New road rider... help with high speed shimmy.

[h2otaco]

Hi there... new to the forum... Very nice by the way, really high quality forum.

First, introductions... I've been riding MTBs for 15 years. Racing XC for the last several. I consider myself a fair cyclist, and a fair bike mechanic. I have 5 bikes in the garage, 2 Roadies (one carbon, one steel) a 29" SS hardtal MTB, a 29" Full suspension XC race rig, and 26" hardtail. I'm new to road cycling, as I'm using it for cross training for the MTB season. haven't thrown my leg over a road bike since 1995 (the steel road bike mentioned above).

So, on with the high speed shimmy/wobble topic.

Last night I went on a group ride with hundreds of participants. I got in a group of fast racer guys and was just hanging on for dear life. We hit a short steep hill and got up to 40mph or so when I noticed my bars starting to shake. It was a little unnerving so I backed it off with a little rear brake, and didn't think anything of it. After about 2 hours of pedal pounding and hanging on to the back of the group I lost contact a little bit. So I used a long descent to try to get back on... In a fog of lactic acid, I had forgotten about the earlier shimmy experience. All was well until I hit about 56mph... I was still gaining speed in a tuck when I noticed the bars start to wobble again. The wobble started small but got worse and worse until I actually started to feel unstable, not a good feeling at highway speeds. First I tried steadying the bars by force, no good, then I tried relaxing... no good. So I started to scrub speed by feathering the rear brake, bringing the speed down to about 30-35. Wobble went away, but I'll never forget it. Just before I got on the brake I felt like I might actually lose control of the bike.

I know people regularly hit these kinds of speeds so this must either be a problem with the bike or with me (the bike I was riding is this: http://quintanarootri.com/QR_bikes/split/split.html), how do I avoid this type of thing happening again. I don't think I'll be able to take my bike up to speed like this again unless I'm certain I can avoid this frightening sensation.

 

[An old Guy]

There are about 12 hills on my 25 mile ride. I can reach 60mph going down any of them. But I keep it below 30.

No bike is safe on public roads at the speeds you are desiring to ride.

And riding at 40 on public roads with a bunch of guys you do not know is really not a great idea.

 

[h2otaco]

I'd really just like the roadbike to feel stable at high speed. Maybe going fast isn't some people cup of tea, and that's cool. For me it's not an issue... I sometimes hit speeds over 30 in races offroad.

As for the group... really it was I that was the odd man out... the group was an elite local road team out training... they were kind enough to let me tag along.

Anyway, I digress... anyone know of how to get rid of this vibration/shuttering?

 

[64Paramount]

I suppose you've done the obvious things like making sure the wheels are true and aren't out of round, etc?

 

[h2otaco]

Originally Posted by 64Paramount .

I suppose you've done the obvious things like making sure the wheels are true and aren't out of round, etc?

Yep... first thing I did when I got home was throw the wheels in the stand. All good.

 

[bmohan55]

The old high speed shimmy, IIRC the bike gets some kind of "harmonic balance" going between tires and wind and creates the shimmy (do a search, someone no doubt has explained it better then me). What has been suggested and worked for me is to grip the top tube with your knees to stop it. Appling brakes (especially the front) can make it worse as it tends to push you harder down on the bike.

Only hit +50mph a few times,scared the hell out of me, tears blinded me, but I didn't reset my speedo for a long time so I could see top speed reading!

 

[Steve_A]

The latest Rivendell Reader (#42) has an interesting article on shimmy by Grant Peterson. He doesn't pretend to solve or explain the problem completely, but it's informative and interesting nonetheless. You can get links (for free) to two different formats for the Reader here: http://www.rivbike.com/products/show/rivendell-readers/24-072 Check out the rest of the Reader while you're there.

 

[Motobecane]

only advice I have is wear a diaper! perhaps wider tires may help but that is really high speed and everything is going to be magnified. fastest I've gone is about 43 mph and it was on a somewhat windy road, it was a bit scary. especially since at 240lbs hitting the brakes doesn't seem to immediately slow me down much!

 

[64Paramount]

Use the forum search function with the keyword "shimmy". Then click the "recent" category to get the threads sorted started from the most recent. You should see this thread in the top 2 or 3 positions, and you'll also a bunch of other threads discussing shimmy and high speed wobble.