Date: Wed, 1 Jul 1998 14:01:51 -0700
Reply-To: David Bayer <bayer@SYBASE.COM>
Sender: Vanagon Mailing List <Vanagon@vanagon.com>
From: David Bayer <bayer@SYBASE.COM>
Subject: Maybe dirt bikes have different springs? Was Re: spacers and
sproings
>| Now on to pre-loading. If I have a spring, and I
>| stick a piece of threaded rod through it and assemble together
>| some washers and nuts and such and clamp the spring down
>| an inch shorter than it normally is, I've pre-loaded it.
>|
>| Suppose I have a non-linear spring, like is on the motorcycle,
>| which at its full extension requires 150 lbs to compress it
>| one inch. Now I clamp it down an inch. A load of 150 lbs.
>| placed on this pre-loaded non-linear spring will now compress
>| it LESS than an additional inch.
In this case, you have to put 150lbs. on it to over come the
restraining force exterted by the threaded rod. As you apply the 150lbs
to overcome the threaded rod, you will compress the spring an infintecemial
<spelling?> amount. The reason being that the rod is a fixed barrier and
will not apply additional force once the weight is moves the spring off the
rod. Even in a nonlinear spring, when the "preload" is caused by a fixed
barrier, as soon as the fixed barrier is removed from the forces acting on
the spring, that fixed barrier can have no effect on the spring.
I just sat around with my boss who races bikes staring at his spring
and listening to him talk about tuning it. His spring and its mounts are
much like the syncro's except the upper perch is on held in place by a
threaded rod and is adjustable. He says that the adjust nut didn't change the
spring's stiffness or softness, just the ride height of the rear of the bike
and the amount of weight it takes to initially push the spring off the bottom
of the shock. When I told him why I was interested in how his rear spring
worked, he asked if the shock length had been increased or were people
thinking about adding a spacer because some shocks are not linear in their
travel and that they can develop a "worn" spot in their travel... This brings
up a good point and I wonder if in raising the ride height of the suspension,
one might not want to get a different set of shocks... Of course, I don't
know anything about this topic, but I wonder if one couldn't match a set
of those Fox shocks that FAT performance used to sell for score Baja bugs
to the westy...
So, to get to my new subject line, I tried to think of a setup that
might allow one to preload a spring like Steve was saying he was on his dirt
bike. I assume the setup will look like most shock/spring combos I have
seen on motorcycles. I also need to define the ends of the shock/spring
thingy - the moving end of the shock/spring assembly is the end on the
shock plunger while the fixed end is on the body of the shock spring
assembly.
Now, what we would need some is an additional
compressive force that would be applied through out the range of motion of
the spring - like another spring. This second spring would be of the pulling
type (I forget the name) instead of the type on the vanagon which is designed
to push. The second spring would be mounted at one end to a screw adjustable
plate and to the opposing end of the main spring. The plate would have to be
mounted to the moving end of the shock so that the "pulling" spring would
travel the same distance as the main spring - this assures an equal
compressive on the main spring. The main spring would need a perch that could
be adjusted to allow the ride height of the bike to be changed as well - this
could be on the fixed end. having it on the fixed end allows this addjust to
be independent of the preload because as I set this assembly up, the pulling
spring and main spring are attached at this end - so what one really adjusts
is the length of the shock extension... Does does this look anything like
a dirt bike setup?
dave
| 
