The Death Wobbles - How to regain control?
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The Death Wobbles - How to regain control?
Postby Arlberg » Sat Nov 17, 2012 7:40 pm
So what is the definitive answer?
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Re: The Death Wobbles - How to regain control?
Postby Dr_Mutley » Sat Nov 17, 2012 7:43 pm
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Re: The Death Wobbles - How to regain control?
Postby Dr_Mutley » Sat Nov 17, 2012 7:45 pm
http://www.cyclingtips.com.au/2011/03/s ... -its-head/" onclick="window.open(this.href);return false;
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Re: The Death Wobbles - How to regain control?
Postby il padrone » Sat Nov 17, 2012 7:55 pm
Generally the best solution when riding is to maintain your position and push your knee against the top tube. This stabilises the steering axis and isolates the wobble action. Long term fixes are a bit of 'suck it and see' and may involve changes to load distribution, alteration of stem length, varying tyre pressures or alteration of saddle position. It's a black art really.
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Re: The Death Wobbles - How to regain control?
Postby ausrandoman » Sat Nov 17, 2012 9:00 pm
To revert to the normal dynamics, you need to increase the drag on the front wheel and reduce the moment of inertia of the trailing castor: lean forward, push one knee against the top tube and lightly feather the front brake.
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Re: The Death Wobbles - How to regain control?
Postby cpical » Sun Nov 18, 2012 7:00 am
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Re: The Death Wobbles - How to regain control?
Postby greyhoundtom » Sun Nov 18, 2012 7:22 am
If you take a car for instance, if a front wheel is out of balance you may not really notice it at 80 km per hour, but once you get to 100 or more km per hour the front end threatens to shake to pieces.
No reason why this should not also occur on a bike at high speed, and the higher the speed the worse the effects.
My son in law who used to race motorbikes is pedantic about ensuring the front wheels on his moto’s are perfectly balanced and can't understand why we never seem worry about that with our racing bikes.
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Re: The Death Wobbles - How to regain control?
Postby ColinOldnCranky » Sun Nov 18, 2012 10:27 am
My solution was simple. Reduce the loads and, when I can't, moderate the speed. Neither solution is applicable to racing of course but fine the rest of the time.
The OP is right - when it happens it is very unnerving.
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Re: The Death Wobbles - How to regain control?
Postby Byke » Thu Nov 22, 2012 3:21 am
I remember reading somewhere - and I haven't got a clue where - that all bicycles have natural resonances and are thus susceptible to speed wobble (but for well-designed bikes that resonance will occur at speeds beyond which the bike is likely ever to be ridden at). It's just that the particular speed that this will occur at will vary for every combination of:
- bike,
- rider, and
- position
The theory went that should you develop a speed wobble, which is a form of simple harmonic motion, you merely need to alter any one of the three factors. When you're hurtling down a hill at 75km/h it's hard to change the bike or the rider, but it's not that hard - once you overcome your fear - to lift your arse off the saddle. Once you do that the speed at which the natural resonance of the bike-and-rider combination occurs will be changed and you'll be fine. (According to this theory the bike-and-standing-rider will still be susceptible to speed wobble, but it will occur at a completely different speed.)
I've had scary speed wobbles twice in my life. Once before I read this. And once after. The first time, some twenty-odd years ago in the Adelaide hills, I can still remember frantically wondering how to minimise the risk of killing myself when I seemingly inevitably fell off. The second, much more recently, I can barely remember. I simply lifted my bum an inch off the saddle and everything magically sorted itself out.
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Re: The Death Wobbles - How to regain control?
Postby clackers » Thu Nov 22, 2012 6:19 am
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Re: The Death Wobbles - How to regain control?
Postby greyhoundtom » Thu Nov 22, 2012 7:07 am
For a resonance problem to occur it still requires something to trigger it..........in other words there has to be some type of vibration in one of the components of the bike, as resonance just doesn’t magically occur without a triggering factor.clackers wrote:I've not encountered this myself but had a chat to an LBS owner who said it was a resonance problem where the frame itself starts cyclically flexinug.
For a vibration to occur in the first place it requires for some component of the bike to be out of balance.........and the component that is out of balance needs to be rotating at a given rotation, which depends on the severity of the imbalance, to amplify the resonance to the extent where it starts to affect the frame of the bike.
The only component that meets those requirements for a speed wobble to start to occur is an imbalance in one of the wheels, the most likely being the front wheel.
However this is aggravated by any static weight high on the bike as this then provides a bounce point for the resonance, increasing the wobbling effect.
Therefore taking the weight of the seat and transferring this to the pedals which are located at a lower point of the resonating frame, instantly reduces the resonating effect.
It is simply a logical cause and effect event.
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Re: The Death Wobbles - How to regain control?
Postby human909 » Thu Nov 22, 2012 7:11 am
greyhoundtom wrote:I still think that in some instances the “death wobbles” has something to do with the actual balance of the front wheel.
There is no debate that it is resonance, though the causes and cures can not be clearly isolated without in depth study. That said phenomenon is less influenced by the frame or wheel balance and more determined by the steering geometry (steering axis, caster, rider interface and location of the centre of mass). However given the nature of resonance EVERY component has some influence.clackers wrote:I've not encountered this myself but had a chat to an LBS owner who said it was a resonance problem where the frame itself starts cyclically flexinug.
The main oscillation is about the steering axis. Whenever you ride a bicycle the steering axis is in a constant state of vibration about a mean position. Normally this is tiny and damped. However when the appropriate conditions are reached this becomes a compounding vibrations which the grows until conditions change.
Also it is not just bikes that has natural resonance, EVERYTHING, has resonance frequencies. This only becomes a problem when the undamped resonance compounds out of control. If you think that cyclists have it bad, check out this!
I believe Byke summarises the best advice for dealing with speed wobbles.
Byke wrote:I remember reading somewhere - and I haven't got a clue where - that all bicycles have natural resonances and are thus susceptible to speed wobble (but for well-designed bikes that resonance will occur at speeds beyond which the bike is likely ever to be ridden at). It's just that the particular speed that this will occur at will vary for every combination of:
- bike,
- rider, and
- position
The theory went that should you develop a speed wobble, which is a form of simple harmonic motion, you merely need to alter any one of the three factors. When you're hurtling down a hill at 75km/h it's hard to change the bike or the rider, but it's not that hard - once you overcome your fear - to lift your arse off the saddle. Once you do that the speed at which the natural resonance of the bike-and-rider combination occurs will be changed and you'll be fine. (According to this theory the bike-and-standing-rider will still be susceptible to speed wobble, but it will occur at a completely different speed.)
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Re: The Death Wobbles - How to regain control?
Postby human909 » Thu Nov 22, 2012 7:23 am
There are hundreds of sources of vibrations hitting a bike at any one time. Road vibration and wind vibration are two significant sources of energy. The rider feedback loop is also a significant source of energy to the system.greyhoundtom wrote:For a resonance problem to occur it still requires something to trigger it..........in other words there has to be some type of vibration in one of the components of the bike, as resonance just doesn’t magically occur without a triggering factor.
(See video for the effect of wind!)
This is not accurate.greyhoundtom wrote: For a vibration to occur in the first place it requires for some component of the bike to be out of balance.........and the component that is out of balance needs to be rotating at a given rotation, which depends on the severity of the imbalance, to amplify the resonance to the extent where it starts to affect the frame of the bike.
The only component that meets those requirements for a speed wobble to start to occur is an imbalance in one of the wheels, the most likely being the front wheel.
Nor is this. Standing up will RAISE the centre of mass not lower it.greyhoundtom wrote: However this is aggravated by any static weight high on the bike as this then provides a bounce point for the resonance, increasing the wobbling effect.
Therefore taking the weight of the seat and transferring this to the pedals which are located at a lower point of the resonating frame, instantly reduces the resonating effect.
Resonance is a heavily studied phenomenon in structural and mechanical engineering. In fact I have recently been dealing myself with resonance calculations for buildings. The importance of controlling it cannot be overstated for large slender structures. This should be fairly obvious from the previous video.
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Re: The Death Wobbles - How to regain control?
Postby Ken Ho » Thu Nov 22, 2012 8:53 am
How about some simple trail-braking, people ??
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Re: The Death Wobbles - How to regain control?
Postby rkelsen » Thu Nov 22, 2012 9:45 am
Agree. Wheel balance on bicycles is a non-issue.human909 wrote:This is not accurate.greyhoundtom wrote:The only component that meets those requirements for a speed wobble to start to occur is an imbalance in one of the wheels, the most likely being the front wheel.
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Re: The Death Wobbles - How to regain control?
Postby Mike Ayling » Thu Nov 22, 2012 11:57 am
Would the two water bottles plus the bar bag have dampened or made the wobble worse?il padrone wrote:Danneaux from the Thorn Forums developed a really bad speed wobble, even at quite low speeds when his touring bike was loaded up. The bike rode fine when un-loaded and he tried everything to solve the problem - in a very scientific manner but to no avail. The manufacturers came to the party and gave him a full exchange of a better-specced bike.
Generally the best solution when riding is to maintain your position and push your knee against the top tube. This stabilises the steering axis and isolates the wobble action. Long term fixes are a bit of 'suck it and see' and may involve changes to load distribution, alteration of stem length, varying tyre pressures or alteration of saddle position. It's a black art really.
Mike
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Re: The Death Wobbles - How to regain control?
Postby sogood » Thu Nov 22, 2012 12:27 pm
RK wrote:And that is Wikipedia - I can write my own definition.
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Re: The Death Wobbles - How to regain control?
Postby Alien27 » Thu Nov 22, 2012 12:59 pm
My 2c which may be helpful;human909 wrote:There are hundreds of sources of vibrations hitting a bike at any one time. Road vibration and wind vibration are two significant sources of energy. The rider feedback loop is also a significant source of energy to the system.greyhoundtom wrote:For a resonance problem to occur it still requires something to trigger it..........in other words there has to be some type of vibration in one of the components of the bike, as resonance just doesn’t magically occur without a triggering factor.
(See video for the effect of wind!)
This is not accurate.greyhoundtom wrote: For a vibration to occur in the first place it requires for some component of the bike to be out of balance.........and the component that is out of balance needs to be rotating at a given rotation, which depends on the severity of the imbalance, to amplify the resonance to the extent where it starts to affect the frame of the bike.
The only component that meets those requirements for a speed wobble to start to occur is an imbalance in one of the wheels, the most likely being the front wheel.
A bicycle's (or motorbike's) front wheel never travels in a perfectly straight line. It instead is constantly undergoing a series of over corrections, i.e. it wobbles from side to side, constantly pulling itself back to the direction the bike is heading in, going a bit too far and then pulling itself back again and again etc... This wobble is very slight and not normally perceivable. From my understanding this wobble is what gives us the 'feel' through the bars. When there is lots of grip available the frequency of this wobble is high as there is a lot of force available to quickly pull the wheel back. When there is lower available grip, for example wet or oily surfaces or if a lot of the available traction is being used for braking or cornering the frequency of the wobble is a slower. When we get to the limit of the available grip the frequency is very slow as and we feel it through the bars as a looseness. It happens subconsciously but this is how we judge the level of grip available.
So the trigger is there in all bikes, they all have a wobble/oscillation around the steering axis all the time. The issue is finding out what particular set of circumstances sets this off into a 'death wobble' (or tank slapper in motorbikes).
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Re: The Death Wobbles - How to regain control?
Postby Alien27 » Thu Nov 22, 2012 1:03 pm
Just read that and it looks like you already covered what i said.human909 wrote: The main oscillation is about the steering axis. Whenever you ride a bicycle the steering axis is in a constant state of vibration about a mean position. Normally this is tiny and damped. However when the appropriate conditions are reached this becomes a compounding vibrations which the grows until conditions change.
+1 on the rest of the post as well
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Re: The Death Wobbles - How to regain control?
Postby rockdoctor » Thu Nov 22, 2012 1:20 pm
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Re: The Death Wobbles - How to regain control?
Postby singlespeedscott » Thu Nov 22, 2012 1:45 pm
I think the biggest problem is today's modern compact frames. The geometry is to tight, the wheel base to short and the frames are to small for the cyclists.
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Re: The Death Wobbles - How to regain control?
Postby human909 » Thu Nov 22, 2012 2:15 pm
Caster (aka trail) ensures that the front wheel is self straitening. This almost essential for any practical bicycle. (though unlike previously thought it is not essential for bike stability). I believe it is this caster force that produces the primary input energy into the resonance.
This affect can be seen quite readily on shopping trolleys and other devices with caster wheels. Many of us have observed caster flutter when a shopping is pushed too fast (by somebody elses kids of course ) Furthermore BMX bikes which have more trail often hit speed wobbles earlier than road bikes. (As a kid everyone in my neighbourhood new of speed wobbles going down a hill.)
Of course the solutions are still similar. Though if you find it problematic on a particular bike then changing forks to decrease trail may assist.
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Re: The Death Wobbles - How to regain control?
Postby ColinOldnCranky » Thu Nov 22, 2012 4:22 pm
Exactly right - it is called sympathetic resonance. It could be resonating sympathetically with the rotation of your wheel or the bumps onthe road. And taking te step you suggest changes the natural resont frequency of the whole systme so that it no longer matches exactly the triggering resonance. Sypathtic resonance requires pretty precise matching, unmatch by just a teeny bit an it dies.Byke wrote:I'm not an engineer but…
I remember reading somewhere - and I haven't got a clue where - that all bicycles have natural resonances and are thus susceptible to speed wobble (but for well-designed bikes that resonance will occur at speeds beyond which the bike is likely ever to be ridden at). It's just that the particular speed that this will occur at will vary for every combination of:
- bike,
- rider, and
- position
The theory went that should you develop a speed wobble, which is a form of simple harmonic motion, you merely need to alter any one of the three factors. When you're hurtling down a hill at 75km/h it's hard to change the bike or the rider, but it's not that hard - once you overcome your fear - to lift your arse off the saddle. Once you do that the speed at which the natural resonance of the bike-and-rider combination occurs will be changed and you'll be fine. (According to this theory the bike-and-standing-rider will still be susceptible to speed wobble, but it will occur at a completely different speed.)
I've had scary speed wobbles twice in my life. Once before I read this. And once after. The first time, some twenty-odd years ago in the Adelaide hills, I can still remember frantically wondering how to minimise the risk of killing myself when I seemingly inevitably fell off. The second, much more recently, I can barely remember. I simply lifted my bum an inch off the saddle and everything magically sorted itself out.
Different materials these days are probably better at dampening the resonances (ie more "dead", they absorb the resonance and convert it to heat) than the older double-butted steel (such as CrMo) frames. I imagine that CF designs would be designed with better damping. But that would still be within sertain parameters (such as not hanging panniers off it). Resonance is a given in any elastic material such as metal. (Maybe we could make bikes out of platicine.)
I remember a friend of mine who had a problem with his Ricardo Elite at not-excessive speeds. We were having a critical look at it when I noticed that they had left out, at manufacturing, that little cross-bar between the chain stays just behind the bottom bracket. As well as holding the kick stand or dynamo, that bar served to dampen resonance.
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Re: The Death Wobbles - How to regain control?
Postby singlespeedscott » Thu Nov 22, 2012 4:45 pm
It's funny you should mention a Ricardo Elite. That's the very bike I use, in a much modified form, for all my mountain rides. It's so stable at high speeds, 70km/hr +.ColinOldnCranky wrote:I remember a friend of mine who had a problem with his Ricardo Elite at not-excessive speeds. We were having a critical look at it when I noticed that they had left out, at manufacturing, that little cross-bar between the chain stays just behind the bottom bracket. As well as holding the kick stand or dynamo, that bar served to dampen resonance.Byke wrote:I'm not an engineer but…
I remember reading somewhere - and I haven't got a clue where - that all bicycles have natural resonances and are thus susceptible to speed wobble (but for well-designed bikes that resonance will occur at speeds beyond which the bike is likely ever to be ridden at). It's just that the particular speed that this will occur at will vary for every combination of:
- bike,
- rider, and
- position
The theory went that should you develop a speed wobble, which is a form of simple harmonic motion, you merely need to alter any one of the three factors. When you're hurtling down a hill at 75km/h it's hard to change the bike or the rider, but it's not that hard - once you overcome your fear - to lift your arse off the saddle. Once you do that the speed at which the natural resonance of the bike-and-rider combination occurs will be changed and you'll be fine. (According to this theory the bike-and-standing-rider will still be susceptible to speed wobble, but it will occur at a completely different speed.)
I've had scary speed wobbles twice in my life. Once before I read this. And once after. The first time, some twenty-odd years ago in the Adelaide hills, I can still remember frantically wondering how to minimise the risk of killing myself when I seemingly inevitably fell off. The second, much more recently, I can barely remember. I simply lifted my bum an inch off the saddle and everything magically sorted itself out.
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Re: The Death Wobbles - How to regain control?
Postby ColinOldnCranky » Thu Nov 22, 2012 9:31 pm
I gather that you don't get the resonance on the way up to 70kph.singlespeedscott wrote:It's funny you should mention a Ricardo Elite. That's the very bike I use, in a much modified form, for all my mountain rides. It's so stable at high speeds, 70km/hr +.ColinOldnCranky wrote:I remember a friend of mine who had a problem with his Ricardo Elite at not-excessive speeds. We were having a critical look at it when I noticed that they had left out, at manufacturing, that little cross-bar between the chain stays just behind the bottom bracket. As well as holding the kick stand or dynamo, that bar served to dampen resonance.Byke wrote:I'm not an engineer but…
I remember reading somewhere - and I haven't got a clue where - that all bicycles have natural resonances and are thus susceptible to speed wobble (but for well-designed bikes that resonance will occur at speeds beyond which the bike is likely ever to be ridden at). It's just that the particular speed that this will occur at will vary for every combination of:
- bike,
- rider, and
- position
The theory went that should you develop a speed wobble, which is a form of simple harmonic motion, you merely need to alter any one of the three factors. When you're hurtling down a hill at 75km/h it's hard to change the bike or the rider, but it's not that hard - once you overcome your fear - to lift your arse off the saddle. Once you do that the speed at which the natural resonance of the bike-and-rider combination occurs will be changed and you'll be fine. (According to this theory the bike-and-standing-rider will still be susceptible to speed wobble, but it will occur at a completely different speed.)
I've had scary speed wobbles twice in my life. Once before I read this. And once after. The first time, some twenty-odd years ago in the Adelaide hills, I can still remember frantically wondering how to minimise the risk of killing myself when I seemingly inevitably fell off. The second, much more recently, I can barely remember. I simply lifted my bum an inch off the saddle and everything magically sorted itself out.
The thing about this effect is that it doesn't occur ABOVE a certain speed. It occurs AT a certain speed. Go past that speed and it settles down again. But at higher speeds the amount of energy being pumped into the system is higher than at lower speeds. Once the energy/second put in is exceeding the energy/second being expended through dampening then the problem manifests itself.
Of course, it does not have to be the whole bike either - we would be in trouble if onlt the wheel did the same thing. Indeed if your front wheel in your car is not balanced properly...
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