Vintage rear suspension

 Okay, I really screwed up here, so be nice to me.

I picked up new Progressive shocks for two vintage bikes yesterday. Both
1974 Honda's, a CR250M and a XL350K. A month or so ago I measured the
eye to eye distance of the shocks on the bikes and the CR was half an
inch longer then the XL, close enough, or so I thought. So I bought two
pair for the CR.

Well, I messed up my measuring, somehow.

Put the new springs on the four new shocks then took an eye to eye
measurement. They are 16 inches. The old CR shocks are 15 1/2 inches eye
to eye. No problem, that extra half inch would do nothing but help. But
the old XL shocks measure 12 1/2 inches eye to eye. This will not work
because the added height will turn down the swingarm, causing the chain
to rub at the swingarm pivot point.

These new shocks have two springs, the one on the bottom is 3 inches
long, the top one is 9 inches long. If I left out the short spring, I
would have the correct height for the XL. Is this possible? Any safety
issues by doing this?  

I can't return the one set because even though they were not mounted,
they are assembled, and no longer considered new.

Thanks in advance for your help.

 On Nov 15, 11:39�pm, AWR7MM...@webtv.net (Mike Corey) wrote:


OK, so that's a way to get the effect of dual rate springs without
actually winding
one spring with different coil spacing at one end. The spring with
closer coil spacing has a stiffer rate, assuming that the wire
diameter is the same.

You need to google up an online spring rate calculator, do the
measurements on the old and the new springs, enter the information and
get the answers and figure out whether you can get away with one
spring or if you need to buy new springs or if you use the old springs
that came on the XL.


Almost anything is possible. If using just the one spring allows the
shock to bottom prematurely, you'll get some erratic steering as the
motorcycle tries to pivot around the rear wheel.

 . wrote:


Not quite. Given identical wire diameters, the closer wound will go coil
bound first. A coil spring is just a torsion bar wound in to a helix. The
wider spaced coils will be stiffer as greater torsion needs to be applied
to achieve the same compression. If you want to stiffen a spring, remove
coils and make a spacer. The closer spaced end is the more compliant. Dual
rate achieved through dual springs and a separation plate work in a similar
manner to progressive springs. If one coil has a rate of 5Kg/mm and one has
a rate of 10Kg/mm, -both- springs will compress under load. As the lower
rate spring becomes coil bound, additional load is taken up by the stiffer
spring.

HTH



Yes. The spring may leave the seat under rebound. The damper rod will be
working at the extreme of its compression range under compression and may
impact internally if its length has been derived from the coil bound length
of both springs. This may lead to less than amusing handling as it smashes
itself apart.

 You will not get away with it.  when you ride over a bump/jump etc and it
unloads the suspension, it will have the 3 inches of un spring slack, if
that doesn't cause the springs to get caught in the assembly and do
something bad, then nothing else your gonna be effectively using the
motorcycle as a sledge hammer on the shocks and swing arm/frame.  what you
could do is reposition the shock mounts, we did it all the time back in the
eighties trying to find an advantage on trials bikes...  surely you could
fudge it a bit at each end, plus how bad is the rubbing?  I mean 90% of the
time you are going to be on the bike (compressing the spring) .
unless you are a small person then this wouldn't apply.  the bikes of today
use heavy plastic to keep the rub from being metal to metal.

then next problem is too, if the rear-end is now jacked way up (static
preloaded height) the front can be soft and dive and really hurt ya.

 PlowBoy wrote:

 > then next problem is too, if the rear-end is now jacked way up (static

Anecdote provided herewith, as if it actually proved something:

I swapped the shocks on my elderly Ducati for Honda shocks 1 inch
longer.  This resulted in serious front-end wobble, the attempted
diagnosis of which put me on the ground at 50 mph and removed nearly a
square foot of skin from my midsection.  Aside from the pain, blood and
expense (also required professional straightening of the triple clamp),
I was really annoyed because the Honda shocks felt so much better.

Suspension may be an art or a science, but it's definitely not a
try-it-and-see-what-happens activity.

--
Cheers, Bev
================================================================
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