Everything about Fat Cat Motorsport Suspension

[E46m3zcp]

EDIT: I am technically inclined and an engineer in the field of automotive, but I am not a suspension expert. What I am commenting on here is what I learned from a research driven by my curiosity and if anyone has additional information I will be happy to learn more.

It's not really clear what FCM means by front/rear bounce rate. From what I know (reference below), the bounce rate is related to how we derive the bounce and pitch frequencies and centers for the coupled front-rear vertical motion of a car. There is not a front and a rear bounce rate. There is just a bounce rate.

The vehicle bounces (up and down) and oscillates (rocks back and forth) about a bounce center and a pitch center, respectively, at a bounce frequency and a pitch frequency. These two modes are superposed to give the final motion. Given the specifications of the vehicle, one can calculate the centers and frequencies and these properties have an impact on comfort.

Cars have higher spring rates in the rear than in the front because it minimizes the pitch motion which is the most annoying mode. According to design principles, not only should the rear rate be higher, but it has to be 30% higher - or the spring center should be at least 6.5% of the wheelbase behind the CG. There are also other guidelines as quoted below. So the "Flat Ride" concept might be vaguely related to these guidelines but it isn't 100% defined by them. "Flat Ride" isn't something that's defined in any vehicle dynamics textbooks, as far as I know.

These principles affect only the ride quality and are unrelated to the cornering ability. I think it is safe to say that cornering ability usually comes at the cost of ride comfort.

Here are the derived design principles from the source mentioned below:



Source: Gillespie, Thomas D., fundamentals of vehicle dynamics (relevant chapter available here: https://edisciplinas.usp.br/mod/reso...php?id=2960518
The relevant portion starts on page 175).

And since I said this was not related to cornering ability, here is a quote from a famous book (Tune to Win, Caroll Smith) about what an experienced racing engineer had to say about spring frequencies.

[Cronenberged]

After 15 years of various adjustable shocks (up to quad adjustable remote reservoir JRZs)... it's honestly been pretty great to not be thinking about adjusting the shocks. The shocks are valved for my spring rates and corner weights, so likely I'd just make it worse anyway.

I've also idiot proofed my tire pressures, with the Michelin track connect setup... so all I really have to think about is camber (which I set via pyrometer).

It's relaxing

What height do you have your car set to?

[K-Dawg]

Makes sense. The Z4 shock body doesn't have to fit down in the kingpin, which is why monotube's on the E46 M are travel limited.

[Obioban]

IIRC the Z4M can have a bit more travel.

[maupineda]

Some data to entertain

Regular E46 M3


CSL M3



Rears are

Regular
587mm - 18" rim
600mm - 19" rim

CSL
589mm - 18" rim
602mm - 19" rim

Yes, surprisingly the CSL is 2mm higher at the rear V Regular M3

Based on the above, I assume the shock travel for the E46 is the same, in fact, it seems that the Z4M is taller (2mm at 607 at the front and 7mm at the rear at 594)

I hope this is useful, someone with actual E46 M3 Billies should confirm in the name of data.

[maupineda]

maupineda are you using E46 M3 or Z4 H&R coilover shocks, or are they the same?

the strut housing is different, and I don't know if the overall length and travel are the same but I would be surprised if they are not very close. the suspension geometry is very alike except for more caster on the E46 M3.

[K-Dawg]

maupineda are you using E46 M3 or Z4 H&R coilover shocks, or are they the same?

[maupineda]

I have those rates, and my car was set up at OE ride height (607mm front / 594mm rear), my dampers are a combination of H&R and Bilsteins shocks (H&R Coilover front strut, and B8 HD shock from the Non-M Z4 at the rear).

At those rates, the car is still very firmly supported (I would not call them soft at all), it really takes some aggressive dips to make the car even try to bottom out (I have not managed to do that yet - but I am not trying as I don't abuse the car), also keep in mind the Z4MC has the 27mm front sway bar, and I am installing the CSL item to reduce roll up front and balance the FRC.

A few reference notes (note this is for the Z4M)...

• The H&R coils have 130mm of total travel until the inverted strut insert bottoms out inside the strut housing
• The internal bump stop is 70mm long
• I would assume a solid block height of ~30mm when the stop is fully compressed

Based on the above, and considering I trimmed 25mm off the stop, my car is set so that when supported the damper travels ~55mm, with a remaining ~60mm of bump travel leaving another 15mm in there for a fully compressed bump stop. With this, the car still has 30mm of bump travel before engaging the BS, and another 30mm of the BS acting as a secondary spring.

I trimmed the stiffer section.

Some images as that are what is always lacking, isn't it?

Full travel without the BS, the insert would bottom out inside the strut housing


OE H&R Bumpstop



Trimmed BS



So, I think I could still lower the car by 10mm and still have a good setup, albeit I would give out some travel, or I can use aftermarket plates to not affect travel at all, but since my last interest is in lowering the car, I just don't care. I set her up with functionality and fast road performance in mind.

But there you go, if you take some time and make the math you can set up the car in a way that will serve the purpose you have in mind better than just slapping new parts on.

P.S. Bilstein HD also use 70mm BS, so they can be trimmed to avoid riding on them all the time (even at stock ride height)

[K-Dawg]

Yeah, metal to metal. The bump stop isn't solid, just trimmed-- and actually just the firm part trimmed.

Right. I meant that at some point the stop is solid and is no longer a spring, and takes up some of the available bump travel.

Other than jumping the car/similar, the way the car is set up it shouldn't engage the bump stop-- it's just there as a safety, not a spring element.

At 250/600/stock weight, you're going to want to be near stock ride height and I'd suspect would want to use a Koni, for the longer travel.

Yeah, I'm going to have to sharpen my pencil a little bit. But it seems like it would be fine if I can get the appropriate length spring. Already have TCK DA, want to try something from Bilstein/FCM on this car. Near stock height is fine with me.

[Obioban]

Thanks.

Your 60/40 split is based on metal to metal travel? So some of your bump travel would be consumed by the bump stop going solid?

My car is full weight, and I'd like to run softer springs (250/600 perhaps), and preferably, OEM strut mounts. Trying to figure out if that's going to be feasible.

Yeah, metal to metal. The bump stop isn't solid, just trimmed-- and actually just the firm part trimmed.

Other than jumping the car/similar, the way the car is set up it shouldn't engage the bump stop-- it's just there as a safety, not a spring element.

At 250/600/stock weight, you're going to want to be near stock ride height and I'd suspect would want to use a Koni, for the longer travel.

[K-Dawg]

IIRC the H&R front damper is either 102mm or 112mm of travel.

Fatcats will do an adjustable shock if you want it-- they just prefer the non adjustable dampers because they're inherently more reliable (simpler).

I have had no bottoming out issues, but obviously some of that will also depend on spring rates, car weight, etc.

FCM does modify the internal bump stops.

Thanks.

Your 60/40 split is based on metal to metal travel? So some of your bump travel would be consumed by the bump stop going solid?

My car is full weight, and I'd like to run softer springs (250/600 perhaps), and preferably, OEM strut mounts. Trying to figure out if that's going to be feasible.

[cobra]

After 15 years of various adjustable shocks (up to quad adjustable remote reservoir JRZs)... it's honestly been pretty great to not be thinking about adjusting the shocks. The shocks are valved for my spring rates and corner weights, so likely I'd just make it worse anyway.

I've also idiot proofed my tire pressures, with the Michelin track connect setup... so all I really have to think about is camber (which I set via pyrometer).

It's relaxing

You know what, now that you mention it I can definitely understand that. I own a BMW S1000XR with more-or-less non adjustable suspension (electronic with 2 modes, and adjustable rear preload only), and it really does let your mind focus on other things since adjusting the suspension isn't even an option. On past motorcycles I was constantly thinking about how it needed a click here and there. And now I just learned to ride it as-is and it's fine for most things.

At the same time, I REALLY wish I could increase the rebound damping in the front. Just a few % stiffer, but since it's a sealed cartridge unit it's not possible.

[Obioban]

I would want adjustability on any aftermarket shock I installed on my car. Even with a really consistent builder and tuner you will want to be able to tweak it slightly once installed based on driver preferences or driving conditions. Seems like doing the non-adjustable route just locks you into repeated services and hassle if you aren't totally happy?

That said the B6 looks like a nice shock. Totally non adjustable so your bump/droop is what it is.

After 15 years of various adjustable shocks (up to quad adjustable remote reservoir JRZs)... it's honestly been pretty great to not be thinking about adjusting the shocks. The shocks are valved for my spring rates and corner weights, so likely I'd just make it worse anyway.

I've also idiot proofed my tire pressures, with the Michelin track connect setup... so all I really have to think about is camber (which I set via pyrometer).

It's relaxing

[Obioban]

FCM only services Bilstein-based nonadjustable dampers. Flat ride theory works with any dampers.

Any more feedback on FCM's services?

I saw on Obioban 's journal that a 60/40 bump/droop ratio puts his car at 14" front ride height. Any bottoming-out at this height?

What is the stroke of the H&R front damper?

Does FCM typically modify the internal bump stop?

While I'm asking questions, what is the stroke of the Bilstein B6 front strut?

IIRC the H&R front damper is either 102mm or 112mm of travel.

Fatcats will do an adjustable shock if you want it-- they just prefer the non adjustable dampers because they're inherently more reliable (simpler).

I have had no bottoming out issues, but obviously some of that will also depend on spring rates, car weight, etc.

FCM does modify the internal bump stops.

[cobra]

I would want adjustability on any aftermarket shock I installed on my car. Even with a really consistent builder and tuner you will want to be able to tweak it slightly once installed based on driver preferences or driving conditions. Seems like doing the non-adjustable route just locks you into repeated services and hassle if you aren't totally happy?

That said the B6 looks like a nice shock. Totally non adjustable so your bump/droop is what it is.