STM slipper clutches offer two major advantages; to assist the rider and help prevent damage to the engine and gearbox.
The slipper clutch assists the rider by reducing the effect of engine braking. Engine braking causes the rear wheel to ‘lock up’ and ‘patter’ when braking hard for a corner. A rider that is too aggressive in this situation suffers, because the rear wheel loses contact with the ground, making continued braking and direction change much harder to accomplish.
The STM clutch also helps save the engine and gearbox, again by reducing engine braking. Most motorcycle engines have a rev limiter, to stop the engine exceeding a certain speed considered safe by the manufacturer. This works fine when the engine is under power, but if a rider is approaching a corner, close to the r.p.m. ‘red line’, and then down-shifts, the engine can easily be forced to a speed it was never designed to reach, often with catastrophic results. Additional benefits are realised as the gearbox suffers less abuse for the same reasons, and it is less important for the rider to provide perfectly co-ordinated gear changes.
Probably the most obvious example of slipper clutch use. In order to ‘back
the bike’ into a corner, a rider needs extremely good clutch control.
The action must eliminate enough engine braking to stop rear wheel patter,
at the same time maintaining some engine braking and stability.
You could not for example, disengage the clutch completely, without disastrous
consequences. Whilst there are always exceptions to a rule, I cannot think of any top
line Supermoto rider who does not use a slipper clutch.
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Again, there cannot be many top line racers that do not use a slipper clutch,
where racing regulations permit. You only have to stand at Brands Hatch’s
Druids Corner, or the hairpin at Mallory Park, to see evidence of who has a slipper
clutch fitted to their bike.
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2005 World Supersport Championship saw STM supported riders finish 1-2-3-4-5!
On the home front, STM supported Leon Camier won the 2005 British Supersport Championship. The Italian company’s racing domination is again illustrated by the following, STM equipped riders:
Not so obvious, but again massive advantages for both rider and machine.
Racers report huge benefits in machine control over braking bumps, when he
rear wheel is trying to lock up. In this situation the bike can easily be
stalled, and is very hard to lay into the turn. With many motocross riders
making the move from two-stroke to four-stroke machines, the mechanical advantages
are enormous. The abuse suffered when an engine is over-revved on the down-shift can
be tolerated by a two-stroke engine, but definitely not a four-stroke. The trouble is,
no one told the riders! The reputation of four-stroke motocross bikes have, as a result,
suffered unfairly and totally unnecessarily. A slipper clutch would help solve the
problem. STM are proud to announce Ricky Carmichael to the already impressive International
line up.
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Very similar technical advantages to Motocross. With the increased race duration of
Enduro and Hare-and-Hound events, any improvements to rider and machine fatigue are
even more valuable.
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A quad benefits from a slipper clutch in the same way a sidecar does. With the increased grip provided by the wide rear tyres, the effect of wheel lock through engine braking is even more severe. This makes rider and mechanical advantages even more pronounced than for a solo motorcycle.
Left to last intentionally, this is probably the biggest application of slipper
clutches. If you think that a slipper clutch is not necessary for a road bike,
ask yourself why Suzuki have fitted one to the new GSXR 1000 K5, and Kawasaki
to the ZX10R. This is surely recognition of the benefits to a road rider.
The standard item fitted to these bikes has its limitations, especially in
adjustability. The improvements of fitting an STM replacement can be enjoyed
in the same way that an Ohlins or WP rear shock is chosen to replace the standard
rear shock, when the very best is required.
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