Bizarre speed wobble, and then messed up front derailleur

[Keith A]

On our group bike ride last night, a pal went over a set of railroad tracks that we have done hundreds of times before and at all different speeds. At some point, he developed a significant speed wobble...so much so that I thought he was messing around and shaking his bike. Once he regained control, his front derailleur was messed up and was rubbing no matter what position he shifted.

The only thing I can figure out is that maybe his body/shoe hit the front derailleur during the speed wobble and knocked it out of place. Maybe pushing it down, which is why is was always rubbing.

So should my pal be worried that something is wrong/broken with his bike? It seems bizarre that this speed wobble happened when we have ridden over these tracks so many times. BTW, this is on totally flat ground.

Thoughts?

[tlittlefield]

Is it a clamp-on FD?

If so it might have just twisted enough to knock it out of alignment with the chain.

[Mark McM]

A speed wobble on flat ground? That's unusual. What speed was he going when the wobble initiated? And how long did it continue, and what stopped it? Speed wobble typically initiates at speeds around 40 mph (plus or minus), and once started, they typically continue until a significant dynamic change occurs on the bike (large reduction in speed, or large shift in the position of the rider on the bike).

A lot of things can cause a momentary wobble that quickly dissipates. A true shimmy typically until a large change occurs.

As mentioned, true shimmies typically occur at higher speeds, and don't necessarily mean that there is anything wrong with the bike. But a low speed shimmy on a bike that has never experienced them before may indicate a problem that needs to be addressed. For example, a friend discovered a low speed shimmy on a bike that had never shimmied at any speed before, and when he stopped to inspect the bike, we found a significant crack in the downtube. (The bike was steel, and the crack tracked in a spiral around the tube. I suspect this tube was made by winding a sheet of steel into a spiral and welding it with a spiral weld, and the crack followed the weld.)

[bking]

he checked all spokes? Could be a broken/stripped nipple? Cracked front wheel.

The derailleur could just be a coincidence. It happens.

[Keith A]

Thanks for the replies so far. My first thought was that the frame might be cracked somewhere, but we didn't stop to inspect it. We were doing about 26 to 28 mph at the time, and I saw the wobble and it was extreme. It continued until he slowed down significantly.

[drewellison]

I'd check the front hub bearings and the front axle. Maybe the bearings are having a breakdown and that can cause some weird wobble. And of course a broken front axle can have a similar effect

[bigreen505]

The problem with posts like this is there are so many things with both bike and rider that can cause a shimmy. I'd start with checking the headset, then go through the frame and fork from dropout to dropout. Cracks can grow over time, so just because something isn't visible and obvious now doesn't mean it's not there.

One thing you can try is riding on a road that's been grooved by heavy trucks. If you turn back and forth across those groves and can feel the frame twist, that's a clear sign the problem is the frame. I had a Wilier that cracked the left seat stay above the dropout. In hindsight, I know exactly when it happened, but it took a couple months of riding before the cracks were visible, or at least before I found them. My dad had a Cervelo S3 that was sketchy downhill. Everyone who rode the bike thought it was fine, but no one got it to the speeds he did. Short version is Cervelo determined that damage was present but not externally visible and replaced the frame.

[Keith A]

Thanks for the additional input.

bigreen505 -- This bike is a Wilier :eek: