Bike unstable downhill at speed
11-27-19, 01:24 PM
#26
Senior Member
i agree with the other responses on checking wheel/frame alignment... and verify there is no twisting of the frame/fork relationship...
and also... is the FORK for sure the correct one, and is it UNBENT?
and look at the frame just behind the head tube... is there ANY hint of deformity, like wrinkles, or cracks in the paint?
"broken spokes"... scary... the bike was previously tortured... check EVERYTHING....
including the state of the lower steering bearing... it could be that while hanging in your stand, it is unloaded, and that bearing may actually be pitted/rusted some... even one bad ball, or some brinelling, can cause issues under load... that lower bearing sees no load when up in a work stand.... the lower bearing is usually the one that gets bad first.
yank the pannier off... go for a test ride... and see how the bike tracks while riding no-handed.... but be ready to grab the bars.
and make sure to check the ultegra wheels for proper centering, too... both with a gauge, and mounted in the bike... eliminate all possible problems... don't assume, find out.
fork is the most likely thing to cause headshake... but multiple issues may be found.
and also... is the FORK for sure the correct one, and is it UNBENT?
and look at the frame just behind the head tube... is there ANY hint of deformity, like wrinkles, or cracks in the paint?
"broken spokes"... scary... the bike was previously tortured... check EVERYTHING....
including the state of the lower steering bearing... it could be that while hanging in your stand, it is unloaded, and that bearing may actually be pitted/rusted some... even one bad ball, or some brinelling, can cause issues under load... that lower bearing sees no load when up in a work stand.... the lower bearing is usually the one that gets bad first.
yank the pannier off... go for a test ride... and see how the bike tracks while riding no-handed.... but be ready to grab the bars.
and make sure to check the ultegra wheels for proper centering, too... both with a gauge, and mounted in the bike... eliminate all possible problems... don't assume, find out.
fork is the most likely thing to cause headshake... but multiple issues may be found.
11-27-19, 02:04 PM
#28
Senior Member
so no pannier, and im not drifting, just having a serious amount of vibration at 40 kmh. i am very interested more-so, as a new bike mechanic, how i would go about checking the frame and fork for mis-alignment. Its not so much a speed wobble either, just a hardcore vibration at a certain speed?
11-27-19, 02:48 PM
#29
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Speed wobble is best described as the bike oscillating or violently vibrating at a certain speed.
OK, then.
If you have checked and ruled out all the possible mechanical causes (loose spokes, loose headset, loose hubs, cracked frame or fork) then it is possible that you have simply found the resonant frequency of your bike. Try and add some weight to the handlebars, like a handlebar bag, I'd bet this will solve the problem as it will greatly alter the resonant frequency of the bike.
just a hardcore vibration at a certain speed?
If you have checked and ruled out all the possible mechanical causes (loose spokes, loose headset, loose hubs, cracked frame or fork) then it is possible that you have simply found the resonant frequency of your bike. Try and add some weight to the handlebars, like a handlebar bag, I'd bet this will solve the problem as it will greatly alter the resonant frequency of the bike.
11-27-19, 02:57 PM
#30
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The fact that the OP had to change the wheels the bike came with due to broken spokes makes me think maddog34 has it right, the bike was pretty badly abused by the former owner and the frame and fork alignment are questionable.
11-28-19, 09:55 PM
#33
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so no pannier, and im not drifting, just having a serious amount of vibration at 40 kmh. i am very interested more-so, as a new bike mechanic, how i would go about checking the frame and fork for mis-alignment. Its not so much a speed wobble either, just a hardcore vibration at a certain speed?
https://www.bikeforums.net/framebuil...-thoughts.html
https://www.bikeforums.net/framebuil...iscussion.html
Here's an album from my Flicker site about aligning. The photos show both specific tooling methods and "home" methods.
https://www.flickr.com/photos/731955...57663801226719
Andy
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11-29-19, 10:00 AM
#34
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Found the old post I was mentioning. Andy https://www.bikeforums.net/bicycle-m...ard-right.html
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