SPEED WOBBLE ON A BICYCLE

Feature Article:

SPEED WOBBLE ON A BICYCLE (THIS IS SHOCK AND AWE)

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

Dale Garvey (3351 4597)