How to avoid a tank slapper or speed wobble

 

A tank slapper is when the handlebars wildly oscillate. They are also called speed wobbles or death wobbles and, as the latter name implies, they can be lethal.

They are usually developed at high speed like the guy in the video above, but they can also happen at low speeds.

According to racing engineer Jeromy More, it’s an “underdamped torsional oscillation of the front wheel/suspension which can be set off by either a road input or input from the rider’s weight creating a steering moment or even a gust of wind”.

That’s why world speed records are set on flat salt lakes in light wind conditions.

However, even on salt lakes, aerodynamic streamliner bikes can develop speed wobbles as seen in this clip from The World’s Fastest Indian.

 

Jeremy says this is an example of “yaw oscillation” which is induced by aerodynamic asymmetry, probably from a gust of wind.

“The whole bike is reacting but in this case no steering damper could help,” he says.

“It is more an aerodynamic stability issue where the stabilising moment created by the tail fins isn’t enough. This is why planes have the tail fin to put the aerodynamic centre of pressure behind the centre of mass so any side wind makes the plane return to its original trajectory and not away from it.”

HOW WOBBLES START

They most often occur on a motorcycle at high speed when aerodynamic drag force moves the load to the rear tyre and un-weights the front tyre.

Jeremy gets awfully technical here, but basically when the oscillation starts the front tyre contact patch is offset from the steering axis and tries to restore itself by swinging back the other way.

Unless you have a steering damper fitted, the steering overshoots and goes past the straight-ahead position and the oscillation is then magnified.

AVOIDING WOBBLES

They can be prevented by working on a bike’s aerodynamics, suspension, steering damping, lighter wheels and basic bike maintenance, especially correct tyre pressures. Motorcycle tyre pressures - nitrogen

Most riders try to calm them down with a steering damper, but be careful as it can be difficult to get the correct amount of stiffness. An incorrectly adjusted steering damper can make it difficult to maintain a straight line or turn corners and can even lead to high-speed weave.

It’s all very technical and best left to skilled technicians such as Jeromy who rides for leisure and works for the Porsche endurance race team in Germany.

Engineers have even found that heavier riders are less susceptible to speed wobbles.

If you don’t have a skilled pit crew, the best ways to avoid speed wobbles are to maintain good motorcycle maintenance, keep your body weight forward over the front wheel and slightly loosen your grip of the bars and bend your arms.

If you grip too hard, your body becomes the steering damper and can actually induce more oscillations. Loosening the grip and bending your elbows reduces that effect.

STOPPING WOBBLES

So what happens if you get into a speed wobble?

There is varied opinion on this.

Some say the answer is to power out of it, others say to back off the throttle.

Some say hit the brakes, while others suggest you leave them alone.

Some say hang on tight with your hands and knees and move forward to weight the front wheel. Others say to loosen your grip and slide back to lighten the weight on the front wheel.

There seems very little popular consensus.

However, Jeromy says the physics principles dictate that you should accelerate and lean back.

“That reduces the weight and hence the movement created by the oscillations until you can then control it with your arms,” he says.

“Quite easy to say on paper but not a natural reaction to want to go faster when the bike is going crazy below you.”

Fuel economy tank slapper
Jeromy Moore

And since speed wobble oscillation is measured at 4-10hz (each change of direction in the handlebars takes from 1/10 to 1/4 a second), he says it is virtually impossible to catch it at the straight-ahead position.

In reality, there is very little you can do about stopping a speed wobble especially when the bars shake so violently they throw your hands off the grips as in the top video. Most people who have experienced them have also crashed.

The best way to stop them is to avoid them.

LOW-SPEED WOBBLES

Even at low speeds, a series of undulations can trigger low-speed wobbles or a tank slapper. They often happen in off-road conditions or on gnarly back roads.

Usually you can safely halt a low-speed tank slapper by simply leaning back and accelerating.

You can also experience speed wobbles under deceleration, especially if you have only one hand on the bars.

They are usually not as wild and can easily be corrected by releasing the brakes and adding a little throttle. Of course, if you are coming up to a stop sign, that’s going to be a problem!

They can be an indication that your front tyre pressure is too low, the tyre is unevenly worn (probably from riding around on it with low pressure), your suspension is worn out, or the steering head bearings are worn, loose or too tight.

Check your tyre pressures and wear. If that’s not the problem, check the play in the handlebars for loose or worn bearings. If so, replace them, but it’s best left to an expert technician.

If it’s not the tyres or steering, it could be your suspension. Again, see an expert.

3 Comments

  1. To complete a junior series, I borrowed a KX125 that had what I called ‘headshake’ alot. The tracks fast back straight had a series of three drop offs that one could gun down and launch off each one. Was my fave part of track. The KX got its head shake going before the first and I had a chance to regain the reigns and recovered on landing, but back it came prior to the second drop off. I was thrown off the side of the track in an old thicketed creek side that was laden with a burial ground for the old cement highway concrete sections. Knocked me out for a bit. Wasn’t my fondest memory of the 92model KX, the fork seals kept blowing aswell., but what sealed the KX wreckairs deal for me was the throttle that kept sticking on. However, I cleared a few jumps that I wasn’t quite ready to master at the time, the landings were hair raising to say the least and I completed the season, shell shocked, but finished.

  2. Used to have a 1981 XJ550 (stock) and it would develop high speed wobble EVERY single time I approached its top speed of 185km/h. Had nothing to do with wind gust starting it off and also made no difference where I was. In fact most of the time I was lying flat over the tank, so as much weight as possible on the front wheel. Also tried fork stabilizers, but nothing helped and it would always go into high speed wobble at its top speed.

    I learned to live with it and it was easy to get out of it. Just before it became too much wobble I trottle off and it stopped every time. Had that bike for 8 years and never crashed it because of that…

    Sure don’t miss that “feature” on my current ride.
    Cheers
    Joe

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