[ST] RE: Skill question on "Tank slap"

["Keith Tynan" <keith.tynan@xxxxxxxxxxxxx>]

> Date: Thu, 13 Jun 2002 20:12:14 +1000
> From: "Chris Maher" <mahervis@xxxxxxxxxxxxxx>
> ...
> Just a question on "tank slap", what causes it, how can you prevent it or
> reduce the chance of it happening and finally has anyone on the list
> experienced it with an ST Sprint??  If so, was it preventable or could you
> have handled it better?
>
> I have never experienced this particular problem but understand that it
> usually leads to highsiding the bike and ouch fall down.

Chris,

What triggers it? - usually a bump or somesuch to upset the steering.

What causes it? - all - as in ALL - bikes are susceptible to it. It is a
consequence of a rear wheel pushing something which has a hinge in the
middle off it (the steering head). This combined system has a resonant
frequency (a frequency at which oscillations become almost
self-perpetuating), which is a function of the mass, CoG, rake, trail, and
particularly the wheelbase (to mention a few). If we just look at the last
property, a long wheelbase makes the speed at which resonance occurs
higher - usually beyond the top speed of the bike. Reducing the wheelbase
lowers the speed at which it occurs, often within the top speed of the bike.
It can be controlled to a manageable level by fitting a steering damper.
Ergo, race bikes with short wheelbases almost always reach speeds where
resonance can occur (if triggered), hence the fitting of steering dampers.

Have I ever had one? - YES, I had one at the back end of last year in the
wet... It started when I hit a bit of overbanding (smooth tar seaming)
coming out of a bend, with some, but very little power on. The rear broke
away a bit, I steered into it (I think), and the whole of the steering went
resonant. The result was a repeated full-lock to full-lock slapper (felt
like 1 per second) for something in the region of 150-200 metres. I have to
admit to never having rehearsed an escape plan for this particular
manoeuvre, but had the distinct impression that shutting off and loading the
front would be detrimental to my cause...hence the 150 metres. I don't
remember to this day whether I resisted or relaxed, but I do remember it
crossed my mind several times that I was: about to lose it, meet with severe
injury or even die... Attendees at the WaleSWeST Malvern Rally last October
might've noticed a slight odour to the company ~:(

Under more normal conditions, the ST (even on track) doesn't need a steering
damper.

Regards,
- --
BRG
mailto:keith.tynan@xxxxxxxxxxxxx
http://myweb.tiscali.co.uk/keith.tynan
'00 BRG Sprint ST 'Wolfram'


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