Old Kaws Never Die - Cranks

People speak of the legendary Z1. What made The Z1 legendary?
It was the first superbike. What made it a superbike? Why did
they run so good in stock form compared to other bikes of that era?
Why did the motors spin up so fast compared to later motors?
For those who are not familiar with what is shown above, There
are 2 crankshafts. The crank on the right is a Z1. The crank on
the left is a KZ1000. For those who are not familiar with the
term inertia, rotational inertia, specifically, note the differences
between the 2 items. The crank on the right has smaller parts. The
higher the inertia, the more power is used to spin up the rotating mass.
An engine with the same power output and less rotating mass will
accelerate faster than one with higher inertia. A clue can also come
from the idle rpm settings of the stock units. 900 rpm for the Z1
and 1000 rpm for the other units. Bottom line; all else being equal,
the bike with the crank on the right will win every time. There is
another competing theory whenever a large rear tire is used. That is
the larger rotating mass allows a better initial launch. This is a
race only deal, again, if you want to race, have a racer guide you.
The info I provide is more for the street than the track.

Here is another photo, which gives another angle on the view. As you
can see both these cranks have been welded in preparation for power
output increases beyond the friction fit of the joints. These cranks
are not single piece units, but are many pieces pressed together. This
simple welding permits outputs of up to 200HP. More than that and you
need more involved welding and some machining. That is best left to the
track pros. The reason Kawasaki went to a more massive crank was to reduce
vibration and give better low-end torque. There is a whole other philosophy
that uses inertia in a different manner. Harley has always used massive
flywheels. This enhances low-end torque and acts as an energy storehouse
that gives excellent initial acceleration. If you have any experience racing
an experienced Harley rider, you will be familiar with this. You both rev up,
and bang you start out and he has the jump on you until you pass him in second
gear or maybe just before you shift to 3rd. That rotating mass needs energy to
spin up, but it also releases energy as it spins down. There are a lot of Harleys
out there that will fly through the 1/8th mile, but would get smoked in the Œ.
Light to light on the street it comes down to the rider, the bike, and the
conditions. There is no speed trap or finish trip light. If you are going to
win you need to make it good.

It may be hard to see in the above photo, but the alternator rotor is held
in place by a woodruff key. Later units are held on by a bolt and depend on
friction to prevent rotation. One problem associated with rapid acceleration
of later bikes is that the alternator rotor can spin on the crank and loosen
the bolt that holds it on. This usually trashes the stator. After it happens
once, material is transferred between the crank and rotor making it difficult
to prevent more problems. Bottom line time again. How much do you gain from
this on a 1200cc bike? What is the actual difference? Hell, I don’t know. I know
I can feel the difference on the street. You will have to get with racetrack
folk to see what their experience is in times and speeds.

These pictures only show 2 cranks. There is a 3rd crank, the MKII.
The MKII crank was 1979-1980, the KZ1000 crank was 1976-1978, and the
900 Z1 crank was 1973-1976. The MKII crank has a larger crank pin,
it is stronger and prefered for output beyond 200HP. It also has
a 20% increase in rotational inertia. When the cranks are prepared for
racing, they are lightened and then balanced. There are advantages
and disadvantages to using a specific crank. A welded Z1 crank is
the way to go on the street. For drag racing applications where
power output exceeds 200HP, then things get more complicated and
expensive. I will leave at that. My Old Kaws Never Die Series is
dedicated to street performance of normally asperated motors. For
other applications you must look elsewhere for information. My
intent here was to show how easy it is to build up a motor for the street.

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