A bit of the wobblies.

[GBC]

I had an ‘interesting’ experience yesterday, and one that I wouldn’t care to have again.
I work from home, spending my day in front of two computer monitors, so come 5pm, like to go out for an hour or so on the bike to clear my head and get some fresh air into my lungs.
Last night, in the last quarter of the run, I was coming down a fairly steep dual carriageway (Carmunnock down to Simshill through Castlemilk if you know the area) which ends at a roundabout. I was on the drops and doing just over30mph and as the road levelled approaching the roundabout, I started to raise my position and was about to move my grip from the drops to the bar and start braking when the bike very suddenly became unstable and the handlebars/ front wheel wobbled rapidly to either side. Fortunately the movement and the speed of it meant that I stayed on a fairly straight course and didn’t leave the lane I was in, and it stopped as soon as I managed to get a finger on the brake lever (the front one was the only one I could reach) and bring the speed down just slightly.
The road surface was in a really good state of repair and the only thing that seemed to trigger it was my slight movement of position.

Any thoughts on what would have caused that? The bike is a steel framed Triumph from the seventies, and I’ve never experienced anything like it before.

 

[TheSandwichMonster]

Loose headset maybe?

 

[Nigel-YZ1]

My 80's Carlton does the same. I always put it down to wheel been a bit squiffy or something. Just sort of accepted it as normal.
When I've got both hands on the bars it's ok.

 

[GBC]

The headset seems to be fine and I honestly don't think it's mechanical. It may sound daft, but to me it was almost a tuning sort of thing in that it kicked in at a certain point, when I moved my position, and stopped as soon as I altered the speed just slightly. The wheel being a bit 'squiffy' could certainly be a factor, although to the eye it looks quite straight. It has though, just reminded me that I was hearing a rhythmic tap from the front wheel earlier which at first I thought was the magnet touching the sensor for the computer, but was able to rule out.
More investigations in that department I think.

 

[Fab Foodie]

Ahhh, shimmy ....

Subject: Shimmy or Speed Wobble
From: Jobst Brandt
Date: June 25, 2004, revised February 25, 2005
Shimmy, a spontaneous steering oscillation of the front wheel, usually occurs at a predictable speed when riding no-hands. The likelihood of shimmy is greatest when the only rider-to-bicycle contact is at the saddle and pedals. This position gives the least damping by hands, arms, and legs. When shimmy occurs on descents, with hands on the bars, it is highly disconcerting because the most common rider response, of gripping the bars firmly, only increases it.
Shimmy is not related to frame alignment or loose bearings, as is often claimed. Shimmy results from dynamics of front wheel rotation, mass of the handlebars, elasticity of the frame, and where the rider contacts the bicycle. Both perfectly aligned bicycles and ones with wheels out of plane to one another shimmy nearly equally well. It is as likely with properly adjusted bearings as loose ones. The idea that shimmy is caused by loose head bearings or frame misalignment seems to have established currency by repetition, although there is no evidence to link these defects with shimmy.
Bicycle shimmy is the lateral oscillation of the head tube about the road contact point of the front wheel and depends largely on frame geometry and the elasticity of the top and down tubes. It is driven by gyroscopic forces of the front wheel, making it largely speed dependent. It cannot be fixed by adjustments because it is inherent to the geometry and elasticity of the bicycle frame. The longer the frame and the higher the saddle, the greater the tendency to shimmy, other things being equal. Weight distribution also has no effect on shimmy although where that weight contacts the frame does. Bicycle shimmy is unchanged when riding no-hands, whether leaning forward or backward.
Shimmy requires a spring and a mass about which to oscillate and these are furnished by the frame and seated rider. Unloading the saddle (without standing up) will stop shimmy. Pedaling or rough road will also reduce the tendency to shimmy. In contrast, coasting no-hands downhill on a smooth road at more than 20mph with the cranks vertical seems to be the most shimmy prone condition.
When coasting no-hands, laying one leg against the top tube is the most common way to inhibit shimmy and also one of the most common ways to coast no-hands. Compliant tread of knobby tires usually have sufficient squirming damping to suppress shimmy. Weight of the handlebar and its extension from of the steering axis also affects shimmy.
Shimmy is caused by the gyroscopic force of the front wheel whose tilt is roughly at right angles to the steering axis, making the wheel steer to the left when it leans to the left. This steering action twists the toptube and downtube, storing energy that both limits travel and causes a return swing. Trail (caster) of the fork acts on the wheel to limit these excursions and return them toward center.
To feel the gyroscopic forces involved in bicycle shimmy, take a front wheel, holding it by its axle in both hands, and give it a spin. Manually steering it from side to side generates strong tilting forces always at right angles to the input. These forces sustain shimmy and are the motions one uses to make quick steering maneuvers while riding no-hands, shifting the hips laterally while firmly seated. The same effect as when wheeling a bicycle wile holding it only by the saddle.
Shimmy that concerns riders the most occurs with hands firmly on the bars and it is rider generated by muscular effect whose natural response is the same as the shimmy frequency, about that of Human shivering. Descending in cold weather can be difficult for this reason. The rider's "death grip" only enhances the incidence of shimmy in this situation. Loosely holding the bars between thumb and forefinger is a way of avoiding shimmy when cold.

 

[wheeliebin]

Happened to me with a hire bike in France years ago, dead scary on a fast downhill - the bike went straight back to the hire shop. Also on a borrowed bike (might have even been an 80s Carlton) - just as scary. The movement was pretty violent, and enough to nearly throw me off. I don't think there was anything "broken" with either bike, as FF says it is some kind of resonance effect. Perhaps more modern bikes are better designed in this respect.

 

[GBC]

That sounds absolutely spot on Fab Foodie, and describes exactly what I experienced and my gut feeling that it was somehow connected to resonance. Many thanks for the info

 

[GBC]

Happened to me with a hire bike in France years ago, dead scary on a fast downhill - the bike went straight back to the hire shop. Also on a borrowed bike (might have even been an 80s Carlton) - just as scary. The movement was pretty violent, and enough to nearly throw me off. I don't think there was anything "broken" with either bike, as FF says it is some kind of resonance effect. Perhaps more modern bikes are better designed in this respect.

Scary indeed at that speed wheeliebin; very much an "are farts lumpy?" moment.

 

[Pat "5mph"]

Scary indeed at that speed wheeliebin; very much an "are farts lumpy?" moment.

Yeah, and so near the Lynn too .... you are going too fast on that road!

 

[Deleted member 1258]

As Fab Foodie said a speed wobble, it might have been sparked off by you sitting up, the movement might have just been enough to unsettle the bike. I rode motorcycles in the 1970's and vaguely remember some small Yamaha's being a bit twitchy and prone to this.

 

[simon.r]

As Fab Foodie said a speed wobble, it might have been sparked off by you sitting up, the movement might have just been enough to unsettle the bike. I rode motorcycles in the 1070's and vaguely remember some small Yamaha's being a bit twitchy and prone to this.

Bloody hell, I didn't think you were that old

 

[Deleted member 1258]

Bloody hell, I didn't think you were that old

whoops, I've put that right now, I proof read before I posted and missed that.

 

[GBC]

Yeah, and so near the Lynn too .... you are going too fast on that road!

I promise I wasn't Pat. The road is the dual carriageway that goes through Castlemilk, recently re-surfaced and a 30 mph limit which is generally adhered to as it's a favourite road for Strathclyde's finest's revenue generation unit.
(And sadly, to paraphrase Mr Connelly, "Too old to go too fast".)

 

[byegad]

Knee into the crossbar next time it helps. It's caused by resonance in the system (Bike/Body) and changing the contact points usually sorts it out.
As mentioned above it will be predictable in that it will happen at the same speed/road surface conditions almost every time.

 

[HLaB]

Touch wood, I've only had similar IIRC in the rear wheel, the front wheel would be scary indeed I've put my wobbles down to a combination of the tyre, greasy surface and my weight distribution at the time, doesn't sound like it was any of them in your case.