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Shimmy AKA Death Wobble

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Dave....
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Joined: 13 May 2016
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PostPosted: 09:27 - 24 Sep 2016    Post subject: Shimmy AKA Death Wobble Reply with quote

As title, my road bike has developed the death wobble at speeds over 40 mph which is a bit unnerving to say the least.
Bike is about five years old carbon frame and the first time it happened about three years ago it was very minor and corrected itself, second time was last year and it did it twice in same day quite bad to the point of thinking I was off into the hedge or worse.
Changed wheels to a tubeless setup and all seemed great again but it happened again recently.
Searched internet endlessly for a solution and have read about every article there is so have tried to check all bearings in wheels, headset even tried balancing wheels (opinion is it`s pointless) different tyre pressures and even considered changing bike as it may have lost its rigidity over time but again it can happen on new bikes apparently.
Can sort it out by shifting my weight over front and even pressing knees on top tube but when it happens the tendency is to brake, death grip then try and correct.
Has this happened to anyone else and what did you do to overcome it?
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ScaredyCat
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PostPosted: 09:47 - 24 Sep 2016    Post subject: Reply with quote

Surely by fighting it you're just making it worse by adding inappropriate input.

Are you going down hill at the time? The reason I ask is, if not, don't brake, don't fight it. Let physics sort it for you.


Is it possible you're pulling back on the handlebars as you peddle and try to go faster so the front end gets lighter?
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Dave....
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PostPosted: 10:45 - 24 Sep 2016    Post subject: Reply with quote

Definitely going downhill, I`m not that fast !
I`m not aware of shifting weight back or pulling on bars and when I want to go faster downhill my hands would be on the front inside of bars with my head down so upper body more forward, weight distribution seems a major factor and the initial reaction is to brake to correct (wrong I know).
Possibly I could be shifting my arse over back of seat without realising it?
I weigh in at 14 stone and six foot tall , does it happen to lightweights, maybe need to lose a bit of weight?
I really wish it just wouldn`t happen rather than trying to fight it as I don`t think my riding style has changed over the years.
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bamt
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PostPosted: 11:00 - 24 Sep 2016    Post subject: Reply with quote

It's pretty much about weight distribution and resonant frequencies.
Gripping the top tube with your knees sorts it out for that reason, just moving your weight will do too.

Moving your arse off the seat isn't a problem itself - I do that to both get more aero and to let my arse breath a bit (anybody who has been on hard multi-day audaxes with me will also claim that I use rocket propulsion as my stomach sorts itself out on the strange demands I put on it...). That won't cause a shimmy, unless it affects the resonant frequency of the bike.
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lihp
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PostPosted: 11:01 - 24 Sep 2016    Post subject: Reply with quote

I assume you're on the drops at those speeds?
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Ste
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PostPosted: 11:40 - 24 Sep 2016    Post subject: Reply with quote

Sell road bike > get mountain bike > winrar. Thumbs Up

What frame is it?
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Dave....
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PostPosted: 13:23 - 24 Sep 2016    Post subject: Reply with quote

Yes on the drops.
I have two road bikes and two mountain bikes so plenty miles on all types.
The problem bike is carbon framed and my local route involves an almost continuous descent of 1000 ft so plenty opportunities to go flat out, which I like doing, although it has never happened on my local and I have never had any bother on my other road bike which is alu framed.


Last edited by Dave.... on 15:45 - 30 Dec 2016; edited 2 times in total
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Ste
Not Work Safe



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PostPosted: 13:33 - 24 Sep 2016    Post subject: Reply with quote

Check that the wheels are true.

You want your weight to be the pedals and not the saddle.

Loosen your grip on the bars.

Pros put a knee against the crossbar to dampen speed wobble, whether or not it's quite that straightforward is another matter. Laughing
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stinkwheel
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PostPosted: 19:07 - 24 Sep 2016    Post subject: Reply with quote

One of several reasons why I wont have a carbon framed bike. My understanding is vagueness and upredictability when descending is a "feature".

Get a steel frame and descend like a bat out of hell.

I'd imagine it's got a lot to do with an overly short wheelbase, tight rake ange and weight forwards riding position. ie. everything that makes it go fast. Some modern racer bikes are so tucked under at the front end, the front wheel hits my toes when I steer.

If you had a set of forks with more rake on them, I bet it would settle down.

EDIT: Just a thought. Does it happen when you're pedalling or when you're coasting?

If you're coasting, the strain in the wheels is being taken up by the leading spokes rather than the trailing ones. So your wheels can be true when at rest and when under power buty move out of true when coasting or braking. Modern bikes tend to have really small numbers of spokes which exascerbates the problem (Mrs stinkwheels race wheels are 16 spoke half-radials which to me is madness on a road bike).

The solution to that is not to coast down hills.
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Last edited by stinkwheel on 19:13 - 24 Sep 2016; edited 1 time in total
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Wafer_Thin_Ham
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PostPosted: 19:11 - 24 Sep 2016    Post subject: Reply with quote

stinkwheel wrote:
One of several reasons why I wont have a carbon framed bike. My understanding is vagueness and upredictability when descending is a "feature".

Get a steel frame and descend like a bat out of hell.



Not all carbon framed bikes are like that. I'm sure a lot of cheap steel framed bikes are pony too.

Never had a problem with any of the carbon frames I've ridden up to 45-50 of descending rock solid. First thing I would check would be the wheels for true-ness.

Interestingly my ally framed MTB did develop a hilarious shimmy at speed when the wheels were out of true.
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defblade
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PostPosted: 10:24 - 25 Sep 2016    Post subject: Reply with quote

Another vote for truing up the wheels.

I actually find the job strangely relaxing....
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Dave....
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PostPosted: 11:22 - 25 Sep 2016    Post subject: Reply with quote

Wheels are bang on and are newish, it also did it on last wheels which is why I changed them.
So it isn`t that, on my bike at least.
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lihp
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PostPosted: 11:25 - 25 Sep 2016    Post subject: Reply with quote

What bike is it?

How old is the bike?

Has it always done it?
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Seb
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PostPosted: 14:34 - 25 Sep 2016    Post subject: Reply with quote

As stated, don't let your legs go limp, if you're heart rate isn't going up on a descent, you're not trying hard enough Laughing

Keep your weight going through the pegs and squeeze the top tube with your knees. Whilst tucking is important, don't put yourself in a crazy position where you're upper body is ends up being rigid to hold it.
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Dave....
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PostPosted: 21:24 - 12 Feb 2017    Post subject: Reply with quote

Solved the problem.











Sold it.

Changed out head bearings first though as I think it may have been that but never got to road test it.
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Ste
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PostPosted: 21:48 - 12 Feb 2017    Post subject: Reply with quote

141 days ago, Ste wrote:
Sell road bike


Wink
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duhawkz
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PostPosted: 22:59 - 16 Feb 2017    Post subject: Reply with quote

I bought a carbon road bike, the seller told me it was VGC, however it's all over the place when going down hill........
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Dave....
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PostPosted: 23:01 - 17 Feb 2017    Post subject: Reply with quote

duhawkz wrote:
I bought a carbon road bike, the seller told me it was VGC, however it's all over the place when going down hill........


Wasn't me Wink
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mudcow007
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PostPosted: 18:05 - 14 Mar 2017    Post subject: Reply with quote

sounds stupid but make sure the tyres are seated on the rims correctly

I had a tiny little egg in the tyre where the bead had slightly come off the rim, which caused the bike to shake got worse the faster you went

Marathon's (tyres) are a bastard to fit when new


**edit**

crap didnt look at the date grr
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Dave....
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PostPosted: 22:20 - 14 Mar 2017    Post subject: Reply with quote

No issues at present with other bike I have as it is attached to a Tacx trainer.
Which presents a shit load of other issues Mad
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UnknownStuntm...
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PostPosted: 09:32 - 15 Mar 2017    Post subject: Reply with quote

Dave.... wrote:
Solved the problem.
...
head bearings

There you go... Small amount of friction in a worn or badly adjusted head bearing will absolutely cause this.

Some people attribute this to be a 'characteristic' of modern lightweight frames, but it really isn't.

More weight in the frameset can mask the issue, as there's more mass to push about before you start to feel it as a wobble.

EDIT: now I see you have a bike on a turbo. Guess what can put stress on headset bearings..... Go on...
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Dave....
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PostPosted: 21:34 - 15 Mar 2017    Post subject: Reply with quote

UnknownStuntman wrote:

EDIT: now I see you have a bike on a turbo. Guess what can put stress on headset bearings..... Go on...


Smile Another bike for sale.
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