Severe Speed Wobble at 50MPH

[Splash]

Hello all.

Just completed a 210km event in Melbourne (Australia) yesterday on the Bianchi Oltre XR.

Zipping down a hill at 80kmph (50mph), both my wheels began to wobble and shake violently.

I fought hard to control it as I just managed to stop the bike safely before going a$$ over tit and breaking bones and the bike...

Can you highlight possible reasons why this may occur and what I need to do to overcome these wobbles?

Splash

[palincss]

Clamp the top tube between your knees.

http://sheldonbrown.com/brandt/shimmy.html

[malcolm]

From a recent thread, scroll down to the shimmy part

http://www.spectrum-cycles.com/geometry.php

[Birddog]

Don't go 50 mph!. Just kidding, I know the feeling as it has happened to me a couple of times. Clamping the top tube with your knees is good for after it starts. Holding one knee (or both) against the top tube helps prevent in the first place. Some theorize, (myself included) that much of the problem is rider input. Make sure you don't have a "deathgrip" on the handlebars. There are several threads about this here and on other forums with lots of theories and very few solid, absolute causes.

[Don49]

My Calfee has always been completely stable on fast descents, although I sometimes clamp the TT with knees as suggested.

Then I added a Caradice bag attached to the titanium seatpost for a multi-day ride. With that 10lbs or so high up and behind the saddle I could now feel a noticeable instability on the same fast descents. Never got to the point of a speed wobble but I could feel the potential was there. Not sure what this means except that relatively small weight shifts can have an effect.

[fogrider]

From a recent thread, scroll down to the shimmy part

http://www.spectrum-cycles.com/geometry.php

Lots of good stuff there. Putting your leg on the toptube is trying to minimize the the affect of the problem after it shows up. One thing that has not been talked about is the balance of the wheels. Think about how would your car handle if you don't balance your wheels/tires? with your bike, when you hold your wheel off the ground and spin it, do you feel that your wheel is off balance? this typically comes from the weight of the stem, and with a aero rims and the use of a valve extender. It just kills me to tape on some weight onto a wheel to balance it, but then it spins smooth. as others have said, there many factors to shimmy, and often there is not much to do, but this is one thing that you can do to avoid it.

[Peter P.]

My Calfee has always been completely stable on fast descents, although I sometimes clamp the TT with knees as suggested.

Then I added a Caradice bag attached to the titanium seatpost for a multi-day ride. With that 10lbs or so high up and behind the saddle I could now feel a noticeable instability on the same fast descents. Never got to the point of a speed wobble but I could feel the potential was there. Not sure what this means except that relatively small weight shifts can have an effect.

Same here.

My commuter bike has a rear rack. I have no troubles at all with high speed shimmy, UNTIL I place a load on top of the rear rack. Five pounds is enough.
If the bike just has panniers on it, the the lower weight does not cause the shimmy.

To answer the OP's question, do a search on this forum for "shimmy" or "speed wobble" and you'll find all sorts of possible causes and remedies.

[endosch2]

There were a couple of threads about speed shakes earlier. I was one who started a discussion. A lot of anecdontes, not a lot of science. I have a bike that shakes at 42mph, I have tried the top tube knee squeeze and other witch craft but I do think it is wind related. Waiting to try some different wheels to see if that does it.

[Ken Robb]

All my thoughts on this topic are expressed in previous threads in the archives. The usual tricks with knees worked for me on any bike that wobbled save one. I had one bike that wobbled no matter what I did. Swapping for wheels with less lateral flex raised the speed where wobbling began by a few MPH but the bike was still not a keeper for me. The guy who bought it couldn't make it wobble even when he tried. He was probably 30 lbs. lighter than me which reinforces the theory about extra weight high on a bike can cause wobbles.

[lhuerta]

As others have said...lots of threads here and on google on this topic.

As for where to start, pinpoint what is the last part you changed (e.g. wheels, stem, bars) or adjustment to your position you made (saddle fore/aft, drop/raise stem)....then work backwards from there.Lou

[Don49]

If the bike just has panniers on it, the the lower weight does not cause the shimmy.Just a side note on panniers, my touring bike has low rider front panniers and I carry 60/40% front/rear weight distribution. With that down low weight acting to stabilize the front end the bike feels infinitely stable, more so than any unladen road bike. My fastest downhill speed was actually reached on the touring bike with panniers.

[oldpotatoe]

Hello all.

Just completed a 210km event in Melbourne (Australia) yesterday on the Bianchi Oltre XR.

Zipping down a hill at 80kmph (50mph), both my wheels began to wobble and shake violently.

I fought hard to control it as I just managed to stop the bike safely before going a$$ over tit and breaking bones and the bike...

Can you highlight possible reasons why this may occur and what I need to do to overcome these wobbles?

Splash

What front wheel? Fat aluminum spoke wheel by chance and gusty winds?

[Bantamben]

Yeah I've had it a lot and I think for me it's having a big flexier frame and having the saddle to far and high back. With weight too rearward it leaves the front end to respond seperare from the back. I think moving saddle forward and getting a longer stem may help. Stiff bike helps too

[palincss]

Yeah I've had it a lot and I think for me it's having a big flexier frame and having the saddle to far and high back. With weight too rearward it leaves the front end to respond seperare from the back. I think moving saddle forward and getting a longer stem may help. Stiff bike helps too

But if the saddle is located where is should be vis a vis the bottom bracket, moving it forward and getting a longer stem isn't going to help anything.

[bikinchris]

I thought carbon bikes couldn't wobble according to a recent thread.

[Bantamben]

I agree but I think you have to compromise for better weight distribution on the bike. If you set saddle by KOPS you may be to far back. I set mine by knee over pedal spindle and have long legs so it put me really far back on bike. I have a short torso and am fairly inflexible so it's hard to get weight distribution right. Since I went to longer stem and a little less setback I feel more balanced on bike. Still if your riding a XL frame they are pretty much all flexy.. Once you move seat forward you may have to raise saddle a little.

A stiff 60cm frame will be a lot flexier then a whippy 55cm frame

[Splash]

Thanks very much for these responses. I have a bit to learn and read up on...

I have never ridden this fast and felt pretty scared. I was enveloped by a class group of 50 or so riders all doing the same or greater speeds than I.

I had the ENVE 6.7 wheel set on, the bike is a 59cm frame. I do not think I was fully over the front and not crouched down enough.

I think there was a cutaway allowing cross winds to smack into you. But, all the other riders dint have a problem, none had the deep rims on that I had and here 2 levels of skills above me...

After reading this thread, I am thinking it could have been a combination of things - body position, loose legs, possibly deep rims and simple lack of experience...




Splash