Clincher Pressure Warning

 

Yesterday a customer brought in an almost new Cervelo where the rear rim of the wheel had literally blown apart.  A big section of the Shimano R500 rim had ripped off about a quarter of an inch from the edge.  Our crew has seen our share of bizarre problems but this was a first.

It simply looked like the tire had blown apart the rim.  When we asked the customer what happened, he said he inflated the tire to 150 PSI (the maximum indicated on the sidewall of the Vittoria tire) and went for a ride.  He was going about 25 mph when his rear wheel exploded.  Fortunately he was able to keep the bike under control and didn’t crash.  My immediate reaction was is this is a simple case of excessive air pressure and poor tire – wheel compatibility. 

Troubleshooting this problem ended up being more complicated than I expected.  Most major wheel manufacturers including Mavic, Zipp and HED clearly state a 125 PSI inflation limit for their clincher wheels.  There are plenty of tires that far exceed this limit (Tufo, Vredestein and Vittoria come to mind).  To restate the obvious, if you destroy your $1000 Zipp wheel from inflating the tire to 150 PSI don’t expect the manufacturer to replace it for free.  But there are more than financial issues at stake here!  There are safety issues and as I will cover later, performance actually suffers from over-inflation.

But back to our problem at hand, we called Shimano’s tech support and asked them what their wheel was rated for.  And they said they didn’t have a specification but that they didn’t recommend 150 PSI.  I couldn’t find a max pressure warning on the wheel.  Nor could I find anything listed in Shimano’s literature or website.  Shimano did issue a return authorization and hopefully the wheel will be replaced at no charge.

Assuming this wasn’t a simple case of a defective rim, this brings up some interesting liability questions.

1. Is Shimano negligent for not stating the allowable limit on their wheels and in their literature?  There is no doubt in my mind that they have tested their wheels for maximum tire pressure, but it is indeed curious why they don’t share the information.
   
2. Is Cervelo negligent for not testing this combination of the maximum tire pressure of the tires and wheels they spec on their bikes? 

The crazy thing about all this is that, despite what most consumers think, there are plenty of scientific studies that show over-inflating tires actually increases rolling resistance of clincher tires (makes you slower).  Over-inflation causes the tire shape to deform and to not contact the road surface smoothly.  There are two other negative side effects of over inflation.  The first is decreased traction and the second is less rider comfort.  Zipp has done some great research in this area (see some of the links below) and they go as far as to list a narrow recommended tire pressure range for their tires (manufactured by Vittoria) based on the rider’s weight and road / weather conditions.

The take away message here is to never inflate clincher tires more than 125 PSI.  Remember that as heat increases so does air pressure.  So cold tire pressure is different that a mid summer’s day riding temperature.  In addition the heat generated from braking will increase the air pressure of the tire.  So it’s best to use a tire designed to work optimally in the 110-125 PSI range and to take care that your tire is inflated to that range.  Be safe, be fast and have some fun out there!

Some more reading on the subject…
http://www.zipp.com/portals/0/support/documents/ZIPP_A6_Wheel%20Manual_0306.pdf
http://www.zipp.com/FAQ/tabid/81/Default.aspx http://www.zipp.com/Support/AskJosh/Wheels/Wheels_tubieclincher/tabid/210/Default.aspx