The Mighty Tankslapper

Posted by pikesan on Nov 26, 2007 in Uncategorized | 0 comments

You have surely seen the image before, a race bike coming hard out of a turn with a light front end and serious headshake. Occasionally, this oscillation gets more violent turning into a full-fledged tankslapper where the handlebars are thrown from stop to stop. Sometimes the result is the rider being catapulted from the bike in spectacular fashion.

Most street riders have experienced the phenomenon that leads to tankslappers to some extent after upsetting the suspension by hitting a rock or pothole (or landing a wheelie a little off center), etc. In most instances, a light grip that allows the bike some freedom to move will allow a shake on the front end to settle down. Now, if you have never experienced a tankslapper then simply imagine amplifying the shake of the previous example by ten fold and multiplying the odds of dirty leathers by at least that much. The purpose here is to briefly describe why tankslappers happen and some solutions to reduce chance of being bucked off because of one.

All motorcycles can experience oscillations in the front suspension, but some motorcycles are much more prone to tankslappers than others. Generally, headshake is more common in motorcycles with low rake and trail angles such as sportbikes. The frequency of the shake will be much higher but not as violent in sportbikes than when it occurs on a cruiser or other bike with high rake and trial angles. Fortunately, the latter bikes are more stable at speed and the forks and tires are much better at dampening a shake than the more rigid equipment on sportbikes.

Cause #1: Irregularity in the Road
A feature in the road, which applies uneven pressure to the front wheel, can start headshake. Rocks, cracks, seams, manhole covers and racetrack curbs. A bike will be more susceptible to this with a light front end under acceleration.

Cause #2: Irregularity in your Stunter Skillz
Just like something in the road can force your wheel to start wobbling, a wheel that is put down a little bit sideways at speed can start the oscillations. As seen here:

Cause #3: Irregularity in your Wheels
An unbalanced wheel can also lead to headshake. If you have a wheel that is damaged and starts to wobble at a given speed, this could be enough to start headshake. It just depends on the frequency again.

So, there are a couple of solutions to help you handle or prevent headshake and tankslappers. The first one is obvious:

Solution #1: Proper maintenance
It’s funny how maintenance is one of the best ways to keep you intact. Make sure your tires are not worn, make sure they are properly balanced. Also make sure that the nut on the steering stem is properly torqued.

Solution #2: Hang Loose
If the front end does start to shake, loosen your grip and don’t fight it. When you grab the bull by the horns you really only add force going in the wrong direction. If you loosen up and just let it shake, you have a much better chance of the bike straightening out on it’s own.

Solution #3: Use A Steering Damper
These little gizmos are worth their weight in gold. I know for a fact that my Ohlins/Scotts damper has saved me from some nasty headshake at least four times on a racetrack. Basically, dampers are shocks that connect the frame to the front forks. They slow down the oscillations so that headshake is stopped almost immediately. It’s a very interesting sensation when you feel it go to work. In fact, the first time I put mine to use the shake was over before I even had time to react and get that familiar “Oh Crap!!!” feeling.