open topic, for anything cycling related.
Death wobbles. Perhaps the scariest thing that can happen on a bicycle. You are flying down a hill and the bike starts wobbling uncontrollably. There is no time to think and you can't think straight anyway. Some say move forward in the saddle to put more weight over the front wheel, others say shift the weight back. Some say loosen the grip on the bars and let the wobble correct itself, others say tighten the the grip on the bars and try and control it. When you are hurtling down that hill with your heart in your mouth thinking its all over, experimenting with various methods is just not feasible!
So what is the definitive answer?
Invariably everyone sits up and takes further weight off the front, which is the opposite to what u should do... When the speed wobble kicks in, lean forward and put more weight on the bars & front wheel, and the vibration/wobble should stop..,
Here's a nice synopsis of the wobble, and what u should do... *courtesy of Wade (AT) Cyclingtips blog
http://www.cyclingtips.com.au/2011/03/s ... -its-head/
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.
Riding bikes in traffic - what seems dangerous is usually safe; what seems safe is often more dangerous.
When you are propelling the bike by pedalling, the head tube pushes the steerer tube (via the bearings) and the fork/front wheel assembly functions as a driven castor. Freewheeling at high speed, the fork/front wheel assembly is no longer driven by the frame. There is more drag, both from rolling resistance and from aerodynamic drag on the rider, on the frame than there is on the front wheel/fork. The front wheel wants to roll faster than the rear wheel. Now, the front wheel/fork is no longer a driven castor. The rest of the bike becomes a trailing castor, with a lot of trail.
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.
Funny, well not so funny actually, this happened to me yesterday as well, going back down from Kinglake to St Andrews. No crash but scared. Not sure I will be able to ride downhill comfortably again. Never experienced this before. The wheels are quite new (Chinese carbon). If it happens again, I think I will go back to my old trusty Eurus, at least for riding in the hills.
I still think that in some instances the “death wobbles” has something to do with the actual balance of the front wheel.
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.
il padrone's line of thinking - I used to have a problem occasionally when the panniers were loaded heavily and I hit a decent down-slope. The racing geometry of most good bikes makes for a shorter chain stay so the panniers can be providing significant weight behind the real axle.
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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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:
- rider, and
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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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.
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.
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.
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.
Last edited by human909 on Thu Nov 22, 2012 6:29 am, edited 1 time in total.
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.
(See video for the effect of wind!)
This is not accurate.
Nor is this. Standing up will RAISE the centre of mass not lower it.
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.
Personally, if I'm going to be altering one variable to change a resonant frequency, I pick speed.
How about some simple trail-braking, people ??
You have officially become your parents.
Agree. Wheel balance on bicycles is a non-issue.
Would the two water bottles plus the bar bag have dampened or made the wobble worse?
Break the resonance is my understanding. Squeeze the top tube and altering the weight distribution makes sense although I have never encountered one. Fingers crossed.
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My 2c which may be helpful;
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).
Just read that and it looks like you already covered what i said.
+1 on the rest of the post as well
I've never encountered this problem (touch wood). However all my bikes are old steel frames which by today's standards are heavy, oversized and sluggish. However said old frames and old 36 spoke Shimano 600/ Mavic MA40 wheels have hit 90km/hr without nary a shimmy.
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.
After further engineering analysis, I have now believe that death wobbles are almost exclusively a caster resonance effect. Not a generic resonance effect as I described earlier. The generic resonance effect described earlier certainly exists but I now don't believe it is the dominant factor.
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.
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.
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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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 +.
I gather that you don't get the resonance on the way up to 70kph.
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...
Unchain yourself-Ride a unicycle
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