speed wobble

gubber12345

So had my first real bad one this morning at about 40 mph....scared the crap outta me.managed to hang on and tried not to panic,just sat up big and let wind resistance do its thing.

So as a matter of interest what actually causes a speed wobble,been at 40 before and no problems but today was all over the shop.dont want to repeat that experience again.

meanredspider

I don't think anybody knows for certain what it is that causes it. 40mph though is slow to be getting this - I was at nearer 60mph when it happened to me. Wonder if your wheels/tyres are badly out of balance...?

rick_chasey

gubber12345 wrote:

So had my first real bad one this morning at about 40 mph....scared the crap outta me.managed to hang on and tried not to panic,just sat up big and let wind resistance do its thing.

So as a matter of interest what actually causes a speed wobble,been at 40 before and no problems but today was all over the shop.dont want to repeat that experience again.

Every object has a natural resonance.

If you're unlucky, with you on it, at certain speeds the natural resonance will match the vibrations you're getting from the road - hence it starting to oscillate much more strongly - hence the speed wobble.

AK_jnr

Anything over the speed I spin out my gearing I tuck with knees against the top tube. I read about it on here. Touch wood I have been fine.

gubber12345

Yeah I've been at higher speeds and no bother just a bit weird....never 60 tho thats just mad lol wheels could be out of balance but how can you tell meanredspider? I'm using mavic krysium elites so a decent e ough wheel but prob as rick chasey says could be road surface...dunno just happy to stay shiny side up.

meanredspider

As Rick says, there's a resonance but, to activate that resonance there needs to be an energy input - it's why chimneys have spiralled vortex shedders or dampers or aren't a constant diameter. With speed wobble, I'd imagine the wobble comes from the wheels - either because they are out of balance or through vortex shedding. You can get into a positive feedback situation where you try to correct but you react too slowly actually adding to the problem (a kind of tank slapper situation).

With the rear wheel you can feel the balance by putting the bike in a stand and swinging the crank in a big gear - you get a vibration from the wheel. From the front wheel, I don't know - seeing how much the wheel want to pendulum into one position might give an indication: they always do a bit. You could play about to see it it will balance. It's only a theory though.

sloppyschleckonds

meanredspider wrote:

I'd imagine

Should have stopped there.

meanredspider

SloppySchleckonds wrote:

meanredspider wrote:

I'd imagine

Should have stopped there.

Sad person

craigus89

I had it once under braking but that was at 50 and I don't usually reach speeds like that. I figured that it could have been a combination of me being a bit nervous at that speed amd braking a bit hard, wheel balance, road surface etc.

wolfsbane2k

40 and 60 mph on a push bike?
No wonder I'm always lagging behind - max I've topped out at is 32mph, and that's tucked in going down hill, but I got a little bit of speed wobble then as well.

I'm not sure quite what speed you begin to get vortex shedding on a spinning tyre - most of the stuff I play around in CFD land is fixed wings! I might see If I can find some time in the wind tunnel to simulate the rolling road & wind at 60mph though . Would be an interesting mid career "divergence" if I could get an extension to the degree out of it.

meanredspider

Wolfsbane2k wrote:

Would be an interesting mid career "divergence" if I could get an extension to the degree out of it.

I'll be expecting an acknowledgement on your paper when you discover the Wolfsbane Effect (Coander, eat your heart out)

wolfsbane2k

meanredspider wrote:

Wolfsbane2k wrote:

Would be an interesting mid career "divergence" if I could get an extension to the degree out of it.

I'll be expecting an acknowledgement on your paper when you discover the Wolfsbane Effect (Coander, eat your heart out)

I recall that Coander effects is very much about direct impinged effects isn't it? I was thinking of the side shedding vortex's, which last I heard was 'too complicated to model'; instead its been researched by someone else again at my own uni. Dang!
http://www.southampton.ac.uk/~aijf197/Wheels.pdf

Since the CFD stuff I play with is mainly rc model driven nowadays, I am out of touch!

wolfsbane2k

Wolfsbane2k wrote:

I recall that Coander effects is very much about direct impinged effects isn't it? I was thinking of the side shedding vortex's, which last I heard was 'too complicated to model'; instead its been researched by someone else again at my own uni. Dang!
http://www.southampton.ac.uk/~aijf197/Wheels.pdf

Since the CFD stuff I play with is mainly rc model driven nowadays, I am out of touch!

Obviously very out of touch http://www.altairhyperworks.com/html/en ... MNGodo.pdf

And thinking properly, that was a really stupid thing to say... There are full on specialists across the world interested in wheel dynamics for world class cycling, what's a hobbyist aerodynamicist with no cash going to add to the world!

Think before opening you mouth!

meanredspider

I'm not sure I followed much of that article but did notice this in the discussion

We propose that the dominant periodic behavior observed for each of the wheels comes from the periodic shedding of fluid off of the rim (and tire) of the rotating wheel