[Fwd: [MC-CHASSIS-DESIGN] Fwd: Steering dampers - a warning.]

Jim Stewart js at strappe.com
Tue Sep 7 21:12:14 PDT 2010


 From the MC Chassis Design list. Tony Foale is an internationally 
renowned motorcycle chassis handling authority. Knows his stuff...

-js



-------- Original Message --------
Subject: [MC-CHASSIS-DESIGN] Fwd: Steering dampers - a warning.
Date: Tue, 7 Sep 2010 20:02:37 -0400
From: Tony Foale <info at tonyfoale.com>
Reply-To: MC-CHASSIS-DESIGN Mailing List <mc-chassis-design at micapeak.com>
To: mc-chassis-design at micapeak.com
References: <AANLkTimfxDYMUkhpbma85gBGXa_hZ-okoHUpHsTrg+Wx at mail.gmail.com>

There has recently been some discussion on the USCRA list about steering
dampers.  The following is a post that I have just made on this subject,
which is also appropriate for this list.

Regards

Tony Foale
info at tonyfoale.com
www.tonyfoale.com


---------- Forwarded message ----------
From: Tony Foale <info at tonyfoale.com>
Date: 7 September 2010 13:25
Subject: Steering dampers - a warning.
To: USCRA at yahoogroups.com


Although hydraulic steering dampers can be useful in reducing the steering
kick back caused by bumps and dropping a misaligned front wheel on the deck
after a wheelie, they also have a dark side which is little known about nor
understood by the average rider.  There are no free lunches.

This is not a suitable forum in which to go into great technical detail, and
this is a very technical subject, so I shall try to keep it as simple as
possible.
There are two oscillatory modes of instability inherent in all inline two
wheelers.  Viz: Wobble and weave.

Wobble is a relatively low speed problem which shows itself as an
oscillation of the handlebars at around 30 to 40 mph.  Some bikes are more
prone to it than others.  e.g. The old 1960s BMWs with the Earles forks were
well known for this.  In general this is more an annoyance or curiousity
rather than a real danger and it can be easily reduced by the use of an
hydraulic steering damper.

Weave, on the other hand occurs at higher speed and has often resulted in
serious injury and many fatalities.  This form of oscillation involves the
whole machine not just the steering.  The bike will roll and yaw side to
side as well as the steering.  Whilst some bikes are known to exhibit this
behaviour it usually remains hidden until a particular set of conditions
sets it off, but ALL bikes have the potential for it to occur.
Unfortunately, a HYDRAULIC STEERING DAMPER INCREASES THE PROBLEM.  In simple
terms, the steering forces provided by the damper occur at a time that
reinforces the weave oscillation.  The forces are out of phase with that
required.

For those who are unfamiliar with just what a weave means I offer this link
to a dramatic video from the IoM.
http://www.youtube.com/watch?v=XiKmPFo6xNA

Here is a layman's description wobble and weave
http://docs.google.com/viewer?a=v&q=cache:uVwYj-iOW0YJ:metrohog.com/images/20020215_NYTimes_Motorcycles.pdf+bray+hill+motorcycle+weave&hl=en&gl=us&pid=bl&srcid=ADGEESiddJprAM8XY0R6Pfok2riB0rggv1NxhkeKPPK8XrhgxyGdE2jw9jEnZJOHHi06X2N0v0GBenMR4NaPwISdnRqeuzNswRV8mEox4vN0pMh1lHVwYhqSI0Dp0CPpfsjTEWAFlxDY&sig=AHIEtbRVqHhMX2FdFqdqHhk7wEpjrqeWOQ

Here is a technical paper on the subject (not suitable viewing for maths
phobes - parental guidance advised)
http://eprints.imperial.ac.uk/retrieve/3175/license.txt

If WERA have mandated the use of steering dampers, then they are seriously
misguided if they believe it to be a general safety enhancement.  As I
stated at the beginning, it can be very useful to reduce the effects of
kickback and that can improve safety.  However, the characteristics of such
dampers is that they make a bike more prone to the very dangerous
possibility of weave.

This is a plea to the USCRA rules committee, to reject any future proposal
to mandate steering dampers under the misguided assumption that they are
always a safety feature.
If a bike shows no sign of kick back problems at a particular circuit then
you are better off to not fit a damper.  On the other hand, if the
handlebars try to fly out of your hands at times then a damper may be the
lessor of two evils.  Not all bikes need them, even on circuits like the
IoM.  I rode there for 5 years and never had one fitted.

Footnote.
There is some hope that in the future devices will be available to reduce
kick back, wobble AND weave.  Micro-computer controlled active systems could
do it, but there is a passive device which analysis has shown to have
positive effects with regard to damping out weave etc.  It has been called
an "inerta",  an hydraulic damper produces a force which is a function of
its compression or extension velocity, an inerta produces a force which is a
function of its compression or extension acceleration, which is 90 deg out
of phase with the velocity.
The reason that your neighbourhood MC shop doesn't stock them is two fold:
1.  It is only recently that their effect has been studied.
2. They are more difficult to impliment in metal than hydraulic dampers.  In
the past we had friction dampers because hydraulic items were too expensive,
so maybe in time we can hope that inertas may become common place.

As regards to dampeners, these were quite common in the days before tearoffs
to clean one's goggles.  They usually took the form of half a tennis ball
clamped to the handlebars, this contained a tethered sponge which had been
dampened before the race.  Their effect on wobble and weave was said to be
minimal, but they worked wonders removing the remains of insects impaled at
high speed.

Regards

Tony Foale
info at tonyfoale.com
www.tonyfoale.com



More information about the SV650 mailing list