Honda TRX 700XX IRS Ride Test and Review
Is Honda's Long Awaited IRS Big Bore Sport Quad Right For You?
August 14, 2008
Honda has dominated in ATV market share over the years. It all started in 1969 when they invented a new way to hit the trails with their US90, (later called the ATC 90 in 1970). Those old enough to have started out in the three-wheeler era probably have many fond memories of the innovative ATC 200X, 350X and 250R. Honda later progressed into four wheeled ATVs, along the way it basically built the foundation of today’s ATV race scene on the back of its mighty TRX250R.
Fast forward to today, even in a sea of available quads, Honda continues to lead in market share. What’s more interesting to this, and annoying to its dealers, is the fact that they’ve been doing this without a big bore sport ATV in their lineup. Honda dealers have been begging for a new big bore model (as well as a SXS, but that’s another story). No doubt this was the weak spot in Honda’s line up. All the other areas are pretty well covered. Their 250EX with its SportsClutch is one of best beginner level quads of all time, and their TRX450R absolutely dominated the race scene in 2007, winning 10 professional ATV racing championships!
Honda’s sport quads continue to be the choice of many ATV enthusiasts world-wide. And it goes without saying, when Honda creates a whole new sport quad category, like they just did with their new TRX700XX, there’s going to be plenty of interested enthusiasts eagerly awaiting a chance to take it for a spin. This is where we come in. You’ll definitely want to try out Honda’s first large displacement Independent Rear Suspended sport quad for yourself. Until then, below is a detailed review that we compiled after Honda allowed us to ride the new XX at the Dumont Dunes Off-Highway Vehicle Area in Dumont, CA.
The heart of the XX is a 686cc dry-sump, liquid-cooled SOHC four-valve single-cylinder four-stroke engine, which traces its heritage to Honda’s Baja-dominating XR650R motorcycle engine. The 700XX engine uses a bore and stroke of 102mm x 84mm, which takes the cake as the largest diameter single-cylinder that Honda has ever mass-produced.
As you would expect the 700XX engine has been specifically designed for the rigors of ATV duty. A gear-driven counterbalancer is used to help neutralize engine vibration. The stroke is longer by 2 mm than its XR650 motorcycle cousin and its bore is 1.4 mm larger.
The XX piston features a slipper skirt design and it strokes through a centrifugally forged, 3.5mm-thick steel sleeve. A forged piston was used to handle the increased mechanical and thermal loads, and it also benefits from an underside shower of cooling oil shot from a jet that protrudes into the center of the engine case. Compression is 10:1. The combustion chamber utilizes a pent roof shape and is fired by a single, centered spark plug, which is fired by the machine’s electric start button.
The engine inhales through a foam air filter fitted in a 7.6-liter airbox. The Programmed Fuel Injection (PGM-FI) system draws fuel from a 3.6-gallon blow-molded, multi-layer fuel tank and delivers it via a single Denso injector through a 44mm throttle body. A Keihin Engine Control Module (ECM) manages both the fuel injection and the transistorized ignition. The engine exhales through a single long-tube header tuned for excellent low-end and mid-range torque without sacrificing top-end power. Gases are expelled through a muffler constructed of 128-grade stainless steel.
Coolant is routed through an aluminum radiator with a 28mm-thick core—the same core size as a CRF450R motocrosser. A special wire-mesh filter in the oil hose connector and a custom bend of the oil return pipe protects the oil from sloshing and aeration in its separate brazed aluminum tank. This dry-sump design eliminates the oil pan, which contributes to a shorter engine height and consequently a lower center of gravity.
Several enhancements are made to the engine to simplify its operation and enhance durability. An automatic decompression system is activated whenever the electric starter is engaged. The starter-motor housing is incorporated into the engine crankcase casting and a disc-type torque limiter minimizes shock to the starter mechanism.
Before we move on to the Chassis and Suspension we need to talk more about Honda’s centered chain drive system, which is an outstanding design as far as we’re concerned. It was designed with the goal of providing optimum power delivery without sacrificing handling. All the while it needed to do this without adding too much weight to the machine. At the end of the day Honda impressively solved a number of issues typically associated with an Independent Rear Suspension (IRS) machine. We feel Honda’s engineers rose to the occasion and delivered the best design since their eccentric chain adjustment system in the eighties, which is now found on most ATVs.
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.
Is this not the most protected rear sprocket and brake mechanism you've ever seen? 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.
The centered chain drive layout allows ample room for long travel suspension high performance suspension. 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.
If you ask us Honda's Centered Chain Drive System is pure genious.