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Technical FAQ with Lennard Zinn: That ticking could be your seatpost

  • By Lennard Zinn
  • Published Sep. 3, 2002
  • Updated Oct. 19, 2010 at 6:32 PM EDT

Question: One of my riding partners had a ticking (not a creaking) sound in his ride.  It would only happen when pedaling hard. Checked all the usual suspects, BB, pedals, and the like.  Turned out that greasing the seatpost stopped it.

–Tommy

Answer: Thanks. We once dealt with a bike that creaked/ticked, and, after much experimentation in and out of the saddle, we isolated it to the seatpost. But in this case, greasing the post was insufficient. The post extended below the seat-tube reinforcing collar and was moving against the inside of the seat tube, which was larger inside than the collar (that fit tightly around the post). In that case, cutting the seatpost off so that it did not extend below the collar eliminated the creak.
–Lennard

Question: Have you noticed the unusual helmet being worn by some members of the Lampre-Daikin squad? It says “Rudy Project” (their sunglasses sponsor) on the helmet, but I spoke with the U.S. Rudy Project people and they know nothing about the helmet. Can you find out who makes the helmet, what is the tech idea behind this new design, and can this helmet be purchased in the U.S.?
–Don

Answer, from Rudy Project: The helmet you are talking about is an original Rudy Project cap called “Stratosphere” and is indeed one the helmets used by Team Lampre. Athletes love to wear it because itis so light and comfortable.
Some more information is available in our website (path: Products/Helmets/Stratosphere) as well as a complete AdobePDF file (easy to download). This helmet is not really a helmet but rather a “cap” and has no certification for the U.S. market. This does not allow us to sell it in America.
–David Michaud, Rudy Project Head of Design

Question: Would you happen to have contact information for Russel Bollig who was written up in the article “Champions’ footsteps“?
– Dave

Answer: You can reach Russel Bollig at Podium Footwear, 303-554-0505.
–Lennard

A follow-up to a previous Q&A segment A comment regarding shimmy:
I really enjoyed Ted Costantino’s “Shake, Rattle and Roll” article in the July 1st issue of VeloNews. I just want to add my two-cent opinion to the subject of front-end shimmy. Let me preface my dime store thoughts with the caveat that I’m not an engineer and have no proof to back up my opinions but hey, the Wright Brothers weren’t either and look where it got them!

I definitely agree with you that front-end geometry is neither the cause nor cure for shimmy. I also agree that frame geometry is the cause, but it’s not faulty geometry that causes shimmy but how a given geometry interacts with a particular rider.

You see, your article offered many examples of what may or may not cause the shakes: Vitus frames with Vitus fork (“rarely”); Vitus frame with carbon fork (“could”); superlight frames (“tend to…but not always”); etc. No one can pin down the source of shimmy because a particular bike under different riders may or may not do it. I find it hard to understand how a lack of top tube rigidity could contribute to shimmy because the fork isn’t twisting in a vertical plane during shimmy but rotating about its axis.

Here’s what I believe is occurring: All frame shave a natural resonant frequency with its wavelength dictated by the wheelbase. When a rider hits a particular bump in the pavement at a particular speed, or merely the pavement surface itself has a unique smoothness that induces a vibration in the frame at a particular speed, it causes the frame to resonate. If the frame’s natural resonant frequency and the pavement inducing vibration are the same frequency, when the frame is steered off-line and the rider corrects (even in the course of normal, straight line riding), this resonance can be amplified until the frame begins to oscillate like a guitar string.

Consider two stones dropped in water: the water ripples up and down away from the impact points. Where the ripples from each stone intersect the other and they are both  “up” or “down”, the amplitudes of ripples will add, making the resultant wave the sum of the two ripples. This is what I believe is similar to what happens when a bicycle starts to shimmy.

Your article listed a couple ways the rider stops shimmy when it occurs; he hits the brakes or shifts his weight to the rear. With my explanation, by braking, the road vibration frequency and the frame’s natural resonant frequency no longer match so shimmy disappears. By shifting his weight rearward, the rider is damping the vibration; his position on the bike changes the resonant frequency of the frame much like clamping the knees against the top tube can stop shimmy – the “hinge” or null point of the oscillation has changed. I also agree with you that rider weight and position can affect shimmy and in fact, I think this is the main factor in causing it.

You see, I ride a particular sport touring bike to work every day. When I ride the bike unloaded, I don’t experience shimmy.

However, when I throw my rear panniers on and ride the bike at higher speeds, the bike will shimmy if I remove my hands from the bars. It’s not bad but it’sthere and it definitely gets worse as the speed escalates, but I can only tell it’s there if I remove both hands from the bars.

Now, if I strap my 10-pound mechanic’s bag to the top of the rear rack (a higher position than my panniers), then the shimmy is so bad that handsfree riding at any speed is out of the question. Hands on the bars is no problem, because my hands on the bars puts changes the resonant frequency of the frame and puts self oscillation out of the range of normal riding speeds.

Here’s what I think the sources are: weight distribution (which you suggest in your article) and center-of-gravity (COG). Why COG; because the problem occurs when I add the higher position weight of the panniers, and is exacerbated when the weight is even higher up with the mechanic’s bag. I believe that a rider’s build affects the potential for shimmy to occur: how heavy is the rider relative to the bike? Do they have a heavy lower body relative to their torso and arms?

And as you suggest, a rider may eliminate shimmy by changing his position (fore/aft weight distribution) on the bike. You are right that changing bike position might cure the problem. Just suggesting a rider buy a different frame without knowing what to change may have a rider wind up with another unrideable frame.

Since I believe COG is the major contributor, I recommend a lower bottom bracket, and a test I would recommend the affected rider try to reinforce my claim is this: locate a hill which they are known to experience shimmy on and reproduce it by riding down. Then lower the seat 5-10mm and retest.

For accuracy, repeat the test three times in the original and modified positions. Since I have this resonant frequency theory of the frame, I also believe you can remove shimmy by changing the wheel base of the frame by purchasing a frame with longer chainstays (most racing frames have chainstays too short to consider seeking even shorter chainstays, so I wouldn’t bother considering that option).

As a final thought that just occurred to me: your assumption that a flexy top tube may contribute to shimmy may just make sense to me. One way to test this theory would be to take our test bike/rider/hill and produce the shimmy. Then, using something like Vise Grips, C-clamp, or hose clamps, secure a couple round shims/splints at the center of the top tube’s length, and repeat the test.

Here again, by stiffening the top tube mid-span you change it’s natural resonant frequency, possibly putting it out of the range of normal riding speeds. If the frame has a replaceable top tube, repairs may be as simple as splicing in an internal sleeve or filling the tube interior of say, a carbon top tube, with perhaps a foam which would do the same thing.
–Peter

Response:
In my experience with building bikes for really tall and heavy riders,who almost universally come to me complaining about the shimmy in their existing bike, there is no question that rider weight and center of gravity play a big role. None of the bikes I have made for myself in the past 11 years shimmy for me, even hands off going down a steep hill. However, when a rider of my same height (6-feet-6-inches) who weighs considerably more than me (295 pounds compared to my 175 pounds) tries one of my personal bikes, they start shimmying at quite low speeds (20mph or so) as soon as he takes his hands off.

Certainly the COG has also gone up in this case. However, I did make a road bike, mountain bike and a tandem for this particular customer which did not shimmy. The change I made was to simply do more of what I had already done on my own bikes to eliminate the shimmy (something I had dealt with my entire riding life until I stopped using skinny-tubed lugged frames).

To eliminate shimmy on my personal bikes, I started lowering the top tubes and increasing the tubing diameters and decreasing the head angles. On this 295-pound customer’s bike, I started from there and further increased the tubing diameter and, in some cases, the wall thickness as well. I did not change the bottom bracket height, chainstay length or wheelbase.
–Lennard


VeloNews technical writer Lennard Zinn is a frame builder, a former U.S. national team rider and author of several books including the pair of successful maintenance guides “Zinn & the Art of Mountain Bike Maintenance” and “Zinn & the Art of Road Bike Maintenance,” and “Zinn & the Art of Triathalon Bikes.”

FILED UNDER: Bikes and Tech / Technical FAQ TAGS: / / / /

Lennard Zinn

Lennard Zinn

Our longtime technical writer joined VeloNews in 1987. He is also a framebuilder, a former U.S. National Team rider, and author of many bicycle books, including Zinn and the Art of Mountain Bike Maintenance 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. He holds a Bachelor’s degree in physics from Colorado College. Readers can send brief technical questions to Ask LZ.

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