Tuesday, March 1, 2011

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

Part 3: Making adjustments
Option (B) So, how does the average scooter owner do this?
The reality is that most riders will replace this highly specialized process with good old street testing. Although seat of the pants testing goes a long way, it can’t be relied on for small gains or the last 5% - 10% of refinement. Using a stop watch (built into your digi-dash), a marked-off flat section of road, and a consistent rider you can dial in your CVT with some trial and error. Pick a section of road that’s safe to travel, is flat and has few ruts, bumps, etc. The length of run should be just enough to allow the SR50 to just come off the CVT, where you notice the rpms starting to climb again as you heard in the video.  Make at least three pulls with your engine fully warmed up and use the average time.
Off the line the rpms should rise quickly and over-rev just slightly. This is due to the rear clutch shoes slipping and finally engaging fully. The amount of slip and the length will vary by rider weight. The only available adjustment for this on the OEM clutch is to replace the springs with different tensions. Once the rpms settle into a steady whine, they should hold steady until the pulleys run out of ratio adjustment then start rising again (rev-out point). If you cannot achieve the ideal engagement with the OEM clutch, a performance clutch such as the Malossi Delta offers not only 6 different spring choices but 3 different spring anchor points, 3 different weight positions and 2 sets of weights. Mathematically, that’s 108 setup possibilities vs the OEM’s 3.

Using a systematic trial and error method, you can then dial in your roller weights. If you notice that your SR50 really starts to accelerate harder after the rev-out point, that's an indication your CVT rpm setting is too low, go to a lighter roller weight. The reason for this is that the CVT is keeping the motor from reaching the peak power rpm, but after the pulleys run out of ratio, the engine finally drives through that "sweet spot". As you keep moving down in roller weights you'll notice your times will improve but begin to flatten out, then start to take an increase again as you pass the peak power rpm. It may be helpful to plot your times against roller weight on some graph paper and draw a curve between your points.   
Click to enlarge

Swapping out your CVT parts can be challenging. First, remove the CVT cover. Some light tapping from the edges of the cover with a soft faced hammer can help to break the seal. Finally, coaxing the cover off the two dowels make take some additional tapping, wiggling, and prying since the steel dowels have a tendency to rust slightly in the aluminum cover.
Removing the front variator is relatively simple if you have an impact wrench. If an impact is not available, various tools are made to assist this. Just spin the nut off the crankshaft and slide off the outer part of the pulley assembly. Next, remove the belt.Last, slip off the inner part of the variator. Make sure all the surfaces are fairly clean and free from burrs.
New rollers in a variety of sizes are available from High Gain Tuning and come in weights ranging from 4 grams to 8 grams per roller. The Piaggio engine uses 19mm X 15.5mm rollers and we have found 4.8gr - 5.3gr are ideal for most derestricted setups with a pipe. Lighter for 49cc and heavier for 70cc. The Morini uses 17mm X 12mm rollers and 6.8gr - 7.2gr work best, again for most derestricted engines with a pipe go lighter for 49cc and heavier for 70cc. To limit the amount of rollers you purchase, you can use three of one weight and three of another (alternating) to get just the right rpm for testing.

To install, reverse the disassembly process and torque the crank nut to the proper specs.  
The rear clutch is a bit more involved. Remove the nut holding the clutch bell onto the shaft. Once the bell is removed, you’ll notice a large nut on the rear fixed pulley assembly about 34mm or so. There are two options. One is a socket and impact wrench, the other is a special tool option. Removing this nut will allow the clutch to be removed, be cautious; it is under heavy spring pressure. With the clutch removed you will now have access to the clutch adjustments and or just springs.

The shoe springs take some finesse to remove and reinstall. When installing new springs, a heavier spring will delay the engagement rpm (higher stall) and a lighter spring will do the opposite.  And lastly, to ease the process of parts swapping, don't worry about installing all the cover screws, three or four will be more than enough for testing purposes. However, do make sure to use the cover. For two reasons: one is for safety, the other is that the cover on a Piaggio has an outboard bearing in the cover that provides critical support to the rear input shaft and the starter drive pilot.

Make sure all your adjustments are evaluated on a fully warmed up engine and transmission and don’t try to make adjustments on days when the outside temperature is extreme or outside your normal riding comfort.

Happy Tuning.