Shimmy while riding with no hands with front load

[bicycletricycle]

I have a few bikes with front racks, I carry a variety of loads on them. Most of them exhibit some shimmy when riding with no hands when some load is present. Usually riding at a medium kind of speed (i know, not very scientific). This shimmy doesn't ever seem to turn into high speed death wobble, just a sort of lazy weaving. Amount of load and position of load seems to effect it some. All of these bikes exhibit no shimmy when unloaded.

looking up shimmy info online yields some advice and some peoples ideas about possible causes and/or fixes. Curiously, it seems to be a bit of a mystery. No rules or definite fixes seem available for the bike builder or bicycle owner. Just a lot of- move load around, try a over tightened needle bearing headset, try a different position, try different air pressure, etc. Zinn, Jan, Grant, Jobst, Moulton, all have interesting articles on the subject.

I am curious and wondering if anyone has a bike that does not shimmy when riding no handed with 10 plus pounds on a front rack.

Thanks for the help
Ryan

[Peter P.]

While I don't have a bike with a front load carrier, I had a friend who used low-rider front panniers for commuting. He found them VERY stable. I'm sure it partially had to do with the load itself, and partially due to the low center of gravity of the load.

For what it's worth, I have a commuter bike with a rear rack. When I place panniers on the bike it rides fine. But if I put weight ON TOP of the rack, the front end will shimmy. To me, it's clearly due to the height of the load. Perhaps that's the source of your problem.

[weisan]

The only bike that I have ridden front and back loaded, hands free without a shimmer is a 26" wheelsize Long Haul Trucker. It was so stable that I took my hands off down a steep descent going at 45 mph...looking back, it was a stupid move.

http://forums.thepaceline.net/attachment.php?attachmentid=1697916468&stc=1&d=1457667262

[bicycletricycle]

The only bike that I have ridden front and back loaded, hands free without a shimmer is a 26" wheelsize Long Haul Trucker. It was so stable that I took my hands off down a steep descent going at 45 mph...looking back, it was a stupid move.

http://forums.thepaceline.net/attachment.php?attachmentid=1697916468&stc=1&d=1457667262

Front handlebar bag and panniers ?

[geordanh]

My experience with this is that front loaded bikes with lower fork trail have a tendency to shimmy more at high speed than higher trail bikes (cue flame war).

Also, down tube parameters seem to play a role as well. My low trail Kogswell with thin walled standard diameter tubing was basically unrideable with a heavy front load going 60 km/h. The over size diameter version was much better, but not nearly as good as a standard high trail bike at speed with a load.

This summer I did a small tour with just a front load on my high trail Waterford RS-14 and it was a dream descending compared to the Kogswell.

I realize my one piece of anecdotal evidence does not compare to Jan Heine's fifty, but I'm reasonably certain that trail plays a role, with lower trail bikes being more prone to shimmy. This intuitively makes sense to me, as the wheel is less stabilized at speed.

[weisan]

Front handlebar bag and panniers ?

Yes sir!

Full on 30 pounds in front and 40 pounds behind.

Fast forward to 2:33 and you will see me executing that stupid move. :D
https://vimeo.com/126438520

[bicycletricycle]

the effect of trail on shimmy does seem to be clouded in mystery, claims on both sides.

almost want to make an adjustable fork to try a direct experiment.


My experience with this is that front loaded bikes with lower fork trail have a tendency to shimmy more at high speed than higher trail bikes (cue flame war).

Also, down tube parameters seem to play a role as well. My low trail Kogswell with thin walled standard diameter tubing was basically unrideable with a heavy front load going 60 km/h. The over size diameter version was much better, but not nearly as good as a standard high trail bike at speed with a load.

This summer I did a small tour with just a front load on my high trail Waterford RS-14 and it was a dream descending compared to the Kogswell.

I realize my one piece of anecdotal evidence does not compare to Jan Heine's fifty, but I'm reasonably certain that trail plays a role, with lower trail bikes being more prone to shimmy. This intuitively makes sense to me, as the wheel is less stabilized at speed.

[DFABob]

The worst shimmy I've ever had was caused by my rear wheel being out of adjustment.

If your bike has adjustable rear dropouts check to make sure your rear wheel is straight in the dropouts. I had this problem once on my cross check. The bike was "mostly" stable unloaded. Once loaded, it thrashed around violently. I suppose the rear wheel was always trying to run away from the rest of the bike, and the front wheel was having a hard time trying to reconcile that fact.

This experience also taught me that if i clamp down on the top tube with both my knees it can stop the "death wobble" in it's tracks.

[geordanh]

The worst shimmy I've ever had was caused by my rear wheel being out of adjustment.

If your bike has adjustable rear dropouts check to make sure your rear wheel is straight in the dropouts. I had this problem once on my cross check. The bike was "mostly" stable unloaded. Once loaded, it thrashed around violently. I suppose the rear wheel was always trying to run away from the rest of the bike, and the front wheel was having a hard time trying to reconcile that fact.

This experience also taught me that if i clamp down on the top tube with both my knees it can stop the "death wobble" in it's tracks.

This is an interesting theory. Do you still get shimmy when the rear wheel is straight? I'd be surprised if you did on that bike.

Could be interesting to do a survey of what bikes people report front loaded shimmy on and then get average fork trail of the do/do not get shimmy reports.

[Mark McM]

The worst shimmy I've ever had was caused by my rear wheel being out of adjustment.

If your bike has adjustable rear dropouts check to make sure your rear wheel is straight in the dropouts. I had this problem once on my cross check. The bike was "mostly" stable unloaded. Once loaded, it thrashed around violently. I suppose the rear wheel was always trying to run away from the rest of the bike, and the front wheel was having a hard time trying to reconcile that fact.

This experience also taught me that if i clamp down on the top tube with both my knees it can stop the "death wobble" in it's tracks.

The problem with most reports of "fixing" shimmy is that they are irreproducible. You say that your bike shimmied when the rear wheel was misaligned, and then stopped shimmying once it was aligned. But did you try to misalign the wheel again to see if it shimmied again?

Shimmy is caused by a system resonance*, which is due to an interaction between multiple components (including the rider). If we get the balance of inertias, spring constants and speeds just right, the bike will shimmy. Changing just one component may stop the shimmy. Unfortunately, we rarely change just one variable, so it is hard to know exactly what change caused/stopped the shimmy.

Because a shimmy is caused by an interaction between the components of a system, it is not caused by a single "faulty" component - a bike which is made of perfectly fine components may still shimmy. Those same components might be used on a differently bike, and won't cause the other bike to shimmy. What this also means is that removing a shimmy can often be accomplished by changing any one of a number of different components. For example, a bike that suffers from shimmy might stop shimmying if either the wheels are changed, or the fork is changed, or the cargo is moved from the front rack to the rear rack (or simply sitting on the bike differently). So "fixing" a shimmy usually isn't a matter of finding the "faulty" component, it is a matter of changing anything that changes the system response of the bike.

There are even ways to change the system response while riding a bike, which might be employed if your bike starts to shimmy under you. One common way is to squeeze the top tube between your knees, which adds damping. Another way is rise up off the saddle, which removes the connection between you and the top of the bike, which will change the elastic response of the system.

*Technically, a shimmy isn't a true resonance, but is actually a Hopf bifurcation (http://velonews.competitor.com/2013/11/bikes-and-tech/technical-faq/technical-faq-bifurcation-and-high-speed-shimmy_309601), in which we cross from one system response realm into another system response realm at a certain critical speed. That's why a bike might be perfectly stable at one speed, and then at a different speed may shimmy wildly.

[bicycletricycle]

the, "lots of things work together to cause this" observation, while interesting and true, is not very helpful.

From a practical perspective, some of these variables are fixed, rider weight, rider position, desired bag locations, touring loads, saddle set back, etc.

some things are more easy to tune, either when ordering a new frame or with a bicycle that you already own and want to improve the handling of.

It seems to me, after reading whatever I could, that even among builders it is not clear what attributes drive this problem more than others, or put another way, they are surprised by builds having this problem while others do not.

All we seem to have is-

mathematical observations about about hopf bifurcation (which provide no practical advice, just, it could be anything and it is everything) and anecdotal information from bicycle riders and some rules of thumb (based on anecdotal observations) from builders.

If I were to call custom builders and ask them for a bicycle that fit my dimensions and that would exhibit no shimmy when riding no handed with a handlebar bag with 15 pounds in it, or I don't have to pay.

Would I get any takers?


The problem with most reports of "fixing" shimmy is that they are irreproducible. You say that your bike shimmied when the rear wheel was misaligned, and then stopped shimmying once it was aligned. But did you try to misalign the wheel again to see if it shimmied again?

Shimmy is caused by a system resonance*, which is due to an interaction between multiple components (including the rider). If we get the balance of inertias, spring constants and speeds just right, the bike will shimmy. Changing just one component may stop the shimmy. Unfortunately, we rarely change just one variable, so it is hard to know exactly what change caused/stopped the shimmy.

Because a shimmy is caused by an interaction between the components of a system, it is not caused by a single "faulty" component - a bike which is made of perfectly fine components may still shimmy. Those same components might be used on a differently bike, and won't cause the other bike to shimmy. What this also means is that removing a shimmy can often be accomplished by changing any one of a number of different components. For example, a bike that suffers from shimmy might stop shimmying if either the wheels are changed, or the fork is changed, or the cargo is moved from the front rack to the rear rack (or simply sitting on the bike differently). So "fixing" a shimmy usually isn't a matter of finding the "faulty" component, it is a matter of changing anything that changes the system response of the bike.

There are even ways to change the system response while riding a bike, which might be employed if your bike starts to shimmy under you. One common way is to squeeze the top tube between your knees, which adds damping. Another way is rise up off the saddle, which removes the connection between you and the top of the bike, which will change the elastic response of the system.

*Technically, a shimmy isn't a true resonance, but is actually a Hopf bifurcation (http://velonews.competitor.com/2013/11/bikes-and-tech/technical-faq/technical-faq-bifurcation-and-high-speed-shimmy_309601), in which we cross from one system response realm into another system response realm at a certain critical speed. That's why a bike might be perfectly stable at one speed, and then at a different speed may shimmy wildly.

[Mark McM]

the, "lots of things work together to cause this" observation, while interesting and true, is not very helpful.

From a practical perspective, some of these variables are fixed, rider weight, rider position, desired bag locations, touring loads, saddle set back, etc.

some things are more easy to tune, either when ordering a new frame or with a bicycle that you already own and want to improve the handling of.

It seems to me, after reading whatever I could, that even among builders it is not clear what attributes drive this problem more than others, or put another way, they are surprised by builds having this problem while others do not.

All we seem to have is-

mathematical observations about about hopf bifurcation (which provide no practical advice, just, it could be anything and it is everything) and anecdotal information from bicycle riders and some rules of thumb (based on anecdotal observations) from builders.

If I were to call custom builders and ask them for a bicycle that fit my dimensions and that would exhibit no shimmy when riding no handed with a handlebar bag with 15 pounds in it, or I don't have to pay.

Would I get any takers?

What you say is generally true, but it is also the unfortunate reality of the situation. Aerodynamics has a phenomenon similar to bicycle called "Flutter", which the airplane wings (or other flight surfaces) can go into an uncontrolled oscillation, which can either render the airplane uncontrollable, or even cause it to break apart. Here is what Wikipedia says about flutter (https://en.wikipedia.org/wiki/Aeroelasticity):

Structures exposed to aerodynamic forces — including wings and aerofoils, but also chimneys and bridges — are designed carefully within known parameters to avoid flutter. In complex structures where both the aerodynamics and the mechanical properties of the structure are not fully understood, flutter can be discounted only through detailed testing. Even changing the mass distribution of an aircraft or the stiffness of one component can induce flutter in an apparently unrelated aerodynamic component. At its mildest this can appear as a "buzz" in the aircraft structure, but at its most violent it can develop uncontrollably with great speed and cause serious damage to or lead to the destruction of the aircraft,[10] as in Braniff Flight 542.

Another famous example of aerodynamic flutter is the Tacoma Narrows bridge (check this out on Youtube if you haven't already seen the videos).

Even for well analyzed structures like airplanes and bridges, flutter isn't completely predictable. However, there are some generalities that are useful in designing shimmy out of a bike:

The stiffer the bicycle is (frame, fork, wheels, etc), the less likely it is to suffer from shimmy. This often means fatter, thicker and/or shorter frame tubes, or wheels with deeper and heavier rims with more spokes.

The more damping there is in the system, the less likely it is to shimmy. This might include fatter tires, or the way the rider sits on the bike (such as clamping the top tube between the legs). Motorcycles sometimes employ steering dampers to reduce shimmy.

For your bike with the front rack, I wouldn't be surprised if it was related to the rigidity of the rack. I have experienced some shimmy on my own otherwise shimmy-free bikes by loading up a not-so-rigid rear rack.

[miguel]

hey dude, lots of TLDR here

low trail fork
needle bearing lower headset cup
figure out where your front center is at and put your body there

[bicycletricycle]

ya, that is why I was wondering if anyone here has front loaded no shimmy bikes, to get an idea of weather it is possible at all or of it just an unfair expectation.

In the better understood structures you give as examples (airplanes, bridges), strategies are used to move the hopf biforcation or resonance of the system to loads unlikely to be seen by the structure or perhaps some kind of active damping is employed.

I wonder if a better understanding of this phenomena in bicycles could result in some better rules of thumb or perhaps gadgets (active steering damper of some kind).

rack stiffness is an interesting avenue to pursue, don't think it is a problem for this particular rack though.

the variety of contradictory solutions to this problem are hilarious.

I have so far read that both tighter and looser headsets can fix it, more and less rake are apparently solutions, bigger tires are both better and worse for shimmy depending on who you ask.

the most common suggestion for a cause is frame stiffness, however, some stiff frames seem to still exhibit this problem, also, it seems convenient that the best solution is one that is very hard to test or experience directly.

That is to say, if you have a customer that has this problem and you try everything and it won't go away than just blame it on the frame. It is unchangeable and chances are no one is going to get the same exact frame made with only a change in tubing diameter to actually test this hypothesis.

It is not unreasonable to assume that frame stiffness has some effect on this problem, but some flexy bicycles don't seem to have it so its not a slam dunk.



What you say is generally true, but it is also the unfortunate reality of the situation. Aerodynamics has a phenomenon similar to bicycle called "Flutter", which the airplane wings (or other flight surfaces) can go into an uncontrolled oscillation, which can either render the airplane uncontrollable, or even cause it to break apart. Here is what Wikipedia says about flutter (https://en.wikipedia.org/wiki/Aeroelasticity):



Another famous example of aerodynamic flutter is the Tacoma Narrows bridge (check this out on Youtube if you haven't already seen the videos).

Even for well analyzed structures like airplanes and bridges, flutter isn't completely predictable. However, there are some generalities that are useful in designing shimmy out of a bike:

The stiffer the bicycle is (frame, fork, wheels, etc), the less likely it is to suffer from shimmy. This often means fatter, thicker and/or shorter frame tubes, or wheels with deeper and heavier rims with more spokes.

The more damping there is in the system, the less likely it is to shimmy. This might include fatter tires, or the way the rider sits on the bike (such as clamping the top tube between the legs). Motorcycles sometimes employ steering dampers to reduce shimmy.

For your bike with the front rack, I wouldn't be surprised if it was related to the rigidity of the rack. I have experienced some shimmy on my own otherwise shimmy-free bikes by loading up a not-so-rigid rear rack.

[geordanh]

I wonder what the r squared value is between a bike's 'plane factor' and 'front load shimmy factor'.

Getting the exact measurements to compute a value might be tricky unfortunately. The only instrument capable of measuring a bike's plane factor is Jan Heine's ass, which is all the way in Seattle, and no one I know is going to take out a ruler to measure death shimmy deflection while in motion. Guess we'll have to leave it to conjecture.

[charliedid]

Maybe it's just exceeded it's ability to handle the weight? What happens if you remove 5 lbs of a load that is exhibiting the shimmy? I've had plenty of bikes shimmy when that load limit is reached.

[charliedid]

Wait, only when riding no-handed? No shimmy otherwise with load?

[bicycletricycle]

ya, none riding no hands, none with no load (on this bike)

amount of load does effect the shimmy on most bikes I have ridden (more load, more shimmy)

any particular bicycle has some weight load that it works best at, more curious about what might make a bike immune to shimmy with front load.

[charliedid]

ya, none riding no hands, none with no load (on this bike)

amount of load does effect the shimmy on most bikes I have ridden (more load, more shimmy)

any particular bicycle has some weight load that it works best at, more curious about what might make a bike immune to shimmy with front load.

Okay Now I get ya. I'm zero help, but curious.

[DFABob]

http://www.bikemag.com/blog/hopey_steering_damper_review/#CJMfj1SjRAXTx0uP.97

http://www.cyclingabout.com/review-hebie-695-steering-stabiliser/

[bicycletricycle]

I was surprised to see that hopey is still around. I always thought those were a cool idea but so ugly.

I wonder if one of those springy wheel stabalizers would work, with my simple and incomplete understanding of physics, I might guess that it could just make it worse. I have had them on dutch style bikes before. hmmmmm

http://i975.photobucket.com/albums/ae236/bicycletricycle/img_0595_zpsesx7ukuo.jpg (http://s975.photobucket.com/user/bicycletricycle/media/img_0595_zpsesx7ukuo.jpg.html)