Motor vehicles – Transmission mechanism – Final drive axle movable
Reexamination Certificate
2001-08-23
2003-06-24
Dickson, Paul N. (Department: 3616)
Motor vehicles
Transmission mechanism
Final drive axle movable
C180S348000, C280S124167, C188S017000, C188S01800A
Reexamination Certificate
active
06581715
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is related to the field of vehicle suspensions and drivetrains. More specifically, the invention relates to semi-independent suspensions and drivetrains for vehicles.
2. Description of Related Art
Numerous designs for suspension and drivetrain systems are known and used in the manufacturing of various types of vehicles. It is known in vehicle engineering that particular designs provide specific advantages in particular applications. Most of the developments in the designing of suspension and drivetrain systems have been centered around automotive applications.
In recent times, smaller specialized all-terrain vehicles (a.k.a. ATVs) have gained in popularity as recreational and utility vehicles. As the popularity of ATVs has increases, so to have the performance demands placed upon them. Consequently, manufacturers of ATVs have responded with performance increases in certain areas, such as, increases in engine power and vehicle size. Such increases in engine output and vehicle size translate into increased inertial effects and extreme dynamic loading. These more powerful, massive ATVs usually require more skill and/or effort by the operator to maintain control during operation. However, ATV manufacturers have had very little success in modifying the previously mentioned automotive suspension and drivetrain designs to optimally adapt them for ATV use.
ATVs require the development of specialized suspension and drivetrain systems to improve operator controllability while continuing to withstand the rugged demands of their off-road application. Typically, ATVs have one or two front wheels and two rear wheels axially mounted on a solid axle in a dependent manner by a swing arm that pivots about a transverse axis of the ATV. Such a system is illustrated in U.S. Pat. No. 4,582,157 to Watanabe. The limitation and disadvantage of this suspension and drivetrain design is that the two rear wheels are mounted on a solid axle, which is axially coupled to a swing arm in such a way that it is only allowed to pivot about, and constrained to be parallel with, the transverse axis of the ATV.
Currently, the three and four wheeled ATVs using the '157's design, yield three undesirable characteristics that have negative effects on vehicle stability. The first two of these undesirable characteristics are in effect during both forward and turning or cornering operations of the ATV. These two characteristics are termed in vehicle engineering as suspensions having a roll center at ground level and possessing infinite roll resistance.
Having the roll center at ground level results in poor roll stability because the center of gravity (a.k.a. CG) of the vehicle can only be designed far above the vehicle's longitudinally oriented roll axis, potentially resulting in increased dynamic roll moment (a.k.a. torque). Infinite roll resistance implies that the suspension doesn't incorporate any roll motions to absorb roll energies. This means that all of the energy that is transferred via the unsprung mass (a.k.a. unsprung weight) from dynamic roll loading, is transferred directly into rolling the sprung mass (a.k.a. sprung weight) of the vehicle. Thus, infinite roll resistance translates into a harsh ATV dynamic roll response that is often difficult to predict and control by the operator. Even during simple forward motion, operating such an ATV can be like riding a twisting, bucking bronco when traversing uneven off-road terrain.
The third undesirable characteristic comes into play when the solid axle drivetrain of the '157 patent is used and the operator is attempting to negotiate the ATV to turn or corner. For the operator to negotiate the ATV around a turn, a sufficient turning moment must be generated by the operator to overcome all resistive turning moments. Usually, these resistive turning moments are primarily caused by inertial effects, which are overcome by the operator simply turning the front steering mechanism of the ATV. This steering action imparts the needed centripetal reaction from the front tires to overcome the inertial turning moments that work to maintain forward motion of the ATV.
This third undesirable characteristic, which is imparted due to the solid axle constraining the rear wheels to rotate at the same speed, is a mechanical counteracting turning moment, and it's contribution is only present while both rear tires are in sufficient traction with the terrain. This mechanical counteracting turning moment causes the ATV to experience a condition termed as understeer. For the operator to better negotiate this ATV to turn, one must overcome this understeer effect. This is typically accomplished by the operator leaning outward to shift the CG of the sprung mass such that a sufficient roll moment is imparted to cause the inside rear tire to lose traction with the terrain, thereby decoupling the mechanical counteracting turning moment caused by the solid axle. Thus, the operator must perilously put the ATV in an unsafe and unstable inertia induced roll condition in order to eliminate or reduce the mechanically induced understeer effect.
The sudden removal of this counteracting turning moment results in a nearly instantaneous transition from a quasi-static understeer condition to a sharp oversteer condition. This oversteer works to worsen the preexisting unstable inertia induced roll condition. Depending on the skill and strength of the operator, this situation can result in a rapid loss of operator roll control and vehicle rollover.
In light of the disadvantages inherent in the above suspension and drivetrain system, it has been recognized that significantly improved vehicle roll dynamics could be obtained if the rear suspension was designed such that the rear axle could also pivot about the vehicle's longitudinally oriented roll axis. These types of semi-independent suspensions offer variably finite roll resistance characteristics which are desirable for increased roll stability and traction.
The most important function of any suspension is to keep the tires in contact with the ground, while maximizing stability. Semi-independent rear suspension motion is all that is necessary for off-road ATV applications because the tires used are of low pressure, and they have rounded shoulders with radical tread patterns extending well into the sidewall region. These tire characteristics nullify the need of having a fully independent suspensions because the tires provide good compliance and traction with the terrain, even if the motion of one side of the suspension moderately affects the other.
In this regard, U.S. Pat. No. 5,845,918 to Grinde et al. discloses an ATV with a semi-independent rear suspension which allows the rear axle to pivot about the vehicle's longitudinal centerline as well as about a transverse axis. This suspension design has been found to substantially improve handling performance of the ATV by giving improved traction on uneven terrain and increased vehicle roll stability. In particular, during cornering, these semi-independent suspensions help roll stability because they postpone the initiation of the transition from understeer to oversteer. For the operator, a quasi-static understeer condition is easier to control than the rapid transition condition to a sharp oversteer.
The suspension system of the '918 patent however, does not totally resolve the third undesirable characteristic explained above, since it too uses a solid rear axle. In addition, the suspension design of the '918 patent severely limits the travel of the rear axle since the travel is limited by the travel of the coil-over shocks, which are displaced in near one to one ratio with the displacement of the rear axle. Thus, the suspension disclosed in the '918 patent is undesirable for ATV applications, especially for high performance applications, where amount of travel in a suspension is considered critical for optimal traction, energy absorption, and operator control.
Furthermore, the drive
Cole Thomas W.
Dickson Paul N.
Fleming Faye
Nixon & Peabody LLP
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