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BMW 3-Series (E90 E92) Forum > E90 / E92 / E93 3-series Technical Forums > Suspension | Brakes | Chassis > Springs debate



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      01-25-2017, 06:29 PM   #1
bmuum3
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Springs debate

If you are on aftermarket shocks, what is everyones preference for springs? H&R are said to be harsh and Eibach doesn't lower the car as much. Swift R spring are out but have really seen a lot on them in term of feel. I think they are linear rather than progressive.
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      01-25-2017, 08:00 PM   #2
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You just answered your own questions.
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      01-26-2017, 12:09 AM   #3
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Well if we're having a debate let's discuss spring rates....Eibachs have the same rates as stock, Swifts are much higher.

http://www.swiftsprings.net/products...ct/spec-r.html

Bmw performance suspension seem to be the same as Dinans which is close to m3 spring rates. So my question is how to decide on the spring rate you want.
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      01-26-2017, 01:08 AM   #4
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How to decide spring rates?

Short Answer:
Pick the rear spring rate you want based on what you plan on doing.
Daily Driver - 460 - 700 lb/in
Some Autocross or Track - 600 - 750 lb/in
Hardcore Autocross or Track - 700 - 800 lb/in

Front spring rate = Rear rate * (145/460). This will give you a smooth ride on a car that will see regular roads. BMW designed them this way for a reason.

You can increase the front rate on a dedicated track car because tracks are smooth and you don't have to worry about bumps ruining your ride as much.

Long Answer:
To decide on the spring rate you want, you first need to determine the natural frequency that you want. The stock m-sport suspension has a frequency around 1.3 Hz for the front and 1.5 Hz for the rear. For a daily driver, I would not go greater than 1.8 Hz in the rear, but if you are going to see a lot of track time, you can go for 2.0 Hz to 2.5 Hz. But the problem for a e9x BMW is that the rear spring rate cannot get high enough to go over 2.0 Hz. The max you can get in the back is 800 lb/in; any more than that and you are going to tear apart the rear. A rate of 800 lb/in will give you a rear natural frequency of 1.95 to 2.0 depending on the weight of the car.

You also want the front's frequency to be lower than the rear's so that your car does not have a harsh ride. BMW designs the rear frequency to front frequency ratio to be about 1.15. So for a car with 1.8 Hz rear frequency, you want a front frequency of 1.56 Hz. The reason you want the front to be lower than the rear is so that when you hit a bump the rear can "catch up" to the front's spring going up and down. That way the car stays flat as it moves up and down. Some studies were done in the 1950s, and the testing showed that a ratio of 1.2 would provide a "flat ride." But cars go faster now, so the ratio should be lower. That is probably why BMW designs for a 1.15.

If you end up with the front frequency higher than the rear, your car will pitch and have a harsh ride. You will need a large amount of dampening force up front to control the pitching, and that will make the ride even harsher.

H&R does not release their spring rates, but my guess is that they have the front too high in relation to the rear. And I would not buy springs from anyone that would not tell me the rate.

So how do you calculate the natural frequency? You need to know the spring rate, the sprung mass over each axle, and the motion ratio of the front and rear. You also need to know all the different formulas for using them. I had to do a lot of reading on this forum and others to make a spreadsheet that I'm using. Just stick to OEM and ratios and don't go over 800 in the rear.

I'm getting coilovers with 224 in the front and 728 in the rear. Based on my estimates and calculations, my rear frequency will be 1.7 Hz and my front will be 1.5 Hz.
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      01-26-2017, 02:16 AM   #5
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Great info and please share the spreadsheet!

Please?
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      01-26-2017, 02:35 AM   #6
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Quote:
Originally Posted by Go Horns! View Post
How to decide spring rates?

Short Answer:
Pick the rear spring rate you want based on what you plan on doing.
Daily Driver - 460 - 700 lb/in
Some Autocross or Track - 600 - 750 lb/in
Hardcore Autocross or Track - 700 - 800 lb/in

Front spring rate = Rear rate * (145/460). This will give you a smooth ride on a car that will see regular roads. BMW designed them this way for a reason.

You can increase the front rate on a dedicated track car because tracks are smooth and you don't have to worry about bumps ruining your ride as much.

Long Answer:
To decide on the spring rate you want, you first need to determine the natural frequency that you want. The stock m-sport suspension has a frequency around 1.3 Hz for the front and 1.5 Hz for the rear. For a daily driver, I would not go greater than 1.8 Hz in the rear, but if you are going to see a lot of track time, you can go for 2.0 Hz to 2.5 Hz. But the problem for a e9x BMW is that the rear spring rate cannot get high enough to go over 2.0 Hz. The max you can get in the back is 800 lb/in; any more than that and you are going to tear apart the rear. A rate of 800 lb/in will give you a rear natural frequency of 1.95 to 2.0 depending on the weight of the car.

You also want the front's frequency to be lower than the rear's so that your car does not have a harsh ride. BMW designs the rear frequency to front frequency ratio to be about 1.15. So for a car with 1.8 Hz rear frequency, you want a front frequency of 1.56 Hz. The reason you want the front to be lower than the rear is so that when you hit a bump the rear can "catch up" to the front's spring going up and down. That way the car stays flat as it moves up and down. Some studies were done in the 1950s, and the testing showed that a ratio of 1.2 would provide a "flat ride." But cars go faster now, so the ratio should be lower. That is probably why BMW designs for a 1.15.

If you end up with the front frequency higher than the rear, your car will pitch and have a harsh ride. You will need a large amount of dampening force up front to control the pitching, and that will make the ride even harsher.

H&R does not release their spring rates, but my guess is that they have the front too high in relation to the rear. And I would not buy springs from anyone that would not tell me the rate.

So how do you calculate the natural frequency? You need to know the spring rate, the sprung mass over each axle, and the motion ratio of the front and rear. You also need to know all the different formulas for using them. I had to do a lot of reading on this forum and others to make a spreadsheet that I'm using. Just stick to OEM and ratios and don't go over 800 in the rear.

I'm getting coilovers with 224 in the front and 728 in the rear. Based on my estimates and calculations, my rear frequency will be 1.7 Hz and my front will be 1.5 Hz.
All this exactly.
The frequency is the biggest thing. The front wheel rate(the effective spring rate at the wheels) is very close to the spring rate, while the rear wheel rate is only about 1/3 of the spring's rate, due to the springs placement on the camber arm. This is why you see such a "high" rear rate coupled with a relatively "low" front rate.

I don't think 800 for the rear is the max. There are guys on here, and m3post that have/are running 1,000 in the rear(this ends up being 316 lb/in at the wheels) some even slightly over that.

My rates were 450 front 800 rear. The car was sort of comfortable, but there was significant pitching when going over bumps. For passengers this meant that their heads were always bouncing off the headrest. Upped the rear rate to 1000, and the ride got MORE comfortable(on all damper settings). Much less pitching, although it is still slightly present. The quality of your dampers is also gonna have a huge impact on how comfy the ride is. You've also gotta make sure that your shocks are valved for the rates you pick, or it will be uncomfortable regardless.
No matter what, you're gonna have to go stiffer the more you lower the car, to keep from bottoming out frequently, and to counter the effect of increased body roll due to lengthening the roll couple.

By my math 1000 rear puts your frequency right at 2.0 Hz, and 350 front puts you at 1.86 Hz. The high front spring rate is desirable to curb brake dive, to keep lowered cars from rolling so much, and to avoid getting into the unfavorable part of the camber curve. You could try to counter this with bump stops, and a big front bar, but this creates it's own problems. Some shock companies don't even want you using bump stops with their dampers.

No matter what you do, you will be compromising in some manner.
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      01-26-2017, 10:37 AM   #7
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Quote:
Originally Posted by bmuum3 View Post
If you are on aftermarket shocks, what is everyones preference for springs? H&R are said to be harsh and Eibach doesn't lower the car as much. Swift R spring are out but have really seen a lot on them in term of feel. I think they are linear rather than progressive.
I have the B12 lowering kit on my 335i. After the 4 month break in period, the springs, shocks and struts settle. With the drop you get with this kit, you wouldn't want to go much farther down, or you will start to limit yourself in terms of the size of tire's you can fit the car with.
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      02-02-2017, 10:55 PM   #8
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Eibach Prokit = 1" drop - .5" on sport models. Once settled you will have about 1-2 finger fender gap on stock tires. Ride is firm, but comfortable for everyday.

H&R - 1.5" drop = no fender gap. People who say they are harsh probably installed them wrong (wrong bumpstops) or have old worn out stock shocks. They are firm, but not harsh if done right. But they are lower than advertised.

Swift Spec R - Great spring! Firm but linear. Best choice if you like to go to the track every now and then, or if you do not have a wife who complains about your firm ride. 1" drop - maybe less. Best looking spring if that matters to you.
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      02-21-2017, 06:22 PM   #9
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So whats the consensus on E93 with the added weight in back. I have and E90 also with Eibach springs and its fine for daily but E93 is like 500lbs more and I'm not sure if same spring is the best way to go..thoughts thx
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Last edited by 335togo; 02-21-2017 at 08:43 PM. Reason: error
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      03-02-2017, 11:00 PM   #10
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