Technical Q&A with Lennard Zinn: Water build-up and Shimmy solutions

By Lennard Zinn

Water Solution
Dear Readers,
It seems that whenever it is raining somewhere in the USA, I get questionsabout drain holes in the bottom bracket and rims. In the past, I have advisedpeople to drill their own if they are not present, but of course that isat great risk of voiding their warranties. However, here is a solutionthat might appeal to those whose bikes are filling up with water as wellas for those who do not want to void their warranties.
LennardDear Lennard,
I recently discovered it’s not difficult to drill a hole down the centerof the set screw that holds the cable guide in place below most bottombrackets. It worked great on my Merlin, which was a sponge before I modifiedit.
IanCheck the alignment
Dear Readers,
As many of you know, we have discussed speed wobble a lot here. Wehave discussed wheels, stiffer top tubes, headset adjustment, increasedfork rake, and losing weight off of the rider as ways to reduce it. I gotthis letter from carbon guru Craig Calfee, however, regarding alignmentof the fork blades being a culprit in high-speed shimmy. Fork alignmentis something that all we framebuilders used to worry about in the era ofsteel forks, as we were building them ourselves, but with suspension forkson mountain bikes and carbon forks on road bikes, neither of which canbe aligned, we all just abdicated responsibility for that and assumed thatthey were right on. Calfee, however, finds that they are not, and he furthermorefinds that he can eliminate bike shimmy by replacing a poorly-aligned forkwith a straight one.
LennardDear Lennard,
Calfee Design has identified a cause of speed wobble (a.k.a. shimmy)and instability that can be prevented. Speed wobble is a dangerouscondition that can cause the rider to lose control of the bicycle and crash.While loose headsets and out of true wheels and frames can contribute tospeed wobble, we have found that fork asymmetry can also cause speed wobble.Fork symmetry is defined as the symmetrical position of the fork dropoutsin relation to the steering axis. Specifically, the equality of thedistances from the dropout faces to the steering axis must be within acertain tolerance for the bike to ride in a stable and confident manner.Traditionally, steel forks were cold set after welding or brazing torealign them after possible distortion caused by the heating and coolingof the metal. A diligent steel frame builder can align the fork bladesto within a millimeter of symmetry.Carbon fiber forks cannot be cold set. They must be molded straightto begin with. We have found that a small percentage of carbon forksby various makers were molded with asymmetrical fork blades. Someare off by a little over a millimeter and others are off by two or more.Forks that are off by over 1.8 millimeters in symmetry have a good possibilityof being prone to speed wobble. A symptom of a fork that is off by1.8 mm or more is a noticeable difficulty when riding no hands at a slowspeed (less than 10 mph). One has to lean to the side slightly tokeep going straight. A bike with asymmetrical forks seems to cornerbetter in one direction but not so well in the other. At speeds of30 mph or more, the bike can develop speed wobble.If your bike has the above-mentioned symptoms, the fork should be measuredfor symmetry. This is difficult to measure without proper tools.Calfee Design measures all forks for symmetry and is equipped to measureany fork. If any Calfee customer wishes to have their fork checked,please send it to us with a letter requesting a fork inspection.Non-Calfee customers may send their forks for inspection for a nominalfee. Replacements may be available for asymmetrical forks, dependingon the individual fork maker’s policy.

Fork Symmetry Measuring Setup:

1. Fork blades must be square to the surface plate
2. Rotate fork in V blocks to measure other dropout
3. Difference between the two measurements must be no more than 1 mm(Calfee tolerance)

Craig Calfee
Calfee DesignSize matters
Dear Lennard,
In your Technical QandA “Ike” talked of his road wobble, and it didbring up a theory I have for this. When I was at college, we didan experiment regarding “Natural Frequency”. It involved a volumeof water and other things I can’t seem to remember at this time havingto do with resonance/vibration. Anyway, it showed that at certainvolumes of water there was more resonance/vibration in the water at certainconsistent intervals. Could this be the same effect most of us feelat certain speeds, i.e. the bikes natural frequency with the road?I have not noticed it on my current bike, but I have on previous ones,where it would happen at couple different specific speeds, but not at allotherwise.
JakDear Jak,
Yes, I am convinced that it has to do with the resonance frequencyof the bike plus rider. A bike that shimmies like mad under a 300-poundrider often will be solid as a rock for a 175-pound rider of the same heightat even twice the speed. I think that you want the resonant frequency ofthe bike plus rider to be as high as possible. Anything that reduces theresonant frequency, like having a bigger frame, which takes more time totwist pack and forth through one vibration cycle, or having a heavier ridersitting on it, increases the likelihood of a dangerous shimmy situation,since the amplitude can build and build at the resonant frequency of theframe until it throws the rider off. And the lower the resonant frequency,the more likely that common environmental factors like wind or bumps onthe road, will set up the shimmy.Shimmy can be quite dangerous, as the numbers of injured riders whohave contacted me after having been thrown off of their violently-shakingbikes can attest to. The one thing I can say is, if your bike starts shimmying,immediately squeeze your knees against both sides of the top tube to dampthe vibration, and apply the brakes carefully, as braking hard can sometimesincrease it. It’s hard to say for sure, but it kind of sounds like thisfatal crash might have been caused by shimmy:
Lennard

---

Technical writer Lennard Zinn is a frame builder (www.zinncycles.com), a former U.S. national team rider and author of several books on bikes and bike maintenance including the pair of successful maintenance guides “ Zinn & the Art of Mountain Bike Maintenance” and “Zinn & the Art of Road Bike Maintenance.”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.