Wheel speed shimmy advice

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NeilL
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Real Name: Neil Lynch

Wheel speed shimmy advice

Post by NeilL » 10 Oct 2016 19:15

Hi today I experienced wheel shimmy coming down Rose Hill, this was a rather frightening experience and I think I now need new bib shorts!

My question is what causes this and can it be stopped?

There is lots of info online but I wondered what the collective advice of the club is?

The front wheel runs true, spokes are tight and the hub also seems OK. The wheel also seemed seated firmly.

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CakeStop
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Post by CakeStop » 10 Oct 2016 19:26

I've never experienced it but when freewheeling fast down hill I tend to have the top bar between my knees simply because it feels more aerodynamic so maybe that prevents it developing in the first place.
Eat cake before you're hungry

NeilL
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Post by NeilL » 10 Oct 2016 20:40

Thanks I'll try it

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jdtate101
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Post by jdtate101 » 10 Oct 2016 22:42

It can happen for a number of reasons, the main one being the frame geometry. Every bike will do it, although most require you to actually push it into oscillation. I've found that clamping the top tube with the knee's if it starts helps, but also descending on the drops prevents it.

If it does happen:

1) Don't death grip the bars..stay loose or you'll aggravate it.
2) Never brake hard, you WILL hit the deck. Brake slowly and favour the rear more.
3) Keep your weight forward, this will help the bike track. If you shift your weight back (a natural reaction) you will unweight the front which allows it to twitch even more.

The speed wobble is usually down to 3 things: Bad bike geometry (head tube angle), poor rider weight distribution or unbalanced braking.

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petemarshall
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Post by petemarshall » 11 Oct 2016 07:43

The usual reason for shimmy on a decent frame is the bars being set to high, particularly using a lot of spacers between headset and stem. If you have more than 15mm ( three standard spacers) this may be the cause.
After that, shimmy may be caused by using a fork with two much or two little rake or trail. If the forks came with the frame this shouldn't be the case.

Shimmy and general poor handling can be added to by the rider . Death grip and holding on the top of bars or the hoods will make things worse. Get in race position on the bars and relax ( yes I know this is easier said than done at 60kmh with a massive artic up your arse :roll:) .

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George
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Post by George » 11 Oct 2016 11:33

jdtate101 wrote:It can happen for a number of reasons, the main one being the frame geometry. Every bike will do it, although most require you to actually push it into oscillation. I've found that clamping the top tube with the knee's if it starts helps, but also descending on the drops prevents it.
That's my experience too. Any bike will do it given enough encouragement, but a top-heavy bike needs less encouragement. Encouragement can take the form of a clumsy shift of weight by the rider, but also a bump in the road or a gust of wind from the side, especially if you have anything that catches the wind, e.g. deep section rims, mudguards, panniers. As James and Pete say, you are always more stable on the drops.
jdtate101 wrote: If it does happen:
2) Never brake hard, you WILL hit the deck. Brake slowly and favour the rear more.
Seconded. My tactic is to feather the back brake. Front-wheel braking makes it worse and heavy back-wheel braking risks skidding.

NeilL
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Post by NeilL » 11 Oct 2016 12:47

Thanks all, this is useful stuff, I will experiment with my position on the bike when descending.

I was on the same decent the day before without issue, I was in the drops when this started but thinking back now I was on / off the brakes a bit more due to a slowing car, this could have set it off, or some other factor.

Its an interesting problem. I have had this bike since January and after tweaking the initial set up it has not changed and this has never happened before. I 'did' have complete confidence in the bike, the geometry is relaxed with the bars high but with only 3 spacers, I have been thinking lowering the bars and this may well be a good time to try this.

Thanks again

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