A number of riders have asked me why their troublesome
clutch slips in the higher gears,
usually (or as badly) in lower gears. It seems obvious to them that, since
in the lower gears, then the clutch should slip more while accelerating
But if their clutch isn’t as healthy as God and Yamaha intended it, then
it tends to
in ever higher transmission gears. Why is that so?
The confusion is a result of applying an “obvious” concept to a complex situation.
For purposes of clarity the following should be kept in mind:
· *On most modern motorcycles
the engine is more than it seems. What we see as the
“motor” is an engine, clutch and transmission. The engine is just the part
crankcase, pistons, connecting rods, crankshaft, and head(s).
*The clutch is “upstream” of the transmission, meaning it receives the engine’s
output directly from the crankshaft and then delivers that same power to
*The torque and horsepower (HP is just torque applied to engine RPM’s) made
the engine doesn’t change with changes in gearing. The torque produced by
engine only varies with RPM’s because the engine’s efficiency varies according
to RPM’s and their relationship to the essential design of the engine and
and exhaust systems. So the torque curve rises to a peak, then falls off
* Few dynamometers can measure engine output, as that would require removing
integral transmission from the “package”. So most engine dynos measure torque
and HP at the transmission output shaft and a rear wheel dyno takes its
at the interface of the rear tire and a spinning drum. With both of theses
of dynos the transmission (and rear wheel drive system in the case of the
dyno) plays a significant part in the resulting power measurements.
There are other types of dynamometers, and variations of the above two,
but these are
the most common.
* The torque measured at the rear wheel drops as gearing between the crank
rear wheel rises (lowers numerically) and power is “wasted” through friction
transfer of power between internal components. With the engine in first
torque gets applied to the ground (OR the dyno) than it does in any “higher
As gear ratios rise (drop numerically), the mechanical advantage of the
(read: measured torque on a dyno chart) over the rear wheel drops.
That’s why you’ll see several roughly parallel lines on dyno charts published
most reputable magazines, when they test a new bike. Each line will represent
the results of dyno-testing each transmission gear available in that particular
motorcycle. And each successively higher gear will fall lower on the chart
the gear before it, as a result of gearing effect (mechanical advantage)
losses. If a dyno chart shows only one curve, it should have a note stating
gear the curve was generated, or if it is actual engine torque output, as
by an engine-dynamometer, with the tranny removed.
So why does a problem clutch slip worse in each higher gear, if the available
drops with each upward shift? The answer is because the clutch is “upstream”
of the transmission. The clutch gets the full brunt of the engine’s power
without the torque reduction that the transmission applies. Higher gear
increases the onset and severity of clutch slippage is because there’s another
factor to consider… traction.
Traction is NOT just a concern between the tires and the road. Traction
friction between any two surfaces as they try to slide against each other.
don’t want any loss in traction between our tires and the road. That’s generally
But there are other places where we want varying amounts of traction. As
our clutch with our left hand, we’re adjusting the traction between the
of the clutch to suit our needs at the time (pulling away from a stop, smoothly
the next gear, etc.). The total available traction between the crankshaft
and the road
is only as reliable as the weakest link in that entire chain of power transmission.
The torque made by the engine will be expended as forward motion or at any
where it can overcome the available traction. If your clutch is healthy
and you’re in
first gear, a twist of the throttle should yield strong acceleration (unless
the rear tire
breaks loose and the engine’s efforts go up in wheelspin and smoke). If
the traction at
the rear wheel holds, the bike goes forward (and/or maybe the front wheel
But, in each successive up-shift, your forward acceleration will be with
because the engine’s power to the rear wheel drops with each successive
(and because air drag rises with increases in velocity, but that’s another
relative to this discussion). If at any time the rear tire should engage
a surface that
causes a significant drop in available traction then wheelspin may result.
wheelspin can occur in any gear, it’s more likely to happen in lower gears,
rear-wheel power is strongest in the lower gears, as shown on the rear-wheel
More power equals more ability to overcome available traction. If the clutch
worn out or suffered damage as a result of abuse or overheating, or simply
it’s useable life, it may slip and allow the engine to gain revs faster
than the apparent
acceleration would warrant.
The reason a less-than-healthy clutch slips more in higher gears, than it
does in lower
is because the engine’s ability to overcome traction at the rear tire is
the higher gears, while the power applied to the clutch has remained constant
to RPM). So, if the rear wheel traction holds, the clutch can become the
link between the crankshaft and the road.
Ken "the Mucker" Sexton.