Tank Slappers

[ozfireblade]

I was trawling through you tube looking at bike footage and came across this clip

a very nasty tankslapper, so I guess I have these questions, what causes them and what do you do when they happen? and if this thread has been done before my bad!!

[faffi]

Found this thanks to google:

 

1. The “tankslapper” is a very frightening experience. Usually occuring when accelerating hard over bumpy pavement, a tankslapper ensues when the front tyre becomes airborne, then regains traction outside the rear tyre’s alignment. The resulting deflection bounces the tyre off to one side, followed by another bounce in the opposite direction as it contacts the pavement again. Unless the bike’s steering geometry is able to damp out the deflections quickly, the resulting oscillations from the front tyre as it bounces back and forth will swiftly gain in strength, causing the bars to swap from side to side with increasing ferocity. The oscillations can be violent enough to rip the bars out of your hands, and fling your feet off the pegs. You can guess what happens next.

 

2. The easy cure for this problem is a steering damper. Many sports bikes now come stock with one, as the radical steering geometry needed for quick handling can otherwise cause some instability in certain situations. While a steering damper is an easy fix, it shouldn’t be a cure-all; if you’re forced to adjust the steering damper’s stiffness (if available) until you can barely turn the bars in order to keep the bike’s handling stable, there is a problem somewhere in your chassis setup. A too-stiff steering damper can also cause handling problems by itself; if your steering damper is adjustable, and you find that your bike won’t hold a line (especially in slower corners), or gets into a small wobble or oscillation in high speed corners, try backing off the stiffness a little and see if it helps.

 

3. Not all sports bikes need a steering damper, however. Many have steering geometry setups that offer quick handling, while still providing the necessary stability to damp out any front-end oscillations. In most cases, one of the biggest contributors to a tankslapper is your body positioning and grip on the bars. Some people ride in a more upright position when carving corners, but when accelerating over bumpy pavement, that upright body position puts even more weight transfer to the rear, which causes the front end to get lighter. Also, the more upright torso means that your grip on the bars is tighter in order to stabilize your upper body. That firmer grip feeds more input into the front end, something it doesn’t need while it’s busy trying to damp out the inputs from the bouncing front tyre. It actually forms a vicious circle: you grip the bars tighter because they’re starting to flap back and forth, but that only feeds more input into the front end, compounding the problem further.

 

4. The easiest way to avoid tankslappers while accelerating over bumpy pavement is to—believe it or not—keep a relaxed grip on the bars. Relaxing your grip on the bars means you must lean forward in order to assist in keeping your torso stabilized. This helps put more weight on the front end, which keeps the front tyre on the pavement. Since you’re not using your arms to stabilize your upper body, get your weight onto the footpegs so that you can get your body as far forward as possible; this also allows you to grip the tank with your knees for more stability.

 

If you do get into a tankslapper, keep your weight forward and—as hard as this sounds—maintain a relaxed grip on the bars. Let the motorcycle’s chassis deal with damping out the oscillations. Don’t try to be a human steering damper; you’ll only make the problem worse. Tankslappers can definitely soil your undies; but if you’re able to deal with them correctly, you’ll usually ride through them before you know it.

 

___

 

On my own account, from what I have read over the years, topped out suspension seems to be a contributing factor. Some bikes, like the Kawa Z1000J, for instance, would have enough lifting force to fully extend the front forks as it neared its top speed with a bolt upright rider. It took very little to set off a serious headshake.

 

Handlebar mounted quarter fairings also often create additional lifting forces. Together with the extra weight carried by the steering bits, they tend to also make bikes far more likely to headshake.

 

Od fashioned steering dampers are not the best idea IMO, because it makes straight line riding - especially at lower speeds - very awkward at best. Modern dampers that offer virtually no resistance around the "zero-point" and become more efficient as loads - and even speed - increase are much more useful.

[Cobie Fair]

Hi Oz,

 

Eirik hasn't been to the school, so hasn't been exposed to the training Keith is doing currently. This whole subject is covered in detail at the school, and also in Twist 2, you could check out Chapter 11.

 

Here is a quick note, I may have put this up here already: one of my coaches was riding over a pretty fast bumpy turn at Barber (turn 3). The thing tankslapped so hard on him he decided to get off the bike. Pretty fast turn, scary! He let go of both bars, was going to bail off the back. The bike instantly corrected and stabilized. He ended up putting his hands back on the bars before running off track.

 

Tank slappers are caused by the rider, and perpetuated by same. Some bikes for sure can use and need steering dampers, but the rider can still mess things up, even with them.

 

CF

[faffi]

One said a tankslapper was the bike trying to get rid of the problem - its rider I'm still amazed that the bike calmed itself from such a violent slapper, though

 

Although I fully accept that these things happens because of the rider's inputs, it still remains that there are things to "assist" the rider in achieving these slappers.

 

BTW, the wobble that typically happens between 30 and 60 mph or so if you let go of the handlebars on some bikes definitely will not right itself on itself, but is easily controlled with a steadying hand or two. Could somebody explain the difference between the two?

[Adam06]

That was a pretty wild ride. Love how he just parks it and turns it off, like "Ok, that's enough for now...."

 

Interesting notes:

 

- It started immediately after a gear change, as the front just popped up slightly.

- If you freeze frame it during the slapper, you can actually see just how tight this guy is holding onto the bars. He's strangling the thing!

- It starts to settle down only once he runs off the track. Perhaps with less traction on the dirt it started to push and slide rather than dig in a shake? Maybe the riders hands were shaken loose?

 

Cobie's story is a good example. You can see any number of video clips of a bike and rider way out of control, totally unstable, then as the rider gets ejected, the bike stops shaking, tracks straight, and usually runs nice and stable until it hits something (tyre barrier etc).

 

I remember I rode a coach bike at VIR that had no steering dampner. It shook it's head a little more than bikes fitted with one, but as long as I was loose on the bars, the shaking stayed up the front and never transferred to the rest of the bike.

[ozfireblade]

Thanks for the response guys,

 

Everything that you have said has confirmed what I thought, gripping too tight with the hands not letting the bike sort its self out. I suppose that even with a dampener on that we still need to maintain a nice relaxed feel on the bars. I had omething similar a few years ago riding on the road in the wet, I crossed over the white lines to overtake on the highway and as I accellareated I had the, felt to me, like the biggest tank slapper in the world. The bike bike was going to stops on either side. I sort of reacted the same way and went to let go of the bars and it sorted its self out

[faffi]

In 30+ years of riding, I have actually never experienced a tankslapper. I have experienced plenty of weaves, though, where the bike feels like it has a hinge in the middle, which I presume stems from insufficient suspension damping and/or a chassis that twists itself only to unwind in a snappy fashion.

[Cobie Fair]

In 30+ years of riding, I have actually never experienced a tankslapper. I have experienced plenty of weaves, though, where the bike feels like it has a hinge in the middle, which I presume stems from insufficient suspension damping and/or a chassis that twists itself only to unwind in a snappy fashion.

 

 

Eirik,

 

I think you had posted elsewhere about riding older bikes (and they did/do wobble a bit when you let go of the bars). It happened on my old FZ-750, with a 16 inch front tire, never really did like that bike, partly due to that. I did put a dampter on it, it helped, but think the geometry wasn't quite right. I wasn't paying that much attention to mechanical aspects back then, alignment could have been off, tires, frame not straight, etc.

 

I still never did like any bike I rode with a 16 inch front though!

 

CF

[Crash106]

I'm glad to see this thread here because until I read it, I always thought the technique for handling a tank slapper was to accelerate hard and make the front tire LIGHTER, or get it in the air, so it could reset and settle down. Of course, I wondered about this "advice" because I wasn't sure how you could gas it with the bars slamming around like that. In the video "Speed on Two Wheels," they put a Goldwing on cruise control and whack the handle bars to cause a nasty tank slapper motion, then let the bike settle down--all No Hands! You can see this segment about 1:06 into the clip below. The oscillation starts to settle in just a few seconds.

 

 

I guess the correct response to a tank slapper is a case of doing more by doing less.

[harnois]

Here's a 200mph tank slapper.

The narration and interviews are so rediculous. "Two little divots in the track is all it takes at 185 mph".... yeah maybe if you have a death grip on the handlebars. Notice as soon as he jumps off, the bike restabilizes immediately even with him hanging off the back of it, so he probably could have just let go of the handlebars for a few seconds, but then he wouldn't have had a documentary.

 

Here's the story in writing:

http://www.pureguts.com/sdm.asp?pg=stories&specific=1

notice 7th paragraph, "My reaction was to tighten my grip on the handlebars..."

[ozfireblade]

Here's a 200mph tank slapper.

The narration and interviews are so rediculous. "Two little divots in the track is all it takes at 185 mph".... yeah maybe if you have a death grip on the handlebars. Notice as soon as he jumps off, the bike restabilizes immediately even with him hanging off the back of it, so he probably could have just let go of the handlebars for a few seconds, but then he wouldn't have had a documentary.

 

Here's the story in writing:

http://www.pureguts....ries&specific=1

notice 7th paragraph, "My reaction was to tighten my grip on the handlebars..."

 

 

Yeh the interview is pretty crappy....nice how when he lets the bike do its thing it becomes stable even when hes hanging off the side.

 

That Juan guy sounds like a candidate for level 1

[faffi]

notice 7th paragraph, "My reaction was to tighten my grip on the handlebars..."

 

Still very natural, a very usual SR. Just like it's very usual for people to lock up the brakes in a panic situation or look at the thing they should avoid. With practice, you can learn to overcome your SR, but when something that appears life-threatening happens for the first time, fighting SR is not what most humans are capable of.

[harnois]

notice 7th paragraph, "My reaction was to tighten my grip on the handlebars..."

 

Still very natural, a very usual SR. Just like it's very usual for people to lock up the brakes in a panic situation or look at the thing they should avoid. With practice, you can learn to overcome your SR, but when something that appears life-threatening happens for the first time, fighting SR is not what most humans are capable of.

 

Yeah, but he doesn't seem to realize (based on any of the videos or the written story) that it was a mistake, and probably the cause of the problem in the first place. He talks about having the frame checked out and finding nothing wrong with it, and about the tiny spots of loose surface, and about changing the setup to get more stability, but he never once talks about the real solution: loosening his grip on the handlebars. This was back in 1998 though, I think, it seems so much easier to find info these days.

[stuman]

One of our instructors (Misti) wrote a great article on the subject of tank slappers. I hope she doesn't mind my posting this...

 

 

Ashen and trembling I maneuvered my California Superbike School Kawasaki 636 off the track and into the hot pits, pulling up next to course control and a few of the other coaches. They took one look at me, my white face and shaky hands and asked “what happened?” “Tank-slapper,” I replied sullenly and they all laughed, “welcome to the club!”

 

I was at Reno Fernley Raceway, coaching with the California Superbike School when I experienced the scariest moment of my riding history that didn’t end up in a crash. I was working with a very fast student, an unassuming guy that I never thought was going to be as fast as he was, but he was blazing, and getting faster and faster each session which meant I had to work!

 

The layout at Reno Fernley raceway (it has since been added to and modified) was fun and fast with a smooth front section that flowed really nicely. The back section however, was extremely bumpy in a kind of wavy and rippled way, and all morning I had been a little nervous and tentative going through it. It happened later on in the afternoon, I was sitting parked off of the track near the front straightaway when my fast guy student screamed past. I pulled onto the track quickly and raced after him, closing the gap considerably in the first few turns. When he pulled onto the back straight however, he was still about 10 bike lengths ahead of me and I needed to close the gap further if I was going to observe him ride. As coaches we usually follow our students for a lap or two and then get in front and lead them for a lap or two. I twisted the throttle harder to close in and everything was going along fine until I hit one of the bumps so hard it bounced me right out of the seat.

 

When I came down the bike jerked and bounced and the bars started to shake from side to side, tossing me back and forth with it. I thought I was going down and for an instant, out of sheer panic I clung to the bars and tried to make it stop. Big mistake, the bars started shaking more violently, I was bounced higher and harder in the seat and as much as I tried to hold on to the handlebars I couldn’t. They were literally ripped right from my hands and started slapping into the tank going from lock to lock.

 

I prepared to eject. I was quite literally about to jump off the bike when an amazing thing happened, the bike started calming down and straightening itself up.

I was so excited, and eager to regain control of my machine that I grabbed at the bars again and the instant I clamped down on them they started shaking wildly again.

 

This time I smartened up to what was happening and implemented some of the very basic things that I teach every day at the California Superbike School about rider input. Basically, as Keith Code says in his book Twist of the Wrist II, “riders create more problems than motorcycles are designed to handle.”

 

What he means by that is that the more you interfere with what the bike is trying to do on its own, by holding on too tight, by trying to muscle the bike around, by squirming and wiggling and messing about, the more difficult it is to have a stable bike.

 

Yes, all these thoughts did zoom through my head as I was flailing around on my bike like a stunned chicken, leaving foot long skid marks trailing behind me.

 

I squeezed the tank with my knees, raised my butt a little bit off the seat to help soak up bumps with my legs (like a jockey riding a horse) loosened my grip, made my arms like wet noodles and tried to relax while rolling on the throttle. After another series of wild jerks the bike calmed down and I had regained control. Whew.

 

Then oddly enough I remembered some random comment that a racer had once said to me that at the time had made no sense at all. “If you ever have a tank slapper, remember to pump up the brakes afterwards because it can cause them to not work at all.”

 

I reached for the brakes as I approached the left hand corner at a high rate of speed and low and behold, no brakes! I pumped wildly, got the bike slowed down enough to make the turn, looked up the track to see my speedy student disappearing in the distance, rode straight for pit exit, and into an awaiting group of shark like coaches who thought my whole experience was funny. Funny?

 

So what exactly is a tank slapper anyway?

 

Let’s break it down.

 

A tank slapper is a rapid, high intensity and unwanted motion of the handlebars back and forth. Literally it is the slapping of the bars from side to side that can get violent enough to actually hit the tank of the motorcycle, hence the name “tankslapper.” The bad news is that they are scary as hell and can cause some pretty nasty crashes. The good news is that there are some very effective techniques you can use to handle them.

 

What causes tank slappers?

 

The suspension system on a motorcycle is designed to make the ride more comfortable for the rider and primarily to keep the tires in good contact with the road surface which can include bumps, cracks, pot holes and all manner of imperfections. This system must work while the motorcycle is straight up and down and also during turning when the bike is leaned over, sometimes at very extreme lean angles.

 

In his book, A Twist of the Wrist II, Keith Code explains, “the process of head shake (which can be the beginnings of a tank slapper) begins when the tire hits a ripple and, along with the suspension, compresses. This throws the wheel slightly off-center. When the suspension and tire release, the wheel is light and flicks back toward a centered position, but again, slightly off-center. Still off-center when it loads again from the next ripple; again it is flicked past its centered position. The cycle of flicking back and forth repeats as the front-end seeks to stabilize through this automatic and necessary self- correcting process. Any bike will do it, and what most riders fail to realize is that this shake is a necessary part of the bike’s suspension system.”

 

The little wiggle in the front of the bike is how the motorcycle self corrects and gets itself back on track. Ever see a motorcycle race where something, either a tank slapper or a big slide causes the rider to either be ejected from, or fall off the bike? As soon as the rider is no longer on the bike it wiggles a bit, straightens out, keeps on going perfectly straight until it runs out of momentum and falls over. This is a classic example of how a bike, if left to its own devices will sort itself out. Code mentions that, “based on the amount of wiggling, squirming and overuse of controls most riders exhibit, the bike would, if it could, surely ask them to leave. Riders create instability on their own mounts.”

 

Head shake can be caused by hitting a bump or a ripple in the pavement or it can occur when accelerating hard out of a corner. Hard acceleration can cause the front end to get light or even wheelie which means that the tire is no longer following the road very well, and when it touches back down it can skip or bounce or be off-center, starting off the headshake. Code explains that, “the good news is that if your bike is basically tight (steering head bearings not excessively worn, forks and shock not sticking etc.) the head-shake stays up front and does not transfer to the rest of the bike.”

 

Eventually, the oscillation will die out on its own, unless we interfere.

 

How Riders make the situation worse:

 

Our normal reactions when the handlebars start to slightly shake are to stiffen up on the bars. Keith Code calls this our “survival reactions,” noting that we do not usually choose to get stiff and tight on the bars, our bodies just do it. When we stiffen up the head shake is transferred through our bodies to the whole bike and that is when the shaking can get more violent. Code says that “too tight on the bars is the most common source of motorcycle handling problems.”

 

How to Prevent a Tank Slapper:

 

Knowing that gripping the bars too tight is what transfers head shake through the bike and makes it feel like a ferocious tiger ripping a piece of meat into shreds, we can work to prevent a tank slapper from ever occurring by maintaining a relaxed position on the bike at all times. Practice sitting on your bike with your knees gripping the tank for more stability. Sit back a little further in your seat so that your arms have a nice bend in them with your elbows pointed to the ground and then flap em like you’re doing the funky chicken. That’s relaxed, and from that position you can easily use your legs to lift your weight off the seat a little bit, like a jockey on a horse, so that your butt is not banging down hard on the seat. Think light as a feather, one with the bike, Zen and the art of motorcycle riding……

 

Installing a steering damper is another way to help prevent tank slappers. A steering damper works to limit the travel and intensity of any head shake that the bike is experiencing by damping or soaking up the excess energy. They are necessary on some of the more modern bikes that have aggressive frame geometry, relatively short wheelbases and powerful engines. Dampers are mounted up front so that there is insufficient leverage to transfer shake through the bike. Keep in mind though that a motorcycle with a damper will still shake if you are tight on the bars, so relax!

What to do if you experience a tank slapper:

If you do find yourself in the unfortunate situation of experiencing a tank slapper first hand don’t try to muscle the bike or force it to stop as it will only make it worse. Try to relax your grip on the bars, pinch the tank with your knees and lift your butt off the seat a little bit. Also, don’t chop the throttle as that will put more weight onto the front and potentially make the situation worse. Ideally you want to continue to accelerate if possible to get the weight further to the back of the bike, or at least maintain a steady and smooth throttle.

Popping a wheelie would eliminate a tank slapper immediately because there would no longer be a front wheel bouncing back and forth in an effort to straighten itself out, but I don’t know too many people that could pull off a stunt like that in the middle of a panic situation.

If all else fails, let go. The bike will try to fix itself.

Another important thing to remember is that occasionally very violent tank slappers can force the front brake pads and brake pistons away from the rotors, causing the brakes to go soft or even to fade completely. So, once you regain control of the motorcycle, check your front brakes and if they feel soft then pump the lever a few times until the pressure returns.

Finding yourself in a situation where the motorcycle you are on is suddenly out of control is no doubt a scary predicament. The more knowledge you are able to arm yourself with, the better equipped you are to handle emergency situations, and the more you are able to practice certain techniques (such as being nice and relaxed on the bike at all times) the more likely you will be to actually do it when it is absolutely necessary. It’s a pretty cool feeling to be able to consciously decide to do something that makes a bad situation better.

 

Ride Safe,

Misti Hurst

www.mistihurst.com

[deadsmiley]

I ride a CBR 954 on the track. It is notorious for tank slappers. Bad ones.

 

My second time at Nashville Super Speedway I had some head shake transitioning from the sweeper onto the high bank (yep, it's a NASCAR track with an infield circuit). I just relaxed my grip a bit and continued to roll on the throttle and the head shake went away.

 

This wasn't a slapper, just a little head shake. Still made my eyes bug out though.

 

Kelly