Open Road

Here's a convoluted question, driven by all the information I've been able to find and (attempt to) synthesize.

As a general concept, I understand that the spring rate for the front suspension is not driven solely by the spring itself; the rubber bushings that are all part of the front suspension (lower control arms, for example) play a role in an effective spring rate. I don't know what those components contribute to effective spring rate as discussions over on RL became heated quickly in certain threads where these kinds of things were starting to be discussed (and, to be fair, there is some trade secret stuff to be cognizant of in the measurement of those things). I understand that Carl measured the stock Porsche, Weltmeister, and Bilstein (?) spring rates and found pretty different numbers between stock and aftermarket.

It also sounds like, per Greg, that moving to Bilstein shocks obviated certain suspension components, the upper spring perch (or concave washer, or spring bearing, whatever; the thing that gradually squishes that Worf investigated on this site a while ago) in particular. Other experienced folks (like Sterling, based on his offering in his store), seem to prefer the Konis, which my reading indicates rely on the upper spring perch to make everything feel good.

Now to my question: Does the stock upper spring perch play a role in the effective spring rate on a stock car? It's a big chunk of rubber so I understand the role in NVH but it's also a big chunk of rubber, so there should be some non-negligible spring rate. Has that been measured?

What I'd really like to get to is a rebuildable option for those parts but I'm trying to, at least, work out the concepts involved, in my head.

If you've read this far, thanks, I'd love to hear your thoughts on things.

Cheers

Zirconocene wrote: ↑Tue Jan 27, 2026 5:06 pm Now to my question: Does the stock upper spring perch play a role in the effective spring rate on a stock car? It's a big chunk of rubber so I understand the role in NVH but it's also a big chunk of rubber, so there should be some non-negligible spring rate. Has that been measured?

AFAIK, no measurements have been posted. So, if it has been measured, the results were kept proprietary.

I purchased new ones in the last few years to replace my original ones. I was surprised at how hard the rubber was on the new ones were. The date on the bag was less than 2 years old so they weren't old stock. I think they act more as a damper to avoid transferring vibration to the chassis. This is how the motor mounts work. My car felt much smoother after replacing them. When I replaced them I didn't change anything else, same shocks and springs.

How do people generally feel about NVH when moving to setups like what KW or GAZ supply? Not talking about sportiness or body roll or damping, just the effects on NVH from significantly thinner damping materials. Modern components that I've looked at pictures of online look to have significantly thinner isolators, even for double wishbone geometries. Is there something that gets lost on that front, when moving away from the Porsche parts?

Talking with PNW 928 folks, it seems like there would be some advantage to suspension travel if you can free up the total thickness of the shock hat and mounting parts, but maybe that makes things less 928-y.

This is all just a thought experiment, as I consider refreshing my suspension and the costs involved for standard parts.

Cheers