100 MPG Madness, Project Aprilia SR50- Modding the driveline

motoiq project aprilia sr50

100 MPG Madness, Project Aprilia SR50- Modding the driveline

By Jeff Naeyaert

In our last installment, we had greatly improved the power of our Aprilia SR50 by installing a Malossi big bore kit ported by AF1.  The big bore kit brought our displacement up to 68cc and improved our acceleration by a bunch.  However, our top speed didn’t change a whole lot, despite our gains in hp and torque as our redline was only increased by a few hundred rpm.  Our top speed was a disappointing 55-56 mph and we were hoping to see 60.  We were also annoyed by a distracting check engine light on our digital dash due to compatibility issues between the Malossi ECU designed for Euro versions of this scooter and our USA spec dash.

To read more about our project click here!

malossi high ratio primary gear for aprila SR50
To improve our top speed we installed Malossi's gear up kit which gives the final drive a higher ratio for more top speed.  The lustrous sheen is WPC treatment which we applied for lower friction.

To make use of our newfound power, make for easier high speed cruising and to increase our top speed we decided to increase our primary drive ratio–the equivalent to changing the ring and pinion on a car.  We used Malossi’s final drive gear set for our SR50.  Before installing it, we WPC treated the gears to reduce friction.  With the gears in place our top speed jumped to 60 mph and our little scooter could easily cruise at 50-55 mph without much stress at less than ½ throttle.  The addition of the gear made everyday commuting practical, even on high speed city streets. The added power and top speed made us able to ride in any lane like a big bike instead of hugging the curb which made riding safer and less stressful. However, our taller gearing cost us some off the line acceleration and we had to make some changes to correct that. 

variator operation
Pictures courtesy of Apriliaforums.com  In this picture you can see the variator and rollers in the low gear position at low rpm.  The drive belt is low on the pulley and the roller weights are low in the ramps of the variator.
variator operation
At high RPM, the roller weights are forced outward by centrifugal force moving the variator pulley over and forcing the belt to ride higher on the pulley.  Changing the weight of the rollers affects how soon this happens.  This is the high RPM, high gear position.
contra pulley operation
The rear pulley must also move to allow the belt position to change to change the gear ratio.  The force of the roller weights is balanced by the tension of the contra or contrast spring.  Changing different weight springs affects how the variator performs as well.  This is the pulley in the low gear. low rpm position.
contra pulley operation
As the belt rises to a higher and higher position in the variator at higher and higher rpm, it drops to a lowerpositionn in the rear pulley, sort of like upshifting on a 10 speed bike.  here is the pulley in high gear position.
transmission belt position
When viewed from the side, the belt position looks like this for low gear low rpm.
cvt belt postion
For high gear and high rpm the belt looks like this.  Turning the transmission is achieved by changing the variator ramp geometry and diameter, roller weight and contra/contrast spring tension.  Transmission tuning to match the engines power characteristics is a very important and often overlooked aspect of scooter tuning.

We replaced the CVT transmissions variator with a Malossi Multivar 2000 unit.  The variator is the part of the CVT transmission that controls the gear ratio of the transmission.  Centrifugal weights in the variators ramp slots move outward at high rpm, wedging between the variator body and backplate  This wedging action causes the variator body to move outward pinching the drive belt between the variator body and the fixed outer pulley.  The drive belt rides higher on the pulley thus raising the gear ratio.

malossi multivar 2000, contrast springs and belt for aprilia sr50
Malossi Multivar 2000, kevlar belt, contrast springs and Delta clutch which we ended up not using.

The Malossi Multivar 2000 has ramp plates cut at a more aggressive angle to hold the engine at lower gear ratios longer, allowing the engine to get into its powerband faster.  The Malossi Multivar 2000 also has a bigger drive face than stock (100mm vs the stock 97mm) allowing a higher top gear belt position for more top speed.  To assure smooth variator action and to help make the variator last longer we treated it and the backplate with the WPC process.

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