Shimmy

[jnm2.0t]

So I was out for a ride yesterday and experienced something a bit unnerving that I am trying to get to the bottom of. I was coming down a hill, coasting with my hands on the handlebar drops, in something of a tuck. Around what was probably 30mph my front wheel began to shimmy left and right. It was not up and down at all, the handlebars were pulling left, then right, then left, etc. It wasn't violent enough to lose control or grip, thankfully, and sitting upright and applying the brakes lightly was enough to end it. It never happened again, but here's some info...

• New bike, Scott CR1 Team. Was checked by an LBS after who found nothing noticeably loose.
• I had my front wheel off before the ride and again between taking it to the above LBS. Why? I didnt want to fuss with the rack and just threw it into the trunk of the Highlander. At any rate it is possible the front wheel was too tight or too loose during the ride if this would be a possible cause. Tips for making sure it is right?
• There was a decent headwind. I was coming down a hill (Page Mill between Foothill and EL Camino, for those that know... not huge 150ft), but there's little between it and the SF Bay. Reading online says this can be a cause.
• I'm a novice rider so it is entirely possible my form was off, putting either too much or too little weight on the drops, if that could be a cause.

Thoughts, tips, and things to make sure to check into highly appreciated!

[GTaye]

In motorcycling this is know as 'headshake' that can cause a 'tankslapper.' (Hit up google for some amazing videos) I've never experienced it either on a motorcycle or bicycle but I can imagine how scary it can be.

From what I can recall, if not caused by mechanical means (loose headset, etc) or weird bike geometry (ruling that out for your Scott), it can be caused by poor weight distribution due to riding position. I've read that unweighting the saddle (not standing up) can remedy the shake, or changing the hand postions. Keeping weight oon the front wheel should help the problem, though. Strange that it ceased when you sat up. I would certainly make SURE your QR is tight on your front wheel, every ride.

Wind as a factor? I guess that's possible. How stiff a wind do you think? Do you have deep aero wheels?

[Patrick Schmidt]

Stand beside the bike and squeeze the front brake. Move the bike backwards and forwards, do you see any play at all in the headset? Pick the front of the bike up 6in or so and drop the front wheel to the ground. Do you hear a solid "thud", or does it sound a little high pitched and rattly?

Are your wheels, especially the rear, true?

As mentioned, weight distrubution also can play a part in shake. If your able to unclinch your sphinchter from the seat the next time it happens, try sliding fore and aft.

I'm not sure what wheels you're running on your bike, but when I ride with 60mm deep wheels descents with any sort of cross wind or doppler effect from passing cars make the handling skeeeetchy!

[jnm2.0t]

In motorcycling this is know as 'headshake' that can cause a 'tankslapper.' (Hit up google for some amazing videos) I've never experienced it either on a motorcycle or bicycle but I can imagine how scary it can be.

From what I can recall, if not caused by mechanical means (loose headset, etc) or weird bike geometry (ruling that out for your Scott), it can be caused by poor weight distribution due to riding position. I've read that unweighting the saddle (not standing up) can remedy the shake, or changing the hand postions. Keeping weight oon the front wheel should help the problem, though. Strange that it ceased when you sat up. I would certainly make SURE your QR is tight on your front wheel, every ride.

Wind as a factor? I guess that's possible. How stiff a wind do you think? Do you have deep aero wheels?

Yes the videos of headshake are exactly it, I found another extreme example of a bike with it below. From this it would seem i had too little weight on the handlebars and too much on the saddle. The reason I sat up was to catch some wind to slow down without hitting the brake, I figured that would probably be the worst thing I could do at that moment. I suppose in the act of sitting up that I pushed off the bars adding force to them and stopping the wobble. I could also have hit a bump or divot that got it going, particularly if I didn't have enough weight on the front.



http://www.cyclingtips.com.au/2011/0...akes-its-head/

Stand beside the bike and squeeze the front brake. Move the bike backwards and forwards, do you see any play at all in the headset? Pick the front of the bike up 6in or so and drop the front wheel to the ground. Do you hear a solid "thud", or does it sound a little high pitched and rattly?

Are your wheels, especially the rear, true?

As mentioned, weight distrubution also can play a part in shake. If your able to unclinch your sphinchter from the seat the next time it happens, try sliding fore and aft.

I'm not sure what wheels you're running on your bike, but when I ride with 60mm deep wheels descents with any sort of cross wind or doppler effect from passing cars make the handling skeeeetchy!

Wheels are just Aksiums, stock with the bike. I'm really leaning (pun) towards poor weight distribution, I feel as if I was sitting back in the saddle way too much.

These guys have way more weight forward than I did, esp 0:35...

[andycooper]

Having experienced exactly the same at some 45+mph, I know just how scary this is. I'm also willing to bet that your headset is loose or just not seated properly - that was the cause of my problem. It only needs to be a little loose to cause shimmy at speed. Given that you've got a Scott CR1, I doubt that it is an issue with the frame (unless, of course, you have a cracked frame somewhere) and unless you were completely unweighting the front end, I'd guess it isn't weight distribution. The other possibility is that your front wheel's skewer was too loose; too tight really won't be an issue here ...

[jnm2.0t]

Ya I will have them take a thorough look at it, probably drop it off with them for the day. The LBS I got shoes from checked it out a bit when they were installing the pedals and didn't see anything that popped out at them.

The skewers... what I do is close the QR, finger tighten the nut, open the QR, tighten the nut a bit more, then close the QR back down. I notice some compression of the fork when the QR closes, i doubt it is too loose... seated a bit askew I suppose would be possible but the rim is so close to the brake I would suspect that would manifest itself in rubbing. Unless it is just the smallest bit askew...

[DJKhakiPants]

Check the headset as others mentioned - ensure cups or bearings are flush and that adjustment is proper. Some frames are susceptible to the speed wobbles because of geometry, even if everything is in adjustment. Depending on the bike it may just be an upsized version of a smaller frame (or vise versa) and the angles of the seat tube and head tube were not adjusted to reflect the frame size.

If it happens again grab the top tube with your knees to help control the bike and slowly bring it to a stop. I have seen people deal with this once as a freak occurrence and others every time they go down hill. If it becomes a continued problem and all of the mechanicals check out it may just be that the frame is not compatible for you.

[jnm2.0t]

Took the bike in today, they again didn't see anything mechanically wrong with it but are giving it a full overview. The guy I spoke to said the stem was too short for that size bike and that he'd throw a longer one on and do some prelim adjustments of the seat height and position for me based on what he saw from my positioning. Going for a ride tomorrow and then a full hour tweaking probably Monday.

[DJKhakiPants]

Any change after the fit?

[jnm2.0t]

I didnt actually get a chance to do the full fitting yet, probably wont for another 2 weeks at this point. They did slide my seat back a bit and give me a longer stem. I've been more conscious of my weight balance and just keeping the top tube between my knees for now. So far I've hit the same speeds and a bit higher with no issue, will just keep incrementally upping speeds. I am guessing it was probably primarily brought on by newbie rider mistakes.