[ST] RE: Fuel consumption

["Keith Tynan" <keith@xxxxxxxxxxxxxxx>]

> Date: Sun, 9 Jun 2002 10:56:11 +0100
> From: "Philip Guy" <bike@xxxxxxxxxxxxx>
> ...
> Forgive my ignorance, but what is the difference in changing the front
> sprocket to 17 as I have done and changing the rear to 45 as u have done.
> Except for the obvious slight differences in performance (I'll
> have to amend
> and have a look at the spreasheet values for the 45), does a bike
> prefer to
> have a rear sprocket change, does it enhance or reduce something?
>  In other
> words, why do some people change the rear and others the front,
> am I missing
> something ?
>
Phil,

(Very) Roughly speaking, a smaller front sprocket results in higher wear
(more stress per tooth), whereas a larger rear results in lower wear.

Assuming you've got all the tools you need, it's my opinion that changing
the front sprocket is easier than changing the rear - though there's not an
awful lot in it thanks to having an external rear sprocket. (I can't
remember whether you need to remove the front sprocket anyway to get the
chain off the rear sprocket.)

Changing any of: {front sprocket, rear sprocket, chain length} results in
different distances between the sprocket centres, which has to be taken up
by rotating the eccentric rear hub. E.g. changing from 18/43/108 (1999
standard) to 17/43 lengthens it; to 19/43, 17/45, or 18/45 shortens it
(least->most). If you have 18/43/108, then the rear axle is near-as-dammit
at the bottom centre of the eccentre when the chain is properly adjusted. If
you have to rotate the hub from this point (because of any change in the
sprocket distances), you will ALWAYS lower the ride height. Depending on
which way you turn the hub, you will also either shorten or lengthen the
wheelbase.

Lowering the ride height gives you less cornering clearance, but brings the
CoG lower (quickening the steering). Shortening the wheelbase gives you
quicker steering but less directional stability; it also changes the
resonant frequency of the chassis and consequently increases the tendency to
tank-slap. Lengthening the wheelbase effectively does the converse. The list
is VERY long... Is it any wonder that even experienced race teams have
difficulty getting suspension set-ups right?

Mr Adams reckoned the answer was 42, but I don't think he had ST rear
sprockets in mind at the time!

FWIW (IIRC), Ducati 9xx typically run 13 or 14 front and 33 or 34 rear. I
can't help but remember Mark Farmer in the '93 TT stripping one or both of
the sprockets on his Duc!

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


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