RE: Digest Number 1126
> Most hot single dirt bikes do not need to idle tooBut Francisco,
> much. You never ride them below 4k rpm.
For the sake of (my) argument, let's approach this from the other end. Perhaps I can
better state my convictions after we get a little more cross-section of where people stand
on this "flywheel mass" question. So, in the interest of further scholarly discourse, let
me ask this:
Would you tell me why a hot 4-stroke dirtbike in the 500-600cc range would have a light
Also, on a slightly different thread of the same concept, Ian's recent telling of the old
flathead Ford V8 hop-up of crankshaft lightening gave light to two polar opposites on this
flywheel issue, yet both were "accepted" without much further questioning.
As for the doubts about flathead crank weight inhibiting acceleration, I tend to believe
it, and here's why.
I've seen many flathead dirt track racers in my childhood, and still remember their sound.
However, I don't have any experience driving them. My point is, my dad's David Brown
tractor (BTW: David Brown is the "DB" in Aston Martin's "DB" model designation!) is a
3-cylinder direct-injection Diesel; no glow plugs, and (I think) no pre-combustion chamber
either. Yet, on the coldest of South Carolina mornings, it fires right up.
Anyway, the flywheel effect on this engine is ENORMOUS! (not related to the starting, in
case I'm being nebulous) From tickover, one can floor it's throttle pedal (which augments
the standard fixed throttle lever) and the engine takes around THREE FULL SECONDS to rev
and make the 2,000-or-so RPM trek from idle to governed redline. My point is this, in the
bottom three gears, the DB's flywheel weight is--by a long shot--it's acceleration
bottleneck. The engine takes longer to rev than acceleration of the whole tractor. Old
flat-twin John Deere tractors (like my Uncle Fred had when I was a young GearHead) were
even moreso. I'm going to guess and say the Deere engine would require 5 or 6-seconds to
rev from idle-to-red while in neutral!
I know Diesels rev slowly and, in the DB's case, I also know why), but the John Deere twin
was a gas engine with a GARGANTUAN external pulley from which one could drive a myriad of
implements, not the least of which was a sawmill; an often-employed device here in the
rural SC of my youth. If I recall correctly, my grandpa's old '50 Ford was similar in that
it seemed to take forever to rev up, even in neutral. Does all this suggest a Fuzzy
Flywheel Formula of sorts?
Of course, in any performance vehicle, the engine should be able to accelerate on its own
AT LEAST as fast as available power could accelerate the vehicle. Otherwise, flywheel mass
is an impediment to performance. But might this also suggest that, in lightening the
crank/flywheel, one reaches a point of greatly diminishing returns? That is, after a
point, going for even less flywheel effect gives next-to-no benefit? ...and brings with it
some very real detriments?
The KTM 540cc (I think) moto-cross bike seemed to have reached this point of diminishing
returns. Any rider could easily sense the potency of its bark, and the snappiness and
instantaneous response both in and out of gear. But, the question is, does it really
accelerate any faster for this? I guess the answer to be, "No." Reference my recent post
on comparing wildly different flywheel weights on the same bike. The perception was wildly
different, but the performance was--surprisingly (to me)--pretty much the same.
I don't wish to brag here, as my successes are only so much water under a very old bridge,
but as a former titled moto-cross rider (500cc class was my specialty as I tamed the
power-of-the-day on the tires-of-the-day, and I worked hard to do it), I feel the 540
KTM's violent throttle response would be a boon in the sand, and NOWHERE else. This is my
opinion and, though an informed one, it is also one I would be happy to abandon since my
quest is one for the working truth, not the canonization of my prose, dogma, or pet
The "perceived power" of the KTM is high; it sounds vicious and menacing as it rips
through the RPM range in neutral. It also feels extraordinarily wicked as one accelerates
in the lower gears as this is where the light-crank's effects are most greatly felt. But I
think that, considering its job as being one of keeping that monster motor connected to
available traction, it's crank weight is, as the Brits so succinctly say, "Over the top."
I also say that it works AGAINST fast lap times, and not FOR.
Happy New Year, fellow GearHeads,
jrc in SC