Motorcycle Forum / General / Technical / October 2007

Harley started building 74 cubic inch engines around 1932, as I recall.
That's 1213 cc, and British twins were only half that size. Harley grew their
engine to
88 cubic inches, or about 1440 cc in the late 1980's, and the gauntlet was
thrown down to see who could build the biggest cruiser.

I seem to have read about a cruiser that's bigger than Triumph's 2300cc
Rocket 3, and I can understand why somebody would want a bigger, slow turning
engine to reduce high frequency vibration.

When Japan INC first got into mass producing motorcycles, they pretty much
followed the FIM's road racing class displacement rules, and there really was
no good reason for doing that with motorcycles destined to be ridden on the
highway, especially American highways, where trips are longer and gasoline
was historically cheaper.

Whatever motorcycle Japan INC designed for any given FIM engine size, they
learned to leave room for expanding the engine, and motorcycle engines grew
and grew, and riders could relax and forget about continously shifting gears.

There is also the wheelbase factor. Motorcycles are suspended by springs, and
springs have a natural frequency at which they rebound. If the wheelbase is
too short, the front springs won't be done with rebounding when the rear
wheel hits the same bump, so the chassis will hobbyhorse on freeway expansion
strips.

An Italian rider once told me that American interstates were much rougher
than the Italian autostrada and that riding here made his butt hurt.

Other riders told me that each section of freeway concrete was intentionally
laid at a slightly differnt angle so truck tires would slap rainwater off the
pavement.

Nowadays, freeways are built by continuously pouring concrete. But the 80,000
pound trucks seem to be able to pound concrete into bumpy pavement.

A larger, longer, heavier motorcycle just works better on long interstate
trips.

Homebuilders used to amuse us with giant touring bikes like the Road Dog that
was 10 to 12 feet long and had a car four cylinder engine in it. We never
thought that Japan Inc would ever build anything like that.

But the Honda Valkyrie is damned close. I was looking at a Valkyrie parked in
front of a local convenience store. The owner had installed oversize tires
front and rear. They were the wrong profile and they made the machine sit
about
two inches higher and it leaned over too far on the kick stand.

What did the rider care? He stood about 6' 8" and weighed 300 pounds.

And there's the final reason for big cruisers: big motorcycles for big men

There have always been big riders who feel like a 750cc or 1000cc motorcycle
is too small for them. They need room to stretch out.

Two smoke compression release is a totally different animal than
on big four strokes.
On old time two stroke off road bikes (at least I have never seen or
heard about one in modern times) the compression release is actually
an air valve to increase engine braking.

On four strokes the compression release is what you expect it to be
and used to make starting easier. I've had 3 generations of these,
the manual, the starter actuated (mechanically linked to kick starter)
and the automatic RPM dependant one). Personally I think the first
and last work well and the one actuated by the kick starter was a
bit problematic.
And I'll kick start a lukewarm thumper any day rather than hand
crank start a fair sized diesel truck engine (used to power a
generator) inside a cramped corrugated iron shed. Talk about
impossible without compression release. And loud. And dangerous.