Explain Bilstein shock “perches” to me

[gwb72tii]

I’ll be installing new sport B8 shocks, and reading through multiple posts about Bilstein shocks, specifically the rears, the topic of perches is discussed without any explanation. Seems the rears have 6 different perches for adjusting ride height. I cannot figure out why lowering the height of the shock perch would affect ride height?

The upper part of the shock and spring are captured by the shock mount bolted to the fender well. No adjustment there. The bottom perch captures the bottom of the spring, compressing it a certain amount. If I move the perch lower, which I assume is possible, it should only affect how much the rear spring is compressed.

How would this lower the rear of the car?

[kojo96]

I’ll be installing new sport B8 shocks, and reading through multiple posts about Bilstein shocks, specifically the rears, the topic of perches is discussed without any explanation. Seems the rears have 6 different perches for adjusting ride height. I cannot figure out why lowering the height of the shock perch would affect ride height?

The upper part of the shock and spring are captured by the shock mount bolted to the fender well. No adjustment there. The bottom perch captures the bottom of the spring, compressing it a certain amount. If I move the perch lower, which I assume is possible, it should only affect how much the rear spring is compressed.

How would this lower the rear of the car?

The higher the perch the shorter the spring, and the ride height is adjusted, very small increments for each groove adjustment. It makes for a firmer ride the higher the perches are.
The lower the perch, the softer the spring rate becomes and allows the car to settle lower on the suspension.

[Shawn D.]

To simplify understanding of this, imagine the whole assembly as only two components (which are the only things that matter with regard to ride height):

A) The length of the spring at ride height (spring compressed length as installed and with the vehicle's weight on its springs), which does not change. Completely remove from your brain the situations where the assembly is uninstalled or is drooped (i.e. when the car is jacked up), because those situations aren't relevant to ride height and the spring extension is solely determined by the extension limit of the shock or its free length, whichever is shorter.

B) The length between the shock eye and the spring perch, which does change during this discussion.

C) The total length of A+B.

B varies as the spring perch is moved up or down on the shock body. If the perch is further up the body of the shock, B is longer, thus C is longer, thus the rear of the car is raised. If the perch is further down the body of the shock, B is shorter, thus C is shorter, thus the rear of the car is lowered.

The higher the perch the shorter the spring, and the ride height is adjusted, very small increments for each groove adjustment. It makes for a firmer ride the higher the perches are.
The lower the perch, the softer the spring rate becomes and allows the car to settle lower on the suspension.

This is not correct with regard to ride height, which necessarily means the weight is on the wheels, not uninstalled or unloaded. The length of the spring when the shock/spring assembly is not installed in the car or is unloaded when in the car is determined by the perch placement (assuming the spring is not so short as to be loose), but under compression due to the weight when sitting on the wheels, spring's extension is not limited by the shock extension limit and the spring length is solely dependent on the weight on it. In no case does the spring rate change.

[gwb72tii]

So this makes sense. When you raise the perch and compress the spring, you are compressing the spring more than the car's weight would on its own, sitting the car lower due to the reduced length of the spring.

[Shawn D.]

When you raise the perch and compress the spring, you are compressing the spring more than the car's weight would on its own, sitting the car lower due to the reduced length of the spring.

No. Please go back and re-read what I wrote.

[gwb72tii]

**light bulb** turns on

As you move the perch higher, the shock is more fully extended as the car is supported by a higher base height of the spring. The spring length with be the same regardless of where the perch is located.
In theory, if you could move the perch as high as you wished, you'd run out of rebound on the shock as the shock would be fully extended at rest.

Thanks Shawn....

How do you move the perches?

[Shawn D.]

Now you've got it! You're welcome.

It'll be obvious once you see the shocks. You move a snap ring to a different groove.