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Featured Tank Slappers and Speed Wobbles – What’s the difference?

Discussion in 'General Motorcycling Discussion' started by Goldenberri, Apr 29, 2015.

  1. #1 Goldenberri, Apr 29, 2015
    Last edited: Apr 29, 2015
    I recently bought my second bike, a lowered VTR250. The previous owner was almost my exact weight and height so it was an obvious choice, since the bike was configured to suit her.

    Both tyres, although road worthy, were squared off and I had the front tyre replaced almost immediately. I mentioned to my mechanic that it felt ‘light’ in the front. He agreed and said that it was too low in the back and I should really have it adjusted. He asked that I not wait too long to get the adjustments made.

    The next day, I missed seeing a deep depression in the road. Not a pot hole as such, as it had bituman in it. It was close on dusk and I had just scanned for kangaroos. I hit it at 100kmph and ended up with what I called the ‘death wobbles’. I came out of it okay, and kept riding, keeping watch for any unusual vibrations. I checked the bike when I got home and there was nothing obvious. The next day I booked it in for the work. New rear tyre, suspension adjustment and a few other odds and sods and a thorough inspection. As my mechanic said, there’s nothing we don’t know about it now and it should be fine.

    Our road, although black top, has these annoying ripples at certain intervals, almost like outback corrugations and I’ve had previous experience with a loaded trailer that produced enough bounce to bend the axle. The other night, I was hitting it a bit harder than usual on the bike and got a similar reaction to my first wobble, but not as bad. Again I came out of it okay.

    So I’ve been researching the dynamics of a ‘tank slapper’ if this is what it is and understand why it happens. Obviously I was at the ‘critical speed’ required to induce the imbalance.

    As a learner, I’ve been unlucky enough to have had two in as many weeks even though the bike is much more stable now. I’ve searched the forums, but there is no existing thread on the subject. At the very least I’m hoping that this thread will add to the knowledge base.

    My question is…..how common are they? Should I have the bike raised back to standard? Is it my bike or is it the road?
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  2. Are you American ? Tires, tarred......
    Can't comment on the wobbles, never had one.
  3. There you go! All fixed McsennaMcsenna
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  4. Was it similar to this?
  5. BjpittBjpitt Halfway through watching it. It's was a front end wobble, certainly not a 'weave' as depicted but both times I was at 100kph and had both hands on the bars. No fancy stuff.
  6. Hey Goldenberri I wont plagiarize but if you google speed wobble Wikipedia has a pretty good explanation
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  7. #7 Goldenberri, Apr 29, 2015
    Last edited: Apr 29, 2015
    Already done that Duke at EeeDuke at Eee but I still want to know if my modified bike is contributing to the problem or whether it is SIMPLY a matter of road surface. I'll own the first one....I wasn't as vigilant as I should have been. But the second one has me flummoxed...edit...because it was raised slightly in the back and cleared mechanically.
  8. Tank slapper, speed wobble are the different names of the same thing. At least that's what I thought (I'll stand corrected if that's not the case).

    It occurs primarily because of the front wheel being turned even a touch in certain situations, such as putting down a whillie or accelerating pretty heavily over a rise (like in the video below, after the first corner):

    (The mistake here is turning and accelerating over the rise; it still can be done with a few techniques).

    It's a bit hard to get a tank slapper from just cruising in a straight line unless you hit something (which you did).

    In that case it may wobble a touch but should settle with no problem unless you fight it.

    The worst thing you can do is to stiffen up and try to fight it. You'll only make it worse.

    That makes me think that you may have a death grip with your arms.

    If it's not the case then it's extremely hard to tell exactly what's going on and would be better to see what exactly you're doing.

    I highly doubt that even big geometry change on your bike will significantly contribute to that and the first thing on my mind is you being tight on the bars.

    If you can eliminate that, then look further. But do it first.
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    • Informative Informative x 1
  9. #9 BENNY THE JET, Apr 29, 2015
    Last edited: Apr 29, 2015
    Should be 100 percent sure your lowered suspension isn't your problem:nailbiting:
    DON'T JUST LOWER ONE END-Maintain The Correct Chassis Attitude
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  10. I'm thinking along the same lines as dimadima If you have a death grip on the bars, your fighting them just exacerbates the issue. Did you experience this in the 125? Or does your son experience same thing on same road?? You need to be really relaxed on the bars. When I approach things like railway lines for example, I'll almost let go of the bars, and just let the bike do what it wants as it goes over the bumps and crap.

    Good Luck working it out.
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  11. I'd say there's every chance it's the modified suspension height. Headshake can often be attributed to the rear shock being too soft allowing the rear to squat too far, shifting too much weight rearward, unloading the front tyre, causing headshake.
    If your bike is already lowered in the rear, the front is already unloaded as the weight has been shifted rearward. You've unloaded the front tyre, before you've even ridden out of the garage.
    When you accelerate, or hit undulation, either of which loads the rear shock, you have even more weight being transferred rearward... Far from ideal.

    You could try stiffening the rear suspension to counteract it, but you'll only be hiding the problem.
    You could also try raising the front forks in the triple (to lower the front) to shift weight forward, but that changes the rake and trail which could lead to other unsavoury handling characteristics.

    I wouldn't suggest doing either of these unless you are prepared to pay a geometry specialist to set the bike up for you.

    If I was you, I'd return the suspension to standard.
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  12. I think the difference is a wobble the front end shakes, a full blown tankslapper the whole bike gets into it. Something like this I found on advrider

    I've only had one, many years ago at 160 on the Hume which I think was exacerbated by a chain on its last legs. Very very much scarier than a wobble. I rolled off gradually and rode out of it but it's not something you forget. But not as scary as that video looks!

    I was thinking about this reading the most embarrassing moments thread, I'd just overtaken 2 Harleys and they cruised back past me before I'd completely recovered - the weave continued till I got down to about 80.
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  13. ^^ That is goddamn scary....
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  14. A tank slapping speed wobble gets your freckle puckered up. Seriously same thing but a tank slapper is when the bars slap the tank side to side. The whole bike will carry on and it can sometimes try to murder you
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  15. #15 Goldenberri, Apr 29, 2015
    Last edited: Apr 29, 2015
    This was not the case. I was extra light on the bars. That, and being a featherweight helped save my bacon on the first occasion. I researched that the heavier you are the worse it can be. So no. I did not stiffen up, I did not fight it and I did not have a death grip.

    BENNY THE JETBENNY THE JET I will ask my mech again, but he evened the bike up. It's still lowered, but I'll report this last incident again and talk about restoring the bike to standard. Thank you.

    No you don't forget it which is why when it happened a second time, I followed protocol and I was more pi$$ed off than anything else. Interestingly, in the video it looks like he was fighting the bars. I raised myself off the pegs on both occasions.

    RRdevilRRdevil .......yep, but it's not going to stop me riding.

    Thanks gents. Back to the mechanic to be on the safe side, but I think (hope) the second time was due to road surface corrugations.
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  16. Sheeeit I so hope I never have that happen whilst I am on the bike! :arghh: reckon I would ditch and let it sort its self out!!!
    but have done a similar death wobble thingo on a pushbike when going hell for leather down 'heartbreak hill' at Bundanoon heading down into Morton Nat Park...smart arse moment turned into sore arse moment very very quickly...you can still hear my swearing echoing in the forests down there...:woot:
    no helmets in those days either so I was very lucky...
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  17. they are even more fun on dirt ... on a road bike ...
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  18. #18 Zim, Apr 30, 2015
    Last edited: Apr 30, 2015
    If the mods done to the chassis have reduced the trail,google it,below about 90mm or so the bike becomes faster to turn in.But less stable,slaps easier. Have the suspension poorly setup, not having the sag right also makes them less stable,,especially on bumpy roads. Most general purpose bikes,less so sports bikes, are plenty stable enough to suit the average user but lower the front or raising the rear moves them towards being less stable.If the rider is outside average weight,70 to 80ks, then the suspension wont be optimized and work well.
    If any off this hits the mark in your case its worth fixing it to work better.And thats if the bike and tyres are in good nic,all bets are off if its not.Learning about this stuff and adjusting your bike to be its best is good fun and a good way to reduce the chances to go down,in a bad way. Forgot to say even down to adjusting tyre pressures has an effect
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  19. #19 Goldenberri, Apr 30, 2015
    Last edited: Apr 30, 2015
    Sorry GoozaGooza I missed this. No I didn't experience this with the 125 but I can't get that thing over 80 anyway unless I have a stiff tail wind. I asked rider boy if he's had any issues but he's proven to be a bit more sensible than his mum and says he doesn't do over 80 on this road. And then I got a lecture. o_O

    It is good fun learning and I'm sure with a bit more tweaking the bike will be great. It's in beautiful condition for it's age so worth pursuing. I'm meticulous about tyre pressure and even carry a gauge.

    I've been giving it some thought and think that most bikes on this road would have a bit of a conniption at 'critical speed' which sends the tyre out of round. I think in my newbie exuberance I was just going a tad too fast on a crappy road and found that speed the second time.

    But raising the bike back to standard is still a good thing to do because at my height I'm going to have to learn how to deal with a higher bike if I want to move on to something bigger.
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  20. I too am height challenged (5'1")but I honestly think that there will be some bikes out there that are really just too tall for us...the RSV4R is way way too high 854mm for my stubby little legs, even with one hell of a lean over...plus with the weight and the lean, physics will always be against me...maybe in 10 years of riding I wouldn't have thoughttwice about ballerina en pointe like riding but I won't be riding at that age :(
    FWIIW i think that if the bike is lowered correctly (front/back suspension and bar risers) lowering shouldn't be an issue on most bikes and far safer...(most be the important word lol)

    PS Just as well you and I don't go riding together...I think we would both be very very naughty and speed demons :p
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