Evo Two Speed Transmission
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image by id10t

Evo's two speed belt transmission is a marvel of scooter technology. This transmission successfully blends the reliability and power of a cog belt driven transmission and the versatility of a two speed mechanism in a relatively worry free package.

The transmission consists of five gears: two for 1st gear, two for 2nd gear and one for the final drive. These five gears power three belts: two for the transmissions 1st and 2nd gear and one for the final output which drives the rear wheel.

This transmission is featured on Evo Powerboards' 2 and 2x.

How does it work?

Centrifugal Clutch (Engine interaction)

This transmission receives power from the engine via a clutch bell bolted to the top shaft. It is a 76mm bell which is quite standard for larger utility engines. Because of this clutch bell, the engine can idle without engaging the transmission.

The 1st gear drive system

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Lower 1st gear pulley with one-way bearing

If you completely ignore the 2nd gear capabilities, this transmission is still quite unique. While in 1st gear, this transmission has two clutch mechanisms: the centrifugal clutch system and the one-way bearing in the lower pulley. Because of these two mechanisms, the engine is allowed to idle without moving the transmission and the scooter is allowed to freewheel without moving the heavy transmission. The 1st gear pulley bearing also has the added benefit of making 2nd gear possible.

The 2nd gear drive system

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2nd gear clutch pressure arm

The 2nd gear drive system is engaged by a clutch mechanism housed inside of the lower 2nd gear pulley. By adding pressure to this pulley, plates inside allow the gear to "catch" the lower drive shaft of the transmission to effectively half the reduction ration which doubles the speed of the lower shaft.

Pressure is produced by a [Bowden cable] which retracts over a large plastic pull attached to the pressure arm. Through a special plate attached to the arm, pressure is transfered into the lower pulley. Since the lower shaft spins faster in 2nd gear that in 1st, the lower 1st gear pulley acts as though it is freewheeling.

Adjustment mechanisms

Short belts' adjustment
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notice the off center nylon cams and the black spacer between them.

The short belts which are housed inside the transmission are adjusted by moving two nylon cams in the top of the transmission. These nylon cams hold the bearings on which the top shaft spins slightly off center. This allows enough movement to properly adjust the belts by rotating these cams. The black spacer allows a perfect finger hold for the process.

Long belt's adjustment
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Long belt tensioner pin sticking out over the final drive.

The transmission sports a bolted pin which extends over the final output shaft of the transmission. This pin will interact with a "puller" which will adjust the location of the bottom of the transmission in order to make adjustments to the belt which runs between the final output shaft and the rear wheel.

Together with the "puller", this setup is referred to as the 1-Touch Tension System.

Maintenance

Maintenance of the Evo transmission is, like any other complicated mechanism, essential to the longevity of operation. With a bit of preventative maintenance and understanding about the workings of the system, this transmission can last a very long time.

See Evo Two Speed Transmission Maintenance for more information.

Modification

There are a few modifications that can be made to both improve the performance of the transmission and lengthen the lifespan of the mechanisms.

See Evo Two Speed Transmission Improvement for more information.

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