2008 Honda TRX700XX
With the introduction of the all-new 2008 TRX700XX, Honda enters the large-displacement Sport ATV market at the peak of the high-performance curve and accelerates into the red zone where no one can follow. Its electric-start, fuel-injected 686cc four-stroke engine traces its heritage to the Baja-dominating XR650R and delivers class-leading power. Just as significant, the drivetrain and suspension of the TRX700XX are state-of-the-art and then some. Specifically, Honda has engineered a brilliant centered chain drive system that permits the use of fully Independent Rear Suspension (IRS) with more advantages than traditional IRS designs.
2008 Honda TRX700XX
Engine:liquid-cooled SOHC dry-sump single-cylinder four-stroke
Transmission:5-speed with Reverse
The adult Sport ATV market in the U.S. continues to mature and today can be classified in three distinct, yet overlapping segments. The first is occupied with 250-400cc models that provide lightweight, tractable power and convenience of operation. The TRX250EX, with its innovative SportClutch, and the ubiquitous TRX300EX are wonderful examples of this category. On the other end of the spectrum are focused, competition-oriented machines like Honda’s TRX450R, which boasts a ready-to-race liquid-cooled, four-valve Unicam four-stroke engine and agile suspension. Straddling the line between entry level and pure competition machines are the high-performance sport ATVs that add season-long capability and versatility to the high-performance formula. The TRX400EX, with its bulletproof XR400-derived air/oil-cooled four-stroke powerplant and cast-aluminum swingarm, slots in well here. In keeping with the increasing expectations of riders in this segment, the new TRX700XX carves out new ground at the pinnacle of the Sport ATV market.
The lifeblood of the 700XX is its 686cc dry-sump, liquid-cooled SOHC four-valve single-cylinder four-stroke engine. With a bore and stroke of 102mm x 84mm, it boasts the largest diameter single-cylinder that Honda has ever mass-produced. Although it is infused with the heart of the potent XR650R mill, the 700XX engine has been purpose-designed and built for the rigors of ATV duty. Given the higher weight of ATVs (compared to off-road motorcycles), torque—and lots of it—was a priority. A longer (plus 2.0mm) stroke and slightly larger (plus 1.4mm) bore accomplish this. But in keeping with Honda’s philosophy of mass-centralization and a low Cg (center of gravity) to achieve optimal handling, engineers determined that a taller cylinder and engine would not do. So they shortened the overall height of the engine by creating a longer-throw crankpin and a correspondingly shorter, carburized connecting rod to shorten cylinder height. The connecting rod is affixed to a ball-bearing crankshaft. Smooth, all-day riding comfort was assured with the fitment of a gear-driven counterbalancer to neutralize engine vibration.
Like Honda’s renowned CRF motocrossers, 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 and the high-tech slug is fitted with a larger 24mm piston pin. The combustion chamber utilizes a pentroof shape and is fired by a single, centered spark plug.
The engine inhales through a foam air filter fitted in a 7.6-liter airbox. Intake valve size is 1mm smaller than those used in the XR (36 vs 37) for better low-rpm flow—and therefore torque—while the exhaust valve diameter remains unchanged at 32mm. 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.
Power is channeled through a five-speed gearbox with Reverse and is transmitted to the rear wheels via an ingenious centered chain drive system. Conceived with the goal of maximizing the architecture of the long A-armed, double-wishbone independent rear suspension, a row of final gears located after the countershaft positions the drive sprocket just 33mm left of the 700XX’s centerline. The proprietary design allows the chain and rear sprocket to be placed virtually in the center of the rear of the ATV. This produces numerous benefits, including the use of longer A-arms and axle shafts. Longer A-arms minimize wheel camber changes as the suspension travels up and down—a handling benefit—and longer axle shafts mean that CV joints operate at less extreme angles, benefiting power efficiency and long-term durability. A further benefit of the design is simplified maintenance.
Two more features of the gearbox and final drive are worth noting. First, top speed in Reverse gear is controlled by monitoring both actual speed and crankshaft speed. When vehicle speed is no less than 12 mph and the engine speed is no less than 3050 rpm (equivalent to 12 mph), fuel delivery to the injector is restricted. The second noteworthy aspect of the gearbox is the engine reduction ratio at the drive sprocket: 1.087:1. This was done to permit the fitment of a smaller-diameter rear sprocket, which in turn allows the sprocket receiver assembly to be located lower on the frame. In addition to lowering the center of gravity, the smaller diameter of the sprocket and its assembly also improve ground clearance.
The frame of the TRX700XX is constructed of steel with a removable sub-frame; each is extremely durable and strong. Independent double-wishbone front suspension uses steel A-arms and premium shock absorbers featuring two single-rate springs per side. This application of dual springs, a first for Honda, separates the function of each. Both the main spring and the short top spring (second spring) are single rate. Bottoming resistance is provided by the heavier main spring and the top spring adds compliance over small bumps. Together, they achieve what a progressive-rate single spring could have accomplished, but in a lighter, more compact package that takes full advantage of the 10.5 inches of useable suspension travel. Front preload is adjusted with screw-type adjusters. The A-arms are attached to aluminum steering knuckles, which in turn bolt up to aluminum hubs and 10-inch diameter aluminum wheels fitted with 21x7R-10 Dunlop radials.
The aforementioned independent double-wishbone rear suspension, made possible by the innovative centered chain drive system, features steel upper and aluminum lower A-arms. The upper and lower A-arms pivot on sealed needle bearings at the frame and support aluminum knuckles via oil-less bushings. The knuckles attach to steel hubs, and the 11-inch-diameter aluminum wheels sport 22x9R-11 Dunlop radials. The reason for steel hubs: greater resistance to side impacts and side loads. These steel hubs weigh less than an aluminum hub with the same strength characteristics. A non-adjustable rear sway bar attached to the frame and lower A-arms controls excessive roll without compromising the independent action of the IRS. Piggyback reservoir shocks provide 9.3 inches of fully usable travel and feature screw-type preload adjusters. Braking action is supplied by a hydraulic single piston squeezing a 200mm disc. The rear brake assembly is mounted on the rear sprocket receiver andissituated33mmright of the 700XX’s centerline—symmetrically opposed to the location of the sprocket/chain assembly to facilitate equal-length A-arms and drive shafts.
An interesting outgrowth of Honda’s goal to fit the longest A-arms and axle shafts possible is the use of special 11-inch-diameter rear wheels (larger than competitive designs). The larger-diameter wheels allow the knuckles and hubs to reach farther into the wheel cavity, which effectively extends the A-arms and axle shafts to the perimeter of the 700XX’s track width. To maintain strength and light weight, the thickness of the rear-wheel wall varies as it extends from the center plate to the outer ring of the wheel— another Honda ATV first. And here’s another: the fitment of replaceable rim scrapers to remove mud and debris from rear wheels, a feature pioneered on Honda’s popular Foreman and Rincon series.
The TRX700XX shows off its high-performance capabilities with aggressive sports styling. A polyethylene push bar, polypropylene rear A-arm protectors and high-impact urethane frame and engine-case protectors are tough, lightweight and replaceable. A cowl-inte-grated headlight with dual 30-watt bulbs provides superb lighting and a highly visible LED brakelight is the most common view competitive riders will have of the new TRX700XX.
With Honda’s legendary fit and finish, coupled with patented innovations such as center chain drive, plus handling and engine performance unrivaled in the class, the TRX700XX recalibrates the standard by which all other sport ATVs are measured.
Centered Chain Drive System
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.
Features & Benefits
The TRX700XX redefines the Sport category of ATVs with a whole new level of confidence, comfort and high performance. With double-wishbone Independent Rear Suspension (IRS), a unique centered chain drive system, and a fuel-injected 686cc engine making monster torque, the TRX700XX is truly the new Sport ATV “King of the Hill.”
- All-new 686cc SOHC liquid-cooled fuel-injected four-valve single-cylinder four-stroke engine.
- Double-wishbone, Independent Rear Suspension (IRS) with tuned swaybar.
- Centered chain final drive system.
- In-gear starting in Forward or easy-to-use Reverse.
- Piggyback rear shocks with adjustable preload.
- Dual-spring front shocks with adjustable preload.
- 11-inch-diameter rear rims with removable mud scrapers.
- Radical new-style bodywork.
- Available colors are Metallic Black/Silver and Metallic Black/Red.
- 686cc SOHC, four-stroke engine generates the highest peak output in its class.
- Liquid cooling provides consistent engine operating temperature in extreme conditions for maximum power and long engine life.
- PGM fuel-injection system with 44mm throttle body for excellent cold-weather starting, lower maintenance and consistent performance at high altitudes (see Technology Section).
- Gear-driven counterbalancer minimizes engine vibration for smooth, comfortable all-day riding.
- Maintenance-free automatic cam-chain tensioner.
- Closed-crankcase vent system safeguards against engine oil contamination.
- Full transistorized ignition with electronic advance.
- USDA-qualified stainless-steel muffler/spark arrester for quiet operation and maximum power.
- Heavy-duty clutch stands up to sport-ATV demands.
- High-efficiency dry-sump lubrication system features large-capacity oil tank for maximum engine cooling.
- Rugged five-speed transmission with Reverse has components and ratios specifically designed for sport-ATV use.
- Unique, centered transmission with its direct-line chain final drive allows optimum IRS double-wishbone A-arm design.
- Independent double-wishbone front suspension uses premium shock absorbers featuring dual single-rate springs that allow a lighter, more compact package with 10.5 inches of fully useable travel.
- Independent double-wishbone rear suspension, made possible by the innovative centered chain drive system, features steel upper and aluminum lower A-arms. Using piggyback reservoir shocks, the system provides 9.3 inches of fully useable travel.
- Steel frame with removable subframe is extremely durable and strong.
- Triple disc brakes for excellent stopping power and weight savings.
- Special, large-diameter 11-inch aluminum rear wheels allow for ideal CV joint angles and A-arm lengths, resulting in superior power delivery, optimum suspension action and improved durability.
- Radial tires with knobby pattern provide superb traction and cornering ability.
- Durable, heavy-duty O-ring–sealed drive chain.
- Easy-to-use dual snail-cam eccentric chain adjusters.
- Engine design meets current California Air Resources Board (CARB) off-road emissions standards.
- Convenient pushbutton electric starting.
- Sculpted, plush seat provides excellent comfort and superb sport riding ergonomics.
- New and uniquely aggressive sport styling.
- Cowl-integrated headlight with dual 30-watt bulbs provides distinctive sport style and excellent function.
- High-visibility, long-life LED brake light.
- Easy access to washable two-stage large-capacity foam air filter.
- Tough, polyethylene push bar up front and polypropylene rear A-arm protectors.
- High-impact urethane skidplate protects frame and engine cases.
- Rear mudguards with rigid footrests.
- 8 AH maintenance-free battery.
- Keyed ignition switch offers added security.
- Simple-to-operate multifunction handlebar switch.
- Left-hand-operated parking brake.
- Tool kit includes tire-pressure air gauge and owner’s manual.
- Transferable one-year, limited warranty; extended coverage available with a Honda Protection Plan.
- Purchase of a new, previously unregistered Honda USA–certified unit by an individual retail user in the United States qualifies the owner for a one-year complimentary membership in the Honda Rider’s Club of America (HRCA).
Honda Genuine Accessories
- Flag Kit.
Engine Type: 686cc liquid-cooled SOHC dry-sump single-cylinder four-stroke
Bore and Stroke: 102mm x 84mm
Induction: Keihin PGM-FI with a 44mm throttle-body fuel-injection system
Ignition: Full transistorized with electronic advance
Transmission: Five-speed with Reverse
Driveline: O-ring–sealed #520 chain
- Front: Independent double-wishbone with dual single-rate springs; 10.5 inches travel
- Rear: Independent double-wishbone with piggy-back reservoir shocks; 9.3 inches travel
- Front: Dual hydraulic 174mm discs
- Rear: Single hydraulic 200mm disc
- Front: 21x7R 10
- Rear: 22x9R 11
Length: 73.9 inches
Width: 46.3 inches
Height: 45.4 inches
Seat Height: 33 inches
Ground Clearance: 10.5 inches
Wheelbase: 50.0 inches
Fuel Capacity: 3.6 gallons
Colors: Metallic Black/Silver, Metallic Black/Red
Curb Weight: TBD
Suggested Retail Price: $7,899.00