Dwonhills - what am I doing wrong??

[Runawaymum]

I'm generally pretty cautious on downhills and tend to stay on the brakes too much and get left behind while everyone else zooms off. I 've been reading up about the correct technique , using the front brake , keeping weight back, looking where I want to go , and practising on some smaller hills.

So today I went down this hill, nice smooth road with a sweeping bend, no potholes etc, little traffic, about 23% gradient ( 229m descent over 1km ) , no brakes...it was all going fine until I applied the front brakes about three quarters of the way down ( there is lights controlled intersection at the bottom) and got major speed wobbles. At this stage I was going about 73k/hr and was only using the front brake , so I kept steady pressure on the front and also light braking on the back . Thankfully I slowed down enough to stop at the intersection,but was very wobbly the whole way down and only just avoided coming off .

Any suggestions what might have caused this, mechanical problem ( wheel not straight/balanced), going too fast, wrong technique - should I have braked harder or let off, something else ??? Please help . How do I respond if it happens again?.

I have had a minor problem with speed wobbles before and had my bike looked at the mechanic said it was fine, get a second opinion??

Note to the gods of cycling....Thank you, thank you, I thought I was going to loses some serious skin there

thanks for you help : )

[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.

[Runawaymum]

Ok I've taken my bike into LBS for a service it probably needed one anyway and from now on I'm using the back brakes first!! Thanks for your advice everyone : ) I'll go find a small hill to practise on.

[SnappyPix]

For me, speed doesn't induce speed wobble (was hitting 70+kph today with no probs) - but now I know what does, as I nearly started a shimmy off again today.
It's definitely when I begin to shift my hands from the hoods to the drops - I've tried sliding them down the handlebar, but this morning inadvertently gave a very, very slight flick as I moved my hand, and the handlebar responded, causing the wheel to start to go. Luckily I'm aware of this now and rectified it immediately by easing off and resting my knees on the top tube.
For me, it's definitely not mechanical - just operator error!

[Runawaymum]

hey Snappy what part of NZ are you in? I'm in Auckland I dont think its the speed, just using the brakes incorrectly duh !! Back to the drawing board

[crazycanuck]

Hey RM-just a quick wave to you in Auckland-We used to live there but are now over here yonder. Have fun with the traffic-i guess you've already figured out that you need to hog the whole lane hey!
Are you a mtn biker as well? No need to worry about Auckland drivers if you are!

Snappy-Sorry to hear you have to leave nz .


c

[RoadRaven]

Remember ladies that the mechanics of bicycles are pretty fundemental, and they are made to go at speed that people can take them too.

When we go downhill, we take bikes to speeds that begin to challenge their frames.

The speed wobbles is most likely a combination of the weight distribution on the bike, the wind resistance or direction, the increasing speed and the way you apply the brakes.

When braking downhill, feather the brakes, don't just apply and hold tight - that will cause a sudden change in resistance and not allow the bike time to adjust.

I also use a combination of braking both wheels, though I don't know if that is recommended, but it works for me.

Have fun experimenting, Runaway... I'm sure those Goddesses of cycling will continue to watch your technique and guide your wheels

[SnappyPix]

hey Snappy what part of NZ are you in? I'm in Auckland I dont think its the speed, just using the brakes incorrectly duh !! Back to the drawing board

Hey - small world! Yes, I live in Auckland!! Wow! Whereabouts are you?

[Runawaymum]

Hi CC bet its alot warmer where you are right now Yeah I'm real good at taking the whole lane when I need too I tend to ride out South where its a bit less hairy ( mostly). I've come to the conclusion that for every 20 -30 K travelled you get one idiot who either doesn't see you or doesn't care !!!!

Snappy pm'd you back

Raven I definetly think both is the way to go, there is another thread on this board that advocates using front brake mostly, having tried that I think I'll go back to both!!

http://forums.teamestrogen.com/showthread.php?t=7729