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Originally Posted by xBlueStreakx

I wish I had a definitive answer to your whole post. I'm still trying to figure things out...

Yeah, I understand. Finding good info on suspension tuning is like finding a needle in a haystack. Orb, Fel1x, and others have provided us with info that is as good as it gets when trying to make informed suspension decisions though.

Here is a pretty good read on the suspension geometry principles Fel1x tested: http://mk3ukr-supra.net/SCC%20-%20su...20part%204.pdf

Originally Posted by xBlueStreakx

With the 672lbs rear, the car is very very planted mid corner and corner exit (understeer... and it drives me nuts). I can get it to oversteer on entry if I'm very aggressive with the trail braking but I don't like this driving style. BTW, I have the stock front swaybar and dont' plan on changing it just yet.

At 6k/12k (336/672), I feel like the front end is unnaturally stiff compared to the rear and on paper it is. I feel like I am driving a boat or something where the rear squats but the front end doesn't budge.

Maybe I am just biased though because I ran my 128i with a spring rate of 200/600. The car was absolutely amazing to drive on track. I got many compliments from instructors on how well balanced the car was. I also used a smaller E92 front sway on that car only in an effort to offset some of the additional camber change caused by running 235 squared tires.

Originally Posted by xBlueStreakx

This topic has been discussed quite heavily by Orb and fe1rx (check out their threads). Orb eventually settled on 1008lbs out back while fe1rx went to 800lbs. Front spring rates were similar to my setup.

Yes, I see where they both attempted to re-balance their cars by moving to a closer F:R effective wheel rate. Seems to have been motivated by feeling the car was pushing too much and then calculating that with rates like 400/700 the rear wheels are still massively under-sprung relative to the front.

Originally Posted by xBlueStreakx

I did further research on this matter and looked at E46 race cars and owners recommend a 200lbs split front/rear no matter what. i.e. 600F/800R, 700F/900R and so forth. Motion ratios are similar, if not identical to our suspension as well which means the theory on wheel rates/frequencies is taken with a grain of salt.

I hate the typical "oh well it worked for this person/car so I'll do it too." I am not sold that many of the people making recommendations have any clue what they are talking about. A blanket "200lb" difference is terrible advice that ignores the proportionality of suspension geometry variances.

There are many people who will swear by tuning suspension by effective wheels rates. By taking the cars suspension geometry into consideration you can directly determine how well your chosen spring rate will work for you. The concept makes sense imo... Choose a neutral spring setup... then tune the rest of the suspension to make the car do what you want. Match how stiff of a spring you choose (frequency) to how much grip your tires can generate (don't choose a 2.5hz spring rate when you're rolling around on all seasons lol)... It's certainly a better approach than "well the x car ran 1000lb front springs and 500 rear." Maybe that car could've shaved 20 seconds off it's lap time too if it had been setup properly lol

A quick search tells me the following:
E30/36/46 front: .94^2 = .88 (1000lb spring gives 880lb wheel-rate)
E36/46 rear: .65^2 = .423 (1000lb spring gives 423lb wheel rate)
E30 rear: .67^2 = .45 (1000lb spring gives 450lb wheel rate)
E46 M3 = .9604/.4356 = 2.20

E46 are 1:2 (.88/.423) ===> 400(.88)=352 and 600(.423)=253.8
E90/E82 are 1:3 (0.9216/0.316969) ===> 400(.9216)=368 and 600(.316969)=190

The E82 and the E46 DO NOT have the same motion ratios. An E82 would have a massively greater variance in F:R effective wheel rates than an E46 would at the same 400/600 spring rate. The E46 would have a frequency of close to 2.1/1.9hz and the E82 would have a F:R frequency of 2.16/1.63... An E82 would need a spring rate of 380/825 to match the same performance as an E46 running 400/600 springs... 400/600 doesn't look all that bad for an E46, but it looks heavily unbalanced for an E82.

To get back to blanket statements like "maintain a 200lb difference," for an E82, a 100/300 split would result in a rear spring that is effectively 6% stiffer than the front (oversteer/flat ride frequency). With a 900/1100 split you'd end up with a rear spring that is effectively -47% weaker than the front (a heavily understeer biased spring frequency).

These examples highlight the importance of calculating wheel rates and frequency to at least get a baseline of how to make meaningful changes to the suspension...

Originally Posted by xBlueStreakx

Other sites I have looked into suggest a very stiff front spring because McPherson suspensions work best when they don't work at all given their bad camber gain characteristics. By not working at all, I mean no body roll (stiffly sprung). This helps support the E46 race car spring rates I've seen.

I absolutely agree that, TO SOME EXTENT, you want to reduce the negative side effects of a McPherson style front suspension. However, how can you determine if you're stiffening the front relative to the rear if you're not taking the cars suspension geometry into account?

Also, I fail to see how keeping the front of the car unnaturally stiff produces "better" handling when coupled with the other typical mods we do to the E82. Yeah sure, by eliminating body roll you prevent dynamic camber changes. However, eliminating DIVE also means you kill all the BENEFITS of a McPherson suspension. Dive under compression causes a favorable change in toe which aids in turn-in (toe-out).

E82 owners are already doing mods that help reduce dynamic camber changes such as M3 control arms, lowering ride height, and sway bars. Stiffening up the front, and reducing bump travel, is also going to reduce dynamic camber change. Why also make the front un-naturally stiff in proportion to the rear?

Originally Posted by xBlueStreakx

I had some back and forth private messaging with Harold at HP Autosport and he flat out said he doesn't recommend going higher than 800lbs in the rear despite the math. Maybe he doesn't want to recommend something that may end up with me damaging the car? At any rate, I heeded his advice and went with 784lbs swifts out back for 2017 to take things incrementally instead of effing up the chassis dynamics and having to backtrack.

No idea... but power oversteer should be controlled with your right foot and not with stiff front springs that cause understeer. BMW already dialed in "reverse rake" from the factory to aid rear traction and prevent snap oversteer. FR-S, BRZ, and Miata owners are using higher effective wheel rates on their lighter RWD cars than us E82 owners are lol... We are more tire limited though for the power we put down (without going staggered).

Originally Posted by xBlueStreakx

TL;DR. I'm at 392F/784R with an itch to also go stiffer at the front given the E46 guys and their setups.

You're following down a bad path with that logic imo. From the threads I've seen, this is what people were doing back in 2008. They were assuming the 1 series had the same motion ratio as the E36 and they were sticking to their stiffer front than rear setups like 600/400. Apparently, even tc kline recommended 450/300 back in 2008... Look how far we've come. Don't go backwards

Fel1x ohlins suspension thread details all the dynamic changes the suspension goes through for a given ride height. After doing the easy performance mods like control arms, sway bars, springs, and lowered ride height, there is no way you need to to also unbalance the spring rates... The deflection Fel1x measured wasn't even that bad in stock form. Static camber alone could pretty much compensate for dynamic camber change even before the aforementioned mods. Dynamic toe changes would be decreased with the stiffening of the front subframe and control arms.

Although Miata's don't have McPherson front suspensions, they typically run higher front spring rates than rear. Why? Because the front motion ratio is LESS than the rear. You need stiffer springs up front than in the rear to create a neutral balance. On an ND Miata, the front motion ratio is 0.69:1 and the rear is 0.75:1. https://www.edmunds.com/mazda/mx-5-m...minations.html

here is another thread discussing tuning Miata race cars: https://forum.miata.net/vb/showthread.php?t=594291. The poster did all the calcs in the first post but was given bad motion ratio numbers. Later posts go on to explain the front motion ratio is LOWER which is why the aftermarket targets higher spring rates in the front for ND Miata's.

On a Miata a, 9kg front springs produce a frequency of 1.96Hz and a 6kg rear spring produces a frequency of 2.12Hz. This is another example of how a stiffer front spring is only used on Miatas to produce a flat ride frequency. It is not used to produce a 2.5hz front spring rate and a 1.5hz rear spring rate like so many E82 owners are doing unknowingly.

An even better place to compare suspension tuning would be the FRS/BRZ forums. They are way more active than here and they too use a mcpherson strut with a multi-link rear. The aftermarket over there seems to be giving them front biased springs as well despite the logic saying you should run a higher rear spring rate. Many are reluctant to move to aftermarket suspensions out of fear of ruining the stock cars excellent balance (131/211 springs from factory supposedly).

If coilover manufacturers add in massive understeer to keep people from crashing their cars than that's one thing. I am looking for a neutral setup, for an already well balanced car, and that makes no sense to me regardless of what other people are doing.