TRX TECH -- MOTORCYCLE OR MYTHMAKER?
BY KEVIN CAMERON
Yamaha has a historic fondness for parallel-Twins. Its first was
the iron-cylindered YD1 two-stroke, granddad to a family that
would include the YDS-2 and -3, the RD350 and RD400, the
water-cooled RZ350, and an entire set of racing cousins that
would end only in 1990. Four-strokes included the long-produced
650 and the novel eight-valve XS500 of the 1970s.
Most recent has come the rather cultish TDM850, produced as a
kind of road-biased dual-purpose machine, but marketed
hesitantly. The world is so filled with high-performance
transverse-Fours that Twins-save for those already owning their
own special myths, Harley-Davidson and Ducati-have become
machines without a niche. The excellent Honda Hawk GT is an
example. Its alloy twin-beam chassis, its sporty engine and its
stirring sound made a fine combination, but it was passed over in
world showrooms for the familiar Fours. "What is it?"
potential buyers seemed to wonder. And the maker didn't tell
them. Good machines went unsold for lack of a suitable myth.
Now the TDM has been reconfigured, and has been given the pants
and shirt of a new outfit. It's up to Yamaha to answer the
familiar question. "What is it?"
The resulting TRX850 is packaged as a sporting Twin, with a
superficial resemblance to Ducati's 900SS. Therefore, the TRX has
a tubular ladder frame, painted white, with tank and seat in red
(yes, a mirror-image version will be available, but surely the
red-tanked model will be the TRX of choice). On paper, this is
pure Ducati. In person, the styling is clearly Japanese, and the
effect is Yamaha.
Yamaha's search for a myth extends farther than frame and color.
The TRX engine has been uniquely altered to give it the
distinctive exhaust beat of a 90-degree V-Twin. Instead of the
usual and traditional 360- or 180-degree crankpin phasing, this
parallel-Twin has its crankpins set at an odd-seeming 90 degrees.
Like a Ducati, the TRX fires at 270-450-degree intervals,
resulting in the irregular, raunchy sound character shared by
aircraft radial engines, V-Eights, and all V-Twins.
But 90-degree phasing isn't all that odd. First, it has a long
history in steam locomotives. They had to be
"quartered," as their special language described the
90-degree crankpin phasing, so that no matter in what position
the engine stopped, it could not be stuck on dead center.
Later, Australian motorcycle engineer P.E. Irving commented on
the special smoothness of a quartered Twin. Consider a
single-cylinder engine, spinning at constant rpm. At top or
bottom dead center, the piston is motionless, and therefore has
no kinetic energy (kinetic energy is that arising simply from
velocity, not from combustion gas pressure). All of the
moving-parts kinetic energy is therefore in the spinning crank.
But at about 78 degrees after and before TDC, the piston is at
maximum velocity, and therefore has maximum kinetic energy. And
where does that energy come from? It comes, of course, from the
crankshaft, which has to slow down slightly in the process of
giving this energy to the piston. And, in the other half of each
stroke, the energy in the piston, now decelerating, returns to
the crank. The crank will accelerate somewhat in the process. As
the engine turns, therefore, there is a constant exchange of
energy between crank and piston. The piston starts and stops,
while the crank's instantaneous rpm varies in complementary
fashion.
This crank-speed variation is part of the reason why the gearbox
of a piston engine must be bigger than that of an electric motor
of equal power. Not only does the piston engine's power come in
energetic punches, widely spaced, but its crankshaft doesn't even
spin at constant speed when it isn't firing. This
"lumpiness" of power and rotation causes higher peak
loads on everything-including the rider's tolerance.
The two traditional phasings for a parallel-Twin are 360 (both
pistons rise and fall together, as in British Twins) and 180 (one
up, one down, as in the familiar RDs). But in each of these
cases, both pistons start and stop together, so the exchange of
energy between crank and pistons is large. The attraction of the
TRX's 90-degree phasing is that one piston is stopped at top or
bottom center, while the other, 90 degrees away, is very close to
its maximum velocity. This means that, instead of exchanging
kinetic energy with the crank, they exchange it with each other.
This leaves the crank turning at a more constant, less lumpy rpm.
It's not a big, earth-shaking effect, but it exists.
While Yamaha's early press release (in Japanese) makes much of
explaining graphically how the TRX's inertia torque is smoothed
by 90-degree phasing, we skeptical, over-marketeered-to veteran
humans remain pretty sure Yamaha did this to make the bike sound
more, well, mythical. They didn't do it for balance, either-the
engine has twin balancer shafts that would be equally effective
no matter what crankpin phase is used.
Aside from its crank phase, the engine is an uprated TDM, with a
claimed 85 horsepower at a moderate 7500 rpm. It has a
reasonable, unfrantic bore-stroke ratio of 1.33, at 89.5 x
67.5mm. Five valves per cylinder and a boosted 10.5:1 compression
ratio give outstanding cylinder-filling-more like a racer than a
streetbike. The major engine quality will be torque, not endless
revs.
Dreaming of a full-liter TRX Twin? Go carefully. These closely
spaced cylinders would have to gain 7.5mm to reach 1000cc by
overboring, and there may not be metal enough. Still, an
interesting thought....
Yamaha officials are now deciding whether or not to import this
interesting and different machine. Before they bring it here,
they'll have to produce an answer to the question, "What is
it?"
If the answer is satisfactory, there is a dandy motorbike to go
with it.