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A Hairy Ride - Motorcycle Physics

  1. Aug 24, 2006 #1
    Had a bit of a Hairy ride this afternoon while riding my motorcycle.

    I was overtaking slow moving traffic at the end of a dual-carridgeway, where the two lanes converge, i had pleanty of power and time to pass and pleanty of space as the end of the carridgeway also had shevrons/markings which would give me pleanty of room for error.

    However, as im passing the two vehicles in an attempt to get past them before the single lane, my Handlebars start to shake violently, oscillating from left to right from about +/-30deg. :surprised

    I back off the power and relax my arms and just hope for the best :P

    Im was trying to think of a cause for this and the only thing i can think of is the shevrons/road markings.

    Code (Text):


    L1   [Truck][Learner Driver]

    L2   [Me]               //////////
    The road markings are the white lines that run at approx 30-40deg angle from right to left and they are slightly raised (only because of the paint thickness).

    My rational is that my front forks were compressed by the lines but this compression was uneven, as one side was hiting the angled lines before the other.
    Could this uneven compression, and subsequent contact with each successive line, have met the shock absorbers natural frequency?

    Should this be taken into account in road marking design and markings be placed perpendicular to the direction of motion?

    If i was to go about showing this is the cause mathematically, how could i go about finding the fork characteristics without experiment? ( i dont have the equipment really, so somewhere to find similar information that i can use to estimate).

    EDIT: I think that some bikes are equipped with a steering shock absorber to damp this kind of motion
    Last edited: Aug 24, 2006
  2. jcsd
  3. Aug 24, 2006 #2


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    If the paint is thick enough that you can actually feel a deflection when you hit it, then your resonance frequency theory is very possible. I've never seen such a thing, but I don't live where you do; our paint is thin.
    Here, the roadbuilders have a trick that they use for making people slow down well in advance of critical intersections. It's called 'rumble strips', which are carefully spaced small ridges in the pavement. They're only about 1/8" high, but are calculated to interfere with the suspension of most vehicles. Although there's no lateral deflection, they set up a very unnerving vertical vibration that pretty much makes high speed impossible.
    If that is indeed what happened to you, I'd seriously consider taking it up with a consumer advocacy group to see about having the practice halted. It's unlikely that you have the only machine susceptible to the phenomenon, and somebody could end up in an accident because of it.
  4. Aug 24, 2006 #3


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    ±30°? And you still maintained control? That's a lot of deflection.

    I can't imagine road markings being raised enough to do what you are saying, but anything is possible. I would think that your front struts would handle any kind of small imperfection like raised paint. However, I would look at something a bit more plausable, like front tire (or tyre as you would say) inbalance. How fast were you going when passing? Do you normally take the bike up that fast?
  5. Aug 24, 2006 #4


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    Good point, Fred. I had taken from 3's description that the vibration occurred only during the time that he was on the chevrons; it was quite remiss of me not to ask. So how about it, 3? Has it happened at similar speed since then? Any other relavent factors that you didn't include?
  6. Aug 24, 2006 #5


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    I have seen rapid acceleration on bikes cause the rapid deflection like you describe. The rider hit the throttle, accelerated up to at least 100km/h then suddenly the front wheel went wild, pitching back and forth and he crashed.

    I would expect that a bump big enough to deflect the gyroscopic interia of the wheel could do that.
  7. Aug 24, 2006 #6
    I'm not sure if this has any bearing but awhile back my truck had a similar problem. For the most part driving was fine except when I got up to higher speeds. Then the steering wheel would rapidly shake back and forth.

    The problem turned out to be the alignment I think but since I have no experience with motorcycles I don't know if it is even possible for there to be a comparable problem (seeing as there are 2 wheels instead of 4).
  8. Aug 24, 2006 #7


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    I had something similar happen on a bike. I attributed it to either worn neck bearings or wheel bearings. (I wiped out a bike going around a corner doing about 70 mph when the front end started oscillating.)

    Try checking your neck bearings by putting it up on the center stand, grab the lower forks, and push/pull, looking for play in the neck. There shouldn't be any.

    Check the front wheel bearings in much the same way.

    You may also find there are other worn parts which allow things to oscilate in unusual circumstances such as the clearance between the shock tubes, loose nuts and bolts, or a badly worn tire or low pressure.
  9. Aug 24, 2006 #8


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    Just thought of something else, but it's probably not relevant. Have you checked the tire itself for abnormalities? I don't know if you have smoothies or something a bit knobbly. What I was thinking is that if you have a knob chewed up, off-centre, it might want to pull the wheel in that direction during a turn or when the tire is compressed enough to put it in contact with the road. The counter-oscillation might possibly have been a subconscious over-correction on your part. (Note that I'm not doubting your riding skill; sometimes things like that just happen before you realize it.)

    edit: Oops! Just saw your post, Q. I see that you covered that.
  10. Aug 24, 2006 #9
    Sorry, i mean +/-15deg possibly a bit more, i.e 30 from left to right.

    Tyres are new and good, (ive recently had both replaced) i wouldnt like to say how fast i was going, but i wasnt hanging around. Nothing stupid mind.

    Never happened before, although ive seen it happen on race bike that push the limits on corners, but this was straight line.

    I will check the front bearing for play, i think i would notice it if it was worn, but any play would make this kind of scenario worse.

    Only other thing i can think of is a peice of debris, but i would expect to (A) Have seen it and (B) It to settle quicker (it must have oscillated about 4-6 times.

    Over correction wasnt really an option :P It was too violent. Ill be thinking 3 times before corssing those things again, thats for sure.

    Thanks for the suggestions by the way, i didnt consider the yolk bearings, i also haent checked if the pressure int he forks is correct.
    Last edited: Aug 24, 2006
  11. Aug 24, 2006 #10


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    Might find this interesting --- the section on stability problems --- if I read it correctly, "wobble" can be damped by playing with the rear suspension? 'Pears to be a common problem, or just the nature of the beasts.
  12. Aug 24, 2006 #11
    This is a truly complicated issue for determining the exact cause.

    My first road motorcycle was a Kawasaki z500, and I can tell you that it could develop "speed-wobbles" in a number of different circumstances; at a constant, high speed; while accelerating; sometimes (but rarely) while decelerating; on a corner (epsecially a bumpy road).

    I have no science whatsoever to describe the cause of this condition, but here are my best guesses based on experience...

    The behaviour of your steering is affected by a huge number of factors, perhaps grouped by:
    Geometry of front forks
    Geometry of rear suspension
    Weight distribution of bike
    Condition of tires (front AND back)
    Road condition
    Weather (especially wind and wind turbulence)​

    Any/all of these can affect your steering. In particular, if the directional control of your bike is naturally "weak", then it might be that your front tire can change orientation slightly without significant impact on the directional force it exerts. If it shifts further, and starts to "bite", the inertia of the bike will try to force the tire the other way. As the wheel passes "straight", it has little directional effect again, but still has its own inertia (wheel, rim, forks, handle bars, everything else mounted on this system) so it keeps going the other way until is "bites" again. Each time it bites, it imparts a small motion to your bike - a minor change in direction. For reasons I'm not scientifically able to explain, this oscillation rapidly escallates, and if nothing occurs to alter the present geometry of the "system", this concludes in complete loss of control. Since riding is a constant balancing act, you subsequently explore new ways to "dismount".

    Whatever the precise cause, it is easy to see why steering dampers limit this effect to a (hopefully) manageable range.

    I do believe the primary issue though is the geometry of your front and rear suspension, and condition (and type) of your tires. Rear suspension and tires are a lesser factor, I believe, but if your rear-end "drifts", even a little, it changes the geometry of the front slightly; maybe enough to start the "wobble".

    My last bike was a Honda Blackbird. 1100cc Sport-Tourer. Under ALL conditions, ALL speeds (I've had it up to 274km/h by the clock), ALL loads (including myself, my girlfriend, and about 50Kg of gear), it never wobbled. Not a hint of it. It was very "light" to steer, yet had a directional stability that was unbelievable. In cross-wind gusts up to approx 40km/h, it DID NOT change direction in the slightest. I swear, the bike leaned into the wind without any input from me. Moreover, the bike swiveled about its centre of gravity. By that, I mean the top of the bike moved into the wind, the tires tracked in the opposite direction, but the centre of mass remained exactly where it was, so when the wind gust ended, and the bike righted itself, I was on exactly the same track as when everything started.

    When I got the bike, it had Bridgestones front and rear. It handled well, but I found the turbulence of passing big trucks made it a little unstable. I changed the front to a Dunlop, and never looked back.

    In conclusion, the advice you got about looking for worn parts is wise. You ought to do that anyway. But even with everything in perfect condition, a wobble is still possible for any of a thousand reasons. Things like changing the angle of your forks, their length, and so on, might help your problem, but like I said, their are so many things that can create the conditions for "wobbles", and figuring out exactly how to "fix" them all is quite a challenge.

    I believe Honda achieved this with the Blackbird, but you can be assured there were a lot of people of very specialized knowledge with a wealth of data available that came up with the exact configuration that resulted in the most stable yet nimble bike I have ever ridden.

    Hope I've shed some light on this for you.
  13. Aug 24, 2006 #12
    Yeah a good example of a wobble in that article, if it was snaking like the other avi i wouldnt even get on it :P

    Heh yeah, "Down Boy" is what went through my mind...
  14. Aug 24, 2006 #13
    Head shakes are common on a lot of street bikes, and even dirt bikes. Steep stearing angles combined with small trail numbers make the problem more prevelant. I have has voilent head shakes before at over 100mph that caused the front wheel to wobble back and forth hitting the stearing lock on both sides several times in a row. This also resulted in a lot of tire smoke, and having the clutch side handlebar ripped lose from my grip, bloodying my nuckels on the clutch lever.

    Sometimes when crossing a painted line as the OP describes, the front tire will become unweighted enough to submit to the riders stearing imput, and actually turn the front wheel slightly off the line of travel. This can cause the tire to grip suddenly and send it violently in the other direction where it finally grips again repeating the process.

    In a high speed wobble the best thing to to is accelerate, as it takes weight off the front wheel lessening the chance of the tire gripping as well. Most people will tend to let off the power when a head shake occurs, and that just makes the problem more severe.

    When crossing painted lines it is best to try to cross them closer to perpendicular than to try to cross in an almost paralell manner. Having the front tire crossing the line at nearly the same time the back tire does will cause a wobble in the whole bike which can also result in a head shake. Accelerating while crossing the lines can help too in my experience, as it takes a lot of weight off the front end.

    Bottom line, modern sport bikes, with their near racing style stering geometrys, are made to turn very quickly, and are not designed with stability in mind. Combine the steep stearing angles with the short wheel bases needed to make fast turning bikes, and you have a machine that is prone to speed wobbles. Keeping tires at the proper inflation and cheecking stearing head bearing head bearings often can help, but sooner or later its going to happen.
  15. Aug 24, 2006 #14
    This seems like a better explanation than the fork compression, now you mention it. Thatnks for the insight.

    What your saying is the lack of resistance momentarily means the steering turns faster than the bike, and when the tire grips again, the bike yanks it all back in line....

    Thanks for the tips.
  16. Aug 25, 2006 #15
    Been there done that. A “tank slapper” is the feared result of the steering forces in a ‘positive feedback’ situation.

    One way of reducing the onset is not to have a ‘death grip’ (with straight-ish elbows) on the handlebars as any slight road input feeds back to your body, moving it, which inturn feeds back into the handlebars, and so on. Relax, let the geometry do it’s stuff.
  17. Aug 25, 2006 #16


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    You can't accelerate forever, and some bikes have a speed range where they get wobble, and you'll have to slow back down through this speed range. I would recommend maintaining speed, and varying the amount of tension in your arms to change the harmonics.

    The last "modern" street bike with a wobble issue was the first year Honda 900 RR, after the first year, they adjusted the triple clamp to move the forks back 3/8" to increase trail, which solved the problem. If a modern bike has an issue, it will some with a steering damper. The really high speed bikes, Hayabusa, ZX12, ZX14 are very stable at high speeds. A video of a souped up, normally aspirated Hayabusa, with stock body work and frame geometry running 211mph (GPS, speedo shows 220mph): busa211.wmv

    Many Harleys use a design that decades old, and are more prone to wobble than other current bikes. Older bikes, like the Kawaski 2 stroke triple 750, and the older still Royal Enfield Atlas, were prone to high speed front end wobbles. Apparently, Harley Davidson, has decided to carry on with this tradition by making clone of bikes made in the 1950's and 1960's, along with all their handling quirks.

    Another cause for a high speed front end wobble is a handlebar mounted windscreen. If you install one of these, install a steering damper as well.

    Regarding all out race bikes, the wobbles you see usually occur from hitting track bumps or dips at high g forces, causing some flex in the geometry, but the bikes quickly recover on their own.

    A front end speed wobble is due to front end geometry, harmonics, and gyroscopic reactions cause the front end to oscillate side to side. What happens is a side force on the front tire cause it to turn into the direction of the force, since the contact patch is behind where the pivot axs would intercept the pavement (it's called trail for a bike, caster for a car). If the chassis is stiff enough, and there's no play in the forks or swing arm, and the trail isn't too large or too small the oscillation will self dampen, but otherwise it can escalate which is what happened to you.

    The last bike I had where I could cause this was a 1984 Suzuki GS1150, bud I had to take my hands off the handlebars and punch one of handles to start the oscillation. With my hands on the handle bars, the oscillation would never occur, since the mass of my arms changed the harmonics. However this is a test I use to determine if a bike is prone to speed wobbles.

    Flex or play in the rear swing arm can also cause wobbles, and worse yet, can results in the rear end hopping up and down and side to side, sometimes bucking the rider off the bike.
    Last edited: Aug 25, 2006
  18. Aug 25, 2006 #17
    I wonder what, if any, effects a shaft drive has in this context. Jeff Reid's mention of the "1984 Suzuki GS1150" reminded me of a ride I took on a shaft drive Suzuki (I can't remember the model). I was showing off, at a moderate speed (maybe around 50-60mph), in a hard corner under full power...

    ...and I hit a bump! I didn't expect a huge reaction, especially on such a heavy bike, but I had no prior experience with a shafty. That back wheel lifted momentarily and displaced way further than I ever would have imagined possible without having felt it for myself. Not an experience I would care to repeat.

    It does however reveal that the torque twist effect under high engine acceleration or deceleration is significant, and I wonder how that affects the overall physics - possibly contributing to the "wobbles".
  19. Aug 25, 2006 #18


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    That bike wouldn't get the wobbles unless you took both hands off the handlebars, and it might have been due to the 16 inch front wheel they used on that bike.

    They have improved these since they first came out, but due to unsprung weight and increased loss of power (versus chain drive) they aren't used for sport bikes. The torque effect is still an issue, but the swing arms are made stronger to reduce this. On some bike, there is still the issue of the back end rising or lowering due to engine torque.
  20. Aug 26, 2006 #19

    Andrew Mason

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    Interesting thread. This http://en.wikipedia.org/wiki/Speed_wobble" [Broken] seems to explain it pretty well.

    There is a link at the end to http://www.flyingsnail.com/Sprung/tankslapper.html" which dramatically shows the problem. It is not just the front end wobbling but the whole machine.

    Last edited by a moderator: May 2, 2017
  21. Aug 26, 2006 #20
    Use a ribbed-pattern Pirelli front tyre, keep other patterns for the rear. You may also need a fork brace, & mild fork damper. Check wheel alignment front-to-rear. Check your fork oil - put in new - & check that your internal fork dampers are not sludged.

    It could be that you were in the process of decellerating slightly as you hit the paint & this set your balance forwards making things a little instable. You may also have been changing direction slightly.

    Some bikes are terrible handlers if the wrong set of circumstances comes up. Stick with bikes with 19" front wheels - the smaller front-wheel bikes loose some gyro effect & can be tricky...
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