Black & Tan 332iT

[heinzboehmer]

Pulling what I wrote in from the sister e46f thread:


So instead of just projecting the upper and lower control arms to a point and assuming that’s roughly where the IC is, I’m projecting those arms to a point, and using that point to define a line that extends outward to define the IC (and in turn, roll center). Very hard to describe in words, I can show you with a model open some time.

Worth noting that jacking is still an issue with the suspension design due to that lower control arm angle, and with the added fore/aft angle I’d been ignoring, jacking looks like it’s even worse. So the butt isn’t lying but it’s not from the roll center itself, just the placement of the members.

Aah I see. Think I got it, but down to peek at your models sometime.

[Bry5on]

Thank you! Instant buy. There's also a guy building a cybertruck vehicle of sorts (channel Project66). He did a phenomenal video talking about building optimal suspension setups, but I feel what's optimal depends on your weight distribution and where your CG is. Another great guy talks about building suspension but leaves the details out - still a great watch (channel XF Motorsports). He's absolutely unhinged and lets his dreams steer his engineering. Very cool stuff

Definitely will be pulling up some of those channels, thanks!

[Bry5on]

What changed in the way you were calculating roll center?

Pulling what I wrote in from the sister e46f thread:

The instant center is basically an imaginary point that defines the roll center when combined with the tire location and track width. It is an important part of handling characteristics. In previous posts I’d estimated the instant center location as the virtual point that the upper and lower control arms make, this was an error as I hadn’t considered the true angle of the upper and lower control arms when looking up/down on the car. As a result, the instant center is significantly farther outboard, which means the roll center is much lower and more reasonably placed than I had originally thought. I’ll have to redo the roll center calculation and update my suspension geometry/roll calculator. I’m disappointed in myself for missing this!

So instead of just projecting the upper and lower control arms to a point and assuming that’s roughly where the IC is, I’m projecting those arms to a point, and using that point to define a line that extends outward to define the IC (and in turn, roll center). Very hard to describe in words, I can show you with a model open some time.

Worth noting that jacking is still an issue with the suspension design due to that lower control arm angle, and with the added fore/aft angle I’d been ignoring, jacking looks like it’s even worse. So the butt isn’t lying but it’s not from the roll center itself, just the placement of the members.

[YoitsTmac]

Race Car Vehicle Dynamics by Milliken and Milliken is my favorite: https://www.amazon.com/Vehicle-Dynam.../dp/1560915269

These days there are also lots of great YouTube videos. More recently, the Suspensions Explained Channel has put out a few great videos.

Ultimately the best way for me to understand is to experiment on my own cars by isolating and changing one variable at a time (you see lots of that in this thread, despite the effort of doing jobs twice or even more times). I had an e31 850i manual that I turned into a corner carving monster that used to clean up e46 M3s at autocross after reading the book above and incrementally experimenting until it was great.

Thank you! Instant buy. There's also a guy building a cybertruck vehicle of sorts (channel Project66). He did a phenomenal video talking about building optimal suspension setups, but I feel what's optimal depends on your weight distribution and where your CG is. Another great guy talks about building suspension but leaves the details out - still a great watch (channel XF Motorsports). He's absolutely unhinged and lets his dreams steer his engineering. Very cool stuff

[heinzboehmer]

What changed in the way you were calculating roll center?

[Bry5on]

Is there a book you'd recommend to those who are not engineers who want to learn more about vehicle dynamics and how all these factors (roll axis, CG, multi link vs trailing, frequency response etc) that affect how a car feels and handles?

Race Car Vehicle Dynamics by Milliken and Milliken is my favorite: https://www.amazon.com/Vehicle-Dynam.../dp/1560915269

These days there are also lots of great YouTube videos. More recently, the Suspensions Explained Channel has put out a few great videos.

Ultimately the best way for me to understand is to experiment on my own cars by isolating and changing one variable at a time (you see lots of that in this thread, despite the effort of doing jobs twice or even more times). I had an e31 850i manual that I turned into a corner carving monster that used to clean up e46 M3s at autocross after reading the book above and incrementally experimenting until it was great.

[YoitsTmac]

Is there a book you'd recommend to those who are not engineers who want to learn more about vehicle dynamics and how all these factors (roll axis, CG, multi link vs trailing, frequency response etc) that affect how a car feels and handles?

[Bry5on]

Well. I'll. Be. Damned.

All this time I've been calculating the roll center height of the rear as something astronomical and offensive at ~12" versus a nicer 4-6". After re-running the analysis, I'm calculating a rear roll center height of about 130mm or just over 5" - just about where it should be.

Turns out I'm the fool and I've got egg on my face for making a bad assumption. The roll center height is *just fine*. The front seems to be about 40mm high, so you might argue that a rear roll center closer to 100mm might be better, but you'd be splitting hairs.

Now, the toe control and ride comfort balance still sucks, and the suspension is still heavy and high inertia so there are still gains to be had from the e39 suspension, but the geometry isn't likely to be one of them...

[Bry5on]

Modeled the e46 for fun, now I can do a real analysis of camber, toe and track changes at some point

[Bry5on]

What would it take to get / make a Mullet tune for those of us w/ CSL airbox + CSL software and different cams (288/280s)? Been doing a LOT of reading about datalogging, tuning, etc.

Mike

Realistically pretty unlikely. Either an engine dyno and a lot of time, or I’d need to daily that setup and iterate for several months. Sorry!

[Bry5on]

If I understand you correctly, you are saying previously you thought the hub pivoted off of the inner control arm joints and the trailing arms just set toe. But in actuality the hub pivots off the front trailing arm location and the control arms just set camber?

Not exactly, but something close to that yes. Previously I assumed that the rear control arms inboard termination point was close to the axis of the hubs. If that were true, you could calculate the instant center and roll center pretty accurately without factoring in the location of the trailing arm bushing.

The e46 hub is always pivoting around an axis that includes the trailing arm bushing on that axis. The second point that defines that axis is a virtual location where the upper and lower control arms meet (I’ll get to this in a second). If you extend that axis in three dimensions, the instant center is the point where that axis crosses an imaginary vertical plane that intersects both rear wheel/hub centers. Then the roll center is the intersection point of the left and right instant centers.

Ok, so that virtual point where the control arms meet (above). It’s always moving with suspension travel, it’s not technically fixed in space like a true semi trailing arm. However! Because the two inner mounting locations are SO close together, that point baaarely moves as we sweep through suspension travel, which effectively approximates a static point, like a semi trailing arm.

So in conclusion, the e46 is not technically the same as a semi-trailing arm rear suspension, but practically speaking it behaves the same with a very small deviation. That amount of small deviation is related to providing the degrees of freedom necessary to easily adjust camber and toe without changing the suspension geometry much at all.

So, for all intents and purposes, a semi-trailing arm suspension that has adjustable camber and toe. Hopefully that was followable.

[nuc1]

What would it take to get / make a Mullet tune for those of us w/ CSL airbox + CSL software and different cams (288/280s)? Been doing a LOT of reading about datalogging, tuning, etc.

Mike

[George Hill]

2) Tonight's realization after modeling the e46 rear suspension: The e46 suspension is just a semi-trailing arm suspension that accommodates camber and toe adjustments. It's really just an evolution of the e30 and prior designs. Suspension behavior is much more in line with a semi trailing arm than a multi-link suspension due to the placement of the members. Can't believe I hadn't realized this until now, duh.

If I understand you correctly, you are saying previously you thought the hub pivoted off of the inner control arm joints and the trailing arms just set toe. But in actuality the hub pivots off the front trailing arm location and the control arms just set camber?

[heinzboehmer]

2) Tonight's realization after modeling the e46 rear suspension: The e46 suspension is just a semi-trailing arm suspension that accommodates camber and toe adjustments. It's really just an evolution of the e30 and prior designs. Suspension behavior is much more in line with a semi trailing arm than a multi-link suspension due to the placement of the members. Can't believe I hadn't realized this until now, duh.

Looking from the bottom of the car, the lower dashed line in this picture depicts the axis of rotation of the 'trailing arm' as the instant center is basically stationary throughout suspension travel:

Woah, mind blown.

[Bry5on]

A few things:
1) Euro Mullet tune v26 seems to be on the money now. I'm happy with cold start operation and a couple days in it still seems to be butter smooth everywhere in the rev band. More seat time to come, but hopefully this is the last rev

2) Tonight's realization after modeling the e46 rear suspension: The e46 suspension is just a semi-trailing arm suspension that accommodates camber and toe adjustments. It's really just an evolution of the e30 and prior designs. Suspension behavior is much more in line with a semi trailing arm than a multi-link suspension due to the placement of the members. Can't believe I hadn't realized this until now, duh.

Looking from the bottom of the car, the lower dashed line in this picture depicts the axis of rotation of the 'trailing arm' as the instant center is basically stationary throughout suspension travel:


3) The hubs are slightly rearward of the diff output flanges, about 20-30mm. This is probably to keep from passing the CV joint through its center axis position if I had to guess.

4) There is quite a lot of anti-squat in the e46, but we already knew this. Wheel hop!