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Ricky Graham's Sacramento-winning RS7500 Honda is a combination of
Japanese and American-made parts. Suspension is made by Showa.
Each combustion chamber is full of valves. with a centrally-located spark
plug. The chamber is rotated off centerline to straighten the ports.
The RS7500's ports are split from the individual valves, joining just
before the intake and exhaust manifolds. This is an intake port.
By John Ulrich
Before the Sacramento Mile, former Grand
National Champion Gary Scott came by the
Cycle News office and said "Honda will
dominate dirt track racing by the year's end."
A few days later Ricky Graharn won the Sacramento Mile
' h
a boar d a H on d a RS750D ,t ree
weeks after that, Bubba Sho-
18
ben won the San jose Mile on another
RS750D; a week later Graham lost
Lhe restarted Ascot half mile by a
wheel lengLh.
For years Honda paid its dues as a
free-spending laughing-stock in din
ttack racing. They ttied CX500 engines enlarged to 750cc and slung
sideways in the frame, and got beal.
They built air-cooled 750s, and got
beal. They'd win a race here, a race
there, but when it came LO winning
race-aher-race and championship-after-championship, that was HarleyDavidson turf.
But it's 1984, not 1983, not 1982.
Honda's winning now, and leads the
1984 Manufacturers Championship.
The bike that made Honda a force
in half mile and mile racing is the
RS750D. It's an air-cooled SOHC,
four-valves-per-cylinder four:sttoke
V-Twm, 45° between the cylmders:
It's compact and busines like makes
93 bhpat 8000 rpm (and 61.2 Ih.-fl.o[
torque at 7000) on the dyno, and
weighs nine pounds less than a Harley-Davidson XR750 engine.
The bottom end is based on the
shaft-drive XV750, a Paris-to-Dakarstyled dual-purpose Honda sold in
Europe. But Lhe RS750D's top end is
a state-of-Lhe-an racing design.
The crankshaft is a one-piece forging with three flywheels and two
throws, one for each forged titanium
Crowerrod; there are two plain main
bearings, each carrying 50mm journals.
The crank throws are offset 45° so
although the RS750D's cylinders are
45° apart, it fires like a goo V-Twin,
improving engine balance and spacing the firing impulses more evenly
than an XR750 engine (which is also
The four-lobe camshafts run directly in the head casting. Epoxy paint
coats the valve train compartment to avoid leaks through the castings.
a 45° V-Twin, but with the cylinders
sharing a common throw.)
Primary drive is a link-plate chain
from the right end ohhe crankshaft to
the basket of the wet clutch. There's a
four-speed ttansmission, wiLh several
differem ratios available for Lhe individual gearsets.
A separate gearcase bolts to Lhe leh
ide of the crankcases; on the XV750
th is case holds hel ical gears transferring power from the transmission
output shaft to Lhe final driveshafl.
The RS750D is chain-driven, and so
its gearcase carries two sttaight-cut
gears, which reverse Lhe direction of
rotation and allow the mouming of a
chain sprocket.
The RS750D's crankshaft rotates
forward; that is, coumerclockwise
when viewed from the left end. The
cI utch and transmission mainshah
also rotate forward, but the transmission output shaft rotates rearward.
That's why theextra'gearcase is needed; the coumershaft sprocket must
rotate forward.
Another way to get the coumershah sprocket rotating in the right
direction would be to reverse crankshaft rotation. But the direction of
crankshaft rotation affects how the
chassis is loaded under acceleration.
If the crank rotates forward, Lhe mo-
torcycle tries to rotate rearward upon
acceleration, lifting the from end and
loading the rear wheel. That's good
for traction.
But if the crankshaft rotates rearward, then acceleration makes Lhe
motorcycle tty to rotate forward upon
acceleration, unloading the rear
wheel. That's bad for ttaction. (Many
tuners believe this theory, but others
discoum it; Yamaha road racers spin
backwards and hook up fine.)
Each cylinder head's lour-lobe camshah runs directly in the head casting, and is powered by a link-plate
chain driven off one end of the crank
(the forward cylinder's cam is driven
off Lhe left side of the crank, the rear
cylinder's cam off the right). The
valves are opened by rocker arms, one
rocker arm LOa valve, with adjustable
screw tappets.
The combustion chamber in each
head is rotated off the engine cemerline to make the ports as srraight as
possible. The imake ports feed Lhe
engine from carburetors moumed on
the right side of the bike, while the
exhaust pons lead to pipes moumed
on the left side of the bike.
It's easy LO rotate a combustion
chamber and to position valves to
match. But a chain-driven camshaft
has to parallel the crankshaft it's
driven off. That means the rocker
arms can't besttaight- the RS750D's
rocker arms aren't- and that could
lead to excessive side loading on each
valve stern, resulting in rapid valve
guide wear.
