Wheel rates with coilover kits = excess pitch motion over bumps?

[//steve\\]

^
I do think people can often go overboard on spring rate. On some level though a "stiffer" ride gives people a better feeling and sometimes the increase in driver confidence does yield a faster car. In the long run maybe not as beneficial but there is something to be said for the car feeling faster whether it actually is or not.

Also, there is a terrible notion that just because the rates are high means the car will ride poorly. Very good quality dampers can make a world of difference. High rates will always ride more firm but I don't think that should be confused with uncomfortable or poor ride quality.

[bbnks2]

Originally Posted by Obioban

All of the below is about non aero cars-- and by aero I mean so much that they couldn't be street driven.

In regard to aero, I have definitely seen people fall back on aero as a possible "explanation" for the spring rates chosen on an E8x/E9x. But I don't buy that either. Aero, especially at the amateur level, is almost always biased heavily rearward which would necessitate more rear spring relative to the front and not less. Just my dissenting opinion. Not necessarily a fact in every case just speaking in generalities.

[bbnks2]

Originally Posted by IamFODI

If flat ride explains the factory setups having higher freqs in the rear, why don't any aftermarket kits that go that way?

If turn-in, transient response, and rear grip explain the aftermarket kits having higher freqs in front, why do basically no factory setups go that way?

I write these walls of text in the hopes of getting the attention of anyone that cares to explain this to me lol. I'd like to learn more about the thought process. Why does every car manufacturer in existence seem to ship cars (even sport cars) with "flat ride" spring rates yet a majority of aftermarket solutions always tend more toward under-steer bias? Is this just a skewed perception because of the BMW specific kits we deal with?! I did just provide an example of how KW ships relatively neutral spring rates for an S2000. Ohlin's s2000 kit provides for a bit more under-steer bias but no where near to the degree that they do for a BMW. So again, is it just our perception of the overall suspension market? Or, is there real science to support a stiffer front end?

This is where autocrossers love to start talking about transient response. Ok I agree to some extent! stiffer front end = quicker response to driver inputs (turn-in feel is improved). That's proven fact. There are formulas to show it. Does that make the car faster through the corner though? Or, is this just a "handling" perception and a subjective feedback from the driver?

The part where my objection comes in is that: why not also increase the rear spring rate?! You make the front end stiffer, the car turns-in with more agility (faster yaw rate and less input lag), but, you're still dragging around the slow to respond rear-end... Are you actually getting around the cones any faster? This is where people get angry and start rattling off their accomplishments and how great of a driver they are so they know what they are doing. OK, but, does that prove your suspension setup is "perfect?" Or, are you just a good driver? I have no issues with acknowledging youre a fast driver guy! No one ever said you were slow!

Anyone have any input on the above? I've read SAE side-step testing and other whitepapers pertaining specifically to transient response so I have done a fair bit of technical research. I can't find anything that discusses this.

[bbnks2]

Found this posted in another thread... watch the whole thing but @55:00 or so he starts mentioning some very relevant concepts:

Question remains I guess is that... is the paper/static math accurate? Seems to be so given the amount of threads asking about handling, under-steer, and pitch.

@1:06:00 he says his first hand experience has been that anticipated behavior of improving front end grip by stiffening the front end has never played out. He drives BMW's with McPherson front struts. He doesn't run sway bars either.

@1:22:00 he explains why frequency is only really important for a road car. Then he continues with why if you do choose a spring rate that has poor pitch moment you'll be relying heavily on your damper tuning and "you better have sway bars to match" to maintain a balanced car. So at first he almost dismisses the idea of using frequency for tuning suspension in racing but then circles back to why it's actually very important to understand when making hardware changes.

[IamFODI]

So I’ve called around a few places and collected an explanation I’m satisfied with. And when I say I collected it, I mean I got virtually all of it from Barry at 3DM, who (barring unforeseen circumstances) thus has won my future business, whatever that’s worth. Bilstein told me they don’t talk shop on topics like this. KW basically told me I was overthinking and everything was fine. Only Öhlins attempted a serious answer to my questions. They then directed me to Barry, who laid it all out.

Barry, if you’re reading this and see that I have anything wrong/incomplete/whatever, please don’t hesitate to correct the record.

Everyone else, please remember that this is my own interpretation and synthesis of what I’ve heard, so if something sounds strange, don’t automatically attribute it to Barry or anyone else.

To recap, here are the basic questions:

Originally Posted by IamFODI

...why don't people with aftermarket coilover kits complain about excess pitching over bumps? Is my understanding incorrect? Is there something else about those suspensions that prevents the problem? Are people experiencing it but not worried about it?

Originally Posted by IamFODI

If flat ride explains the factory setups having higher freqs in the rear, why don't any aftermarket kits that go that way?

If turn-in, transient response, and rear grip explain the aftermarket kits having higher freqs in front, why do basically no factory setups go that way?

Few things to consider:

1. While flat ride is always nice to have, it’s not equally important in all applications. For a street car, it’s of paramount importance. For a track/race car, it’s secondary in importance to handling balance.

2. Handling balance depends a lot on personal preference and driving style, so there’s plenty of room for different companies (with different test drivers etc.) to have different opinions about what’s best.

3. The stock suspension must account for extra weight in the back on occasion, which tilts it further in the direction of stiffer rear springs.

4. Because the stock setup was designed as a whole, the engineers could pick the spring rates for flat ride and then tune the handling balance with sway bars etc. The kind of person who would buy a coilover kit and use off-the-shelf spring rates probably isn’t even going to try that kind of tuning, and if they do, they probably won’t do it well. So, if you're the coilover manufacturer choosing default spring rates, you have to focus on what you can control, i.e. springs and dampers – and you're probably going to want to set the springs for a safe understeer margin in a variety of setups, and lean on damping to keep the ride flat if necessary.

5. Lowering is hell for E9x front suspension geometry. Even moderate lowering can drop the front roll center quiet a bit, necessitating much more roll stiffness.

6. The front suspension being a modified MacPherson strut, it needs a lot of static camber when it’s run at the track. That can tend to increase oversteer. So, depending on the driver and the rest of the car’s setup, it might be worth adding front roll stiffness to bring the car closer to neutral or slight understeer.

So, if you’re selling a coilover kit meant to be suitable for casual track use to the kind of person who would run the default spring rates, what to do?

Not everyone is going to have the exact same opinion, which presumably is why there’s variation. For example, among the Bilstein PSS10, KW V3, and Öhlins R&T, Bilstein has a front:rear wheel rate ratio much higher than 1, followed by KW and Öhlins closer to 1. However, it makes sense that they’ll all err on the side of raising the front rates much more than they raise the rear rates. It’s just the most sensible thing for their target market.

Hope that makes sense / is helpful.

[Obioban]

Originally Posted by IamFODI

So I’ve called around a few places and collected an explanation I’m satisfied with. And when I say I collected it, I mean I got virtually all of it from Barry at 3DM, who (barring unforeseen circumstances) thus has won my future business, whatever that’s worth. Bilstein told me they don’t talk shop on topics like this. KW basically told me I was overthinking and everything was fine. Only Öhlins attempted a serious answer to my questions. They then directed me to Barry, who laid it all out.

Barry, if you’re reading this and see that I have anything wrong/incomplete/whatever, please don’t hesitate to correct the record.

To recap, here are the basic questions:



Few things to consider:

1. While flat ride is always nice to have, it’s not equally important in all applications. For a street car, it’s of paramount importance. For a track/race car, it’s secondary in importance to handling balance.

2. Handling balance depends a lot on personal preference and driving style, so there’s plenty of room for different companies (with different test drivers etc.) to have different opinions about what’s best.

3. The stock suspension must account for extra weight in the back on occasion, which tilts it further in the direction of stiffer rear springs.

4. Because the stock setup was designed as a whole, the engineers could pick the spring rates for flat ride and then tune the handling balance with sway bars etc. The kind of person who would buy a coilover kit and use off-the-shelf spring rates probably isn’t even going to try that kind of tuning, and if they do, they probably won’t do it well. So, if you're the coilover manufacturer choosing default spring rates, you have to focus on what you can control, i.e. springs and dampers – and you're probably going to want to set the springs for a safe understeer margin in a variety of setups, and lean on damping to keep the ride flat if necessary.

5. Lowering is hell for E9x front suspension geometry. Even moderate lowering can drop the front roll center quiet a bit, necessitating much more roll stiffness.

6. The front suspension being a modified MacPherson strut, it needs a lot of static camber when it’s run at the track. That can tend to increase oversteer. So, depending on the driver and the rest of the car’s setup, it might be worth adding front roll stiffness to bring the car closer to neutral or slight understeer.

So, if you’re selling a coilover kit meant to be suitable for casual track use to the kind of person who would run the default spring rates, what to do?

Not everyone is going to have the exact same opinion, which presumably is why there’s variation. For example, among the Bilstein PSS10, KW V3, and Öhlins R&T, Bilstein has a front:rear wheel rate ratio much higher than 1, followed by KW and Öhlins closer to 1. However, it makes sense that they’ll all err on the side of raising the front rates much more than they raise the rear rates. It’s just the most sensible thing for their target market.

Hope that makes sense / is helpful.

So you'll be going with Ohlins with flat ride spring rates at near stock ride height with a big and adjustable front sway bar?

... that is exactly where my car sits

[IamFODI]

Originally Posted by Obioban

So you'll be going with Ohlins with flat ride spring rates at near stock ride height with a big and adjustable front sway bar?

... that is exactly where my car sits

Something like that. We’ll see when the time comes.

[Captain Starbucks]

What sway bar do you have?

I'm going to be getting the R&T in the next few months and was thinking the coilovers would be fine by themselves? Street use only for me.

[Obioban]

Originally Posted by Captain Starbucks

What sway bar do you have?

I'm going to be getting the R&T in the next few months and was thinking the coilovers would be fine by themselves? Street use only for me.

I'm in an e46 M3, but the nice thing about frequencies is you can have these discussions across cars of different weights/motion ratios/track widths.

But, Ground control front sway, OE CSL rear sway, with adjustable rear end links I made myself (because they don't exist for the stock sway and I need to able to eliminate preload).

[gmx]

Originally Posted by //steve\\

One of the other guys I've chatted with a fair bit settled on 8/14k and seems to be quite happy there and is running similar tires to me. I'm starting to think that might be where I'd like to start.


I don't recall who commented on why these suspension companies offer setups that are less than ideal out of the box and I think the answer is that they are leaving a margin for "safety". Typically a car that understeers is better for a beginner/novice than one that oversteers. Same reason why most cars from the factory behave this way.

I said that and it was an assumption. I cannot really think of another reason. Maybe manufacturing.
For example, Bilstein springs all their CS kits for each M3, E36, E46, E92 to 100/140NM (about 570/800). Maybe this means they can valve all the dampers the same for these cars? I really do not know on a economic scale. Consider the weight of all these cars is for sure different.

Originally Posted by Obioban

All of the below is about non aero cars-- and by aero I mean so much that they couldn't be street driven.

Most aftermarket setups are unnecessarily stiff-- stiff to the point that it's probably detrimental to performance rather than beneficial. IMO they are this stiff because people are convinced that's what they want/need... perhaps because...

Most people lower their cars more than they should-- so they need to make up for their limited travel and sub optimal camber curves by having huge levels of roll stiffness, especially up front.

The stiffer the springs, the stiffer the dampers you need. The stiffer the dampers you have, the less time suspension events take, so pitch matters less and less.

Stock springs on the e92 are, what, 160 lb/in, IIRC? So a car with 320 lb/in springs up front is already DOUBLE the stock stiffness. How many people are running something as "soft" as double stock stiffness? Puts things in context, imo.

I think many people would be better served with stocker ride heights and softer front springs-- in terms of lap times, comfort, time spent in pitch, and feeling of control over the car. People seem to forget that softer springs = more grip, until you NEED more spring (self induced or otherwise).

Also agree with this big time. I've seen the "chase" for static neg camber and no consideration for geometry correction except guys with specific SPL parts - and we're not even sure/assured they work except in the description of the product and faith in SPL knowing what they're doing (hope so).

What you're saying kind of works for Japanese cars (multiply the front/rear and get away with it) but not in M cars, it's not that straight forward. This is a hint and why FCM's own worksheet lists the riding on stop rate at least the for E46 M3, during cornering

Copy the GTS heights (E92). They're on TIS, you can't make a mistake here. Apply for necessary wheel diameter. This should serve as a baseline starting point. I haven't checked the end link part numbers, another assumption: These heights may be a combination of optimal roll centre in consideration of the supplied/as-is endlinks as well as for the "Street use". Street use being the E92 GTS street ride height settings which are the same as ZCP/Comp pack IIRC.

There's also been some debate around "soft spring = more grip". I can't recall who, but one person dug into this theory and couldn't find anything scientific about it. In fact, he found it was just blindly repeated quote, theoritical or linear graphs which don't apply in the real world. Also, it was the-thing-to-do during the 60s where they're using far different tyre construction than today. I've actually seen the opposite from some engineers involved in motorsport and from James Clay himself at BW justifying it's use in the rain/wet as well.

+1 for bbnks2 aero comment. It looks like all the cars with some sort of aero move the aero balance rearwards. I've spotted posts throughout the giant daily track chat thread where people run minimal AOA to regain "balance". There's a wide variety of splitters, ride-heights in there. Honestly a lot of variables and reliance on "feel" which could be absolutely wrong/placebo etc.

As for FR, I've bounced around between "makes-sense" as well as reverted to skeptical. However, I will try a softer front spring and see how I go. It looks like I cannot really find a stiffer rear (>14.3kg) spring in 250mm length and I really don't want to use a helper to get there. At this point, I don't think there exists a magic number though. The magic number changes for the each use case; ie. straight line highway cruising and some mountain use vs all-out race use.

But I will once again refer to BMW's own racecars. They're homologated as we established. The spring rates, lengths, ID are all there as part numbers in realOEM and sometimes in the PDF docs from BMW themselves. They all run a rear C/O which we know at least the E9x chassis motion ratio is 0.813. We know the weight of the car (1430kg IIRC). We know they use a BBS wheel and AP caliper, disc, Ohlins TTX. All the rest of the unsprung weight is the same as the road car except specific bushings which would mean we have a really good base to estimate the frequencies they run. What we don't know, without asking teams that ran them is what config they usually set the car up with. There's a total of 9 combinations they could run (3 spring rate options front and rear each). Now, if you follow the diagonal through this matrix, they run a 25-28% bias to the front. Diagonal being:

1. soft front/rear
2. medium front/rear
3. hard front/rear

As I said, I don't know whether it was popular for teams to run soft/medium front/rear for example. I also don't know the swaybar arm length as they were triple adjustable but the same diameter as per sanctioning/homologation rules.

I don't know the damper motion ratio of the F8x cars, so I'll leave that assumption out of this.

For predominant road use, flat-ride seems to be a no-brainer. Weight transfer characteristics on track, I remain skeptical and personally just have to try it out making sure I don't get coil bind and corner-balance the car each time (RHD sucks). My current front bias is 31%, too much as per my feel. I'll drop to 24% and then 18% and see how we go. Predominately though, I need to figure out how to stiffen the rear. My frequency is too low there but too high on the front (atm) for any street tyre except those cheater 200TW, maybe.

I can't find the specific YT material, but Kyle.engineers is a good channel to go through.

[Obioban]

The softer spring = more grip is reasonably easily proven out in tweaking oversteer/understeer ratios.

E.g. if your car understeers, and you want to address it with a spring rate change, the fix is to soften the front spring or stiffer the rear spring. This either increases the front grip or decreased the rear grip, moving you away from understeer.

No?

[gmx]

Then why not stick with the 160lbs/in front rate? Why bother stiffening the car at all? This line of thought suggests we're paying money to decrease grip, yet we end up faster.

Understeer/oversteer ratio is about rotation and balance, not grip. I truly believe the terms are so badly misused. It's up there with "weight transfer". You can't transfer weight. You can transfer load though.

We stiffen to reduce deflection such as dynamic motion ratios as well as the tyre itself which is why what tyre you run is extremely important in deciding a baseline of rates. We need the ability to manage lateral/longitudinal forces. Where we go too stiff [in general], you give up tyre life or chew through them usually due to getting too hot. There is a point where stiffness gives you that load control of the contact patch and a drop off where it doesn't. In turn, you end up doing the former; overheating the tyre. I'll take the R888R/NT01. The "updated" R888R is notorious for being poor on most heavy cars (modified or not), whereas really light cars such as the 86/MX5, Clubmans can get away with minimal or zero changes.

I will admit, this line of thinking and other circumstances prompted me to soften the front first rather than stiffen the rear. My woeful <2hz rear remains... woeful. If I didn't need to tackle front tyre clearance, and search harder for an ideal barrel spring for the rear - I'd have done that instead. Even without the aero, I can use the frequency on the tyre I'm about to run. Some other calcs taken from elsewhere, I've "lost" about 41lbs/in from eliminating some bushes, I'd assume most of that is in the subframe and potentially inboard camber joint.

[Obioban]

As you point out, 160 lb/in is for the stock PS2 levels of grip. We go stiffer for the stickier tires. It's the degree of stiffer I think people are overdoing.

I didn't realize you were <2hz in the rear. That does, indeed, seem a bit soft to me if you're on stickier R comps/slicks.

I'm currently at 1.92hz front, 2.06hz rear, running whatever R comps are cheapest/mile whenever I need new tires :P

And, fwiw, with the GC front sway and CSL rear sway, that gives me ~4647 lb-ft/deg (off bump stop) of front roll stiffness and 1237 lb-ft/deg (off bump stop) of rear roll stiffness (vs 1688/809 off bump stop stock). "~" because it depends exactly where I set the sway stiffness... pretty much dialed that in by ass.

[bbnks2]

Originally Posted by gmx

I said that and it was an assumption. I cannot really think of another reason. Maybe manufacturing.
For example, Bilstein springs all their CS kits for each M3, E36, E46, E92 to 100/140NM (about 570/800). Maybe this means they can valve all the dampers the same for these cars? I really do not know on a economic scale. Consider the weight of all these cars is for sure different.

Thanks for giving Bilstein as another example of how the "shelf" rates on some of these coilover kits aren't some magic setup that they "R&D" but rather they are simply halfway decent rates that they landed on and adapted to multiple chassis. Produce a damper and spring combo and machine mounts to adapt it to multiple vehicles. I can't see any other reason why these rates are used on all three generations other than it's being done out of ease of manufacturing, distribution, and costs. "Economy of scale" is likely your answer lol.

Not arguing anything here... just putting numbers to how using Bilstein clubsports will affect the balance of each generation of BMW noted above:

E36 M3:
Front Motion Ratio - .94
Rear Motion Ratio - .67

To calculate Front wheel rates: SR*(.88)
To calculate Rear wheel rates: SR*(.44)

Bilstein Clubsport: 502 / 352 (59% / 41% spring wheel rate bias)

E46 M3:
Front: 0.938
Rear: 0.7

To calculate Front wheel rates: SR*(.88)
To calculate Rear wheel rates: SR*(.49)

Bilstein Clubsport: 502 / 392 (56% / 44% spring wheel rate bias)

E8x/E9x:
Front 0.960
Rear 0.563

To calculate Front wheel rates: SR*(0.92)
To calculate Rear wheel rates: SR*(.32)

Bilstein Clubsport: 525 / 256 (67% / 33% spring wheel rate bias)

If you're not taking into consideration how effective the spring rates are for your specific chassis than you're missing out on an opportunity to reduce quite a bit of trial and error in swapping out springs and then custom valving etc. Or, working backwards through handling issues... Now consider the above and also the fact that almost no one will touch the rear bar on any e8x/e9x or even an E92 M3. Yet, they'll quickly toss on a front bar that is +40 - +100% stiffer. You can easily push a car that came off the factory floor with 60% (designed to under-steer to be "safe") front roll couple to 80%+.

Originally Posted by IamFODI

Not everyone is going to have the exact same opinion, which presumably is why there’s variation. For example, among the Bilstein PSS10, KW V3, and Öhlins R&T, Bilstein has a front:rear wheel rate ratio much higher than 1, followed by KW and Öhlins closer to 1. However, it makes sense that they’ll all err on the side of raising the front rates much more than they raise the rear rates. It’s just the most sensible thing for their target market.

Hope that makes sense / is helpful.

Sounds like Barry could've just simplified his reply with "IDK." Not trying to offend in any way. It just doesn't sound like he gave you any real explanations at all but rather a bunch of generic statements at least in regard to his own 8k/14k rates.

If you want to argue that it's necessary to do relatively stiffer front spring rates because the car is lowered by some crazy amount then so be it. There is a basis in reality to that statement, but, also consider that there is also zero chassis specific empirical data to show it. It's purely anecdotal textbook theory. Plot things out in wingeo and you'll likely find that it's theory that is largely blown out of proportion. McPherson does have negative camber recovery but at any ride height that keeps the chassis from banging into the ground static camber can offset the negative camber recovery in roll. Not to mention that modern street tires are built with slight positive camber in mind to produce optimal contact patch.

It almost sounds like Barry is saying off-the-shelf springs rates are oriented around hellaflush cars with no camber adjustment and no driving skills. If that's the case, then the people in this thread looking at spring rates from a more performance oriented perspective would want to reconsider the off-the-shelf rates many coilvoers for BMW's are shipping with. They produce a lot of rake and slam the car. Looks cool, sells well, but it's not how I have my car set up at all and for good reason. And even if you do slam your car this is really just a big excuse and isn't really a reasonable explanation at all for why the rates are what many might consider to be "botched" like the 571/800 example above. The affects of lowering a car even 2" up front just isn't as severe as stated. You're talking maybe necessitating dialing in 1* more static camber... not running 300lb/in higher front spring rates.

[IamFODI]

Originally Posted by bbnks2

Sounds like Barry could've just simplified his reply with "IDK." Not trying to offend in any way. It just doesn't sound like he gave you any real explanations at all but rather a bunch of generic statements at least in regard to his own 8k/14k rates.

I didn't feel that way at all. If you do, maybe it's because I did a poor job conveying what he said.

To be clear, I wasn't necessarily looking for anyone to tell me "here are all the exact reasons why all these companies chose these spring rates." That would have been a clear request for proprietary information that no one has any reason to disclose to me, and it wouldn't even be possible in the first place to get such info on multiple kits from one party. All I really needed was a plausible story that could help inform my own decision making, and a plausible reason not to think either all OE suspensions or all aftermarket suspensions were stupid. I feel I got that in spades.

Originally Posted by bbnks2

It almost sounds like Barry is saying off-the-shelf springs rates are oriented around hellaflush cars with no camber adjustment and no driving skills. If that's the case, then the people in this thread looking at spring rates from a more performance oriented perspective would want to reconsider the off-the-shelf rates many coilvoers for BMW's are shipping with. They produce a lot of rake and slam the car. Looks cool, sells well, but it's not how I have my car set up at all and for good reason.

Again, I wouldn't pin this on Barry. Your perception could just be due to my presentation. He never said anything like that.

I'll agree with you on one thing: people in this thread probably are not the target audience of the default spring rates in these kits, so it makes sense that we'd find those rates a bit weird.

[IamFODI]

Originally Posted by Obioban

As you point out, 160 lb/in is for the stock PS2 levels of grip. We go stiffer for the stickier tires.

To expand on this a bit:

My understanding is that a sticker tire requires more roll stiffness to keep the car off the bumpstops and keep all wheels on the ground, and permits more roll stiffness for quicker turn-in and transitions. Correct?

[gmx]

Originally Posted by Obioban

As you point out, 160 lb/in is for the stock PS2 levels of grip. We go stiffer for the stickier tires. It's the degree of stiffer I think people are overdoing.

I didn't realize you were <2hz in the rear. That does, indeed, seem a bit soft to me if you're on stickier R comps/slicks.

I'm currently at 1.92hz front, 2.06hz rear, running whatever R comps are cheapest/mile whenever I need new tires :P

And, fwiw, with the GC front sway and CSL rear sway, that gives me ~4647 lb-ft/deg (off bump stop) of front roll stiffness and 1237 lb-ft/deg (off bump stop) of rear roll stiffness (vs 1688/809 off bump stop stock). "~" because it depends exactly where I set the sway stiffness... pretty much dialed that in by ass.

Everyone running 800lbs/in rears is on the E8x/E9x. Worse on the heavier M3s. All off the shelf kits seem to have "stalled" at this number (JRZ, Bilstein range, KW sans their recent update). I think I've only seen MCS dampers be paired with 1000lbs/in rears (non rear-C/O). Without going on a tangent, MCS sells dampers but others such as BimmerWorld "help" put them in kits. One I saw for sale at the moment in the classifieds uses a 200m 1100lbs/in 8" Eibach spring and GC weight jacks. When I was talking to BW a while back I showed them my cart (because I had nfi), and asked if I missed anything - they said nothing about GC weight jacks. The height of those would probably make up for an 8" length spring and no helper.

I've come across some 10" length, 2.5" ID Hyperco available up to 950lbs/in. There's heaps more in 9" length and Ohlins seems to have spares available through their retailers, including the 230mm length ones with a 65mm ID.

[fsmtnbiker]

You will not be able to get the rear ride height low enough with a 10” long spring of that rate in the stock location. I run an 8” 1000lb spring w/ the original KW helper and that gives a nice range of height adjustment.

[gmx]

Yeah I'm starting to realise that. This Bilstein CS spring is similar to what some GT3 cars run factory; like an integrated helper at the top of the coils which explains why they can get away being 250mm free-length. They're also more akin to a barrel type spreading out to a 3" ID.
It's a shame the rate is low as I like the spring design, superior to anything else off the shelf.

And I have to think how this affects the damper travel, bit in over my head and probably suited to another thread. Might just slap in a 9" swift or try an Ohlins rear (230mm, no helper).

KW sizes/cert here: https://www.m3post.com/forums/showthread.php?t=1541716

Original kits with 800lbs/in rears have a 200mm free-length and helper with a 30mm block height.

[bbnks2]

Originally Posted by gmx

Yeah I'm starting to realise that. This Bilstein CS spring is similar to what some GT3 cars run factory; like an integrated helper at the top of the coils which explains why they can get away being 250mm free-length. They're also more akin to a barrel type spreading out to a 3" ID.
It's a shame the rate is low as I like the spring design, superior to anything else off the shelf.

What leads you to believe that the Bilstein CS rear spring is similar to the E92 M3 GTS spring? Hard to find info on the GTS but all the pictures I see show a 14k linear spring. It does not change diameter anywhere and the coil distance is evenly distributed. I see that the GTS got custom KW clubsports. I can't find the front spring rates/lengths posted anywhere so I don't want to assume it's the same 9k/14k setup as the off-the-shelf KW clubsports. The Bilstein use a progressive rear spring hence why it is barrel shaped.

What exactly are you trying to accomplish?

Spring length is largely going to be dictated by the strut length (speaking relative to the rear with a divorced spring setup). KW rear struts are not adjustable length and I don't think Bilstein are either. The strut bodies are not threaded. That makes spring fitment very particular to the strut specifications (strut tube length and stroke). Too short of a spring will require a helper to keep it taught when the wheels are in full droop (generally springs get shorter as they get made stiffer). Too long a spring will max out the height adjustment and cause the rear to have limited "lowering" ability. So you need to pick a spring that works with your Bilstein dampers... If they came with 250mm springs then that is fairly long. They might be running a longer spring because of the aforementioned progressive spring rate. The car will not sit insanely high despite the longer spring length requiring a lot of pre-load in the lower camber arm (presumably).

Originally Posted by gmx

And I have to think how this affects the damper travel, bit in over my head and probably suited to another thread. Might just slap in a 9" swift or try an Ohlins rear (230mm, no helper).

KW sizes/cert here: https://www.m3post.com/forums/showthread.php?t=1541716

Original kits with 800lbs/in rears have a 200mm free-length and helper with a 30mm block height.

You're chasing your tail if you're basing decisions based on random threads/numbers. Like stated above, you need a spring that matches your rear struts and spring perch height etc. and not just because KW supposedly moved to a longer rear spring at some point. Spring length is largely inconsequential so long as you have adjustment range at the spring perch left to work with (you can achieve your target ride height). You can rely on pre-load of the rear spring a LOT to gain ride height since they don't need to have a lot of stroke...

I am not so sure that thread is entirely accurate to begin with. It is true that if you go on KW's website you see two separate E92 M3 "3-way" coilovers listed. One says 9k/14k (2011) and another has a data sheet that says 11k/12k (2014). Does that mean the "newer" kit is "better?" Is it just a mistake? Does anyone have KW clubsports that came with 11k/12k rates? Every site I see lists them as 510/800. 11k/12k seems like a pretty awful move considering everyone else is going the opposite direction (like Ohlins moved from 6k/7k to 8k/14k).

I would speculate that the move to a longer and softer rear spring would have more to do general complaints about "harness" than anything else. Making the spring longer to compensate for the loss of ride height (more pre-load on the longer spring regains that lost ride height). There is no performance difference to chase there from what I can see... No reason for you to "want" the 220mm spring just because KW is using it. Strut travel would be exactly the same so long as you adjust the spring perch to get back to the same ride height as prior to the spring change.

[tsk94]

Flat ride for a street car = good. Flat ride for track applications, not optimal. It can be made to work but it's not ideal.

I did a lot of research prior to choosing suspension for my race car build. Tl;dr version was I couldn't find any professional race teams (where spring rate info was published) or OEM homologated race cars (such as BMW's own E92 GT4 and M4 GT4) that were running a 'flat-ride' setup. All of them were front biased. Coupled with my experience now with my own car, I'm a strong believer that flat-ride should and will remain a street orientated setup. For reference I'm running 800F/1150R giving me an approx. ~25% bias to the front. This seems to be about average for typical track setups for this chassis, common setups are 500/800, 600/900, 700/1000 and 800/1100.

For anyone interested, I'm running custom KW Competition suspension. I worked directly with KW for my setup. They have lots of data on the E9X chassis, both in stock configuration and GT3 and GT4 race cars. All of which have been put on their suspension chassis dyno for analysis. They have optimal setups based on the cars modifications, weight, aero, tire setup and intended usage. Initially they recommended 900# in the front but we dropped it down to 800 since I was using a larger, adjustable sway bar vs. an OEM one like the GT4's did.

In terms of what would be optimal for street usage, I'm not much help there. I thought the stock EDC suspension, which I believe the spring rates are 160F/550R, was quite good. Perhaps 10-15% stiffer rates with a good quality shock would work quite well. For street car I think flat-ride makes sense. Could also add a bigger front bar to add additional roll stiffness for performance 'street' driving to work well with flat-ride setup springs.

EDIT:
I will also add that, while the spring rates I'm running on paper look quite stiff, they offer more compliance over bumps and particularly kerbs on track compared to the stock setup. I used the car twice on track with stock EDC suspension, just R-comps and upgraded brake pads and cooling before starting the build. The car is noticeably more composed and compliant on track with the new suspension vs. OEM. Whilst also providing supreme improvement in body control.
That comes down to the shocks. Quality dampers are amazing things. Don't cheap out on your shocks!

[Obioban]

Originally Posted by tsk94

Flat ride for a street car = good. Flat ride for track applications, not optimal. It can be made to work but it's not ideal.

I did a lot of research prior to choosing suspension for my race car build. Tl;dr version was I couldn't find any professional race teams (where spring rate info was published) or OEM homologated race cars (such as BMW's own E92 GT4 and M4 GT4) that were running a 'flat-ride' setup. All of them were front biased. Coupled with my experience now with my own car, I'm a strong believer that flat-ride should and will remain a street orientated setup. For reference I'm running 800F/1150R giving me an approx. ~25% bias to the front. This seems to be about average for typical track setups for this chassis, common setups are 500/800, 600/900, 700/1000 and 800/1100.

For anyone interested, I'm running custom KW Competition suspension. I worked directly with KW for my setup. They have lots of data on the E9X chassis, both in stock configuration and GT3 and GT4 race cars. All of which have been put on their suspension chassis dyno for analysis. They have optimal setups based on the cars modifications, weight, aero, tire setup and intended usage. Initially they recommended 900# in the front but we dropped it down to 800 since I was using a larger, adjustable sway bar vs. an OEM one like the GT4's did.

In terms of what would be optimal for street usage, I'm not much help there. I thought the stock EDC suspension, which I believe the spring rates are 160F/550R, was quite good. Perhaps 10-15% stiffer rates with a good quality shock would work quite well. For street car I think flat-ride makes sense. Could also add a bigger front bar to add additional roll stiffness for performance 'street' driving to work well with flat-ride setup springs.

EDIT:
I will also add that, while the spring rates I'm running on paper look quite stiff, they offer more compliance over bumps and particularly kerbs on track compared to the stock setup. I used the car twice on track with stock EDC suspension, just R-comps and upgraded brake pads and cooling before starting the build. The car is noticeably more composed and compliant on track with the new suspension vs. OEM. Whilst also providing supreme improvement in body control.
That comes down to the shocks. Quality dampers are amazing things. Don't cheap out on your shocks!

The GT4s have real aero-- changes everything.

FWIW, everyone who has gone to flat ride on M3Forum (e46s, on which non flat ride is the traditional aftermarket track setup as well) has gained at least a second of lap time after. Now, I can't definitively state if that's because the cars are easier to drive or because they have higher limits, but people are getting faster times out of their cars by softening the front.

That said, I'm not sure what your definition of "street car" is. Perhaps that includes non aero tracked cars, in which case we're in agreement.