Tank slapper

[Rough Edge racing]

 Have any of you had a front end wobble or what's known as a tank slapper? What type bike and what was the bike and rider doing when the problem came on.
  A group of bikers was discussing this and it seemed to happen to just about any bike if the conditions were just right... and sometimes there seemed to be no reason for it. There was no agreement as to the causes...
 I never experienced a true tank slapper but have a few swiggle their heads after unloading the suspension while cresting hill at high speed...

[Aaron D.]

I have never owned a bike that did it, but I have ridden a few.

One in particular was a Suzuki 500 twin, the 2 stroke. Taking hands off the bars caused a massive wobble. Oddly my brother's 500 didn't do this.

[RayB]

I never had a full tank slapper, but I did have a high speed wobble ( the precursor of a tank slapper ) on my BMW. I made a pass around a semi on an interstate at around 80 to 85 mph with the bike loaded with camping gear. There was also about a 30 mph quartering tailwind. As I broke through the truck air blast and was moving into the lane ahead of the truck she went into a wobble ( the BMW rubber cow ) and head shake. Scared the sh*t out of me. It eventually stabilized ( or I wouln't be here to talk about it now )

When I arrived at the BMW rally I was headed to, we talked about it and the first thing people said was "ah your steering head bearings are to loose" - nope. Never did figure out the cause. I did, however, put new tires on the bike a week before this incident and the replacement tires have a more round profile on the front and, to be honest, I was running pressure higher than normal

When I got home I spent a lot of time researching the subject and I found that yes, under the right conditions, and two wheeled vehicle ( bicycles included ) can go into a wobble and possibly an unstable wobble that would be called a slapper and down you go. Look for tank splappers on You Tube. There's a few clips on this subject. There's also one about Harley baggers going into the death wobble at speeds around 40mph.

[redrider]

Here ya go: https://youtu.be/z3OQTU-kE2s

[LeakyLogic]

When I was a young adult I became aware of the importance of proper tire pressure. Tank slapping happened to me while riding a CR250. On acceleration at approximately 50MPH on an irregular dirt surface the handlebars suddenly oscillated from one side to another. This back and forth movement increased so fast that it caused me to really clamp on the grips. There was no stopping it by physically overpowering it. My grip on the throttle cause a sort of whiskey throttle that increased my speed and tank slapping motion. All this time I continued to go straight. I was able to recognize that my increase in speed was making it worse and focused on trying to release the throttle without losing my grip. I was able to slow down. I actually don't know if I was truly able to release the throttle to end the condition, the bumps on the ground changed, or drag from my rear brake finally got grip. This all happened for a few seconds at about 50-60MPH.

After I calmed down,I realized it was caused by too much air in the front tire because the front end seemed to bounce instead of absorb the bumps prior and during the event. I severely overfilled it and will never fill a tire by feel again. I learned the hard way.

[Gliderjohn]

In the late 70s on my Suzuki GS400. I had not owned the bike long and had a local dealer put on a new front tire for me while getting some other service done. Later I passed a car between 60-70mph. When I leaned a bit to enter back into my lane the bike went into a full blown tank slapper. Somehow kept it upright and was able to stop successfully. Started checking things and found all four fork pinch bolts loose! Never since then thank God. It does get ones attention.
GliderJohn

[RayB]

The first time I saw the results of a real tank slapper ( not a high speed wobble ) was in the late 60s at Santa Fe Speedway outside of Chicago. Guy on a sportster was coming out of a turn in front of the stands and went into a real slapper-the kind you can't control. He went one way and the sportster went airborne.

Bike wrecked and he had a broken leg and arm

[Sasquatch Jim]

  I have had many occasions of beginning slappers that I got under control.  The worst one was on a 650 Triumph during a TT race long ago.  It was a full on slapper that took me right out of the competition although i did not crash.
 Having owned and ridden sidecar rigs since 68, I can say that the tendency is much greater with them than with solo bikes.
  With solo bikes, the most common cause is a bit of slack in the steering head bearing and tightening it up just a bit usually fixes it.  There are many other things that can be a cause but that is by far the most common.
 On my Triumph in the race, it was a loose rear axle nut.

[RayB]

Another common cause is a squared off rear tire

[Gliderjohn]

Quote from RayB:

Another common cause is a squared off rear tire

Ah crap! I live in Kansas! Almost always end up with a squared rear tire before it is really wore out. Never had nary a wobble, guess knock on wood.
GliderJohn

[Sheepdog]

I have never owned a bike that did it, but I have ridden a few.

One in particular was a Suzuki 500 twin, the 2 stroke. Taking hands off the bars caused a massive wobble. Oddly my brother's 500 didn't do this.

Funny that you mention the T500...mine would do the handlebar-hula on occasion, also. Heavier fork oil and a set of S&W shocks improved that bike a great deal...

[Dick]

Yep, had one while accelerating hard on my Yamaha XS1100 right after changing tires. Hit a small deviation in the road surface doing about 90MPH and wham, just like that the bars were slamming from stop to stop. Lasted about a second or two and it ended. Felt like minutes. I thought hitting the pavement was inevitable, but not. I honestly believe that while accelerating hard I may have been holding the bars too tight and that may have contributed. I went back to the spot and could see the rubber skid marks left in the road from the front tire. After that I never held a death grip on the bars and never owned a bike without a steering damper installed. 

[steven c]

 On my CB550 Honda right after I picked it up from having something done to the front wheel( i can't remember ) at about 65 full on tank slapper, rolled off the gas and got it back, Started doing most of my own work after that.

[RayB]

My conclusion on the tank slapper bit is that there are too many variables to boil this subject down to a few. Suspension, rider error, tires, road conditions, chassis design all enter into it. Our ignorance on the subject makes one re-think the risks we face riding motorcycles.

[Kiwi_Roy]

I had a low mileage Goldwing almost throw me off at about 90, I put it down to the screen vortex shedding.
The Goldwings had a heavy weight fitted to the steering column behind the headlight, I think it was to avoid tank slap




Ray B,
         Yes I did a few times but I was always nervous about it.
The bike was practically new when I sold it, about 4K miles
I found a picture of the weight, lets see if I can't post it.

Note the warning notice which says

WARNING
Do not remove this weight when riding with faring to prevent an accident
Without this weigh steering inertia moment will be reduced resulting in
loss of control and motorcycle stability

IMHO the California II of the same year is light years ahead of the Honda in the handling department.
The Honda on a bumpy sweeper was like riding a garden gate against the Guzzi riding on rails.

[RayB]

Kiwi Roy

Did you ever take it back up to 90 after that? If so, did it wobble again?

I wonder if fairings don't provide a little lift on the front end and unload the front tire some at certain speeds. Honda's weight on teh steering stem could have been a preventive measure against lift.
I have a WindJammer on my R100 and was thinking about lift...I don't have any aerodynamic data and I doubt if Mr. Vetter ever ran wind tunnel tests.

[drlapo]

Worst one was on a Hodaka Combat Wombat
Full throttle on an abandoned rail line
Lock to lock with the back wheel stepping out 4 feet +/- to each side
It was massive fun

[Guzzistajohn]

I've had it happen a few times rolling downhill with no throttle with my hands off the bars. It'll wake you up pretty quick. Take you to oh......pucker factor 6 of 7.

[Arizona Wayne]

I've had a tank slapper or 2 but don't try to diagnose why.  When it happens my dirt track riding experience comes into play to live to ride another day.  I think it's when for a moment your whole bike gets light and somewhat airborne  for whatever reason.  Or if you ride over something slippery upright you can go into a TS.  If TS happens to me repeatedly then I will look for reasons why.

Headshake is a different phenomenon and can happen to any MC front end at a certain speed for your bike/weight distribution when coasting.  When operating a sidecar rig you're dealing with headshake a lot because your rig is not balanced.

[DaSwami]

Never had anything remotely like that happen, not even a wobble. 

I understand the phenomena is replicated on the front wheels of a shopping cart, you know when you push the cart and the wheels wobble rapidly side to side.  There is "zero" steering angle on those wheels, so it makes sense it would be worse with bikes with sportier front steering angles

[redrider]

"My conclusion on the tank slapper bit is that there are too many variables to boil this subject down to a few. Suspension, rider error, tires, road conditions, chassis design all enter into it. Our ignorance on the subject makes one re-think the risks we face riding motorcycles."

Well said, Sir.

[Rox]

My conclusion on the tank slapper bit is that there are too many variables to boil this subject down to a few. Suspension, rider error, tires, road conditions, chassis design all enter into it. Our ignorance on the subject makes one re-think the risks we face riding motorcycles.

Best explanation yet.

[drums4money]

My conclusion on the tank slapper bit is that there are too many variables to boil this subject down to a few. Suspension, rider error, tires, road conditions, chassis design all enter into it. Our ignorance on the subject makes one re-think the risks we face riding motorcycles.

+1
Had that wonderful phenomenon occur on the MV.  Before attempting to mask the problem with a damper, I went for a basic setup.  Stock settings were more appropriate for a 5' jockey rather than a 6' 185# mouth-breather like me.  Made an impressive difference. 

http://www.europeancycleservices.com/

[canuguzzi]

Kiwi Roy

Did you ever take it back up to 90 after that? If so, did it wobble again?

I wonder if fairings don't provide a little lift on the front end and unload the front tire some at certain speeds. Honda's weight on teh steering stem could have been a preventive measure against lift.
I have a WindJammer on my R100 and was thinking about lift...I don't have any aerodynamic data and I doubt if Mr. Vetter ever ran wind tunnel tests.

Vetter included instructions to mount their fairings so that at a certain speed, there was actually a down force rather than up lift, at least for the Quicksilver fairings. He even referenced the amount the fairing should load the suspension so you could measure it.

[POP0404]

Once on a Yamaha 1700 cruiser. Just coming out of a slight downhill bend. No warning it just happened. I never discovered the cause, (but suspect the front end slipped a little) I rode the bike for 120,000km's and it only ever happened once. I would rather not experience it again.

[guzzisteve]

The only time on a Guzzi was on an 85 LM 1000 w/16" wheel, stood on all brakes and it subsided once I got back down to 90mph. Thought I was going to hop off for a few seconds

[JBBenson]

I have never experienced one, but I always understood that you were supposed to hit the throttle to settle a tank slapping event.

Reading this thread it seems that most hit the brakes and came out of it. Maybe coincidence?

Hmm.

[oldbike54]

 Most real tank slappers , as opposed to weave , happen so fast the rider is only along for the ride .

 Dusty

[charlie b]

I heard horror stories about the dreaded 16" wheels on mine.  I can induce a 'wobble' if I load it up on the rear so the front end unloads.  It is kinda mild and only happens if I take both hands off the bars.  With the bike loaded 'normally' it doesn't happen at any speed (tested up to 115  :)  ).

I did have one of those 'moments'.  Fast sweeper with tire ruts.  I took it a bit fast so I drifted to the outside.  As the front wheel 'hopped' over into the outside tire rut (unloading the front tire almost completely) it started a really bad wobble.  I had just started to wonder how bad it was going to hurt, going off into rocks at 80mph, when it calmed down and just carried on through the rest of the corner.  I did nothing to correct it.  I froze in position.  I just let the bars do their thing.  I think I let go of the throttle, I did not apply brakes (have heard conflicting stories about that method of correction).  My assessment was that as soon as the front end loaded up again, it went back to a stable condition.

PS if you watch race bikes, when they crest a hill and the front end comes down just a little off, they get a good slapper going.

[LowRyter]

https://www.youtube.com/watch?v=LW0UAgzntao

this would be better if he fell on his ass.


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