A Sideways Shimmy Thing....

[FriendlyFred]

I was interested in the thread discussing high speed shimmys on road bikes. For the first time ever I've experienced some sort of sideways shimmy, and it's very disconcerting. I have a BMC Road Racer that is a GREAT bike. However, when doing a fast desent, the bike feels like it's going to shimmy to the side. The actrual feeling is that the rear of the bike feels like it's going to slide out from under me to the left or right. I have to easily brake and bring the bike under 30 mph to get rid of that feeling. It's the same sort of feeling as when a really strong cross wind hits you when you're doing a desent. I've tried different wheels (I'm running Mavics normally) and different tires...same result. It seems to be the frame. The frame has never been crashed and is in great shape (I had the local shop go over it).

I have a Trek (5200), a Cervelo (Dual), and a BMC Team Machine and none of them exhibit this sideways shimmy, just the BMC Road Racer. It's a fantastic bike, except I'm getting really leary of doing desents with it. Any ideas what it could be? Has anyone else experienced this with a Road Racer?

thx.
FF

 

[dhk2]

Never really heard of this before, but from your description, this sounds like the "oversteer" feeling caused by a rear tire going down after a puncture.

Has the bike always done this since new, or as long as you've owned it? Does it have the original fork? Is the headset in good shape?

You seem to have checked out all the probable causes, but still seems there is some loss of stiffness or play somewhere. Have you tried grabbing the wheel at the top and pushing it back and forth? If there is excessive play in the frame, axle or bearings, it should be apparent. Also, what about when you push on the side of the BB?

 

[FriendlyFred]

Never really heard of this before, but from your description, this sounds like the "oversteer" feeling caused by a rear tire going down after a puncture.

Has the bike always done this since new, or as long as you've owned it? Does it have the original fork? Is the headset in good shape?

You seem to have checked out all the probable causes, but still seems there is some loss of stiffness or play somewhere. Have you tried grabbing the wheel at the top and pushing it back and forth? If there is excessive play in the frame, axle or bearings, it should be apparent. Also, what about when you push on the side of the BB?

Hi-

Thanks for the reply.
The feeling of oversteer when a tire is going down with a puncture is exactly the feeling that you get when desending high speed. That's what I thought had happened the first time, but the tire is fine.

The bike has done this since I got it. It has the original Easton EC90 fork and both the fork and headset seem to be in good order. I've tried checking on excessive play throughout and there doesn't seem to ike be any. Physically, the bike seems perfect. It doesn't exhibit any other signs at all when riding. the only time there's any of this weird sideways shimmy is when you're desending at a speed over 30 mph. My first inclination was it had to be the tires or the wheels. I put recently trued Bontragers (race lite) on and got the same effect. I then thought there must be something funky with the rear stays, perhaps damage where they join the seat stay. I took it to the shop and they really checked it over and could find no damage or sign of damage.
It's such a great bike otherwise......I wish I could figure this out.

 

[dhk2]

Have never encountered this, but there must be a reason only this bike out of your fleet of race bikes does this. I'm not familar with this frame, but since everything else checks out, and you've tried different wheels, have to wonder if the frame is flexing to much. If you lean the frame over, and push on the crank spindle with your foot, does it flex more than your other bikes? Also, if you push the saddle away and pull the stem towards you, how about the torsional stiffness between the head tube and seat tube?

You didn't mention your frame size and weight, but that could be a factor. Obviously, a 58 or 60 cm frame has more flex than a small size if built with the same tubeset.