Technical Q&A with Lennard Zinn - Buffing out those scratches

Published October 28, 2008 03:23AM

Dear Lennard,
All it took was one chain drop on my Dura Ace 10 speed to scratch up the jewel-like finish on the crankarm. Being the type who hates scratched up stuff, I broke out the # 0000 steel wool and to test out a thought, I lightly rubbed the inside surface of the opposite crank arm where there were a few little scratches from the Allen wrench used to install and remove pedals. What happened was, I got a dull finish where I rubbed. I rubbed a little more, not very aggressively, and the dull finish, which I just produced, came off. It looked really shiny and nice and then I went over the crank spider. I have to say, it looks like new but my question is, what did I do? I hope I didn’t ruin anything.
Richard

Dear Richard,
Great question! As for your work on the back of the crank, no harm done. But working on the spider, well, you may have produced a whole lot of work for yourself to maintain that finish. Your crank, like most aluminum bike parts, is anodized. Anodization is the black or gray surface on rims, for instance. On many parts, the anodized coating is clear, and that’s the case with Dura-Ace up until the 2009 Dura-Ace 7900 group, which is black.

Aluminum bike parts are usually tumbled in ceramic media first to smooth them off, and often polished afterwards, perhaps even hand polished. The process of anodization deposits aluminum oxide on the surface. Anodization doesn’t hide anything other than color, so if it is shiny when it goes into the tank, it is shiny when it comes out, but if it is rough, it will still look rough after anodizing.

Aluminum oxide is harder than aluminum, but you can rub through it with steel wool. It’s hard to know if you did so without seeing it. Certainly you must be familiar with what it looks like when a shoe rubs the anodized coating off across the middle of a crank. The shoe may even shine it rather than scratch it, but the bare aluminum thus revealed will look different than the anodized surface.

If you did rub through the anodization with your steel wool, then that bare aluminum surface you have rubbed down to is much softer than the surface of the rest of the crank, and once it forms a little oxide on it, the edges of the factory anodization will be visible around it. Which means that to keep it all looking shiny, you will constantly be rubbing it, always pushing back the retreating edge of anodization as you go, making the raw aluminum surface ever larger.

Aluminum on its own forms an aluminum oxide coating that inhibits further oxidization, protecting the metal a bit, but not against caustic attack, like from a salt-water bath. That natural oxide coating is easy to remove and polish to a high gloss, with a fine abrasive like Simichrome Polish. The thin anodized coatings on bike parts in the 1970s resulted in many riders constantly working on their cranks, hubs, rims, etc. with Simichrome to rub off the anodization and then to make the bare aluminum super shiny.

We (at Zinn Cycles) have wrangled over the anodization issue ourselves. At much expense and hassle, we have always sold our square-taper custom cranks with a high polish and clear anodization over it. But, like any cranks, with enough use, they get scratched and look crappy, and there is nothing you can do about it due to that hard anodization covering everything except the worn-off area. It took a lot of convincing, particularly of me, but we have decided with our new integrated-spindle custom cranks to not polish them (revealing the machining marks of which we are actually quite proud) and not to anodize them, either. It’s easy to scratch them, since they don’t have a hard-anodized coating, but on the other hand, it’s easy to remove the scratches with steel wool or a Scotch-Brite pad. And if somebody wants to polish them up to a mirror sheen, they can do that, too. Corrosion is generally not an issue, and a little abrasion would remove that, just like the bare aluminum braking surface of your rims always stays free of corrosion.

During anodization, a thick coating of aluminum oxide is obtained through immersion of the part in an electrolytic solution and passing an electrical current through it (using the part as the “anode;” hence the term “anodization”). The process is akin to chroming, but it deposits aluminum oxide, rather than chromium, on the surface. The anodized layer is nearly colorless, and it is porous enough be dyed different colors somewhat permanently. Colored ski poles are a good example, as are the purple and green bike parts of the 1980s and early 1990s. I said “somewhat,” because colors, particularly reds, can fade. Perhaps you have some pink mountain bike bar ends as evidence of this fading problem. After dying, if the part is placed in boiling water, the pores seal up.

Maybe you’re somebody who hates scratched up parts enough that you want to buff them down to the bare metal and anodize them at home! Here are some instructions: astro.neutral.org or www.turborick.com or www.warpig.com/. You have to not mind having strong acids sitting around in containers in your house, though! Maybe you don’t mind those scratches that much …
Lennard

Dear Lennard,
In one of your recent columns you mentioned that you use Scotch-Brite or a Scotch-Brite polishing wheel to polish stuff. I have been unable to find out what Scotch-Brite is and what its hardness is (diamond being 10, etc). I’ve looked everywhere on the net and cannot find anything. Being someone who is certainly more knowledgeable about the technical world, do you know?
John

Dear John,
3M’s Scotch-Brite brand is now 50 years old, according to its Web site, but I’ve never investigated its hardness. You might also look up Fibertex, which is a similar product that we cross-country skiers use to smooth and de-hair our ski bases. Perhaps you can find something about its hardness online.
Lennard

Dear Lennard,
I am a huge fan of road tubeless. I have converted my Ksyriums and my Spinergy FCC full carbon clinchers to tubeless.

I have looked at the Lew Pro VC-1 carbon clincher and I am amazed at how the clincher tire clings to the rim. I’ve even read the Lew “White Paper” that explains how the tire adheres to the rim, but it still is a bit unclear.

That being said, it appears to me the the Lew Pro VC-1 would be very good candidate for converting to road tubeless (with road tubeless tires of course), assuming the spoke holes can be easily sealed with Stan’s rim tape.
Randall

SRAM 1090-R2C (Return to Center) bar-end shifter
When I was at the Ford Ironman World Championship in Kailua-Kona, Hawaii a couple of weeks ago, I wrote a lot about the new electronic Dura-Ace bar-end shifters, but, for various reasons, my earlier posting about another new bar-end shifter did not go up, so I’ll post it now.

SRAM is right when it says this part is ignored. Campagnolo still has no 11-speed bar-end shifter to go with its new groups.

The new SRAM shifter has been in development for over three years, and it has a ratcheting design. Make a shift and release it, and it returns to lined up straight ahead. Kind of like RapidFire, wouldn’t you say? The shift levers always stay aligned with the wind instead of one of them being flipped down across the wind when you’re in the big gear. That’s a small amount of drag, but the hands also always stay lined up in the wind and make smaller movements to shift.

The photo shows Michelie Jones’s prototype ratcheting shifter that she has used for a number of months atop a press release with the rendering of what the actual thing will look like. The new

shifter currently is available only in aluminum and is not as light as the Force carbon levers.

I’ll be at the Milan bike show for my column next week.
Keep the rubber side down,
Lennard

Technical Q&A with Lennard Zinn - Buffing out those scratches

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