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Old 08-31-2013, 05:35 AM   #935
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More from Motorcycle Handling and Chassis Design the art and science, by Tony Foale

More detail
Having considered the basis of initiating a turn in fairly general terms, let’s now look at this very
important aspect in more detail.
Consider a racing bike approaching a corner and the rider needs to heel over as fast as possible. He
strongly applies counter-steer and the machine starts to lean over rapidly, but getting a quick roll
acceleration also means we need a quick roll deceleration. We start off upright with no roll velocity and
we end up at 45-50 degrees lean, again with no roll velocity. In the process the roll velocity must have
increased up to a maximum value somewhere around about half of the final roll angle, and then
decelerated back down to zero roll velocity at the final lean angle. So the whole lean-in process is not
just as simple as a bit of counter-steering followed by straightening out at the end. Basically, we use
counter-steer to lean the bike in, at about half way through the roll we have to remove it and possibly
give it some "pro-steering" to cause the roll deceleration.
For a long time those that read various motorcycle magazines may have been somewhat confused by
the rather conflicting “explanations” often given for this process. There would appear to be two
conflicting theories and the adherents of one seem to deny completely any possibility of validity in the
other. We might term these two theories
• Gyroscopic or precessional theory. Where it is taken as read, that at least the majority of the
lean-in torque comes from gyroscopic reactions.
• Steering out from under theory. Which basically assumes that as the front tyre steers out from
under the CoG., gravity will then continue the lean as the steering straightens up.
I imagine that most open-minded people interested in this subject would be inclined to the view that there
are probably some truths and untruths in both points of view with reality lying somewhere in a
combination of both. We shall see that this is indeed the actual situation, but we shall also see that the
physical mechanisms from either theory alone is capable of explaining the motorcycle lean-in. However,
neither theory alone properly explains all the observed phenomenon. Both theories however, require
that we use “counter-steering” i.e. the initial rider’s input is counter to that necessary for a very slow
speed turn.
The whole process of establishing a stable cornering attitude is extremely complex and to understand it
properly needs a mathematical explanation outside of the scope of this book, but the following is a
detailed description of the process using graphical rather than mathematical results from computer
dynamic simulations. In order to fully understand what’s happening some of the simulations represent
impossible situations, but are never-the-less useful. For example, in the first simulation we consider the
case in which the tyres produce no lateral force, thus leaving us with only gyroscopic reactions to lean
the machine. Another simulation is done with no gyroscopic effects, this approach allows us to clearly
see the individual forces and is useful to test the two theories above. The simulations allow us to
investigate the effects of parameter combinations that just aren’t possible with a real machine, and this
can provide valuable insights to the detail behaviour. All the simulations are for a bike travelling at 100
km/h, and the rider is aiming for a final lean angle of 44 degrees. The bike data is not for any specific
Balance and steering 4-9
bike but, except where noted, are typical of an average large capacity machine. Before tackling the
following, the reader is advised to read Appendix 4 describing the mechanisms of gyroscopic effects.
The following text is quite detailed and possibly tedious to read, but is included for those that want to
better understand the detailed mechanisms of the lean-in process.
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