ClayMan28

I'm aware that Bilsteins come with 70mm ID springs - people talk about modifying them to fit standard springs (e.g., 60mm and 65mm) but I haven't seen any pictures or further discussion of it actually done. What exactly needs to be changed and how much work is it to do? Looking at pictures of the coilovers it looks like there's plenty of room between the 70mm and spring and piston shaft to use a tighter spring.

I've searched and seen discussion about "cutting the spring perch" - I assume this means putting the perches on a lathe because there is an inner ridge of some sort on the perch that prevents smaller springs from sitting? Is there anything else that needs to be done? Will the OE top hats fit?

scareyourpassenger

The better option is switching to english measurement springs. Way more options than the metric sizes.

2.75" springs may just work or modify to use 2.5" springs.

User 121020

Here's the only image I have of my machined lower perches. The rest of the hardware in the image is for a custom top hat design.

AyJay

The easiest thing to do would be to remove the helper springs and the spring collars that they require. You would then only have to accomodate the lower adjustable spring perch (which is flat and no problem) and the tophat which is made to work with a 70mm spring. I saw somewhere on this forum that a member was successfully using 65mm springs with the OEM tophats but I haven't tried this.

User 121020

Do you have your lower perches on upside down or are you using 2.75" ID springs? The standard Bilstein lower perches will not accommodate 2.5" ID springs without chucking them up on a lathe.

AyJay

I'm working off of memory here but I recall examinging the black perches that rotate along the threaded shock body to be flat. I remember briefly looking at them and thinking I could probably run a smaller ID spring if I needed too. If you have tried it and say it will not work then I would probably defer to you...

I've never actually tried to sit a smaller ID spring on there.

My car is currently up on jackstands cuz I got lazy and only did my front track brakes last night. While I'm putting in the rear pads tonight I will examine the spring perch again.

***Edit***

I see that nmrado actually has a pic posted that was blocked by my work's internet software... clearly he's tried it before so I retract my previous statement.

ClayMan28

A picture speaks a thousand words! The next question is what top hats to use...

User 121020

If I'm not mistaken some people have just continued using the OE top hats. Maybe others will post up with their top hat solutions.

I made my own, but wouldn't recommend using my design unless you have access to free machining. Material and off the shelf hardware cost me a few hundred dollars and a couple of weekends of machine work. Though, I'm not a very efficient machinist.

User 121020

Also, here is a plot of the OE valving for the PSS dampers.



Here's my damping curves for 900 lb/in front and 800 lb/in rear springs. They need about 100 to 150 more lbf in rebound. Otherwise they work great and have gotten me to a few top PAX finishes since December. The compression feels good.

Front


Rear

ClayMan28

nmrado, you are a great resource to the community! What's your view on the ideal / maximum rate that the stock PSS valving can take? I understand they're overdamped for the stock PSS springs (345# front, 285# rear)... I am piecing together a more street-friendly set up (relative to yours), and trying to choose the best rates without revalving:
1. 9kg/mm (504#) front with 8kg/mm (448#) rear
2. 8kg/mm (448#) front with 7kg/mm (392#) rear
3. For even more front bias: 8kg/mm (448#) front with 345# rear (the front PSS spring)