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Confession: I found this information on
Renthals'
website so make sure you take a look there, they've a great range of
quality products including sprocket/chain kits. Introduction to TORQUE, WORK and POWER
Torque is the twisting force about a point, sometimes called a
'moment'. The torque is defined as the force multiplied by the
distance from the pivot perpendicular to the force.
Torque = Force x Perpendicular Distance to Pivot
For example: One foot pound of torque is the twisting force
necessary to support a one pound weight on a weightless horizontal
bar, one foot from the pivot. You might directly measure torque when
tightening a nut to a specified torque using a torque wrench. Here,
a twisting force is applied to the nut, until the resistance to
rotation of the nut is equal to the torque required.
Work is the the transfer of energy. The work done is equal to the
force applied multiplied by the distance travelled in the direction
of that force.
Work = Force x Distance Travelled
Power is the rate of doing work, the amount of work done in a unit
of time. The power produced is the work done divided by the time
taken.
| Power = |
Force x Distance Travelled |
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Time |
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For example: If a weight is fixed solidly to the
floor and you try to lift it, you are applying force. However the
weight cannot move, so no work is done on the weight. Although force
is exerted by your arms, no energy is transferred to the weight. If
you lift a one pound weight one foot, then by definition one foot
pound of work has been done. If you take one minute to do this then
you will be producing power at one foot pound per minute.
One horsepower is 33,000 foot pounds per minute. To find the
horsepower of an engine, the torque produced by the engine is
measured and the horsepower calculated. This is done using a
dynamometer which is essentially a brake with a measuring device -
hence the term brake horse power (bhp) which is often used. A torque
curve is produced by plotting the torque measured against the engine
speed.
With torque in foot pounds:
| Horsepower = |
Torque x RPM |
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5252 |
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Using this equation a power curve can be produced
from the torque curve.
How does this apply to a motorcycle?
For the rider, torque is the all-important factor. A bike will
accelerate at a rate that matches its torque curve (ignoring rolling
/ air resistance). The torque peak is the point at which the bike
has maximum acceleration, either side of this peak it is less. For a
given torque at the rear wheel, the acceleration of the bike is the
same, irrespective of the engine speed. Horsepower increases with
the engine speed until well after the torque peak, and only peaks
when the decreasing torque compensates for the increasing rpm. (look
at the equation.) The acceleration at the torque peak is greater
than that at the power peak.
So why do we talk about horsepower so much? Consider a large
waterwheel. While it's obvious that the water wheel generate a large
torque, its rotational speed is very slow and hence its power (the
ability to do work over time) is low. A waterwheel is therefore not
generally very powerful. A powerful engine with lots of horsepower
is one which produces high torque at high rpm.
Theoretically, producing torque at high rpm is better than producing
torque low rpm, as at high rpm you can use gearing. A powerful
engine is useful because it can then be geared down - you don't want
the rear wheel of your bike doing 8000rpm anyway! Gearing down
reduces the speed at the rear wheel with a corresponding increase in
torque. This does not affect the power of the engine apart from
frictional losses. Incidentally a properly lubricated chain drive is
98.5% efficient, significantly better than a geared drive. For road
racing, this theory closely matches reality, but for offroad the
above is not the only consideration. (still awake?!...)
But what does that mean about gearing...
The stock gearing of your bike is likely to have been determined by
choosing a compromise ratio based on what worked best for test
riders in "average" conditions. As soon as the bike is taken out of
average conditions - by engine tune, terrain, track design or rider
style the stock gearing might no longer be the optimum solution - a
different setup might get you round the track faster.
Maximum speed occurs when the driving force is exactly
counterbalanced by the air and rolling resistances. At this point
the acceleration has fallen to zero.
Setting up the gearing of any vehicle is a trade-off between
acceleration and top speed.
Gearing a bike up to produce higher top speed with less acceleration
is done using a larger countershaft (gearbox) sprocket or a smaller
rear sprocket.
Gearing a bike down giving it more acceleration with lower top speed
is done using a smaller countershaft (gearbox) sprocket or a larger
rear sprocket.
The ratio chart shows the gearing ratios for different numbers of
teeth on the gearbox and rear sprockets. The numbers given are the
number of revolutions of the gearbox sprocket required to cause one
complete revolution of the back wheel. These figures are calculated
by dividing the number of teeth on the rear chainwheel by the number
of teeth on the gearbox sprocket.
From the table it is clear that changing one tooth on the gearbox
sprocket has a significantly larger effect on the gearing than
changing one tooth on the rear sprocket. To make a small change in
gearing it is therefore necessary to change the rear chainwheel size
by one tooth, as changing the gearbox sprocket makes a far larger
difference in gearing.
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