I was watching a motorcycle race last week and one bike went out of control. The announcer was talking about how bad tank slappers can be. While I saw it on TV, it took me back to a few times when it happened to me while riding.
What is a tank slapper? A tank slapper refers to a 4 to 10 hertz oscillation to the steerable wheel of the motorcycle.The cause of the tank slapper is difficult to assess but can be prevented with a proper motorcycle set up.
No matter how you look at this topic, experiencing a tank slapper is no fun and actually scary. Let’s try to demystify this phenomena.
How Common Are Tank Slappers?
It seems tank slappers pop up like unpredicted thunderstorms on a beautiful sunny day. You are riding along and then suddenly, there it is. Bars whipping violently back and forth.
Many of the videos where I see this, the bike is on the road or racetrack moving at high speeds.
This is not true all the time. There is a video of a Harley going down the highway and then suddenly, the bike’s front wheel is whipsawing, it flung the rider over the handlebars, then both are sliding down the highway.
My point is there does not seem to be any limitations on the type of bike on which the tank slapper can happen. In my case, it happened on a dirt bike several times.
After doing some reading, it seems tank slappers can happen also on bicycles. This was a surprise to me. Thank slappers should happen less on mountain bike and dirt bikes, because of knobby tires and suspension.
The occurrences I had with the dirt bike did not fit some criteria of the tank slapper.
What Causes A Tank Slapper?
There seems to be as many causes for the tank slapper as there are people who have an opinion on them. I am more interested in the technical aspect of tank slappers so I can work on ways to minimize them.
| Tank Slapper Cause | Note |
| Out of round wheel | Feels like the out of round bicycle wheel as a kid. |
| Traction & aerodynamic asymmetry | The load of the bike has been transferred to the back wheel |
| Potholes | Watch for these during the spring and early summer months |
| Out of balance rims | Look for wheel weights that may have fallen off |
| New painted lines on the road | Be careful of the difference in traction between paint and pavement |
| Coming down from a wheelie | Don’t wheelie at high speeds |
| Suspension Settings | Take the time to adjust it for your weight |
The technical description of the tank slapper is a 4 to 10 hertz oscillation to the steerable wheel of the motorcycle. Upon seeing this description, I could not remember what a hertz was other than it was something from my high school physics class.
A hertz is one cycle per second. There is an electrical connection, but for our purposes we are looking at 4 to 10 oscillations per second. That is quick.
Out Of Round Wheel Causing The Tank Slapper
As a kid, did you ever ride a bicycle with an out of round wheel. Maybe you even rode a motorcycle with an out of round wheel. Remember the feeling it created while going slowly and then speeding up?
As the speed increases we put a cyclic load onto the bike’s frame. This load them matches the resonant frequency of the bike creating the speed wobble.
The natural force of the bike is to stand up. AKA the gyroscopic force, keeps the bike up. The frame and lateral forces counter the frequency pushing the front wheel the opposite way. The wheel goes too far and the front end swings back again.
The best way to understand the resonant frequency is by watching this famous clip of a bridge falling apart. The wind started blowing the bridge around and the resonant frequency was found. Once the frequency was reached the bridge destroyed itself.
In our case, it is not like the bike will blow apart when reaching the resonant frequency. I want you to understand upon reaching the frequency, the front end will continue to slap back and force until an outside for changes things.
Traction and Aerodynamic Asymmetry
A second explanation I found on the tank slapper relates to traction and aerodynamic asymmetry. Usually these happen at high rates of speed and found more on the track or raceway.
I may get this wrong to some extent, but here it the way I understand the scientific explanation.
The load of the bike moves forces or “weight” from the front tire to the back tire. This lessens the effect of the front tire contact patch. The steering axis attempts to correct itself creating a torque in the opposite direction swinging the front end the other way.
This continues, but the swinging gets worse until you have a full-fledged tank slapper.
Now that we have talked about the technical causes of the tank slapper lets put a few things out there to which riders can relate.
Potholes Causing Tank Slappers
I live in the midwest and hate riding in the city or on the highways around the city during the spring and early summer months. Potholes are everywhere.
Some of them are huge and unavoidable. It is not uncommon to see three or four cars on the side of the road with flats from where they hit a deep pothole. Imagine what it would be like hitting one of those things on a motorcycle. It must feel like a failed motorcycle jump.
During a ride this spring, I rode a slalom course while going down the road. Traffic picked up, and I had to pay more attention to traffic than each specific pothole on the road.
Our speed picked up, and I hit a big hole. My handlebars started osculating, but luckily they stopped as traffic slowed down again.
There are two points to take away from this experience on how potholes can create a tank slapper.
First, as you just read from my experience the hole created a moment with a light contact patch on the front wheel. This was all it took to start the making of a tank slapper. This point relates to the suspension settings that we will get to later.
Second, these potholes can bend your rims making the wheel out of alignment. A while back my dad bought a bike and hit a pothole one day that bent his rim. He did not have time to take it to the shop to get it fixed.
For reasons I don’t remember, I had to ride the bike to the shop. The ride was like the one I described earlier with an out of round rim on your bike. (I have one of those now on my mountain bike and it sucks.) This ride was much worse do to the increased mass of the bike rim. Long story short, I made it, but with a lot of anxiety.
When you hit a big pothole, make sure you pull the bike over to check out your equipment.
Out of Balance Rims
If you notice the rim is bent, it may make sense to have the bike trailered home or to the shop. When this is not possible, do not get on and ride like everything is normal.
Ride on the shoulder at a low speed to see how the out-of-balance rim affects the handling. This allows you to get a feel on how fast you can safely travel. One of my mistakes from riding my pop’s bike was not riding slow enough for a longer duration.
Losing wheel weights is another way your rims become out of balance.
Last year, I had the tires replaced on my bike. When I got it back, those ugly wheel weights were on the back rim. Luckily I did not have any on the front.
If you have weights on the front, make sure you add an inspection of them as part of your pre-ride checklist. Not only should you make sure to visually see them, but touch them to see if they might be loose. It’s better to catch them now versus seeing them flung off while riding at 70 mph.
Painted Lines Causing Tank Slappers
How many times do you sit in traffic slowly going down a busy street and do not understand the cause for the backup? One of my favorites? You drive to and from work on the highway 5 days a week and wonder what the construction crews are doing.
As for sitting in traffic? You eventually get by the slow moving truck who is laying down new paint.
Have you ever ridden across this new paint and felt the motorcycle get wobbly? This can be a cause for a tank slapper.
Not because of aerodynamics, an out-of-balance rim, or pothole, but a change in the friction between the road and tire.
The sudden slick surface can make the tire slide across the painted area. As the tire faces the increased friction of the actual roadway again, the tire gets pushed beyond the standard straight position.
The bike frame and steering geometry try correcting the situation only to have it overcorrect and the oscillating begins. Before you know it, the tank slapper is in full force.
Pro-Tip: When riding and scanning the sides and roadway in front of you, pay attention to the lines. Are they nice and shiny or look like new paint? If so, be careful.
I also pay attention to the lines in the middle of the road. The dashes for passing can also be slippery.
Coming Down From A Wheelie Can Cause A Tank Slapper
We all see those crazy guys who wheelie down the road. I thought it was cool until I got my family.
Riders bring a wheelie down and start a tank slapper. The difference in the front wheel spinning, what it lands on, or having the handlebars turned slightly can all affect the conditions creating a slapper.
Not to mention landing the wheelie hard so it bends the rims or fork tubes, these can create slappers.
Just don’t do wheelies on the street. Do them in a controlled environment and at a slower speed. (Now I just sound like a grumpy old guy. lol)
Improper Suspension Setting Causing A Tank Slapper
Your bike suspension provides more than a smooth ride. As odd as it sounds, it is an important element in giving you traction.
You probably think I am crazy on the second point, but follow me for a second.
When your tires do not contact the riding surface, you don’t have any traction. When the tire has excessive force put on it from the compression and dampening, the contact patch will also have more and less pressure. The contact patch may even get bigger or smaller depending on the suspension movement.
The pressure change and contact patch change all affect the amount of traction you get. When the suspension is not tuned correctly, the rapid changes in the suspension create the favorable conditions for a tank slapper.
Tank Slapper Recovery On A Motorcycle
The first and best thing to do in addressing the tank slapper is prevention. Take the time and properly maintain your bike. Most of the root causes listed above can be significantly reduce the chance of a tank slapper with routine maintenance and pre-ride checklist.
It does not require taking the whole bike apart or large amounts of mechanical knowledge. What you cannot fix should probably be taken to a mechanic anyway, like the suspension.
Removing Mass From The Bike
Aside from maintenance, we add gear to our bikes adding weight and changing the balance. You may try removing any weight you added to the bike and see if any wobbles go away.
Steering Dampener
Many of the articles you find on the internet tell you to put a dampener on the bike to prevent the tank slapper. This may not be a good idea.
When the cause of your problem is one of the above items. The problem gets masked and in fact may put you in more danger.
If you think it fixes the problem while riding at a low speed, you are likely to increase your riding velocity at some point. The dampener cannot mask the problem at every speed. There comes a point the tank slapper will return.
Stopping A Tank Slapper In Progress
The tank slapper gremlins may still show even when you’re riding safely. The best way to try to stop it is leaning forward. You move the center of the bike mass forward putting more weight on the front wheel increasing traction and naturally increasing dampening.
You may also look at changing the speed of the bike by slowing it down. This also changes the frequency of the front wheel to reduce the chance of a tank slapper get worse.
Slowing the bike down does not mean jamming on the brake. A sudden change in speed like that can make things worse or induce a worse slapper. Slowly change your speed.
Now it is time to get out there and go for a ride.
