The Motorbike Archives

Quality two-stroke trail bikes have been the backbone of Yamaha's motorcycle sales for years. Yamaha's enduro series has consisted of mediocre handling chassis wrapped around sundry two-stroke single cylinder engines. They have all qualified as great trail bikes but not as professional-level enduro machines. While engine development has been very rapid for Yamaha's two-strokes, chassis and suspension improvement has lagged.

This year's enduro is basically an assemblage of the MX-model components with street trimmings and a detuned engine. And for the first time in its seven year history the Yamaha enduro chassis has been completely changed; fortunately, for the better. The new frame is the same as used for the 250, 360 and 500 MX series.

This particular chassis was mediocre, at best, for motocross, but thousands of the MX machines were bought and converted into excellent trail, enduro and cross-country bikes. The prime reason for the popularity of the MX bikes for these conversions was the superior handling characteristics provided by a chassis and suspension units much improved over the enduro models.

The new frame is longer and heavier than that of the old enduro. The added 1 1/2" of wheelbase, the 30° head angle and over 5" of trail makes the DT360A handle much steadier than the machine it replaces. The old enduro models would thrill the leathers off Hakan Andersson when pushed past its limit, which was about 40 mph in all but the smoothest dirt. The DT360A can go 10 to 15 mph faster through a rough section than last year's enduro with less rider effort.

Gone are most of the wild side-to-side gyrations and nasty attempts to high-side the rider in whoop-de-doos or on steep downhills. On fast trails and fire roads the improved tracking and straight-line running make the new enduro a joy to ride rather than the constant thrill it once was.

The new chassis has its limitations, as does any motorcycle. Its soft suspension springing and damping lets the 292-pound bike wallow when pushed hard in the rough. Moderate power output makes straightening out the bike, when you do get in trouble, a bit hair-raising at times. The new chassis is tremendous for brisk trail riding but it doesn't make the DT360A a racer. Better it is; perfect it isn't, although the handling improvement is enough to make an only average enduro rider feel like a good one.

Much of the handling improvement can be directly attributed to the new frame's lower center of gravity. The engine and swing arm pivot have been relocated about two inches below where the RT3 carried these items. With a lower CG the machine has less tendency to teeter and doesn't require as much balancing by the rider. This lower CG also lessens the machine's tendency to high-side in choppy turns.

The development process of improving the forks and shocks has been one of prolonged evolution. No one at Yamaha could give you a step-by-step listing of every modification and improvement made in the enduro and MX forks. There has been what amounts to a complete redesign of the fork assembly in the last seven years. The steel sliders are now aluminum; the fork legs are made of tempered alloy-steel tubing rather than pot-metal pipe; the chrome doesn't peel off the legs in 30 days; oil doesn't squirt all over the place after a hard day's ride; the damping works half-way decently; and the springs work fairly well.

The DT360's Thermal Flow shocks are also a pick-up from the MX series and they are a tremendous improvement over the old ones. The damper unit is a one-piece aluminum casting that contains the main cylinder bore body and a separate, finned oil reservoir. The primary benefit conferred by this reservoir is its larger fluid capacity and the cooling area it provides. This combination lets the Thermal Flow damper perform with pleasing consistency. The shock springs are in stacked pairs, soft and stiff, to provide two-rate action to accommodate the average range in rider size and most, if not all, variations in terrain.

The front and rear suspension units are well matched for dual, on/off road, noncompetitive riding. The springing is soft and not made harsh by excessive damping. Heavier riders and/or fast riding in rough terrain are enough to bottom the 4"-travel shocks. Cranking up the spring tensioners compensates for more weight or harder riding conditions, but the extra preload collapses the soft springs, leaving the stiffer coils to overwhelm the gentle damping. We found a compromise between overloaded damping and suspension bottoming with the spring tensioner set in the middle position: that proved best for all-around use.

The forks perform just like the shocks. The spring tension is soft and so is the damping. This combination gives the rider a cushion in most rough low-speed stuff and over high-speed washboard. The forks are not equal to the rigors of most enduros and would need stronger springs and perhaps heavier oil. Replacing the stock springs may become a necessity for most riders. The coils in our test bike were partly collapsed at the 800 mile mark and had robbed the forks of their full performance potential. Serious enduro riders will experiment with the forks and shocks anyway. The bike hasn't been made that comes with suspension units perfectly matched to all riders and all conditions.

Chassis and handling improvements are matched by the tremendous overall gain in engine performance. The engine is a detuned copy of the MX and is the most versatile big-bore, two-stroke enduro power-plant Cycle has tested. It's not the most powerful or smoothest two-stroke single. It does, however, produce an exceptional amount of low-speed power—the kind of power that does little to spin the rear wheel and a lot to provide acceleration. For a combination of uses—plonking, climbing, pulling, fire roading or asphalt scratching—the engine is ideal.

The engine's ability to generate enough power to get the rear wheel churning away starts at 2500 rpm. The rate at which you accelerate is directly related to the amount of throttle applied. The DT360A's muscular engine takes most of the hassle out of dirt riding. It doesn't have to be wound up tight in order to pull the bike over a snag or out of quagmire. The block-pattern tires, tractionless terrain or tall street gearing are the only limits to the engine's pulling potential. Because of ease of riding gained with this extra low-speed power, it's certainly worth the additional investment to get the larger 360cc engine in place of a smaller displacement machine.

Attempting to spin the engine above 6000 rpm for more power is futile, but staying below this rev limit still gives the rider a 3500 rpm power range, and that's more than enough to overcome the broad gear spacing. Riding in the rough, for expert and novice alike, is made enjoyably easy by this wide power spread. Gear shifting doesn't have to become a preoccupation. When you want to accelerate, rolling open the throttle another notch generally brings the speed up. The engine doesn't have to be spun like a buzz saw and is seldom worked hard. It's entirely possible that this engine would last an entire enduro season without appreciably wearing engine internals, because of low crankshaft speeds and moderate work loads placed on them.

Yamaha has designed and built all its present two-strokes with reed valve induction as a prime feature. These reeds act as a one-way gate that opens and closes the inlet tract during the vacuum and pressure phases in the crankcase. Reed valves are especially effective in keeping low speed performance clean. With correct jetting and a proper heat range spark plug, low speed running is crisp because the engine doesn't load up and wet-foul its plug. The reed valve system doesn't give the engine more power; simply cleaner performance. High engine speed breathing is limited with the reed valve and fuel economy is no better than that of most 360cc two-stroke singles.

The DT360A's cylinder casting is vastly larger than the RT3's, as is the reed port cavity. This opening accepts a bigger reed assembly (six reeds replacing last year's four) and would seemingly dump more fuel into the engine. Such is not the case. Last year's machine used the same 30mm Mikuni as the DT360A, yet the 1973 RT3 engine required a #230 main jet where the new enduro comes with a # 180 standard. The DT360A proved slightly more economical than last year's enduro. During a 3-day, 500-mile test jaunt with both a '73 and '74 enduro, the DT360A averaged 38 mpg while an accompanying RT3 recorded 35 mpg. The most surprising operational difference between the two enduros was in the oil consumption. On a trip in which the RT3 gobbled a quart of oil every 180 miles, the DT360A consumed not quite the same amount after 240 miles.

Perfectly matched to the engine's performance capabilities is the reliable driveline. The huge oil-bathed clutch is driven off a helical-cut pinion gear on the right crankshaft end, just as Yamaha has done with all the enduro engines. Clutch action is smooth and the engagement point is broad enough to keep the plates from grabbing suddenly and causing the bike to lurch. On one occasion we became bogged in a soft sandwash studded with huge boulders. The clutch was slipped for ten minutes while climbing over the rocks. The result was a temporary loss in adjustment at the lever and difficulty in shifting gears. But after a few minutes of less abusive riding the normal disengagement and clean shifting returned.

The gearbox is a dream for dual purpose riding and enduros. First gear is down in the ratio basement, to let the engine pull strongly at crawling speeds. Second gear is spaced some distance from first, but seldom presents any problem for the engine in making the jump. The middle three gear ratios, second, third and fourth, are equidistant, with fifth staged to serve as an overdrive. First gear is perfect for those impossible situations; the middle three cogs are nicely spaced for trail needs; and fifth is just right for fast roads and highway. With this broad span of ratios the engine is as happy to grind away at a trails section as it is to cruise the asphalt at 65 mph.

Overall gearing is taller than with the RT3, although internal gear ratios are the same for both models. The change isn't much: the DT360A goes 41 mph at 3000 rpm in top gear compared to 37 mph for the RT3. The DT360A produces its useable power at a lower crankshaft speed so this doesn't inhibit the bike's performance at all. For all-around riding, the overall gearing is ideal. Most enduro riders, particularly in the mountains and woods areas, will find it advantageous to drop the secondary gearing by installing a 14-tooth countershaft sprocket in place of the standard 15-tooth. Acceleration and low speed performance will improve.

The #520 D.I.D. chain that is stock on the DT360A seems to be one heck of an improvement over the rubber bands that used to come on Japanese enduro bikes. With moderate care, proper lubrication, occasional cleaning and by maintaining correct wheel alignment we had to snug up the chain only three times in over 1200 miles of riding. The countershaft sprocket cover casting serves no purpose other than decoration. Removing it will make for easy on-the-trail maintenance if the chain should happen to break.

This year, for the first time in their on/off-road bike line, Yamaha has installed a CDI ignition on the 360cc enduro to replace the conventional flywheel magneto and points. Eliminating the points and condensor will, barring unforeseen problems, improve dependability and decrease maintenance. Externally, the crankshaft-mounted flywheel looks like it might house a conventional magneto. Inside the flywheel there's a lighting coil, primary ignition exciter coil and trigger or sending unit.►