• Bikes & Tech

Technical Q&A with Lennard Zinn – Keeping it simple

by VeloNews.com

• April 22, 2009
• Comments Off

By Lennard Zinn

Dear Readers,
A bike tech column is going to naturally gravitate toward discussing new equipment. I regularly get letters, however, extolling the virtues of a more sustainable approach with less emphasis on the latest equipment.

Given that it’s Earth Day, I thought it a good time to give voice to a couple of these letters. And for Earth Day (and every day), ride your bike and avoid motorized transportation as much as you can.
Lennard

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Dear Lennard,
With just about every bicycle part being made out of carbon fiber these days, I was wondering if the carbon fiber could be recycled. I like the idea of steel, aluminum, and titanium being able to be recycled and reused. I’d hate to see all this expensive carbon fiber end up sitting around in landfills for generations to come.
Cynthia

Dear Cynthia,
Well, I called Eric Lombardi, director of Eco-Cycle, Boulder County’s groundbreaking (and country’s largest community-based) recycling program that recycles an amazingly diverse number of materials. He says that he knows of no recycling that is being done with parts made out of carbon fiber, and he’s as much of an expert on the subject as anyone I know.
Lennard

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Dear Lennard,
I’d like to see you and the other cycling media pick up on the pro-sustainability, anti-disposability vibe. There’s a reason so many attend the NAHBS, and/or ride single-speeds or fixed-gears. The bicycle is a simple, durable machine but the high-end stuff has gotten more finicky, short-lived, and high-maintenance. High-end stuff now focuses on a second-tier property that is easily measured – weight – oftentimes at the expense of first-tier qualities like reliability, fit, and handling. Those qualities are more important but not so easily quantified.

Note to the manufacturers: we don’t want yet another cog on the cassette, which needs complete replacement when a single cog wears out. Or narrower chains that evaporate in a few thousand miles.
Please weigh the merits before tossing out yet another headset or BB standard. There are plenty of us customers out here that want our perfectly-good bikes to last, so please keep them repairable and rebuildable with a minimum impact on the environment that we are so lucky to ride through.
Todd

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Dear Lennard,
Where do you stand on going double butted or not with titanium frames? I find it interesting that Kent Eriksen goes straight tube solely, but recognize lots of smart and accomplished builders make use of double butted 3/2 and 6/4.

Does it make more sense for certain applications and less for others or other riders? I’m 6-foot-2 and 190 pounds (at least in winter I’m that heavy).
Steve

Dear Steve,
I think it’s useful to explain how double butting came into being. It was created in the age of steel lugged bikes as a way to make the bike lighter without making the tubes so thin in the heat-affected zone that they would be damaged during brazing, and providing more thickness where the stresses are highest, namely at the joints. The ends were thicker than the center sections on all of the three main tubes except the top of the seat tube, since that area would be reamed anyway and you can’t have two diameters when you’re trying to fit a seatpost inside it.

Double butting is also used on aluminum bicycle tubes as well, for the same reasons, except that in this case, the tubes are welded, not brazed.
Welding is the process of melting the surfaces of the two parts to be joined to each other, usually along with some filler (in rod or wire form) of the same material.

By contrast, brazing (and soldering) is the process of melting a metal with a lower melting point onto the joint between parts made of a higher melting point metal, thus adhering them together.

Aluminum and magnesium are welded with an electric arc of alternating current (AC), rather than with a direct current (DC) arc (which is used to weld steel and titanium). With AC welding, the current moving toward the surface heats and welds, and when the current direction switches (usually 60 times/second), it pulls the oxide scale from the aluminum surface. Aluminum and magnesium oxidize so quickly that without this frequent scale removal, they could not be welded. Welding consumes some length of the tubes, partially explaining the large welds on aluminum and magnesium joints relative to TIG-welded steel and titanium joints. Due to this consumption of material, thicker tubes near the weld is a benefit.

On welded steel bikes, since the frame is not heat-treated afterward as it is with many aluminum alloys, the steel is weakened near the joint due to the high heat of welding – far higher than what is used in brazing or silver soldering – removing much of the temper the steel gained during drawing, butting and heat treating. A thicker tube near the joints bolsters the tube that’s been weakened at its highest-stress point; so again, butting makes sense, because a steel tube would weigh a lot if it were the thickness required for a strong joint throughout its length.

A butted tube, however, is less stiff than a straight-gauge (continuous thickness) one. Grab a yardstick (or meter stick) at the ends and flex it. Notice that the majority of the bowing happens in the center of its length, not at the ends. That’s what happens in a tube as well.

So, with a titanium tube, you can cut some weight by butting it, but you introduce more flex to a tube that’s already more flexible than a steel one of similar dimensions. Titanium welds, like steel welds, are small, and a good welder can weld titanium tubes that are just as thin at the ends as steel tubes. But the lower density of titanium allows more of it to be used and still be lighter than steel. So the necessity for butting is reduced due to the lower density, and a gain in stiffness is consequently achieved by using straight-gauge tubing. That’s why you see such a difference of opinion about using butted titanium tubes or not among Ti builders, whereas all steel builders will always choose a butted tube if it’s available. And for a rider of your height and weight, the stiffness of straight-gauge tubes would be beneficial.
Lennard

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Dear Lennard,
A friend cracked the top tube on his carbon frame. He brought it back to the local mail order outlet, and they told him “it will fail, but it’s fine to ride now because there’s not much stress on the top tube.” He has since purchased another and given this frame to his sister. This whole thing is driving me crazy because of A) the bike shop’s response, B) his apparent disregard for the safety of his kin, and C) the discussions we have about A and B.
What’s the deal here?
Otto

Dear Otto,
Well, I would be loath to call a bike with a cracked top tube safe, but of course I have not seen the bike. The top tube does have less stress on it than the down tube or seat tube, in general, but this one obviously did have enough stress on it to crack it at one time. If a stress big enough to fail the down tube came along, given that the frame no longer has the integrity of the top tube for support, there would be little to stop the entire front triangle from breaking in two, an occurrence that would most likely bring negative consequences to the rider as well, like landing hard on the pavement. And jagged edges of carbon fiber created by breakage of the tube that your face and torso are closest to raises other awful possible scenarios.

I had a teammate back in the 1980s who raced one race on his steel frame even though he knew it had a small crack in the top tube. He was a small, albeit strong guy, and it was “just” a criterium. Sure enough, the top tube snapped early in the race in the normal course of racing, not from crashing. He was lucky to avoid being gashed by the torn ends of the tube.

I guess I just can’t see how it would pay to send one’s sister out on such a bike. I don’t condone throwing out perfectly serviceable things, and I write maintenance books so people can keep their old parts running longer, but when it comes to safety, some things are not the worth the risk.
Lennard

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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.