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      01-10-2010, 06:15 PM   #23
Andrew 912
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Very Nice write up.... informative and details... I am sure it would so much helps to our member friends (You have the same goal as to many of us).
Thank you for your time to write in this forum. Please keep your update post!
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      01-10-2010, 09:29 PM   #24
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Originally Posted by JAJ View Post
The perch-perch distance at full droop can't be changed on the stock setup. The shortest spring you can install must be long enough to get from one perch to the other with no gap. Otherwise the spring will come adrift when you jack the car up.

So, the length is what it must be. The only other parameter is the rate, and the spring rate is determined by how far you need it to compress at static position. The softer the spring, the lower the ride.

Because BMW set the suspension up with preload, they created design flexibility to shorten the spring and stiffen it to about 200# per inch while keeping the stock ride height.

At that point the damping will be out of tune. However, if you now reduce the spring rate to lower the car, the damping comes back into tune as the car gets lower. The reduction of travel is a whole other matter - the best damper tuning can't compensate for the rapid climb in spring rate created by the bump stop.

The only "get out of jail free" card in this equation is the prospect of doing a bimodal progressive wind on the spring. It starts with a softer section that allows the car to settle most of the way to static, then that section goes into bind and the stiffer section takes over and controls the motion in the narrow stroke range above and below the static height. The Bilstein PSS10 spring shows this exact winding design.

However, the progressive spring strategy requires new dampers because the stock dampers are tuned for much lower spring rates. The car would be pretty bouncy on stock dampers. Of course, now that you have to replace the dampers, you might as well make them height adjustable - oh, wait! We've just designed a Type Two (street sport) kit!
I don’t follow you here. Building on your data, assume linear springs and motion ratio = 1, and use a “lowering” spring that barely bridges the perch-perch gap (preload spring compression = 0"; thus, shorter than stock spring), which drops the car by 0.25” (steady state suspension compression = 4.25”). That would result in k = 188 ft lb (800 lb/4.25 in), which is 13% stiffer than stock. So, you can have a shorter and stiffer spring resulting in a net drop over stock.
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      01-11-2010, 01:44 AM   #25
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Originally Posted by lucid View Post
I don’t follow you here. Building on your data, assume linear springs and motion ratio = 1, and use a “lowering” spring that barely bridges the perch-perch gap (preload spring compression = 0"; thus, shorter than stock spring), which drops the car by 0.25” (steady state suspension compression = 4.25”). That would result in k = 188 ft lb (800 lb/4.25 in), which is 13% stiffer than stock. So, you can have a shorter and stiffer spring resulting in a net drop over stock.
We're doing exactly the same math. Zero preload, zero drop = 200#, zero preload, 1 inch drop = 160#.

According to the advertising, Eibach drops the front 0.8" (yielding 166#), H&R drops it 1.0 # (yielding 160#) and Dinan drops it 1/2" (yielding 177#).

Dinan is the only one of the three with the opportunity for stiffer than stock springs, and there's no way to know if they actually stiffened them or if they did a shorter stock-rate spring and a smaller pre-load.
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      01-11-2010, 01:49 AM   #26
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Quote:
Originally Posted by JAJ View Post
My opinion (I promised to keep them out of this, but it's just too tempting) is that Dinan springs and suspension parts matched with Bilstein's Sport dampers would work really well together.
What do you think about Eibach springs matched with Bilstein Sport dampers? They have internal bump stops on the front. No idea how much shorter they are then stock.
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      01-11-2010, 01:54 AM   #27
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Originally Posted by akina335 View Post
What do you think about Eibach springs matched with Bilstein Sport dampers? They have internal bump stops on the front. No idea how much shorter they are then stock.
It would probably "look" a lot like the RDSport suspension. How it would "work", I can't guess.
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      01-11-2010, 08:16 AM   #28
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Quote:
Originally Posted by JAJ View Post
We're doing exactly the same math. Zero preload, zero drop = 200#, zero preload, 1 inch drop = 160#.

According to the advertising, Eibach drops the front 0.8" (yielding 166#), H&R drops it 1.0 # (yielding 160#) and Dinan drops it 1/2" (yielding 177#).

Dinan is the only one of the three with the opportunity for stiffer than stock springs, and there's no way to know if they actually stiffened them or if they did a shorter stock-rate spring and a smaller pre-load.
OK. The reason I specifically picked 0.25" drop is because I think that is what the Dinan spring does. My understandings is that the other 0.25" is supposed to come from the shaved guide support (I haven't taken one of these assemblies apart, so I am not certain of the details), and the total drop ends up being 0.5". The Dinan springs indeed feel slightly stiffer than stock. I jacked up the car last night, and they do have preload--not sure how much. So, my guess is that there is some preload to keep them in there, so they are about 10% stiffer.
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      01-11-2010, 08:21 AM   #29
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I have ordered Bilstein coilovers... I have heard good things about them
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      01-11-2010, 11:19 AM   #30
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Quote:
Originally Posted by JAJ View Post
We're doing exactly the same math. Zero preload, zero drop = 200#, zero preload, 1 inch drop = 160#.

According to the advertising, Eibach drops the front 0.8" (yielding 166#), H&R drops it 1.0 # (yielding 160#) and Dinan drops it 1/2" (yielding 177#).

Dinan is the only one of the three with the opportunity for stiffer than stock springs, and there's no way to know if they actually stiffened them or if they did a shorter stock-rate spring and a smaller pre-load.
I would like to point out a few facts:

1. Based on my hands-on experience, H&R is stiffer than Dinan, and Dinan is slightly stiffer than stock.

2. H&R is shorter than Dinan, and Dinan is shorter than stock.

3. All three have some type of preload. Stock over 2", Dinan roughly 2", and H&R around 1.5".

4. Static compression on stock springs is not 4". It is actually around 3" max. The shock only has 5.75" of travel; the bumpstop compresses to around 1" max. Therefore we would only get 5.75-4-1=0.75" of bump travel based on 4", which is unrealistic.

So how is it possible that H&R is stiffer and create a drop vs. stock? The key is preload.

According to #2 above, the stock spring is preloaded by more than 2", which means about 300 to 400 pounds or roughly 50% of the unsprung weight per corner "disappears", which means only 50% of USW is actually used to compress the spring from its perch-to perch-length.

H&R has a lot less preload, which means more unsprung weight is used to compress the spring from its perch-to-perch length. This is how it gets away from being stiffer and providing a drop.
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      01-11-2010, 12:09 PM   #31
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rldzhao, if the stock spring preload compression is indeed 2" (JAJ's numbers suggest that is more like 0.8"), then that creates even more room for using shorter and stiffer aftermarket springs, and would explain how H&Rs are stiffer and result in 1" drop. I haven't taken any measurements on any of this, so I'm going with what you guys have posted here so far. All I can measure is the natural length of the stock springs as they are in my basement. I believe the fronts are ~11.5".
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      01-11-2010, 12:50 PM   #32
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      01-11-2010, 03:50 PM   #33
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This def needs to be stickied.
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      01-11-2010, 05:04 PM   #34
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Quote:
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This def needs to be stickied.
Agreed. Done.
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      01-11-2010, 10:13 PM   #35
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Quote:
Originally Posted by lucid View Post
rldzhao, if the stock spring preload compression is indeed 2" (JAJ's numbers suggest that is more like 0.8"), then that creates even more room for using shorter and stiffer aftermarket springs, and would explain how H&Rs are stiffer and result in 1" drop. I haven't taken any measurements on any of this, so I'm going with what you guys have posted here so far. All I can measure is the natural length of the stock springs as they are in my basement. I believe the fronts are ~11.5".
From my fresh measurement:

Stock front spring: ~12"
Front spring (mounted) perch-to-perch distance: ~8"
Preload compression: 12"-8"=~4"


Which means that the front suspension experiences a preload of roughly:

147 lbm/in * 4 in = ~588 lbm

Now, this number may be slightly high because the spring stiffness during the initial 20% of travel is less than 147 lbm/in, which means the preload will be <588 lbm.
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      01-11-2010, 10:18 PM   #36
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Excellent post, thanks for taking the time to do this.
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      01-11-2010, 10:31 PM   #37
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Most of this stuff I don't understand but I find it fascinating. Thankss JAJ for all of your efforts.
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      01-11-2010, 10:33 PM   #38
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JAJ: Thanks for the extensive write-up.

Just curious, was the EDC option completely out of the question for you when you decided on your vehicle? In other words, were you against it from the beginning from a technological standpoint, or just knew you wanted something that would exceed the EDC's track performance? Do you expect to get similar (or better) comfort or civility with daily driving in your new set-up, or have you foregone using it as a DD all-together?


Sorry for all of the questions, but your posts got me to thinking...
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      01-11-2010, 11:02 PM   #39
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Quote:
Originally Posted by rldzhao View Post
From my fresh measurement:

Stock front spring: ~12"
Front spring (mounted) perch-to-perch distance: ~8"
Preload compression: 12"-8"=~4"


Which means that the front suspension experiences a preload of roughly:

147 lbm/in * 4 in = ~588 lbm

Now, this number may be slightly high because the spring stiffness during the initial 20% of travel is less than 147 lbm/in, which means the preload will be <588 lbm.
Thanks. Knowing the perch-perch distance helps.
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      01-12-2010, 12:51 AM   #40
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Gentlemen

First, let me say I didn't have a charitable bone in my body when I did the work - I did it for me! I wanted to upgrade my suspension. It was only after I'd put the data together that I realized it might be useful for others.

Second, my measurements are as best as I could do them with the suspension still on the car. Some, like the droop of 4", I'm pretty sure of. Others, like the depth of the front and rear bump stops, I'm good within 1/4 inch, but probably no better. Other members, like RLDZHAO, will do better because they can measure the components when they're off the car.

Third, my assessment of the four "types" of suspension are generic and not intended to either criticize or endorse any particular product. Suspensions, at least at the level of "track day" and below, are not particularly finicky. I once ran 400# per inch rear racing springs (an increase of more than double the stock rate) on my E39 M5 with street-spec non-adjustable dampers (Koni FSD). The car rode like a limousine on the road and was superb on the track in the rain; only when the track dried was it obvious that the rear damping was softer than optimal.

The damper settings for compression and rebound, even when they're perfect, are a compromise. The needed "resistive" settings vary as the square root of the spring rate, so a 20% increase in the spring rate only changes the damper settings by 10%, which frankly is probably less than the margin of error in the manufacturing tolerances.

What's more, the car's damping needs change depending on whether you're cornering or going straight. The front sway bar couples both front sides together. Your carefully chosen spring rate and damper setting go out the window when the (undamped) sway bar drags the springs and dampers on the other side of the car into the calculations as you go around a corner. What's more, the rear suspension gets involved too. If it rolls more easily and farther than the front, it transfers weight forward in a turn. The damper setting has to be good enough, but perfect is just not feasible, even with four-way adjustable racing kit.

My point is that it's way too easy to over-think suspension design and settings for street-track vehicles. Your choice of seats will have as big an effect on lap-times as your suspension. Pick a reputable supplier, follow the instructions, and once you know how the kit works, make incremental changes one at a time with care.

YMMV, naturally.

Last edited by JAJ; 01-12-2010 at 01:12 AM.
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      01-12-2010, 01:08 AM   #41
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Quote:
Originally Posted by OC kid View Post
JAJ: Thanks for the extensive write-up.

Just curious, was the EDC option completely out of the question for you when you decided on your vehicle? In other words, were you against it from the beginning from a technological standpoint, or just knew you wanted something that would exceed the EDC's track performance? Do you expect to get similar (or better) comfort or civility with daily driving in your new set-up, or have you foregone using it as a DD all-together?


Sorry for all of the questions, but your posts got me to thinking...
I've owned some "first generation BMW electronics" before. I've become careful, let's just say.

When I bought the third or fourth E92 M3 delivered by my dealer in March 2008, just as the cars first hit the market in Canada, I knew I didn't want anything they hadn't delivered in production before (except for the S65, and that was a really just copy of the S85 with the VANOS fixed).

The car was inbound when I chose it, and I didn't have the option of EDC. At the time I thought that was a good thing, having just removed the Ground Control suspension from my E39 M5 so I could sell it.

I was pretty sure that I'd eventually upgrade the M3 suspension, and I just couldn't imagine EDC making it easier. Because the car was early delivery, I have 6MT too, since DCT didn't come out for another six months. Again, I'm happy that I don't have it, regardless of how good it finally became after two years of bug fixes.

So the answer to your question is "I was skeptical at the time, so I bought the simplest configuration that was available" on the assumption it would also be the most trouble-free. I wasn't disappointed; the car has been flawless since I picked it up.

As for DD use, in the summer of 2007 I decided that the $3,000 a year I was spending on gas, insurance and parking was a waste of money (it could be better spent on R-Comps, rims and upgrades) so I switched to public transit for commuting. I drive the M3 about 8,000 miles a year now, and a lot of it is highway or track time. I DO want a car that's pleasant to drive, though, even if I don't drive it all the time. Vancouver roads are nasty in some parts of the city, and I want my creature comforts intact.

Last edited by JAJ; 01-12-2010 at 01:35 AM.
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      01-12-2010, 01:24 PM   #42
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JAJ, as I mentioned before, the information and opinions you posted in this thread are highly appreciated. That said, I think follow up discussion aimed at the clarification/correction of some of what has been posted is constructive.

You make a good point about a statically calibrated system not being able to achieve optimum damping at all times in real world conditions. That is somewhat different for a dynamically calibrated system, and I wonder about the degree of adjustability of the OEM EDC dampers and what kind of inputs the programming considers apart from just the velocity across the element.
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      01-13-2010, 01:53 AM   #43
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Quote:
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JAJ, as I mentioned before, the information and opinions you posted in this thread are highly appreciated. That said, I think follow up discussion aimed at the clarification/correction of some of what has been posted is constructive.

You make a good point about a statically calibrated system not being able to achieve optimum damping at all times in real world conditions. That is somewhat different for a dynamically calibrated system, and I wonder about the degree of adjustability of the OEM EDC dampers and what kind of inputs the programming considers apart from just the velocity across the element.
The more measurements the better! Sorry if I sounded defensive, I just wanted to be clear that the posts were nothing more than generalizations, and they're going to be a bit smudgy when it came to the details.

In other news, I've spent some spare time in the last couple of days shopping for accelerometer chips and data capture boxes. I'm going to have a go at figuring out how to adjust the dampers by combining measured dynamic data with seat-of-the-pants feel.

This is going to be fun. I'll keep y'all posted...
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      01-13-2010, 07:36 AM   #44
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In other news, I've spent some spare time in the last couple of days shopping for accelerometer chips and data capture boxes. I'm going to have a go at figuring out how to adjust the dampers by combining measured dynamic data with seat-of-the-pants feel.

This is going to be fun. I'll keep y'all posted...
This will be interesting!
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