Monday, February 28, 2011

Aprilia SR50 DiTech Transmission Tuning… Do I need an Engineering Degree?

Part 2: Understanding the fundamentals

There are two options for CVT tuning, (A) a scientific method as I explained in part 1 using a 1:1 gear drive and Dynamometer or (B) a not so scientific method (The butt dyno).

Option (A) Reviewing of the power curve with the 1:1 drive will reveal where peak power occurres. The CVT assembly is then reinstalled in the SR50’s transmission. From a standing start, the throttle is again held fully open and the SR50 begins to accelerate to top speed.
If the clutch is doing its job optimally, then the rpm’s will rise, and then hold at the peak power rpm while the variator adjust the ratio, and then finally continue to rise again once the maximum range of the CVT adjustment has been exceeded. This video will let you hear and see a properly setup CVT... you will hear the rpm rise, hold then rise after the CVT has completed its transition
If the shift-speed rpm is off, then an adjustment is made to the roller weights. This acceleration profile is useful in explaining the overall function of the CVT system. As the throttle is applied from rest, the engine speed quickly rises, spinning the front variator assembly. At this stage the belt is resting against the innermost part of the front variator, and is pushed outward on the rear assembly by the squeezing force of the large torque driver spring. As the rpms rise, the rollers in the front variator are flung outward in their slots and have the effect of squeezing the front pulley halves together, thus gripping the belt and starting it to move.

Click to enlarge

As the belt moves it begins to rotate the rear clutch. The rear clutch begins to spin and accelerates as well. But the SR50 hasn't started moving yet. Inside of the rear clutch are three brake shoes that are held in place with extension springs. Once sufficient rpm is achieved centrifugal force starts to move the shoes in the rear clutch outward against spring tension of the three little extension springs. The shoes engage on the drive bell and the SR50 begins to move.

The SR50 now begins to accelerate and the rpms briefly over-rev as the shoes seat and settle in to the “shift-speed”. The elegance of the CVT is in this ability to maintain the rpm independent of road speed. When the rpms go above the shift speed then the increased centrifugal force pushes the variator rollers out farther, squeezing the front pulley halves together more, thus slightly lowering the ratio of CVT (higher gear) and therefore dragging the engine speed back down. 

Think of what happens when you shift a manual transmission from 1st to 2nd, the rpms drop. The opposite also applies, when the load increases (like going up a hill) and the rpms drop, the clutching automatically compensates by easing the front pulley halves back apart, increasing the ratio (lower gear), until the shift-speed is achieved again. In practice, this happens so quickly, and on such a minute scale, that these adjustments are completely unnoticeable.

The transition of the CVT: 

The variator roller weight is the primary means to adjust shift-speed. A higher weight will have the effect of squeezing harder at a given rpm, therefore decreasing the ratio (increasing the “gear”) and decreasing rpm. Likewise, a lower weight will increase the ratio as well as shift-speed. 
Finally, once the adjustment range of the CVT has been exceeded (when the belt is all the way out on the front pulley, the SR50 can continue to accelerate, but no longer at constant rpm. The transmission will essentially act like a manual gearbox stuck in top gear and the engine will continue to accelerate the SR50 until it runs out of power, or hits the rev limiter.  It's important to understand this since the power of your engine will fall off quickly after the peak power is achieved.

Since the Euro laws have limited scooters to 30 mph, they will reach this "rev-out" point fairly early.
To reach higher speeds will require modification of the final drive gearing such as new transmission primary or secondary gears. Otherwise the motor will be spinning at 10,000 rpm +, possibly way past its peak power, assuming the rev limiter will even let it.
The rear torque driver spring (contrast spring) also has a very specialized function which is to maintain enough tension on the belt to keep it from slipping. This also is a bit of balancing act because too much belt tension translates to inefficiency. It’s kind of like over-tightening the chain on your bicycle, power is wasted by over-stretching the belt. On the other hand, too little tension and the added power of your recent engine modification will vanish as a slipping belt won't transfer the power to the rear tire. The trick is to find just the right tension without overdoing it.
A secondary influence of the contrast spring is that it has a slight impact on shift speed since the belt tension needs to be reacted against by the front pulley halves. The higher belt tension tends to push the belt deeper into the front sheaves making the ratio higher, and revving the motor higher. Therefore, for a given desired rpm you would need to compensate with a higher roller weight to bring the rpms back down. After a contrast spring change, it’s good practice to re-optimize the roller weights.
On a factory clutch, the three shoe springs are really the only clutch parts that enjoy the freedom of rider preference. But their role in ratio adjustment is short lived once the SR50 has started to move it’s all up to the rollers and contrast spring. Installing a very tight, stiff set of springs will provide a higher "stall" speed, effectively raising the rpm during launch. Alternately, a softer set of springs will provide a little easier, smoother engagement. After the shoes have engaged, there’s little if any effect of the shoe-springs.