Quote:
Originally Posted by Christ
What a CVT should be = two mechanical centrifugal clutches, one that opens with speed, one that closes with speed, and an idler pulley.
This is what my PUG had... 3200 RPM, in high gear, was about 28 MPH.
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I've seen a PUG, but it's been years. As I recalled it had a garden-variety "snowmobile clutch" mechanical CVT, but you're either misunderstanding this type of trans or describing something entirely different.
Rubber belt CVTs have a centrifugal drive clutch that closes with speed against spring pressure and a driven clutch that is not centrifugal in nature that opens along a ramp against a spring responding to belt tension. Sometimes the post of the drive clutch has an idler bearing to reduce transmitted torque at idle speed. Sometimes the centrifugal clutch is mounted to a go-kart-like starter clutch to disengage it from the engine at idle speed. Neither of these last two are specifically required nor common on low-end machines like PUGs and similar contraptions.
Coming off idle the centrifugal drive closes a little putting pressure on the sides of the V-shaped drive belt, transmitting torque along a small radius of the sheave faces. Because belt tension is low, the belt rides along the largest radius of the driven clutch sheaves yielding a numerically low transmission ratio. This allows the vehicle to begin moving as the engine spins up to "shift RPM" (where the clutch begins to shift ratios) at which point you are typically generating as much horsepower as you can for the conditions (speed/load). Assuming you keep your foot planted to the floor the vehicle speed increase allows the engine to begin to speed up above shift RPM at which point the drive clutch's centrifugal force increases, increasing belt tension, pushing the belt up the drive sheave faces and pulling the belt down the driven sheave face, continuously decreasing the numerical drive ratio. This increases load, pulls engine speed in line with shift RPM, and so continues the cycle. Once maximum drive ratio is achieved (assuming WOT all the way) the engine can speed above shift RPM up to engine governed RPM.
If you settle into part-throttle cruise as some speed between idle and full RPM at full shift-out, the decrease in horsepower (and therefore torque) of the engine decreases the tension in the drive belt allowing the drive clutch to balance the rpm-specific centrifugal forces against the driven-clutch's spring pressure and ramp angle. This shifts into numerically lower ratios, pulling down engine RPM. Engine load as a percentage of available torque will increase as the RPM decreases and CVT shifts out, all while the vehicle is traveling at a steady speed on flat ground. Encountering a hill or increasing throttle changes this equation and the CVT back-shifts to bring up engine RPM and increase reserve torque.
Automotive CVTs are all computer-controlled electro/hydraulic contraptions these days. Computer programming controls how they respond, whereas with snowmobile clutches it's all simple physics and geometry. I've driven a Caliber CVT rental and I adapted to it quickly since in my experience it behaves very much like a mechanical rubber belt CVT. I would definitely recommend driving such a vehicle with a scangauge. I think throttle modulation and driving style, along with frequent neutral coasting, would benefit you greatly. I do recall the Caliber had a bit too much engine braking dialed in (something CVTs rarely do well, so some computer controlled ones over-simulate it).