Rear Flat Tire induced Tank Slapper

Discussion in 'The Perfect Line and Other Riding Myths' started by stefer, Jun 14, 2013.

  1. stefer

    stefer Been here awhile

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    Hi,

    I've done a few searches to no avail. I'm hoping to gain some insight from the responses.

    Anyways today I was driving on the interstate (~65mph) on my 06 KLR650. I started getting a weird vibe about how my bike was handling. Something felt a little loose. So I gave a mild blip of steering input to initiate a mild countersteer, just to kind of test and see what was going on. What resulted was a mild tank slapper which was gentle and very easy to control. I gradually slowed down and pulled onto the first exit ramp and stopped rubber side down :clap (this entire process probably happened within a quarter mile -- the description may make it seem longer, but my decision making was very rapid). When I was driving it felt like my front forks were kind of twisting around. Other people on the internet are describing it like they were sliding on ice -- however they are also describing much greater loss of control

    Anyways, it turns out my rear tire was flat. But I am curious -- why would a flat rear tire cause a tank slapper? What is the theory behind that? I'm really glad I didn't get hurt -- reading some horror stories online now.

    Worth noting: I had 40 lbs of gear on the cargo rack, but I'm light so me and my gear weigh ~200lbs. Also, this was my first rear flat tire while driving (had others leak overnight and I've had front tires go on me) and my first flat tire at interstate speed.

    Any ideas on the theory behind this? Again Google searching didn't help. This is a fairly common problem...someone should know, right?





    PS: while I was on the side of the highway I had three bikers offer me help. It is really cool to be a part of this brotherhood, so to speak.
    #1
  2. joexr

    joexr Banned

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    The tank slapper part makes no sense. Normally countersteering a little with a rear flat would just make the rear kind of go out to the side a little opposite the side of the bar you pushed forward.
    #2
  3. larryboy

    larryboy Chopper Rider

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    Rear flat tire drops the back of the back of the bike which increases the rake and trail of the front end...hold on, it's tank slapper time.
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  4. LuciferMutt

    LuciferMutt Rides slow bike slow

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    If it was "gentle and easy to control," it wasn't a take slapper.

    What happened to you was a weave, caused by the rear of the bike riding too low in comparison to the front. The normal invisible gyroscopic procession process that keeps that bike upright at speed gets slowed down too much resulting in a nasty weave. You were probably also feeling the back end of the bike floating side to side at the same time resulting in the "twisted" feeling at the forks.

    Glad you got stopped OK. Why'd the tire go flat?
    #4
  5. lnewqban

    lnewqban Ninjetter

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    What happened to you is normal for flat tires at speed; however, those 40 lbs of gear on the cargo rack magnified the head shake.

    Any mild tank slapper is an oscillation; hence, it has a pivot point and a mass.
    The pivot point is the steering column, the more and farther form the pivot the mass, the stronger the oscillation.
    The tires and our arms are the dampers that help keeping that oscillation at bay.
    Normally inflated tires are more rigid and flexible (good damper), while flat tires are not.
    Probably the same would have happened without a flat tire if you had removed your hands from the bar.
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  6. stefer

    stefer Been here awhile

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    Perhaps it wasn't a tank slapper. It may have been was a weave (as suggested below) or something else -- but the bike was certainly oscillating back and forth and not moving off to the side.

    I can see this, but would changing the angle by an inch or two make that much of a difference in handling?

    This explanation makes sense to me, however I'm curious -- what would cause the balancing process to slow down?

    Not sure why the tire went flat. Will find out Monday (first chance I get to fix it).
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  7. stefer

    stefer Been here awhile

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    A tank slapper has the pivot point at the rear wheel's contact patch with the yaw (and some roll) developing instability, right? I think we are on the same page, just different meanings to the phrase "pivot point."

    Before this I had 30 miles in on the highway, as far as I could tell handling was perfectly normal, except for weak gen 1 KLR braking which is exacerbated with extra weight.







    For the record if I am coming across as argumentative, that is not my intention. I am getting a number of good ideas and am trying to better understand them.
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  8. stefer

    stefer Been here awhile

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    I think LuciferMutt's explanation makes much sense. Look at this wikipedia animation of Weave: Weave Animation

    Vs the wikipedia animation of a slapper: Tank Slapper Animation

    When you look at these two animations, notice the rear wheel lateral movement associated with the Weave. Based on that it would seem that steering input cases side forces on the rear wheel. Since the wheel is flat, it kind of floats to either side, then the motorcycle compensates and an exacerbated weave results.

    I think this is a very reasonable explanation.
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  9. Jailor8

    Jailor8 Long timer

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    I would say a wobble or weave is a better term, just really loose in the back, squirmy.

    I have a 05 KLR and was being told by my butt and brain that something wasn't right as it was my first moving rear flat also. 2 lane highway headed for the Interstate at night. Weird part is I would get a bit of a squirm or wobble, just enough to say what was that?? And it would go away, played that game for a mile. Stared up the overpass so I could go down the on ramp and when my speed dropped to about 50, I started wobbling and to recover (I call it a recover, at the time that's where the bike wanted to go and stay up right!) I had to leave the highway to the right and cut the corner on the ramp into the grass down a slope and back up at a angle. No doubt what was wrong in the grass you could feel the tire flopping on the rim and the side angle of the slope I was on didn't help. Made it back to the ramp shoulder ok. Luck would have it I didnt have my compressor or spoons (short trip of about 60 miles total) called my son 6 miles away and said bring my spare rear rim and tire out here to me. Pull axle, push axle and back on the road!!:deal

    Glad you landed yours ok too!!!:clap

    jailor8
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  10. larryboy

    larryboy Chopper Rider

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    Yes, on top of the weirdness of riding with a flat tire.
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  11. joexr

    joexr Banned

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    This backwards until you get towards chopperesque rakes.
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  12. trc.rhubarb

    trc.rhubarb ZoomSplat!

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    The only thing that had anything to do with what happened was a flat tire.
    You had a loose rearend which caused a slight weave more than likely exacerbated by stiffening up and transferring more to the bars.

    The 40lbs on the back didn't make it happen. How to fix it in the future? Pay attention to your gut and remain calm,

    Tank Slappers require strong changes in the input to correct, bars can be ripped from your hands. Watch some racing videos to see how crazy they are.

    Here's a good short video of a tank slapper http://www.youtube.com/watch?v=LZ1srcQMa_0
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  13. lnewqban

    lnewqban Ninjetter

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    Like with airplanes, the yaw happens around the combined CG of the bike+rider+cargo.
    The steering column is a hinge of the mechanism.
    Without that hinge (let's weld the bearings temporarily), the oscillations would be very limited (and stability impossible).

    I find the explanations in this video very interesting:

    http://www.youtube.com/watch?feature=player_embedded&v=fvsDIq3WwVA

    With the bike in a vertical position and on both tires, grab the cargo rack and shake it sideways.
    Observe how the oscillation of the frame ends up oscillating the front tire and handlebar.

    http://books.google.com/books?id=rJTQxITnkbgC&pg=PR1&lpg=PR1&dq=Motorcycle+Dynamics+%28Second+Edition%29&source=bl&ots=DYdOEbaNwD&sig=DgYmBjiYesMSXdDfAL59hy-QLNc&hl=en&sa=X&ei=TERdT4rPM4SXtwfOhMnjDg&ved=0CGEQ6AEwBg#v=onepage&q=Motorcycle%20Dynamics%20%28Second%20Edition%29&f=false

    There is a complicated leverage (horizontally and vertically) among the trail of the steering geometry, the axis of the steering column, the CG of the bike, the rear contact patch and the mass on the cargo rack.
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  14. larryboy

    larryboy Chopper Rider

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    You've never followed a lowered KLR with luggage on it, wiggler, take off the lowering link, no more wiggler. Drop the rear end four inches with a flat and you'll be wondering if it's gonna go full tank slapper because it'll wiggle pretty bad.
    #14
  15. joexr

    joexr Banned

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    We were talking tank slappers before.:deal
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  16. bigdog99

    bigdog99 CJ's bitch

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    The bike walks across the flat tire carcass back and forth. Because the rear contact patch is not "fixed" any longer, the corrective countersteer is a delayed reaction which quickly becomes an overreaction, which feels like a mild "tankslapper". Riders who grew up with the early tube-tired heavyweight bikes in the '70s got to know that initial sink then weave and to quickly react. Most of us had the toes beveled off of our Red Wing work boots from dragging them as outriggers until we could get the whole mess stopped.
    #16
  17. corndog67

    corndog67 Banned

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    Larry's reply to the OP is backward. If the back of the bike is lower, less tankslapper, more straight line stability.
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  18. joexr

    joexr Banned

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    .
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  19. LuciferMutt

    LuciferMutt Rides slow bike slow

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    To a point. Eventually, (especially with heavy luggage hanging out in a top box), the geometry gets so relaxed, that normal gyroscopic procession doesn't function right anymore and the bike goes into a weave -- it literally can't hold itself up anymore. That's why I was saying it's not really a "tank slapper" per se...

    BTW, tank slappers often happen when a n00b rider hammers the throttle WFO, and hangs on to the bars tightly, while all the weight goes to the rear wheel. The front tire can't do it's think right anymore, and goes all over the place.
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  20. Bill Harris

    Bill Harris Confirmed Curmudgeon

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    And the front-end shimmy/wobble may be the bike telling you to check the head bearings, wheel bearings and swingarm bearings. Above and beyond the rear flat and the uber-loaded topbox.

    --Bill
    #20