May 30, 2014

Bike Tire Pressure Discussion

This from Mark Mullen – Team Z Ride Leader

One question I hear a lot from newer cyclists and even from a few experienced ones is: what tire pressure should I be using?  And it is a little confusing.  Most tires give you a range (and it can often be quite wide) or simply a maximum pressure.  That implies there is room for variation, but how do you know what will work for you?  Well, you could take one of the following traditional approaches:

1) “I always pump my tires up to the max!” Why? “Because maxxing out everything is awesome, dude!” You mean, like your credit cards?  “Well, um. . .”

2) “I pump my tires up to whatever my friends tell me.”  Why?  “Because I always do what my friends tell me to do.”  You mean, like smoking, and watching Real Housewives of Reston?  “Well, um. . .”

3) “I pump up my tires until they explode in the transition area five minutes before the race is scheduled to go off.”

As an alternative, you could try research and science!

 Here’s a link to an article from Bicycle Quarterly  that includes a helpful table showing recommended inflation pressures for various width tires at various weights.  BQ  has been at the forefront of busting a few treasured cycling myths: that higher pressure tires are faster (after a certain point they aren’t, and may even be slower due to the energy lost by your body as it gets hammered by the harsh ride), that wider tires have greater rolling resistance (they don’t; they may even be faster in some case because they stay planted to the road rather than being jolted all over the place; narrower tires may however convey a slight aerodynamic advantage).

Three things to note:

A) The text makes this point, but please note that the weights in the chart are not rider weights, but wheel weights.  So you simply weigh your bike and yourself, and then use a percentage for each wheel.  For your average road bike, the article suggests a weight distribution of 45% front and 55% back.  If you have a touring bike the load distribution may favor the rear a little more.  On a tri bike, where more of the rider’s weight is over the front wheel, the distribution may be closer to 50/50.  However, using 45/55 is still not a bad idea since you can get a much more comfortable ride if you take a few PSI out of your front tire relative to the back.

B) The table gives recommended starting  values.  If you want to be a little faster and don’t mind getting the crap hammered out of your body, you might put in a few more psi.  If you want a little more comfort you might take out a little more.  But the table offers a sensible baseline against which to experiment.

C) The table shows very clearly why for heavier riders wider tires are better.  You see that for given wheel weights the necessary inflation pressure for narrow tires quickly rises above the point that most tires are engineered to withstand.      

The following text provided in response to this note by Ray Nancoz – who teaches a few of our technical bike/gear clinics.  

This is the chart I present in the Beerz & Gearz Clinic.   I too have found it useful as a starting point.   They call the chart “Optimized Tire Pressure” though they don’t say Optimized for what.   They chart is based on allowing a 15% “tire drop” which is an estimate of how much the tire will squish as it rolls under you.   I believe that the suggested pressure is what Bike Quarterly would consider the sweet spot compromise between comfort and speed.   I find that the recommended pressures work well on my commuter but on my race bike I get a better feel at a little higher pressure.   I start at the recommended pressure and then raise it little at a time till I start to feel the higher frequency road vibrations (buzz).