Monday, 29 November, 2021

How To Fix Motorcycle Death Wobble


motorcycle-death-wobble

It seems like every day in various motorcycle groups, the same question comes up: How To Fix Motorcycle Death Wobble.

Motorcycle Death Wobble, Speed Wobble, Head Shake, Tank Slapper, whatever you call it. They’re all slang terms for the action of the front forks (and handlebars) wobbling back and forth rapidly.

Example of a wobble.

First, some Technical Jargon

A motorcycle’s front caster is its front wheel and steered assembly, pivoting on the steering-head bearings. The front wheel follows behind the steering axis by a distance known as “trail.” The rear caster is the rest of the machine—frame, engine, swingarm, and rear wheel—also pivoting around the steering-head axis.

Wobble is the rapid oscillation (8+ cycles per second) of the front caster.

Weave is the slower side-to-side swing (2–3 cycles per second) of the rear caster.

So, motorcycle death wobble typically falls under the “Wobble” category.

Motorcycle Death Wobble Causes

As you’re probably aware, a motorcycle naturally wants to go straight. The front wheel doesn’t want to steer any other direction but straight, unless the bike is banked or leaned over. So, if  some disturbance deflects the front wheel from its centered position, it naturally wants to move back towards center. If there isn’t enough damping force (friction) in the system, it can wind up moving beyond it’s center axis, at which point it will want to move back towards center again, and a wobble develops.

Wobble typically occurs in the speed range of 35–40 mph first. Riders may experience a bit of wobble if they take both hands off the bars at these speeds to adjust their face shield, gloves, or some other item. In most cases, the damping effect of putting their hands back on the bars is enough to stop it.

Tire Related

Tires – Tire Size/Type, and Condition can all effect the stability of your bike. A motorcycle is engineered around a certain tire size/design. Running tires of a different size or design than came on the bike originally can cause stability issues. Tire wear, cupping, or imbalance can also cause problems.

Tire Pressure is an often overlooked variable in stability. A tire’s pressure changes it’s contact patch. The contact patch provides most of the damping (friction in the system) that is responsible for maintaining stability. So, too much pressure can result in not enough damping (friction), which can create a wobble. Likewise, not enough pressure can create irregular loading/wear, which will cause problems. Refer to your service manual, tire placard, or the tire manufacturer for correct pressures on your specific model.

Belt/Chain Deflection and Rear Wheel Alignment – In some scenarios, a drive belt or chain that isn’t set correctly, or a rear wheel out of alignment can cause stability issues. Be sure to check these items periodically.

Chassis Related

Structural Looseness Or Flexibility – Other possible causes of instability are loose pivots—steering head bearings, swingarm pivots, and wheel bearings, wire wheel spokes, and even damaged frame sections. You’ll want to inspect all of these things carefully, and refer to your motorcycle’s service manual for specific testing procedures.

Aftermarket Accessories – Panniers, Saddlebags, Fairings, Windshields, etc. and anything in or on those items all change the weight distribution of the motorcycle. Adding mass to the motorcycle’s steered assembly may make wobble more likely. So, you can try temporarily removing these items one at a time to see if that solves the problem.

Rider Related

Abrupt Inputs – Too much throttle or maneuvering too quickly momentarily unweights the tires. The effect is similar to having too much tire pressure. The contact patch (and resulting damping force) is reduced, and stability suffers. Remember: Slow is Smooth, Smooth is Fast. Control your throttle & steering inputs so that they’re less abrupt, then increase your inputs incrementally until you know how much is too much for a given scenario.

The Motorcycle Death Wobble Fix:

If you’ve checked over all of the above variables, they all check out, and the death wobble persists, a steering damper may be what you need. A motorcycle steering damper is a device that adds damping force to the front caster, specifically to control death wobble.

Motorcycle Weave Causes

The slower “weave” is a little bit different than the tank slapper, but no less annoying. Commonly, this signifies an issue with the rear suspension, rear tire, swing arm pivots, rear wheel, or something along those lines.

If your motorcycle’s engine is rubber mounted, this can be a sign of a bad motor mount, too. The powertrain has the ability to move in the frame to an extent, and the sudden shift in weight distribution can upset the bike’s stability.

This is a known issue with Harley Dynas, but isn’t necessarily limited to Dynas, or Harley Davidson Motorcycles.

For Dynas specifically, a powertrain stabilizer is available to help keep the powertrain and swingarm in line with the frame.

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