Technical FAQ with Lennard Zinn: Shimmy testing and fixing tips
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
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First, some follow-up from our last column A few more reader comments on the creaking bottom bracket (finding thesources of these things can be a bear!):
1. On April 25th, you included a letter from a guy who had a click in his BB that would not go away. I had a similar problem that was undiagnosed by some pretty good shops. I fixed it by replacing the down tube water bottle holder. It had a broken weld that could not be seen, unless you flexed it a little.
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 derailleur clamp band creaking against the frame. I ended up lightly greasing the clamp band where it contacts the frame. Unfortunately, I had already tried all the things you suggested (including a bottom bracket overhaul 3 or 4 times) before I discovered this. Granted, the Bianchi is probably painted vs. raw titanium (which might make the creak less likely), but it’s an easy thing to try.
Now 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 53×15 I get a vibration going in the bike.I specifically notice it in my water bottle cage. The bottle actually shakes and 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.
Answer: 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 stem up. That is the lightest point in the wheel. You know how rough the ride in your car would be if you did not have your tires balanced with those little lead weights.
It’s the same on a bike, and if you put it in a bike stand 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 do this. With lighter wheels, the problem can be bigger because the imbalance is often bigger. (The mass of the splint or the weld material at the rim seam is still just about the same weight, no matter how light the rest of 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. With a long, threaded valve stem, you can thread on a bunch of the valve collar nuts. 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 see if it doesn’t go away.
Question: I have a Lemond 853 frameset with a carbon fork and have noticed shimmy at speeds around 35mph, mostly on long straight downhills. I have gotten conflicting answers to the causes of this, from its a problem with the fork to it is inherent in bicycle design. How do I deal with this and what is the best position for coasting on long downhills.
Answer: In my experience, front-end shimmy is most commonly associated with an overly flimsy front triangle, especially with a heavy rider sitting on it. It is for this reason that it is so common on large frames. I know nothing about you, but I am willing to bet that you do not have a tiny frame, either for your bike or of your body.
The fact that I had a nasty shimmy on some (64cm+) bikes I raced on in the early 1980s had a lot to do with my deciding to become a frame builder, since shorter riders did not seem to be having shimmy problems on descents like I was.
If the front triangle is easily able to twist side to side, it creates an opportunity for shimmy to happen. As a frame gets larger, especially if it is built with the same tubing (same diameters and wall thicknesses) as smaller frames in the manufacturer’s line, it becomes increasingly easier to twist the frame back and forth. The time it takes for a single back-and-forth twist (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 twisting cycles per second – i.e., the reciprocal of the period) tends to be low enough that a resonance (an oscillation at the resonant frequency that builds in amplitude) can be built up in the frame when encountering normal road 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 also dependent on the weight of the rider. A frame that will not shimmy for a light rider often will shimmy under a heavier rider. I find that increasing the diameter and wall thickness of the main-triangle tubes, particularly the top tube, greatly reduces the incidence of shimmy.
I also reduce shimmy likelihood by shortening the seat tube and lowering the top tube and/or angling it upward. A shallower head angle and/or longer fork rake tends to allow the fork to absorb more shock that might otherwise be translated into twisting oscillations.
Finally, almost all bicycle wheels are unbalanced, as I mentioned above. Balancing one’s wheels in the manner I described also reduces shimmy problems. Also, obvious maintenance problems can play a role in shimmy, like a loose headset, out-of-true and out-of-dish wheels, and a tire glued on crookedly. And shimmy effects are exacerbated by strong winds.
Question: Somewhere on your Web site there used to be an article by Mr. Zinn that described how to replace various parts in Campy Ergo power shifters. It appeared long ago in a printed edition of the magazine, but I have long since tossed that. Does that piece still reside somewhere in cyberspace? Can I still view it or print it? Do you even know what I’m talking about?
Answer: 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 chapter of “Zinn and the Art of Road Bike Maintenance.” It is all in there, with all of the various generations and numbers of speeds.
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.”