About Motorcycles

[Guest selphie]

I am reading this website and I thought maybe it would be of interest to bikers...

About the history and construction of motorbikes

 

However, about the weaving and damping part I am so confused I wonder if anyone can explain in layman terms...?

Chassis stability

A good motorcycle chassis has no stability problems.

 

There could be three kinds of stability problems with motorbikes:

 

-Capsizing is well known in low speeds, and easy to overcome by going a bit faster.

-Wobbling (AVI movie) is a high frequency (7-9 Hz) oscillation of the front wheel. It is often relatively harmless but annoying (and quite frightening if previously inexperienced). It can appear at moderate speeds.

-Weaving (AVI movie) is a low frequency (2-3 Hz) oscillation of the whole vehicle. It can become unstable at higher speeds with fatal results.

Based on The control and stability analysis of two-wheeled road vehicles:

 

-The weave oscillations damp out once the rider reduced the roll angle.

 

-Tyre (tire) characteristics and inflation pressures are important variables in the behaviour of the motorcycle at high speeds.

 

-From a stability point of view it is desirable to make the lateral stiffness as large as possible, with the possibility of an optimum value for the torsional stiffness of the rear frame.

 

-Common levels of lateral stiffness at the wheel spindle deteriorates the wobble mode damping substantially with significant changes in the wobble frequency as well, and slight reduction in the weave mode damping at high speeds.

 

-Lateral distortion should be opposed as much as possible by locating the front fork torsional axis as low as possible.

 

-The largest contribution to the weave damping came from the cornering and camber stiffnesses and relaxation length of the rear tyre and not so much from the same parameters of the front tyre.

 

-Amongst others, stiff frames, a long wheelbase, a long trail and a flat steering head angle were found to increase weave mode damping.

 

-Degraded damping of the rear suspension, rear loading and increased speed amplifies cornering weave tendencies.

 

-Rear load assemblies with appropriate stiffness and damping were successful in damping out weave and wobble oscillations.

[kurarensu]

2 chim 4 me selphie..

[Cyanide14]

Very deep into technical details, I also can't understand.

[Strong Eagle]

Have you ever pushed a shopping cart with a front wheel that wobbled back and forth? That front wheel has 0 degree rake angle and a lot of trail... a design that tends to amplify weave or wobble.

 

Weave or wobble on a motorcycle begins with something in the bike that produces oscillations. It could be an underinflated tire, a tire that is out of round, a bad tread design, loose rear shock mounts, or loose steering head bearings.

 

When the front wheel is moved from a straight line the contact patch moves from the center of the tire to the side and by laws of gyroscopic motion the tendency is for the motorcycle to get the contact patch under the center of the tire. If there is nothing to damp the movement, then the oscillation back and forth continues... the weave or wobble that is sometimes felt.

 

The real dangerous situation is when the weave or wobble starts a harmonic wobble in the bike. Kind of like pushing a swing... you keep pushing on it, putting more energy into it and the swing goes higher and higher. A frame that flexes can start a harmonic weave and old Harleys are famous for tank slappers... the harmonic weave becomes so violent that the handlebars literally hit the tank. Not good. The old joke was that Harley frames were made with a hinge in the middle.

 

So, a good design damps harmonic oscillations instead of amplifying them. For example, a stiff frame stops oscillations and this is why high performance sport bikes have a very stiff frame... to counter the low rake angle. Keeping the rear tire solidly inflated prevents the tire sidewalls from flexing and amplifying wobble while keeping a lower pressure in the front tire tends to damp oscillations.

[GerardSantaCruz]

thanks for the "translation", eagle man.