The 2008 TRX700XX enters the big-displacement Sport ATV category with a number of innovations. The most notable is the centered chain drive system. Designed with the goal of providing optimum power delivery and class-leading handling in a lightweight package, the centered chain drive system solves a number of issues associated with Independent Rear Suspension (IRS) that have compromised competitive designs.
Honda’s engineers faced a perplexing challenge. Namely, how to get the for
midable power of the TRX700XX engine to the ground in straight-line situations, cornering conditions and when negotiating uneven terrain. IRS was an attractive choice for its ability to plant all four wheels on the ground in tough and challenging topography, but despite its empirical advantages, there were drawbacks. IRS designs can add weight and often extract design compromises. Ideally, IRS is configured with long, equal-length double-wishbone suspension “A-arms” configured in a manner that minimizes camber deflection at the extremes of suspension travel, maximizes squat under acceleration, minimizes pitching at high speeds and neutralizes understeer when cornering. To reduce weight, lightweight aluminum alloys can be employed. But to overcome potential design compromises, a thorough examination of drive systems was required.
A compact, centered shaft-drive system similar to the IRS-equipped TRX680FA Rincon could allow the use of equal-length A-arm rear suspension, but it was ruled out for three reasons. First, a shaft drive and its corresponding differential are heavier than a simple sprocket-and-chain drive system and the IRS-spec’d TRX700XX needed to be lean and nimble. Next, shaft drive is not as efficient as chain drive when transmitting power, a decided negative for a high-performance platform.
Clearly chain drive was the solution. The problem: traditional engine side-mounting of the countershaft sprocket meant that the location of the rear sprocket and chain would intrude into the travel space occupied by the A-arms. Some IRS basics help explain the conundrum.
Double-wishbone suspension is ideal in an IRS setup because it offers longitudinal and lateral strength and rigidity, allows for smooth shock absorber action and provides optimal control of rear wheel travel at both low and high speeds. By making the upper and lower A-arms as long as possible, camber changes are kept to a minimum as the suspension arcs through its travel. This translates to a more consistent tire contact patch when traction is at a premium. The best way to achieve this is to attach the A-arms as close to the center-point of the vehicle as possible. But when a large sprocket and chain occupy the space needed for the movement of the A-arms, compromises must be made. If the A-arms are shortened so that their frame mounting points are outside of the chain line, significant camber changes occur when the suspension is fully compressed or extended, minimizing and moving the tire contact area. Another approach is to lower and/or tilt the inboard frame mounts of the lower A-arms (reducing ground clearance) and utilize a single-beam I-arm as the upper locating link. But an I-arm is less able to resist the twisting and flexing forces exerted on it; indeed these forces can be transmitted to the rear shock as well, causing it to bind. A more radical strategy employed by some competitive machines involves attaching the upper I-arm at a point farther back and lower on the frame. “Tucking the tail” of the rear suspension in this manner results in a mechanical control angle (the plane of the rear control arm mounting points relative to the plane of the drive-and-driven sprockets) as severe as 15 degrees—5 degrees is optimal—which effectively shortens the wheelbase when the suspension is fully extended and lengthens it when the suspension is fully compressed. The result is a fore-and-aft pitching over undulating terrain that can significantly impede predictable handling.
It was clear to Honda’s engineers that a centered chain drive system was the solution because it allowed for the ideal length and placement of the A-arms. The first step was to move the drive sprocket from its traditional left-side position on the ATV. To do this the engineers designed an ingenious set of gears and shafts. First, a final-drive gear was attached to the end of the countershaft—where the drive sprocket is normally mounted. Then, the final-drive gear spins a final idle gear which, in turn, spins the final driven gear. (The idle gear is needed to maintain the counter-rotating direction of the drive sprocket.) The final driven gear is mounted to a shaft that extends inward toward the centerline of the ATV and on its end is mounted the drive sprocket. This amazingly compact gear unit locates the drive sprocket just 33mm to the left of the 700XX’s centerline, giving the chain a straight run back to the rear driven sprocket. (The rear disc assembly sits an equal distance to the right of the centerline.) The driven sprocket is affixed to a compact receiver to which the equal-length rear wheel axle shafts are attached via constant velocity (CV) joints. So configured, the centered chain drive layout provides ample room for the location and placement of the desired A-arms.
Another benefit of the centered chain drive system is the ability to employ longer axle shafts. In the same manner that the up-and-down travel of long A-arms is circumscribed in a relatively small arc, so too is the effect with longer wheel axles. The positive benefit of minimal wheel axle travel is that the CV-joint angles are never extreme, thereby reducing power-robbing friction and the build-up of life-sapping heat.
Centered chain drive also contributes to the TRX700XX’s mass centralization. When a rider initiates a turn, the ATV rotates around its roll axis—a fore-and-aft horizontal line drawn through the center of mass of the ATV and rider. Placing the major masses (engine, fuel, rider) closer to this roll axis results in an ATV that reacts more quickly and smoothly to control inputs at the handlebars. Centering the chain drive and moving the A-arm mounting points, CV joints and rear disc assembly as close as possible to the 700XX’s centerline contribute to the application of the principle.
The centered chain drive system of the TRX700XX is an elegant solution to an engineering problem that has puzzled designers of sport ATV IRS systems for years. Leave it to Honda to apply sound principles and common sense to achieve an uncommon solution.