Fork Spring Question

I have a 82, Honda CB450SC Nighthawk (AKA The SOB450). The suspension
is shot and I cannot find replacement fork springs (I went to Greg at
Santa Clara cycle and he spent a lot of time calling suppliers to no
avail). The bike isn't worth taking to a suspension shop - so I have
two options:

1) Find fork springs from another bike which will work - note: they are
comparable to the fork springs from any early 80's Honda Twin ~400cc
bike... according to Clymer's
diam = 0.92 inches (say 1")
Length Spring A = 9.4"
Length Spring B = 14"

2) Find industrial compression springs which fit. In which case I need
some approximate spring constants. [If I go this way, I'll buy a
variety of springs and mix & match the springs until I get the ride I like]

Can anyone help me out on these?

 

Hmmmm. Interesing equation, but I'm suspicious because it doesn't
specify the material or the units (it is inches or mm?) -- I presume
that 1.2x10^7 is some scaling factor which would change if I measure the
spring in different units - or use different material.

Can you tell me where you got this equation? I'd like to find out more
about it so I cna use it confidently.

 

Aha! I found a webpage which shows that equation:
http://www.efunda.com/DesignStandards/springs/calc_comp_designer_eqn.cfm

And it indicates that instead of using a constant number (1.2x10^7) one
needs to supply the shear modulus, G, for the material used. (and from
the units of the shear modulus, the other units may be determined).

Alas, I cannot easily measure the shear modulus of my old springs - It
is easier to measure the length, then put a 10kg mass on top of the
spring and measure the compressed length to determine K (which is in
units of N/mm - or Lbs/in). Of course, this does not tell me what the
spring constant of a NEW spring would be...

 

there are a range of alloys that are used, differing amounts of nickel,
etc, can change the constant +/- a fair amount.

 

Hmmm. I suspect that range is appropriate for my weight, but I doubt
Honda put springs that stiff into a little 450 Nighthawk.

Has anyone experimented with industrial springs in their forks?

 

One may not, I but I have seen brass, titanium, even PLASTIC springs. I
understand racing suspensions use titanium alloys (since money is no
object). [Yes, I know that this is quibbling - I am not putting
plastic or titanium springs in my Nighthawk] However, repeated heating
changes the hardness of steel, which will change the shear modulus - so
over use spring steel softens and becomes ... well, sprung steel

Cool! You have allayed my suspicions. The shear modulus in your
equation is probably typical for the coil springs used in suspensions.
So I can probably measure it (as you said) and determine what the old
values
were and add 50% spring rate to account for the fact that I'm a pretty
big guy!

 

Why not shim them up with some pvc pipe spacers for the correct amount of
sag and see how they work?

You could also post the length and diameter of your springs and those of us
with a stash of old springs could see if we have something close.

RickB

 

I tried it and the amount of force required to compress the springs
makes it very difficult to start the threads properly on the fork cap.

Cool. Here are the specs...
diam = 0.92 inches (say 1")
Length Spring A = 9.4"
Length Spring B = 14"

 

Wow. The pre-load on my little 450 is < 1" (how far the cap screws in.)

I wonder why no one makes fork caps with an adjust for pre-load. I
mean, all ya need is a threaded hole with a screw (and a lock-nut). I
could mod the caps and allow for an additional 2 or 3 inches of preload
.... maybe I need to track down a spare set of fork tube caps...

 

Hmmm.