MarinS4

The RS uprights have a profound difference on front end grip. Look at lower control arm position and you’ll have your answer. When the LCA is setup properly you have camber gain through the suspension arc. When wrong (too low for OEM parts) you experience camber losses through the suspension arc. Everyone talks about bumpsteer for these yet the greatest impact comes from correcting LCA position on a lowered car. When the tie rod/LCA are in the sweet spot of the suspension arc it REALLY helps with front end grip!

My car is not very low. Well above RS spec. I was shocked at the improvements.!

Pull the trigger and have no regrets.

nothingbutgt3

I recently installed the RS uprights, together with RS monoball top mounts and new A-arm harder rubber inserts, I am waiting for the FVD lowering kit to fix the height issue (because I am using Bilstein PSS10 + the collar spacer OEM RS), but the mechanic has already told me that it is "lovely" the way the geometry looks like just watching the car from underneath, because even in static position he sees the arms working with the right alignment.

I am considering to go front approximately around 110/115mm, which is 10 mms less than the medium value of the RS specs.

In few days I will be able to say something more, after having driven the car a little bit.

Mark in Baltimore

Another factor to keep in mind is that if you're running Motons, MCS, JRZ, et al., the design of those types allows less suspension changes over bumps versus, say, PSS10s. I had to ditch my RS carriers when I was racing to stay legal in my class, and I had zero bump steer. With the PSS10s, I do have bump steer. Have a set of RS carriers in the box and just need to put them on.

TheOtherEric

You didn't mean to say that, because we all know that bump steer gets really bad down at RS height! Perhaps I'm reading it wrong, or it didn't come out right.

+1. With stock carriers, the bump steer at/below RS is unbearable, but at RS +10 (i.e. ROW standard -20; ROW Sport -10), there's little if any bump steer. That's 134mm front ride height, and I run 275F/500R springs with JRZ custom-valved shocks, so those aren't even stiff.

Bill Verburg

yes I did mean that. What you "know" isn't borne out by the facts

The 2 things to look at are the
toe curve; best case is a vertical line w/ no toe changes from wheel travel, worst case is a horizontal line
camber curve; best case and worst case are debatable and depend on multiple other factors but you do want some increase in negative camber w/ wheel compression, how much you want varies

The toe curve is the most concerning as far as bump steer is concerned, it has the largest slope ~ US ride height then flattens as the car is lowered but the slope is fairly constant meaning that after the initial lowering where most of the additional bump steer comes in, additional lowering doesn't add a lot of additional damage

the camber curve on the other hand gets steeper causing larger neg camber gains as the car is lowered or the wheel compressed, again good or bad depends on a lot of other things

It's no accident that the curves are closest together in the ride height region from RS to US

again what the RS wheel carriers do is lower the axis to put the suspension geometry back into the sweet spot

TheOtherEric

With all due respect Bill, what you're saying defies logic and common knowledge. We all know bump steer gets very bad at/below RS ride height (BTDT!), and if the curves show otherwise then either they're wrong or you're interpreting them wrong. Why else are people installing RS carriers? Why else do race cars with MacPherson struts so commonly use bump-steer kits? Trying to be polite here, but I don't want people making the costly mistake of going to RS and lower with stock carriers and expecting it to be driveable. It won't be.

Bill Verburg

The curves are from the factory, you are the one that needs to rethink what you know

Every wheel height has a correlated point on the curves. The static ride height is indicated by the 3 axis lines but bump steer is dynamic so it's best to think of wheel position ,US static is RoW 20mm droop, RoW static is RS 20mm droop. RoW static is US 20mmm compression, RS static is RoW 20mm droop etc..The slope of the curve at given point indicates the rate at which toe is changing, it is an inverse relation meaning the greater the slope the less toe change per mm of wheel movement and the smaller the slope the more toe change at that point for each additional increment of wheel movement, when the slope is constant it means that the toe Δ increment has not changed.

The most damaging part of the curve where there are large changes in toe for a small change in wheel height is where the curve is changing from steep to flat, this occurs on a 993 w/ stock wheel carriers when the wheel goes from -20mm droop to 0mm droop, beyond that out to 80mmm or more of compression the toe Δ is fairly constant(there is still bump, steer but it is not getting worse) once the slope becomes close to constant(close to a straight line) yes there is toe change w/ additional compression(bump steer) but it isn't a lot worse. BTW normal for a toe change curve is a slope of -1

ToSi

@ TheOtherEric - Bill gets grumpy when you challenge what he thinks he knows. The slope of the toe curve is ~1.5 to 2x as steep at RS vs. ROW height but 'steer' is more than just toe. The loss of camber gain & decreasing trail in bump exaggerate the feeling of 'bump steer' at low ride heights that you & anyone else who's lowered one of these cars a bunch has experienced..

Ivan J

Just an observation.... If we consider the suspension moving in both directions about the static ride height (compression and unloading of the suspension), there is definitely more toe change at RS height compared to ROW, because the slope gets much steeper when the suspension is unloaded at ROW height.

This may or may not be relevant...

When the wheel is unloaded, it isn't "connected" to the road very well, so it seems reasonable to assume that the bump steer effect may not contribute much to what the driver feels.

Perhaps it is that transition from being unloaded to loaded, when good traction is restored, that contributes to what we feel as more bump steer at lower ride heights. If that is the case, it would explain why there is more perceptible bump steer at RS height.