Speed wobble! What's the cause?

[Gayboy]

I nearly kakked myself coming down Suikerbossie the other day, and it didn't make the rest of the ride very comfortable. Can anyone give any pointers on what might be causing it? It'll be much appreciated!

[bruce]

I nearly kakked myself coming down Suikerbossie the other day' date=' and it didn't make the rest of the ride very comfortable. Can anyone give any pointers on what might be causing it? It'll be much appreciated! [/quote']

this is also called a high speed shimmy.

 

It is caused when the bikes vibration reaches resonance, and the vibration is amplified by the geometry of the bike.  This amplification caused the speed wobble.

 

It is very easy to stop - just lift your butt off the saddle.  there is nothing generally wrong with your bike - it is just a combination of frame, rider mass, wheels, etc etc.  The road and your saddle are the two anchor points around which the oscillation takes place.  Like a bow string, the bike starts vibrating between these two points.  By lifting yourself off the saddle, you release one side of the bow string and the resonant vibration stops.

[CycleJunkie]

Another way to stop it is to clamp your top tube between your knees. Has the same effect as what Bruce said.

[Gayboy]

Awesome! Thank you both - I'm hugely relieved it's that simple. I had nightmares of expensive new wheels and tyres and endless services coming up. 

[Spinnekop]

Please note....Bruce is an EXPERT on this....saw him wobble a COUPLE of times.....

[ChrisH]

I did what Bruce suggested and lifted my bum off the saddle. Unfortunately I may have gone too far because I also lifted my feet off the pedals and my hands off the handlebars.

I need new shorts.

[kosmonooit]

aah resonance. We got shown this when being introduced to this phenomenon in Physics I:

 

kosmonooit2010-02-05 09:46:18

[Minion]

 

aah resonance. We got shown this when being introduced to this phenomenon in Physics I:

 

Without even clicking the link, I will bet that that's the video of the Tacoma Narrows Bridge.

 

[kosmonooit]

So ..... did you take a look yet?

[The_Break]

Awesome! Thank you both - I'm hugely relieved it's that simple. I had nightmares of expensive new wheels and tyres and endless services coming up. 

 

Hi Gayboy

 

it is not as simple as saying this is how you overcome it when it happens. You are treating a symptom and not the cause. A correctly engineered bike should not do this. Take a look at the guys when they descend the mountains in the big tours, they are doing over 100km/h at times and you will almost never see this happen.

 

There are a few causes and yes some of them are incorrect geometry of the bike (which is unfortunately only the designers problem and hard to prove) but here is what I would check in order, but first a few questions:

 

When the bike started wobbling were you breaking or free wheeling/pedaling?

Was there a cross-wind of any type?

How long have you had your bike and have you changed anything recently?

Have you gone this fast before on Suikerbossie or a similar angle and road surface?

 

My guess is it is one of the following that you could check very quickly:

 

If you were breaking, check that your breaks are set corectly, i.e. that when you pull the brakes that they do not make the wheel move to one side. Rim should sit exactly still.

If you were not breaking, could have just been a side wind, this can happen even with short section rims but worst with deep section rims.

One of the worst causes if that guys put new forks on bikes or swap forks out between bikes that are not of the correct rake angle and thus cause a change in the wheel base. If the change causes a too short a wheel base then it has a trolley wheel effect and hence the wobble. Check that the bike has the original fork on it.

There are a few other factors that could contribute, but they are most probably not the cause. Check these 1st and let us know what you find.

 

Good luck! I had the same problem a few years back and was so scared to ride my bike down a hill I could not go over 40km/h. Took me a year to get it sorted and my confidence back.

 

Cheers.

[The Saint]

What The Break said, summed it up perfectly.

 

 

 

Also a stiff upper body can cause you to "wobble" the key its to just relax and let the bike follow its natural course.

[HeartCoppi]

Was the wobble in the hands or on the back wheel?

[David Marshall]

High speed shimmy.

 

Two things I know about this. 1) It happens 2) It's terrifying.  There is a lot of info on the web (sheldon brown,Calfee design and a lot more).

 

It will happen on every bike at some point when the right conditions are met (rider weight and posture, bike "balance", road surface, wind), It is just that on a "stable" bike these conditions are met at a speed greater than you cycle at.

 

If it happens on a bike that you are used to riding and it has not happened before I would  worry because it could be a sign of something failing.  If it happens after you have changed something the cause is usually obvious.

 

However shimmys usually manifest on bikes that are new to you and are mechanically sound. What you now need to do is alter the conditions that allow the frame to resonate at that point.  As mentioned in previous posts that while riding this can be as simple as moving your weight, clamping your knees on the top tube or even just resting one knee on the tube.  You can also play around with the bike (Wheels with more spokes, changing the fork, shortening the effective steerer length and so on).  The problem is that it is difficult to reproduce the conditions that caused the shimmy in the first place, so measuring the effectiveness of any change is hard.

[Johan Bornman]

aah resonance. We got shown this when being introduced to this phenomenon in Physics I:

 

Without even clicking the link' date=' I will bet that that's the video of the Tacoma Narrows Bridge.
[/quote']

 

My thought exactly. And no, I haven't clicked the link.

[Johan Bornman]

it is not as simple as saying this is how you overcome it when it happens. You are treating a symptom and not the cause. A correctly engineered bike should not do this. Take a look at the guys when they descend the mountains in the big tours' date=' they are doing over 100km/h at times and you will almost never see this happen.

 [/quote']

 

It has noting to do with high speed. It can happen at any speed. As  Bruce alluded it is a combination of a specific rider on a specific bike on a specific road. This cannot be planned (engineered) for.

 

There are a few causes and yes some of them are incorrect geometry of the bike (which is unfortunately only the designers problem and hard to prove) but here is what I would check in order' date=' but first a few questions:

 

 [/quote']

 

I don't like to think of standard bikes as having incorrect geometry. The geometry of the modern bicycle has been arrived at from a century of development, refinement, testing and riding by billions (I think, maybe many millions) of people. Bicycle frame geometry is not a science exclusive to a few people working for exclusive bicycle companies. It is in the public domain and the notion of secret angles and combinations spoken about in bicycle magazines is nonsense.

 

 

When the bike started wobbling were you breaking or free wheeling/pedaling?

Was there a cross-wind of any type?

How long have you had your bike and have you changed anything recently?

Have you gone this fast before on Suikerbossie or a similar angle and road surface?

 

 

I hate to be a pedant on this one' date=' but the word is BRAKING or BRAKE. Nothing is breaking until he falls.

 

 

My guess is it is one of the following that you could check very quickly:

 

If you were breaking, check that your breaks are set corectly, i.e. that when you pull the brakes that they do not make the wheel move to one side. Rim should sit exactly still.

 

 

This is true, but for a completely different reason. Brake callipers that arent' centred pulls wheels to one side as you point out, but this is bad for the spokes in that they tend to go loose from this movement.

 

The wheel bends at the brakes only and the bottom tracks in its original position. It is tempting to assume that the rim shooting back to its non-bending position induces a sort of wave in the wheel that then transfers to the frame, but the fact that shimmy starts without braking too, makes this theory unlikely.

 

 

If you were not breaking' date=' could have just been a side wind, this can happen even with short section rims but worst with deep section rims.

One of the worst causes if that guys put new forks on bikes or swap forks out between bikes that are not of the correct rake angle and thus cause a change in the wheel base. If the change causes a too short a wheel base then it has a trolley wheel effect and hence the wobble. Check that the bike has the original fork on it.

There are a few other factors that could contribute, but they are most probably not the cause. Check these 1st and let us know what you find.

 

 

[/quote']

 

A sidewind has no effect on shimmy. It happens on rainy days, sunny days, Mondays, Thursdays, still days, gusty days hot days and cold days.

 

What do you mean by Rake Angle? Rake is Angle or Slope. Rake Angle is Rake Rake. In cheesy novels you'll find a hero who walks into a room and charms the chicks with his hat "at a rakish angle". That is wrong. The hat is rakish or raked. Forks have rake. 

 

Nevertheless, there is no such thing as a wrong fork angle. There are only fork angles for specific applications. Igf I have to ask you to list the wrong angles, what would you say?

 

Rake has nothing to do with shimmy. Shimmy is a complex phenomena but there is some solid understanding of its cause. We do know that it is caused by a harmonious wave in the frame probably emanating from a torsional movement at the head tube. We also know that by destroying a node in that wave, the shimmy dissappears (but could appear again at say the one-octave-up equivalent of that wave).

 

You mention Suikerbosrand. This is an excellent place to induce and practice shimmy strategies. At the top of the section approaching the Kareekloof gate, there is an incredibly long gentle downhill without turns. Use that to test your shimmy strategies. Ride no-hands, gently picking up speed until your bike starts to shimmy. Now stand up, balancing the bike by pinching the saddle between your thighs. The shimmy immediately dissappears. Sit down and it comes back etc etc. To me this indicates that the wave is not pefectly horizontal through the bike but probably at a 45-degree angle.

 

Whatever your conclusion, if you do the route often on different bikes you'll notice that they all shimmy. Just the onset-speed differs slightly and when riding no-hands, the onset speeds all fall within a narrow band.

 

Good luck! I had the same problem a few years back and was so scared to ride my bike down a hill I could not go over 40km/h. Took me a year to get it sorted and my confidence back.

 

 

 

Here I agree with you. It is an experience that can put you off riding a bike. The good news is that you can practice to instinctively counter the onset of shimmy. You should routinely induce shimmy so that you can get used to it.

 

 

 

[The_Break]

it is not as simple as saying this is how you overcome it when it happens. You are treating a symptom and not the cause. A correctly engineered bike should not do this. Take a look at the guys when they descend the mountains in the big tours' date=' they are doing over 100km/h at times and you will almost never see this happen.

 [/quote']

 

It has noting to do with high speed. It can happen at any speed. As  Bruce alluded it is a combination of a specific rider on a specific bike on a specific road. This cannot be planned (engineered) for.

 

There are a few causes and yes some of them are incorrect geometry of the bike (which is unfortunately only the designers problem and hard to prove) but here is what I would check in order' date=' but first a few questions:

 

 [/quote']

 

I don't like to think of standard bikes as having incorrect geometry. The geometry of the modern bicycle has been arrived at from a century of development, refinement, testing and riding by billions (I think, maybe many millions) of people. Bicycle frame geometry is not a science exclusive to a few people working for exclusive bicycle companies. It is in the public domain and the notion of secret angles and combinations spoken about in bicycle magazines is nonsense.

 

Johan, bikes are designed with specific angles, if youu deviate from the original design it will not function as designed and hence may do things you do not want it to do. Such as give a speed wobble.

 

When the bike started wobbling were you breaking or free wheeling/pedaling?

Was there a cross-wind of any type?

How long have you had your bike and have you changed anything recently?

Have you gone this fast before on Suikerbossie or a similar angle and road surface?

 

 

I hate to be a pedant on this one' date=' but the word is BRAKING or BRAKE. Nothing is breaking until he falls.

 

Are you German?

 

 

My guess is it is one of the following that you could check very quickly:

 

If you were breaking, check that your breaks are set corectly, i.e. that when you pull the brakes that they do not make the wheel move to one side. Rim should sit exactly still.

 

 

This is true, but for a completely different reason. Brake callipers that arent' centred pulls wheels to one side as you point out, but this is bad for the spokes in that they tend to go loose from this movement.

 

The wheel bends at the brakes only and the bottom tracks in its original position. It is tempting to assume that the rim shooting back to its non-bending position induces a sort of wave in the wheel that then transfers to the frame, but the fact that shimmy starts without braking too, makes this theory unlikely.

 

 

If you were not breaking' date=' could have just been a side wind, this can happen even with short section rims but worst with deep section rims.

One of the worst causes if that guys put new forks on bikes or swap forks out between bikes that are not of the correct rake angle and thus cause a change in the wheel base. If the change causes a too short a wheel base then it has a trolley wheel effect and hence the wobble. Check that the bike has the original fork on it.

There are a few other factors that could contribute, but they are most probably not the cause. Check these 1st and let us know what you find.

 

 

[/quote']

 

A sidewind has no effect on shimmy. It happens on rainy days, sunny days, Mondays, Thursdays, still days, gusty days hot days and cold days.

 

Have you ever ridden a set of deep section at high speed and hit a cross wind? This is exactly what will happen. Your bike can give a speed wobble. Try it, it fun!

 

What do you mean by Rake Angle? Rake is Angle or Slope. Rake Angle is Rake Rake. In cheesy novels you'll find a hero who walks into a room and charms the chicks with his hat "at a rakish angle". That is wrong. The hat is rakish or raked. Forks have rake. 

 

Nevertheless, there is no such thing as a wrong fork angle. There are only fork angles for specific applications. Igf I have to ask you to list the wrong angles, what would you say?

 

Rake has nothing to do with shimmy. Shimmy is a complex phenomena but there is some solid understanding of its cause. We do know that it is caused by a harmonious wave in the frame probably emanating from a torsional movement at the head tube. We also know that by destroying a node in that wave, the shimmy dissappears (but could appear again at say the one-octave-up equivalent of that wave).

 

Johan, if you say rake has nothing to do with a bike being stable then you will argue with nearly all bicycle designers. Getting the correct wheel base, steerer tube angle, rake, etc is what makes good bikes handle well and bad bikes handle poorly. Making sure you are using the correct fork for your frame is like making sure you put the correct fuel or oil in your car.

 

You mention Suikerbosrand. This is an excellent place to induce and practice shimmy strategies. At the top of the section approaching the Kareekloof gate, there is an incredibly long gentle downhill without turns. Use that to test your shimmy strategies. Ride no-hands, gently picking up speed until your bike starts to shimmy. Now stand up, balancing the bike by pinching the saddle between your thighs. The shimmy immediately dissappears. Sit down and it comes back etc etc. To me this indicates that the wave is not pefectly horizontal through the bike but probably at a 45-degree angle.

 

Whatever your conclusion, if you do the route often on different bikes you'll notice that they all shimmy. Just the onset-speed differs slightly and when riding no-hands, the onset speeds all fall within a narrow band.

 

I still think bottom line is that whether I got the spelling incorrect or the terms incorrect, the bloke needs to check those points on his bike. All you did was rip the text apart, you never helped the guy solve his problem.

 

Good luck! I had the same problem a few years back and was so scared to ride my bike down a hill I could not go over 40km/h. Took me a year to get it sorted and my confidence back.

 

 

 

Here I agree with you. It is an experience that can put you off riding a bike. The good news is that you can practice to instinctively counter the onset of shimmy. You should routinely induce shimmy so that you can get used to it.

 

 

 

 

I agree on speed wobble induction, it is a great way to learn and can be done at lower speeds.