Bicycle Mechanics - Why Does My Bike Have A Front End Shimmy?
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My 1982 Peugeot PSVN has a bad front-end shimmy (wobble) when riding hands-off above roughly 15 mph. Doesn't matter if I am pedaling or coasting, and the pavement can be smooth. It stops if I put a hand on the bars. I've never noticed a shimmy at low speeds or if I have a hand on the bars.
Possibly related, the bike does an odd thing if I ride over a little bump (like a pavement ripple) when leaned over in a turn, this is with hands on the bars - the bike gives one little "shake of its head" then continues on its way. It isn't something I feel with my other bikes.
The bike is 58 cm, no fenders or other attachments. This is how it is equipped.
I am 5' 11", 186 lb. and usually have a messenger bag (it is my commuting bike).
What are some things I should check? So far all I've done is checked that the headset is not loose, if that even matters. I have searched here, but didn't find too much, other than to make sure fenders aren't loose - which is not applicable.
05-14-12, 04:01 PM
Shimmy is very common; placing your knee on the top tube will usually stop it. Search on "bike shimmy", here's a Jobst Brandt article to get you started: http://www.sheldonbrown.com/brandt/shimmy.html
05-14-12, 04:37 PM
That link is cool, the last paragraph is amazing.
That said, it can be mechanical as well:
Check for loose bearings- headset and hub, also check to see the tire is seated on the rim properly (is the nut on the presta stem? Might be too tight and the tube isn't letting the tire bead seat properly), and finally check for trueness and roundness of the wheel.
Also check for pitted bearings, a pit in a headset can do this as well.
Okay, basically check it all!
I'm going to build a new wheelset (vintage Mavic hubs and rims) so that'll assure a true wheel.
05-14-12, 10:45 PM
Wheel shake can be caused numerous things:
- loose front and/or wheel-bearings
- out-of-round or laterally un-true wheels
- loose headset
- mis-aligned fork
- mis-aligned/bent frame
Here's a good article on wheel-shimmy: Dave Moulton - Shimmy re-visited (http://davesbikeblog.blogspot.com/2006/08/shimmy-re-visited.html)
Also your shift-cables look a little short. Can you turn your handlebars 90-degrees or more easily? Aside from all the other harmonic frequencies bouncing around on the bike, you don't need your shift-cables acting as springs pushing your handlebars back & forth.
Hmm, I am not sure - I can turn them to 90 degrees, but haven't noticed if there is resistance from the cables as I do so. I'm not home, but when I get home later this week, I will check that too. Thanks.
05-14-12, 11:05 PM
Apply your front brake hard, and then push the bike forward and backward with downward pressure on the front wheel. That should allow you to feel if the headset is loose.
05-15-12, 12:51 AM
Check the spoke tension also. I had a wheel that visually appeared true but the spokes were under-tensioned and it created a low speed shimmy.
There's also an outside chance the frame or fork is slightly bent (sorry Sheldon, but it can be the case), or the dish on one of the wheels is bad which will put the wheels out of alignment. Unlikely, but it's not too hard to check with a straight edge or a string line if everything else checks out ok.
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