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      10-30-2011, 07:31 PM   #45
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Interesting to see this thread after almost 3 yrs.
I don't understand motion ration and susp frequency and don't really want to try.
I am interested in why aftermarket susp tuners don't emulate BMW's tripling the rear spring rate from the front (the 135i, 335i and M3 oe rates are all tripled in the rear from the front). Anyone know why?
A favorite vendor on this forum recommends doubling the rear from the front w/Swift springs. Other manufacturers don't even double the rear rate from the front.
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      10-31-2011, 01:13 PM   #46
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Quote:
Originally Posted by cvc 22349a View Post
Interesting to see this thread after almost 3 yrs.
I don't understand motion ration and susp frequency and don't really want to try.
I am interested in why aftermarket susp tuners don't emulate BMW's tripling the rear spring rate from the front (the 135i, 335i and M3 oe rates are all tripled in the rear from the front). Anyone know why?
A favorite vendor on this forum recommends doubling the rear from the front w/Swift springs. Other manufacturers don't even double the rear rate from the front.
Hi CVC! I don't think anyone knows for sure exactly why, except for each individual vendor. However I will try to answer anyway; I hope I'm not adding to the noise...

I think vendors don't triple the rear springrate because they can't. That is, they can't raise both the front rate 3X and the rear rate 3X. Because, the way the rear spring is positioned it would pry the subframe away from the body instead of behaving like a spring.

If the rear were a coil over spring design, like the E39, then we would see vendor wheelrates that are: 1) BMW-like, with slightly higher rear, 2) the same front to rear, and 3) slightly higher in front.

It may be possible to reduce understeer on a 400#/800# car by using long bump stops in the rear. Add 150# at the rear shock to the 800# spring and you're at 360# rear wheel rate. You've "added" to the spring. I think this is what BMW is doing with the non-M cars.

Motion ratio is easy to understand intuitively, it's just leverage. Put a spring in a pair of hedge clippers, and the spring is easier to compress depending where you put it.

Motion ratio is also easy to calculate. Divide rear springrate by 3 and you have the approximate wheel rate.
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      10-31-2011, 03:28 PM   #47
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^ Interesting ! Maybe we can hear some more opinions from people that race their cars with different spring rates (like HP Autowerks) ?
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      10-31-2011, 04:18 PM   #48
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Quote:
Originally Posted by luckyu View Post
Hi CVC! I don't think anyone knows for sure exactly why, except for each individual vendor. However I will try to answer anyway; I hope I'm not adding to the noise...

I think vendors don't triple the rear springrate because they can't. That is, they can't raise both the front rate 3X and the rear rate 3X. Because, the way the rear spring is positioned it would pry the subframe away from the body instead of behaving like a spring.

If the rear were a coil over spring design, like the E39, then we would see vendor wheelrates that are: 1) BMW-like, with slightly higher rear, 2) the same front to rear, and 3) slightly higher in front.

It may be possible to reduce understeer on a 400#/800# car by using long bump stops in the rear. Add 150# at the rear shock to the 800# spring and you're at 360# rear wheel rate. You've "added" to the spring. I think this is what BMW is doing with the non-M cars.

Motion ratio is easy to understand intuitively, it's just leverage. Put a spring in a pair of hedge clippers, and the spring is easier to compress depending where you put it.

Motion ratio is also easy to calculate. Divide rear springrate by 3 and you have the approximate wheel rate.
I didn't mean increasing the front spring rate x3; just tripling the rear rate from the front as BMW has w/the 135, 335, e9X M3.
Examples of doubling the rear spring rate from the front instead of triplling it can include KW's street comfort c/o 286 lb F/573 lb R or HPA's c/o kits with 336 lb F/672 lb R.
In trying to make sense of what 'Orb' has written; if the rear spring rate isn't tripled from the front, will it further promote understeer and the car won't settle at the same time front to back when going over a bump? In the second example, wouldn't a 224 lb F/ 672 lb R spring set up be more congruent with the way the car was engineered?
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      10-31-2011, 05:59 PM   #49
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Quote:
Originally Posted by cvc 22349a View Post
I didn't mean increasing the front spring rate x3; just tripling the rear rate from the front as BMW has w/the 135, 335, e9X M3.
I gotcha, it was just coincidence I chose those numbers.

Quote:
Examples of doubling the rear spring rate from the front instead of triplling it can include KW's street comfort c/o 286 lb F/573 lb R or HPA's c/o kits with 336 lb F/672 lb R.

In trying to make sense of what 'Orb' has written; if the rear spring rate isn't tripled from the front, will it further promote understeer and the car won't settle at the same time front to back when going over a bump?
Right. The settling behavior could be "fixed" with shocks, but my understanding is that would cause other problems.

Quote:
In the second example, wouldn't a 224 lb F/ 672 lb R spring set up be more congruent with the way the car was engineered?
Yes.

The 336/672 setup does two things: it messes up the nice flat ride you get with BMW's rates, and, it causes understeer (unless that's corrected somehow). The flat ride is not important for racing though.
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      11-01-2011, 10:39 AM   #50
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Quote:
Originally Posted by cvc 22349a View Post
Interesting to see this thread after almost 3 yrs.
I don't understand motion ration and susp frequency and don't really want to try.
I am interested in why aftermarket susp tuners don't emulate BMW's tripling the rear spring rate from the front (the 135i, 335i and M3 oe rates are all tripled in the rear from the front). Anyone know why?
A favorite vendor on this forum recommends doubling the rear from the front w/Swift springs. Other manufacturers don't even double the rear rate from the front.
What is your source for the spring rates? The figures I've seen do not agree with your theory about tripling the front rate.

Stock 135i:

• Front: 120 lb/in
• Rear : 350 lb/in

135i BMW Performance:

• Front: 160 lb/in
• Rear: 420 lb/in

Stock M3 e92:

• Front: 160 lb/in
• Rear: 550 lb/in
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      11-01-2011, 11:01 AM   #51
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Quote:
Originally Posted by GaryS View Post
What is your source for the spring rates? The figures I've seen do not agree with your theory about tripling the front rate.

Stock 135i:

• Front: 120 lb/in
• Rear : 350 lb/in

135i BMW Performance:

• Front: 160 lb/in
• Rear: 420 lb/in

Stock M3 e92:

• Front: 160 lb/in
• Rear: 550 lb/in

The first page of this thread..

Adding:
E9X 335i:

Front 145 lb/in
Rear 460 lb/in

135i and 335i are approx. tripled in the rear from the front. The M3 is more than tripled in the rear.
As previously stated, I'm just wondering why the aftermarket doesn't try to emulate that formula, instead choosing to go with much higher front rates and doubling (or not even doubling) the rear rates, and what effects do the aftermarket rates have on understeer and front to rear bump absorbtion?
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      11-01-2011, 11:09 AM   #52
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Quote:
Originally Posted by GaryS View Post
What is your source for the spring rates? The figures I've seen do not agree with your theory about tripling the front rate.
I think CVC is using tripling as an approximation, to avoid having to calculate wheel rates. :-) After all, as long as one is focusing on one particular car, you can make comparisons by comparing spring rates, without having to go to wheel rates. Makes sense.

It is true there are differences between the M and non-M cars in the rear, and the performance setup uses a stiffer front than BMW's ZSP setups.

But all the cars you showed have wheel rates that are very close front to back. CVC is talking about various coilover setups that have significantly higher wheel rates in front. What are they up to?

Like the Ohlins DFV setup, for example. Their standard springs give 110 pounds per inch wheel rate in back? What led them to choose such a low rate relative to the front, and how do they overcome the disadvantages? My theory about the mechanical limit in back does not explain their choice.
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      11-01-2011, 11:12 AM   #53
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Quote:
Originally Posted by tscdennab View Post
^ Interesting ! Maybe we can hear some more opinions from people that race their cars with different spring rates (like HP Autowerks) ?
I agree, it would be great to hear from those who set up and race the configurations we're talking about!
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      11-01-2011, 02:14 PM   #54
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Quote:
Originally Posted by cvc 22349a View Post

135i and 335i are approx. tripled in the rear from the front. The M3 is more than tripled in the rear.
As previously stated, I'm just wondering why the aftermarket doesn't try to emulate that formula, instead choosing to go with much higher front rates and doubling (or not even doubling) the rear rates, and what effects do the aftermarket rates have on understeer and front to rear bump absorbtion?
Oh, okay. I guess you saw the answers that HP Autowerks and Orb gave to a similar question earlier in this thread. In any case, that's the main reason why I chose BMW Performance springs. I couldn't find enough information to have confidence that any other aftermarket brand was trying to achieve what I wanted and knew how to achieve it.
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      11-01-2011, 02:51 PM   #55
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Very interesting information indeed!

How do you calculate the frequency of the springs? I've lost track of that!
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      11-01-2011, 03:27 PM   #56
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Quote:
Originally Posted by GaryS View Post
Oh, okay. I guess you saw the answers that HP Autowerks and Orb gave to a similar question earlier in this thread. In any case, that's the main reason why I chose BMW Performance springs. I couldn't find enough information to have confidence that any other aftermarket brand was trying to achieve what I wanted and knew how to achieve it.
I read what Orb wrote about susp frequencies, motion ratio and wheel rates. That lead to my question why tuners aren't tripling the rear spring rate from the front. Haven't seen anything from a susp tuner or vendor on this.
I've also considered the BMW PS springs to maintain BMW engineered motion ratio; just couldn't get their rear bumpstops seperate from the kit, so I'm still 'rolling' on the zsp springs as opposed to increasing spring rates which aren't congruent with the way the car was engineered..
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      11-01-2011, 06:44 PM   #57
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Quote:
Originally Posted by vasillalov View Post
Very interesting information indeed!

How do you calculate the frequency of the springs? I've lost track of that!
The natural frequency in Hz is:

sqrt(386.4*K / M) / 6.28

K = spring rate in lbs per inch
M = corner weight in lbs (remember this is unsprung -- EDIT: sprung, not unsprung -- weight)

386.4 is a magic constant to get the units to work out.
6.28 is 2pi
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Last edited by luckyu; 12-18-2011 at 05:38 PM..
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      11-01-2011, 07:03 PM   #58
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Quote:
Originally Posted by GaryS View Post
Oh, okay. I guess you saw the answers that HP Autowerks and Orb gave to a similar question earlier in this thread. In any case, that's the main reason why I chose BMW Performance springs. I couldn't find enough information to have confidence that any other aftermarket brand was trying to achieve what I wanted and knew how to achieve it.
The question was never really answered. Orb's answer suggested that the other vendors' cars are underperformers, and Harold's answer suggested that the other vendors were using trial and error, and were pursuing the goal of limiting travel in front to avoid undesirable camber change. At least that's how I read "using experience from previous chassis" (that's a paraphrase).

The performance suspension by the way does not give the "flat ride" that the ZSP does. The front rate is much too high.
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      11-02-2011, 03:20 AM   #59
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Quote:
Originally Posted by luckyu View Post
The performance suspension by the way does not give the "flat ride" that the ZSP does. The front rate is much too high.
The problem with the BMW Performance suspension rates is that the rates you see discussed were estimated by Mr. 5 with a lot of assumptions and never tested on a bench. He also got the suspension with M-Sport dampers, while others in Europe get it with different dampers.

Also, different people receive different front springs depending on their car's options.

So the BMW Performance spring rates are not at all reliable (unless you have a better source for the rates than this estimation ?).
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      11-02-2011, 05:28 AM   #60
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Quote:
Originally Posted by tscdennab View Post
The problem with the BMW Performance suspension rates is that the rates you see discussed were estimated by Mr. 5 with a lot of assumptions and never tested on a bench.
I always thought those figures for 335i Performance suspension looked wrong. The figures for 135i Performance Suspension were confirmed by jah29, and they agree with the BMW advertising that they are 34% firmer than stock.

Now I see that BMW advertising says the same thing about the 335i Performance Suspension: "Spring rates are increased 48% compared to standard suspension and 34% compared to the sports suspension." So assuming the figures posted for e90 ZSP are correct, that makes it:

Front: 145 x 1.34 = 194.3 lb/in
Rear: 460 x 1.34 = 616 lb/in
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      11-02-2011, 08:53 AM   #61
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Quote:
Originally Posted by luckyu View Post
The natural frequency in Hz is:

sqrt(386.4*K / M) / 6.28

K = spring rate in lbs per inch
M = corner weight in lbs (remember this is unsprung weight)

386.4 is a magic constant to get the units to work out.
6.28 is 2pi
Thank you! I love magical constants
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      11-02-2011, 09:04 AM   #62
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This is a great thread with some really important technical information here!

So here is an obvious question from an end user who does not want to spend time and money experimenting with different suspensions: what suspension should I get? ...and I realize this is a loaded question and there are too many variables. So here are some more details:

CURRENT SETUP:

* E90 335i with ZSP, 6MT and full bolt ons
* Sport suspension with all OEM components.

PURPOSE AND GOALS:

* Improve handling of the car on AutoX and track events. I plan on attending about 20 AutoX and 3-5 track events per year.
* Maintain sufficient ride comfort for long trips
* Maintain sufficient clearance and comfort for pot-hole ridden streets of Chicago

I realize that those goals may be potentially mutually exclusive or at least difficult to achieve, but I am hoping that all of the suspension nerds will be able to put together a "shopping list" of suspension modifications that can help me achieve those goals.

Thanks in advance.
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      11-02-2011, 11:18 AM   #63
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Quote:
Originally Posted by tscdennab View Post
The problem with the BMW Performance suspension rates is that the rates you see discussed were estimated by Mr. 5 with a lot of assumptions and never tested on a bench. He also got the suspension with M-Sport dampers, while others in Europe get it with different dampers.

Also, different people receive different front springs depending on their car's options.

So the BMW Performance spring rates are not at all reliable (unless you have a better source for the rates than this estimation ?).
That's true and I was skeptical about them too, but I'm beginning to think they are close to the correct numbers.

GaryS, we believe the 160/420 numbers Jah29 gave for the 135i performance springs, but those also do not give a flat ride. To get a flat ride optimized at 65mph, you would need 564 in back, and optimized at 75mph you need 542 in back.

160/420 gives equal natural frequencies front and rear, and on the 135i they are 1.38 Hz. (The "flat ride" tuning yields a slightly higher nat. frequency in the rear.)

195/505 (I'm doing this from memory now, numbers may be slightly off) gives the same thing, nearly identical frequencies (1.47 Hz / 1.45 Hz on the 335i manual).

So, I think for performance suspension BMW is making the frequencies equal, for whatever reason. This is a more "race-like" setup, used in race cars that have spring frequencies around 2 Hz.

One more thing. I came across tested numbers for the 1M coupe. 130/480. These numbers give frequencies of 1.24 Hz / 1.48 Hz on the 1M, and yield a flat ride optimized at about 47 mph.

The E92 M3's frequencies are 1.31 Hz / 1.49 Hz which yield a flat ride optimized at 67 mph.
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      11-02-2011, 05:18 PM   #64
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I'm lost with the whole frequency thing. So the higher the frequency the less 'race like' a setup it is? I'm looking at getting the Eibach Pro Street S for my car. It's pretty much dead on with ZSP with exception of the progressive springs up front that go up to 271lbs. I'm a n00b when it comes to suspension. Anyone know what I can expect out of that setup?

I would do the math for the frequency myself if I knew what the corner weight was.

Eibach Pro Street S
Front: 134 - 271 lb/in (Progressive)
Rear: 456 lb/in (Linear)
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      11-03-2011, 12:18 AM   #65
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Quote:
Originally Posted by tag824 View Post
I'm lost with the whole frequency thing. So the higher the frequency the less 'race like' a setup it is? I'm looking at getting the Eibach Pro Street S for my car. It's pretty much dead on with ZSP with exception of the progressive springs up front that go up to 271lbs. I'm a n00b when it comes to suspension. Anyone know what I can expect out of that setup?

I would do the math for the frequency myself if I knew what the corner weight was.

Eibach Pro Street S
Front: 134 - 271 lb/in (Progressive)
Rear: 456 lb/in (Linear)
Race cars tend to be 2 Hz, or higher. Road cars tend to be 1.5 Hz or lower. Books say that people find >1.5 Hz uncomfortable.

You can compute corner weight from the wet weight BMWUSA gives, and estimating unsprung weight. See the thread "M3 on 328i" for examples.

Front corner:
wetweight * frontweightdist / 2 - 105

Rear corner:
wetweight * (1 - frontweightdist) / 2 - 135
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      11-03-2011, 06:38 AM   #66
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I just learned the KW V3 spring rates from another thread:

Front: 190-257lb/in
Rear: 543-571lb/in

These look very well chosen to me, from the perspective of posted formulas.
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