Bicycle "Shimmy" Causes
- Thread starter Punx0r
- Start date
dogman dan
1 PW
Interesting read. Take note of the idea that a heavier tire and wheel increases shimmy. Then consider that your hub motor weighs how many pounds?
The other one ignored by way too many noobs, I have myself felt the impending failure of a cheap Wallmart bikes alloy frame. I found the cheap bikes good for at best 2000 miles, then they start to ride like they are made of cooked spaghetti. Even new, the poor stiffness caused shimmy above 30 mph. Carry the battery or cargo on a rack that flexes some, and you are really screwed. I've also seen rim brakes stop working because the frame would bend out instead of clamping the pads to the rim.
I found it amazing that he did not discuss wheel base, which is a bit long anyway on road bikes. Longer wheel base bikes like beach cruisers not only have stiffer frames, but longer frames than mtb's. Longer frames definitely seem to shimmy less, nearly all motorcycles have about 6" longer wheelbase than bikes.
The other one ignored by way too many noobs, I have myself felt the impending failure of a cheap Wallmart bikes alloy frame. I found the cheap bikes good for at best 2000 miles, then they start to ride like they are made of cooked spaghetti. Even new, the poor stiffness caused shimmy above 30 mph. Carry the battery or cargo on a rack that flexes some, and you are really screwed. I've also seen rim brakes stop working because the frame would bend out instead of clamping the pads to the rim.
I found it amazing that he did not discuss wheel base, which is a bit long anyway on road bikes. Longer wheel base bikes like beach cruisers not only have stiffer frames, but longer frames than mtb's. Longer frames definitely seem to shimmy less, nearly all motorcycles have about 6" longer wheelbase than bikes.
friendly1uk
1 MW
I don't think a hub motor will be a negative effect. It would slow the oscillation and maybe even damp it out. It is light wheels with less torsional strength that can get a wobble on.
Edit: This just makes sense. I dunno what is in the article.
Edit: This just makes sense. I dunno what is in the article.
The fingers
10 TW
An unknown function of cosmic rays causes speed wobble whenever someone's velocity on two wheels exceeds their intelligent desire for self-preservation. 
:lol:
:lol:chvidgov.bc.ca
10 kW
I was experiencing nasty front shimmy on my recumbent trike triggered by hitting bumps of a small size.
What affected it?
Toe in not equal to zero mm was worse
Low inflation pressures on tires worse than higher. Big Apples had much worse shimmy than thinner Marathon Racers.
No hands riding would make it critical.
Loose hands better than tight control.
How did I fix it?
Switched to Maxxis hookworms at moderate PSI (60psi).
Added 1 and 1/8" O rings around the steerers just inside the headset caps, to confer more stiction to the headsets (similar to Cane Creek type).
Tightened up the Headsets, nice and tight.
Problem solved for over 2000 miles no shimmy. The greased O-rings seem to be holding up so far, may need to replace them at some point.
My readings suggested that shimmy is not clear cut causation-wise and is a very complex phenomenon, usually affected by headset quality, tires, and often weight distribution. Causes are probably somewhat different on bikes vs trikes.
What affected it?
Toe in not equal to zero mm was worse
Low inflation pressures on tires worse than higher. Big Apples had much worse shimmy than thinner Marathon Racers.
No hands riding would make it critical.
Loose hands better than tight control.
How did I fix it?
Switched to Maxxis hookworms at moderate PSI (60psi).
Added 1 and 1/8" O rings around the steerers just inside the headset caps, to confer more stiction to the headsets (similar to Cane Creek type).
Tightened up the Headsets, nice and tight.
Problem solved for over 2000 miles no shimmy. The greased O-rings seem to be holding up so far, may need to replace them at some point.
My readings suggested that shimmy is not clear cut causation-wise and is a very complex phenomenon, usually affected by headset quality, tires, and often weight distribution. Causes are probably somewhat different on bikes vs trikes.
dogman said:
Plus the wide section tyre, thick downhill tube, slime...
I also had a problem with shimmy that was helped by tightening (from recommended to torque, to "tight") the headset after I changed it.
The last time I road my rear air shock seal gave out and I ended up riding home with it bottomed out. This slackened the head angle, which I would have thought would make the bike more stable, but it made it twitchy, like when you learn forwards over the handlebars and give it some steering input. It is a bit complex sometimes.
dogman dan
1 PW
It's very complex, and all I understand is what I have seen on the road. Not all shimmy is the tank slapper kind. On motorcycles, it's true high speed wobble that the rider causes with steering overcorrection. It's not the frame bending.
But on bikes, you can get wobble even at very low speeds, that is caused by the overloading of the bike. I'm talking about rear racks or panniers. This is why we push so hard to get people to carry the battery in the frame triangle if that is at all possible. Any sway in a rack will amplify it.
Loaded heavy enough, you can upset the balance of the bike with just pedal strokes, and every pedal stroke causes the rear load to wag like a dogs tail. If the frame is cheap thin alloy tube, you will see a definite flexing of the frame in every stroke. This flexing may actually be desirable for an unloaded bike. The old cromoly frames would whip like a spring with a hard enough stroke and riders loved this in a sprint.
But as ebikers, we need to think like cargo bikes. We need very stiff frames, so a steel mtb or beach cruiser frame is good, because the steel frames are stiff, even in the cheap bikes.
Lengthening the wheelbase really helps, which is why longtails kick ass as cargo bikes. My longtail has minimal tail wagging, until I load it with more than 100 pounds. At 75 pounds of load, it rides fine even at 30 mph.
But on bikes, you can get wobble even at very low speeds, that is caused by the overloading of the bike. I'm talking about rear racks or panniers. This is why we push so hard to get people to carry the battery in the frame triangle if that is at all possible. Any sway in a rack will amplify it.
Loaded heavy enough, you can upset the balance of the bike with just pedal strokes, and every pedal stroke causes the rear load to wag like a dogs tail. If the frame is cheap thin alloy tube, you will see a definite flexing of the frame in every stroke. This flexing may actually be desirable for an unloaded bike. The old cromoly frames would whip like a spring with a hard enough stroke and riders loved this in a sprint.
But as ebikers, we need to think like cargo bikes. We need very stiff frames, so a steel mtb or beach cruiser frame is good, because the steel frames are stiff, even in the cheap bikes.
Lengthening the wheelbase really helps, which is why longtails kick ass as cargo bikes. My longtail has minimal tail wagging, until I load it with more than 100 pounds. At 75 pounds of load, it rides fine even at 30 mph.
Here's a thought, could be completely wrong: Most of use ride "upright", but actually bent forwards. At high speed this would create some degree of aerodynamic lift on your chest (angle of attack). I guess this would create a rotation moment about the back axle, the effect being to change the weight distribution and change the headtube angle (if suspension forks). The effect could perhaps be observed using a camera to film the front fork, seeing if the suspension extends as speed increases (having averaged out the inevitable bumps).
