Tank Slap

I had a genuine tankslapper at about 110 in the wet on the banking of the Pocono Raceway (out in PA) while crossing the bump. It was like a giant shook me and threw me off the bike. I slid on the wet pavement, rolled a few times and stopped about 5 ft from the wall. I walked. The bike was a bit banged up as it slid on its left side, and hit the wall with the top of the bike. I did a lot of thinking, and as I read through the posts I see some valid points and some misconceptions on the reasons why tankslappers happen, and how to prevent them. Here are my thoughts on this.

Why do tankslappers happen? The bike's forks (and any car's front wheel shocks for that matter) are at an angle which we all know about. The angled forks move the contact patch of the front wheel slightly behind it. As a result, the entire momentum of the bike moving forward and the ground moving in the opposite direction create the force which wants to keep the front wheel straight. Which is a good thing. It's mostly this force (and some of the gyroscopic force) which resists turning the front when you try to make a steering input. It's the same force which straightens the front wheel if left alone. Let's call it the straightening force.

During normal riding when the front wheel is never too far off the alignment, this straightening force is very useful. However, the amount of this straightening force grows dramatically when the front gets more than 5 deg out of alignment. Setting down a wheelie with a croocket front wheel is a typical example when this might happen. This however is an easily fixable example - just land with the front mostly straight.

There is a bigger problem because it may happen during seemingly normal riding. Believe it or not, but in order to stay in turn, we have to constantly apply steering input (pushing the inside bar, or pulling the outside bar) - whether you realize it or not (ask me for proof if you don't believe it, I don't want to crowd already lengthy explanation). So, when the bike stays in turn, the force of your steering input is balanced with the straightening force of the front wheel. When/if the front gets light even for a fraction of a second (a bump, wet, light front on acceleration out of a turn), the straightening force of the front wheel is greatly reduced for that fraction of a second. But your steering input (which tries to turn the front wheel to the outside of the turn) stays the same. As a result - the two forces which were in balance, but now are not - the front gets croocked (turned a little too far to the outside of the turn), and when the next moment you passed the bump, the front is no longer as light as the moment before, and the straightening force (which is far greater because the front wheel is turned to the side more) is trying to straighten the front wheel back into alignment. That's the start of a tankslapper.

The only thing that can truly slow down the swing of the front wheel is your steering damper - not your hands, not the throttle which has been ripped from you hands.

Let's pause for a moment and think. What can we do? First, let's think. The tankslapper happened because the front got out of alignment too far. And it got out of alignment too far because our steering input pushed it there when the straightening force of the front wheel disappeared for a moment when the front got light. OK, so if we could somehow reduce the amount of steering input we need to apply to go through the turn, that would help, right? OK, we could reduce the amount of steering input to keep the bike going through the turn by... moving our upper body to the inside of the turn! Don't even bother to hang off if you don't want to. Just move your upper body to the inside. You would be able to take the same turn with the same speed, but with less lean and with less steering input required to stay in turn.

As a side point here is that tensing on the bars creates unwanted steering input which can only contribute to the problem, and relaxed grip minimizes it.

But, what if you already got into the tankslapper? Frankly, there is nothing you can do. The fact that you got into it indicates that the oscillations are increasing and not subsiding. That's where a steering damper is the only thing that can introduce enough resistance to make the oscillation to subside. That's why it is so important to either replace the stock oil in the damper with 80W gear oil, or get the aftermarket damper. It's simple physics. A little more resistance and oscilations subside. A little less, and they increase.

Another thing is that this may not happen for years. When people say they installed a damper and immediately feel the difference, it just makes no sense. A little headshake on uneven surface is normal - it's the straightening force of the front wheel which works its way through uneven surface. The steering damper only matters if it saved you from a tankslapper. Otherwise, it's pretty useless.

So, to sum it up. Refill your damper with 80W, or upgrade it. Minimize steering input in turns by shifting your upper body to the inside, even before the turn. Staying relaxed minimizes your steering input to only what is necessary.
 
I forgot to mention another small detail. I ride year around and wanted to make sure 80W oil would not create problems in sub-freezing temperatures. So, I put the damper with 80W oil in the freezer for a few hours. Then I took it out and tried to move it by hand. It was noticeable more difficult to move it then at room temperature, but it was moving - not stuck. This proved to be not a problem in follow up rides in sub-freezing temperatures.
 
Had it happen once at about 120mph when I hit some little potholes while accelerating wide open. Scared the hell out of me, but it all turned out ok.
 
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