Perch settings

[Giblet]

I am not a setup wiz and I don't fully understand what perch is and how it effects handling, especially at high speed on a stiffly sprung car like a simulated Skip Barber RT 2000 with a very stiff roll bar as well.

[Jersey Tom]

Do you mean spring preload?

[Giblet]

Spring Perch Offset. Its only on the front end of this car.

[garygph]

Spring perch offset - For cars with coil‐over spring/damper units and conventional suspension, the spring perch offset is the distance from the spring seat of the spring perch (or shock collar) on the shock body to the rod end of the shock body. With no other spring changes, reducing this offset will extend the shock (raising the ride height at that corner of the car), whereas increasing it will collapse the shock (lowering the ride height). This number simply represents the lengthening or shortening of the spring with zero being a baseline starting point.

Is this what you are reffering to?

[gato azul]

I think, Gary has posted a good summary, as what it is and what it does, but I may want to add one thing.

I don't know the suspension of an Skip Barber, out of the top of my head, but assuming it's an single seater, what adjusting the perch height (spring preload) does is, that it moves the location of the piston inside the damper.
This changes the ride height, but you can readjust/compensate this with the ride height adjustment on the push/pull rods.
So, you may come back to the same ride height, but you have changed the droop setting of your damper/suspension.
Which means, how much can your damper extent, before the piston inside touches his max/min (top out/bottom out stop) position.

This can be used to affect the handling of the car, in some single seater series (Formula Ford in particular), it is very common, to run a "zero droop" front damper/suspension setting.
Because if one damper reaches his stop, during operation, it lead to a rather "sudden" change in stiffness at this axle, which in turn will affect the load tranfer. This can be a bad thing, but it can also be used as an advantge, under certain conditions.
If the damper is fitted with an "rebound spring" or elastic "droop limiter", changing the perch height will affect, the point when the rebound spring comes into contact, or if it is already in contact at ride height, will change the preload setting of the rebound spring.

this photo shows, what you would call a "perch height setting" for a coil over shock/damper.


In bikes (racing), this parameter is commonly refered to as "sag", you may find some infos about this, when you google for it.

[Jersey Tom]

Preload can do interesting things.

Personally I probably wouldn't go to the spring perch to do ride height adjustment if possible. Better to do with a push/pull rod adjustment.

[munks]

Preload can do interesting things.

Care to elaborate any further than what blue cat did?

[hardingfv32]

Preload can do interesting things.

Roughly, like what?

Brian

[Jersey Tom]

Think about the impact it has on elastic load transfer, on pitch and roll... if the shock can't travel until a certain amount of load (preload) is exceeded.

[marcush.]

can of worms anyone? if you happen to have pushrod/pullrod ,altering the length of the rod (or the damper installed length will also alter the motion ratio which is usually not a linear thing to begin with...lots of things can do interesting things.

[gato azul]

some of the aspects are discussed here and here.

and here for the "zero droop" case, metioned earlier.

Have a read, think about it, do some calcs/maths, see if it makes any sense and than make up your own mind.

What Marcus says is correct too, not sure if it is neccessary a "can of worms", can be a quick way to alter spring stiffness/rate by a very small increment (smaller then the gap between two "off the shelf" springs), by altering the motion ratio slightly, due to the different rocker position.

Sure, you should keep track of all the different variables you change, as with any other setup change, which affects more then one parameter of your car/setup.

[marcush.]

the can of worms starts when altering settings with implications you are not aware of.

e.g.ride height change causing a change of wheel rate ...
or any car with ride height sensitive aero will act practically as a spring in the system and so without having access to a aeromap and all necessary geometric info a ride height change can throw you EASILY out of any useful setupwindow. ..so you also need to know the magnitude of impact of your change as well so you are able to do reasonable steps as well as being able to decifer what caused which ...