Wednesday, February 18, 2009

Make or Brake

Taking a break from the ATOC for a post, I have been doing a lot of work on my mountain bike, and will be sharing some of my wrenching skills here over the next week or so. Taking time out to make videos and good macro still shots slows things down a lot, but with Sheldon Brown gone I'd like to offer some help to those interested in learning how to maintain their own gear.

Don't be alarmed by my rapid hand motion at the end of the cut. The Dremel is not nearly powerful enough to move your hand out of the way that fast. I was moving my hand to try to prevent the cut-off blade from getting caught and breaking. This doesn't happen very often, but with the light so close and the camera lens to worry about getting showered with sparks it was a bit harder to position myself in the usual way.

Having the use of both hands is also a big help, but if you have a friend to hold the end of the cable and pull on it a bit, and move it slowly so as to open the cut up, it will go very, very smoothly. Just be patient and go slow. If you are doing a lot of cutting (IE: having a re-cabling party with friends) put down an old rug or some cardboard to protect the floor and wear old clothes.

Many, many bikes come assembled with improperly cut cables, being too long or too short, and occasionally, being so poorly cut that it makes brakes and shifters perform poorly. Cables that are too short create kinks, while those that are too long create unnecessary drag and can be a danger for snagging.

Brake cable housing and shifter housing are not the same, and using a shift housing for a brake housing will result in catastrophic failure. This is due to the way the wire is wound, or not wound, as it were, on shifter cables, where the wires run lengthwise from end to end to minimize compression. This makes shift cable much less compressible, but putting a few hundred pounds of load on it will make it burst through the plastic sheathing and leave you without brakes at a really bad moment. Always double check to make sure you are using brake cabling and housing when re-cabling brakes.

Here's a short video showing the simple cutting station I set up with a brief discussion of the important features.

These photographs are from two ends of the same cable. One end I cut off with a Dremel cut-off wheel, and the other is the end the bike shop that sold me the cable made when they cut it off a large spool from the mechanic's bench. As you might imagine, the torn, stretched and cork-screw shaped end from the bike shop will not seat properly, will want to bend and "walk" its way around inside any fitting, turning the cable.

This brake cable would make for very spongy braking with the cable moving around a lot when put under load. If it were a shift cable housing the shifting performance would be very poor, resulting in noisy gears and jumping of gears on the rear dérailleur.

When you cut the actual cable, use some metal duct tape (no, not the plastic stuff, the metal stuff) to wrap the cable with during the cut. You don't want a cable strand coming loose while you cut. The heat of the cut will weld the cable strands together and make a much more durable end. Use the cut-off wheel as a grinding wheel and put a nice dull point on the cable. Again, the heat will weld the cable strands together. It makes for a perfect cut and a cable that will thread perfectly.

I prefer a piece of metal duct tape cut in the shape of a little flag to the little metal ends you get from the bike shop. Squeezing those ends is usually what breaks off a strand, which you don't find out about until you take it off, and then *poof*, you have a frayed cable you'll end up having to replace. If you cut the cable on the frame, which is perfectly acceptable, use something to protect the frame from hot sparks, which will pit paint and carbon fiber clear-coats.

It takes a bit more effort to do this work yourself, but personally, I wouldn't trust something as life and death as brakes to anyone else. Also, I really want my shifting to be as crisp and clean as is humanly possible, so I am happy to do this work myself so I get perfect cabling.

No comments: