Coilbind and Perch height

[Scott_Higashi@necelam.com]

So here's the scoop on coilbind...

Any given spring can only squish a finite amount before the coils come
together.  Some of that squish is taken up by just lowering the car onto
the springs.  The rest is available to soak up bumps and compress as the
number of people change or cornering forces act on the car.  Say your car
has 2" of strut travel at a given ride height.  Say the spring has 3" of
available "squish" left when static.  In this instance should you drive up
and hit a big speedbump, you'll bottom out your strut before you coilbind.
Of course, bumpstops are a good thing...

Now, say you realize that you don't like bottoming out and you raise the
car 2".  You just gained 2" of travel right?  WRONG!  Remember, regardless
of ride height you only have as much travel as the spring OR strut,
whichever is less.  So really even though you raised the car 2", you only
gained 1" of travel.  Should you hit the same speedbump, instead of
bottoming the strut, you'll coilbind.

On the TEIN setup where they say you must lower the front at least 2.1",
it's probably due to minimizing coilbind and the fact that you may
internally top out the strut.  Not to mention that if you don't have enough
of the strut shaft engaged within the tube, you put BIG side load/stress on
the piston.  This is something you don't have to worry about with shocks,
BTW.  Now if the topping out wasn't a problem with the TEINs, then a longer
spring could fix the problem.  Too bad they don't use standard 2.5" ERS
type springs! ;^)

This is why when running shortened struts it is important to realize that
you MAY need longer springs to take advantage of the extra travel you just
gained.

Now, between springs of the same length, the stiffer springs generally have
less travel to coilbind due to the thicker wire needed to wind them,
however, they have a greater force necessary to coilbind.  Add to that that
you use up less initial "squish" when dropping the car onto the springs it
usually works out to be pretty close as to whether or not you lose overall
spring travel when you go up or down in rate.

Hope that cleared some stuff up...

-Scott