Automotive Engineering - April 2023 - 28

DESIGNING FOR
200 BMEP AND
22,000 RPM
GP miniaturization: Tiny 39mm piston, pin and clip from the
Honda 250cc RC165, next to a 35mm film canister for size
comparison.
Attacking friction
Irimajiri attacked sources of friction in his new 50cc RC115.
Honda's original 250cc GP fours were constructed
much like Japanese production-bike engines. A
pressed-together crankshaft with full-circle flywheels
turned on large roller main bearings. Bearing ODs were
clamped between an upper and lower horizontally split
crankcase, as were the gearbox shafts. Also similar to
production, the crankcase closely encircled the crank's
flywheels. Holes in the lower case allowed spent oil to
drain back to a sump or scavenge pump below.
The RC-115 no longer enclosed circular flywheels in a
close-fitting horizontally split crankcase. Irimajiri
switched from large diameter ball or roller mains to
the smallest possible caged needle bearings, made as
" pillow blocks " bolted to the underside of the upper
crankcase/cylinder casting. The lower case, now having
no structural function, became magnesium. Main
bearing oiling came from a transverse gallery in the
lower case, delivering to each main bearing by upward
projections sealed by O-ring.
Three benefits resulted:
* Removing the spinning crankshaft from any possibility
of acting as a " viscometer, " shearing oil between
itself and the nearby inner crankcase walls
* Minimizing resistance to the pumping of crankcase
air between neighboring cylinders
* Minimizing the diameters of all bearings
The aim of #3 above was to reduce the velocity at
which bearing rolling elements " squished " through
any oil film on their races, thereby reducing bearing
friction torque. We've all driven at highway speed onto
a thin layer of standing water: you feel the car decelerate
as its " rollers " squirt the fluid ahead of them. Joe
Craig, Norton's 1930s racing engineer, noted that bigend
roller bearing temperature rose much faster from
too much oil than from too little.
28 April 2023
In examining a disassembled RC-165 250cc six-cylinder crankshaft,
the author saw tiny out-pumping chevrons embossed into the ODs of
main bearing needle cages. Such chevrons are often seen on oil seals.
The crankshaft was radically altered. The previous full-circle webs
were replaced by minimum-mass narrow rectangular bars. Each carried
two cylindrical tungsten balance weights opposite the crankpin.
Bearing learnings
Radically low crankshaft polar moment gave these Honda GP engines
their distinctive " yelping dogs " exhaust sound during warm up. When the
mechanic (or an inexperienced rider) closed the throttle, the engine
stopped almost instantly. Journalists during that era argued that the light
cranks boosted acceleration but the real reasons are more interesting.
Two experienced race-team managers, the late Dick O'Brien of
Harley-Davidson and Rob Muzzy of Superbike racing fame, told the
author that they could never get engines with light cranks to topend
properly at Daytona. A technical paper from Honda's F1 experience
('Development of Reciprocating Parts and Crankshaft in
Honda's Third Formula 1 Era,' T.Gotou and N. Yanagisawa, Honda
R&D Technical Review, 2009) explains. The engineers discovered
that crankshaft torsional activity plus cyclic rpm variation could
cause valve float to occur as much as 1600 rpm below the threshold
found in rig testing.
The great enemy of crankshafts is torsional vibration - cyclic,
back-and-forth twisting caused by the crank's elasticity being excited
by cylinder firing impulses. This is complicated if, as in all Honda GP
race cranks of this period, it is pressed together. The torque capacity
of the joints has limits. Mr. Honda believed rolling-element bearings
to be essential at high rpm, but the 20,000-rpm plain-bearing engines
of Formula 1's V10 era refute this. All of the crankshafts in
Honda's 1960s GP engines were pressed together to allow use of
one-piece connecting rods and main and crankpin rolling bearings.
Another friction-reducing change made by Irimajiri was to move lateral
location of the connecting rods from the big-end (rotating) to
the small-end (oscillating).
AUTOMOTIVE ENGINEERING
Pressed-together crankshafts in Honda's 1960s GP engines (RC165 six-cyl. shown)
allowed use of one-piece con rods and rolling-element bearings. The central gear
drove the clutch through a jackshaft.
BOTH IMAGES: JOHN OWENS
Automotive Engineering - April 2023

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