Lowering Overview
When I shortened the Magna, I took into consideration some basic
concepts. IMO the front fork is too long, and the same for the wheelbase. The longer
front fork weakens the suspension, and the fork travel is excessive. Honda did this to
give it more of a 'cruiser' style, and also, I guess, to fit a wider variety of people. So
I postulated that 'shortening' the front fork would have the effect of strengthening &
tightening the front end. It would also shorten the wheelbase minutely. If the stock
ratio's were kept, (i.e., shortened the rear as well as the front, by the same amount) the
cg would be lower, while keeping the stock front/rear ratios. Combining the lower cg and
'stiffer, more stable' front end, the bike should handle the same, if not better. Also,
for us shorter riders, the height reduction is a very welcome thing.
With the above in mind, I did some measurements and
calculations, over about a month period. I measured the stock height, and both calculated
and measured the effects of shortening both the front & rear. After these thorough
measurements, I decided to go on with the shortening. I'm glad I did.
The result was a better handling bike. I am very happy with
the handling now. I think the combination of tires/suspension mods tunes the Magna to its
peak.
Calculating ride height changes from shock length is not an
easy task. I found it geometrically challenging, to say the least, and used some
mathematical approximations and empirical data on both the front and the rear geometry.
Rear Suspension
The rear modification is self evident. Shorter shocks. The stock
Magna comes with shocks that are 14.25" long. The Progressive Suspension 17000 comes
in various sizes, including a 14.25" length for the stock Magna. Due to the angle of
the shock, and the position on the swingarm, shortening the shock by a certain amount does
not lower the bike by the same amount. I figured out the actual amount the bike would be
lowered by analyzing the angles and lengths, and came up with the same number PS gave me
of 1.2. This is the multiplication factor that relates the amount you shorten the shock to
the amount the actual bike is lowered. So, a 13.5" shock would lower the rear end by
1.2*(14.25-13.5)")= .9". PS also informed me that their shocks (17000 series
anyway) all had the same length when fully bottomed out. To empirically verify that a
13.5" shock would lower the suspension without causing any clearance problems, I took
a stock shock, and measured its normal height, referenced to the mounting bolts. Then I
mounted a single shock, and took measurements with that. I let the shock compress all the
way until it bottomed out. No clearance problem. Then, I let the suspension up to the
point where it would be .75" below its stock height (approximately 13.5"). This
seemed great, but when I let it up 1.25" (to maybe lower it a little more with a
13.0" shock) it seemed a bit excessive. Therefore, a 13.5" shock it is. There
was one factor I overlooked, and that was spring stiffness. The PS spring is stiffer than
stock, so that the bike actually sits a little higher in the back than I calculated. I
estimate about .1" higher.
Front Suspension
The front, geometrically is a little easier. The fork makes an
angle of about 30 degrees, its rake, so the actual lowering will be multiplied by the
cosine of that angle, which is 1.7". This is a crude approximation, but I found it
sufficient. Notice that the number is 1.7", which is more than the rear. This was
because when I initially did the calculations, I multiplied by the sine of 30 degrees,
which is incorrect. So lowering the front fork by 2" lowers the bike by about
1.7". These are approximate numbers, and the calculations for even a simple case are
quite complex. For a more exact analysis, the trail must be taken into consideration, and
other factors. Being an engineer, I love these puzzles. Note that when the bike is lowered
by 1.7", the wheelbase will be shortened by .5". I lowered the front forks by
2" before I realized the error in my calculations, but it worked out great anyway.
The way I have the spacers set up, it would be easy in the future to change the 2"
spacer to a 1", which I may yet do.
Front Suspension Technical
Details
To lower the front suspension, I made the decision that I didn't
want to change the stock spring/spacer rate. I wanted the preload and geometry the same,
just a little lower. This resulted from my road education in the suspension disaster. Upon
discussions with PS on how to do this, they related to me details about kits they sell for
more popular bikes. These kits consist of spacers and replacement top out springs, which
when inserted into the forks, cause the front to sit lower, and keep the stock preload.
This is a hard thing to imagine, not knowing that much about the pieces of the forks, so
the following paragraphs will describe in detail exactly how the front is lowered. The
following photo shows the major pieces of the fork assembly.
I want to stress that I did not just cut the
spring spacers. This would lower the bike, but it would also cause the front springs to be
compressed less than stock . . . not a good idea, if you read my horror story.
| The way the suspension is set up is
that the fork tube is free to move up and down with respect to the slider (or Visa-versa,
depending on your reference point). Attached (Bolted) to the bottom of the slider is the
piston, which sits inside the fork tube inside the slider. The piston is just another
tube, with a bolt hole at one end, and a "stop" at the other end. This bottom of
the piston (with the help of the "top out spring") stops the fork tube from
moving all the way up, and is the top bound of the suspension travel (fully extended). The
top of the piston is where the fork spring sits, and the fork spacer sits on top of that.
There is a small spring, called the "top out spring" (about 1" long) in
between the bottom of the stop and the bottom of the fork tube, so that when the
suspension is fully extended, the assembly will hit the spring, not the piston. There is
tension in this at all times, and the spring usually only comes into play when the
suspension is fully extended. If the top
out spring were longer, this would cause the fork tube to sit lower in the slider, but
would also lessen the space between the top of the fork tube and the top of the stop,
effectively compressing the fork spring more than normal. The trick is to insert spacers
between the top out spring and where it sits on the piston. This lowers the fork tube in
the slider by the exact amount of the spacers. Now, to keep the "stock" spring
compression, the fork spring spacer must be cut off by the same amount as the length of
the spacers. For example, on my V65, I inserted two 1" spacers in between the top out
spring and the slider tube (in the assembly). Then I cut the fork spring spacer down by
2". This sat the bike 2" lower, and kept the stock spring tension/rate. Below
are drawings of the front fork, which should make this discussion easier to
understand.
|
|
 Spacer
and Top-Out Spring |
 Spacer
On Spring
|
Stock Fork
Modified Fork
The spacers I described above are made out of cylindrical
brass stock, but aluminum would be just as good. It is advisable to have some made in
increments of ¼", say one ¼" long, two ½" long, and one 1" long for
each side. This way you can fine tune the height of the suspension. For each top out
spring spacer insertion, the fork spring spacer must be equivalently shortened.
Remember, that now that the bike is sitting
lower, the brake lines will have a tendency to loop around, since they have to travel 2
less inches. This wasn't a big difficulty on mine, even though I have the braided steel
lines, which are a lot less forgiving than the stock rubber ones.
The modification is completely reversible, even if the spring spacers are cut, you can
easily make up the space with any type of spacer, even the cut pieces of the original one.