CVT Scooter Transmission
Continuously Variable Transmission CVT
How does it work?
Scooters were
meant to be easy and so a lot of complication has gone into making them
simple. Here’s how to demystify the transmission without even getting
your hands dirty. The biggest development
in the scooter world since, well the scooter world began is the now
ubiquitous Constantly Variable Transmission (CVT), and since that is one
of the major driving forces behind concessional scooter licences and
the scooter boom in sales – pause for breath – almost no one knows how
it works. Let’s fix that right here.
The
transmission laid bare. At rest the belt sits between the front pulley
halves, while the rear pulley halves (hidden by the concentric clutch)
are held together.
Scooter
engines, like all automotive engines, need gearboxes to make them more
efficient at driving the vehicle. Without a gearbox, a scooter’s speed
would be completely dependent on the engine speed as the ratio between
the engine and the rear wheel would be fixed – like an old singlespeed
bicycle. Good enough to chug along on the flat, but remember how you’d
run out of legs downhill, and uphill? Getting off and walking was often
easier.
Remember
getting your first three-speed hub-gear bicycle? And what a difference
it made? Then you traded up to a 10-speed derailleur machine. Remember
how much easier life became? Sure, you still had to shift manually, and
you could get caught in the wrong gear at the wrong time, but when
things went right, you could really fly.
That was the
thing about having 10 gears. You could be in the right gear more often,
and maximise your pedalling power. A modern scooter is the same, but
instead of 10 gears it has only one, though that single gear is
infinitely flexible, taking you from tickover to top speed. Here’s how
it works.
You can see the two rear pulley halves and the belt sitting
betweenthem here.In his right hand is the clutch inner.
Inside the variator, you can see three of the rollers and, in the
emptyspaces,the ramps they have to climb to change gears.
The two rear pulley halves, with the loading spring
and the centrifugal clutch before it’s all bolted in
Your 10-speed
bike had two gears (or chain rings) at the front and five at the back.
By operating the gear levers, you shifted the chain between the chain
rings, changing the gearing. There were a lot to choose from, but the
basic rule was a small gear at the front and a large gear at the back
was low gearing, suitable for climbing hills and low speeds (because
each pedal stroke caused less rotation at the rear wheel), while a large
cog at the front and a small cog at the rear was the highest gear
(because each pedal stroke turned the rear wheel more). Moving the chain
from cog to cog required extra, spring loaded chain length to fit the
different sized gears. While scooter transmissions run on the basic big
cog/little cog premise, there are differences.
There
are no separate gears and no extra length to the drive belt to
accommodate gearshifts. That is how the transmission works as a
Constantly Variable Transmission (CVT) system.
The
front and rear cogs of your pushbike have been replaced by a pair of
pulleys in your scooter, each pulley made up of two, conical, sliding
halves with a fixed length belt running between them. The front pulley
is driven by the engine, via gears from the crankshaft. The rear pulley
drives the rear wheel, again via gears. In between that and the rear
wheel is an automatic clutch (so the engine can idle when the scooter is
stopped).
It
is the sliding nature of the pulleys that creates the really clever
CVT. At rest, the front pulley halves are held open by springs, while
the rear pulley halves are held shut. This gives the ‘small gear at the
front and large gear at the back’ effect of the lowest gear on a
10-speed bicycle. As you accelerate, the front pulley halves close
together and that forces the rear pulley halves apart. Remember, the
belt that runs between the pulleys has a fixed length, so when the front
pulley closes up, the belt has to force the rear pulley apart. When the
front pulley halves are forced completely together, the rear ones are
forced completely apart and then you have the large front cog and small
rear cog of a 10-speed bicycle’s highest gear.
The
clever part is that, as the pulleys move apart and close together, the
gearing shifts constantly so that two things happen: you never have to
change gears; and you never find yourself in the wrong gear. Brilliant,
huh?
So
how does the scooter know how to change gear? With another clever
little piece of kit called the variator. This is a disc-shaped thingy
that is fitted to the same shaft as the front pulley. It has a sloped
section and carries a number of weighted rollers in a cage around the
disc. As the assembly spins, these rollers react to centrifugal force
and try to move outwards. As they do so, the assembly climbs the ramp
and forces the front pulley closed, which in turn forces the rear pulley
open and thus raises the gearing.
When
you roll off the throttle, the centrifugal force is reduced and the
springloaded rollers drop back, allowing the front pulley to open
slightly, which allows the belt to ride lower within the spring-loaded,
sliding halves of the pulley, which in turn allows the rear pulley to
close up and lower the gearing.
It sounds
complex to describe, but it makes riding a scooter far simpler than the
old hand-shift set-up. And that’s a good thing. Be happy.
What is gearing?
Gearing, in this instance, is simply a means of
changing the amount the rear wheel turns in relation to the engine’s
rotation.
The variator
The variator is a disc containing weighted rollers
disposed radially in a cage and fixed to the sliding front pulley. As it
spins faster, the weights climb ramps, forcing the front pulley halves
together, and raising the effective gearing. The variator does not in
itself change the gearing – that is fixed by the size of the two pulleys
– but it does affect the rate at which the gearing changes. The weight
of the rollers may be changed to fine-tune the transmission and is a
popular performance modification for scooter owners.
The clutch
The clutch allows the scooter to idle with the engine running by
isolating the rear wheel from the transmission. It operates in much the
same way as the variator, being of a centrifugal design. As you twist
the throttle, the engine increases its speed and the belt drive spins
the clutch. As the rotational speed increases, centrifugal force causes
the clutch inner to expand, and begin driving the clutch outer, which is
connected to the rear wheel.
CVT
Twist and go, baby!
As published in TW SCOOTER MAGAZINE - 19/12/2005
MICHAEL DRABIKOWSKI
Please note that the pictures in the above article are not necessarily from a Roketa Scooter CVT but are general pictures to demonstrate how the CVT functions on your scooter.
Peace to you
Roketa Scooter Man