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

[tsk94]

Originally Posted by Obioban

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.

My post mainly is referring to track or pure race cars where lap time is the only goal. My car falls into this category (overall very similar to an OEM GT4 build), so take what I say with a grain of salt. The E92 GT4's do have aero, but it's nothing like a GT3 car or even TCR car.

In terms of what works best as a hybrid setup, ie. something that is driven on the road but also tracked, I don't know. I've moved away from that as finding a balanced compromise is nearly impossible. If it's good on the street, it will be too soft and leave you wanting more on track and visa versa. I did that with a fairly extensively modified (suspension wise) E46 M3 - KW club sports, Hotchkis sway bars, control arms, bushings etc etc. Decent on the street and decent of the track - not optimal for either.

I'm sure many people have gained time going to a slightly stiffer setup while maintaining flat-ride. Good shocks could be adjusted to make less then perfect spring rates (ie. flat ride setups on track) perform decently well. Also, so many other factors come into play as well. Sway bar sizes and settings, alignment, ride heights, tire setups, diff type and lock-up, and lastly, personal preference. Overall, spring rates are a very small part to the whole equation. But I'd still want to optimize them for my situation. Again, I'm talking in terms of on track perform, faster lap times.

Hope that clears some things up.

[Obioban]

Originally Posted by tsk94

My post mainly is referring to track or pure race cars where lap time is the only goal. My car falls into this category (overall very similar to an OEM GT4 build), so take what I say with a grain of salt. The E92 GT4's do have aero, but it's nothing like a GT3 car or even TCR car.

In terms of what works best as a hybrid setup, ie. something that is driven on the road but also tracked, I don't know. I've moved away from that as finding a balanced compromise is nearly impossible. If it's good on the street, it will be too soft and leave you wanting more on track and visa versa. I did that with a fairly extensively modified (suspension wise) E46 M3 - KW club sports, Hotchkis sway bars, control arms, bushings etc etc. Decent on the street and decent of the track - not optimal for either.

I'm sure many people have gained time going to a slightly stiffer setup while maintaining flat-ride. Good shocks could be adjusted to make less then perfect spring rates (ie. flat ride setups on track) perform decently well. Also, so many other factors come into play as well. Sway bar sizes and settings, alignment, ride heights, tire setups, diff type and lock-up, and lastly, personal preference. Overall, spring rates are a very small part to the whole equation. But I'd still want to optimize them for my situation. Again, I'm talking in terms of on track perform, faster lap times.

Hope that clears some things up.

Clears up what you were saying, but goes contrary to the result people are getting on the e46 side.

[tsk94]

I should add, I've also tracked a 135i with upgraded suspension utilizing 'flat-ride' spring rates for 2 seasons prior to building the M3. So my views are not baseless - I've tried and experienced both. In my opinion it is not the optimal setup for track use if lap time is the ultimate goal.

[tsk94]

Originally Posted by Obioban

Clears up what you were saying, but goes contrary to the result people are getting on the e46 side.

I have limited experience with the E46 chassis compared to the E82 and E92. The vastly different suspension geometry of the E46 compared to the E9X and E82's could explain some of the difference seen in these results. That being said, if you talk to people like Turner, Bimmerworld, Falline, all professional teams raced both these chassis for years, and won, they will tell you that they found front biased setups to be faster - hence why they run them.

[gmx]

OT: Yeah I ended up measuring shock stroke and LCA <-> unibody length inc adjustment range at full droop. I've seen some kits come with huge adjusters, it'll change everything...

FWIW, old FIA GT4 and current SRO GT4 are very different. The current regs allow more aero freedom within BoP. The E92 GT4 is basically an extended GTS front lip, the same under-engine panel as the road-car, and a more effective rear wing. But not as huge or downforce points as the 9livesracing(?) or APR wings people are using here, which mind you, tend to be much wider than the roofline of the car.
So yes, while it does have aero, it's subtle and road-car relevant which is what point of GT4 initially was.

[tsk94]

Originally Posted by gmx

OT: Yeah I ended up measuring shock stroke and LCA <-> unibody length inc adjustment range at full droop. I've seen some kits come with huge adjusters, it'll change everything...

FWIW, old FIA GT4 and current SRO GT4 are very different. The current regs allow more aero freedom within BoP. The E92 GT4 is basically an extended GTS front lip, the same under-engine panel as the road-car, and a more effective rear wing. But not as huge or downforce points as the 9livesracing(?) or APR wings people are using here, which mind you, tend to be much wider than the roofline of the car.
So yes, while it does have aero, it's subtle and road-car relevant which is what point of GT4 initially was.

Ya I was referring to the original E92 GT4. Like you mentioned the aero on it is actually quite low. My car (and most modern E92 builds you see today), with a larger front splitter and much larger rear wing, will actually have significantly more aero then the GT4.

[bbnks2]

I'm at a complete loss for why you are saying neutral/balanced spring rates are "less than perfect" and need to be fixed with damping... you seem to associating "flat ride" with "undersprung" or "soft" and that's just incorrect.

All I can say is read back as a lot of your points have already been addressed.

If you're going to bring up the e92 m3 gt4 car then please post the spring rates and frequency calcs to support your statements. Otherwise, you're just making assumptions based on your own bias.

The part you're definitly missing is that the e92 m3 gt4 car weighs 2400lbs. How do you gut that much weight from a production vehicle? It all comes out of the rear. You cant make the engine any lighter. The 3700lb m3s 52/48 weight distribution becomes 60/40... the e92 m3 gt4 car also gets a true rear coilover. The 900/800 spring rates that car came with does come out to be relatively neutral when you actually start banging numbers into the calcs... very crude estimate puts 900/800 springs at 3.5/3.6hz...

So yeah if you're running e92 m3 gt4 spring rates on a regular old e92 m3 you're doing it wrong and being mislead. Your proof that these are good rates because they are being used on race cars is way off base whether you think they are faster for you or not.

As already discussed, 800/1150 would be neutral/flat ride if you have a true rear coilover. It's an understeer oriented setup on cars with stock divorced rear springs. People don't understand the difference so you see the same rates on both setups. Both drivers will post here saying that their car is fastest car ever and all the real race cars "run these rates."

[gmx]

https://www.m3post.com/forums/showth...=358384&page=6

Second link official pdf brochure lists as 1380kg. In my notes I have 1430kg wet for some reason, probably this potentially false post:
https://www.m3post.com/forums/showthread.php?t=393417

Even the GT2 car was 1246kg - that thing is completely different and has barely any road going resemblance.

Last link has the full parts list. From the seals Pn, maybe you could find the AP caliper to nail down it's weight or estimate it based upon their latest motorsport offerings. Disc ring size, you could probably find out. Hat looks unknown. Spacers + BBS motorsport wheels. Rest of the GT4 rules meant the suspension arm material, pickup points had to be the same as the road-going version. You could only change bushes or actual components based on grounds of safety /strengthening. This does not make it difficult to nail down unsprung weight at the very least.

[tsk94]

Originally Posted by bbnks2

I'm at a complete loss for why you are saying neutral/balanced spring rates are "less than perfect" and need to be fixed with damping... you seem to associating "flat ride" with "undersprung" or "soft" and that's just incorrect.

All I can say is read back as a lot of your points have already been addressed.

If you're going to bring up the e92 m3 gt4 car then please post the spring rates and frequency calcs to support your statements. Otherwise, you're just making assumptions based on your own bias.

The part you're definitly missing is that the e92 m3 gt4 car weighs 2400lbs. How do you gut that much weight from a production vehicle? It all comes out of the rear. You cant make the engine any lighter. The 3700lb m3s 52/48 weight distribution becomes 60/40... the e92 m3 gt4 car also gets a true rear coilover. The 900/800 spring rates that car came with do come out to be neutral when you actually start banging numbers into the calcs... very crude estimate puts 900/800 springs at 3.5/3.6hz... flat ride brah

So yeah if you're running e92 m3 gt4 spring rates on a regular old e92 m3 you're doing it wrong and being mislead. Your proof that these are good rates because they are being used on race cars is way off base whether you think they are faster for you or not.

As already discussed, 800/1150 would be neutral/flat ride if you have a true rear coilover. It's an understeer oriented setup on cars with stock divorced rear springs. People dont understand the difference so you see the same rates on both setups. Both drivers will post here saying their car is fastest car ever and all the real race cars "run these rates."

I have the calculations and have posted them in numerous thread in which you've been active in previously. If you're that interested in specifics, ask.

The E92 GT4 is not 2400lbs, they were around right around 3150lb from the factory. Throw in full fluids and a driver and you're looking at 3300-3400 race weight. My car is a bit heavier at 3500 w/ driver but I have room to take out about 200lbs still if I want to get serious. At my current weight, with my spring setup I'm running 2.97/2.26, ~25% front bias. And the weight difference is minimal. Do you really think the engineers at BMW Motorsport would run such a front biased setup if something like flat-ride was faster? It had NO WHERE near flat-ride from the factory. Even if you assumed using the stiffest rear spring rate it was offered with (970 coilover) and the softest front spring (900), from the factory it would still have an approximate front bias of 10%.. And from speaking to BMW motorsports themselves in Germany, no one (at least no one fast) was running or buying the cars this way.

Moving beyond that, no my setup is not understeered biased. It was actually initially oversteered biased because handling balance goes far beyond just wheel rates and frequencies. My setup is a rear coilover setup, the rear spring rate is 570 - which works out to ~1150 in a divorced setup. I state 1150 as divorced is the norm and most people wouldn't know what 570 in a coilover translates to.

Why would I lie saying professional teams run such and such rates, or this ratio front to rear? I'm trying to help people, like you. I spent 3 months researching, talking to people from these companies, and professional teams to work out what would be best. I didn't want to spend my money and then realize a better option existed to just spend more money again - do it once and do it right.

I never claimed to be copying spring rates either, not sure how you got that idea. I've used other platforms and what fast, race-winning teams have used as REFERENCE and worked from there. I know you love your flat-ride setup approach, you do you. I've experienced both and have my opinion - which happens to be the shared opinion of professional race engineers that are a lot more in the know then us, and by better drivers then either of us.

Edit:
Can a flat-ride setup be made to work decently well on track? Yes, of course. If you have a good shock to accompany flat-ride spring rates, and adjust suspension setup according (no rear sway bar, big front sway bar), tweaked alignment compared to 'traditional' spring rate setup, and so on. I believe I've stated many times, that a flat-ride style setup is not optimal for serious track/pure race use. I never said it was absolute garbage or the car will be undriveable.

[Obioban]

Originally Posted by tsk94

Can a flat-ride setup be made to work decently well on track? Yes, of course. If you have a good shock to accompany flat-ride spring rates, and adjust suspension setup according (no rear sway bar, big front sway bar), tweaked alignment compared to 'traditional' spring rate setup, and so on. I believe I've stated many times, that a flat-ride style setup is not optimal for serious track/pure race use. I never said it was absolute garbage or the car will be undriveable.

Normal front sway and no rear sway, or large front sway and stock sized rear sway, is generally how it goes with a flat ride setup-- Not large front sway, no rear sway.

I'd also suggest that you're too focused on shocks as a part of this-- good (and properly matched) shocked will benefit either setup vs crappy shocks. That said, one of the nice things about a flat ride setup is your less dependent on shocks to get the car settled down quickly.

[tsk94]

Originally Posted by Obioban

Normal front sway and no rear sway, or large front sway and stock sized rear sway, is generally how it goes with a flat ride setup-- Not large front sway, no rear sway.

I'd also suggest that you're too focused on shocks as a part of this-- good (and properly matched) shocked will benefit either setup vs crappy shocks. That said, one of the nice things about a flat ride setup is your less dependent on shocks to get the car settled down quickly.

I think it depends on the car, but yeah what you're saying is likely true. With that being said, sway bar rate differences are usually minor in the grand scheme of things. When I was researching suspension and I looked into FCM, he suggested I'd likely be running a front sway bar (all of which are pretty big, even OEM) and no rear sway, or upgraded front with an OEM rear. So that follows what you've said, but not far off of what I mentioned earlier - at that point personal preference of handling balance could come into play as well.

You can never over look the difference a shock will make, especially a high quality one vs. a mid tier one and certainly compared to lower end ones. Theres a reason a similar double adjustable Ohlins TTX costs twice as much as a comparable MCS double adjustable. Similarly why a MCS DA costs 50%+ more then something like a Fortune Auto or a DA Koni. You pay for what you get. Yes shocks are a bit outside of the questions of what wheel rates might be best, but to ignore the importance and/or value of a proper quality shock would be ignorant - which is why I've been bringing it up.

[Obioban]

Originally Posted by tsk94

I think it depends on the car, but yeah what you're saying is likely true. With that being said, sway bar rate differences are usually minor in the grand scheme of things. When I was researching suspension and I looked into FCM, he suggested I'd likely be running a front sway bar (all of which are pretty big, even OEM) and no rear sway, or upgraded front with an OEM rear. So that follows what you've said, but not far off of what I mentioned earlier - at that point personal preference of handling balance could come into play as well.

You can never over look the difference a shock will make, especially a high quality one vs. a mid tier one and certainly compared to lower end ones. Theres a reason a similar double adjustable Ohlins TTX costs twice as much as a comparable MCS double adjustable. Similarly why a MCS DA costs 50%+ more then something like a Fortune Auto or a DA Koni. You pay for what you get. Yes shocks are a bit outside of the questions of what wheel rates might be best, but to ignore the importance and/or value of a proper quality shock would be ignorant - which is why I've been bringing it up.

I'm not saying shocks don't matter-- I've run JRZs, Motons, TCKs (underrated IMO), and currently Ohlins. It's just not the topic at hand, and distracts from it.

[bbnks2]

Originally Posted by tsk94

I have the calculations and have posted them in numerous thread in which you've been active in previously. If you're that interested in specifics, ask.

Yes, I asked if you can post them. Then we can actually compare/contrast things on a factual basis on not a a "word-of-mouth" basis. I kept my calcs simple as they were just conceptual and didn't look at weight distribution and other factors, but, you would definitely need those factors to get realistic GT4 numbers.

Originally Posted by tsk94

The E92 GT4 is not 2400lbs, they were around right around 3150lb from the factory. Throw in full fluids and a driver and you're looking at 3300-3400 race weight. My car is a bit heavier at 3500 w/ driver but I have room to take out about 200lbs still if I want to get serious. At my current weight, with my spring setup I'm running 2.97/2.26, ~25% front bias. And the weight difference is minimal. Do you really think the engineers at BMW Motorsport would run such a front biased setup if something like flat-ride was faster? It had NO WHERE near flat-ride from the factory. Even if you assumed using the stiffest rear spring rate it was offered with (970 coilover) and the softest front spring (900), from the factory it would still have an approximate front bias of 10%.. And from speaking to BMW motorsports themselves in Germany, no one (at least no one fast) was running or buying the cars this way.

I can't seem to find the source again where I got the M3 GT4 weight from @24xxlbs. I might've grabbed GT2 weight or something. That was not the point though. The point was that the weight distribution of the E92 M3 GTS, and race cars in general, is more forward vs the production car. Same as if you gut an E92 M3 for tracking. Weight reduction moves the weight distribution (especially with driver) forward which then affects how much spring you need in the front and rear, obviously. So everyone harped on that because I wrote the wrong number but missed the point as to why it's relevant in regard to choosing spring rates...

E92 M3 GT4:
Front 0.960^2
Rear 0.8133^2 (true-rear coil-over)

To calculate Front wheel rates: SR*(0.92)
To calculate Rear wheel rates: SR*(.66)

Spring rates per E92 M3 GT4 catalog(http://www.bimmerfile.com/wp-content...talog+US.pdf):
Front: 140-60-160 main spring (.2k helper spring) with optional rates of 18k and 20k
Rear: 160-50-130 main spring (2k tender spring) with optional rates of 15k and 17k

I do see the official weight was put at 1430kg = 3152lb. I'll use that weight and figure weight distribution with 150lb driver to be around 57/43.

So, using the shelf rates of 16k/13k I get
3.16/2.66Hz or 18.80% stiffer front

Using the stiffest combination of 20k/17k I get:
3.53/3.04 or 16.12% stiffer front

Using the softest front and stiffest rear of 16k/17K I get:
3.16/3.04 or 4.51% stiffer front

Using the stiffest front and softest rear 20K/13K I get:
3.53/2.66 or 32.70% stiffer front

Quite frankly, I am making a lot of assumptions to even get this far, and excluded things from the calculations which would make the actual effective spring rates more accurate, so, I make no claim that these numbers are perfect for this specific car. I only post numbers to show the relativity of a given spring rate on different chassis... To show that Bilstein Clubsport rates used across three different chassis will produce three differently handling cars... to make the point that if you're not considering these factors then you may end up with a much differently handling vehicle than you intended.

Originally Posted by tsk94

Moving beyond that, no my setup is not understeered biased. It was actually initially oversteered biased because handling balance goes far beyond just wheel rates and frequencies. My setup is a rear coilover setup, the rear spring rate is 570 - which works out to ~1150 in a divorced setup. I state 1150 as divorced is the norm and most people wouldn't know what 570 in a coilover translates to.

Why would I lie saying professional teams run such and such rates, or this ratio front to rear? I'm trying to help people, like you. I spent 3 months researching, talking to people from these companies, and professional teams to work out what would be best. I didn't want to spend my money and then realize a better option existed to just spend more money again - do it once and do it right.

I have no idea how your car handles. You didn't share any relevant details for me to make assumptions on your cars overall handling beside spring rates. I am merely discussing spring rates and how spring rates affect pitch motion.

Going back to what I said:

So yeah if you're running e92 m3 gt4 spring rates on a regular old e92 m3 you're doing it wrong and being mislead. Your proof that these are good rates because they are being used on race cars is way off base whether you think they are faster for you or not.

You run a true-rear coil-over so the statement: "if you're running e92 m3 gt4 spring rates on a regular old e92 m3 you're doing it wrong" doesn't apply to you...

I shouldn't have said "your proof" in a reply. It was a general statement. A general statement said within the context of this thread and other threads where people advocate for running spring rates that they do not understand because "race cars run these spring rates." Dive into it and these race cars are NOWHERE near as under-steer oriented as some of these enthusiast builds and off-the-shelf coilover spring rates. That's a fact. BC called for 10k/10k. Ohlin's called for 7k/6K? and now 8k/14k. Bilstein called for 10k/14k... JRZ calls for 10k/14k in a divorced rear spring rate... MCS calls for 10k/14K in a true-rear coil-over. The spring rates are all over the place. Who is right? Which one did you call and speak to? Slap your own custom springs onto these coilovers and now the damping is all screwy...

I also have 0 confidence that the people you spoke to necessarily understand half this stuff either. You also assume I've never spoken to anyone? My anecdotal experience counts for nothing but yours counts for everything because??? Every race book in existence mentions tuning around spring frequencies and how to do it. Anyone can be a race car driver/pit crew member. All it takes is racing fees and enough money to buy a production race car. There are race car drivers with 0 experience competing in SRO. Being on a race team doesn't mean they understand whether or not the spring rates the car runs is under-steer biased or not. Believe me... I've spoken to race teams too and they can be just as clueless as the rest of us. Same goes for the representatives of businesses. You think the chief engineer grabbed the phone at KW to discuss spring rate choices with you? Sorry to present this negative viewpoint, because there are knowledgeable people out there too, but it's reality.

Let's continue with the E92 M3 GT4 example and compare it to how you have your car sprung:

Using your cars 800/571 (roughly 14k/10k) spring rates, 3550 weight, and a presumed 53/47 weight distribution I get:
2.99/2.21 or 35.29% stiffer front

Yes, you are at the front biased end of the spectrum of what a GT4 car would've ran (spring rate wise) and then some. This thread is not about race cars though nor is it about how to set up your M3 to be a replica GT4 spec car... Op asked about pitch and why off-the-shelf coilover kits ship with really low rear rates that you would think would induce pitch problems. Op is correct. The spring rates are likely a compromised to keep the average enthusiast happy with their street car on rough roads. Got nothing to do with race-cars or performance. Damping can make up for poor rates whether it be on the street or the track.

Originally Posted by tsk94

I never claimed to be copying spring rates either, not sure how you got that idea. I've used other platforms and what fast, race-winning teams have used as REFERENCE and worked from there. I know you love your flat-ride setup approach, you do you. I've experienced both and have my opinion - which happens to be the shared opinion of professional race engineers that are a lot more in the know then us, and by better drivers then either of us.

Again, I was just making a general statement that people blindly copy other cars spring rates and that they have no real understanding or justification of WHY those cars run the rates that they do. The only reason why we are even mentioning race cars (as we did in the other thread too) is because people like to say "oh well race cars run these rates so they are the best." Uhh not exactly... you drive a regular E92 M3 with a divorced rear spring and none of the other modifications that a race car does that actually warrants running the quoted rates... That's the perspective I am writing in. Or the "well the [insert non-equivalent chassis here] ran these rates." It just doesn't work that way.

I also never said anyone must run flat-ride. I wouldn't even know how to calculate flat-ride. The "10%" split everyone likes to mention is just a general rules of thumb and it's actually quite a bit more complicated than that. Hitting curbs at 110mph might actually require a higher front natural frequency for all I know. Depends on the car geometry quite a bit too.

I am only looking at frequencies, and crunching the numbers, to show RELATIVE stiffness of car A vs car B. This done to show people how they can expect their car to handle rather than blindly making statements like "race cars run these rates. I know because I asked them." If you want to argue that under-steer is faster than so be it. I could re-iterate some of the counter-points I've already presented to that notion but you can read back and watch the physics and vehicle engineers explain it first hand. Or, read some of the race car engineering books like I have.

Originally Posted by tsk94

Edit:
Can a flat-ride setup be made to work decently well on track? Yes, of course. If you have a good shock to accompany flat-ride spring rates, and adjust suspension setup according (no rear sway bar, big front sway bar), tweaked alignment compared to 'traditional' spring rate setup, and so on. I believe I've stated many times, that a flat-ride style setup is not optimal for serious track/pure race use. I never said it was absolute garbage or the car will be undriveable.

Not sure what you're referring to when you say a "flat-ride" cars have to be tweaked compared to a "traditional" setup. Unless by "traditional" you mean "race car setups" and by "not optimal" you mean that it is YOUR interpretation that all of these traditional race cars run under-steer oriented spring rates to produce faster lap times... I think it's clear that I disagree with that interpretation. But again you can read-back to some of my counter-points to that.

Other considerations... The M3 GT4 car would've ran 3.5* front camber and a 9.5" front wheel with ET25 offset. Rear wheel was 10" with ET40 offset. This moves the front track width out up front and the rear track width is made narrower. This was to fit FIA wheel size regs as well as the factory body panels. This is something that tracked E92 M3's are also forced to do. Both track width and camber will play minor rolls in effectiveness of the spring rates but the point is that the GT4 options make the front springs less effective. Yes, you're right that there is much more to consider than just basic frequency calcs when it comes to overall roll couple distribution and "handling." But that is not the point of the thread.

Also consider the M3 GT4 ran a different rear sway bar but the same stock front bar. I am seeing it was made by Eibach? I can only imagine that the GT4 sway bar is solid and stiffer. I can't imagine any other reason they would run an aftermarket rear sway bar that is the exact same diameter as the hollow stock rear bar (22.5mm). Then again, I see people saying the Eibach bar is hollow too. Doesn't make as much sense but it's a pretty baseless observation that it's said to be hollow. Again, just to continue the discussion around the E92 M3 GT4. There is quite a bit different about the car despite it being a modestly modified class. I don't know of anyone only replacing the rear bar of the E92 M3, but, that is what BMW did with the GT4?

[tsk94]

Originally Posted by bbnks2

Yes, I asked if you can post them. Then we can actually compare/contrast things on a factual basis on not a a "word-of-mouth" basis. I kept my calcs simple as they were just conceptual and didn't look at weight distribution and other factors, but, you would definitely need those factors to get realistic GT4 numbers.



I can't seem to find the source again where I got the M3 GT4 weight from @24xxlbs. I might've grabbed GT2 weight or something. That was not the point though. The point was that the weight distribution of the E92 M3 GTS, and race cars in general, is more forward vs the production car. Same as if you gut an E92 M3 for tracking. Weight reduction moves the weight distribution (especially with driver) forward which then affects how much spring you need in the front and rear, obviously. So everyone harped on that because I wrote the wrong number but missed the point as to why it's relevant in regard to choosing spring rates...

E92 M3 GT4:
Front 0.960^2
Rear 0.8133^2 (true-rear coil-over)

To calculate Front wheel rates: SR*(0.92)
To calculate Rear wheel rates: SR*(.66)

Spring rates per E92 M3 GT4 catalog(http://www.bimmerfile.com/wp-content...talog+US.pdf):
Front: 140-60-160 main spring (.2k helper spring) with optional rates of 18k and 20k
Rear: 160-50-130 main spring (2k tender spring) with optional rates of 15k and 17k

I do see the official weight was put at 1430kg = 3152lb. I'll use that weight and figure weight distribution with 150lb driver to be around 57/43.

So, using the shelf rates of 16k/13k I get
3.16/2.66Hz or 18.80% stiffer front

Using the stiffest combination of 20k/17k I get:
3.53/3.04 or 16.12% stiffer front

Using the softest front and stiffest rear of 16k/17K I get:
3.16/3.04 or 4.51% stiffer front

Using the stiffest front and softest rear 20K/13K I get:
3.53/2.66 or 32.70% stiffer front

Quite frankly, I am making a lot of assumptions to even get this far, and excluded things from the calculations which would make the actual effective spring rates more accurate, so, I make no claim that these numbers are perfect for this specific car. I only post numbers to show the relativity of a given spring rate on different chassis... To show that Bilstein Clubsport rates used across three different chassis will produce three differently handling cars... to make the point that if you're not considering these factors then you may end up with a much differently handling vehicle than you intended.



I have no idea how your car handles. You didn't share any relevant details for me to make assumptions on your cars overall handling beside spring rates. I am merely discussing spring rates and how spring rates affect pitch motion.

Going back to what I said:

You run a true-rear coil-over so the statement: "if you're running e92 m3 gt4 spring rates on a regular old e92 m3 you're doing it wrong" doesn't apply to you...

I shouldn't have said "your proof" in a reply. It was a general statement. A general statement said within the context of this thread and other threads where people advocate for running spring rates that they do not understand because "race cars run these spring rates." Dive into it and these race cars are NOWHERE near as under-steer oriented as some of these enthusiast builds and off-the-shelf coilover spring rates. That's a fact. BC called for 10k/10k. Ohlin's called for 7k/6K? and now 8k/14k. Bilstein called for 10k/14k... JRZ calls for 10k/14k in a divorced rear spring rate... MCS calls for 10k/14K in a true-rear coil-over. The spring rates are all over the place. Who is right? Which one did you call and speak to? Slap your own custom springs onto these coilovers and now the damping is all screwy...

I also have 0 confidence that the people you spoke to necessarily understand half this stuff either. You also assume I've never spoken to anyone? My anecdotal experience counts for nothing but yours counts for everything because??? Every race book in existence mentions tuning around spring frequencies and how to do it. Anyone can be a race car driver/pit crew member. All it takes is racing fees and enough money to buy a production race car. There are race car drivers with 0 experience competing in SRO. Being on a race team doesn't mean they understand whether or not the spring rates the car runs is under-steer biased or not. Believe me... I've spoken to race teams too and they can be just as clueless as the rest of us. Same goes for the representatives of businesses. You think the chief engineer grabbed the phone at KW to discuss spring rate choices with you? Sorry to present this negative viewpoint, because there are knowledgeable people out there too, but it's reality.

Let's continue with the E92 M3 GT4 example and compare it to how you have your car sprung:

Using your cars 800/571 (roughly 14k/10k) spring rates, 3550 weight, and a presumed 53/47 weight distribution I get:
2.99/2.21 or 35.29% stiffer front

Yes, you are at the front biased end of the spectrum of what a GT4 car would've ran (spring rate wise) and then some. This thread is not about race cars though nor is it about how to set up your M3 to be a replica GT4 spec car... Op asked about pitch and why off-the-shelf coilover kits ship with really low rear rates that you would think would induce pitch problems. Op is correct. The spring rates are likely a compromised to keep the average enthusiast happy with their street car on rough roads. Got nothing to do with race-cars or performance. Damping can make up for poor rates whether it be on the street or the track.



Again, I was just making a general statement that people blindly copy other cars spring rates and that they have no real understanding or justification of WHY those cars run the rates that they do. The only reason why we are even mentioning race cars (as we did in the other thread too) is because people like to say "oh well race cars run these rates so they are the best." Uhh not exactly... you drive a regular E92 M3 with a divorced rear spring and none of the other modifications that a race car does that actually warrants running the quoted rates... That's the perspective I am writing in. Or the "well the [insert non-equivalent chassis here] ran these rates." It just doesn't work that way.

I also never said anyone must run flat-ride. I wouldn't even know how to calculate flat-ride. The "10%" split everyone likes to mention is just a general rules of thumb and it's actually quite a bit more complicated than that. Hitting curbs at 110mph might actually require a higher front natural frequency for all I know. Depends on the car geometry quite a bit too.

I am only looking at frequencies, and crunching the numbers, to show RELATIVE stiffness of car A vs car B. This done to show people how they can expect their car to handle rather than blindly making statements like "race cars run these rates. I know because I asked them." If you want to argue that under-steer is faster than so be it. I could re-iterate some of the counter-points I've already presented to that notion but you can read back and watch the physics and vehicle engineers explain it first hand. Or, read some of the race car engineering books like I have.



Not sure what you're referring to when you say a "flat-ride" cars have to be tweaked compared to a "traditional" setup. Unless by "traditional" you mean "race car setups" and by "not optimal" you mean that it is YOUR interpretation that all of these traditional race cars run under-steer oriented spring rates to produce faster lap times... I think it's clear that I disagree with that interpretation. But again you can read-back to some of my counter-points to that.

Other considerations... The M3 GT4 car would've ran 3.5* front camber and a 9.5" front wheel with ET25 offset. Rear wheel was 10" with ET40 offset. This moves the front track width out up front and the rear track width is made narrower. This was to fit FIA wheel size regs as well as the factory body panels. This is something that tracked E92 M3's are also forced to do. Both track width and camber will play minor rolls in effectiveness of the spring rates but the point is that the GT4 options make the front springs less effective. Yes, you're right that there is much more to consider than just basic frequency calcs when it comes to overall roll couple distribution and "handling." But that is not the point of the thread.

Also consider the M3 GT4 ran a different rear sway bar but the same stock front bar. I am seeing it was made by Eibach? I can only imagine that the GT4 sway bar is solid and stiffer. I can't imagine any other reason they would run an aftermarket rear sway bar that is the exact same diameter as the hollow stock rear bar (22.5mm). Then again, I see people saying the Eibach bar is hollow too. Doesn't make as much sense but it's a pretty baseless observation that it's said to be hollow. Again, just to continue the discussion around the E92 M3 GT4. There is quite a bit different about the car despite it being a modestly modified class.

I think the misconception you have is that front biased spring rates mean understeer orientated setups - we both know it's not that simple.

I appreciate the time and effort you put into these posts, but I'm not sure what exactly you're trying to say, other then my setup (spring rate wise) is inherently flawed.

I've done my fair share of reading as well. There is plenty of documentation to support the notation that front-biased setups are optimal over a flat-ride setup for track use. This is also supported by countless conversations I've had with people from BMW Motorsports, KW, MCS, Bimmerworld, Turner, Falline and so on. Does that go beyond the scope of this thread, perhaps. But in my initial posts I was referring to track setup and clearly stated that for more street orientated use flat-ride definitely has it's place and purpose.

As this has gotten off-topic from the original thread, I'd be happy to discuss it further with you over PM if you're interested. You're clearly knowledgeable and make good points but we seem to have different philosophies and approaches to suspension setup.

[bbnks2]

Originally Posted by tsk94

I think the misconception you have is that front biased spring rates mean understeer orientated setups - we both know it's not that simple.

I appreciate the time and effort you put into these posts, but I'm not sure what exactly you're trying to say, other then my setup (spring rate wise) is inherently flawed.

I think you're reading everything I said wrong lol. I feel like I am repeating myself but again, I am only talking about the springs in isolation. And again, I never said your car under-steers. Not sure how else I can better hedge my comments as I always do.

Do you have any scientific or objective data you can quote to support that a relatively stiffer front spring rate produces faster lap times? This is off topic indeed but since you specifically mentioned that as a benefit of moving away from neutral balanced spring rates I'd like to learn more.

[tsk94]

You make a good point that a lot of the 'professionals' are clueless. That is true. So my blanket statement of how speaking to them as being useful can be misleading. While plenty are clueless, I have spoken to individuals from the teams/companies that have been knowledgeable, smart people - like anything it's about sorting relevant information for your application and being able to apply it properly for yourself. Hope that clears it up a bit..

Edit:
To your above comment, if you'd like to discuss it more then send me a PM, happy to talk.

[bbnks2]

Originally Posted by tsk94

like anything it's about sorting relevant information for your application and being able to apply it properly for yourself. Hope that clears it up a bit..

That's all I am trying to do is make random bits of info provided relevant.

Nothing is clear when you have aftermarket solutions providing for such a wild array of rates lol. Doesn't seem like any other platform varies this much.

[gmx]

If you take it to PM, be happy to share my calcs... I assumed a 53:47 weight distribution for the GT4. I have not included fuel (110L tank for endurance).

I'm not going to quote the posts but some things I feel need correction. I agree about scrub radius, except they also ran spacers; 5mm & 15mm respectively so that offset is true for the wheel itself only and not including the spacers. Missing details is really going to lead you down a bad path when and if you start simulating.
When you say this:
"Front: 140-60-160 main spring (.2k helper spring) with optional rates of 18k and 20k"
-What do you mean 18k (& 20k)? It's a 140mm, 60mm ID, 160N/mm main spring with a 60mm length, 60mm ID, 2N/mm helper. It's not that straight forward calculating dual-rate like that. Simplistically, rate will be a step function based on compression height, main spring rate alone then the combined rate (dual-rate formula). Or, since you play with wingeo/susprog3d - maybe those model it?
Any citation for the Eibach hollow/solid bar? The rules IIRC said same bar as factory. The only change that could be made was the arm length for triple adjustable and that was it. Could've been a pretty good / interesting move to change the gauge of the tube or go full solid to circumvent this rule somewhat. That's pretty much motorsport, as I said above, the details.

FWIW, the KW/Bilstein kits also have a 20N/mm rate on the helper/tender, not sure about others. People that have removed the helper/tender then say the car handles the same are a prime example of why grassroots/casual track day attendee feedback on forums/elsewhere should really be taken with a grain of salt. I use both helper/tender because even the industry disagrees on this. Some will call a "low-rate"/zero-rate a tender for some reason. Eibach refuses to and calls it a helper instead.

I will finally measure stuff like rear unsprung weight with a linear spring since weighing individual components then applying rule of thumb ratios is a bit hit or miss. Well not really, I was only 365grams off on the front. The rear has half shafts and crap etc.
FWIW, with a Bilstein CS damper and OE discs/calipers/pads (down 6mm), my front is 54.2kg. That's with a Rays ZE40 ET21 18x9.5 wheel & 265/35/R18 AD08R

[bbnks2]

Originally Posted by gmx

If you take it to PM, be happy to share my calcs... I assumed a 53:47 weight distribution for the GT4. I have not included fuel (110L tank for endurance).

I'm not going to quote the posts but some things I feel need correction. I agree about scrub radius, except they also ran spacers; 5mm & 15mm respectively so that offset is true for the wheel itself only and not including the spacers. Missing details is really going to lead you down a bad path when and if you start simulating.
When you say this:
"Front: 140-60-160 main spring (.2k helper spring) with optional rates of 18k and 20k"
-What do you mean 18k (& 20k)? It's a 140mm, 60mm ID, 160N/mm main spring with a 60mm length, 60mm ID, 2N/mm helper. It's not that straight forward calculating dual-rate like that. Simplistically, rate will be a step function based on compression height, main spring rate alone then the combined rate (dual-rate formula). Or, since you play with wingeo/susprog3d - maybe those model it?
Any citation for the Eibach hollow/solid bar? The rules IIRC said same bar as factory. The only change that could be made was the arm length for triple adjustable and that was it. Could've been a pretty good / interesting move to change the gauge of the tube or go full solid to circumvent this rule somewhat. That's pretty much motorsport, as I said above, the details.

FWIW, the KW/Bilstein kits also have a 20N/mm rate on the helper/tender, not sure about others. People that have removed the helper/tender then say the car handles the same are a prime example of why grassroots/casual track day attendee feedback on forums/elsewhere should really be taken with a grain of salt. I use both helper/tender because even the industry disagrees on this. Some will call a "low-rate"/zero-rate a tender for some reason. Eibach refuses to and calls it a helper instead.

I will finally measure stuff like rear unsprung weight with a linear spring since weighing individual components then applying rule of thumb ratios is a bit hit or miss. Well not really, I was only 365grams off on the front. The rear has half shafts and crap etc.
FWIW, with a Bilstein CS damper and OE discs/calipers/pads (down 6mm), my front is 54.2kg. That's with a Rays ZE40 ET21 18x9.5 wheel & 265/35/R18 AD08R

The spec sheet shows three available front springs and 3 rear springs.

160N/mm (16.3 kg/mm) in 160mm length and a 60mm ID is the stock front spring
180N/mm
200N/mm

They are not dual rate springs but I understand what you mean that the helper spring influences the spring rate (but only until it is bound). The front helper spring has no meaningful impact on spring rate at all. It has like 10lb/in spring rate and will be bound instantly. It's there to reduce bump travel. Something many people overlook. Put a 800lb spring up front and you'll have 4.5" of bump travel at the wheel and only 1" of droop travel (just a rough estimate off the top of my head). The 60mm helper will reduce bump travel by a good 1.5" or so bringing you closer to a 55/45 bump/droop distribution.

The rear tender is about 100lb/in. It's going to be bound as soon as you begin lowering rear of the car onto the ground too. The rate as I understand it is more for bump/droop distribution. No, I dont think you would "feel" removing the tender spring at all just driving around as it plays zero role at static ride ride height and in bump. But toss the car into a corner and the rear will have much more bump travel in roll than it did than with the tender in place. With the tender, assist spring, helper, whatever you want to call it in place the rear will get into the bump stop sooner which changes how the car will handle when loaded in a corner (stiffer). Just conceptual... not saying I know what the bump/droop travel is of the e92 m3 gt4.

I should measure the unsprung weight as I have my entire car in pieces to finish up the m3 spindle conversion. Just got my struts back from being revalved and converted to M3 mounts. I'm more interested in putting it back together at this point than tearing it apart more just to weigh crap. I'm over it at this point lol.

[gmx]

Ah ok, thought you were adding 2kg (or .2kg) to the main spring (rounded / integer) of 16kg. Np, read it wrong.

[//steve\\]

I've found reading all of this interesting but I sure hope some n00b doesn't stumble across this because they will be thoroughly confused.

All this info is interesting to me but it seems like you're down to details that are chasing after those last tenths.

The hard part is that bad setups can still often be driven very quickly in the right hands so it's easy for someone to come in and say hey I have some of the fastest lap times at track X and this is what I run. Maybe they are just a better driver who could be even faster on a proper setup.

It is quite interesting to me to see that rates are all over the place. I was just chatting with someone I following in Europe who recently upgraded his KW Clubsports to the same rate front and rear. Maybe it's a bandaid to a poor setup, maybe not.

I'd really love to know what rates the Team Schirmer cars run. It's hard to argue their success.

[IamFODI]

Originally Posted by //steve\\

I'd really love to know what rates the Team Schirmer cars run. It's hard to argue their success.

Aren't they also running some custom suspension geometry, or do I have them confused with someone else?