Speed wobble on London to Brighton

I had this on my first 'proper' bike last year. Scared the bejesus out me as I careered towards the centre line with traffic coming the other way.

Loads of research found out the dreaded name but was then unable to travel fast down any steep hill: cadence braking, weight forward, death-grip on the crossbar, soft hands, all to no avail. I even went into a slightly worn patch of road on a hill at c 5 mph and the shakes started.

Took it to the LBS where they said the headset was loose. Great. Took it down another hill, round curves, no problem, then touched the front brake in the straight: massive wobble, uncontrolled bike, brakes barely functioning and car coming the other way. Somehow put it into a broadside which stopped everything, literally.

Bike return to factory was not a problem. Bought an upgrade and never a problem since

The "death wobble" is terrifying isn't it!  I've encountered it a couple of times (just survived on each), both I was convinced something was loose on my bike but having looked into it and my bike being absolutely fine I realised it was me.

Generally down to gripping far too tightly and rigidly on the handle bars when things started to get really fast and likely poor weight positioning on my part too.

I tend to try and relax my arms (still firm grip, but less tense arms) and consciously consider my weight position.  I also read that you can hold your knees/thighs inwards against the frame to give some more stability if you feel the wobble coming on.

Other than that I practised alot on local (less steep descents) to get my confidence back, gradually increasing speed.  Ruined my confidence initially but I'm getting back.

Tend to agree what others mentioned about body positioning. If you experience speed wobble, and if you have a lot of weight high and out front, or high and way back, I would consider pulling the weight toward the frame, downward, including inside the main triangle. Try this, see if your wobble problem goes away.

One thing you didn't mention is your body weight. With spoke count at minimal (18F & 21R) there might be a body weight limit. Best to ask Vision Tech Support for an answer. http://www.visiontechusa.com/support/

Last option is to ensure your wheels are true, balanced and spokes are at proper tension.

As others have mentioned, the weight distribution is a key factor. The shimmy can be worse on some bikes than on others, depending on the harmonics of the frame, wheel and tyre combinations. Stiffer frames don't always make things better as the resonance can be shifted to certain areas, exacerbating the problem. The relationship between the performance (and stiffness) of the frame and the forks and headstock is particularly important.

Shifting forward should help as other comments point out. If you're off the saddle, your legs act as shock absorbers and the bike is free to move under you, so the harmonics are quite different as the resonance isn't 'locked in', allowing the shimmy to disappear. This is why BMXers and MTBers will stand on the pedals over the rough stuff.

Descending fast calls for nerve and the speeds top riders can attain (120km/h) on a fast downhill stretch in the TdF I find astounding. That's a lot of trust to put in two tiny tyre patches on the road; not for me these days.

When I did work as an engineer this harmonic frequency stuff was an area I covered a great deal and I found it fascinating. I think one of the reasons I'm less keen on carbon fibre as an engineering material is that I've less of a 'feel' for its engineering properties. Carbon frames can respond in very different ways even from each other, depending on their profile and how they're manufactured. I like steel frames in particular because they're much more predictable, but even aluminium or titanium frames and magnesium components are easier to understand. I know some engineers love carbon fibre because of its strength and weight saving properties, but for me I'd rather use an alloy as a crucial structural member and keep the carbon fibre for lesser structural roles where its weight saving is still a benefit.

Incidentally, this shimmy can be truly terrifying if it happens when you're riding a motorbike. It's not so bad if it's at the rear end, but if it occurs at the bars then you've got a 'tankslapper' and it can easily spit you off, unless you have the nerve either to accelerate through it or slow down under it, as need be. Some motorbike models are notorious for it.

yeah ... back off the "death grip" and shift forward ... I'm lucky that my cock-eyed

shape means an inline post so now i'm slightly forward anyhoo but a good stable

design is also handy (Cheers Dom )

Scary isn't it! I used to ride a hill a lot which wasn't particularly fast and I could guarantee by the bottom of it it'd have white buckles. I did figure out what was going on but still prefer not to go down it even though I decend far steeper these days. Nearly went under a bus once.
First instinct is it's the bike must be something wrong but in general it's not the bike it's you. I kept on thinking the bike must be too big but the reality (after a conversation with a bike fitter) was my weight was to far rearward. The rippled road surface would induce it but shifting weight forward cured it. Really difficult to move your weight forward when your scared but practice on something less challenging and relax. If you can work up to decending in the drops and putting all your weight on the pedals not the seat to lower your CG. I've only started doing this for the last couple of seasons but it's great fun especially on the brakes into corners I'm still not as fast as many but I have fun and feel safer which is most important.

Usually down to poor weight distribution. You're probably sitting too far back on the saddle, then when the wobble happens, shifting even further backwards and exacerbating the problem.

Next time it happens, try not to death-grip on the bars, shift your weight forward and press your knees against the top tube.