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Technical Q&A with Lennard Zinn

Published: May. 16, 2002

VeloNews technical writer Lennard Zinn is a frame builder, a formerU.S. national team rider and author of several books on bikes and bikemantenance. This is the third of Zinn's weekly VeloNews.com column devotedto addressing readers' technical questions about bikes, their care andfeeding and how we as riders can use them as comfortably and efficientlyas possible. Readers can send brief technical questions directlyto Zinn. We'll try to print a representative sample of questions eachThursday.


First, some follow-up from our last columnA few more reader comments on the creaking bottom bracket (finding thesources of these things can be a bear!):1. On April25th, you included a letter from a guy who had a click in his BB thatwould not go away.  I had a similar problem that was undiagnosed bysome pretty good shops.  I fixed it by replacing the down tube waterbottle holder.  It had a broken weld that could not be seen, unlessyou flexed it a little.
Sean
 
2. I had what seems like an identical noise with my Merlin frame. Not sure if the Bianchi SL has a clamp-on or braze-on front derailleur(mine was clamp-on) -- but for me the problem was the front derailleurclamp band creaking against the frame.  I ended up lightly greasingthe clamp band where it contacts the frame.  Unfortunately, I hadalready tried all the things you suggested (including a bottom bracketoverhaul 3 or 4 times) before I discovered this. Granted, the Bianchi isprobably painted vs. raw titanium (which might make the creak less likely),but it's an easy thing to try.
ChadNow to this week's questions:Question: After I replaced my 8-speed gruppo with a 9-speed DuraAce gruppo, I have only been testing it on my Cycle-Ops fluid trainer.When I get pedaling with like a 53x15 I get a vibration going in the bike.I specifically notice it in my water bottle cage. The bottle actually shakesand makes a lot of noise. It does it in both cages, and they are not loose.What is causing this vibration? I have never had a problem quite this weird.
JimAnswer: I imagine it is an unbalanced rear wheel.If you let the wheel spin down without a load (or a chain) on it,assuming the bearings are in good shape, it will stop with the valve stemup. That is the lightest point in the wheel. You know how rough the ridein your car would be if you did not have your tires balanced with thoselittle lead weights. It's the same on a bike, and if you put it in a bikestand and spin the crank so that the rear wheel is spinning very rapidly,it will jump up and down along with the stand. Almost all bikes will dothis. With lighter wheels, the problem can be bigger because the imbalanceis often bigger. (The mass of the splint or the weld material at the rimseam is still just about the same weight, no matter how light the restof the rim is, and there is less overall angular momentum at the same speed).Correct the imbalance by stacking up weights at the valve stem. Witha long, threaded valve stem, you can thread on a bunch of the valve collarnuts. The wheel is balanced when it spins down to a stop at random spots,rather than with the valve stem up. Then ride the trainer again and seeif it doesn't go away.
LennardQuestion: I have a Lemond 853 frameset with a carbon fork andhave noticed shimmy at speeds around 35mph, mostly on long straight downhills. I have gotten conflicting answers to the causes of this, from its a problemwith the fork to it is inherent in bicycle design.  How do I dealwith this and what is the best position for coasting on long downhills.
John Answer: In my experience, front-end shimmy is most commonlyassociated with an overly flimsy front triangle, especially with a heavyrider sitting on it. It is for this reason that it is so common on largeframes. I know nothing about you, but I am willing to bet that you do nothave a tiny frame, either or your bike or of your body.The fact that I had a nasty shimmy on some (64cm+) bikes I racedon in the early 1980s had a lot to do with my deciding to become a framebuilder, since shorter riders did not seem to be having shimmy problemson descents like I was.If the front triangle is easily able to twist side to side, it createsan opportunity for shimmy to happen. As a frame gets larger, especiallyif it is built with the same tubing (same diameters and wall thicknesses)as smaller frames in the manufacturer’s line, it becomes increasingly easierto twist the frame back and forth. The time it takes for a single back-and-forthtwist (the period) tends to be much longer than for small, tight, frames,which snap back very quickly relative to a large frame.The frame’s frequency of oscillation (the number of back-forth twistingcycles per second – i.e., the reciprocal of the period) tends to be lowenough that a resonance (an oscillation at the resonant frequency thatbuilds in amplitude) can be built up in the frame when encountering normalroad surfaces and wind effects at oft-encountered speeds.I test for shimmy by riding with no hands down steep hills (checkout the "2002 Road Bike Gear Guide" on OLN TV). The oscillation is alsodependent on the weight of the rider. A frame that will not shimmy fora light rider often will shimmy under a heavier rider.I find that increasing the diameter and wall thickness of the main-triangletubes, particularly the top tube, greatly reduces the incidence of shimmy.I also reduce shimmy likelihood by shortening the seat tube and loweringthe top tube and/or angling it upward.A shallower head angle and/or longer fork rake tends to allow thefork to absorb more shock that might otherwise be translated into twistingoscillations.Finally, almost all bicycle wheels are unbalanced, as I mentionedabove. Balancing one’s wheels in the manner I described also reduces shimmyproblems.Also, obvious maintenance problems can play a role in shimmy, likea loose headset, out-of-true and out-of-dish wheels, and a tire glued oncrookedly. And shimmy effects are exacerbated by strong winds.
LennardQuestion: Somewhere on your Web site there used to be an articleby Mr. Zinn that described how to replace various parts in Campy Ergopowershifters.  It appeared long ago in a printed edition of the magazine,but I have long since tossed that.  Does that piece still reside somewherein cyberspace? Can I still view it or print it?  Do you even knowwhat I'm talking about?
JonathanAnswer: I am pretty sure it is not anywhere on VeloNews.com(nor any other site that I know of).If you can find the old issue, it is VeloNews, Feb. 28, 2000.It exists in print in an even more complete form within the shifter chapterof "Zinn and the Artof Road Bike Maintenance." It is all in there, with all of the variousgenerations and numbers of speeds.
Lennard

VeloNews technical writer Lennard Zinn is a frame builder, a formerU.S. national team rider and author of several books including the pairof successful maintenance guides "Zinn& the Art of Mountain Bike Maintenance" and "Zinn& the Art of Road Bike Maintenance."