[Closed] High speed road wobble - cause?

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I used to get this on an old road bike, ****ing terrifying when it happens, could have had me off a couple of times. Ended up cutting through the frame and chucking it.

later read something which sounds like it's not all that uncommon, it just hits some resonant frequency and sends it into a tank slapper. The cure is apparently to tuck your knees right in tight on the frame. Although i would prefer not to have to find out if that works.

Mine was on a steel frame, I think it might be less likely on stiffer modern frames, at a guess.

Could be a wheel out of true. If that is the cause, you'd probably be able to notice it at lower speeds, but it just might not get so out of hand until your 60kph.

I've certainly had that problem on mtb.

quite a few factors at play.

http://www.tandfonline.com/doi/full/10.1080/00423114.2011.594164#tabModule

Abstract
Wheel shimmy and wobble are well-known dynamic phenomena at automobiles, aeroplanes and motorcycles. In particular, wobble at the motorcycle is an (unstable) eigenmode with oscillations of the wheel about the steering axis, and it is no surprise that unstable bicycle wobble is perceived unpleasant or may be dangerous, if not controlled by the rider in time. Basic research on wobble at motorcycles within the last decades has revealed a better understanding of the sudden onset of wobble, and the complex relations between parameters affecting wobble have been identified. These fundamental findings have been transferred to bicycles. As mass distribution and inertial properties, rider influence and lateral compliances of tyre and frame differ at bicycle and motorcycle, models to represent wobble at motorcycles have to prove themselves, when applied to bicycles. For that purpose numerical results are compared with measurements from test runs, and parametric influences on the stability of the wobble mode at bicycles have been evolved. All numerical analysis and measurements are based on a specific test bicycle equipped with steering angle sensor, wheel-speed sensor, global positioning system (GPS) 3-axis accelerometer, and 3-axis angular velocity gyroscopic sensor.

Used to get that on my old Cervelo Soloist when descending past 40mph. No idea why, maybe the nasty Wolf forks it came with or something. Was properly terrifying!

It's happened to me a couple of times. ways to stop it are keeping peddling (you don't need to be putting any power in, just keep the pedals turning), or camping the top tube with your knees. I've used both these ideas since the last wobble, and hey presto, no wobble, could be a coincidence though!

All good advice. Also try to relax. Tensing up can apparantly make it happen. However i find it hard to take my own advice, I get the death grip.

Change of shorts, please.

Wheel balance and roundness as per your car wheels around 40mph is a nice spot to get the resonance effect where your wheel will be jumping around.
Other contributors can be your position on the bike, tiredness causing you to over correct a small wobble.
I made the mistake of going above 40mph with spokey dokeys on my front wheel new shorts were most definitely needed.

In bikes, I think it's a combo of front end trail and frame stiffness hitting a resonance point at a certain speed. Frame flexes, puts the steering out, trail self-corrects in harmony with the frame flex. Shorter-trail, flexier steel frames do it most easily. Try changing the trail with a bigger front tyre, see if that stops it? Sounds daft but if you sprayed expanding foam into the top tube you'd stiffen it and deaden the spring, probably killing the wobble resonance.

Wow 😯

never knew bicycles could have tank slappers too!

for me, stable high speed tucked position descent comes from a light grip on the bars and clamping the seat between my thighs quite tightly and pressing my knees into the top tube of the frame.

I think a fierce grip on the bars can add to the issue as each of your arms are then fighting each other in pushing the steering from side to side.

You could also try playing about a bit with your position on the bike, I know that as an MTBer I have a tendency to shift my weight back when descending, I have to consciously make an effort to stay centered (or even a little over the front wheel) on a road bike.

Wheel balance or bike tracking?

HTH

😉

I've found that be never exceeding 20mph, up, down or on the flat, that I don't have this issue.

I thought it was just me!

Well, I just use it as an excuse to mince down hills these days. 😳

[url= http://singletrackworld.com/forum/topic/scary-road-momentthoughts ]Last time[/url] this came up, headset adjustment was also mentioned.

By me*.

*[url=

]"Dude, I told you to do the bending once in a while!"[/url]

Assuming wheels are true, bearings in hubs and headset are fine then check your wheel line. On my motorbike I was getting a speed wobble at 90-95 which could power through but quite scary non the less. Completely removed by correcting the wheel alignment.