Posted October 13, 2007 - 02:16 AM
The reason I'm asking is because I'm trying to teach myself how to set up a bike and head shake is a annoying problem and I cant see a steering damper as a solution
Posted October 13, 2007 - 02:42 AM
The oscillations must have 2 factors working, an underdamped second order system and a positive feedback mechanism.
The underdamped second order system is the nutation of the front wheel. An example of an underdamped second order system is a spring and mass system where the mass can bob up and down (oscillate) when hanging from a spring.
The positive feedback mechanism is the bike+rider, approximating a second order damped system. At a critical bike speed, the front wheel nutation frequency matches the bike+rider natural frequency; if the bike+rider side-to-side system is insufficiently damped, the movement provides a positive feedback to the front wheel second order system, amplifying or sustaining the nutation.
Nutation is the torque-free tendency of a freely spinning rigid body's spin axis to oscillate around the average spin axis. Ideally, it is a second order behaviour with no damping. In real cases, the oscillation will decay due to damping in the system.
Nutation is easily demonstrated by suspending a spinning bicycle wheel by the axle such that the axle hangs vertically. If the rim is given a sharp bump parallel to the spin axis, the spin axis will oscillate in a cone-shaped movement around the average spin axis. This is nutation, not to be confused with precession.
A bicycle or motor bike front wheel has two degrees of freedom in an effective gimbal system. The inner gimbal is the steering fork, free to rotate around the steering axis. The outer gimbal is effectively the entire bike, able to tilt to either side along the longitudinal axis. This gives the front wheel the required two degrees of freedom necessary for nutation to occur.
If a front wheel is disturbed into nutation, the effect on the bike is to rapidly steer from side to side, as well as to tilt the bike from side to side. These 2 motions are rotations around the two degrees of freedom of the effective gymbal system described earlier. The effect is manifested in two fundamentally different modes, depending on how much the rider is a part of the gimbal system:
The first is commonly known as shimmy and can easily occur while riding no hands on a bicycle. It is a relatively harmless side to side shaking of the head tube combined with a side to side steering wobble as the front wheel pivots around the ground contact point. The rider is not a major part of the nutation system. The rider on the saddle provides a firm anchor point for the rest of the frame to pivot around.
The second case is less common and can result in a crash. It is variously called a death wobble or weave. It occurs at a higher speed with a firm grip on the bars. The longitudinal pivot is not near the ground contact patch as in shimmy but closer to the rider's body. In this case the rider is part of the gimbal system due to a firm grip on the handle bars. The effect is that the entire bike+rider weaves rapidly from side to side, as if the riders is riding a mini slalom course, as opposed to shimmy in which mainly the head tube and handlebars are shaking.
The damping coefficient (the inverse of Q factor) of the second order nutation system is a function of the angular momentum. The higher the wheel speed, the higher the nutation Q factor (less damping), other factors remaining the same. With a higher Q factor, less positive feedback is needed for instability to occur. This explains why shimmy and death wobble only happens above certain speeds - the Q factor needs to become high enough for steering instability. Motor bikes are often equipped with a steering damper to increase the gimbal damping, lowering the nutation Q factor.
All 2nd order systems have some amount of damping due to frictional effects. A wheel on its own, if disturbed into nutation, will damp out by itself. To get shimmy or death wobble, a feedback system is required for the nutation to be sustained. The feedback system is provided by the bike+rider.
The feedback factor
For positive feedback to exist, there needs to be a system such that nutation is amplified enough for any damping effects to be cancelled. A second order system can provide such feedback at a certain frequency or range of frequencies. If a bike+rider approximates a second order system, then at a certain frequency or range of frequencies, the phase response will be such that positive feedback is established. Since a spinning front wheel is a lightly damped system, only a small amount of positive feedback is required for instability.
Hope this helps..
Posted October 13, 2007 - 03:15 AM
Posted October 13, 2007 - 05:57 AM
I'd like to know some direct reasons like too soft, too steep etc etc
Frog man did a very good explaination of why it happens. Poor body position will lead to head shake. That is one of the main reasons why head shake happens.
Posted October 13, 2007 - 06:16 AM
Head shake comes from the spinning of the wheel getting unbalanced at the same "rate" as the bike and the rider, it is then transfered to the whole package (front wheel and fork, rider and the rest of the bike) by the rider or the bike resisting the shake. That other forces than the rider steers the wheel is not as relevant as I thought it would be, it would be no problem if the bike/rider is balanced. If the tire has grip enough a head shake from steering stop to steering stop would be pretty much impossible to save?
That would mean to me that it is critical to have a good balance between the front and the rear of the bike and to have a front end that follows the ground as good as it can. Rebound is probably a lot more important than compression in this case?
How does steering angles and offset play a part in this?
I think I got to understand why we use smaller wheels on a supermoto by reading this text and being forced to use wikipedia
Posted October 14, 2007 - 07:11 PM
Posted October 14, 2007 - 09:14 PM
At certain dynamic frequencies the motorcycle/rider combination will have a tendency to reach a critical frequency where resonation will occur, in the off road world this resonation can be initiated by nutation caused by irregularities in terrain, such as the example given of the tap on the spinning bicycle wheel.
What the damper does is move the bike/rider resonation frequency somewhere else, way up higher where it doesn't affect him under the conditions and speeds he rides at normally.
But even with a damper, at really high speeds, the front wheel of a motorcycle will still oscillate. I've seen vids of riders going for speed records with full knowledge of this hazard still miscalculate and the results were disastrous.
Posted October 14, 2007 - 10:52 PM
Posted October 15, 2007 - 01:09 AM
Posted October 18, 2007 - 05:34 AM
i notice head shake big time when I'm in the later part of my motos and i get lazy or tired, my body english is not correct and my weight is not distributed correctly.
as SlickitySloan said staying loose and your keeping your weight where it needs to be will keep you straight and right on track!
Posted October 18, 2007 - 06:29 PM
to answer his question about rake and how it effects it. first let me say that dropping from a 21" front tire to a 17" front drastically changes the weight transfer angle of the bike which can contribute to headshake out of a corner depending on things like your chain pull angle and swingarm angle but no need to get into that as your asking more of a general question.
Its generally accepted that a larger rake angle will be more stable at higher speeds while a smaller rake angle will steer quicker, you need to account for trail as well. if you draw an imaginary line from your steering stem to where it hits the ground then a line straight down from your axle(tire contact patch) the difference is your trail. This trail is the "righting" effect number partially. so basically your tire contact patch "slides" around this point. its easy to tell if you set your bike upright and steer left to right and see how your front end moves.
I dont think your rake angle will cause problems unless you make drastic changes to your frame geometry, your headshake is more likely a problem with your tire tread, air pressure, suspension settings(front and rear) etc. bikes tend to headshake when the front end gets light and this is usually the main reason for adding dampers to them. keeping loose on the bike and your weight in the right place will reduce these events. wheelying out of corners leaves your front tire spinning alot slower at take off then on landing and speeding back up once it touches down can also be an ill feeling.
in what situation are you getting head shake?
Posted October 20, 2007 - 06:01 AM
Posted October 21, 2007 - 01:30 AM
what speed? the poor aerodynamics at high speeds might be causing front end lift from your front fender. could also be a wheel balance problem
Probably around 60mph
It is very bumpy at the end of the straight also. I get it sometimes during braking in high speed also, if its bumpy
Posted October 21, 2007 - 04:38 AM
there is not much you can do besides a damper for the braking while bumpy situation. If its bumpy enough your shifting your tire contact patch forward and backwards rapidly changing your trail numbers while keeping your rake constant(so to speak). you might try a few different tire psi settings to see if it helps.
Posted October 21, 2007 - 09:06 AM
Posted October 21, 2007 - 12:27 PM
Adjust your height of the forks in the triple clamp- top of the forks level with top of triple clamp will give you more stable high speed ( less headshake ) but bike will not turn as sharp- slide the forks up- less rake- quicker turning but less high speed stability ( increase head shake)
Posted October 30, 2007 - 09:29 AM