We Dig into a Clutch Sheave Upgrade
It's time we revisited the guys at Rigid Industries in Mesa, Arizona, to see their latest addition to the '08 Yamaha Rhino 700 they're providing for our giveaway. Their building the UTV with some cool goodies, then it's all to be given away to some lucky reader of our UTV specific mini-mag SideBySource. In each issue we're detailing a piece of the build up and you can go to Rigid's website (www.rigidindustries.com) to fill out an online entry form. The winner will be drawn at random at OFF-ROAD Magazine headquarters on September 10, 2009. Limit one entry per household address.
In our last issue we showed you the tuned performance package that Rigid installed. This kit included an intake with K&N filter, Dynatek Digital Ignition ECU, and DMC dual exhaust for a substantial bump in power output. Now it's time to optimize the transfer of that newfound power to the tires.
The Rhino uses a continuously variable transmission (CVT) to provide the gearing needed from slow speed takeoff to high speed cruising. A belt is driven by a set of sheave pulleys that have the ability to change their effective size (and thus overall gear ratio) by allowing the tapered pulley faces to change width, making the belt ride on a larger or smaller pulley circumference.
A clutch sheave on the primary (drive) pulley sandwiches the belt between the tapered pulley faces. At low speeds, the pulley halves are fully separated and the drive belt sits low on the faces emulating a small pulley diameter. This provides slow speed and high torque. As the rotational speed of the engine (and pulley) increases the pulley faces move closer, creating a larger effective pulley diameter for the belt. This make the Rhino go faster.
The transition of the pulley faces on the primary sheave is controlled via weights that move outward centrifugally as engine speed increases. This affects the rate at which the transmission drive ratio is increased. The weight size is chosen to match the power output of the engine.
When the engine power is modified, it's best to also change the gearing curve of the clutch assembly to get the best power transfer. This is done with custom machining of the clutch sheave assembly. Rigid recommends installing a modified sheave to increase top speed and gain a small increase in bottom end torque. They say this is the drivetrain upgrade that gives you the biggest bang for your buck. A modified unit can increase top speed by 7-8 mph, and a few hp more with other engine mods. Follow along with our installation this month as Matt Weiss upgrades the Rhino and shows you what it takes to install a sheave kit.
We just found out that GBC will be supplying the tires for this build and we'll include info about them in a future article. Next month, we'll show the addition of a full rollcage and soft top to the UTV so keep watching ... and don't forget to enter to win the Rigid Industries built Yamaha Rhino! SBS