Hi All,
This was posted to the Ducati list.
Here we go; more and more mysteries neatly explained. ;-)
"There is real science going on; stuff we can explain and hopefully sort out."
LSR guys; I've found the name for my bike; "speedman's wobble" ;-)
Watch those 'hazards in tinkering with a bike's settings'.
Seriously, we will have to see what they come up with. Tony, have they been
in touch?
Cheers,
Rick
AVOIDING TANKSLAPPERS
A potentially fatal phenomenon that has been haunting motor- cyclists
for decades could soon become history. Steve Farrar reports
Although he was just 30 seconds into the race, Paul Orritt's Honda
Fireblade was touching 150mph as it crested Bray Hill. A highly
experienced rider, Orritt was looking forward to almost two hours of
trouble-free, high-speed racing in the 1999 Manx Grand Prix. But as he
guided his bike smoothly around the gentle bend that led down into the
hill, his plans came to a violent end. Without warning, the front
wheel started to thrash from side to side and Orritt found himself
battling just to stay on. Bikers call it a "tank slapper" because the
handlebars suddenly seem intent on battering the fuel tank into
submission.
Orritt had faced such episodes before. But this time it was
different. "It just wouldn't come out of the tank slapper," he
recalls. "I was no longer in control." The bike fishtailed down the
hill, eventually throwing Orritt into the road. He flew for another
100m before smashing into a fence. When he awoke in hospital, he had
six broken ribs, two smashed shoulder blades and a mangled right hand
that no longer had an index finger. That he survived at all was
surprising.
Orritt has his own theory as to what triggered the catastrophe - he
remembers that the trouble began immediately after he ran over a
couple of bumps in the freshly laid road surface. Yet no other
competitors reported suffering similar problems. Orritt's experience
is, however, by no means unique. It is impossible to know just how
many motorcyclists have fallen victim to a bike that abruptly takes on
a life of its own, but there have been a few high-profile cases. In
1993, two police motorcyclists on a high-speed training exercise came
off their bikes as they negotiated a sweeping bend on the A3 north of
Guildford. One died. In 1996, Suzuki recalled a new model after
several people were killed in similar circumstances Statements were
subsequently issued in both cases noting that uncontrollable
oscillations set in after the bike hit bumps in the road while taking
a corner at speed. But exactly what was happening and why most riders
seemed to get away with it remained a mystery - until now.
Engineers at Imperial College, London, believe they have found the
solution. The answer they have come up with shows how a host of
seemingly unconnected elements can sometimes combine to devastating
effect. David Limebeer, professor of control engineering at Imperial,
has worked on the project with colleague Robin Sharpe, professor of
automative product engineering at Cranfield University, for nearly
five years. He is very happy with the new analysis. "It shows these
accidents are not acts of God," he says. "There is real science going
on; stuff we can explain and hopefully sort out."
Limebeer has studied video footage of Orritt's last few moments in the
race and noted the distinctive features of the bike's behaviour that
led to the crash. But the engineers' action has been largely confined
to the latest computer-modelling techniques. Vast quantities of
simultaneous calculations are needed to unravel the complex
interactions of the forces and geometry of bike, rider and road. A
powerful multibody modelling programme called Autosim, devised by
scientists at the University of Michigan in the United States, has
enabled Limebeer and his colleagues to recreate the circumstances that
lead to the tank slapper in unprecedented detail.
All the important features of this type of accident can be replicated
and tested without putting anyone at risk. The problem has been known
to science since it was described as "speedman's wobble" in a journal
paper of 1922. Yet it has proved remarkably resistant to the attention
of engineers. Unravelling its many elements was impossible without
powerful modern computing techniques. In fact, there are two linked
forms of motorcycle instability: wobble, which occurs at low speed and
causes the front wheel to rapidly flip back and forth like that of an
unruly shopping trolley; and weave, the fish-tailing effect that
happens at high speed and that caused Orritt to crash. Both,
potentially fatal, share many general characteristics.
It is clear that an outside influence is needed to spark the
phenomena. Subtle undulations accidentally introduced into a road
surface during its construction can be enough. These need be no more
than a centimetre in height and stretch over just a few metres, so
long as they have precisely the right wavelength between peaks to
"tune in" to the wobble or weave dynamics of the bike and its rider -
values determined by intrinsic mechanical properties.
Other factors need to be present too. The bike has to be travelling at
just the right speed. It must also be taking a corner, with the bike
leaning at a particular angle. Together, these elements enable the
vertical oscillations caused by the bumps in the road to become
amplified and transmitted to the bike's steering - like a tuning fork
applied to a guitar string. The magic combination sets loose the
unseen forces behind the tank slapper.
Engineers have experimented with ways to influence the extent of the
problem. They have found that heavier riders are less susceptible to
the problem than lighter ones. More significantly, hazards inherent in
tinkering with the bike's settings have emerged. While increasing the
steering damping reduces the risk of wobble, it increases the chance
of weave. "It needs a holistic approach," Limebeer says.
The results have been sent to motorbike manufacturer Triumph as well
as to the Metropolitan and Thames Valley police forces. Aspects have
been published and discussed at major conferences, while an
overarching paper has recently been submitted to a leading journal.
The next task is to look at how an understanding of the science behind
weave and wobble can be used to construct a motorbike that is less
susceptible to the problem. Limebeer admits that designing them out is
a delicious prospect. When he experienced the problem soon after he
started riding motorbikes some eight years ago, he was lucky enough to
be able to ride through the weave. But he is acutely aware that many,
including Orritt, have not been so fortunate.
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