Dwonhills - what am I doing wrong??

[SnappyPix]

I had a problem with speed wobble (or "shimmy" as it's also commonly known) a few weeks ago, on a sharp descent.
It's terrifying - I literally feared for my life and thought my front wheel must have come loose and was about to wobble off.
In hindsight, what actually caused the wobble was me shifting position - moving my hands from the hoods to the drops.
Braking only makes it worse, as does gripping the bars with a death-like hold!

I'm not much of a physicist, but understand it occurs when the bike reaches a certain frequency. The best way to stop it is to loosen the grip on the handlebars a little and rest your knee(s) on the top-tube. This alters the natural frequency causing the shimmy.

Despite actually knowing all this, when it happened to me, my mind went completely blank, I panicked and careered down the hill looking like I'd drunk the local tavern dry!

Some bikes are allegedly prone to wobble more than others (apparently dependent on the frame), although I'm not sure there's any substance to this arguement.
I'm now so paranoid about getting "the wobbles" again that I now grip the top tube between my knees as I descend, which seems to work as it hasn't happened again (and I've ridden the same descent several times since).
When doing sweeping turns, I also try and make sure one knee is resting on the tube.

[margo49]

Must have shaken you to have spelt it like that!
Seriously,
I'm no expert but I agree with that Knee thing Snappy said

[DebW]

I've read that speed wobble can be cause by the fork. Apparently some carbon forks are prone to it, but not the most expensive ones. Gripping the top tube is good advise, or find the speed at which wobble begins and stay under it. I've never experienced it so can't say more - I ride a steel frame/fork and have never been over 61 km/hr.

[Tater]

I have a carbon fork on my Dulce and have had her up to 43.5 mph/69.6 kph, but haven't had a shimmy. I do loosely grip the bars (was in the drops for the 43.5 mph) and tuck my knees into the tube, though. That's an interesting situation, runawaymum. Could it have even been the road surface you were on? Stupid question, I know, but I know on chip seal, I feel as if the front wheel wants to vibrate right out from under me!

[Nanci]

My bike doesn't do this, luckily, but I've read that to make it stop, you just need to touch the top tube with one knee. Maybe someone who has this problem can comment. I'm always thinking about it as I approach higher speeds, just in case. (But it hasn't happened up to 49 mph- and I doubt I will be going much faster than that!!!)

When I apply the brakes, both for road and MTB, I use three fingers on the rear lever and two on the front, and apply equal pressure, but I don't jam on the brakes, I start very gently and increase pressure as needed and as I see how my bike responds. On the road, you can lightly feather the brakes, or, if going fast, where you don't want to ever let off completely, you can apply more pressure, then less. I, for some reason, am afraid of overheating my brakes/wheels/tires, and I don't know if this is a logical fear or not, but I try to give them a break intermittantly, not hold a steady pressure the whole way down.

Also, I try to plan ahead, and slow down _before_ a curve, so I can then just glide through it without having to brake as much.

The more you descend like this, the more comfortable it will get for you- especially if you can follow other riders through- but do your own thing at the speed _you_ are comfortable with.

Nanci

PS, I have a stock/came with the bike carbon fork.

[cosc]

I have experienced speed wobbles. The shimmy usually starts if I turn the wheel, or when I brake. The bike mechanic said some of it was because my brake was loose and not toed in right. I haven't experienced the shimmy since he adjusted the brake. I hope you can figure out the problem. A terrible frightful experience!

[Kathi]

I hit 42 mph the other day on Candisc in N. Dakota. I didn't notice any wobble and I was in my drops. In fact, I didn't realize I was going that speed until I checked the bike computer after the ride.

I also feather both brakes on downhills. That's the way I learned and it feels safer.