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

[IamFODI]

My understanding is:

1. For most applications, you ideally want higher wheel rates in the rear than in the front to reduce pitching over bumps. This is part of what some call "flat ride".

2. Based on the motion ratios listed here, the spring rates for most off-the-shelf coilover kits from reputable companies are such that the wheel rates will actually be higher in front than in the rear. E.g., Bilstein PSS10: 514/799 spring rates, yielding 474/260 wheel rates (front/rear in lb/in).

If all of this is true, 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?

[thakid22]

Tuning spring frequency for flat ride is important. It sets the basis for how the chassis responds to varying conditions.
However, the bushings and shocks play a huge role here too. Flat ride simply means the chassis is inherently more stable.
Even with spring rates that don't conform to flat ride principles, a vehicle can ride flatly.
It simply means that you'll need more shock valving and accuracy of that valving to achieve a level ride.
If you get excess pitching, return the coilovers and get something else as they are broken, damaged, or defective.

[IamFODI]

Originally Posted by thakid22

Even with spring rates that don't conform to flat ride principles, a vehicle can ride flatly.
It simply means that you'll need more shock valving and accuracy of that valving to achieve a level ride.

Makes sense.

Any idea why Bilstein (or any other reputable coilover manufacturer) would sacrifice flat ride spring rates by jacking the front rates way up and then lean on damping to keep the car riding flatly? Sharper turn-in, better transient response...?

[gmx]

I've explored this a little and concluded the following:
- It's McPherson strut. You need the front roll stiffness (combination of springs + ARB). They cannot assume you have a stiffer ARB. Also, just because a given car has the same effective roll stiffness with various combination of springs/ARBs, doesn't mean it'll drive the same.
- Bilstein just like OE has a reputation to uphold. No one wants to be known as the 911 widow-maker.
- Stiffer front will less likely lift the diagonally opposite rear wheel in yaw. If you know how our diff works, you really don't want this situation. It's not as bad as open diff/torsen but it's still bad. I've not seen this talked about, could be wrong. Just a personal thought.
- Edit: No one seems to go over 140nm (800lbs) on the rear except possibly MCS only? I think the camber arc caused by the less than stellar motion ratio has a lot to do with this. The motion ratio varies throughout it's entire range which is why you see some suspension builders referring to "wheel rate spikes, unpredictable handling" It's also difficult, probably impossible to get TUV certified converting to rear C/O for more desirable wheel rates. The strut tower/floor are still unknowns, mostly a bunch of anecdotes where people have no issues or "haven't heard anything". Yeah we all know how those anecdotes worked out for the E46 M3. As an example, people never checked the floor properly. The subframe HAD to be lowered to inspect it properly. If you saw cracks without disassembly, the car was pretty much toast and needed a new floor. It shocked me that it seemed to take the community almost 10 years to finally accept this, check it[,mitigate] properly and stop the "I dOn'T HaVe aNy IssUes" bs because the fact is it happens to all of them.

Interestingly... KW recommends even stiffer front, 120nm esp on their competitions. Bilstein went to 100nm on their CS kit, up from 90 on the PSS10 & B16(?). I'm about to try 90nm and go shorter at the same time, which brings me to my next point:
- it is difficult to get tyre clearance to the lower spring perch. For example, I probably cannot use a 140mm 80nm spring on this damper as the travel taken up at static ride height would be almost 50% and potentially result in coil-bind. 140mm, 90nm I can probably get away with.

[//steve\\]

Interested to see where this thread goes. I'm new to the E9x chassis but suspension setup/tuning is probably the thing I'm most interested in learning.

Spring rates seem to be all over the map and I'm not quite sure just yet what rates are more street vs track. I know the shocks have a LOT to do with it.

FWIW my car is currently on Bilstein B16 damptronic (which should be the same springs/rates as the PSS10). To the OP I believe Bilstein runs progressive springs on all of these so the listed rate is hard to compare to the linear rates that nearly everyone else uses.

One of the things I've been trying to search after is what do those progressive rates translate too when compared to a linear rate spring in terms of performance driving. Ultimately with enough weight on the spring it's going to take on that listed upper spring rate but what does it take to get to that point?

I'm mostly after this answer because I'm not sure the B16 will be "motorsport-ish" enough for what I want out of the car but I'm having a really hard time trying to determine what spring rates would be ideal for me on a standard linear rate spring.

[IamFODI]

//steve\\, how flatly does your car ride? Any pitch over bumps at any speed?

[//steve\\]

Originally Posted by IamFODI

//steve\\, how flatly does your car ride? Any pitch over bumps at any speed?

I just haven't had enough time behind the wheel to make any kind of accurate statements about the way the car drives. I got it back in October and I've realistically only driven it 300 miles since then. I've been taking my time undoing a few things from the previous owner (namely supercharger removal) before I really get it out and see what the car can do.

Also being entirely new to the E9x I'll likely take it a little easy at first. I've spent the last 10+ years in S2000s so I'll need a bit of a reset. I'd say I'm a good driver but my personality lends to me slowly ramping up how I push instead of going ham the 2nd time I drive it.

Maybe I'm being skeptical for no good reason but the B16 damptronic rides so amazingly well on the road for "normal" driving when the suspension is left soft that I feel like it would be impossible for it to feel as precise as I want when pushing the car.

I ran Ohlins R&T on my S2000 for the last several years and absolutely loved them. Ride was firm but very compliant running what was about a 15-20% increase in spring rate over the off the shelf springs. 572/458 > 650/550.

I'm probably putting myself in a tough spot because I LOVE how great the car rides on the road but my benchmark is a perfectly balanced car that was easily 500lbs lighter so it's just not going to be the same no matter what I do although I'll be doing my best to try.

This car is also a weekend toy for me so I'm more than willing to give up a bit of street comfort for ultimate performance.

[gmx]

As you know you can't compare spring rate from car to car.
Figure out the wheel rates on your S2000. You spring the car for the tyre you run initially then work from there. Ie, a S/R-comp and slick (and type of slick for sure) will all have a different frequency "window"

[bbnks2]

Originally Posted by gmx

I've explored this a little and concluded the following:
- It's McPherson strut. You need the front roll stiffness (combination of springs + ARB). They cannot assume you have a stiffer ARB. Also, just because a given car has the same effective roll stiffness with various combination of springs/ARBs, doesn't mean it'll drive the same.
- Bilstein just like OE has a reputation to uphold. No one wants to be known as the 911 widow-maker.
- Stiffer front will less likely lift the diagonally opposite rear wheel in yaw. If you know how our diff works, you really don't want this situation. It's not as bad as open diff/torsen but it's still bad. I've not seen this talked about, could be wrong. Just a personal thought.
- Edit: No one seems to go over 140nm (800lbs) on the rear except possibly MCS only? I think the camber arc caused by the less than stellar motion ratio has a lot to do with this. The motion ratio varies throughout it's entire range which is why you see some suspension builders referring to "wheel rate spikes, unpredictable handling" It's also difficult, probably impossible to get TUV certified converting to rear C/O for more desirable wheel rates. The strut tower/floor are still unknowns, mostly a bunch of anecdotes where people have no issues or "haven't heard anything". Yeah we all know how those anecdotes worked out for the E46 M3. As an example, people never checked the floor properly. The subframe HAD to be lowered to inspect it properly. If you saw cracks without disassembly, the car was pretty much toast and needed a new floor. It shocked me that it seemed to take the community almost 10 years to finally accept this, check it[,mitigate] properly and stop the "I dOn'T HaVe aNy IssUes" bs because the fact is it happens to all of them.

Interestingly... KW recommends even stiffer front, 120nm esp on their competitions. Bilstein went to 100nm on their CS kit, up from 90 on the PSS10 & B16(?). I'm about to try 90nm and go shorter at the same time, which brings me to my next point:
- it is difficult to get tyre clearance to the lower spring perch. For example, I probably cannot use a 140mm 80nm spring on this damper as the travel taken up at static ride height would be almost 50% and potentially result in coil-bind. 140mm, 90nm I can probably get away with.

Just like you're saying people's comments have been anecdotal like saying "I don't have any issues" your own statements made in this post are just as anecdotal. Not to mention some of it is just plain incorrect.

Fact of the matter is that a more neutral balanced car is harder to drive. It makes the cars behavior much more dependent upon driver inputs. People's driving styles vary wildly but driver confidence is still needed to drive a car fast. I don't think I've ever come across any factual science to support that higher front spring rates relative to the rear makes a car go faster. So yeah I personally think ignorance of motion ratio and suspension geometry combined with an already common gravitation toward wanting under-steer characteristics sometimes ends up producing HEAVILY under-steer biased setups and head scratching spring rates...

Originally Posted by gmx

As you know you can't compare spring rate from car to car.
Figure out the wheel rates on your S2000. You spring the car for the tyre you run initially then work from there. Ie, a S/R-comp and slick (and type of slick for sure) will all have a different frequency "window"

You actually can compare spring rates between cars. So long as you understand the basic differences between various suspension designs... Wheel rate and natural spring frequency calculations are a static look at the effective spring rates of a vehicle. It's not perfect but by considering the vehicles corner weight and motion ratio you absolutely can get an idea of how relatively stiff one car is compared to another. Doesn't mean that they will behave the same dynamically but it does give you a static look at relative stiffness.

S2000 run fairly even spring rates front and rear... and they have fairly even motion ratios and weight distributions front and rear. Go figure! No one in an s2000 is running 400lb front spring and 800lb rear springs. They run 100N/mm / 80N/mm (ohlins rates) and motion ratio is .7/.67 or something like that. Weight distribution being fairly close to 50/50. KW runs 80N/mm F:R and their clubsports run 100N/mm F:R. Again... makes perfect sense for that car.

To get your M3 to be similarly sprung as an Ohlins S2000, you would run 100N/mm / 165N/mm accounting for the motion ratio and weight of the M3. That is roughly 572lb/in front and 950 lb/in rear. That is a slightly under-steer oriented setup as is. Anyone else surprised that I ended up at 572/950?! To be closer to the S2000 KW Clubsport rates of 100/100N/mm rates you would run 450/950!

There is absolutely no reason you can't run 1000lb spring out back. Why people are so scared to is beyond me. I will add my anecdotal experience that I have 336/896lb springs in my 135i that has a complete E92 M3 rear-end in it and it feels fantastic. yes, playful and I can rotate the rear of the car by simply lifting off the gas mid-corner. This is completely normal behavior though hence the caution to "never lift" in a rwd car... The rear-end is still relatively soft though, overall, and I get down onto the bump stops just accelerating down the road (verified with a camera)... 1000lb is NOT stiff since the rear motion ratio is something around ~.35. The only issue is that once you start getting over 1000lb/in the spring gets very hard to select as the usable stroke gets smaller. You either have to move to a larger diameter spring or a longer one... and that is generally not an option. Moving to a true rear coil-over allows the use of lower spring rates (more stroke), and even helper springs (dial in bump/droop), but that requires reinforcing the rear upper strut mount. This really only becomes relevant for cars that require effective wheel rates of over 400lb/in... cars with aero and slicks really need to be running upwards of 1500lb/in springs with the stock divorced spring setup. True rear coilover pretty much becomes a necessity. Yet we see e92 M3's with big wings and rcomps running 600lb rates lol... these people think they are doing them-self a favor by keeping the rear-end "soft" but in reality they are probably riding around on the rear bump stops from pure aero down-force alone and basically running infinite spring rates. Then they can;t figure out why their car is so hard ot drive and squirrelly at the limit...

Also, E46's tore the chassis floor regardless of spring rates. It was the simple fact that there was a structural issue with the rear-end. Track an E46 with bone stock spring rates and you'd crack the rear floor pan just as quickly as if you ran 1000lb springs.

Originally Posted by gmx

- Edit: No one seems to go over 140nm (800lbs) on the rear except possibly MCS only? I think the camber arc caused by the less than stellar motion ratio has a lot to do with this. The motion ratio varies throughout it's entire range which is why you see some suspension builders referring to "wheel rate spikes, unpredictable handling" It's also difficult, probably impossible to get TUV certified converting to rear C/O for more desirable wheel rates.

MCS runs a true rear coil-over so their 800lb rear spring rate produces the same effective spring rate as if you ran around a ~1500 lb spring in the stock location... So if you're happy running 600/800 MCS spring rates then you should be happy running 600/1500 spring rates in stock location. Yet, that is somehow thought of as being too stiff and impossible! lol. How anyone could advocate for running an 800lb spring supported by the weak un-reinforced strut tower while also refusing to run over 800lb/in in the stock location because of non-existent rear chassis failures is beyond me...

Motion ratio varies throughout it's range? Yes, it absolutely does change dynamically but idk where you are getting from that it varies massively. This is pure conjecture on your part. BMW builds a remarkably well designed suspension. The multi-links keep everything moving in a fairly constant arc. If you don't believe me, go look up the empirical data provided by fe1rx on 1addicts. The rear multi-link does have significantly better camber recovery than the front mcpherson strut. That would have more of an impact on your static alignment settings than your choice of spring rates though...

TUV ratings? not even sure where you pulled this one from but if TUV was an issue then Ohlin's and other European manufacturers wouldn't be selling kits with 60/160 N/mm and much higher rates for the M235i and other F-series cars. The M235i uses the same HA-5 multi-link rear end and has damn near the same motion ratio and vehicle dynamics as an e8x/e9x so the spring rates between these two chassis should be very similar.

[//steve\\]

My gauge comparing the S2000 to E92 actually had nothing to do with the actual rates used more so than saying Ohlins determined that X is the ideal rate for the S2000 for a do it all off the shelf kit and they determined Y is the idea rate for the E92 off the shelf kit.

I know I'm making a big assumption here but if I had to guess the Ohlins R&T is mostly geared to feel about the same from one car to the next. I can't imagine that for say the S2000 they tune them to be super sporty but then on the E9x they tune them to be super soft and comfortable. That said I know the Ohlins R&T is really IMO a mostly road, little bit of track type setup out of the box and if you want it to be more equally road and track you'll need to bump the spring rates a bit. Probably doesn't apply to a total novice driver that isn't comfortable pushing the car but with a little experience you'll find the off the shelf rates just too soft if you're running any kind of serious summer tire.

To further the point in the previous post, the F8x Ohlins R&T off the shelf run something like a 1200lb rear spring. That should sound crazy and you'd think based on rate alone it would ride terrible on the road but I can't fathom that out of anything Ohlins produces. Likely comes down to motion ratio of the F8x suspension.

My gut tells me I'm just going to have to get something and try it out and then make a change if needed. It seems like the vast majority of people don't really want to put that much thought into it and just slap something on the car that (insert company name) has deemed to be ideal. Nothing wrong with that but nothing off the shelf is ever perfect.

[bbnks2]

Originally Posted by //steve\\

I know I'm making a big assumption here but if I had to guess the Ohlins R&T is mostly geared to feel about the same from one car to the next. I can't imagine that for say the S2000 they tune them to be super sporty but then on the E9x they tune them to be super soft and comfortable. That said I know the Ohlins R&T is really IMO a mostly road, little bit of track type setup out of the box and if you want it to be more equally road and track you'll need to bump the spring rates a bit. Probably doesn't apply to a total novice driver that isn't comfortable pushing the car but with a little experience you'll find the off the shelf rates just too soft if you're running any kind of serious summer tire.

I wouldn't assume ANYTHING... You can't assume that Ohlin's put even 1/2 a second of thought into the spring rates that they chose for the e8x/e9x. The off-the-shelf ohlin's rates were 343/400 lb/in since 2007 lol. Yet, here we are in 2020 and they have gone back and changed the off-the-shelf rates to 448/762 for the e8x and 1M... they apparently claimed that they never intended the 343/400 rates to be used on a 1-series as it's their e9x spring rate. This all new kit is the proper rate for the 1-series. People actually are buying into that explanation but anyone with half a brain knows that an e8x and an e9x are damn near the same suspension, bushings, ratios, yadda yadda. A 1M is an E92 M3 with a different shell lol. They just sold kits with poor spring rates plain and simple (imo). 400lb/in out back is actually softer than many e90 cars came with off the factory floor so how they can use this excuse is beyond me... I personally think 448/762 is still a poor spring rate but that's just my opinion.

Originally Posted by //steve\\

My gut tells me I'm just going to have to get something and try it out and then make a change if needed. It seems like the vast majority of people don't really want to put that much thought into it and just slap something on the car that (insert company name) has deemed to be ideal. Nothing wrong with that but nothing off the shelf is ever perfect.

The problem with "make a change if needed" is that the more you move away from the spring rates that a suspension shipped with the worse off your damping is going to be. Re-valving, changing spring rates, installation, alignment... it all becomes a lot of added expense and time of trial and error to fix something you paid a lot of money for...

[//steve\\]

Originally Posted by bbnks2

I wouldn't assume ANYTHING... You can't assume that Ohlin's put even 1/2 a second of thought into the spring rates that they chose for the e8x/e9x. The off-the-shelf ohlin's rates were 343/400 lb/in since 2007 lol. Yet, here we are in 2020 and they have gone back and changed the off-the-shelf rates to 448/762 for the e8x and 1M... they apparently claimed that they never intended the 343/400 rates to be used on a 1-series as it's their e9x spring rate. This all new kit is the proper rate for the 1-series. People actually are buying into that explanation but anyone with half a brain knows that an e8x and an e9x are damn near the same suspension, bushings, ratios, yadda yadda. A 1M is an E92 M3 with a different shell lol. They just sold kits with poor spring rates plain and simple (imo). 400lb/in out back is actually softer than many e90 cars came with off the factory floor so how they can use this excuse is beyond me... I personally think 448/762 is still a poor spring rate but that's just my opinion.

The problem with "make a change if needed" is that the more you move away from the spring rates that a suspension shipped with the worse off your damping is going to be. Re-valving, changing spring rates, installation, alignment... it all becomes a lot of added expense and time of trial and error to fix something you paid a lot of money for...

So what would you run for rates? Street tires only for what would be a 50/50 street/track car. I won't ever run massive aero and I won't even run slicks.

If I buy Ohlins I'm buying through 3DM who actually offer one free revalve so yes there is time lost in changing if they suck but what I would anticipate is making an educated guess at what might be ideal for me, getting some seat time with it, and then making one change after based on my seat time to get me pretty close to exactly what I want. I'm not trying to win anything but doesn't mean I don't want the most ideal setup I can put together.

This shouldn't be that hard to get to 95% good enough.

[bbnks2]

To answer the OP questions though:

If all of this is true, 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?

Imbalanced springs will oscillate in a naturally poor manner in isolation. Yes, dampers can be used to control pitch behavior and initial tire loading and transient behavior. What you cant do with dampers is change the cars behavior in steady state which is where mid-corner under-steer/over-steer characteristics will rear their ugly head. So yeah, spring frequency is only a small part of the tuning picture, but, it's almost always mentioned in handling discussions conceptually because it is the base that people should be working from to make informed decisions about how to change the car to handle the way they want.

This thesis goes through the math of suspension modeling within the context of damping: https://pdfs.semanticscholar.org/5f6...62a900cd26.pdf

[bbnks2]

Originally Posted by //steve\\

So what would you run for rates? Street tires only for what would be a 50/50 street/track car. I won't ever run massive aero and I won't even run slicks.

If I buy Ohlins I'm buying through 3DM who actually offer one free revalve so yes there is time lost in changing if they suck but what I would anticipate is making an educated guess at what might be ideal for me, getting some seat time with it, and then making one change after based on my seat time to get me pretty close to exactly what I want. I'm not trying to win anything but doesn't mean I don't want the most ideal setup I can put together.

This shouldn't be that hard to get to 95% good enough.

Barry is recommending the 8k/12k rates. He did some shaker rig testing which resulted in some custom valving to match those rates. I can't speak for why he oriented things around 8k/12k.

I know 6k/16k works well for me personally. I also have the stock e92 M3 sway bars on the car. If i felt I wanted more front roll stiffness I would throw an aftermarket front sway bar on the car. Easy change. These rates produce an even front and rear bump/droop travel distribution for me as well. I can't speak for the same kind of dynamics in regard to the Ohlin's. Not sure what the strut stroke is or the length of the bump stops. Ohlins M235i spring rates were released as 60n/mm / 160N/mm so maybe Google around for how peopel feel running ohlins with those rates. I know that some people ended up dropping the rear spring rate down to 12k purely because they felt the car was too stiff on the street. Not that leaving the front at 6k and having a pogo-stick rear-end helps with that but that is their choice.

[IamFODI]

Originally Posted by bbnks2

Imbalanced springs will oscillate in a naturally poor manner in isolation. Yes, dampers can be used to control pitch behavior and initial tire loading and transient behavior. What you cant do with dampers is change the cars behavior in steady state which is where mid-corner under-steer/over-steer characteristics will rear their ugly head. So yeah, spring frequency is only a small part of the tuning picture, but, it's almost always mentioned in handling discussions conceptually because it is the base that people should be working from to make informed decisions about how to change the car to handle the way they want.

This thesis goes through the math of suspension modeling within the context of damping: https://pdfs.semanticscholar.org/5f6...62a900cd26.pdf

Thanks for coming back to the original topic.

This all makes sense, as do gmx's comments about the potential value of higher front frequencies. What I don't understand is why so many (all?) aftermarket kits for this car seem to produce higher front frequencies while the factory setups on almost all cars have the higher frequencies in the rear.

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?

If the off-the-shelf spring rates in the aftermarket kits can be dismissed as incompetent, why hasn't a single aftermarket kit corrected that oversight?

[gmx]

Originally Posted by bbnks2

Just like you're saying people's comments have been anecdotal like saying "I don't have any issues" your own statements made in this post are just as anecdotal. Not to mention some of it is just plain incorrect.

Fact of the matter is that a more neutral balanced car is harder to drive. It makes the cars behavior much more dependent upon driver inputs. People's driving styles vary wildly but driver confidence is still needed to drive a car fast. I don't think I've ever come across any factual science to support that higher front spring rates relative to the rear makes a car go faster. So yeah I personally think ignorance of motion ratio and suspension geometry combined with an already common gravitation toward wanting under-steer characteristics sometimes ends up producing HEAVILY under-steer biased setups and head scratching spring rates...
The homologated race cars all do this (proper factory backed operations, not what you guys as a 1 person show call a "racecar" there) and books like racecar engineering mentions it. Anyway, I was looking for shorter , softer springs and went to my local Eibach dealer to pick them up. You really don't want to know what a former Carrera Cup Germany/Aus, V8SC Aus pro picked for his GTS. And for sure, no question and no doubt he is better than you. Also, they measured the KW GTS spring rate, it is nowhere near what is said on this forum and all over the internet.


You actually can compare spring rates between cars. So long as you understand the basic differences between various suspension designs... Wheel rate and natural spring frequency calculations are a static look at the effective spring rates of a vehicle. It's not perfect but by considering the vehicles corner weight and motion ratio you absolutely can get an idea of how relatively stiff one car is compared to another. Doesn't mean that they will behave the same dynamically but it does give you a static look at relative stiffness.

S2000 run fairly even spring rates front and rear... and they have fairly even motion ratios and weight distributions front and rear. Go figure! No one in an s2000 is running 400lb front spring and 800lb rear springs. They run 100N/mm / 80N/mm (ohlins rates) and motion ratio is .7/.67 or something like that. Weight distribution being fairly close to 50/50. KW runs 80N/mm F:R and their clubsports run 100N/mm F:R. Again... makes perfect sense for that car.
So you can compare spring rates after you calculate the effective wheel rates. Yeah, exactly what I said, lol


To get your M3 to be similarly sprung as an Ohlins S2000, you would run 100N/mm / 165N/mm accounting for the motion ratio and weight of the M3. That is roughly 572lb/in front and 950 lb/in rear. That is a slightly under-steer oriented setup as is. Anyone else surprised that I ended up at 572/950?! To be closer to the S2000 KW Clubsport rates of 100/100N/mm rates you would run 450/950!
You're ignoring the double wishbone front of the S2000.

There is absolutely no reason you can't run 1000lb spring out back. Why people are so scared to is beyond me. I will add my anecdotal experience that I have 336/896lb springs in my 135i that has a complete E92 M3 rear-end in it and it feels fantastic. yes, playful and I can rotate the rear of the car by simply lifting off the gas mid-corner. This is completely normal behavior though hence the caution to "never lift" in a rwd car... The rear-end is still relatively soft though, overall, and I get down onto the bump stops just accelerating down the road (verified with a camera)... 1000lb is NOT stiff since the rear motion ratio is something around ~.35. The only issue is that once you start getting over 1000lb/in the spring gets very hard to select as the usable stroke gets smaller. You either have to move to a larger diameter spring or a longer one... and that is generally not an option. Moving to a true rear coil-over allows the use of lower spring rates (more stroke), and even helper springs (dial in bump/droop), but that requires reinforcing the rear upper strut mount. This really only becomes relevant for cars that require effective wheel rates of over 400lb/in... cars with aero and slicks really need to be running upwards of 1500lb/in springs with the stock divorced spring setup. True rear coilover pretty much becomes a necessity. Yet we see e92 M3's with big wings and rcomps running 600lb rates lol... these people think they are doing them-self a favor by keeping the rear-end "soft" but in reality they are probably riding around on the rear bump stops from pure aero down-force alone and basically running infinite spring rates. Then they can;t figure out why their car is so hard ot drive and squirrelly at the limit...
"Requires" reinforcing? Right, so who has actually torn one or how did they conclude this? They probably saw the GT4 with a cage tied into those points and drew such conclusion. And then like usual forum misinformation it spreads on like parroting you're doing right here. Nice and conditioned. But hey, at least you're not conditioned into running stiff front rates others do!
Also, E46's tore the chassis floor regardless of spring rates. It was the simple fact that there was a structural issue with the rear-end. Track an E46 with bone stock spring rates and you'd crack the rear floor pan just as quickly as if you ran 1000lb springs.
Yet it took people 10 years to find out the hard way. Everyone on the internet was "checking" the floor the wrong way. I don't even know why it's called a sub-frame issue.



MCS runs a true rear coil-over so their 800lb rear spring rate produces the same effective spring rate as if you ran around a ~1500 lb spring in the stock location... So if you're happy running 600/800 MCS spring rates then you should be happy running 600/1500 spring rates in stock location. Yet, that is somehow thought of as being too stiff and impossible! lol. How anyone could advocate for running an 800lb spring supported by the weak un-reinforced strut tower while also refusing to run over 800lb/in in the stock location because of non-existent rear chassis failures is beyond me...
Not all the time, I've seen people run 700/1000 on non CO MCS rears. You say the un-reinforced strut tower is weak. Where's the evidence? Because someone else said so, right, got ya. On the other side, we have TT, AST, Intrax saying no issue

Motion ratio varies throughout it's range? Yes, it absolutely does change dynamically but idk where you are getting from that it varies massively. This is pure conjecture on your part. BMW builds a remarkably well designed suspension. The multi-links keep everything moving in a fairly constant arc. If you don't believe me, go look up the empirical data provided by fe1rx on 1addicts. The rear multi-link does have significantly better camber recovery than the front mcpherson strut. That would have more of an impact on your static alignment settings than your choice of spring rates though...
It's been calculated and the info is out there by fe1rx himself. Read and comprehend his thread properly. It was measured at 13 degrees between extremes which no one would probably face. Read the thread again. And what's better than a camera? A pot on the suspension actually measuring it which has been done here on a M4 running slicks/aero.
Never said anything about the multi-link camber recovery, weird flex but ok.

TUV ratings? not even sure where you pulled this one from but if TUV was an issue then Ohlin's and other European manufacturers wouldn't be selling kits with 60/160 N/mm and much higher rates for the M235i and other F-series cars. The M235i uses the same HA-5 multi-link rear end and has damn near the same motion ratio and vehicle dynamics as an e8x/e9x so the spring rates between these two chassis should be very similar.

You cannot change suspension pickup points nor location of the suspension (ie divorced stays divorced) and expect to breeze through TUV certification. F-series also have a MR of 0.67 and likely articulate better. Maybe that comfortably allows higher rates? Do you even realise what a product like "GC Weight jacks" really do?

Fundamentally I don't disagree with you although you have a very argumentative attitude - no clue where it comes from. Other times you're plain wrong. At least you've learned not to quote motion ratios as an exponent of 2 (or simplify this figure) ... which is not a ratio at that point anyway, lol


EDIT: And further replies in this thread are proof. You don't just "pick" spring rates, you pick a frequency. Any pro outfit will ask first, what tyre? Your answer will give them a frequency to shoot for. Aero being the only exception. Then go figure out your motion ratios, unsprung, sprung weights then pick your rates.
I was going to try flat-ride purely because I'm skeptical. It made sense at first, then further theory-crafting - not really but definitely not as much as aftermarket kits are offering. Then, I can't really change the front rate easily with my tyre/wheel combination and risking coil bind. I've started doing calcs on a dual/twin spring setup but this becomes ridiculously complex. My big question is trail braking. And most people don't brake late nor hard enough to give good feedback about it. I'd trust it if there was CAN/Motec output though.

[IamFODI]

Few more questions -- not trying to argue, just curious:

Originally Posted by gmx

The homologated race cars all do this (proper factory backed operations, not what you guys as a 1 person show call a "racecar" there) and books like racecar engineering mentions it.

Aren't these all much more heavily damped than off-the-shelf "street/track" coilover kits?

Originally Posted by gmx

Also, they measured the KW GTS spring rate, it is nowhere near what is said on this forum and all over the internet.

...Well? Are you going to tell us or what??

Originally Posted by gmx

My big question is trail braking.

How do you expect this to relate to front-vs.-rear suspension frequencies?

[bbnks2]

Originally Posted by gmx

Fundamentally I don't disagree with you although you have a very argumentative attitude - no clue where it comes from. Other times you're plain wrong. At least you've learned not to quote motion ratios as an exponent of 2 (or simplify this figure) ... which is not a ratio at that point anyway, lol

EDIT: And further replies in this thread are proof. You don't just "pick" spring rates, you pick a frequency. Any pro outfit will ask first, what tyre? Your answer will give them a frequency to shoot for. Aero being the only exception. Then go figure out your motion ratios, unsprung, sprung weights then pick your rates.
I was going to try flat-ride purely because I'm skeptical. It made sense at first, then further theory-crafting - not really but definitely not as much as aftermarket kits are offering. Then, I can't really change the front rate easily with my tyre/wheel combination and risking coil bind. I've started doing calcs on a dual/twin spring setup but this becomes ridiculously complex. My big question is trail braking. And most people don't brake late nor hard enough to give good feedback about it. I'd trust it if there was CAN/Motec output though.

I'm not being argumentative you're just merely choosing to read it that way. All I did was provide counter points for other reading the thread because I felt like your initial reply was perpetuating assumptions. Albeit some of what I wrote was written in a snippy way toward the end. It's the internet so text doesnt translate well to banter... i was expecting you to read past the sarcasm a bit. You cant be annoyed at my ancedotal counter point about the strut tower when I was replying to your post purely to point out the hypocrisy of the ancedotal comments you made about the stock spring location and the chassis breaking. Is this a mind fuck or what? I'm actually paraphrasing from Turner regarding the strut tower. Turner being one of the closest teams associated with bmws factory race car production.

I disagree with much of what you replied but no point in turning it into an argument because i think we're actually both on the same page. I do think you need to take a step back from what youre posting and realize you're writing a lot of assumptions, especially about the m235i, to try to prove me wrong/debate with me about things that we actually both seem to agree on. Something I have a bad habit of doing myself... debating the logic of "what ifs."

Not sure what 13* is referencing off memory, specifically, but, the angle of the spring has a minimal impact of motion ratio (if spring bowing is what your referring to). It's usually completely disregarded altogether by most people to simplify things because of how minimal the impact is. I'd think it has more affect on spring fatigue than changing the motion ratio dynamically. Yes, I know what articulating spring perches do (like ground control produced). They can compensate for 6* I believe of that spring bowing. Motion ratio was literally plotted though on a 1/4" basis and was shown to be fairly constant across the wheels arc. Definitely a complete hyperbole to insinuate that everyone runs completely botched spring rates in the rear because the suspension kinematics are just so poor that the motion ratio varies wildly as the wheel moves through it's dynamic range. That's just simply not the case. Especially not on an E92 M3 with balljoints in place of mushy bushings.

I agree that you don't just pick spring rates. I personally am sticking with spring rates that work well with my coilovers. That also turns out to be a decent spring rate for a non-aero car with decent tires. Not crazy high at all... the car is still soft in the grand scheme of things. I could easily double these rates. But like I said, then I'd need helper springs up front and I'm not anyone even sure anyone makes a 1800lb spring to use in the rear lol

My comment about being wrong on some stuff was mostly referring to your spring length and rate comments. Making a spring stiffer reduces its stroke. Making it shorter reduces its stroke. If you move to a softer front spring rate to try to get closer to flat ride then you'll be moving further from spring bind... not closer. Just because the spring is now softer doesn't actually mean is has less use-able stroke. Just means you'll be banging off your bump stop with less load. The spring itself will have more use-able stroke. Unless of course you preload the crap out of the softer spring to compensate for a loss in ride height or something.

Not sure what exactly I'm wrong about as none of your replies actually corrected me on anything. I am always open to criticism. It's just that you replied with a bunch of conjecture. Your comments about "what we do here" i am guessing is referring to here in the usa. "1 man show" doesn't mean anything to me without more context and just sounds bitter. You seem to be paraphrasing from things I said in a 1addicts thread about how most race cars actually have fairly neutral balanced spring rates. This was a comment made purely in response to the red herring argument made by people that: "all race cars run higher front spring rates."). I literally quoted public knowledge pertaining to m235iR (m240i evo) and m4 gts homulgated spring rates from world challenge to use as real world examples. World challenge being a sanctioning body that literally races these cars across the globe. World challenge literally being oriented around running factory backed race cars to keep costs down for race teams. M235iR was built by BMW pretty much just to compete in world challenge TC class. So, insinuating that these cars spring rates that I quoted are just screwy because of some "1 man usa racecar team" or whatever is just plain ignorant. BMW built the car. People bought them to race as is for 96,000 usd. Sorry to get off topic but you took things here lol.

If you recall, my point in that thread was that the lower rear spring rates the bmw's run is actually pretty high because they are all running true rear coilovers. So again, we both understand why that's significant and we are both on the same page there. Not much to argue. If people are running 700/1000 overseas with stock location springs then yeah my personal opinion is they don't know jack. Like you already said, an arbitrary 300lb/in "split" is absolute nonsense. No argument there.

Idk what you're looking at with the m4 gts rates but the off the shelf suspension sold to "convert" a regular m4 into a "GTS" are NOT the same rates as the factory production m4 GTS gets. Some people are running significantly different rates on their "M4 GTS" than BMW does. It's just further proof that no one seems to actually have an answer for what a "good" spring rate is on these cars.

[//steve\\]

I'm going to oversimplify everything from the few replies above into a single question that I can use to do my own further homework


So from what you guys are saying based on these principles I'd want 400/700 or 400/800 as my rates (don't put much thought into the actual numbers, it's more the gap between f/r I'm looking to understand the logic on). This is keeping in mind standard divorced rear setup.

[bbnks2]

Originally Posted by //steve\\

I'm going to oversimplify everything from the few replies above into a single question that I can use to do my own further homework


So from what you guys are saying based on these principles I'd want 400/700 or 400/800 as my rates (don't put much thought into the actual numbers, it's more the gap between f/r I'm looking to understand the logic on). This is keeping in mind standard divorced rear setup.

6k/16k is 336/896 which is more on the "neutral" end of the spectrum and is close to 2hz f:r. It's hard for me to say "these are the rates you should run" because there are so many wildly different spring rates being used on these cars and people all say their setup is fantastic. Some argue they want even more front spring rate than the 10-12k they already have up front. I am not debating that as with aero and slicks I can see it being needed... I am merely debating why they arent also increasing the rear to match...

For the e92 M3 I think 8k/18k (448/1008) would be good rates as it is a hair heavier than a 135i/1M. If the car really doesn't feel right you should be able to move around by +/- 2k without needing a re-valve at all. Maybe speak with Barry and get his 2c. When I asked why he oriented using around 8k/12k he said his shaker rig testing was not looking at handling and relative stiffness but rather developing his damping curve.

[//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.

[Obioban]

Originally Posted by IamFODI

Thanks for coming back to the original topic.

This all makes sense, as do gmx's comments about the potential value of higher front frequencies. What I don't understand is why so many (all?) aftermarket kits for this car seem to produce higher front frequencies while the factory setups on almost all cars have the higher frequencies in the rear.

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?

If the off-the-shelf spring rates in the aftermarket kits can be dismissed as incompetent, why hasn't a single aftermarket kit corrected that oversight?

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).