spring/shock rates/coilovers...

[Scott Williams <sfwilliams@home.com>]

David & Michelle Barwick wrote:

> Mushy on the street?  If you mean compliant, maybe I will spring (get it?)
> for the revalving.

David, I want you to delete this email unless you really want to learn the
parameters of suspension set up. I'm warning you...

When I say "mushy" I mean that the transient response is not very sharp. I'll
explain. First of all it is the springs that really determine how stiff the
ride "should" be. I say "should" because the struts/shocks (dampeners) will
optimally be set to a rate that is no greater than that necessary to control
the oscillation rate (the "boing-boing-boinginess") of the springs. Put
another way, the dampener's rate is only a function of the spring rate. The
stiffer the spring the greater the amount of control force the dampeners need
to exert. Thus, once again,  it is the spring that really determines how stiff
the ride "should" be.

Okay, do you get that? Alright now... the spring rate determines how *far* the
car can roll on it's suspension. Meanwhile, the dampening rate determines the
*speed* of the suspension movement. With a stiff set of dampeners the
suspension slowly rolls from one side to the other side. When the dampeners
are soft the car will roll faster.

Still following? Here's your answer to why my suspension is mushy: My springs
are very progressive in rate. They're long travel and go from about 75lbs/inch
to about 350lbs/inch. In contrast, the dampening rate is rather linear
throughout the travel. This feels great when landing jumps; the car just soaks
up the impact and then regains it's composure. However, the dampener's
stiffness also slows the movement of the suspension when cornering
-particularly the last couple of inches of travel which have a very soft
spring rate. Turn the wheel of my car and it doesn't react instantly. There's
just a bit of delay.

So, rather than razor sharp handling that a street or race car has, my car
feels "mushy" on the street. On the dirt or snow, however,  there is so little
grip that the car doesn't really lean all that much and I don't notice the
slow response. Rallyists steer as much with the brake and throttle as we do
with the steering wheel -sometimes more.

> Did you give them your car weight and spring rates, or is Group N
> essentially the same thing?

Group N is an FIA homologation classification. The dampener's design follows a
specific formula that was approved for competition. I didn't really specifiy
anything other than what spring rates I'd be using, desired ride height, and
required suspension travel -extension and compression. The Bilstein engineer
did the calculations and decided on the rates.

> I have been hearing a ton of positive things
> about coilovers; replacing the springs being quick and no re-alignment,
> light weight etc..  Do you have any experience with the coilovers?

Yep, I've got coilovers on the back of my car which provides some control over
the oversteer characteristics of the car. On pavement, I'll lower the rear
just a little. On dirt or snow I'll raise it a smidge. Anytime you raise or
lower the front, though, you'll have to realign the suspension. As for the
weight difference, there really isn't much to the coilovers. You can pick and
choose spring rates as you please, though. That's the huge advantage.

Well, I hope that helps!
--
Scott F. Williams
NJ Scirocco nut
SCCA ProRally driver
Hotrod Rabbit GTi

Check out our rally team's website!
http://www.usrallyteam.com



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