Everything about Fat Cat Motorsport Suspension

[cobra]

cobra, thanks for your interesting post. I always trust direct measurements on the exact platform for maximum accuracy. The M3's front MR appeared very slightly higher than my non-M ( Obioban asked me to verify the front before doing his setup!). If the rear spring MR is closer to 0.63 than 0.7 then the rear spring rate needs to be even higher to achieve Flat Ride. It's quite disjointed to have a very low rear spring MR and a VERY high damper MR. Too easy to screw up the damper and have that behavior override the spring, changing the car's characteristics. I think a lot of M3s (and E46/E36 in general) suffer from poorly-optimized damping. The newer platforms with lower rear damper MRs don't suffer as much.

I also find it useful to physically bounce the car to get a sense of how much friction / stiction is present. To illustrate this, just a couple weeks ago I met up with Andrew ("Farkle") to get started on an FCM Elite build for his M2C track toy. He's currently testing Ohlins DFV that came with pitch and was hating the ride frequencies. Did some calcs (we measured motion ratios on his car together a few years ago) and swapped in softer rear springs. He was expecting the front to go from ~2.3 Hz to about 1.8 Hz front. The rear would stay at around 2.3 Hz. I suggested we do a physical bounce test in his garage and at least with the displacement we could induce into the suspension, we obtained about 2.1 Hz front and 2.2 Hz rear.

Before the test I bet him $100 that the front would be higher than he had calc - turns out I was right! The front was nearly 20% higher than calc'd and the rear was about 5% lower. He still had Flat Ride though at a much lower ratio than he was expecting. Also, IIRC he has monoballs on most of his suspension arms and his sway bars are articulating smoothly per his comments. But, there's still some unexpected friction in the system increasing the effective front ride frequency. I think it was eye-opening to him. I was reminded how important friction is when setting up a VW Touareg a couple years ago. The calculated OE frequencies SHOULD have given a marsh mellow ride (~1.0 - 1.1 Hz!) but the real-world results were in the 1.5 - 1.7 Hz range. That car on OE bushings had a TON of stiction! The calculated MRs were really just a rough starting point in that extreme case.

Did you do physical bounce tests as well to verify what your calculations suggested? Can you use your data acq setup on various road surfaces to see how the damped ride frequencies compared to ideal / calculated over small / medium / larger bumps? Your raw data would be most interesting to see if you're willing to share it.

Bry5on cool note on the rear IC and jacking. This may be why using Flat Ride is particularly helpful to keep the jacking under control, provided the dampers / bump stops aren't inducing more vertical motion than needed via the rear suspension bottoming out then popping up. Do you know if other earlier platforms that rear axle behavior? The E36 I imagine would be very similar.

Exactly what I noticed - I found the rear end jacking down and was confused why the front end felt fine, but rear did not. After adjusting the MR's in the calculator and substantially reducing the rear rebound damping, it finally felt normal in terms of grip and comfort. I did not do a bounce test, but that's a great way to measure it directly and also extracting the non-shock damping/friction.

Bry5on, I will take a look at the MR curves, and want to do more data in the future that I will happily share!

[Bry5on]

Bry5on cool note on the rear IC and jacking. This may be why using Flat Ride is particularly helpful to keep the jacking under control, provided the dampers / bump stops aren't inducing more vertical motion than needed via the rear suspension bottoming out then popping up. Do you know if other earlier platforms that rear axle behavior? The E36 I imagine would be very similar.

I know the e36 is substantially similar but I’ve not measured one myself, so I can’t say for certain. The dead giveaway is how close the inner bushings are between the upper and lower control arms.

Effectively this means you have a very stable IC and RC which actually serves to hurt more than it helps because there is a substantial jacking force pushing the rear of the car up. A properly setup e46 front suspension should have very nearly zero jacking force, so this gives the feeling of the rear end ‘lifting’ on steering inputs, as well as providing some inherent front end grip or rear end looseness on turn in or suspension load-up before achieving steady state. It’s even worse if you’re lowered more than an inch or so as your front will actually jack DOWN and pull you into the bump stops/require more sway bar.

Personally I think this is the biggest downside of the e46 suspension and is the primary reason that folks can find grip with stiffer than otherwise optimal ride frequencies, as stiff springs will serve to mitigate the displacement of the chassis upward from the jacking force. A lower rear roll center would get you much closer to the static suspension behavior during dynamic events. Effectively your front/rear roll stiffness is always changing rather largely, and not due to your springs or dampers.

Would you guys comment on the impact of that rear roll center and motion ratio for divorced spring vs rear coilover setups?

Does applying the motion ratio to your spring calculation really account for all of the differences in suspension performance between these two rear setups?

I believe that the e46 rear spring motion ratio is designed purely to mitigate jacking of the rear suspension. With a stiffer inboard vs softer coilover spring, the car will actually lift/jack less and keep more stable rear suspension geometry. I would never recommend anyone to run a true rear coil over for this reason, your car will literally lift up more in the rear and cause additional unwanted geometry changes.

I honestly don’t know why they chose such a high rear roll center and such an inboard instant center. The only two things I can think of are 1) that it makes the car have a tendency to break rear traction on quick steering/throttle inputs (think Scandinavian flick), 2) that it will make the front end really bite hard when you wrench the wheel (versus say a 911 which is setup opposite), or 3) that the car is designed to be really weighted down in the rear with passengers and gear and they did this to mitigate smashing the bump stops and upsetting the car. Some day I’d like to make a rear subframe that corrects this geometric ‘flaw’ … okay off the soap box.

edit: here’s a photo of my model

[cobra]

I did each one multiple times to get an average. The front is easy, but the rear can be tricky to measure spring displacement just due to the tight packaging of it.

[Suspension Decoder]

I did not do a bounce test, but that's a great way to measure it directly and also extracting the non-shock damping/friction.

Keep in mind that the rod force / damper gas force (especially on a higher-pressurized monotube) WILL contribute to the suspension's friction as it behaves like a capacitance / damper 'preload' that must be overcome before the suspension moves into compression. The rod force + very low speed compression can combine to create some nasty jerk... This rod force CANNOT be ignored except at the cost of grip and ride quality.

What setup are you using, btw? damper rod diameters front and rear? Do you have know the rod force on your dampers, and seal drag? I just started measuring the latter a few years ago - very revealing. And have you done any frequency-based damper force analysis?

[Suspension Decoder]

cobra Some good science right there! Thanks for sharing your data. I'll update the E46 M3 spreadsheet rear damper and spring MR with the values you obtained.

EDIT2: I tried to do a quick and dirty HTML edit on the MRs but looks like I need to regenerate the JS function calls via Spreadsheet Converter, which I don't have on this new computer. I'll get that taken care of soon as possible. The original version is what's in place and I'll advise when v2.0 is available.

[Bry5on]

cobra do you have solid subframe bushings, stock, other? There’s some vertical displacement in the bushings that will technically effect MR too. Not much but good to have in the thread record. Thanks for the data! Subframe and other bushings will also change your ride frequency as they are springs too of course

Everything is a spring if you push hard enough!

edit: it just occurred to me that you probably measured without the spring in place, in which case the bushing question is irrelevant.

[cobra]

cobra do you have solid subframe bushings, stock, other? There’s some vertical displacement in the bushings that will technically effect MR too. Not much but good to have in the thread record. Thanks for the data! Subframe and other bushings will also change your ride frequency as they are springs too of course

Everything is a spring if you push hard enough!

edit: it just occurred to me that you probably measured without the spring in place, in which case the bushing question is irrelevant.

My car is stock other than the shocks/springs. I removed the spring and disconnected swaybar for these measurements.
Towards full droop, the lower control arm touches the spring at quite an angle. I was measuring at the center of the spring mount

[Suspension Decoder]

My car is stock other than the shocks/springs. I removed the spring and disconnected swaybar for these measurements.
Towards full droop, the lower control arm touches the spring at quite an angle. I was measuring at the center of the spring mount

Over what range did you measure the motion ratios? I generally exclude wheel vs. damper travel data below static height and focus on static height to full bump. Any non-linearity going into droop would skew the calculated MR.

[cobra]

Over what range did you measure the motion ratios? I generally exclude wheel vs. damper travel data below static height and focus on static height to full bump. Any non-linearity going into droop would skew the calculated MR.

This was pretty much the full range of shock travel. My shocks are a little shorter than stock hence only 120mm and not 140-150. Either way, it looks pretty linear so I didn't bother clipping the ends.
You asked earlier what shocks - I am running a custom monotube setup with piggyback reservoirs and base valves, 22mm front and 16mm rear shaft diameters, at about 75psi.

[bigjae46]

I called Suspension Decoder a couple of days ago. The one thing that you'll immediately notice is that he loves talking suspensions! Really had a great back and forth and answered all of my questions. Even when I asked about some terms like jacking down he explained them in a manner I could understand. I liked how he brought up relatable real-world examples which allowed me to connect them to my own experiences. Now this thread makes a lot more sense to me. I think the consult fee is absolutely worth it just to talk to Shaikh. I walked away with a clear picture of how I can improve the car, the why behind those steps, and an expectation.

I liked how he brought up his experience from making really stiff setups then moving to the rally tuning. Then I thought about how every student I instruct in a modern M car, I always have the conversation of driving in M mode vs comfort. Every time, the driver is far better in comfort. I observed one beat his previous PR by 3 seconds. I knew about flat ride which is mostly springs but Shaikh helped make all of the other stuff fall into place like gas force, jacking down and up, etc.

So the plan is a Stage 2, GC front bar and then test it. The only unknown is the rear bar. I really liked the difference it made. If I'm still getting a lot of understeer, I will need to replace the rear subframe to install the rear bar. I cut the brackets off to save a 1/2 lb lol

[digger]

Do all the H&R coil over kits for BMW models use a Bilstein mono shock branded H&R?

[maupineda]

Not sure if all. But z4m kit is.

[Cronenberged]

Do all the H&R coil over kits for BMW models use a Bilstein mono shock branded H&R?

I believe the street performance is, that's the one FCM modifies because the internals are similar to Bilsteins. Suspension Decoder correct me if I'm wrong.

[Suspension Decoder]

Do all the H&R coil over kits for BMW models use a Bilstein mono shock branded H&R?

From what I've seen, any H&R Street Performance coil-over (or RSS) will be a Bilstein-based monotube that is rebuildable. The H&R strut tube (with the 40mm polished chrome surface) has 'Bilstein' clearly stamped near the top of the tube, just below the chamfer where the threaded shaft begins.

[Suspension Decoder]

I believe the street performance is, that's the one FCM modifies because the internals are similar to Bilsteins. Suspension Decoder correct me if I'm wrong.

Yes, H&R is essentially white-labeling the Bilstein. The internals are exactly Bilstein. H&R likely makes their own strut housing or has Bilstein make one to their specs (not sure which one). H&R seems to specialize in making custom springs. The ones that come with their coil-over kits I only sometimes find useful / usable for my customers. The front is typically too short in length and too stiff in rate, having you live in the front bump stops (which are usually 200 lb/in and fairly long). Sometimes the rear spring is reusable, depending on your application, but typically you'll want to change both. And, if your corner weights are different enough left vs. right, you might want to use different rates as well!

As a side note, I'm the process of updating the spreadsheet as I do another iteration to Christina's suspension (mostly focusing on the front end). I may soften springs all around or just up front to increase the Flat Ride relationship (there's very little right now). Based on real-world bounce results, I think using a rear spring MR of 0.63 (as came from the direct measurements by cobra) would under-estimate the bounce frequency. I do believe the rear damper motion ratio need to be increased for better accuracy. I find (and OEMs have known this for a while) that while geometric measurements get you in the ballpark, especially when it comes to system friction, the real-world constants often differ from theory. What the tires actually experience is what matters most, in my mind and experience.

EDIT: I would be very interested in some feedback from drivers who use the spreadsheet, know corner weights, and can perform bounce frequency tests with dampers at soft (if adjusters are available).