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Wobble while descending

 
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mattsurf




Joined: 28 Sep 2016
Posts: 191
Location: salisbury

PostPosted: Tue Mar 07, 2017 11:06 am    Post subject: Wobble while descending Reply with quote

On a recent ride, on a couple of the very fast, steep descents, my bike decided to wobble alarmingly while descending.

This was my first ride on the road bike since November.

Over the winter, with hours on the turbo, I have changed my saddle and saddle position, I am now using an Adamo on both road and TT bike and I have moved the saddle forward on my road bike, could the more forward riding position be causing the wobble?

The roads were wet and slippers, could this be affecting my tyres to the extent that the bike wobbles

I was using carbon wheels - the brakes were really grabby (cold and wet), could this be causing the wobbles?

I checked the bike and the headset - can see any issues and headset was tight (I did tighten it a little though)

It was slightly unnerving experience, and I don't recall this happening in the past, any thoughts would be welcom
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Jorgan




Joined: 12 Nov 2007
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PostPosted: Tue Mar 07, 2017 11:09 am    Post subject: Reply with quote

How deep were the rims? Could be the wind. Tyre pressure? How low are you crouching?
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explorerJC




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PostPosted: Tue Mar 07, 2017 11:50 am    Post subject: Reply with quote

poorly designed, manufactured or set up of bike or too much weight over rear (not uncommon when descending)

without gripping the bars, put more weight through the bars (with bent elbows) to dampen the wobble
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hammerer




Joined: 19 Nov 2007
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PostPosted: Tue Mar 07, 2017 12:32 pm    Post subject: Reply with quote

speed wobble, or as a motorcyclist calls it a tank slapper. its to do with the characteristics of the bike, the wind and a few other factors. its not uncommon. i used to get them at about 35mph with PX 50mm wheels and my road bike. they say you must accelerate out of it and relax but when you a\re hanging on for life i think that is easier said than done Wink
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moonmonkey02




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PostPosted: Tue Mar 07, 2017 1:11 pm    Post subject: Reply with quote

hammerer wrote:
speed wobble, or as a motorcyclist calls it a tank slapper. its to do with the characteristics of the bike, the wind and a few other factors. its not uncommon. i used to get them at about 35mph with PX 50mm wheels and my road bike. they say you must accelerate out of it and relax but when you a\re hanging on for life i think that is easier said than done Wink


WHS^^^

Usually when descending you naturally shift weight backwards therefore making the front lighter: move weight forward slightly usually helps.

Other causes would be poor headset adjustment or knackered bearings.

Sometimes it just happens no matter what: I had it once on a bike doing 50 downhill but never happened again, unnerving but if you hang on for a few seconds it sorts itself out.

Happens on motorbikes when accelerating hard, front end rises and skips over the road: original Fireblade was notorious and only fix was to fit a steering damper. I'd like to see someone on a pushbike cause this due to acceleration.
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Andy916




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PostPosted: Tue Mar 07, 2017 2:22 pm    Post subject: Reply with quote

hammerer wrote:
speed wobble, or as a motorcyclist calls it a tank slapper. its to do with the characteristics of the bike, the wind and a few other factors. its not uncommon. i used to get them at about 35mph with PX 50mm wheels and my road bike. they say you must accelerate out of it and relax but when you a\re hanging on for life i think that is easier said than done Wink


Combination of steep steering geometry and bumps when heeled over commonly leads to a bit of a wobble through the bars. Stay loose and a decent setup will just sort itself out. If you hold on rigidly the energy is transferred throughout, escalating the wobble markedly.

Easier said than done (relaxing on the bars) when it happens descending Wrynose on 50mm rims, but a background of racing late '80s 2-strokes round the likes of Oulton Park helps.
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mattsurf




Joined: 28 Sep 2016
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Location: salisbury

PostPosted: Tue Mar 07, 2017 3:43 pm    Post subject: Reply with quote

Thanks for the comments - I didn't have this issue last season, so it came as a shock.

I have adjusted my brake pads, I think were angles in towards the rim slightly, leading to the brakes grabbing badly - this was probably made worse due to cold damp day

Also since most of my road rides recently have been on the CX bike, with disc brakes, and it descends beautifully, I did not give my road bike a proper degree of respect.
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SwimsLikeAWalrus




Joined: 30 Apr 2007
Posts: 1452

PostPosted: Tue Mar 07, 2017 10:55 pm    Post subject: Reply with quote

Andy916 wrote:
hammerer wrote:
speed wobble, or as a motorcyclist calls it a tank slapper. its to do with the characteristics of the bike, the wind and a few other factors. its not uncommon. i used to get them at about 35mph with PX 50mm wheels and my road bike. they say you must accelerate out of it and relax but when you a\re hanging on for life i think that is easier said than done Wink


Combination of steep steering geometry and bumps when heeled over commonly leads to a bit of a wobble through the bars. Stay loose and a decent setup will just sort itself out. If you hold on rigidly the energy is transferred throughout, escalating the wobble markedly.

Easier said than done (relaxing on the bars) when it happens descending Wrynose on 50mm rims, but a background of racing late '80s 2-strokes round the likes of Oulton Park helps.


I had a near death experience with some PX50s on a medium descent. I thought a crash was a certainty and the bike was all over the place. I just relaxed and awaited the impact, which never happened.
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TRO Saracen




Joined: 18 Aug 2010
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PostPosted: Wed Mar 08, 2017 11:32 am    Post subject: Reply with quote

Happened to me in Lanzarote at 48 mph. Some went faster than me, no idea how, I was cacking it! As noted, it soon went away although I can't remember doing anything specific to stop it.

Things that are scarier;

Aquaplaning on a motorbike in the fast lane of a dual carriageway at 75mph.

The time I was doing another screaming descent at about 45 mph and I was experimenting with the drop down onto the crossbar pro aero position. Then my arse got stuck under the saddle, so I was trying to ease it out forwards without making any sudden movements which would have probably have spat me off.
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mattsurf




Joined: 28 Sep 2016
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PostPosted: Wed Mar 08, 2017 11:11 pm    Post subject: Reply with quote

No wobble today, same hill, went down faster braked harder, all completely stable and wobble free. It's a mystery
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jibberjim




Joined: 15 Aug 2008
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PostPosted: Wed Mar 08, 2017 11:16 pm    Post subject: Reply with quote

mattsurf wrote:
No wobble today, same hill, went down faster braked harder, all completely stable and wobble free. It's a mystery


It's probably about resonant frequency, which means going faster, altering the weight distribution on the bike etc. are all able to remove it. So going faster alone would be expected to stop it.
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tuckandgo




Joined: 03 Sep 2012
Posts: 272

PostPosted: Wed Mar 15, 2017 12:01 pm    Post subject: Reply with quote

[quote="mattsurf"]Thanks for the comments - I didn't have this issue last season, so it came as a shock.

I have adjusted my brake pads, I think were angles in towards the rim slightly, leading to the brakes grabbing badly - this was probably made worse due to cold damp day



The brakes aren't causing speed wobbles.

Or they aren't speed wobbles.

If your bike is destabilising under braking then that is one thing. (braking too harsh, late, whatever)

If your bike is simply wobbling at speed (a known phenomenon) that that is something else, specific to your bike, your set up, you and the specific speed. You need to go faster, or slower.
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user134098




Joined: 29 Nov 2004
Posts: 10732

PostPosted: Wed Mar 15, 2017 3:37 pm    Post subject: Reply with quote

This used to be a thing for Giant owners insofar as
- I had a speed wobble
- Do you ride a Giant?
- Yes

I thought it was because their early aero forks had very little rake
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