Motor vehicles – Having four wheels driven
Reexamination Certificate
1999-06-24
2002-12-10
Lerner, Avraham (Department: 3611)
Motor vehicles
Having four wheels driven
C475S088000
Reexamination Certificate
active
06491126
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a straddle-type all-terrain vehicle and, more particularly, to a straddle-type all-terrain vehicle with a limited-slip differential.
BACKGROUND OF THE INVENTION
In recent years, there has been a growing demand for four-wheel drive straddle-type all-terrain vehicles as opposed to the simpler two-wheel drive variety. This has been largely due to the desire to drive these vehicles in increasingly rough off-road conditions, which require improved traction and improved capability to transmit engine torque to the wheels.
Presently on the market are two types of four-wheel drive straddle-type all-terrain vehicles (ATV's). The first type employs a regressive limited-slip differential at the front and a fully locked axle at the rear. The front differential is either of the speed-sensing or torque-sensing variety. Examples of regressive differentials are the Sure-Trac™ and the Detroit Locker™. These types of differentials will be discussed in greater detail later. The second type of drive train found on the market today uses an over-running clutch in the front and a fully locked axle at the rear.
The regressive limited-slip differential permits engine torque to be transferred equally to the front drive axles provided that the tires have equal traction. A regressive limited-slip differential transfers torque from the slipping wheel to the wheel with traction as a function of the differential wheel speed. However, the transfer of torque diminishes as the wheel speed differential increases, eventually reaching a plateau beyond which no more torque can be transferred (hence the name “regressive”).
The over-running clutch functions according to the difference in wheel speed between the front and rear wheels. When the front and rear axles are turning at the same speed, the vehicle is powering only the rear wheels (2×4 mode). However, if the difference in axle speed increases beyond a threshold value, the over-running clutch couples the front drive axle to the rear drive axle (4×4 mode). Since the vehicle runs mainly in 2×4 mode, steering is easy. However, when wheel slippage occurs and power is transferred also to the front wheels (4×4 mode), vehicle performance becomes less than optimal.
Most current 4×4 ATV's are equipped with a solid rear drive axle. This provides optimal traction because torque is delivered to both rear drive wheels all the time irrespective of the ground conditions. Due to the short wheelbase, it is not necessary to employ a rear differential because the rear inner wheel tends to lift during cornering, allowing natural, non-detrimental wheel slippage. However, when hauling heavy loads on the rear of the ATV, the downward load precludes the natural wheel lift during cornering, thus making it necessary to employ an open differential at the rear or else suffer the consequences of inner rear tire drag. Some ATV's have a manually selectable locking rear axle that allows the user to select either an open rear differential or a closed, locked axle depending on the traction requirements.
In order to fully appreciate the present invention, it is preferable to understand the structure and functioning of a differential.
When cornering, the outer wheels of an straddle-type all-terrain vehicle travel a greater distance (and thus rotate at a greater angular velocity) than the inner wheels. For a non-driven axle, the left and right wheels are free to rotate independently of each other and thus each wheel follows at its own angular velocity. However, for wheels that are coupled to a driven axle, it is necessary to employ a differential (or “diff”) to allow the outer wheel to turn faster than the inner wheel while still transmitting torque to both wheels.
In the absence of a differential, the inner wheel during cornering will drag because most of the weight is on the outer wheel and thus the inner wheel is forced to rotate at the same angular velocity as the outer wheel even though the inner wheel has a shorter distance to travel. The partial dragging of the inner wheel is detrimental to tire wear, comfort and control and furthermore promotes understeer (the tendency of the vehicle to continue in a straight line). Hence the necessity for a differential capable of permitting the inner wheel to rotate at a lesser angular velocity than the outer wheel during cornering while still distributing torque to both the inner and outer wheel.
Engine torque is transmitted through the clutch and transmission gears to a drive pinion mounted at the end of the drive shaft. This drive pinion is typically a bevel gear (either straight or spiral) meshed with a perpendicular (also beveled) ring gear. A differential case (or housing) is fixed to the ring gear so that the case revolves with the ring gear about an axis defined by the drive axle. Inside the differential case is typically a pinion shaft mounted to the differential case and rotatable therewith in the plane perpendicular to the axis defined by the drive axle. Mounted on the pinion shaft are two differential beveled pinion gears which revolve with the ring gear and case but which are also free to rotate about the pinion shaft axis. The differential pinion gears are meshed with a pair of beveled side gears which are each splined to a drive axle. This arrangement is termed an “open differential”. These devices are also colloquially known as “force balancers” since they distribute an equal amount of torque to each drive axle irrespective of the rotational speed of each axle. In operation, the rotation of the drive pinion causes the ring gear and the differential case to revolve about the axis defined by the drive axles. Since the differential pinion gears are mounted to the pinion shaft, they revolve in a planetary fashion around the aforementioned axis and, in so doing, drive the meshed side gears (thereby driving each of the drive axles).
When traveling in a straight line (and not exceeding the traction limit of the tires), the differential pinions and side gears move with the case. There is no relative movement between the teeth of the differential pinions and those of the side gears.
When cornering, the differential pinions still revolve with the ring gear and differential case. Since the outer wheel must now rotate faster than the inner wheel, the differential pinions must now also rotate about their shaft axis. The rotation of the differential pinions compensates for the differential rotation of the outer and inner axles. Nevertheless, the rotation of the differential pinions does not affect the transfer of torque to each of the side gears. Thus, the differential pinions continue to exert a torque on each of the side gears, which in turn, transmit torque to the drive wheels.
An open differential can compensate for any variation in axle speed up to the traction limit of the wheels. In other words, when one wheel begins to slip or spin, the differential case continues to revolve (assuming the torque being transmitted from the drive pinion to the ring gear remains constant, i.e., assuming constant engine torque). As the differential case revolves, the differential pinions continue to revolve around the side gears. However, since the side gear coupled to the free-spinning wheel presents a much smaller rotational inertia, the differential pinions will transfer all of the available torque to the side gear coupled to the free-spinning wheel. Thus, no torque is transferred to the wheel with the greater traction resulting in a complete loss of traction. In the industry jargon, it is said that the differential pinions “walk” around the axle connected to the wheel with the greater traction. In other words, the differential pinions “take the path of least resistance” by driving the side gear splined to the free-spinning wheel instead of driving the wheel with traction.
Thus, although the open differential distributes torque equally to both wheels when cornering, the open differential is susceptible to wheelspin (and thus loss of traction and control) wheneve
Gagnon Claude
Morin Vincent
Robison Gary
Bombardier Inc.
Lerner Avraham
Pillsbury & Winthrop LLP
Winner Tony
LandOfFree
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