Shakey wobbler at speed

[Andrew_Culture]

My old man (by which I mean my old tourer) has always been very good to me, and as I ride to work every day I try to take reasonably good care of the old chap.

But...

This evening nipping down a hill just over 30mph I took care and gently applied the front brake, and the whole front of the bike shook and wobbled about in a very worrying manor. Why?

All bolts and nuts are tight, tyres (Marathons) are pumped, wheels are almost perfectly true and I've tinkered with the brake pads to get them close and even.

So why the wobbles?

 

[Pauluk]

Did you feel the shaking/wobbling through the handle bars?
Have you recently removed the front wheel? (Sorry if this sounds obvious).

 

[Ian H]

It could be almost anything, up to and including a cracked frame. First thing to try might be to grease and adjust the headset. But have a good look over the bike. Were you carrying anything usually heavy?

 

[Andrew_Culture]

Did you feel the shaking/wobbling through the handle bars?
Have you recently removed the front wheel? (Sorry if this sounds obvious).

No need to apologise! I'll check how true the wheel is before riding to work tomorrow.

 

[Andrew_Culture]

It could be almost anything, up to and including a cracked frame. First thing to try might be to grease and adjust the headset. But have a good look over the bike. Were you carrying anything usually heavy?

My work rucksack is surprisingly heavy. The bike is really old so gawd knows what hairline fractures there could be.

I'm a bit concerned as I'm doing the Hackney Dunwich run this Saturday and my only other option is an old Raleigh MTB that probably weighs more than I do and has tractor tyres on it.

Mind you we're aiming for a cruising speed of 12mph so the bike hopefully won't be under so much stress.

 

[Fab Foodie]

http://www.sheldonbrown.com/brandt/shimmy.html

 

[MrJamie]

Ive had a couple of wobbles, one at about 35mph when i braked and swerved a bit for a big pothole and the bike didnt seem to want to calm down for a few seconds. The 2nd time was again at about 35mph when i tried to look behind me to see if my friend was keeping up and it just started the wobble a bit. Id describe it as if the front of the bike was wobbling like it was resonating somehow and scarey because it seems to get worse before it settles, so feels like if it continues itll hit the deck. That was probably a year ago and nothing like that has happened since so i figured its probably more me than the bike.

 

[ColinJ]

I did the Kirklees Sportive/Brian Robinson Challenge two years on the trot with the same rider and he got very violent shimmy both times at the same place - at the bottom of the descent off Holme Moss towards Woodhead Pass.

It can be a scary descent at the best of times if you really let go (I had to keep feathering the brakes to keep my speed down to 50 mph) but when your bike starts wobbling like the wobbliest thing from planet Wobble ... He was absolutely terrified.

I think it was the way he was braking - I think he was nervous and had rigid arms. I was braking with my arms bent and pretty relaxed, and I was slightly raised out of the saddle so both pairs of limbs were acting as shock absorbers and damping out any vibrations before they built up. I also tend to keep one leg against the top tube on descents like that because I've read that it helps to avoid shimmy.

 

[Andrew_Culture]

I might have been a bit tense, I'll take the same route home today and see how I get on.

Thanks as always for your help.

 

[Andrew_Culture]

http://www.sheldonbrown.com/brandt/shimmy.html

What a brilliant link!

I recognise a lot of the causes there, and the frame on my bike is huge.

I think this could be don to my lousy technique and pushing a bike that was built for comfort to go too fast.

 

[Fab Foodie]

What a brilliant link!

I recognise a lot of the causes there, and the frame on my bike is huge.

I think this could be don to my lousy technique and pushing a bike that was built for comfort to go too fast.

Welcome.
Jobst Brandt and Sheldon Brown are prime myth-busters in most things cycling and a damn good reference source.

 

[Ian H]

http://www.sheldonbrown.com/brandt/shimmy.html

I hesitate to disagree with the great Sheldon, but I did have a bike (still have the frame) that was marginal on stability on a particular descent. If the headset was just slightly slack it would shimmy; adjust it and the bike was stable again. But I think the point is that small things can tip the balance.

Edit: just realised it's Brandt, not Sheldon. I'm less worried about disagreeing with him.

 

[Fab Foodie]

I hesitate to disagree with the great Sheldon, but I did have a bike (still have the frame) that was marginal on stability on a particular descent. If the headset was just slightly slack it would shimmy; adjust it and the bike was stable again. But I think the point is that small things can tip the balance.

That's not unreasonable because a part of the system is disconnected, i.e the fork/frame interface and so the resonant frequency of the system becomes change. However, I still maintain based on Jobst/Sheldon and my own anecdata that most shimmy is due to rider tension or death-grip rather than simply letting the bike have it's head.

 

[Andrew_Culture]

As an update I tried feathering the front brake again today, same braking technique and same speed as yesterday, and apart from the brakes being a little slack there was no shimmying at all!