The increased demand in recent years for off-road and all terrain vehicles has led to tremendous developments in those types of vehicles. Many of the developments have centered around making the vehicle more adaptable to changing road conditions, e.g., dirt roads, pavement and gravel. As the road terrain changes, it is desirable to vary the driving capabilities of the vehicle to more efficiently navigate the new terrain. Prior four-wheel drive and all terrain vehicles were cumbersome since they required the operator to manually engage and disengage the secondary drive shaft, e.g., by stopping the vehicle to physically lock/unlock the wheel hubs. Improvements in vehicle drive trains, such as the development of automated systems for engaging and disengaging a driven axle, eliminated many of the problems of the prior designs. These automated drive systems are sometimes referred to as "on-the-fly" four wheel drive. These systems, however, require the vehicle to be in either 2-wheel or 4-wheel drive at all times.
Generally, all four-wheel drive vehicles include a differential for transferring torque from a drive shaft to the driven shafts that are attached to the wheels. Typically, the driven shafts (or half shafts) are independent of one another allowing differential action to occur when one wheel attempts to rotate at a different speed than the other, for example when the vehicle turns. The differential action also eliminates tire scrubbing, reduces transmission loads and reduces understeering during cornering (the tendency to go straight in a corner). There are four main types of conventional differentials: open, limited slip, locking, and center differentials. An open differential allows differential action between the half shafts but, when one wheel loses traction, all available torque is transferred to the wheel without traction resulting in the vehicle stopping.
A limited slip differential overcomes the problems with the open differential by transferring all torque to the wheel that is not slipping. Some of the more expensive limited slip differentials use sensors and hydraulic pressure to actuate the clutch packs locking the two half shafts together. The benefits of these hydraulic (or viscous) units are often overshadowed by their cost, since they require expensive fluids and complex pumping systems. The heat generated in these systems, especially when used for prolonged periods of time may also require the addition of an auxiliary fluid cooling source.
The third type of differential is a locking differential that uses clutches to lock the two half shafts together or incorporates a mechanical link connecting the two shafts. In these types of differentials, both wheels can transmit torque regardless of traction. The primary drawback to these types of differentials is that the two half shafts are no longer independent of each other. As such, the half shafts are either locked or unlocked to one another. This can result in problems during turning where the outside wheel tries to rotate faster than the inside wheel. Since the half shafts are locked together, one wheel must scrub. Another problem that occurs in locking differentials is twichiness when cornering due to the inability of the two shafts to turn at different speeds.
The final type of differential is a center differential. These types of differentials are used in the transfer case of a four wheel drive vehicle to develop a torque split between the front and rear drive shafts.
Many differentials on the market today use some form of an overrunning clutch to transmit torque when needed to a driven shaft. One successful use of an overrunning clutch in an all terrain vehicle is disclosed in U.S. Pat. No. 5,036,939. In that patent, the vehicle incorporates overrunning clutches directly into the wheel hubs, thus allowing each wheel to independently disengage when required.