Canyon Aeroad Disc - Shimmy

Discuss light weight issues concerning road bikes & parts.
Bordcla
Posts: 184
Joined: Wed Dec 20, 2017 3:42 pm

by Bordcla

Very nearly ended up on the ground at around 40 mph yesterday as I was descending. Stock Canyon Aeroad Disc in size L, with the stock Reynolds Strike wheels. Not clear, reading on the web, what causes this shimmy / speed wobble. It felt like it started with a slight gust of wind, and then first thing I knew, I was zig zagging down the pavement at 40 mph, holding on for dear life. Managed to get it back under control, but it felt like the bike really wanted to throw me off. Quite amazing the force that can store and spring back.

Are these frames known to be sensitive to shimmy, or could mine be just a bad exemple? I don't find this very confidence inspiring, especially considering that it is quite easy to descend quite a bit faster than 40 mph given the right terrain, and wind is something ever present when descending.

Will a change of wheelset affect the dynamics of the bike such that it can prevent (or worsen?) the tendency to oscillate?

I must admit, I'm a bit apprehensive of riding this bike at the Vermont Gran Fondo, descending the famous Vermont Gaps. I also planned to ride that bike in the 2019 Haute Route Alps, but if I have to be on the brakes to keep my speed below 40 mph on the descents, I'm starting to think it's not a good idea.

Any concerns to have with this bike or is this just the sort of thing that could be a "one off" or "perfect storm" occurrence?

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wheelbuilder
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Joined: Wed Feb 08, 2017 2:10 am

by wheelbuilder

Wow. I have no information to help you, but that sure sounds scary.

by Weenie


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TonyM
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Joined: Thu Jan 22, 2015 4:11 pm

by TonyM

Something similar happened to me once. It came from the tension of the spokes. Did you check these?

alcatraz
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Joined: Mon Aug 29, 2016 11:19 am

by alcatraz

Sounds like a loose headset, stem, wheel axle. But best would be to check the steerer for cracks to be on the safe side. Also check your brake caliper bolts.

It's possible that as you are braking your left fork leg is a bit flexy and if you have play somewhere or some funky brake pad/disc action you'll get some oscillating fork flex left/right. This could perhaps be aggravated by braking even harder which might be the instinctive reaction.

Try the rear brake more and less front, should it happen again (unless you are cornering).

I'm just brainstorming here. I don't use disc brakes nor have I encountered the problem myself.

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Beaver
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Joined: Wed Aug 18, 2004 6:06 pm
Location: Lower Saxony - Germany

by Beaver

Do you (or maybe a friend) have another wheelset to try? Those Strike SLG wheels don't behave well in crosswinds.

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Miller
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Joined: Thu Sep 21, 2006 8:54 pm
Location: Reading, UK

by Miller

Grip the top tube with your knees, no chance of shimmy then.

Bordcla
Posts: 184
Joined: Wed Dec 20, 2017 3:42 pm

by Bordcla

TonyM wrote:
Mon May 28, 2018 5:13 am
Something similar happened to me once. It came from the tension of the spokes. Did you check these?
Out of curiosity, how were you able to determine that?

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TonyM
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Joined: Thu Jan 22, 2015 4:11 pm

by TonyM

I checked the tension of my wheelset as I usually do every 2-3 months and several spokes had a very different tension than the others. I had to tighten then. Ad then everything was ok.

Bordcla
Posts: 184
Joined: Wed Dec 20, 2017 3:42 pm

by Bordcla

TonyM wrote:
Mon May 28, 2018 4:20 pm
I checked the tension of my wheelset as I usually do every 2-3 months and several spokes had a very different tension than the others. I had to tighten then. Ad then everything was ok.
Was it shimmying regularly before you made that adjustment?

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TonyM
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Joined: Thu Jan 22, 2015 4:11 pm

by TonyM

Bordcla wrote:
Mon May 28, 2018 5:09 pm
TonyM wrote:
Mon May 28, 2018 4:20 pm
I checked the tension of my wheelset as I usually do every 2-3 months and several spokes had a very different tension than the others. I had to tighten then. Ad then everything was ok.
Was it shimmying regularly before you made that adjustment?
Yes it started to be like this on the descents.
You have anyway to check the tension of your spokes regularly, especially if you ride a lot or ride on bad roads, descents etc.. It is for your own safety.
Same applies for all bolts etc.....I know so many stories of people not checking all these points and having all kinds of problems or even accidents and injuries.

Bordcla
Posts: 184
Joined: Wed Dec 20, 2017 3:42 pm

by Bordcla

TonyM wrote:
Mon May 28, 2018 5:22 pm
Bordcla wrote:
Mon May 28, 2018 5:09 pm
TonyM wrote:
Mon May 28, 2018 4:20 pm
I checked the tension of my wheelset as I usually do every 2-3 months and several spokes had a very different tension than the others. I had to tighten then. Ad then everything was ok.
Was it shimmying regularly before you made that adjustment?
Yes it started to be like this on the descents.
You have anyway to check the tension of your spokes regularly, especially if you ride a lot or ride on bad roads, descents etc.. It is for your own safety.
Same applies for all bolts etc.....I know so many stories of people not checking all these points and having all kinds of problems or even accidents and injuries.
So if I read you correctly, the Strike wheelset is the culprit? Do you have a tension gauge for the spokes? How tight do they need to be? Was the front or the rear causing the problems, or both?

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TonyM
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Joined: Thu Jan 22, 2015 4:11 pm

by TonyM

In my case yes that was the front wheel. I don’t have a tension gauge but more than 30 years of experience in checking the tension of the spokes regularly. You can tell if a spoke is loosen or not. The tension gauge is more necessarily when you built a wheelset yourself. I did it once, just to try, with a friend who was a mechanic but you have to have a good experience on that.

As someone wrote earlier the shimming can come from various factors. But checking the tension of your spokes of your wheelset (and bolts on your bike etc...) is definitely something you should do regularly if you are serious about biking and about your safety.

Hexsense
Posts: 497
Joined: Wed Dec 30, 2015 12:41 am

by Hexsense

While it doesn't seem to matter or effect normal riding condition, have you tried to balance your wheel? Maybe speed wobble from unbalanced wheel are amplifying things. Try spinning the wheel fast on work stand. If it shake violently, you should weight balance them.
https://silca.cc/products/speedbalance- ... net-system

Also, in that condition, use rear brake only. Just a slight pull from tail end of the bike make your bike straight again.

sungod
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Location: it's raining, it must be uk

by sungod

whatever the cause, as mentioned above you can probably control or even prevent shimmy, you just need to be ready and act calmly (yes, it can be difficult to be calm, but it's what you need to do!)

- pressing one/both knees against the top tube, this may be the most effective thing you can do - i have a habit of putting one knee against the top tube on fast descents with strong/changing crosswind as i know those conditions can trigger shimmy for me
- slight *rear* braking to alter speed, or pedal hard to alter speed
- slightly unload weight from saddle, just a few cm, not standing
- try to stay relaxed, tensing/fighting the bars can make things worse - your reaction time is not fast enough, you could even amplify the effect

Bordcla
Posts: 184
Joined: Wed Dec 20, 2017 3:42 pm

by Bordcla

sungod wrote:
Mon May 28, 2018 8:12 pm
whatever the cause, as mentioned above you can probably control or even prevent shimmy, you just need to be ready and act calmly (yes, it can be difficult to be calm, but it's what you need to do!)

- pressing one/both knees against the top tube, this may be the most effective thing you can do - i have a habit of putting one knee against the top tube on fast descents with strong/changing crosswind as i know those conditions can trigger shimmy for me
- slight *rear* braking to alter speed, or pedal hard to alter speed
- slightly unload weight from saddle, just a few cm, not standing
- try to stay relaxed, tensing/fighting the bars can make things worse - your reaction time is not fast enough, you could even amplify the effect
Actually, when this one occurred, it was toward the end of a hard ride, and I was descending casually, very relaxed (too relaxed?) with the hands on the hoods instead of the agressive position in the drops I usually adopt whenever descending. That may or may not have contributed. In any event, I've descended that hill several hundreds of times on my previous three bikes, in all kinds of posture, and never had this happen to me. I was lucky not to get thrown off, as the bike sure did its best to try and get rid of me! To the point where I hope that the violence of the shaking did not hurt anything...

by Weenie


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