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

[Tbonem3]

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

[Cronenberged]

I believe the B6 is OEM replacement or equivalent

[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.

[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?

[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

[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

[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]

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!

[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!

[Obioban]

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.

[Bry5on]

Okay, I think I'm starting to get it.

So, it's not that the springs act against the jacking force per se; it's that, if the suspension is set up around very stiff springs, the relative contribution of any jacking force will be less? Is that about right?

Still not sure how spring position plays into this. If the function of a spring is to manage the position of the wheel WRT the body or vice-versa, wouldn't all forces on the spring be affected by the motion ratio?

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.

[IamFODI]

Yeah, you’re on the right track, just got the spring force and reaction a little backwards. Simplifying for the concept (the math isn’t exactly this simple). So the springs put out a linear force, let’s say 400lb/in, so if you have 200lb of jacking force, the springs will be pushing with the commensurate 200lb less force (since your car’s weight hasn’t changed and the tires are still supporting the full car’s weight, but some of it is going to jacking). If the springs are pushing with 200lb less force, they’ll be compressed by 1/2” less. This is where the jacking height (due to the force) increases relative to spring rate.

Now let’s say you have an 800lb/in spring and you still have the same 200lb jacking force. You’ll now be decompressing the spring half as much, just 1/4”. Since the force is the same, based on CG, roll center and cornering force, it doesn’t change (much) based on spring rate choice. If your springs are stiffer, the suspension will change height less due to jacking, in a similar way that the body will roll, dive and squat less with a stiffer spring. Hope that makes sense, I’m shooting from the hip a bit here.

Okay, I think I'm starting to get it.

So, it's not that the springs act against the jacking force per se; it's that, if the suspension is set up around very stiff springs, the relative contribution of any jacking force will be less? Is that about right?

Still not sure how spring position plays into this. If the function of a spring is to manage the position of the wheel WRT the body or vice-versa, wouldn't all forces on the spring be affected by the motion ratio?

[Bry5on]

Thanks for this, and for your patience.

Here's where I think I'm confused. The idea that stiff springs mitigate jacking implies that, at some point, the spring force opposes the jacking force, right? But if I put a jack under my subframe and lift, the suspension goes into extension, which is exactly what the springs are trying to do. IOW, the spring force and the jacking force are acting in the same direction, not opposing each other – which would mean adding spring rate would make jacking worse, not better. Does that make sense? What am I missing/misunderstanding?

Yeah, you’re on the right track, just got the spring force and reaction a little backwards. Simplifying for the concept (the math isn’t exactly this simple). So the springs put out a linear force, let’s say 400lb/in, so if you have 200lb of jacking force, the springs will be pushing with the commensurate 200lb less force (since your car’s weight hasn’t changed and the tires are still supporting the full car’s weight, but some of it is going to jacking). If the springs are pushing with 200lb less force, they’ll be compressed by 1/2” less. This is where the jacking height (due to the force) increases relative to spring rate.

Now let’s say you have an 800lb/in spring and you still have the same 200lb jacking force. You’ll now be decompressing the spring half as much, just 1/4”. Since the force is the same, based on CG, roll center and cornering force, it doesn’t change (much) based on spring rate choice. If your springs are stiffer, the suspension will change height less due to jacking, in a similar way that the body will roll, dive and squat less with a stiffer spring. Hope that makes sense, I’m shooting from the hip a bit here.

[IamFODI]

Think of jacking just like placing a jack under your rear subframe and lifting. That’s essentially where the force is acting, pretty near the middle of the car, so the whole suspension is lifted roughly equally Left-Right (why the inside lifts so much).

This will be a little over simplified, but not by too much: Because the lower control arm can only take force along its axis, and the control arm points up and in, when you corner, there’s a cornering force along the ground, and an upward force. Add the size of those two forces to make a triangle where the hypotenuse angle is the lower control arm angle. The inner wheel does a little bit to help reduce jacking, because the inner lower control arm is in tension (opposite direction forces), but it doesn’t generate as much grip, so you end up with a net jacking force.

I’m approximating it for the sake of simplicity and tried to simplify it in my model to only the net contribution by doing the math on the outside tire and knocking the number down to account for the contribution of the inner tire. It’s enough to be close and simplifies the calculations.

Thanks for this, and for your patience.

Here's where I think I'm confused. The idea that stiff springs mitigate jacking implies that, at some point, the spring force opposes the jacking force, right? But if I put a jack under my subframe and lift, the suspension goes into extension, which is exactly what the springs are trying to do. IOW, the spring force and the jacking force are acting in the same direction, not opposing each other – which would mean adding spring rate would make jacking worse, not better. Does that make sense? What am I missing/misunderstanding?

[Bry5on]

Trying to understand this and not finding much success. My amateur brain would have thought springs would exacerbate jacking given that they operate in the same direction.

Does the right-side spring resist jacking on the left side, and vice-versa?

Think of jacking just like placing a jack under your rear subframe and lifting. That’s essentially where the force is acting, pretty near the middle of the car, so the whole suspension is lifted roughly equally Left-Right (why the inside lifts so much).

This will be a little over simplified, but not by too much: Because the lower control arm can only take force along its axis, and the control arm points up and in, when you corner, there’s a cornering force along the ground, and an upward force. Add the size of those two forces to make a triangle where the hypotenuse angle is the lower control arm angle. The inner wheel does a little bit to help reduce jacking, because the inner lower control arm is in tension (opposite direction forces), but it doesn’t generate as much grip, so you end up with a net jacking force.

I’m approximating it for the sake of simplicity and tried to simplify it in my model to only the net contribution by doing the math on the outside tire and knocking the number down to account for the contribution of the inner tire. It’s enough to be close and simplifies the calculations.

edit: the real math uses roll or instant centers and tire contact patch, but thinking about the control arm angle is probably way easier to get the concept right.