What causes speed wobble?

Finding the cause

I ride a 30 year old, vintage Motobecane 10-speed that is in excellent condition. Six months ago, while making a high speed turn in a 90-degree corner (about 22 mph) the front end and wheel began an uncontrollable wobble and shimmy, causing a crash, resulting in a bent rim and handle bar.

I replaced front wheel and bike seemed to ride fine. Did notice some front wheel wobble when riding no hands above 20 mph, but not bad. While riding to work one day, a van ran a stop sign, barely missing me, but my evasive maneuver, again provoked major front wheel wobble, resulting in another crash. My local mechanic cannot seem to find the problem, as new wheel is true.

What could be the cause of this condition, and a can you suggest a potential cure. I am an old guy, and do not want to subject myself to this kind of bike instigating crash.
– Pat Hudgins

WHAT CAUSES SPEED WOBBLE?

Speed wobble is caused when something, either an inherent misalignment of the frame or components, or an outside force, that causes the wheels to track divergent paths. The frame flexes to allow this to occur until the action is overpowered by the self-correcting forces created by the bike’s steering geometry. The energy stored in the frame then causes the steering to over-correct in the opposite direction, which starts another round, and thus the oscillation begins. We call it a speed wobble.

Speed wobbles can also start on a perfectly aligned frame. This can happen because the cyclist is not firmly connected to the frame, so the right conditions can cause the rider to pendulum from side to side, out of sync with the frame and steering geometry. This type of speed wobble is more aggravated, abrupt-and usually precedes disaster. In both cases gripping the handlebars and bracing the upper body allows more energy to be stored in the flexing frame, which exacerbates the wobble.

FINDING THE CAUSE

To help find the cause I’ll need to know a few more things: Have you owned the bike forever, or is it a recent acquisition? I need to understand if the wobble is caused by a recent event in the bike’s history ? like a frame or rear-wheel misalignment. What size is the frame? Tall riders on big bikes ? especially steel framed models are more prone to wobbles because they can develop oscillations excited by frame flex. Have you made any adjustments to the headset bearings or is there any excessive play in the wheel bearings? These questions will provide a good starting point for diagnosing the cause of the wobble. Otherwise: I have three words for you: ‘Can the bike.’ You have already hit the asphalt twice with no warning; the third time is your fault.

Discovering the cause ? and then fixing it may take the faculties of a top-notch frame builder or a pro shop that can align the frame and check the compatibility of its components. After that your problem still may rear its ugly head. Get a newer bike that fits you well and get off that Motobecane before you break some bones.

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