Re Springs and handling

[David Ziglin <davidz@domain.elided>]

Any helpful commentary will be much appreciated.

I recently installed sports springs ( 20% stiffer over stock ) and Koni
yellow adjustables on my '66 Duetto.  Interestingly, the car was raised not
lowered.  The discarded springs and rear shocks were well worn.  For
aesthetics and lower center of gravity for the track, I had considered
cutting some length off of the coils.

In response to this idea, I received the following from a friend.  Again, I
would very much appreciate your sharing your expertice re this response.  I
have heard, by the way, from a long time Duetto racer who was pleased that
I had not installed stiffer than 20% over stock springs.  FWIW, I love my
present setup, a wonderful improvement; however, I loved the car when the
springs sagged and the dampers bottomed.

Ultimately, my question is, what should I do?  Note that recent writers
have expressed positive results after the snip. ?????

Thanks in advance.

David


When I wanted to know everything there was to know about motorcycles with
sidecars, I read a lot, especially about suspensions and handling.  Also, at
that time, lowered choppers were just starting to become popular so there
were lots of reasons to learn about suspensions and springs.  Many people
tried to lower their motorcycle and sidecar by "snipping" off the springs
thinking that the rig would handle better.  I know that young people have
lowered their street cars this way for many many years to make them look
better.

But, it is not advised in any serious situation.  It almost always makes
things worse.  Here is why.

A coil spring is really nothing other than a long bar of metal that, at this
moment, just happens to be rolled up in a coil.  A torsion bar is really
nothing more other than a long bar of metal that, at this moment, just
happens to be NOT rolled up in a coil. It is straight. They both do the same
thing and they both work basically the same way.

When a "twisting" force is applied to one end of the torsion bar, and the
other end is held firm, the bar twists and the amount of the force that is
applied is taken up all over and all along the surface of the bar during the
twisting.  (Just imagine that you have painted the surface of a torsion bar.
When it twists, all the point comes off.  That is because the entire surface
of the torsion bar twists under the force and loosens the paint.)

When the coil spring is compressed, it is not the compressing of the coil
that takes up the force. It is the same principle as the torsion bar.  The
amount of the force is taken up over the surface of the coil spring, even
though it is rolled up.  As the coil spring twists to take up the force, the
"coils" are then drawn closer together until eventually they touch, and the
spring becomes "spring bound".  At this point the spring can't twist anymore
and your reach the end of the springs ability to absorb force.

Anyway, the amount of force that can be taken up in the twisting of either
the torsion bar or the spring is direction proportional to the surface area
of the spring.  The more surface area you have, the more absorption can take
place.  The thicker the spring (the greater the diameter of the bar), the
more surface area you have.  The longer the spring, the more surface area you
have.  So, if you want to absorb more force, make the spring thicker or
longer or both.

If you cut off the spring for some reason, (like to lower the car), you
actually shorten the length of the "bar" and you reduce the surface area of
the spring.  The spring cant absorb as much force.  Your spring rate goes up
like a rocket ship. You can shorten the length of the spring until it is so
short that it won't absorb any force at all.  You stop having a spring and
you get a rod at that point!

A spring that is nice and soft at full length gets has hard as a rock if you
cut 2 inches off.  That is two inches of length of the spring, not height of
the coil.  If you want to lower the height of the coil 1 inch, like you said
you wanted to do, you might have to cut 8 inches of spring off, depending on
the diameter of the coil.  You get a vehicle that rides like a lumber wagon.

You could wind up with something that handles so bad, it won't corner at all.
 Talk about taking trips into the weeds.

And, don't forget, once you cut off the spring, you can't glue it back on.
And, if the height of the coil spring is now shorter, you must also find
shocks that are exactly the same amount shorter.  And, the rebound and
compression dampening rates of the new shocks have to take into consideration
the change in the spring rate that you made when you whacked off the spring
in the first place.  Just selecting the proper shock could be a major
exercise in frustration if you wind up with some goofy shock length

There are other much better ways to lower the car.  You do not want to change
the spring rate or change the way the suspension works.  (Unless, of course,
that is the reason for shortening the spring in the first place. But, you
said you just want to lower it.)  Maybe for your car there are replacement
stub axle carriers or hubs that have the axle actually located higher up.
This will lower the car and you don't make one tiny bit of change to the
suspension at all.  If not, it is not totally unrealistic to have somebody
make some for you.

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End of alfa-digest V7 #740
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