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Technical Q&A with Lennard Zinn – In search of a magic bullet

By Lennard Zinn

Shifter cable for brakes?
Dear Lennard,
A Dura-Ace shifter cable is slightly narrower than a typical brake cable. On a triathlon bike that uses bar end shifters and brake levers in the basebar, can a shifter cable be used in place of a rear brake cable to decrease friction through the housing? Or is it too weak to hold up to braking forces?

Dear Mark,
I don’t think it’s a good idea, unless you’re testing it either on somebody who is very light in weight, or somebody whom you don’t like.

According to Ric Hjertberg, soon to be former employee of FSA, writes: “JIS says the gear cable should support 100 kgf and brake 150 kgf (when pulled from the lug).

“So, by those numbers, a gear wire would have no problem doing brake chores. However, that’s not useful because gear cable makers value longevity and linear stiffness more than strength, which stacks the deck against using gear wires for braking.

“FSA makes a cable set for Vision aero levers that’s got a gear wire lug but a brake cable diameter. While we’ve all seen compromised arrangements like a brake connected to a bar end shifter, the best policy is full function.”

And Wayne Stetina from Shimano says, “Seems to me the greatest difference might be the strength of the cable end caps? As you know, if that fails, it’s total brake failure during maximum application of brake force.”

And Shimano’s Devin Walton writes, “The bottom line answer would be no. The reason is that the testing for the two applications is different and the cable heads are different. I think it would be irresponsible to allude to any tensile strength differences as that might indicate that there would be an appropriate substitution if they were similar. The best ways to avoid friction are to keep a clean cable and avoid sharp bends in the housing. Also, not all housing are created equal as far as the materials that they are lined with.”

Re: scratched cranks
Dear Lennard,
I read your column on polishing anodized cranks this morning, and it brought up another question. I got a new CX bike this fall, with SRAM Force components. In the heat of battle, I’ve managed to kick off the top layer of material on the right crankarm with my cleat in some places. I’m not worried about polishing it; I’m concerned with how much damage I may have done to the structure of the crankarm. Any insight on this?

Dear Rich,
Well, I would guess that you just chipped off the clear coat. If you don’t have torn fiber ends sticking up, I wouldn’t worry about it.

Dear Lennard,
Now that you’ve explained how to remove scratches from aluminum parts, could you please give me some tips for dealing with scratches on carbon parts?

Dear Darrell,
Scratches are not in the carbon, they are in the clear coat covering the carbon fabric. It’s the same as a scratch in a painted bike or car. You can use rubbing compound, available at automotive paint supply places, to rub out the scratches. Turtle Wax or other car wax with abrasive content will take out small scratches.

Dear Lennard,
I read your response regarding the use of steel wool to clean scratches from aluminum crank arms, and I feel the reader has done damage to his crankarms. The readers’ concern was potential damage to his crankset from his actions taken with steel wool. One major problem created by introducing steel wool to aluminum is “dissimilar metal corrosion” or galvanic action between dissimilar metals; this case aluminum and steel wool.

In aviation (my profession), it is forbidden to use any steel tool i.e. wire brush, steel wool, etc. to remove corrosion from aluminum. The fear is tiny steel particles becoming embedded in the aluminum and result in a process in which ions move from metal to another. When those ions move, the metal losing ions, aluminum, decays. The end result is the metal breaks down in the form of corrosion, and structural integrity of the crankarm is compromised. To kick off the process just add moisture. To really accelerate the process, add salty moisture; sweat will do the trick.

I would agree where the shoe rubs on the cranks the metal will always be shiny and free of corrosion or other protective barrier. However, where there will be no contact, corrosion will form. If using an abrasive pad stick with Scotch-Brite; white or blue will work on a bike red is almost too coarse. Or, one could use aluminum oxide based sandpaper or emery cloth.

Technical writer Lennard Zinn is a frame builder (www.zinncycles.com), a former U.S. national team rider and author of numerous books on bikes and bike maintenance including the pair of successful maintenance guides “Zinn and the Art of Mountain Bike Maintenance” – now available also on DVD, and “Zinn and the Art of Road Bike Maintenance,” as well as “Zinn and the Art of Triathlon Bikes” and “Zinn’s Cycling Primer: Maintenance Tips and Skill Building for Cyclists.”

Zinn’s regular column is devoted to addressing readers’ technical questions about bikes, their care and feeding and how we as riders can use them as comfortably and efficiently as possible. Readers can send brief technical questions directly to Zinn. Zinn’s column appears here each Tuesday.

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