Terrifying wobble under braking

[winjim]

Riding the new bike today down a pretty fast descent, it developed a nasty wobble under braking. So much so that I was afraid to brake any more firmly and had to slide off the saddle and put my foot on the floor to slow down.

Bike is an old Peugeot mixte which is a couple of sizes smaller than would be ideal. Steel rims and cheap single pivot Lam brakes. So what's the culprit and how can I fix it??

 

[Yellow Saddle]

You sure it is not just plain garden-variety shimmy?

 

[winjim]

You sure it is not just plain garden-variety shimmy?

Don't know, I've never had it on my more modern bikes. Whatever it is, I worry that it's being amplified by my weight distribution as the bike's really a bit too small.

 

[Yellow Saddle]

Don't know, I've never had it on my more modern bikes. Whatever it is, I worry that it's being amplified by my weight distribution as the bike's really a bit too small.

OK, my guess is that it is standard shimmy. It isn't the brakes that caused the shimmy but slowing down to the critical speed where shimmy naturally occurs on that particular bike. Speed at which shimmy happens is very critical in the onset, but has a bit of range in the slow-down. In other words, you sped up right past the point but slowed down nicely into it and once it started, going slower and slower kept it there until you destroyed the wave node by getting off the saddle and slowing down beyond it's critical range.
What to do? Test it under safer conditions. This can easily be done on a long, slight slope. Ride no-hands, going faster and faster until the shimmy occurs and then lean forward and grab the bars. It will be immediately arrested since the real speed where it occurs with hands on the bars is much higher. In other words, test it no-hands at low speed to confirm that it was shimmy.
There is no condition that I know of where braking introduces a wobble.

 

[Yellow Saddle]

Oh, and to answer your question. You can't fix it.

 

[winjim]

I am still getting used to the bike, it handles very differently to my modern road bikes. I'll get out and do some testing. If I had just yanked the brakes on, do you think that would have successfully and safely slowed the bike?

As long as I can predict it, I don't necessarily need to fix it.

 

[Yellow Saddle]

I am still getting used to the bike, it handles very differently to my modern road bikes. I'll get out and do some testing. If I had just yanked the brakes on, do you think that would have successfully and safely slowed the bike?

As long as I can predict it, I don't necessarily need to fix it.

No, that would not have arrested the shimmy. Slowing down is pretty limited and the slow-down range of the wobble is large. You have to test it and know what's going to happen. The best way to cope with it is to learn how it behaves at an artificial induction speed as I suggested.

 

[Smokin Joe]

Pressing your knees against the top tube is said to cure it, whether true or not I don't know.

 

[winjim]

Pressing your knees against the top tube is said to cure it, whether true or not I don't know.

Top tube? It's a mixte!

 

[wifflebat]

Ride no hands...

I'm 39 years old and cannot do this.

...there, I said it.

 

[Dogtrousers]

I'm 39 years old and cannot do this.

...there, I said it.

If you'd said "I'm 15 years old" that might have been a problem. As you didn't, it isn't. Over the age of about 16 riding with no hands is an unnecessary skill.

 

[Racing roadkill]

Good old speed wobble. It's scary when you get a good one.

 

[Salty seadog]

If you'd said "I'm 15 years old" that might have been a problem. As you didn't, it isn't. Over the age of about 16 riding with no hands is an unnecessary skill.[/QUOTE]

Not if you need to zip up a jersey or pop a buff on. I'm 43 and do it regularly, without a helmet too....

 

[wifflebat]

Yes, some day I could be powering over the summit of a great mountain and, instead of casually extracting my wind vest from a back pocket and zipping it up no-hands style, i would have to prop my bike against the team car and rummage around in my basket for a cardigan :/

 

[winjim]

Good old speed wobble. It's scary when you get a good one.

It was less the wobble itself, than working out exactly how I was going to slow the bike down before I reached the two lanes of stationary traffic at the bottom of the hill. It helped that firstly I was wearing regular shoes, not cleats, so sticking my foot on the floor worked as a braking mechanism, and secondly the bike has no top tube so I could leap off the front of the saddle without worrying about hitting myself in the goolies.