6. Centreline

Now the main thing you need to concern yourself with, when
measuring up for the fittment of the new 17" ZZR600 rear wheel
into the original swingarm of your GPz900R, is the centreline.

You can take it as given that this wheel does in fact fit into the
original GPz900R swing arm, even though it is 4.5" wide and
the original 18" wheel is 3.5" wide.
The tyre that was fitted to this 4.5" wide rim is a 160/60/ZR17.
Although from the finished installation it may be possible to fit
a 170/60/ZR17 onto the rim and still have enough clearance.
Also, you would need to consider the effect this would have
on the handling of the bike.
I have not done a comparison yet, however I can tell you the
160/60 rides pretty well.
And the bike tracks well, and doesn't seem to push into corners.
A 170/60 may lift the rear a little and make the bike turn in more.
It may also have a little bit wider camber, so you be the judge.

Once you establish this centreline, you will know how much to
shim and rework the axle spacers to give you the correct offset
to ensure your new wheel centreline is where the 'original'
wheel centreline was.
What I mean by wheel 'centreline' is the middle of the rim.
The midpoint of the wheel from the the left rim to the right rim.

So, with both wheels removed, the axles installed and all the
bracketry, and spacers that are internal to the swingarm,
I measured the total width of each.
The ZZR setup was a full 10mm wider from spacer to spacer
 - internal to the swingarm.
 - GPz900R axle spacer to spacer = 239mm.
 - ZZR600   axle spacer to spacer = 249mm.

So, if I am to use the original GPz900R axle, I need to reduce
the total width of the spacers and brackets by 10mm, so it all
fits into the original swingarm.

Axle and Spacers - Full Setup. (NB: only one washer used) 

Left rear axle Spacer. (gets reduced by 2.5mm)



Right rear axle internal Spacer, and Brake bracket Sleeve-Spacer.

I measured the 'uncompressed' swingarm - from internal (R#)
to internal (L#) and found I could save about 3mm from when
the swingarm is compressed with the original GPz900R wheel
installed and everything tightened.
 - Uncompressed internal swingarm = 242mm.

Uncompressed GPz900R Swingarm.

So, now I only had to lose 7mm from the total ZZR600 axle
spacers, and get the wheel centred.

I did not want to expand the swingarm to gain anymore space,
as doing this would mean I would lose available threads from
the axle. And I have already lost 3mm based on not compressing
the swingarm.
Also I did not want to stress the swingarm welds by expanding.
However based on the fact that it does compress the swingarm
by 3mm, there may be some flexiblity there to expand slightly.

To regain 1mm extra axle thread, I used a thinner washer for
the axle nut, external to the swingarm.
There are plenty of threads being used for tightening the axle
nut, so no issues here.

Right Side Axle Nut - with NEW 'thinner' washer, and OLD 'original' washer.

When I measured the offset, I found the centreline of the
ZZR600 wheel to be actually 5mm offset to the right of the
original GPz900R wheel.
This means that Kawasaki has the centreline of each wheel
at the exact centre point of of each axle including the spacers.
The wheel centreline is exactly in the middle of the swingarm.
Good to know.

This may not always be the case.
A wheel does not have to be centred in a swingarm.
The wheel centreline needs to be centred based on the whole
chasis with respect to the front wheel.
The rim distances may not necessarily measure the same
from both sides of the swingarm.
You must measure the axle spacers to rim, on both the left
and right side to be sure.

So, in my case, if I need to take 7mm from the spacers,
then I need to take 3.5mm from each side and retain the
correct wheel centreline.
And then mill down the spacers and brackets accordingly.

Next to measure spacers.