[El Guapo]

I know this has been hashed out before, but please bear with me. I currently own a Univega Modo Volare (Thron steel). The bike feels plenty stable at normal cruising speeds. 49 mph used to be the magic number ... hit that and the wobble began. Soon that barrier became 46. Now it is 40. I have tried different positioning on the bike (weight forward, weight backward, off the saddle, on the saddle). The wobble isn't some mildly disturbing annoyance, but more akin to "bucking". Imagine your rear tire catastrophically deflating. The rear of the bike goes completely loose at speeds over 40. God forbid I have to corner. That feels like the BB is cantering. Could my frame be "loosening" up. I know this sounds ridiculous, but I can't come up with any other explanation. I have a Bianchi Limited from '86 that descends steady as a rock (50+ with NO wobble). I have been assured by my LBS that my Campy Proton wheels are tensioned to spec and my tires (Conti Ultra 2000's) are inflated properly. I understand why speed wobble happens, but no one can explain to me why that speed threshold continues to decrease.

 

[Kerry Irons]

A number of factors

You have a bike that is prone to speed wobble, probably due to lack of stiffness in the head tube/top tube/down tube joining. The change the speed of where this starts could be triggered by a number of things, including head set adjustments, tire weight, tire pressure, bearing adjustment, your weight, etc. Can you prevent/control the problem by clamping the top tube between your knees? This is the standard fix. It is very unlikely that your frame is changing, short of a developing crack that would be visible on inspection.

 

[El Guapo]

Thanks for responding. I have tried clamping my knees on the top tube (one at a time and both at the same time). I have checked the headset adjustment and all seems well there. Whats wierd is that my old Bianchi has the same size tubes, but is solid as solid can be. Would it be possible that the fork might be the culprit? Maybe it would be akin to bottom bracket creaks actually resonating from somewhere else in the frame. I have inspected the frame pretty thoroughly. Would cracks show up through the paint? Or, could they reside under the paint?

 

[eclipse1]

some things to check

are you lightly on the brakes. if so they may be deflecting the wheel, this was a problem and my bike bucked 10-12 inches to each side, tough to stay on it took me a long time thought it was water in the rim. you might want to try a set of wheels, then it would narrow down problem. is the head tight, any recent crashes, is the frame straight. are the back of the dropouts in a right angle to the frame. these are things i would check. stange how it is getting lower at speed. that kinda say it is something that is getting worse. but what ???
good luck maybe post when you find the answer

 

[froze]

Seems everyone covered all the good stuff, so make sure your computer magnet is attached to a spoke on the opposite end from the valve stem. If that's good, then just for kicks try moving the tire on the rim 1/4 of a turn and see if anything changes-let us know.

 

[curlybike]

Seems everyone covered all the good stuff, so make sure your computer magnet is attached to a spoke on the opposite end from the valve stem. If that's good, then just for kicks try moving the tire on the rim 1/4 of a turn and see if anything changes-let us know.

In most wheels, the side opposite the stem is the heavy part. They install a joiner in the rim that is heavier than the stem. Adding the magnet there will make the imbalance worse.

 

[CycloPathic]

I know this has been hashed out before, but please bear with me. I currently own a Univega Modo Volare (Thron steel). The bike feels plenty stable at normal cruising speeds. 49 mph used to be the magic number ... hit that and the wobble began. Soon that barrier became 46. Now it is 40. I have tried different positioning on the bike (weight forward, weight backward, off the saddle, on the saddle). The wobble isn't some mildly disturbing annoyance, but more akin to "bucking". Imagine your rear tire catastrophically deflating. The rear of the bike goes completely loose at speeds over 40. God forbid I have to corner. That feels like the BB is cantering. Could my frame be "loosening" up. I know this sounds ridiculous, but I can't come up with any other explanation. I have a Bianchi Limited from '86 that descends steady as a rock (50+ with NO wobble). I have been assured by my LBS that my Campy Proton wheels are tensioned to spec and my tires (Conti Ultra 2000's) are inflated properly. I understand why speed wobble happens, but no one can explain to me why that speed threshold continues to decrease.

It could be you, really. If bike doesn't have enough trail and unstable, a bit of "shivering"/subtile input from upper body may induce shimmy. If that's the case start riding rollers.

But first I'd double check stem/handlebar/fork/hubs and headset for play, hubs/headsets need adjustment sooner or later. Other then above replace fork with less rake?

 

[froze]

In most wheels, the side opposite the stem is the heavy part. They install a joiner in the rim that is heavier than the stem. Adding the magnet there will make the imbalance worse.

Not that I'm disagreeing with you, because you are right about the joiner BUT I have found on some rims on friends bikes over the years as well as my own that if you relocate the magnet it took away the imbalance. My point here is to at least try it before ruling it out, because it might work, and if it doesn't so what? just put the magnet back..

 

[curlybike]

Not that I'm disagreeing with you, because you are right about the joiner BUT I have found on some rims on friends bikes over the years as well as my own that if you relocate the magnet it took away the imbalance. My point here is to at least try it before ruling it out, because it might work, and if it doesn't so what? just put the magnet back..

I should have added that you were right in suggesting the magnet move, just not where you suggested.

 

[Coolhand]

There are a ton of good articles on Velonews.com by Zinn on this issue. You should check them out.