Variator Rampability

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Wheelman-111
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Variator Rampability

Post by Wheelman-111 »

Greetings:

Variator tuning is a balance of physical forces. The gram weight of the rollers is a part of the balance. Engine crank RPM is another. So greater engine RPM and/or heavier rollers make the scooter CVT want to shift "into 5th gear" more readily.

The counteracting force comes from the rear pulley "Contra" spring, always pushing the rear pulleys closed, which tends to push the belt outward, which in turn shortens the slack toward the front. Thus the belt in turn will tend to spread the front pulleys, dropping back into "first gear" as soon as the RPMs decrease. The absolute strength of the Contra spring will determine how much RPM it takes to make a given set of rollers move all the way to the edge of the variator.

However consider Midrange performance. Once the rollers have "maxed out", what does it take to get them moving back to the variator's middle? How quickly that happens, and under what circumstances is also determined in part by the physics resulting from the surface angles confining those rollers. (One more factor is the cam action of the rear pulley. Those come in several "flavors" too, but the effect is beyond the scope of this discussion.)

The angle of the roller ramps may be different - steeper - on the SEF ramp plate.
Polini makes several different plates, each with the ramp angle value stamped into it. (I've seen 15 and 25 degrees, making me suspect there's also a 20 out there somewhere...) I suspect the same is true for other variator makers. Ultimately, the plate ramp angle has to be a good complement for the variator ramps, which unlike the ramp plate have a curve cast into them.

In a given Variator, a flatter 15 degree ramp offers more resistance to the spring/belt force driving the rollers back to center. When you release and then reapply throttle, the RPMs don't jump up very quickly. The engine buzzes along at a steadier RPM, hopefully close to but below the range where it makes best power. This is good for relaxed, steady-state cruising at constant speeds, good mileage, and low Drama.

A 25-degree plate offers less resistance to the contra spring/belt centering forces. Because of this, the rollers scurry back to center more quickly, so off-to-on throttle adjustments result in a quicker gain in engine RPM. This is better for racing or playing cat-and-mouse in the twisties. The engine can gin up its best HP very quickly, at the expense of gas mileage.

So the "Right" plate for you depends on where and how you ride. Unfortunately, I have yet to see anywhere you can buy a Polini 15, 20 and 25-degree ramp plate on its own. I for one would like to experiment more with this tuning variable.
Wheelman-111
Most of my money is spent on scooterparts. The rest is just wasted.
"ISO": '03 Vespa ET4 Malossi187 74MPH
Flash 9: 2001 Elite SR Contesta 72 ZX Tran, 9:1 Gears, Stock Airbox/Carb/Pipe 58.8 MPH
Punkin: 2010 Vespa/Malossi S78, 61MPH
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