I come over the top of the Lammermuirs from Gifford (hard work on a 17% gradient into a very strong headwind) and saw Whiteadder Reservior below me. Great!

Turned East and down, cross-wind, road a bit bumpy. Built speed up to about 33mph.

Suddenly the bike started vibrating/wobbling uncontrollably. I tensioned my arms and braked as hard as I dared (braking seemed to make it worse!) and started looking for a place to crash – not the road preferably.

I got it to a halt, and it continued to wobble right down to low speed. I suspect the carbon fork or the cheap wheels. Certainly something to do with the cross-wind.

Bloody Scary.

On the way back down, with a tail wind and better road surface, I got up to 41mph without wobbles so it’s not “just” the bike.

It’s a Kaffenback with a carbon fork. If I give the bars a shake deliberately, there is a distinct wobble.

Should I ditch the fork?

al.

I’d have thought that the resonance needed to generate a speed wobble will probably need a combination of factors to coincide to get a repeat. The bike, rider weight, wind speed/direction would all influence it. Of course changing one element of the bike setup may be enough to stop it happening, but it may just change the combination of cricumstance which cause it a bit – so it still happens, but in slightly different circumstances.

I’ve only experienced deliberately induced speed wobbles (not at any great speed) when exploring the handling of bikes. First checks to try and minimise it would be on wheel build/tension and fork/frame integrity/rigidity. Unfortunately unless you can find away of deliberately inducing it you’d never be sure if any changes would have had an effect.

Apparently you’re supposed to relax the grip on the bars and either stand up on the pedals, or grip the top tube with your knees… rather you than me 😉

<armchair theorist>

Is the carbon fork significantly shorter than a steel Kaff fork (which are long axle-crown) and affecting the geometry?

Purely idle speculation on my part I must admit.

</armchair theorist>

My road bike had developed a slight speed wobble over the last couple of months and I couldn’t figure out why. I’ve just replaced the rear tyre that had worn flat on the centre section. Its now wobble free!

I’ve got a Kaff fork in the garage, I’ll bang it on. I just rode to the postbox at the end of the street and back, giving the bars a deliberate shake. Something is distinctly wobbly.

Spokes are tight, QR’s tight, headset isn’t loose… I’ll try changing the fork.

As for standing up and letting go of the bars – no way! I’d have a skint face for sure.

al.

It’s called shimmy. It happens because of resonance of a flexible structure. It’s very hard to pin down what causes it.

Tighten QRs, make sure tyres are seated and inflated right.

Lots on it here http://www.ctc.org.uk/DesktopDefault.aspx?TabID=3813

Twohats: tyres eh? Conti 700x23c at 100PSI, used but new-ish. As I say I can feel this at 2MPH if I shake the bars, so I don’t think it’s that for me. After I change the fork, if it’s still wobbly I’ll try the tyres.

Actually I just realised I have another front wheel so I’ll try that first (and avoid crown-race whacking!).

al.

Thanks Brant.

Both cases involve something which acts as the “final straw”: an extra factor that when added to bike that is already somewhat prone to shimmy, tips it over the edge. We’d all be safer if bike designers would steer a bit further from that edge to start with!

So, they reckon it’s your fault!

😉

al.

Dont tense up..- it makes it worse

When this happens just try and relax on the bars, sounds daft, but it works..

Happens a lot for me, and i find, if I relax my arms, it dies down..

Agreed, it is pretty scary tho..

No idea what causes it..

So, they reckon it’s your fault!

Off the top of my head, bikes with short trail (steep/fast) will be close to “the edge”, but equally slack/long bikes don’t have as much weight on the front wheel, so can go all tank-slappery in theory.

Brant: So, I’d say my Kaff is steep and long! Hence rigidity of your average trifle.

Mr Armchair Theorist says the Kaff fork is long. I’ll go and measure the carbon (on-one) fork against the Kaff fork.

Back in a tick.

al.

Which carbon fork on there? on-one branded cross or mtb fork?

Brant: On-One one that came with the Pompino Pro build.

Well, that’s a remarkably tricky thing to measure. All in all, I’d say the length is exactly the same give or take a mm.

al.

Check your headset as well.

When it happens, move your knees to gentle grip the top tube.

Reminds me of my Proflex 854 with Vector forks…. It didn’t like 25/26 mph at all… ok above that though!

How many times have you pedalled like stink down a steep and straight sealed road, tucked into an aero position and watched the speed ramp up? It’s fun isn’t it? What is your best ever speed?

Let me now spoil these idyllic thoughts – there is a dark, dark side to this practice, the rare but dreaded death wobble.

Under certain conditions (over which you have little control) the front wheel and handlebars can start to shimmy or wobble. This very quickly develops into an uncontrollable shaking, which becomes so violent that you could be catapulted off the bike when the front wheel wobbles too far. To make matters worse, this tends to happen when you are going at your fastest speed ever (picture yourself hitting gravel at 84 kph)

The only relief I can offer from this frightening scenario is that it apparently happens rarely with MTBs (more instances with road bikes) and only at very high speed. I have experienced it only 3 times in over 10 years of serious mountain biking and each time it was very scary. The first time was at 60 kph on my first mountain bike, the second was on a road bike at 65kph, and the third was last Thursday at 84 kph.

Your immediate instincts are to suspect something wrong with the bike (loose head set, loose or broken skewer, loose wheel bearings/cones, cracked frame etc) Each of those bikes were top line models in perfect mechanical condition and adjustment. It was never repeated with the first bike in over 3 years of riding, I never rode the second again (it wasn’t mine) and so the phenomenon was far from my mind, until last Thursday – the nightmare had returned.

Have you experienced it? If you have not, be warned, it is seriously frightening. If you do get it, I guarantee that you will remember reading this article. You will recognize it immediately “this is what Dale wrote about at such length, now just what did he say to do?”

My Thursday’s incident involved coming down an exceptionally steep hill off the Toowoomba ranges (part of the 100k Epic Challenge held on 25 October 2003) I hit this hill doing about 70kph from the previous hill. As you speed over the crest you look down a dead straight, sealed and deserted, very steep road – go, go, go. You then see that it turns to gravel at the bottom transition point- a tightening of your rear end follows (gulp….that is where you will be doing your max speed!)

I was thrilled to see 80kph come up at some stage and then everything became quiet, smooth and dreamy – like breaking the sound barrier? Then the first small tremor of the handlebars crept in: this quickly became violent and I knew real shock and awe. I moved back on the saddle a little and this calmed things down a bit until I hit the gravel, where it came back in earnest. I didn’t brake at any time, just held on in fear. To my great relief, and no help from me, it eventually subsided into minor tremors. I checked all the fore-mentioned items and all were OK – no apparent reason for the violent wobbles.

I checked the stability of my forks, “Skareb” Super, against the other two rider’s forks (front wheel clamped between knees and handlebars moved side to side) “Fox” forks were stiffer than “Sid” World Cup which were similar to mine. The other 2 riders did not experience this problem, yet both did over 80 kph.

This incident left me uneasy about my nice new “S Works” FSR bike. What was the problem, frame geometry, Skareb forks, Mavic Crossmax wheels? I thought of E- mailing each company to seek comment and solutions, but decided to do my own research on the Web. – but where to start? Surprisingly, I got heaps of very specific technical info.by typing “Speed Wobbles on Bicycles” into the search engine. I waded through all this.

We are indebted, once again, to the world famous wheel guru Jobst Brandt for the most useful and comprehensive coverage of the problem.

It is a well recognised problem with bicycles at high speed. There is nothing wrong with the bikes – any bike can do it given the right circumstances. It has to do with the dynamics of forward motion and the gyroscopic forces of wheels. It happens rarely with MTBs because these generally operate on rough surfaces, at moderate speeds using knobbly tyres. All these factors militate against frame resonance, which is the culprit. The smoother everything is, the more likely it will happen. It is said that the shivering of a cold road rider can set it off because human shivering is very close to the natural resonance of a bike frame!

The explanations and physics behind this are really fascinating but the important issue to us is: –

WHAT DO YOU DO IF IT HAPPENS?

Concentrate now, the highlighted bits are the ones that you must remember.

All articles broadly had 2 recommendations: –

1. UNWEIGH YOUR SEAT – that is, stand up on the pedals for a moment or two

The tremor from the front ultimately causes the frame to increasingly oscillate (wobble). The rider’s weight on the seat acts to maintain these wobbles as an anchor point for them. If you take your weight off the seat, the wobbles dissipate.

An analogy I see, which further illustrates the principle, is that of an arrow fired into a tree. The arrow quivers (just like your bicycle frame when it gets the shakes) If you could suddenly take the tree away from the arrow (allow me a little latitude here) the quivering would stop because the arrow is no longer anchored to the tree. The bike frame similarly stops quivering when you remove its anchor point by rising off the seat!

2. SQUEEZE YOUR KNEES/LEGS TIGHTLY INTO THE TOP TUBE – that is, stop it vibrating.

I have a problem with the second method. My top tube is too far below my knees to do this, although I could get the calves of my legs against the top tube.

I would try recommendation No.1 first, as it is the easiest to do positively, followed by the leg squeezing for good measure. The big question is, “will it work?” I don’t know, and I’m not itching to try it out. I am just offering this info to you in the unlikely event that you come up against this scary problem. No guarantees are offered.

I was pleased, however, to learn that it is not something wrong with the bike and that it is a well-known and researched phenomenon. I will be a bit nervous now on fast smooth downhills (do I brake at 75kph, or let it run out?) but I do know that I now have a plan of action with the above advice indelibly etched into my brain. I suggest that you do likewise, particularly if you are entering the 100k event on 25 October.

I would be interested in any feedback on this matter

Wheeeeeeeee……..judder…judder..judder.judder…..that’s all folks

Well, that was a long reply! Wish I’d read it last week…

The shpeel from Chris Juden on the CTC link Brant supplied says the same thing – stand up and get your leg against the top tube.

A bloke at work was thrown from his MTB suddenly when descending a Glentress fireroad at 35mph. I think we now know why…

I was lucky, and it sounds like there’s fek all I can do to avoid it in future. But I’ll know better if it happens again.

Thanks all.

al.

yup, scary indeed. As mentioned above, move one leg inward (don’t think you need to clamp with both knees) so that your thigh is firmish along the top tube, this should resolve it.

PS, if your top tube is nearer to/below your knees than your thighs then you have got the wrong size road bike; assuming you were on a road bike Joxster (worms, can, opens)

MTB-Idle – Member

PS, if your top tube is nearer to/below your knees than your thighs then you have got the wrong size road bike; assuming you were on a road bike Joxster (worms, can, opens)

Depends if it is a Compact, Semi Compact or Traditional frame. I can’t remember where my tubes end and my knees,thighs begin

I’m a bit surprised that anyone would go that kind of speed without standing up – even on the road I take the weight off the saddle, shuffle back a wee bit and push my heels down behind the pedal axles. So much easier to control the bike if you whack a pothole, or indeed if a wobble develops.

Basically, never freewheel one pedal up and one down with all your weight going through the top of the bike (saddle and bars). Mega important off-road, but you might as well go with the same practice on road too because it is so much more stable.

I find popping a wheelie, a bunny hop or a rolling endo usually stops it.

Watch how the best pros descend – never sitting on the seat at top speed. There may be a reason other than aerodynamics for it.

AK – the is a nice wee hill there. You may have hit a change of surface which may also have given this effect.

moving a leg onto the top tube helps as speed wobble is usually caused by a combination of geomtry and front end stiffness, where the resonant frequency of the frame’ss flex and the steering geometry settle together and set up a wobble. wheel weight seems to influence it too but isn’t a cause.

can be cured by a different fork length, heavier / lighter wheel and also squirting expanding foam into the top tube to stiffen it – not reccomended at all but it proved the frame was the base cause with an old bike i had that wobbled. stiffer frames don’t tend to suffer from wobble but can also loose the spring that many riders like in a steel / ti frame.

BigJohn – I assure you popping a wheelie at that stage was not an option!

Cheers, al.

I didn’t say it was a happy ending, just an ending.

I used to like how Bjarne Riis descended, way off the back, just his stomach on the saddle.