e46 M3 suspension setup, or how to not downgrade your car with suspension mods

[IamFODI]

A little hard to explain briefly (I maybe over simplified this too much originally, in retrospect) but the motion ratio is the square of the upper control arm spring lever ratio. The wheel acts at the motion ratio because the lever arm reacts in a squared way (derivation here: https://enderw88.wordpress.com/autom...g-rate-theory/ ). When you apply a force to the frame of the car, it only reacts with the lever arm (d1/d2) and not the square of the lever arm as there is no additional ratio of displacement to multiply it through a second time.

This would be much easier to explain with a whiteboard and some free body diagrams (and more time), but hopefully the link plus description above helps. I’ll bet there are some good YouTube videos that show the derivation of wheel spring rate with motion ratio out there.

Thanks again for your efforts.

If you ever decide to make a whiteboard video for YouTube, I'd watch it, for what that's worth!

[Bry5on]

I increasingly feel like the more I know about suspension, the less I know about suspension.

... and that deviating from stock as little as possible (but as much as necessary) is the best strategy-- especially in ride height and geometry.

I feel this way about so many things! I wish it were a bit easier to experiment with roll center heights, I’d really like to get a feel for what big changes there do. Porsche has a very different strategy for roll center heights on the 911 and cayman and those cars (cayman in particular) feel really great. We also haven’t even started down the road of the effects of roll, pitch and rotational inertia!

[Obioban]

I feel this way about so many things! I wish it were a bit easier to experiment with roll center heights, I’d really like to get a feel for what big changes there do. Porsche has a very different strategy for roll center heights on the 911 and cayman and those cars (cayman in particular) feel really great. We also haven’t even started down the road of the effects of roll, pitch and rotational inertia!

Just because I happened to be watching this while reading your post and he amusingly encapsulates the situation:

https://www.youtube.com/watch?v=sCRiUbucTts&t=400s

[Bry5on]

Just because I happened to be watching this while reading your post and he amusingly encapsulates the situation:
https://www.youtube.com/watch?v=sCRiUbucTts&t=400s

His videos are great. I almost bidded on his old house, recognized it right away from his earlier YouTube videos! It’s a small world

[D-O]

I increasingly feel like the more I know about suspension, the less I know about suspension.

... and that deviating from stock as little as possible (but as much as necessary) is the best strategy-- especially in ride height and geometry.

As Bry5on pointed out, this applies to a bunch of things. It can be a hard lesson to learn. As the most talented mechanic I know often says, "I am not smarter than the engineer who designed this car."

D-O

[Drewrivera]

So B4 with Eibach springs ideal for Street and comfortable ride with perhaps some strut and sway bars to complete the oem+ feel? I assume there's no need to do the two different set ups from rear to front if so as well?

[tlow98]

So B4 with Eibach springs ideal for Street and comfortable ride with perhaps some strut and sway bars to complete the oem+ feel? I assume there's no need to do the two different set ups from rear to front if so as well?

Unless you’re on an extreme budget, Bilstein B4s are kinda meh. I’d recommend either stock Sachs or Koni yellows. The B4s are an odd middle ground I’ve found and in every case I’ve used them (twice now) I’ve regretted it. They’re a bit harsher than OE, but also kind floaty at high speed.

if you’re going through all the effort to swap them Yellows are better in every way; comfort & performance.

[Cronenberged]

I believe the B6 is OEM replacement or equivalent

[Tbonem3]

B6 (monotube) is sport. B4 is OE replacement (twin tube).

[JokerElite]

+1 on B6 being a harsh. I was running the B12 pro-kit for years which is B6 + Eibach lowering springs and it would crash on the slightest bumps. I changed to new Sachs with OE springs and I love it. Imo, the look of the slight drop wasn't worth the harshness as I do not track the car.

[maupineda]

I have H&R coils on my Z4M, and though a different chassis, i think the consensus is clear. Bilsteins are harsh. The H&R coils use Bilstein shocks, and to make them livable i had to Swiss cheese the setup

- linear front springs - Swift 225#
- linear rear springs - TCK 600#
- rear B8 from non M Z4, supposedly less aggressive and shorter (less droop)
- shortened bump stops - cut to give 1.5in of free travel before stop compression.

what I found, though not backed up with solid data, is that between H&R version vs Bilstein branded shocks, is the latter set have digressive valving. I did feel the back of the car a bit more compliant after the B8 install v H&R variant.

I kept OE height to avoid geometry changes, the springs are about 10-15% stiffer than stock, and I added a CSL front sway. Did I gain anything over stock? More likely “nada” other that learning 😂.

i am a firm believer that the high pressure gas is what makes Bilsteins jarring.

when roads are smooth and not sharp bumps the ride and body control is beautiful. The key to improve ride was to ensure there was free suspension travel before engaging the stops just as the OE setup. Lowering these cars screw up the ride if not done properly.

[Drewrivera]

Koni yellow and H and R Springs was my set up before and before that was Status Gruppe coils and i have to say, Status Gruppe was smoother to me. I had harsh road feels with the Koni, but I can't exactly point out what went wrong because i installed the set up, didn't refresh the entire set of rubber bushings or pads for the suspension and i bought the dampers from a website that had them for cheaper. And to date , without realizing that Dinan and H and R are the same spring, i ended up putting on b4 with Dinan, which i have absolutely no travel at this point, 😜😜, so i now have to get back to restoring this suspension for better feel. I still have the dampers and springs from previous set ups but not sure what to do with them. I thought i could rebuild but that may be the same amount as new purchasing of dampers. What did the alpina have in there set up by the way, was it just springs with Sachs?

[Maxima SE]

I have H&R coils on my Z4M, and though a different chassis, i think the consensus is clear. Bilsteins are harsh. The H&R coils use Bilstein shocks, and to make them livable i had to Swiss cheese the setup

- linear front springs - Swift 225#
- linear rear springs - TCK 600#
- rear B8 from non M Z4, supposedly less aggressive and shorter (less droop)
- shortened bump stops - cut to give 1.5in of free travel before stop compression.

what I found, though not backed up with solid data, is that between H&R version vs Bilstein branded shocks, is the latter set have digressive valving. I did feel the back of the car a bit more compliant after the B8 install v H&R variant.

I kept OE height to avoid geometry changes, the springs are about 10-15% stiffer than stock, and I added a CSL front sway. Did I gain anything over stock? More likely “nada” other that learning 😂.

i am a firm believer that the high pressure gas is what makes Bilsteins jarring.

when roads are smooth and not sharp bumps the ride and body control is beautiful. The key to improve ride was to ensure there was free suspension travel before engaging the stops just as the OE setup. Lowering these cars screw up the ride if not done properly.

I have the B12 Pro kit on my Z4M with 19" tires and the drop is lovely but hitting the smallest pothole makes it sound like you slammed into a brick wall. Even the oem suspension with 18" tires sounded like that. Just a very stiff chassis. Everytime I hit a pothole I feel like I bent a wheel. On smooth roads its a comfortable ride otherwise and the thing handles amazin !

[maupineda]

I have the B12 Pro kit on my Z4M with 19" tires and the drop is lovely but hitting the smallest pothole makes it sound like you slammed into a brick wall. Even the oem suspension with 18" tires sounded like that. Just a very stiff chassis. Everytime I hit a pothole I feel like I bent a wheel. On smooth roads its a comfortable ride otherwise and the thing handles amazin !

I feel you! With the changes I made I was able to “round off “ the edges lets say. But before it was actually pretty bad.

[bigjae46]

Maybe I missed it but how low can you go in the rear before you start compromising suspension geometry?

I’m 13.5 left and 13.75 right fender to center of wheel in the front but 12.75 left and 13 right in the rear measured with no driver. I’m right at 50/50 crossweights with the driver. My seating position is far enough back so my fat arse distributes more weight to the rear. So I’m sure the rear heights with me in it are lower.

Im really happy with the car’s performance as is. I’ve lucked out and been able to balance the car out with aero. I’m wondering if I can get a little more mechanical grip in the rear so I can take off the gurney flap which adds a ton of drag with my Amazon.com wing that I bought with Hilton points 🤣