Rear Sway Bar Options

[Ginger_Extract]

Hey y'all, thinking about playing with the rear bar, again, on my 1'er. I currently have the 15mm OEM 335xi bar, and while it's an improvement in the car's handling balance, I'm still fighting push. I'm already running a 700 lb-in rear spring, and I'd prefer to not go stiffer there. Going to also try stepping down front spring rate, again, this time from 336 to 280 lb-in.

That said, I wanted to confirm what adjustable bar options are available to us. Is UUC the only game in town for an adjustable bar?

[berns]

Originally Posted by Ginger_Extract

Hey y'all, thinking about playing with the rear bar, again, on my 1'er. I currently have the 15mm OEM 335xi bar, and while it's an improvement in the car's handling balance, I'm still fighting push. I'm already running a 700 lb-in rear spring, and I'd prefer to not go stiffer there. Going to also try stepping down front spring rate, again, this time from 336 to 280 lb-in.

That said, I wanted to confirm what adjustable bar options are available to us. Is UUC the only game in town for an adjustable bar?

Was looking around today as well. Could try the Eibach E9X m3 rear bar, it's an adjustable 23mm I believe.

My front end feels incredible now with the big rear bar, as I was telling you. But it came at the cost of rear grip. Pretty confident going wide rubber out back will bring it all together.

[bionicbelly]

Just my opinion, but my philosophy is to never take away grip to balance a car. If you are pushing, find grip in the front. Camber, spring, shock setting, alignment, etc.

You can also adjust your driving style. My Z3 always pushed like a pig, but I figured out that if I braked before the corner, and completely released before turn in, then used a very slow turn in(edit: Initial turn is is slow, but once it sets, can dial in steering like normal), the front would stick like glue. Basically, the front end did not like fast inputs. dial steering in slowly, and it would stay planted, and I could come out on throttle, and get some rotation if needed.

Obviously, I have no experience with your car, but in my experience, adding spring in the rear will not help, it will just make the rear slower.

What are your alignment settings?

[bbnks2]

Originally Posted by Ginger_Extract

Hey y'all, thinking about playing with the rear bar, again, on my 1'er. I currently have the 15mm OEM 335xi bar, and while it's an improvement in the car's handling balance, I'm still fighting push. I'm already running a 700 lb-in rear spring, and I'd prefer to not go stiffer there. Going to also try stepping down front spring rate, again, this time from 336 to 280 lb-in.

That said, I wanted to confirm what adjustable bar options are available to us. Is UUC the only game in town for an adjustable bar?

You have a square tire setup? I'd look into tweaking something else other than reducing the front spring rate because dropping below 300lb/in with 7in springs is going to introduce bind and eat away at available bump travel.

[lowside67]

Agreed. I have 400lb front springs on my 128i and it is too soft for even autocross use - I would definitely not be going down further.

How much camber do you have in the front? Going from -2.8 to -3.6 was a game changer, the car went from being truly pushy all the time to neutral with a hint of oversteer, the difference was pronounced.

I also find the car is very sensitive to rake - my car goes from oversteer to understeer by changing the back ride height by 1/2".

-Mark

[bbnks2]

Originally Posted by lowside67

Agreed. I have 400lb front springs on my 128i and it is too soft for even autocross use - I would definitely not be going down further.

How much camber do you have in the front? Going from -2.8 to -3.6 was a game changer, the car went from being truly pushy all the time to neutral with a hint of oversteer, the difference was pronounced.

I also find the car is very sensitive to rake - my car goes from oversteer to understeer by changing the back ride height by 1/2".

-Mark

Whether or not 400lb/in is stiff or not is subjective and depends on 100 other factors, so, I left that out of it. I simply meant that Coil-over design does dictate spring rate to some extent as strut length, spring stroke, and strut stroke are fixed variables. You can tweak the car 100 different ways to dictate its behavior thereafter.

[Kgolf31]

Originally Posted by bbnks2

Whether or not 400lb/in is stiff or not is subjective and depends on 100 other factors, so, I left that out of it. I simply meant that Coil-over design does dictate spring rate to some extent as strut length, spring stroke, and strut stroke are fixed variables. You can tweak the car 100 different ways to dictate its behavior thereafter.

...waits until 6.5 inch spring come into the conversation about "coil bind"

OP - I ran a UUC bar. It's the only bar I'd consider running because it's not massive, but provides just enough for what is needed.

I had good success with it.

[Ginger_Extract]

I started with 400/700, the car handled and rode terribly. Awful, unrelenting understeer until corner exit where the boost would make the rear end come out. Tried three different front bars - 135i, E90 M3 and Eibach, and two different rear bars - 135i and 335xi, before giving up on that set up. The car has an LSD, too.

Alignment was -2.8* up front (camber plate maxed out, stock arms), -1.6* outback. The understeer chewed up front tires and the car sucked to drive. Started with 255 square, later switched to 255/265 stagger, as it proved much faster.

So my wheel set up is 17x9 +35 w/ 255/40 up front and 18x9.5 +50 w/265/35 out back. It took a lot of trial and error to get to this setup, and it has proven to be the fastest one that will work without wide body. Bumpers, fenders and the fuel filler hose have all been modified heavily to make this setup work.

Switched to 336/700 springs. Car still drove like shit with the Eibach and M3 bar, even after fixing the binding. Swapped in a 15mm 335xi rear bar when doing solid subframe bushings, made a marginal dent in the issue. Switched back to the 135i OEM front bar and the balance further improved.

Tried unhooking the front bar altogether on this setup and it was a complete oversteer/drift machine. Fun, and quicker than constant understeer, but very edgy to drive. Felt like an AP1 S2000 on blown shocks. That tells me the front end has the grip, but there was too much front roll stiffness relative to the rear.

Reinstalled the 135i front bar, added M3 front control arms, alignment set to -3.3* up front with 0 toe and -2.3* out back with 1/8" total toe-in (maxed out both ends). Also helped a touch, but we're talking minor, minor differences in terms of absolute limits handling balance and lap times.

That brings me to present. Since unhooking the front bar got rid of any and all understeer, but made it too hard to drive, I'm going to try unhooking both the front and rear bar together to see how it affects the handling balance. All of my steps are made slow and methodically, to eliminate variables and potential inconsistencies in results.

Additionally, the switch to an 8"/280lb-in front spring may induce pre-load or binding issues, but the front end of the car is simply too low, even with the front coil-over perch set to maximum height. The car needs more front ride height, because the addition of the M3 control arms, and the added caster those provide means I am now rubbing on the front bumper when I used to clear it previously. At this point, I can't reform/roll/modify the front fenders and bumper attachment any further short of cutting the wheel opening and installing fender flares. There is simply not any more give left there for modification.

[Ginger_Extract]

Originally Posted by Kgolf31

...waits until 6.5 inch spring come into the conversation about "coil bind"

OP - I ran a UUC bar. It's the only bar I'd consider running because it's not massive, but provides just enough for what is needed.

I had good success with it.

My thoughts exactly. I want to use the least amount of rear bar possible to correct the car's handling deficiencies. I already have enough issues powering out on corner exit, but at this point, I've already proven that the car is faster oversteering than understeering.

[fe1rx]

Originally Posted by Ginger_Extract

I started with 400/700, the car handled and rode terribly.

You have described the handling balance, but how was the ride terrible?

[Ginger_Extract]

Originally Posted by fe1rx

You have described the handling balance, but how was the ride terrible?

Broken pavement, freeway expansion joints and the like caused a terrible front-to-back bobbing motion that no amount of shock tuning would remedy. Additionally, any time the car hit a decent-sized bump, it felt as if the shock towers were being blown out, while the rear of the car responded more normally. The front-to-rear ride comfort/balance was all fucked up.

When I pulled the 400 lb-in springs to replace them with the 336 lb-in springs, I checked for coil bind. After about 1.5 years of track work, and 10,000 street miles, there was maybe a touch of paint transfer on the coils, but it was marginal at most. Switching the 336 front springs alleviated about 80% of the ride quality detriments I was experiencing. My hunch is that the ride quality would further improve moving to the 280 lb-in front spring.

Keep in mind, this isn't my daily driver. It's a track car that is driven to the track, and occasionally to work. A stiff ride doesn't bother me. A brutal ride quality does, as it indicates a problem.

This video corroborates my experiences, and confirms that the front-to-rear spring rate balance was simply not right at 400/700.

[fe1rx]

Originally Posted by Ginger_Extract

Broken pavement, freeway expansion joints and the like caused a terrible front-to-back bobbing motion that no amount of shock tuning would remedy. Additionally, any time the car hit a decent-sized bump, it felt as if the shock towers were being blown out, while the rear of the car responded more normally. The front-to-rear ride comfort/balance was all fucked up.

When I pulled the 400 lb-in springs to replace them with the 336 lb-in springs, I checked for coil bind. After about 1.5 years of track work, and 10,000 street miles, there was maybe a touch of paint transfer on the coils, but it was marginal at most. Switching the 336 front springs alleviated about 80% of the ride quality detriments I was experiencing. My hunch is that the ride quality would further improve moving to the 280 lb-in front spring.

Keep in mind, this isn't my daily driver. It's a track car that is driven to the track, and occasionally to work. A stiff ride doesn't bother me. A brutal ride quality does, as it indicates a problem.

This video corroborates my experiences, and confirms that the front-to-rear spring rate balance was simply not right at 400/700.

Here is what "Race Car Vehicle Dynamics" (Millikan and Millikan) has to say about front-to-rear ride frequencies: "From experience, we know that on most rear-drive race cars the front natural frequency is higher than the rear, based on front roll stiffness requirements." The exact opposite is required to achieve the no-pitch ride quality you and the video you posted are talking about. It is worth noting too, that no pitch can only be achieved at one speed, and that it really only has applicability to normally (softly) sprung street vehicles. To achieve it on our cars at 60 mph with your 336 lb/in front springs would take 1500 lb/in rear springs - which is to say, it is neither practical nor possible in a track car.

Stiff springs alone will not make it feel like your strut towers are being bashed out. That is coil binding and/or severe bump stop impact. Given the very short front springs that must be used in our cars for packaging reasons, and the short useful strokes inherent with short springs, we are severely constrained when it comes to front springs. I am using Swift 60 N/mm fronts (Z65-178-060) which I translate as 343 lb/in x 7.0". Likely what you are using too. (The difference in rate arises depending on if you translate from 6.0 kg/mm or 60 N/mm.) Good choice, in my experience. I couldn't get any stiffer spring to work without coil binding.

For me, getting the Swift 343 lb/in x 7" springs to work without coil binding required essentially zero preload, which means zero ability to adjust ride height with the lower spring perch. To get the higher ride height I wanted required moving the spindle down on the strut, and building some additional height into my camber plates.

I have tried 120 N/mm (686 lb/in) rear springs but have settled on 140 N/mm (800 lb/in). I use the OE front bar and a 20 mm H&R rear bar, so counter to the conventional internet wisdom. I have to thank Kgolf31 for leading the way on stiffer rear bars. This only works with an LSD (I have a MFactory helical). Like yours, mine is a track car. It feels stiff on the street, but not brutal. Balance is delightful.

Bottom line, I am suggesting for a track car don't go softer than 343 on the front, go stiffer on the rear with bar and/or springs to get the balance you want. Also, address front suspension travel / bump stop geometry first, then address ride height by means other than adjusting the lower spring perch.

[bionicbelly]

This is really good stuff. I was planning on going no bar in the rear. But, this is a very different car than I am used to.

Also, just a note, I called AST about spring rates as the shocks I bought had springs (650 front 450 rear) and they said that should be about perfect.

Luckily, the fronts and rears are the same length (coil over rear), so I can get a little more experimentation done without having to buy quite so many springs.

[bbnks2]

Originally Posted by Kgolf31

...waits until 6.5 inch spring come into the conversation about "coil bind"

Yes, you will experience coil bind with 6.5" springs and 280lb/in. Hands down no doubt about it. Are you saying I am wrong in warning him not to reduce spring rate for this reason? I really don't ever understand your logic or the point of your backhanded statements that have 0 sound reasoning behind them.

The other big issue would be ride height... which he mentioned himself in follow-up. the car will sit too low with that spring rate even with the coil-overs at max ride height. I know all this because I've tried it.

280 lb/in up front IS STIFF. It's over 200% stiffer than stock. However, that doesn't make it an ideal spring rate when used with short springs on coil-overs that are really only designed to work with one spring rate give or take a bit.

Originally Posted by fe1rx

Here is what "Race Car Vehicle Dynamics" (Millikan and Millikan) has to say about front-to-rear ride frequencies: "From experience, we know that on most rear-drive race cars the front natural frequency is higher than the rear, based on front roll stiffness requirements."

I would disagree with this. Spring rates are THE driving factor in how load is distributed Front to Rear in roll. It is one of the FEW things you can change that will have a significant impact on load distribution. If you imbalance the springs, you basically need to balance the car back out with sway bars since that is the next most influential factor. However, doing so brings with it ride quality issues. You can throw a stiff rear sway into the car and run soft rear springs, but now the car is going to handle curbing very poorly and AERO tweaks like wings and splitters are going to become an absolute HEADACHE to tune right. You'll have nice grip for powering out of corners, but at all other times you'll be dragging the rear of the car around like an anchor and the front-end will be light.

Also, consider the typical setup on these cars... stiff front springs and relatively soft rear springs. Sure, you can use sways to balance the car back out in roll like said above; however, you are also significantly changing how the car is going to handle in other aspects like corner entry. With stiff front springs you won't get as much dive under braking and less weight will transfer over the front wheels. The car is going to skip/skate on entry (understeer) since vertical load distribution is essential to corner entry grip (diminishing returns dictate you still want to make the car as light as possible to improve grip instead of adding ballast to the car). Without forward weight distribution under braking you are actually REDUCING the amount of turn-in grip the car has. It is not until all 4 wheels are in roll (mid corner) that you want load distribution to be as even as possible across all 4 wheels (spring and sway choices are working in roll to achieve a neutral wheel rate).

Not only are you affecting corner entry, but you are also affecting corner exit. A soft rear spring rate = more squat on exit. More squat on exit = power-on under-steer at corner exit. Isn't what I am saying beginning to sound REALLY freaking familiar to all the complaints about under-steer on this forum?

For every article you can find that says don't worry about keeping spring rates neutral there as just as many technical articles that state neutral spring rates are THE foundation for a neutral car (and a neutral car is fastest per science). This is obviously just conceptual thinking and "packaging restraints" and things like coil-over design cause us to deviate from this at times.

Originally Posted by fe1rx

I am using Swift 60 N/mm fronts (Z65-178-060) which I translate as 343 lb/in x 7.0". Likely what you are using too. (The difference in rate arises depending on if you translate from 6.0 kg/mm or 60 N/mm.) Good choice, in my experience. I couldn't get any stiffer spring to work without coil binding.

For me, getting the Swift 343 lb/in x 7" springs to work without coil binding required essentially zero preload, which means zero ability to adjust ride height with the lower spring perch. To get the higher ride height I wanted required moving the spindle down on the strut, and building some additional height into my camber plates.

This is what I am using as well. The YCW strut length gives me 4.75" of ground clearance at max ride height with these springs which I find to be just about ideal. When I tried running softer springs the car was obviously slammed and I had to do the same thing. I added spacers to my camber plates to increase ride height without changing ride characteristics with spring pre-load. However, coil-bind was still present so I had to move back to a stiffer spring. I ditched the H&R swaybar I had been running for the stock sway because the stiffer front springs (336lb/in) introduced under-steer with my particular setup.

Originally Posted by fe1rx

I have tried 120 N/mm (686 lb/in) rear springs but have settled on 140 N/mm (800 lb/in). I use the OE front bar and a 20 mm H&R rear bar, so counter to the conventional internet wisdom. I have to thank Kgolf31 for leading the way on stiffer rear bars. This only works with an LSD (I have a MFactory helical). Like yours, mine is a track car. It feels stiff on the street, but not brutal. Balance is delightful.

Limited droop travel combined with the jacking effect of a larger rear bar is probably why people find the LSD so necessary with a large rear sway. On braking and turn-in you can easily use up your 2" of rear droop travel (even less droop available with stiffer rear springs). The inside rear wheel will get even lighter under roll with the rear bar pulling it off the ground as well = 1 tire fires with the e-diff on an un-weighted wheel.

Originally Posted by fe1rx

Bottom line, I am suggesting for a track car don't go softer than 343 on the front, go stiffer on the rear with bar and/or springs to get the balance you want. Also, address front suspension travel / bump stop geometry first, then address ride height by means other than adjusting the lower spring perch.

I don't understand what peoples obsession is with super stiff springs except that they are ignorant of what they are even achieving... I never found this mentality at a roadcourse from seasoned drivers. I only find it at autocross where people want to put 900 lb springs on 2000lb Miatas. NASCAR runs spring rates off 300/in up front...

The typical autocross person also doesn't seem to realize that running WIDE tires and a WIDE track width means that they'll be sacrificing slalom ability for stability in sweepers. You don't gain back that slalom agility by stiffening the car even though it feels that way. Go back to a narrower track width and you'll instantly feel the go-kart effect without making any spring changes and you'll pick up time in the slaloms by sacrificing some stability in sweepers.

[Kgolf31]

I ran 400/700. Ran a UUC bar up front and a UUC bar in the rear. My ride height, for reference



Also, shocks play a big role in how a car is going to handle. Don't think YCW is going to be your best option...Locking in on a spring rate and further fine tuning handling dynamics on shocks/bars is going to be my recommendation. No need to beat a dead horse on all this other stuff

[lowside67]

Originally Posted by Ginger_Extract

Broken pavement, freeway expansion joints and the like caused a terrible front-to-back bobbing motion that no amount of shock tuning would remedy. Additionally, any time the car hit a decent-sized bump, it felt as if the shock towers were being blown out, while the rear of the car responded more normally. The front-to-rear ride comfort/balance was all fucked up.

I find this very interesting as I run the identical rates to you on a lighter 128 and while I am looking to tweak my spring rates for competitive purposes but I genuinely find my car to ride very well on the freeway. It is not stock, but I actually find the car feels quite composed over sharp impacts like expansion joints.

Usually ride quality is more closely correlated with damper quality and settings than spring rates. Not exclusively, but more closely.

To the original question, I am going stiffer on springs in the front as I run a relatively moderate sized front bar (Hotchkis). I find the car has far more fore-aft pitch change between throttle and brakes than I would like and am hoping the slightly stiffer springs will help the weight transfer quicker to avoid this back and forth.

-Mark

[Kgolf31]

Originally Posted by lowside67

I find this very interesting as I run the identical rates to you on a lighter 128 and while I am looking to tweak my spring rates for competitive purposes but I genuinely find my car to ride very well on the freeway. It is not stock, but I actually find the car feels quite composed over sharp impacts like expansion joints.

Usually ride quality is more closely correlated with damper quality and settings than spring rates. Not exclusively, but more closely.

To the original question, I am going stiffer on springs in the front as I run a relatively moderate sized front bar (Hotchkis). I find the car has far more fore-aft pitch change between throttle and brakes than I would like and am hoping the slightly stiffer springs will help the weight transfer quicker to avoid this back and forth.

-Mark

Springs control the amount a car can pitch, shocks control the rate at which a car can pitch

[berns]

I haven't run many different setups on my car, since I built it pretty specifically off the bat. My only two test days have been at Buttonwillow CW13.

I run KW Clubsport dampers (V3, basically) and Swift springs. 6k front and 12k rear.
M3 lower arms, every bushing, mount, and bearing you can think of, for the most part.
Started out with the E92 M3 front bar and stock rear bar.
Tire setup was: 18x9" squared with 255/35 Nexen Sur4G

Rear grip was great and the car was relatively easy to power-on out of corners, which surprised me. Understeer was annoying, both turn-in, mid corner, and everywhere really. At this point, I wish I just put the stock front bar back in for comparison, but I instead opted to go for the E92 M3 rear bar, as the 1M came this way as well and the car displayed pretty awful roll. Here are some photos of cornering from that first event. Best time was 2:00.3, on fresh tires.

Lifting the front inner tire, and look at that squat.




At this point, I dropped the rear subframe again (worst idea) and installed the E92 M3 rear sway bar to match. Truthfully, I should have definitely gotten another alignment and properly set up the adjustable end link, so there is some room to play still. The front end now feels amazing. Turn-in is immediate, the grip is there with -3* up front, and mid corner wash-out is in the past. However, the rear-end is SUPER playful, and I don't have enough tire to handle it. I also think I'm in major need for damper setting adjustments.

Despite the nice front-end, I could barely get on the gas all day out of any corner, which resulted in a 2:01.5 as my best time. Over a full second slower around Buttonwillow CW13.

Both front tires are on the ground now. Roll doesn't look as severe. Not totally sure what to take away from all of this, but I feel that with some more rear grip, this will be the faster setup. So I'm going to try to run staggered, 18x9 front, 18x9.5 rear, 265 front and 275 rear.




The KW dampers have a ton of threads left. At my current ride height, the top of the tire is still below the lower perch up front. The rear has a ton of room, it seems. I did the math and it looks like I'll have to run a 10mm spacer out back to get the 9.5's out to where my 9's are, which will leave me some room to the fender edge and enough clearance inboard to avoid the fuel filler.

Ride height for reference. Car will probably come up some.

[Ginger_Extract]

Originally Posted by fe1rx

Here is what "Race Car Vehicle Dynamics" (Millikan and Millikan) has to say about front-to-rear ride frequencies: "From experience, we know that on most rear-drive race cars the front natural frequency is higher than the rear, based on front roll stiffness requirements." The exact opposite is required to achieve the no-pitch ride quality you and the video you posted are talking about.

- I think we need to better understand what they are defining as a race car. That sounds too generalized. The video above features a street car-based track car with a strut type front suspension, and multilink rear, like our cars.

I am using Swift 60 N/mm fronts (Z65-178-060) which I translate as 343 lb/in x 7.0". Likely what you are using too. (The difference in rate arises depending on if you translate from 6.0 kg/mm or 60 N/mm.) Good choice, in my experience. I couldn't get any stiffer spring to work without coil binding.

- Yes, I am currently on the same front spring. I am thinking of moving to Z60-203-050, 50 N/mm springs, but 8" tall. I would ideally like to see another 1/4" in front ride height. At least on my TCK coil-overs, I can lower the spring perch about 1/2" and still clear my 17x9 setup.

I have tried 120 N/mm (686 lb/in) rear springs but have settled on 140 N/mm (800 lb/in).

- Got a part number on those rear springs? A bit hesitant to try stiffer springs in the back, simply because I'm unsure how well my TCK S/A dampers can control that much spring rate, but springs are easy to swap out and experiment with, the rear bar is a pain.

My responses are in bold, deleted some things for brevity's sake. This conversation highlights that there are always multiple takes on how to address suspension tuning.

Still feel compelled to try the 280 lb-in/8" tall front spring. I'm going to go out on a limb and guess that not all coil-over bodies are threaded the same. An 8" spring might work without a ton of pre-load, we shall see. If it sucks, I would let you all know. Not apples to apples, I know, but I drove an M4 with 280/750 springs and really enjoyed the way it rode and responded.

[Ginger_Extract]

Originally Posted by bionicbelly

This is really good stuff. I was planning on going no bar in the rear. But, this is a very different car than I am used to.

Also, just a note, I called AST about spring rates as the shocks I bought had springs (650 front 450 rear) and they said that should be about perfect.

Luckily, the fronts and rears are the same length (coil over rear), so I can get a little more experimentation done without having to buy quite so many springs.

Try swapping the springs around and running no sway bars at all. 336/700 with no front bar prompted massive oversteer. Low speed rotation was amazing, zero push. With 450/650, it might work.

[Ginger_Extract]

Originally Posted by berns

I haven't run many different setups on my car, since I built it pretty specifically off the bat. My only two test days have been at Buttonwillow CW13.

I run KW Clubsport dampers (V3, basically) and Swift springs. 6k front and 12k rear.
M3 lower arms, every bushing, mount, and bearing you can think of, for the most part.
Started out with the E92 M3 front bar and stock rear bar.
Tire setup was: 18x9" squared with 255/35 Nexen Sur4G

Rear grip was great and the car was relatively easy to power-on out of corners, which surprised me. Understeer was annoying, both turn-in, mid corner, and everywhere really. At this point, I wish I just put the stock front bar back in for comparison, but I instead opted to go for the E92 M3 rear bar, as the 1M came this way as well and the car displayed pretty awful roll. Here are some photos of cornering from that first event. Best time was 2:00.3, on fresh tires.

At this point, I dropped the rear subframe again (worst idea) and installed the E92 M3 rear sway bar to match. Truthfully, I should have definitely gotten another alignment and properly set up the adjustable end link, so there is some room to play still. The front end now feels amazing. Turn-in is immediate, the grip is there with -3* up front, and mid corner wash-out is in the past. However, the rear-end is SUPER playful, and I don't have enough tire to handle it. I also think I'm in major need for damper setting adjustments.

We need to get together and bounce some ideas around when we aren't at a busy track day.

[bbnks2]

Originally Posted by Ginger_Extract

I have tried 120 N/mm (686 lb/in) rear springs but have settled on 140 N/mm (800 lb/in).

- Got a part number on those rear springs? A bit hesitant to try stiffer springs in the back, simply because I'm unsure how well my TCK S/A dampers can control that much spring rate, but springs are easy to swap out and experiment with, the rear bar is a pain.

I don't understand this mentality. Increasing the rear spring rate by 114lb/in is only an EFFECTIVE wheel rate change of ~40 lb/in. You're talking almost an imperceptible difference. Additionally, the rear struts themselves have a separate motion ratio from the springs since they are mounted separately. The strut is mounted in a position that is MORE effective than the spring (Felix can speak more to it than I can). The strut is mounted closer to the wheel than the spring is... I believe I saw the motion ratio of the strut put at around .7 vs the spring being only .3x? Therefore, my guess would be that the actual impact of increasing the rear spring rate by such a marginal amount would have only a marginal impact on damping. Put the rear struts 1 click stiffer?

Why do people think that it is ok to add significant roll stiffness via a sway bar but doing the same with springs would be a sin? Damping is the only legitimate reason I see so I am glad you at least used that as your basis for not wanting to do it.

I guess you could argue that you want to keep the rear spring rate soft so the car can SQUAT under acceleration load. To that I would say... 800lb/in is still relatively soft compared to spring rate people are running up front in the 400-500lb/in range. The car will still squat HARD under acceleration. I have 900lb springs out back and the car still squats on acceleration quiet a bit. In corner, you might just have to learn to NOT TO TOUCH THE GAS until you've begun opening the steering wheel up on exit. Corning takes up all available grip and it's not until you begin opening the steering wheel that more grip becomes available to begin squeezing on throttle...