Speed Wobbles

SACH Central

A strange and frightening thing happened to me yesterday wile out training. I got what I think was a speed wobble. In fact the whole frame wobbled. I didn't think aluminium and carbon could, frankly, wobble so much.

I was coming down a fairly fast decent (I reckon 1: 10) on a narrowish road. I'm not a nervious decender (at least up until yesterday!) but I'm not one of these kamikaze decenders either. I was in the drops freewheeling. There was a strong, gusty,cross wind and a car approached. I applied both brakes evenly as the car approached just to be sure...and then it happened. The whole bike just started to shake under me. Of course I was afraid of crashing and braked harder, which might have made matters worse. I managed to stop the bike completly without falling off but it certainly frightened the bejasus out of me.

I checked my speedo, I was doing 35.5MPH so I wasn't going fast. I also checked the headset....rocking the bike forwards & backwards with both braked on full. The was no play in it. The bike handled fine before, and again after, although I did take it easier on any more downhills.

Anyone got any ideas as to why it might have happened?

kona

when o a dh descent, i always put the back brake on a fair bit before the first.

id say a crosswind had alot to do with it, was it a wobble, or was the frame actually vibrating??

doesnt sound that bad, although my bikes weigh a fair bit more than a racer!!! and i suppose are more stable!

my guess would be that you were caught between the crosswind and the wake of the car colliding with you in the middle.

SACH Central

kona wrote: »

doesnt sound that bad, although my bikes weigh a fair bit more than a racer!!! and i suppose are more stable!

my guess would be that you were caught between the crosswind and the wake of the car colliding with you in the middle.

A vibration, or vibrations, would be too tame an expression for it. I remember looking down at my top tube......waves rather than vibrations is the word that comes to mind. It seemed that the whole bike was just shaking. It seems that the harder I applied the brakes the worse it became. I was seriously s******g myself. Riding through it/out of it wasn't an option, at least for me. I had both brakes full on and managed to go from 35 MPH to stop on a steep downhill. The cross winds weren't that bad in fairness, I just got one bit of bad 'buffetting' from a gust and the car...that was coming up the hill towards me and wasn't going that fast as it was a steepish hill with a couple of bends.....so I don't think that was the cause.

RtD

Follow this link here, sounds like the same problem, might give you a bit more of an explanation.

http://www.bikeradar.com/fitness/article/qa-speed-wobble-14776

SACH Central

Thanks for that RtD. I'll try those suggestions if it ever happens again.

uberwolf

that's a low speed for that to happen allright.

I never narrowed down a cause, just speed. Clamping the frame between my knees always reduced the impact of it though

studiorat

Scary Stuff.
Changing your weight off the saddle helps apparently, it moves the pivot point back. If you can keep the presence of mind to do that you're a better man than me...
For motorbikes a steering damper is used to stop tank slappers, maybe your head set is loose.
If it's not a mechanical failure, like a loose spoke or the headset, it's to do with resonant frequencies between the wheel and the frame. Is there anything attached to the wheel that would unbalance it at speed? If something throws a wheel off centre it will automatically rebalance while spinning, I believe the wobble comes on because the rebalance over compensates and sets it off again. A cycle if you will ;o)

I had a motorbike that used to get a bit vague around 65 mph but would pull through it. In my experience bicycles get more stable as the wheels spin faster. Though obviously not in your case!

There's an interesting read about bicycle dynamics here

Raam

What Uberwolf says works for me.

Clamping the frame between my knees always reduced the impact of it though

It's especially noticeable in crosswinds

kona

thats insane!!!

If it applies to bikes it must apply to cars?? how do they overcome the problem?? different materials?

does anybody know the natural frequency of carbon???

you could work it out as to what speed it happens at, although it wouldnt take into account resistance, etc....wed be here all day!:D

RtD

The natural frequency is not just a function of the material used, it also involves the shape of the object amongst other things.

kona

RtD wrote: »

The natural frequency is not just a function of the material used, it also involves the shape of the object amongst other things.

there must be a constant though?

kenmc

kona wrote: »

there must be a constant though?

nope, otherwise all wind instruments made of the same material would sound the same irregardless of other factors.
It's damn complex maths - functions of length, width, thickness of the material etc etc.
Oh and cars can exhibit speed shakes also at certain speeds, but they have 4 points of contact on the ground, so less likely to fall over.
happens to motorbikes too - ever see them get into a tank-slapper in a race? scary stuff.
it's basically a case of constructive interference, or a feedback loop like where you whistle into a microphone, and the microphone picks up the sound from the speakers and resends it and you get that horrible feedback squeal.

niceonetom

kona wrote: »

thats insane!!!

If it applies to bikes it must apply to cars?? how do they overcome the problem?? different materials?

does anybody know the natural frequency of carbon???

you could work it out as to what speed it happens at, although it wouldnt take into account resistance, etc....wed be here all day!:D

doesn't really happen to cars as they have multiple steering pivots, but it does happen with cars and caravans, and trucks and trailers etc. where there's only one pivot.

harmonic motion for complicated structures is a bastard to actually mathematically model and it's usually easier just to build the thing and test it. famous example being the bouncy bridge outside the tate modern in london - not steering wobble but another example of simple harmonic motion.

i've never really had it happen on a bike. touch wood.

kona

bit of integration and differntiation will sort that out no problem





who wants to do it:D:D:D

NOT ME!!! IM FINISHED MY EXAMS!!!!:D:D:D:D

DirkVoodoo

Natural frequency is a function of the mass of the object and the stiffness of the material.

In addition to Niceonetom's example, the most famous example of harmonic resonance in a man made object was the Tacoma Bridge:

Youtube Linky

studiorat

Raam wrote: »

What Uberwolf says works for me.



It's especially noticeable in crosswinds

There's an excuse to buy new wheels if ever I saw one!!!

Diamondmaker

Hi

I found this thread, its a bit old but have just come across it myself - twice.

Both times it has happened on the same descent, its a 10% gradient and I was 50ish kmh. Again the whole bike began to shake and braking seemed to increase the problem in my case. I had a dead end ahead of me but decided I had no option but to let off the brakes, resteady and try again in much less distance....( cliff at dead end ! :eek: )

It was real skin of the teeth stuff and I did not cycle ( this amongst other reasons ) this hill for a while after..

....Happened again today ! I did the 10% descent this time without scare, far more reserved than in the past but then there is a 9% descent, with a hill after, so I pushed this one. Always in the back of the mind that I could stop going up the hill.

Just the same, crazy wobbles, so I let go of the brakes completely and just held tight ( car behind me ! :eek: ) and waited for gravity to start working in the other direction, sweet sweet other direction, as I climbed the hill.

My bike is a decent enough Hybrid, but I detected a possible slight imbalance when I stood it up side down and spun the rear wheel..also the brakes dont have the same travel to reach similar pressures, is this a factor ?

Another ideas that I could have checked out.

Im new to cycling and I love hill climbing but the Descent is now becoming a real monkey on my back !

Im training for a tri and hope to buy a racer by then, I plan to not "kick about" so much on it if I do.

Cheers if anyone can help contribute to saving my bacon in the future !:)

DM down under.

tart

I used to get wobbles untill i lowwerd my stem(took out two spacers the bars are now 3" lowwer than my saddle). it might not work for you but it worked for me.

ED E

niceonetom wrote: »

simple harmonic motion.

I go online to get away from schoolwork and then what do I see but SHM:rolleyes:

:P

I've had bikes wobble where the frame would tilt to both sides but the centre would stay on the same axis and the front wheel would rotate slightly. Hasnt happened in ages though and slowing ended it.

Paul Kiernan

Interesting theories on SHM and Shimmies but maybe you simply have a loose headset?

But then, of course, you checked that first!