[tscdennab]

Hi,

I'm trying to figure out the best spring rates for my upcoming suspension and I would like to gather various opinions first. I will use the car for street and drag racing, and not for circuit racing (who knows though). I am looking to replace my aging M-Sport suspension with an adjustable suspension in order to hopefully dial it in better for 60ft and eliminate wheel hop.

So my calculations brought me to the following numbers: 224lb/in front, 616lb/in rear. I know most people here use much higher front spring rates, but I am just looking for "perfect balance". So how did I reach these rates ?

Well, all the BMW springs (including the M3 springs) have triple the spring rate on the rear compared to front. And I found the following formulas in one thread:

The motion ratio for the car:

Front: 0.96^2
Rear: 0.563^2

Calculation for wheel rates:

Coil spring rate * Motion ratio = Wheel spring rate

So in order for the car to have the same wheel rate front/back, the rear spring rate should be triple the front spring rate. Since I don't want an extremely stiff rear spring rate, I think that 616lb/in in the rear would be sufficient for me, this leads to 224lb/in front spring rates. These spring rates are about 15% stiffer than BMW Performance yellow springs. I would like to use these springs with AST 4200 dual-adjustable dampers.

So what do you think ? Will these combination of spring rates / shocks give me a "balanced" suspension ? Another question is, can the AST 4200 dampers work well with a front spring rate of just 224lb/in or will the suspension feel "over-damped" ?

Thanks !

[subieworx]

You don't necessarily want the front and rear to have the same wheel rate although that all depends on what you are going for. 224 is definitely soft in the front. I run 450/900 on my car with custom valved AST 4100's. Works well, but you definitely feel the bumps in the front with the spring rate and camber plates.

The only way to know how soft you can go on the AST's is to see shock dyno plots. I would think that spring rate would be too soft for the 4200's given the springs they come with and their adjustment range. I think a revalve would be required.

[neoduffer]

I just had 4100's installed with 392F/672R spring rates and Ground Control street camber plates. Overall the car feels "lively", but not harsh. One noticeable downside is that the new suspension accentuates the buzz and squeaks from the interior trim.

I've only driven it about 10 miles from the shop back to the house, so there isn't much else to say at this point. More feedback to come after an alignment and corner balance this week. I was looking for something to improve the handling of the car for HPDE use and I think this is what I was looking for. I'm not sure if I could tolerate this setup for a daily driver after riding on stock suspension for that past 3 years. Perhaps 336F/616R is a better match for mixed daily driving and light track use.

Tires are 245/40R18 PS2's up front and 265/35/R18 Conti DW's on the rear. I immediately notice a cushier ride with the stock suspension after changing the wheels and tires. The higher spring rate seems to match well with the non-RFT setup. I would be more worried about the higher spring rates if I were on the stock RFT's still.

[tscdennab]

Originally Posted by neoduffer

I just had 4100's installed with 392F/672R spring rates and Ground Control street camber plates. Overall the car feels "lively", but not harsh. One noticeable downside is that the new suspension accentuates the buzz and squeaks from the interior trim.

I've only driven it about 10 miles from the shop back to the house, so there isn't much else to say at this point. More feedback to come after an alignment and corner balance this week. I was looking for something to improve the handling of the car for HPDE use and I think this is what I was looking for. I'm not sure if I could tolerate this setup for a daily driver after riding on stock suspension for that past 3 years. Perhaps 336F/616R is a better match for mixed daily driving and light track use.

Tires are 245/40R18 PS2's up front and 265/35/R18 Conti DW's on the rear. I immediately notice a cushier ride with the stock suspension after changing the wheels and tires. The higher spring rate seems to match well with the non-RFT setup. I would be more worried about the higher spring rates if I were on the stock RFT's still.

Can you tell if the "liveliness" comes from more from the front or from the rear ? Or both ? I suppose you had the dampers set to the softest position, right ?

Doesn't sound like the setup I would like. I will surely limit my spring rates to something lower like Swift 224/616 or Eibach 250/600. But what I'm afraid of is that the AST 4200 dampers are made for stiffer springs than low rate springs like 200-250.

[tibra1]

As I mentioned to you a comfy street setup to minimize understeer will be 336/672..392/784 would be a dual purpose street/track more geared for track

224 is extremely low in the front...these dampers can AND should be run with a higher spring rate than this.

[tscdennab]

Originally Posted by tibra1

these dampers can AND should be run with a higher spring rate than this.

Unfortunately, this may be true. I will try to get a definite answer about this from AST. If indeed they are too much for these springs and I cannot do custom valving for correct handling of these springs, I will look at other options which work with those spring rates. Maybe Ohlins, I know for sure that they do custom valving. They are also cheaper since I don't need to convert the rear links. But I lose the double adjustability

I really don't want to give up on the 3x spring rate ratio between the front and rear. Since all the BMW springs, including the M3, have this ratio. There must be something to it...

Also why do you think that 224 is extremely low since the BMW Performance spring is 195. I would really not want much more than the BMW Performance spring rate.

[tibra1]

May I ask why you want to use such a low rate in the front..the comfort level will be acceptable when using a higher spring rate b/c the damping power of the ASTs is there and you can still tweak compression to fine tune..

If you are set on the 224 spring rate..then perhaps another damper is the way to go for you. As I mentioned comparing BMW to these is not feasible..also keep in mind the swift thrust sheets will aid tremendously in comfort level even at these higher spring rates

[tscdennab]

Originally Posted by tibra1

May I ask why you want to use such a low rate in the front..the comfort level will be acceptable when using a higher spring rate b/c the damping power of the ASTs is there and you can still tweak compression to fine tune..

Firstly, I will use the car for street and some drag racing. Drag racing requires soft(ish) front springs and adjustable dampers. My M-Sport dampers are a bit tired after 45k miles on our bad roads. No circuit racing for me because we don't have tracks in our country...and AutoX like events are very rare. And why should I want a stiff suspension when it is not needed ?

On the other hand, you mentioned that the compression setting can be tweaked, I'm not sure how much does this contribute to ride quality over bumps, I guess I will have to wait for your review

I would like a range of adjustment between "a bit softer than stock" to "a bit stiffer than BMW Performance suspension". I am not sure if I can achieve that with 336 springs, but I will keep researching...

[luckyu]

Originally Posted by tscdennab

Hi,

I'm trying to figure out the best spring rates for my upcoming suspension and I would like to gather various opinions first. I will use the car for street and drag racing, and not for circuit racing (who knows though).
Thanks !

I think it's good you're looking carefully at what spring rates you want. Since you're going with AST -- which is great kit -- you should keep in mind that many people who buy this stuff race on a track -- they are not you. They are using much higher spring rates than are tolerable for a daily driver. The AST 4200 shocks are great, but they are do not use discrete valving for high speed and low speed like high end shocks do. If you go to higher spring rates and adjust the shocks to match, you will hurt. :-)

Regarding balance and that tripling thing. There are two things going on here. You have to decide how important each one is to you.

1) Neutral steer. Making wheel rates approximately the same encourages a neutral steer car. Neutral steer cars achieve the best mechanical grip/comfort ratio. Matching spring rates alone are not sufficient for neutral steer, but they set the stage so to speak. N.S. is often a good choice for road cars and fits BMW's 50/50 weight distribution nicely. I'm a fan.

2) Flat ride tuning. BMW sets up the rear wheel rate to be a little higher than front, just a little higher, so that when you go over a big looong bump on the freeway at 70 mph, the rear of the car will bounce a little faster, and "catch up" to the bounce in the front. The result is significantly less pitch while cruising. This tuning has to do with harmonic frequencies -- the weight of your car matters. It is the frequencies that are tuned, not the wheelrates.

Neither of these is of much concern in drag racing, I imagine. They are both important in a nice road car.

Good luck!

[mike-y]

it really depends on how the damper is setup. Eiback Multi-Pro R1s use 200/500 lb/in springs F/R, and the ride is pretty harsh/stiff because the dampers are setup with stiff valving, and adjust for compression and rebound at the same time. That same spring rate on another damper will feel and handle differently depending on how they are valved.

Your best bet is to talk with AST or maybe HP Autowerk, as they are familiar with these systems to find out what would work best for you.

[tscdennab]

Originally Posted by luckyu

I think it's good you're looking carefully at what spring rates you want. Since you're going with AST -- which is great kit -- you should keep in mind that many people who buy this stuff race on a track -- they are not you. They are using much higher spring rates than are tolerable for a daily driver. The AST 4200 shocks are great, but they are do not use discrete valving for high speed and low speed like high end shocks do. If you go to higher spring rates and adjust the shocks to match, you will hurt. :-)

Regarding balance and that tripling thing. There are two things going on here. You have to decide how important each one is to you.

1) Neutral steer. Making wheel rates approximately the same encourages a neutral steer car. Neutral steer cars achieve the best mechanical grip/comfort ratio. Matching spring rates alone are not sufficient for neutral steer, but they set the stage so to speak. N.S. is often a good choice for road cars and fits BMW's 50/50 weight distribution nicely. I'm a fan.

2) Flat ride tuning. BMW sets up the rear wheel rate to be a little higher than front, just a little higher, so that when you go over a big looong bump on the freeway at 70 mph, the rear of the car will bounce a little faster, and "catch up" to the bounce in the front. The result is significantly less pitch while cruising. This tuning has to do with harmonic frequencies -- the weight of your car matters. It is the frequencies that are tuned, not the wheelrates.

Neither of these is of much concern in drag racing, I imagine. They are both important in a nice road car.

Good luck!

Nice info, thanks ! I just noticed that Eibach Multi-Pro R1 coilovers use 200/500 spring rates, so I suppose I'm not weird in wanting such rates

I didn't pay much attention to separate valving for high-speed and low-speed...are you saying that AST 4100/4200 cannot provide at least M-sport-like comfort with appropriate spring rates ?

[neoduffer]

Originally Posted by tscdennab

Can you tell if the "liveliness" comes from more from the front or from the rear ? Or both ? I suppose you had the dampers set to the softest position, right ?

Doesn't sound like the setup I would like. I will surely limit my spring rates to something lower like Swift 224/616 or Eibach 250/600. But what I'm afraid of is that the AST 4200 dampers are made for stiffer springs than low rate springs like 200-250.

The dampers are set on full soft. I didn't check myself, but that is what I was told when picking up the car. The front is definitely more responsive than before. I can't tell how much the rear contributes, but it does seem different from before. Sorry, but I really can't quantify it right now.

Added: 250/600 sounds like a good combo for what you are looking for. MY front is probably over sprung for general driving, so I may go to a lower spring rate in the future if I don't like the 400 fronts.

[luckyu]

Originally Posted by tscdennab

Nice info, thanks ! I just noticed that Eibach Multi-Pro R1 coilovers use 200/500 spring rates, so I suppose I'm not weird in wanting such rates

I didn't pay much attention to separate valving for high-speed and low-speed...are you saying that AST 4100/4200 cannot provide at least M-sport-like comfort with appropriate spring rates ?

That is a really tough question to answer, particularly because I hate the M-sport characteristics I can't tell you how the ride will be -- and I've never ridden on AST shocks, ever -- all I meant is to caution when reading "Oh man these AST coilovers with 3453/85694 springs are great!" to take it with a grain of salt. The question to ask AST owners is, Would you want to drive it every day?

In connection with that, I brought up the high end shocks because some people are getting comfortable results on very high spring rates. That is only possible with high tech shocks.

One combo to look at, if you're looking at Swift springs, is 196/672. This would yield nice frequencies on my 328i, I don't know on your 335i. Also, I would pick Hyperco over Eibach if it's available where you are.

[cvc 22349a]

[QUOTE=tscdennab;10722926]

I really don't want to give up on the 3x spring rate ratio between the front and rear. Since all the BMW springs, including the M3, have this ratio. There must be something to it...

+1 but if you go w/the 224 lb/in front spring, you'd have to go w/a 672 lb/in rear spring to maintain that motion ratio, no? If yes, wouldn't Koni sport dampers be sufficient?

[tibra1]

Originally Posted by luckyu

The AST 4200 shocks are great, but they are do not use discrete valving for high speed and low speed like high end shocks do. If you go to higher spring rates and adjust the shocks to match, you will hurt. :-)

I dont agree with this..street driving is my primary function and in looking at spring rates..you MUST also consider which springs..Swifts springs when paired with their thrust sheets. have an excellent reputation for providing comfort with control...even at what is percieved as high spring rates

With that said you will definitly know you have coilovers but it will by no means "hurt". If something w modest control and OEM like driveability is what you are after..there are the Ohlins Road and Track..or Konis..all good dampers..or simply a spring shock setup.

Tscdenabb is specifically asking about the AST 4200s..and IMO no way should these be run w such a soft front spring

[luckyu]

Originally Posted by tibra1

I dont agree with this..street driving is my primary function and in looking at spring rates..you MUST also consider which springs..Swifts springs when paired with their thrust sheets. have an excellent reputation for providing comfort with control...even at what is percieved as high spring rates

With that said you will definitly know you have coilovers but it will by no means "hurt". If something w modest control and OEM like driveability is what you are after..there are the Ohlins Road and Track..or Konis..all good dampers..or simply a spring shock setup.

Tscdenabb is specifically asking about the AST 4200s..and IMO no way should these be run w such a soft front spring

All right I will defer to your knowledge, and "hurt" was the wrong word. I would urge Tscdenabb to ask AST owners their thoughts!

(sorry, now I'm going to rant about Swift springs...this really isn't directed at you Tibra1.. it just kinda came out...)

I think the benefits of Swift springs are sometimes overstated on the internet. Swift springs absolutely have specific technical advantages, like lower weight and longer usable stroke. They are remarkable and good products. But I want to caution anyone not to think that they are softer than they are.

What happens is that a racer uses some spring outside its usable stroke, and doesn't realize that the true springrate is not the nominal rate printed on the spring. It's much higher. Then they swap in a Swift, whose true rate is much lower, much closer to the rate on the box. Result? Swift feels softer.

What this all means is that some people -- some -- are abusing their springs, using them outside their linear stroke. It doesn't mean that a Swift will feel any different from a properly used Hyperco.

There are plenty of situations that demand longer stroke or can benefit from longer stroke. The front spring of the e90, for example! But again, just to make another example. If I use a Hyperco spring knowing that in the last inch of jounce it will ramp up 10% over its nominal rate, then, no problem, I just design that 10% out of the bump stop. Because in the last inch of bump travel, I'm deep into the bump stop anyway, and the total desired spring rate is highly progressive!

As for the weight advantages of Swift springs -- which are remarkable -- someone will have to explain to me how it makes more than a 1% difference in "reaction time" in the rear (or front) suspension. We're talking about 0.5 pound out of 65 pounds total, and the 0.5 pound is at an install ratio of 0.56. It hardly moves at all, compared to the shock, the arms, the wheel. How that 0.5 pound can possibly affect reaction time one iota is beyond me.

I want to qualify what I'm saying. I'm an engineer, and I love technology. Where would we be if we weren't saving a gram of unsprung weight here, a gram there? We'd never make progress. Swift springs are great, they're a step forward. They come in nice colors.

All I'm saying is that there are many many applications where if you swap out an Eibach for a Swift you wouldn't be able to tell any difference in feel or performance. There are also many applications where only a Swift can be used to get the desired behavior! And yes, there are applications where you will notice a difference with Swift, because the spring is progressive far beyond the rate someone thinks it is, or the spring is being used inappropriately.

I read a story yesterday about a Miata that during a race experienced coil bind, causing the lower perch to drop onto the axle, where it held the car up sort of and allowed the driver to finish the race and drive part way home. Point is, some racers just aren't very careful about the max stroke of their spring. That they're racing on. At 100 mph.

When someone comes along, and tells you hey this product is softer or has better reaction time but can't explain what that means, and the supposed benefits can't be measured or even articulated, then you have to consider that it's a heck of a lot more likely the guy is telling stories, than it is he's discovered magic springs.

(Alright I feel better now. Again this is not directed at you tibra1, or at anyone here.)

[tibra1]

^I have never quite dissected on that level...maybe I should defer to your knowledge here.

[mike-y]

wow, someone actually took into account the installed movement ratio of the spring. the tech level of this thread just jumped up by a whole bunch.

that said, I think too much emphasis is being put on thrust sheets. their primary function is to allow rotational movement of the spring and prevent binding as it coils/uncoils with load, which helps maintain a more constant spring rate. they help with rate consistency over the range of travel, which might make the ride a bit more compliant, but it won't make a 500 lb spring suddenly feel like a 300 lb spring. The compression valving on the damper will make more of a difference in that respect, so you still need to choose your rates carefully, based on what you are working with in respect to the damper.

and with that, I will just say the OP needs to talk to someone who is smarter than me (and very familiar with AST) about what rates will work with the the AST dampers to give him the ride quality and handling he wants.

[tscdennab]

Originally Posted by mike-y

and with that, I will just say the OP needs to talk to someone who is smarter than me (and very familiar with AST) about what rates will work with the the AST dampers to give him the ride quality and handling he wants.

I asked Vorschlag about it and they said that the dampers are made to work with rates between 350lb/in and 700lb/in but "it would work" with 224lb/in. I think I will have to ask if custom valving is available, or just look at other options. Maybe monotube dampers are not for me and I should go with twin-tube which (from my research) seems to be more confortable on the street.

[tibra1]

Originally Posted by tscdennab

I asked Vorschlag about it and they said that the dampers are made to work with rates between 350lb/in and 700lb/in but "it would work" with 224lb/in. I think I will have to ask if custom valving is available, or just look at other options. Maybe monotube dampers are not for me and I should go with twin-tube which (from my research) seems to be more confortable on the street.

This may make the most sense for your needs..twin tubes will be more comfortable.

[HP Autosport]

Must you have 4200 for your 335i? I believe AST stop making the pin to pin 4200 rear because they had so many issues with it.

As for 4100, 336F and 672R are the rates for most if not all the kits leaving our door.

[tscdennab]

Originally Posted by HP Autowerks

Must you have 4200 for your 335i? I believe AST stop making the pin to pin 4200 rear because they had so many issues with it.

As for 4100, 336F and 672R are the rates for most if not all the kits leaving our door.

I want dual adjustable dampers because I do mostly drag racing and I think I need independent compression adjustment. And the price difference is not very high between single adjustable and dual adjustable. I got a very good price from someone in Europe, AST 4200 set with Eibach springs of my choice and custom revalving for $200 cheaper than a typical set of KW V3s. This would somewhat offset the cost of rear M3 shock mounts conversion which costs about $500 in itself. So I would get dual-adjustable monotube shocks for $300 more than the dual-adjustable twin-tube shocks.

But I am concerned about the ride quality, shock travel and ride height issues, since no one has used 224lb/in springs in front with AST 4200 dampers, like I want to use. So in the end, unless I will be sure 100% that there will be enough shock travel and ride height will be close to stock, I think I'm going with KW V3.