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Tue 4 Mar 2008, 12:08 pm UTC

Q&A: Speed wobble

By Jez Loftus

Q: I have recently upgraded to a Cannondale 2008 Synapse 105 after riding a flatbar Trek 1000 for the last four years, and have found my riding experience greatly enhanced by the new, lighter bike.

Last weekend I decided to test out my new bike on one of the few big hills around here and found the ascent much easier than before. However, on the long descent (a 10% slope over about 1.2 miles), on reaching about 37mph, the handlebars began to wobble quite violently and for a time I seriously thought I was going to come off.

After managing to brake to reduce my speed, the wobble stopped and I regained ‘control’ of the bike, but the experience had quite literally shaken me as coming off at that speed could have caused serious injury to both me and my bike.

Now I am nervous about attempting another descent at such a speed in case this recurs. Is it due to the stiffer carbon frame, the set-up of the bike or just that having ridden a flat bar for years I still need to acquaint myself properly with the new riding position?

Damon Taylor

A: Unsurprisingly, what you’ve described is called a speed wobble, Damon. It’s something that all bikes exhibit but thankfully few of us experience. If you’re interested in the physics behind the speed wobble there’s a wealth of debate on the internet – just type ‘bicycle speed wobble’ into a search engine.

Speed wobbles are a complex phenomenon and any vehicle with a single steering pivot is capable of exhibiting one. All things have a resonant frequency and when the resonance frequency of the front wheel interacts with that of the frame you get a speed wobble.

Strangely enough, you’re most likely to get a speed wobble on smooth roads. Mountain bikes rarely exhibit speed wobbles because of the varying terrain, and knobbly tyres make it hard to reach the resonant frequency. It’s even been said that the shivering of a rider can cause a speed wobble as it is the same as the resonant frequency of the frame.

We know the Cannondale Synapse well and it’s a good bike. The trouble is, we’ve never experienced any wobble on it, and it’s not the sort of thing you can go out and create – nor one that we’d actually want to practise curing! – which doesn’t help your situation.

Suggestions of ways to combat the problem include, when in a wobble, unweighting your seat by standing up on the pedals, the theory being that your weight acts to maintain the wobble, so taking the weight off should dissipate the oscillation; another is to squeeze your legs tightly against the frame’s top-tube to stop the vibrations.

There are so many variables it’s hard to pinpoint one cause, and you may never experience the wobble again. It’s unlikely that there’s anything wrong with your bike, though you shouldn’t overlook doing some safety checks on your headset, hubs and fork alignment. It could be as simple as swapping the tyre, moving your body position or even just loosening your grip on the bars.

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User Comments

There are 10 comments on this post

Showing 1 - 10 of 10 comments

  • This has happened to me in the past, in my case it was because the spoke were not tight enough.Have your wheels checked if they are of the spoked type.Good luck btw scary is nt it...

  • Just recently happened to me whilst flying down Holme Moss towards Glossop. It took my bike onto the opposing carrage way (luckily no cars coming up at that moment) and as I gradually worked my bike back to "my" side of the road, the idea of bailing out into one of those posts that are at the side of th road did not appeal. Shorts almost went smelly I can tell you. Fortunately for me, the day was wet and very misty so brakes were not functioning properly. I believe that saved my bacon as I very very gradually slowed down to the point where I could get off the bike as a quivering wreck. The rest of the hill was taken much more sedately I can tell you.

    It would take a brave man who is experiencing the wobble at speed to "stand up on the pedals" or "grip cross bar with knees" Yes it might do the job but having the b*lls to do it at speed???? Not me!

  • This is an awful and unforgettable cycling experience. There seems to be plenty of debate about the causes and cures on the web.

    After this had happened to me, my LBS eventually found a horizontal hairline crack at the top of the carbon fibre front fork. It was just visible to the naked eye but when the front wheel was removed and the fork flexed - it really opened and closed. The front wheel was also slightly out of true.

    Maybe this didn't cause it but it hasn't happened since i fixed it. I now get my bike checked out on a more regular basis. However, in theory, this can happen on a perfectly operating machine. For example, it looked like one of the TdF riders had one the other day.

  • Happened to me about 4 weeks ago.

    It bloddy hurts when you hit the deck at 38mph!

    Body's broken but repairing slowly, and the bike too - not likely to be back on the road until the new year.

    No amount of 'relax your grip on the handlebars' or 'stand up on the pedals' enters your head at the time. Logic has long gone and blind panic sets in.

    Not looking forward to a repeat any time soon.

  • Speed wobble solution

    When it happened i thought i was dead! The frame is a Merlin Ti (Magia) and the thought of it happening again was not good. As an engineer i thought there must be a solution and if i looked at any modern bike the head tube is very stiff, mine was not.

    My idea was to stiffeen the head tube.

    Using polyurethane expanding foam (the sort used by builders) I filled about 1/3 of the top tube and 1/2 of the down tube (back from the head tube). I then epoxy resined a length of matting along the length of the inside of the head tube which covered about 1/3 of the circumference.

    After rebuilding the frame was noticeable stiffer at the front end and more importantly I have since built up speed out without any wobble.

    Not saying this will work on all occasions but it has worked for me and saved me scrapping an otherwise very nice frame.

  • Do I know about tank slappers or what ! 15 years ago I had a touring bike that built up a good tank slapper (motor bike term) on every decent from Norway through France Spain and beyond. The instant cure which works up to whatever speed you can reach down an alp is to lift the front wheel up off the ground for as long as you dare(a fraction of a second should be fine). This does the trick and gives you the chance to hit the brakes for a few seconds before you pick up another wobble. you may care to practce this (in a safe place) if it is going to be a regular occerance. Permanent cures ...I met a German cyclist in Corsica that bolted a 1/2 inch thick metal plate either side of the head tube/top tube and down tube each plate was about eight inches long this stffened up the front end no end as it were which I think gives a clue to the cause either way would make an interesting addition to a carbon frame!

  • I experienced this recently. However, it was the rear wheel that was the problem. My first thought as I was belting down the hill at 45mph was that I had a puncture and it was VERY scary! When I managed to stop, I checked and it was fine.

    When I got home, I checked the bike over and found the minutest ammount of play in the rear wheel bearings (Mavic Aksiums) which I adjusted out.

    I went down the exact same hill a week later at the same speed without any problems. I guess if the play had been in the front wheel it would have shown up as a severe speed wobble.

    Moral of the story - check play in wheel bearings/hubs! (and headset bearings)

    Gary.

  • Interestingly enough when I discussed speed wobbles with Geoge Longstaff (frame builder) some years ago he was of the opinion that if the frame was straight it must be the wheels that are out and causing it. As I had been building wheels for 30 years I didnt think much of this idea at the time. One of the odd things on a loaded touring bike is that some times the slightest re-distribution of weight can have a dramatic effect on wobble potential. So an un-scientific conclusion might be that possible causes are.

    1/ lack of stiffnes in the area of the head tube.

    2/ Out of track /alignment of the frame or wheels.

    3/ Excessive play in the wheel bearigs/ Headset'

    4/ Weight distribution critical or poor.

    5/ Wheels/Tyres badley out of balance

    6/ Too much wine/beer.

  • I had a client here in Tenerife who claimed he experienced a speed wobble on one of my rental bikes descending Mt Teide.

    Naturally I swapped the bike over for another one but could not find anything immediately wrong with it. Eventually, I found out that the headset bearings had settled or else worn in alightly, affecting the preload. I since have tightened the headset and now re-check this periodically on all my bikes.

    It's something else to watch out for...

    Les from Tenerife-Training

  • Tony Foale and Vic Willoughby wrote a book about motorbike design that has probably the best insight into why a single track two-wheeled vehicle (bike) wobbles.

    Firstly as the handlebars/forks etc turn (wobble) from side to side the front end of the frame will rise and fall. This can be seen when stationary if kneeling at the side of the bike facing the bike (looking at the head tube top tube) and turning the bars slightly to-and-fro. You should be able to see that the headtube toptube fall slightly 3 or 4 millimetres when the bars are turned left or right and rise as the bars are returned to the centre/staightahead position. In a speedwobble this includes the rider and frame rocking up and down slightly, pivoting about the rear hub.

    That was the easy bit to explain. Now for the gyroscopic effect of the front wheel. Take a front wheel hold the ends of the axle in each hand between thumb and index finger now spin the wheel a quickly. Still holding the axle use forefinger of one hand grasp spokes and spin wheel..... easier to do than to describe. You now have a big gyroscope in your hands, try to tilt the wheel to one side the wheel tries to counteract this movement. The gyroscopic effect.

    Does this seem complicated? That is because it is.

    You have now observed two of the components of the speedwobble. Firstly if the handlebars start to turn away from straight-ahead position then the weight of the rider will act to push down on the frame (standing up or sitting down has no effect on gravity). Secondly the inherent stability (ironically it is the stability of the bike that causes the wobble) of the bike created at speed by the two wheels spinning as gyroscopes create a restorative force that brings the front wheel back to the straightahead position. Unfortunately this restorative force is so great (too much) that it overshoots (over compensates) and takes the handle bars too far to the opposite side. This cyclical (almost pendulum) effect then increases in intensity to produce the speedwobble.

    People have been killed in the Isle of Man TT by speedwobble tankslappers. There have been devices to try and dampen the effect but bikes still wobble.

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