100 MPG Madness, Project Aprilia SR50- Modding the driveline


Malossi roller weights, contrast springs, torsion controler tuning aids
To tune our drivetrain, these are the calibration parts, roller weights, contrast springs and torsion controller bearing.

Variators are tuned by using rollers of different weights.  Lighter rollers upshift slower causing the engine to rev out to a higher rpm under acceleration.  Heavier rollers upshift faster, dropping the rpm.  The trick is to test various roller weights to find one in which the shifting rate matches the engine’s power characteristics.  After much trial and error we settled on a set of 7 gram weights which seemed to give the best acceleration.

Malossi multivar 2000, wpc treated and dr pulley sliders
In our final state of tune, we WPC treated our variator and swapped our rollers for 7 gram Dr. Pulley sliders. Just as on the gear up kit, these parts are also WPC treated.  It helps the variator work with more smoothness and consistency and decreases wear on the drive face and ramps.

At this time we replaced our stock drive belt with a Malossi Kevlar reinforced one.  The Kevlar belt gives greater grip with less slippage and a longer life, important with our much increased level of power.

After we discovered what weights our bike liked we finally swapped in a set of Dr Pulley sliders to replace the Malossi rollers. The Dr Pulley sliders are cam shaped instead of round which allows the ramp plate to sit 0.020″ lower in the variator at rest.  This gives a lower gear ratio out of the hole, improving low speed acceleration.  The Dr Pulley sliders have less sliding friction then the round rollers giving a smoother and consistent variator action. 

After some research, we decided to try the Dr Pulley clutch. We knew that we would need a lot of tuning flexibility and this clutch is the most adjustable on the market. The Dr Pulley clutch is awesome because you can adjust the stall speed via the centrifugal springs on the shoes. These control the clutch’s engagement RPM. We wanted to raise the launch rpm to make up for the sluggishness caused by the higher gear ratio. This would get our SR50 moving right as the engine was getting into its powerband. What makes the Dr Pulley cool is that it can totally lock up solidly even with a high engagement speed. With other race clutches, when stiff springs are used to raise the launch RPM, the clutch loses centrifugal locking force. This makes them slip more, not exactly a good combo for top speed or acceleration. Dr Pulley clutches have a cam wedging device that mechanically locks the clutch shoes on the drum once they are engaged by using the engine’s torque driving force to help lock the clutch. Thus, the Dr Pulley is the only clutch that can have both a high launch RPM and a solid top end lock up.

Dr Pulley sliders
We got an assortment of Dr Pulley sliders for tuning.  Yes these bits can be expensive but tuning the drivetrain is just as important for performance as tuning the engine.

The rpm at which lock up occurs and how hard the clutch locks up is adjusted using an additional set of springs called pillow springs. The pillow springs counter the engines torque and reduce the cam wedging action. The stiffer the pillow springs, the higher the lock up rpm and the smoother the lock up will be. The Dr Pulley clutch has a much more durable semi metallic friction material as opposed to the cardboard like stock stuff.

Dr Pulley clutch
The Dr. Pulley clutch is the most tuneable on the market, it comes with this large assortment of pillow and shoe springs to tune both the stall speed and the lock up percentage.  You can have both slip and grip with this clutch.

When testing the clutch we could immediately see that this had a lot of potential as we could detect no slip and the launch speed was raised slightly. We had to do more tuning. The Dr Pulley clutch comes with a big assortment of springs for tuning the launch and lock up rpm. The clutch is shipped with the softest blue spring in place. This spring started to engage at 5000 rpm, and grabbed at 6500 rpm. We needed more to get the engine into its powerband and opted for one step stiffer, the red spring. This spring started to engage at 5500 rpm, and grab hard at around 6800 rpm. We also ran the soft purple pillow or lock up spring entirely for a hard lock up.

Leave a Reply

Your email address will not be published. Required fields are marked *