Author Topic: Speed wobble  (Read 869 times)

finch

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Speed wobble
« on: 05 May, 2024, 01:49:13 pm »
Today I was coasting down a hill that I go down regularly , on the same bike I ride regularly and experienced a little bit of speed wobble which personally I found quite frightening.

There have been a couple of changes since my last ride down the hill which include - different tyres - 30c down to 28c and probably much more supple running at their max of 87psi - this could be a touch higher I’ll need to check

It was a bad day weather wise and I started getting a little touch of the dreaded Mavic howl at high speed coasting - it was happening when I got the speed wobble and the chain was hanging down inside the stay - obviously I was looking down and assessing this when it started - and I’m wondering about possible causes

The front wheel is true , the tyre is seated properly and the headset “feels” fine

Could it have been from the drag and chain suck and my weird position looking down at it ?

Perhaps tyres slightly too hard ?

Re: Speed wobble
« Reply #1 on: 05 May, 2024, 01:55:29 pm »
Must be an old Mavic wheel if it's howling.  Get a couple of 5mm Allen keys, remove the freehib, clean the inside then lubricate it will disc brake mineral oil, or wetlube.

finch

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Re: Speed wobble
« Reply #2 on: 05 May, 2024, 02:06:36 pm »
I’ve got the correct mineral oil etc and know how to do it and I even have a spare fts-l Freehub but I’ve obviously let this one get a bit dry - just wondering whether that could be a contributor

Re: Speed wobble
« Reply #3 on: 05 May, 2024, 02:07:38 pm »
Doubt it. Have you checked headset?

finch

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Re: Speed wobble
« Reply #4 on: 05 May, 2024, 02:10:22 pm »
Yeah , mind you I was doing maybe 30-35mph looking down at it and could have been slightly off balance

Re: Speed wobble
« Reply #5 on: 05 May, 2024, 02:17:28 pm »
If max pressure, then sounds like 28c is too narrow for your weight.

Regarding speed wobble. It’s to do with harmonics that can multiply in the frame for certain speed ranges and surfaces. If it occurs again, press a knee against the top tube, which can often be enough the dampen it, and bring it under control.

Another cause of speed wobbles is a misaligned frame.

finch

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Re: Speed wobble
« Reply #6 on: 05 May, 2024, 02:20:03 pm »
Hmm - food for thought it was fine with 30s and 25s are fine on my other bike but I imagine these things are specific to a bike

I know it’s never been crashed and it’s a top quality Alu frame

Re: Speed wobble
« Reply #7 on: 05 May, 2024, 02:31:54 pm »
I've had speed wobble, shimmy, with a lightweight steel frame. Road surface or wind can make the frame go into resonance and induce speed wobbles. Shivering can also induce it. Also, weight behind the saddle such as a large seatbag, or a rack.
When I have to carry a large bikepacking style seat bag for brevets, I purposely over tighten the headset so there is a bit of resistance.  Probably not good for bearing life but effective in my case.
 There is a headset available to prevent speed wobble, viscoset by cane creek.



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T42

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Re: Speed wobble
« Reply #8 on: 05 May, 2024, 03:56:58 pm »
I corrected shimmy on the Ti Warhorse by adding a bar bag, which changed the bars' resonant frequency.

You can also change the bike's resonant frequency by gripping the bars tightly and keeping your arms rigid.  If you get it just right the bike will start to shimmy, which will cause you to grip more tightly and make your arms stiffer yet and the shimmy worse.  BTDT on the 10% slope down off the top of Mt. Ventoux towards Malaucène and scared myself silly.
I've dusted off all those old bottles and set them up straight

finch

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Re: Speed wobble
« Reply #9 on: 05 May, 2024, 05:28:47 pm »
Tyres now 80psi
Tightened headset a little to just the point before it gets “not smooth” even though there was no play.
Serviced Freehub
Checked hub bearings and nipped them up as far as they’ll allow while spinning freely
Dropped the stem 10mm and whapped the 2 spacers on the top
Greased and double checked skewers

Will report

Re: Speed wobble
« Reply #10 on: 07 May, 2024, 09:26:22 am »
I'd a bike for years which had shimmy problems.  Tried everything, including new headset, umpteen different tyre brands, new rims of front and back, professionally built..

The bike was supplied new in 1985 with Campag hubs and of course it couldn't possibly be anything to do with them.  Then I had the bike serviced again and the LBS told me my hubs could not be serviced because they couldn't get the parts any longer, or some such advice.  So I had new hubs.

Bingo!  After that the bike was steady as a rock.  I haven't ridden any other bike with shimmy, which is just as well in North Yorkshire. 

I corrected shimmy on the Ti Warhorse by adding a bar bag, which changed the bars' resonant frequency.

You can also change the bike's resonant frequency by gripping the bars tightly and keeping your arms rigid.  If you get it just right the bike will start to shimmy, which will cause you to grip more tightly and make your arms stiffer yet and the shimmy worse.  BTDT on the 10% slope down off the top of Mt. Ventoux towards Malaucène and scared myself silly.

Have you watched the 2015 film Ventoux?  It was made the year I visited 'the giant' but fortunately I hadn't watched it before I made my very enjoyable whizz down the mountain.   
Sheldon Brown never said leave it to the professionals.

Re: Speed wobble
« Reply #11 on: 07 May, 2024, 07:09:48 pm »
I've had bad shimmys twice.  Both times I felt like I was going to die!

The first time I was about 10 years old so don't remember clearly what brought it on. 

The second was on a long descent in the Himalayas and it was brought on by having to brake sharply when another rider pulled out in front of me when I was about to overtake. 

I've had it again since, but now I know about the knee on top tube trick, I'm able to stop it straight away.  But tightening the headset is an interesting idea.