Monday, February 28, 2011

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

Part 1: Background and Basics

If you have ridden an Aprilia SR50 DiTech just once you already know it is a twist and go scooter. Twist and go meaning there is no need on your part to shift or change gears while riding and no clutch to disengage at a stop. This is achieved by using a CVT (continuously variable transmission). The first CVT friction belt transmission like the SR50 uses was patented in 1886 by Daimler and Benz and they continue to be very popular today in many industries. The SR50’s CVT is not extremely complicated but there are computer controlled CVT’s that require an engineering degree to understand. The SR50 CVT is about a 3 on a scale from 1-10 in complexity and can be maintained and setup by the owner with a little knowledge of its design.

Your Italian made SR50 was shipped to the US with the CVT already setup. Unfortunately was it not only setup in a restricted mode mechanically (to comply with European laws) it was setup to operate with an electrically restricted ECU. One of the first changes we make to the SR50 here in the US is remove the Euro restrictions, mechanical and electrical. Since the CVT was setup to operate with the restrictions in place you now will need to tune the CVT to operate best in its new derestricted mode. If you continue past the basic derestriction process and add any number of performance parts available for the SR50 you will need to re-tune the CVT.

The clutch is one of your transmissions components and before we start tuning the transmission, we need to first understand what the clutch does and why it was designed the way it was. The automatic CVT is designed to continuously vary the transmission gear ratio while holding the engine speed constant. If you were to examine the power curve of your SR50 2 stroke engine it would rise, and then fall very quickly near its peak power output.

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To get the best performance from your 2 stroke SR50 you want it to operate at the rpm which peak power occurs and there is the beauty of the CVT transmission and its ability to hold the engine there. To identify this rpm the quickest on a CVT scooter a device which replaces the CVT with a 1:1 gear drive is needed. This device would be much like running a gear case transmission scooter in its final gear. A Dynamometer is the ideal place for doing this since you have no slippage after installing this and it becomes very difficult to start and stop making street testing dangerous.

When you modify your SR50’s engine by simply derestricting it and adding a performance pipe you've shifted that peak power rpm. Without readjusting the clutch to realign the controlled rpm with the new peak power, full advantage of the engine mod may not be achieved. As in a restricted SR50, the stock clutch would be adjusted to hold the engine at 5000 rpm from the factory.

If the clutch wasn't readjusted to run at the rpm necessary for the pipe to perform best only a small percentage of improvement would be gained since there's another .84HP available at 9000-10000 rpm. If the clutch was properly adjusted then max power available to the SR50 would be achieved.

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