Q2 2008 - Reading Cycling Club

SpeedWobbles(aka ‘The Tank Slapper’) By Steve BaleI own a lot of bikes and in my collectionthere is one with an engine. In motorcyclinga ‘tank slapper’ is one of the mostfeared riding incidents and bikes that areprone to them quickly get a bad reputation.The tank slapper term is derivedfrom the fact that the handlebars oscillateso wildly the bar-ends hit the fuel tank.Two incidents have prompted me towrite this article, and thankfully neitherinvolved myself. The first was witnessingPaul Orritt crash at the 1999 Isle ofMan TT races and the second wasLloyd McInally’s crash at a road racelast year. Both riders were victims of aspeed wobble, a phenomenon where anundesirable oscillation at the front endof the bike causes the handlebars toviolently shake from side to side which, ifleft, will throw the rider off.Feet out – now that’s got to be a bad sign!For a very graphic illustration of what I’mtalking about look up ‘Paul Orritt’ on theinternet and you should come across avideo of his crash, a moment of my lifeI will never forget. Racers at the Isle ofMan tend not to survive 150mph crashes.Thankfully Paul did.So what is going on and why do bicyclesbehave this way sometimes?Speed wobbles occur because the frontwheel is free to turn about its steering axiswhile at the same time the whole bike canmove from side to side along a horizontalaxis with the pivot points being the frontand rear wheels in contact with the road.When you ride the bike is moving fromside to side as well as the wheel rotatingaround the axle. This means the wheel isin ‘nutation’.‘Rotation’ is an object spinning around afixed axis while ‘nutation’ means the axis(or axle in the case of a wheel) is alsomoving as the object is spinning. Think ofthe Earth’s axis and its seasonal wobbleand then project that thought of theEarth’s axis to your front wheel and thinkhow the movement at the poles would affectthings. (Mavic) Cosmic man!This nutation is a constant and naturaloccurrence caused by the movementof the rider pedalling, side winds,bumps in the road, etc. Under normalcircumstances you will never noticethis because all sorts of things dampthe effects out, principally the fact thatyou, and more importantly your frontwheel, are objects in motion and youwill continue on your trajectories unlessanother force says otherwise. You alsoconstantly damp the nutation with yourhands on the handlebars keeping it undercontrol. However, very occasionallysituation normal is suspended andcircumstances conspire to work againstyou. At a critical speed, the front wheelnutation frequency matches the frequencyof the bike and rider either sustainingor amplifying the nutation. “Hang on”, Ihear you say, where did this frequencystuff come from? Well you, your frame,its stiffness, its geometry, its weight, yourweight, your position, in fact everythingabout you and your bike combines to giveyou, the cycling unit, a latent resonantfrequency. The classic resonant frequencyexample is that of glass resonatingwhen the fat lady sings. In terms of thisdiscourse when your frequency matchesthat of your ever wobbling front wheelsuddenly you’re wobbling together.Rotation (R), Precession (P) and Nutation (N)of the EarthNow you’re having a wobble, usually forno immediately apparent reason. Duringa high speed shimmy the front wheel isnot just fluttering back and forth about itssteering axis but is also moving side toside in the horizontal plane shaking thehead tube violently from side to side; therider’s weight on the saddle provides ananchor point, the rear wheel on the roadis another making a pivot point for thefront end of the bike to move from. Thesituation can now go one of two ways:better or worse. Let’s look at the worsescenario so as to better explain the betterone.In the worst case scenario the rider isexacerbating the problem. Faced withhandlebars that have a mind of their ownthe rider grips them hard to take controlback or use the brakes. By holding thebars tightly the rider is now connected ineven more harmony and thrown side toside (remember the poles/your axle). Thisextra, larger mass moving side to sidewith a high centre of gravity amplifies theproblem and a crash is becoming almostinevitable. With motorcycles, where thebike’s mass is greater than the rider’s,the rider is likely to lose the fight and bethrown off. Once the motorbike is free ofits pesky rider the bike may well stop itsoscillation and continue on upright in astraight line. For the bicycle rider gravitywill do its stuff and once you have beenthrown to one side beyond the point of noreturn you’re going down and taking yourbike with you.So now we know not to grip the bars witha death grip (easier said than done) whatare the do’s and don’ts to get us to a betterposition.Don’t panic and do relax your grip.Pressing your knee against the top tubewill often stop a shimmy; in doing soyou have damped the shaking top tubethrough the muscles and tissues in youleg without actually connecting the leg tothe top tube. This is also a clue that therider needs to be holding the handlebarslightly so that you are damping thenutation rather than being connected to itby grasping the handlebars tightly. Applythe brakes and you’re gripping the barsso try not to. However as you’re likelyto be hurtling down a hill at 40mph youprobably don’t have a choice! A delicatetouch is required and remember it is theconnection between you and the bikethat is causing the shimmy and the moreconnected you are the worse it will get.Also lift your weight from the saddlewithout actually standing up. This willtransfer your weight to the pedals whichare a much lower and further forwardpoint of contact. Keep one pedal downand most of your weight on that pedalswitching pedals as you corner keepingthe lower pedal on the outside. This willmake the point of contact between youand the bike very low, but also off centreof the frame.And what about your bike? Is it to blame?Is it dangerous? Well, as with motorcy-