Roll center ball joint spacers and how they relate to ride height.

[DukeNeverwinter]

Hey-O.

So months back i purchased Wise fab spindles as part of a reason to convert to 5x114.5 and sti hubs.

The front spindles have a built in 50mm roll center correction. What a roll center spacer does is space out the lower ball joint farther away from the center of the hub bearing. I won't get into geometry stuff...you can search, etc.

Well, the front shocks are mounted to the lower control arm, when you space the ball joint away from the hub the effective ride height of the vehicle drops.

So, options are to lengthen the shocks or make a new lower bracket. I opted for a lower bracket. I had a local fab shop make some spacers for me to play with. One was 5/8in offset, one was 1in offset. I ended up using the 1inch offset one to get back to the 4.25nch original ride height. So, I think, for every 1inch of shock length you get approx 1.5 inchs of ride height, maybe a less, I didn't make a scientific model or measurement. 20240901_183118.jpg20240901_183142.jpg

[driveslikejehu]

I did a reasonably complete layout of the front and rear geometry and that's what I got.
Looking at the lower control arm to shock pickup and center of tire contact, I calculated 1.46 to 1.
Since the shock is angled, the "wheel rate" is ~1.7:1. So 500# measured at contact patch is 850# on the shock/spring.
I'm happy to be corrected, if anybody has better info.

[J R Jones]

DNV, There have been failures of that shock mounting bracket documented on this forum. It appears that the welding failed. Be sure your new brackets are adequate. Since these brackets are taller, there is space for weld inside and outside.
This upright changes the LCA angle and the tie rod location. Since bump steer is an issue in the original FFR configuration, I would access your bump steer with these components.
jim

[DukeNeverwinter]

This post was more to makecpeople awarecof the ride height effect of roll center spacers. But good discussions are welcome.

I'm probably going to have the guys revisit it. And move the shock bolt more inward towards the car and brace the inner walls. I plan to go aluminum sti arms, so they'll be coming off 'soon' anyway.

As is, I had to flip the shocks and run spring on bottom. The current brackets cause the spring to hit the front sway bar if in original orientation.

[J R Jones]

DNV, With the exception or roll center (more on that later) no geometry has changed. The spring rate and wheel rate are the same. The ride height is dropped at the upright and re-raised by mounting the coilover higher on the LCA. It would have been possible to adjust the ride height with the spring pre-load.

The description "50mm roll center correction" is a bit ambiguous. Is that measured at the upright or the actual roll center?
The roll center is a calculated point on the longitudinal centerline of the car based on control arm angles and tire contact patch. If the 50mm is measured where the lower ball joint is located on the upright, the roll center changes at the centerline ~25MM.
With a hand built kit car, driven on the street, I predict that it would be difficult to perceive a roll center change of 25MM.
I did mention bump steer which some 818 builders tolerate or do not find troublesome. I do find it troublesome and I would strive to know how this modification affects the bump steer.
jim

[DukeNeverwinter]

Rollcenter correction. (Copied from the Wisefab webpage)
-50mm drop from the knuckle / upright

If the spring is pushing on the bottom arm. And the spindle has effectively 50mm spacer on the bottom between the balljoint and the spindle, then the ride height changes becasue the center of the hub is moved farther up away from the spring mounting point.. Thst's the only purpose of this post.. I was not going to get into a geo discussion, that has been discussed elsewhere.


When I installed these spindles, with no other changes, the ride height dropped to something like 3inches, maybe even lower. As soon as I installed these new brackets, I got back to 4.25in or so.