Friday, September 19, 2008

Exploding Rims and Derailleur Hair



Yesterday this wheel came in on a mid-1990s Gary Fisher. The rider reported that it exploded AFTER he had completed a precipitous descent.

"I felt a thump-thump-thump and stopped, but the spokes were all tight, so I started up again. A minute later, BLAM, it blew," said the rider.

I'll guess it did.

He did not crash.

In case you don't know, this sort of failure started becoming common during the mountain bike boom. Powerful rim brakes in an abrasive environment wear away rim sidewalls probably ten times as fast as road rims used to wear.

Today my associate across the bench drew this piece of pubescent componentry on a vintage Columbia road bike. Sorry the pictures are fuzzy. So is the derailleur.

5 comments:

greatpumpkin said...

My LBS reports that a commuter customer switched to disk brakes because he was tired of wearing out rims several times a year. Perhaps more effectove (higher friction) pads have something to do with this too. And could the rims themselves be less durable than old rims were?

cafiend said...

I wondered myself whether the alloys and treatments used in rims now affected abrasion resistance. The coatings and anodizing of rims in the 1970s did seem to make braking less effective until the wheels had seen a few miles. Since I started running Mathauser pads in 1980 and have run salmon ever since, I would expect to see maximum abrasion. Mathauser used to say that the pads needed a break-in period because of their durable compound. The only rims I wore near to failure were the first set on my 1991 Stumpjumper. I built new wheels for that in about 1995. I'd put Mathauser pads on when I got the bike.

I have old Weinmann concave rims that show very little wear. On my 'cross/commuter I run a Mavic MA2 laced up in 1995 on the front wheel (my primary braking wheel). It shows little wear. That front wheel was on two or three different road bikes, in continuous use. I may even have built it before 1995.

Newer, heat-treated rims seem to die of fatigue failure before brake pad wear becomes apparent.

Ron said...

Good pictures. Nice blog, I started following your posts today. Which bike shop do you work at, and where?

I'm not an expert but I think this whole anodizing deal just creates a coat of removable oxide layer over the rims that crack and fail before the wheel takes the loads. It appears to be cosmetic as opposed to having any structural advantages for the rim.

This is my blog btw. On the left, side, as you scroll down, you'll find a list of "Failed Bicycle Ideas". I have already recognized the anodized mavic MA-40 rims as a failure because of its bad track records. :)

cafiend said...

Ron, I'd guess with your background in mechanical engineering you have a better handle on the technical issues than I do. I've just logged a fair amount of mileage and I learn from anyone who brings more knowledge. Anodizing doesn't seem to be a huge issue these days because it is a lot less prevalent than in the 1980s. I never happened to own any dark rims when I was trying to race back then. I also avoided the color craze of the 1990s.

I'll havae to see about linking out to your blog when I'm not poaching computer time at work.

I disguise my shop ID somewhat because "the views of the management are not necessarily reflected in this editorial."

Ron said...

Fair enough! :) Yes, not many anodized rims out today. But enter the funny carbon composite spokes.