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Wednesday
Nov202013

More about shimmy

Hold a bicycle wheel at arm’s length in front of you, hold the hub spindle with your left and right fingers. Now move your hands in the motion of pedaling a miniature bike. The wheel may be spinning or not for the purpose of this demonstration.

This is the motion of the bicycle or motor cycle’s front wheel during a speed wobble of shimmy. It is not simply a fluttering back and forth in the horizontal plane, about the bike’s steering, but it is fluttering, or wobbling in a two dimensional fashion in both the horizontal and vertical planes, violently shaking the whole bike and the rider.

I used to think this was a design flaw, but now I am inclined to think it is a natural occurrence that happens because it can. On a two wheel vehicle the front wheel is able to move in both planes, whereas on a four wheel vehicle the front wheel can only move left and right about its steering axis.

However, when the steering bearings wear out on an older car, the wheel can then move in both planes, and wheel flutter will occur. This will happen at a definite speed. (It might be 60mph.) Drive beyond that speed and the fluttering stops, only to return again as the car slows, and the speed reaches that certain critical level, then will subside again at a lower speed.  

Most bicycles seem to shimmy coasting downhill at around 45mph. A loose pannier, or saddle bag will often cause a bike to shimmy at a lower speed. Therein lays a clue. It seems if there is something flapping around loose, it amplifies the shaking. The tighter the rider grips the handlebars, he then becomes this fluid extension of the bike.

This is especially true of motorcycles where riders have been tossed around like a rag doll. Some have even broken arms. Expert motorcycle riders have demonstrated that one can ride “No Hands,” induce a shimmy by tapping the end of the handlebars, then stop it by simply leaning forward.

Tall riders riding bicycles with large frames seem more prone to shimmy. Why? The seat tube slopes backwards usually at an angle of 73 degrees. The taller the frame the more the rider’s weight is directly over the center of the rear wheel. This provides a near vertical pivot between the rider’s weight on the saddle, and the rear wheel contacting the road. The front end of the bike can now shake about this pivot. (See above.)

If the rider is sitting fairly upright, the pressure of wind on his chest is forcing even more weight onto the rear of the bike. If the rider were to lift his weight from the saddle, that weight is now on the pedals. Lower and further forward. Get down into a low tuck position, and the weight is now towards the front of the bike. Pressing a knee or leg against the top tube will often stop a shimmy. This dampens the shaking without being actually attached to the frame thereby increasing the problem.

Go to this YouTube video for a montage of motorcycle shimmy’s where the riders quickly get out of it. In most cases it does not even seem to faze them. It is the same with bicycles, the bikes that are shimmying are the same ones that the pros use in the Grand Tours. The pros do not seem to have a problem descending mountains, reaching speeds as high as 55 -60 mph.

Finally the Fuso frames I built did not shimmy. (As a general rule, there have been rare occasions.) So what did I do different? The frames all had the stiffer Columbus SP chainstays. This gave the frames more lateral stiffness.

If a bike and its frame are anchored at the rear by the rider’s weight, and the front wheel starts to wobble, (As it seems it will at a certain critical speed.) it will only do so if it can. In order for the whole bike to shake, either the frame or the wheels are flexing. Add lateral stiffness to the frame and/or wheels and the front wheel can’t shake.

Move the rider’s weight forward and you are effectively holding the front wheel so it cannot wobble. It is okay that the wheel can move left and right about its steering axis, in order to wobble the wheel has to move in both the vertical and horizontal planes.

I was prompted to write about this subject again because I read this article. Written by a mathematician, it was a little beyond my understanding, so this is an attempt to look at the problem in simple terms. When I built frames I never had this issue, so I never addressed it. It is only in recent years I have started to study this and my views are still evolving.

I’m sure if the motorcycle manufacturers had all the answers they would fix the problem shown in the video link, but it is a complex matter. Feel free to add your views and ideas.

 

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Reader Comments (12)

This is a very interesting post. Myself, as a tall rider riding a 64cm frame, experience this problem a lot. I usually carry weight in front of the bike, during randonneuring events. I noticed that a handlebar bag reduces the tendency of shimming, while attaching weight to the frame itself (strapping a bag behind the very long steering tube) increases this tendency. It seems to me that this is due to the resonance of a mass-spring system (http://en.wikipedia.org/w/index.php?title=Spring_mass_system&redirect=no), which is affected by the mass itself, as well as the place where it is attached in the bike. The loose panier you mentioned is a good example, either because it amplifies the shaking itself, as because it lowers the resonance frequency. Also, bags can sometimes act as a damper (handlebar bag surely does) or not, depending on the bag and its content.

November 20, 2013 | Unregistered Commenterheltonbiker

Dear Mr. Moulton,

Thanks for your thoughts on speed wobble. I have experienced shimmy twice on one particular bike.

The bike is a Calfee Tetra 60cm that I purchased used. I have put about 7,000 miles on this bike, and have had it shimmy on me twice. Both times was when I was descending on a smooth road at about 30 mph. I was not able to stop the shimmy, but was able to slow down and stop the bike safely.

I am a large rider, 6'1" 210 lbs, 60 years old, and not particularly athletic. I have two other road bikes that I have ridden a similar amount of miles on each, never having experienced a shimmy on either one.

I have read a number of articles on shimmy, and most agree that one of the causes can be too much flex in the frame. In my case, Calfee frames are known to be very strong and stiff. It sure feels that way: were I can induce chain rub on the front derailleur by standing and pedaling my Ibis Spanky steel frame bike, I have never had that happen on the Calfee.

One thing that I have noticed on the Calfee, and not on the other steel framed bikes, is that when I hit a rock or bump in the road, off center with the front wheel, I can feel a mild shimmy come up though the fork. It only lasts about a second or two and dampens right out. I have never felt that on either other road bike, one with a steel fork and one with a carbon fork.

I really like riding the Calfee, it takes a lot of the sting out of the crappy roads here, but I don't want to experience any more shimmy. One thing that I have tried is swapping the wheels. The Calfee came with Campy Electron wheels, 22 spoke radial front and 24 spoke rear. Now I am using Mavic Open Pro rims with 36 spoke 3 cross spokes. I can't say that I notice much difference.

I have contacted the people at Calfee, and they suggested that we could rebuild the frame with a larger diameter head tube (currently 1") and a beefier fork.

I really enjoy reading your blog! I wonder if you could share any thoughts on this subject.

Thanks in advance.

Yours truly,
Frank Simmons

November 20, 2013 | Unregistered CommenterFrank Simmons

The motorcycle link you posted doesn't show what I would consider "shimmy", but wobbles induced by the rear wheel losing traction and stepping out. However, I have experienced actual shimmy on a motorcycle when the front tire was going soft. Probably something to do with the trail changing. Another time I got into a tankslapper on a motorcycle was when the back tire went down with a passenger and high mounted duffle bag on the back. That time I high sided at low speed on the shoulder and broke my collar bone and twisted my ankle. (My passenger got off lightly.)

November 20, 2013 | Unregistered CommenterJohn B

I also had problems with shimmy on my 1954 Hetchins Magop2 Only way I could control it was with the knee against the top tube

November 21, 2013 | Unregistered Commenterjohn crump

Re, My other post. The thing that I wanted to point out, is that the bike I had that got the shimmy was a 1953 Hetchins WITH THE CURLY STAYS. MAYBE that is what made it shimmy?

November 21, 2013 | Unregistered Commenterjohn crumpo

I used to get shimmy on my '91 Cannondale SE2000 (Yes the first "full suspension with the Frankenstem!) Long top tube and a lot of weight on the bars. It only happened when I was riding with no hands and I could stop it with my knee.

I am also a framebuilder. I have thought about this problem some as I have built a few bikes for people over 6'7". I think the key is the stiffness and length of the steer tube. Tall bikes obviously have long ones. The longer they are the more easily they will twist. Combine this with a particular rake and trail at speed and a harmonic is set up. Usually it can be stopped by grabbing the bar or top tube but I think with extreme examples the wheel itself can carry on in its harmonic tantrum.

That video is scary. I bet he doesnt ride that bike at speed again without getting very nervous.

I try and limit the length of the head tube by using as much rise in the stem as I can get away with. I also have taken Daves advise into account and I use the stiffest DT and TT that I can...

My 2 cents.

November 22, 2013 | Unregistered Commenterhalekai

I would like to chime in on this discussion because I have a Fuso that kicks into shimmy mode at around 35mph, sometimes less, and often now. This is on the most trusted going downhill bike I have ever owned. I was never hesitant to let it hit 60 mph on the steep slopes of a drop from close to 11,000 ft. to the 6,000 ft elevation of the town I live in. Then one day it cut loose and I stared death straight in the face.
In accordance to your theory that large bikes have a bigger tendency to shimmy my Fuso is a 64cm. My son has a 65cm Fuso and his still can bomb the downhills without any problem.
As I own numerous bikes I have discovered that all of them went downhill as if on a rail until I used them on a trainer, both front/rear wheel attached ones and just rear wheel attached trainers.
So my question to you Dave and other responders is, can it be that big bikes get out of alignment by having their wheels fixed on a trainer and subsequently cause them to be prone to shimmy after?

November 23, 2013 | Unregistered CommenterBernie R.

I would describe myself and a slower rider, around 5'8" and circa 70kg. I have experienced the shimmy on a couple of occasions but put the causes down to two non-scientific things. Sometimes I have hit a pot-hole in the road which has caused the vibration. Other times I believe it has been my own nervousness that has been the cause in that I can be too tense and my body too rigid and if a car has passed me by unawares or some other 'shock' it caused me to 'jump' a little which has sent the bike towards a wobble. I suppose its the fear of hitting the deck, but with each ride the confidence builds.

November 24, 2013 | Unregistered Commenter@weybridgegibbo

Bernie R,
Check the rear dropouts for a hairline crack.
Dave

November 25, 2013 | Registered CommenterDave Moulton

Here is my solution to the shimmy problem...

http://www.flickr.com/photos/44613580@N03/10065998466/

Id be especially curious to hear what Dave has to say about this type of fork. Extremely stiff and weighs in at 1.8 pounds...not too bad.

November 27, 2013 | Unregistered Commenterhalekai

An interesting concept. I can see where that would stiffen up the whole front fork, head tube area, making it almost impossible for any sideways movement of the front wheel.
I could see this develop as an after market attachment for bikes that have a shimmy problem.
Dave

November 27, 2013 | Registered CommenterDave Moulton

I know nothing about shimmy on normal bikes with normal riders. I'm 6'6" - 203lbs and I experienced really bad shimmy on almost all bikes I have ridden (well... abused too), be it a road bike with 700c or a mountain bike 29er full suspension.
The problem comes part from trail/rake/headtube angle like Haleika (that I know personally) mentioned. But it is for really tall people the problem of short chainstays. The industry's marketing guys sold us the equation "short chainstays = faster/better/cooler bike". This can be true for several applications, like cyclocross racing, Mtb that will eat only switchback while racing, racing BMX etc. It is not a good idea for someone tall (tall start at 6'4" IMO) who wants a relaxed (normal, read not racing) position on the bike. The straighter the torso, the more weight you put on the rear wheel. Simple. Simple as putting the rider in the center of the bike when this rider is tall and don't race. My solution for tall riders (I'm the founder of a bike company) is using a bigger wheel size (36" wheels) and having a geometry that put the rider centered in between the two wheels when riding (not racing). Who needs a "snappy" bike if it can't handle a fast downhill without shimmy? If you have a really tall rider who wants to experience a safe ride, send it to DirtySixer, the only big bike for tall riders.

December 3, 2013 | Unregistered CommenterDavid

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