unstable front end?

[Geoff]

What causes the front end of a bike to become unstable at speed? I was riding yesterday and once I hit a good down hill at about 40mph the front started to get really squirlly, I almost lost it. I have had some shimmy before but nothing this bad. Also I have never noticed it on a sprint lead out only down hill. The bike is a Lemond Victorie with the issue fork and it fits well.

Any answers, comments, or wise cracks wanted.

Geoff

[e-RICHIE]

Originally Posted by Geoff

What causes the front end of a bike to become unstable at speed? I was riding yesterday and once I hit a good down hill at about 40mph the front started to get really squirlly, I almost lost it. I have had some shimmy before but nothing this bad. Also I have never noticed it on a sprint lead out only down hill. The bike is a Lemond Victorie with the issue fork and it fits well.

Any answers, comments, or wise cracks wanted.

Geoff

often it's caused by a momentary lapse in concentration.

[Geoff]

Its possible that my little mind has trouble staying focused, but the shimmy is always there on this bike and I have not noticed it on my Atlanta.

It could be that my lack of concentration causes me to exagerate the movement.

What would could be a cause of the shimmy to begin with? Fork, Fit, Geos?

Maybe its just me.

Geoff

[dirtdigger88]

try and get a copy of the latest Rivendell Reader- if need be pm me a fax number and Ill send you the pages-

there is a good article explaining the shimmy-

number #37 I think-

Jason

[e-RICHIE]

Originally Posted by Geoff

What would could be a cause of the shimmy to begin with?

my highly opinionated post from a 2002 CR list thread atmo.

and a follow-up post yo -

[Geoff]

e-RICHIE,

Well I never knew that I was chickin!

However, I had left my helmet at work on Friday and had been riding with out it, which I rarely do these days and maybe I was being more concerned that other wise. Normally I love the down hills I was just at the Tour of Georgia and hit 55 mph and loved it.

I will agree with what I think is a premiss to your satement, that you really need to attack downhills, leadouts, etc. for them to be successfull and for you to be focused at that kind of effort.

Does the knee on the top tube work?

Dirt,

I will see if I can find it.


Know if you will I will curl up in the corner and cower while I wait for my computer to refresh and see if there are anymore posts of this.

[Bradford]

I’ve had a couple of shimmy problems in the last few years and I’m pretty sure it was pilot error. I sure don’t know what causes shimmy in general, but I think I know what caused it with me.

For me, it seems to have been how I was sitting on the bike. I never had a bike shimmy until I started riding a tandem. Since my wife doesn’t like to ride aggressively, spending a lot of time on the tandem made my decent less aggressive as I started to sit up and back more. In addition, it really bothers her to have one pedal down and one pedal up, so I started descending with both feet even. After having a bad shimmy incident at Open House last year, Ron Kiefel suggested that the problem was that I needed to put the outside foot down to put some tension into the bike. I tried it, and it also put me back into a more aggressive posture like I used to use before I started with the tandem.

I may not have enough experience to know if this makes sense or not, but Ron does, and it works, so I’m back to a more aggressive posture with the outside foot down and a little pressure on the pedal. So far, I’m back to shimmy free.

[Ray]

Geoff,

There are a few different possible sources of shimmy. In my cases, it's almost always been because of high winds on a fast and hairy descent and my nerves in that situation causing too tight a grip on the bars and too much tightness in my upper body (its happened twice in the last year, same basic situation each time). Once I remember to relax, I'm almost always fine. But when it happens, yeah, I've always found the knee against the top tube works, although sometimes I've had to clamp the tube with both knees.

Good luck,

-Ray

[e-RICHIE]

Originally Posted by Bradford

So far, I’m back to shimmy free.

sweeeet

[Birddog]

I've had it occur on several occasions, different bikes too. I was able to determine that a particular bike had a tendency to shimmy at a particular speed. The shimmy usually (but not always) started after hitting a small irregularity in the road surface and sometimes would turn violent. What works for me is to loosen my grip on the bars. I found that I had a "death grip" on the bars, and that was a major contributor to the problem. Since becoming aware of the "grip" issue, I've had nary a problem, but the knee trick works. Sometimes it takes a leap of faith to lighten your grip as the shimmy increases. Try and set up the shimmy on purpose and see if this helps.

Birddog

[Eric E]

So, if I can quote e-richie:

"for the rest of us, when it comes to the issue of shimmy, and descending speeds, and the related issues, i say it's all pilot error."

I suspect that there are cyclists that can conquer all vibrational instabilities. Not me - I do possess some imagination as to what can happen at high speed, and thus will brake and thus tighten up going down many of our Colorado mountains.

On an earlier version of my Ti Rapid Tour (custom geometry w/ 72 degree seat tube), braking and other "pilot errors" caused my bike to shimmy. Serotta, to their credit, worked with me on this, getting me to increase the spoke tension on my custom wheels (their low tension made the shimmy life threatening to me), get a stiffer fork, and finally, when my bike was happily shimmying away while being passed by Huffies on a MS150, replaced the frame with one with a stiffer tube set (paticularly the rear triangle and top tube, I think).

Since that time, with the same semi-incompetent pilot, my bike has not shimmied - even with the sort of hard. panic braking like when a parked car pulls out in front of you on a downhill - this would bring it on the earlier frame.

Enjoy, Eric

[TimD]

With all due respect to e-Ritchie, I don't believe shimmy is exclusively due to pilot error, from personal experience.

I once owned a Trek 5000 which was exceedingly nervous over 35 MPH.
Behavior was independent of wheelset, position on the bike (with a caveat, see below), force on the bars, crosswinds, etc. Same road, same hill, same rider, three other bikes - no problem.

There's a theory that the front end can only oscillate if anchored by a large
mass, and lifting your butt off the saddle temporarily will stop the oscillation.
Since my current bikes don't exhibit "death wobble", I haven't tested this
theory.

My $0.02
TimD

[bironi]

Geoff,

Make sure your head has not come loose. This will cause shimmy.

[e-RICHIE]

Originally Posted by bironi

Geoff,

Make sure your head has not come loose. This will cause shimmy.

i can agree with this atmo -

[11.4]

In addition to a loose head, also check your headset. That can cause the same problem.

Seriously, I've found some frame or component issues that can contribute to shimmy. On frames, I've seen poorly built frames with excessively flexible top tubes that don't hold the head tube and steer stable on certain kinds of roads. It takes quite a screwed up frame to do this, however. On components, I've seen loose headsets and significant mis-dishing of wheels cause problems. These are all problems that tend to start up gradually and can be managed, albeit with inconvenience. There's also of course the selection of too high a profile rim for the wind conditions and a rider's handling skills -- especially on the front, having to stabilize a wheel that the wind wants to turn can create instabilities as well. The usual response is to tighten up on the bars.

Positioning is actually one of the biggest issues. On the track, if you have too much weight distribution towards the front, you'll have trouble keeping stability on the banking -- you'll feel very twitchy and tend to swing off the pole. Too much weight on the front not only encumbers steering a bit, but also takes weight off the rear -- the result is that your front wheel is doing too much work and your rear wheel isn't stabilizing your line of travel. If you start wobbling, in some ways it's actually coming from the rear wheel, not the front, because the rear isn't doing what it needs to help out the front wheel. This same kind of issue shows up on the road, of course, and when you're pointed downhill, you're naturally weighting the front more.

How to handle this? First, just learn how to expect it and handle it. Second, shift your weight on a downhill to minimize the effect. Third, don't pick a position for the most absolute aero position on the flats if you're going to have to ride that position on downhills as well. Your position is always a compromise and you have to take downhill stability into consideration. You might note that this shimmy problem has become more publicized since threadless headsets came into existence, leading me to suspect that people are setting their bars lower and not able to make the simple quill stem height adjustment that cures the issue. You'll notice that the great descenders do not have a huge drop from the saddle to their bars, and they're able to ride more in their drops because of this as well. Everyone wants a position like Tom Boonen's, but we don't all have the arms and torso to suit. Plus, Tom isn't the greatest descender with his position either. There's a lesson to learn in every bike you look at.