Aug 14, 2007

CVT vs Variomatic: a clarification

A CVT, constantly variable transmission, is not a variomatic. The latter is a simple system. A centrifugal clutch is the heart of the operation. The clutch itself is like a brake drum, but with more shoes. As the engine spins up the clutch, the centrifugal force expands the shoes until they start rubbing against the drum surface. The vehicle moves forward when the shoes generate enough friction to start the drum rotating, which in turn, starts the driven wheel(s) rotating. As you can see, full shoe-drum contact happens after a certain speed and until then, a fair amount of engine power is wasted. The details, as usual, can differ. There's a number of forms the 'shoes' take, including hooked levers and stuff, but the principle is the same. That's the reason your 7 bhp Kinetic Honda DX is no match for your 7 bhp Hero Honda Splendor. In effect, the Kinetic Honda's transmission is a centrifugal clutch, not a variomatic transmission.

The CVT, on the other hand, is a far more efficient and far more complex device. As the name suggests, the CVT (which is actually a development of the DAF Variomatic transmission) is like a gearbox with an infinite number of gears. Usually this takes the form of a tapering cone that drives a belt/chain etc. As the engine speeds and vehicle speeds rise and fall, the belt/chain travels up and down the cone for maximum efficiency. From the saddle/driver's seat, a CVT is quite disconcerting because the engine revs right up to somewhere above the torque peak where the engine performance translates best to motion. And as soon as the engine does this, the CVT holds revs constant while the actual speed rises. You have this disconcerting feeling that you're gathering speed with no extra effort as the engine speeds don't rise. Very common on snowmobiles, the damn things will hit a straight at, say 6,500 rpm, and with a constant engine note, the things will gather frightening speed as they thunder down the strip. For more technical detail, look for Audi Multitronic, which I recall had a very clear description.

[Note: I haven't researched this, so this is how I understand it. Correct me if I am wrong please.]

2 comments:

JulianPaul said...

Whoops! Looks like i provoked a big post. No seriously, I thought we referred to CVTs as varios and vice-versa.

Someone once told me that the transmission in the Kinetic Honda has the cone-belt system, which lead me to believe that it had a very efficient CVT system. That's why it was considered miles ahead of regular centrifugal clutches like on the sunny and other crap.

can you confirm the kinetic had only a centrifugal clutch and NOT the belt-cone thingy ?

Glifford said...

Julian is right.

Transmission in scooters like the Activa, KiHo, Scooty, Wave, Kriztal, Dio and even the "Mighty" Blaze is a combination of the centrifugal clutch and the belt and cone mechanism.

The centrifugal clutch is used to ensure the engine is disengaged (from the wheels) at starting and for idling and can rely on the twist of the accelerator (rising engine RPM) to ensure smooth engagment, rather than the skill (and fingers) of the rider (as in a manual clutched vehicle).

The cone belt is simply used to vary the ratios in order to get good pickup and high RPMs and low speeds and steady cruising at low rpms and high speeds.

On the other hand there were several vehicles which infact had/have only the centrifugal clutch and cone-belt. These include the Kinetic Luna (older), TVS 50 and Bajaj Sunny. Here the clutch is used precisely for the reasons above..

Once engaged the vehicle speeds are in direct proportion of the engine speed as they have only one ratio (fixed down by a chain and sprocket). Hence the engine is tuned to give a compromise of both initial torque and high speed performance. That is why people complain of either the pick up of their fixed speed vehicles of the top end performance.

Incidentally the centrifugal clutch was also used in the (manual geared) Hero Honda Street.