Abstract: A four-wheel drive straddle-type all-terrain vehicle has a progressive limited-slip differential that functions as an open differential when the wheel speed differential is less than a predetermined threshold, or cutting point, and functions as a progressive limited-slip differential when the wheel speed differential exceeds the cutting point. The progressive limited-slip differential is able to transfer torque to the wheel with greater traction as a generally linear function of the wheel speed differential, meaning that the more one wheel slips with respect to the other, the more torque is transferred from the slipping wheel to the wheel with traction. The progressive limited-slip differential comprises a rotary shear pump that drives a piston against a pack of clutch plates. The force of the piston against the clutch pack locks the left and right axles together to provide optimal traction in slippery conditions.

Abstract: The present invention is directed towards a motorcycle or off-road use with an improved engine, air inlet, exhaust and engine control management. The engine is an improved four-stroke engine with a monocoque cylinder and crank case. The air inlet has been moved to the head tube of the motorcycle frame. The engine is reverse flow with the exhaust coming out the rear side and being shortened to less than about 50 inches. The engine control management has been modified to include an ignition interrupt, gear based ignition timing and after market tuning capabilities.