pressure over a short amount of time. Some tires will lose as much as 25 to 40 % of their air in a week. If you air your tires up the day before you will probably have a little less air in your tires by race start. This would be a bigger reason why to air them up race day.

In short, letting the air out of your tires before an event will not prevent flats overnight, but if you like to do it because it makes you feel better, that is fine. Sports are surrounded by all kinds of pre-event habits and superstitions. Letting your air out can be chalked up to that.


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