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      07-14-2018, 06:31 PM   #1
Shadowfax028
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On a quest!!! Spring rates

BMW in there infanant wisdom will not provide spring rates in its product description. I would like to know if anyone on this forum has the spring rates for the M235i with adaptive suspension f/r, along with the MPerformance (Red Spring) rates?
I'm hoping someone might have a spring dyno and have quantitative information or specifications.

Last edited by Shadowfax028; 07-14-2018 at 07:41 PM..
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      07-15-2018, 10:37 AM   #2
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Did you try emailing aftermarket vendors? They probably spec'd those as part of their R&D.
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      07-27-2018, 12:46 PM   #3
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Bump!
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      01-01-2019, 09:00 AM   #4
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I found this a few weeks ago while wondering the same question. I'm not sure if US/UK Spec M235i have the same spring rates but the guys in the UK have done a bit of work to figure this out. Rates for rears are below, as is a link to the discussion on babybmw.net. It seems like the fronts are 120-180 lb/in depending on the model of your car or aftermarket springs you buy. If someone here knows better, please share! OP - Hope this helps!

H&R M135i Rear = 348 lb/inch
OEM M135i Rears = 383 lb/inch
Eibach Rear = 384 lb/inch
'AC Schnitzer' Rear = 384 lb/inch
H&R M235i Rear = 392 lb/inch
OEM M235i Rears = 417 lb/inch
OEM M140i Rears = 587 lb/inch (~same as OEM M3/M4)

https://www.babybmw.net/forum/viewto...44195#p1344195
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      01-01-2019, 10:33 AM   #5
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Quote:
Originally Posted by ghost.M View Post
I found this a few weeks ago while wondering the same question. I'm not sure if US/UK Spec M235i have the same spring rates but the guys in the UK have done a bit of work to figure this out. Rates for rears are below, as is a link to the discussion on babybmw.net. It seems like the fronts are 120-180 lb/in depending on the model of your car or aftermarket springs you buy. If someone here knows better, please share! OP - Hope this helps!

H&R M135i Rear = 348 lb/inch
OEM M135i Rears = 383 lb/inch
Eibach Rear = 384 lb/inch
'AC Schnitzer' Rear = 384 lb/inch
H&R M235i Rear = 392 lb/inch
OEM M235i Rears = 417 lb/inch
OEM M140i Rears = 587 lb/inch (~same as OEM M3/M4)

https://www.babybmw.net/forum/viewto...44195#p1344195
They seem to have done a lot of analysis on the rear spring rates with plausible results, but the rates for the front that were thrown out are highly suspect, a lot less than expected and would lead to handling problems if only 1/3 or 1/4 the rate of the rear. They also don't take into account any variable rate effect as the coils bind or the bump stops are reached.
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      01-02-2019, 08:46 PM   #6
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Quote:
Originally Posted by aerobod View Post
They seem to have done a lot of analysis on the rear spring rates with plausible results, but the rates for the front that were thrown out are highly suspect, a lot less than expected and would lead to handling problems if only 1/3 or 1/4 the rate of the rear. They also don't take into account any variable rate effect as the coils bind or the bump stops are reached.
You're right - there is less data on that thread about front rates than rear rates, and they don't mention the other factors you've highlight above. Seeing as no-one else (at least not yet found on this forum) has done the work to calculate, the rate estimates on that thread are about as good as you'll find anywhere though. Also, the OEM F80/F82 front spring rates aren't very stiff either.

Reading the below thread you'll find the F80/F82 crowd as well as the engineers at Swift Springs would disagree with you about the handling characteristics of high spring rate variances between front and rear on F-Series bmws. In addition, the AFE Control Coilovers for F30s (couldn't find any reviews on these) have an even higher front rear variance than the Swift Springs! Not trying to argue here, just suggesting the data seems to go against your intuition on this and maybe the front rate estimates on babybmw aren't too far off.

https://f80.bimmerpost.com/forums/sh....php?t=1054041

185 lb/in Stock M3/4 Front Springs
575 lb/in Stock M3/4 Rear Springs

268 lb/in M3 Swift Spec-R Front Springs
715 lb/in M3 Swift Spec-R Rear Springs

279 lb/in M4 Swift Spec-R Front Springs
726 lb/in M4 Swift Spec-R Rear Springs

https://afepower.com/afe-power-430-5...oilover-system

275 lbs/in Front, 1100 lbs/in rear
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      01-02-2019, 11:40 PM   #7
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Quote:
Originally Posted by ghost.M View Post
You're right - there is less data on that thread about front rates than rear rates, and they don't mention the other factors you've highlight above. Seeing as no-one else (at least not yet found on this forum) has done the work to calculate, the rate estimates on that thread are about as good as you'll find anywhere though. Also, the OEM F80/F82 front spring rates aren't very stiff either.

Reading the below thread you'll find the F80/F82 crowd as well as the engineers at Swift Springs would disagree with you about the handling characteristics of high spring rate variances between front and rear on F-Series bmws. In addition, the AFE Control Coilovers for F30s (couldn't find any reviews on these) have an even higher front rear variance than the Swift Springs! Not trying to argue here, just suggesting the data seems to go against your intuition on this and maybe the front rate estimates on babybmw aren't too far off.

https://f80.bimmerpost.com/forums/sh....php?t=1054041

185 lb/in Stock M3/4 Front Springs
575 lb/in Stock M3/4 Rear Springs

268 lb/in M3 Swift Spec-R Front Springs
715 lb/in M3 Swift Spec-R Rear Springs

279 lb/in M4 Swift Spec-R Front Springs
726 lb/in M4 Swift Spec-R Rear Springs

https://afepower.com/afe-power-430-5...oilover-system

275 lbs/in Front, 1100 lbs/in rear
There is a lot of information missing in effective spring rates, as the engineered bump stops BMW uses are in use over much of the suspension compression, so the steel spring rates are only applicable over the initial compression and all of the extension. Without looking at all the components, not a lot will be learnt about the effective suspension stiffness. The low front steel spring rates are quite similar to my fun car that weighs 1/3 as much as the M3/M4. One of the conclusions likely is that changing the front steel spring rate will have little effect when not addressing the bump stops and front anti-roll (sway) bar too.

The compound spring rate under hard cornering is going to be a lot higher than the quoted rate. The polymer bump stops also have a progressive spring rate that increases with compression, compared with a steel spring being either linear, or progressive when coil-bound with variable pitch coils. The F20 OEM front struts only have about 10mm of compression at normal ride height until the polymer bump-stop adds to the spring rate.

Another issue is going to be the effect of the rear anti-roll (sway) bar with stiff rear springs relative to the front, ensuring that it isn’t too stiff to give balanced handling and good traction under acceleration, too stiff and the inside rear wheel will easily lift while the front outside wheel is too heavily loaded.

If the front to rear rates are this disparate, then I wouldn’t change the strut / coil-over springs without understanding the total effective rate and adjusting the other components too.
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      01-03-2019, 11:52 AM   #8
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Some interesting input here.
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      01-03-2019, 01:30 PM   #9
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From a qualitative evaluation of suspension stiffness, here is some information on perceived suspension stiffness:

The natural frequency of road car suspension is usually between 1 Hz and 2.5 Hz, with the following ride qualities:
"Pillow soft" - 1.0Hz is a soft traditional Cadillac ride
"Normal but a bit stodgy" - 1.5Hz is typical of a modern SUV or family sedan
"Quite hard and sometimes uncomfortable" - 2.0Hz is typical of many sports cars
"Teeth rattling harsh" - 2.5Hz is stiff and somewhat harsh, typical of the stiffest OEM suspensions.

With a sprung corner mass of 350kg (close to that of an M240i with driver and tank of fuel, minus unsprung mass), the effective spring rate when hitting a speed bump squarely and not invoking the anti-roll (sway) bar for each of these frequencies will be expressed as f = √(k / m) ÷ 2π, or k = (2πf ^ 2)m.
k = spring rate in N/m (1 N/m = 0.00571 lb/in)
m = sprung mass in kg
f = natural frequency in Hz

For
1.0Hz - 13,821N/m (79 lb/in)
1.5Hz - 31,089N/m (178 lb/in)
2.0Hz - 55,270N/m (316 lb/in)
2.5Hz - 86,359N/m (493 lb/in)

Track cars can go as high as 5.0Hz if bump absorption is not a significant consideration, leading to spring rates as high as 345,000 N/m or 1,970 lb/in.

From the perspective of tyre impact absorption, a typical tyre at 2 bar / 30psi pressure will deflect about 10mm under a total mass of 400kg (sprung + unsprung), leading to a spring rate of about 400,000N/m (2,284 lb/in), so will not affect the perceived ride quality when hitting large bumps to any significant degree, instead being responsible for high frequency / small deflection bump absorption.
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      01-03-2019, 04:04 PM   #10
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Quote:
Originally Posted by aerobod View Post
With a sprung corner mass of 350kg (close to that of an M240i with driver and tank of fuel, minus unsprung mass)
Do you assume that to be the average sprung corner mass for each corner of the car? My corners, with ~4 gallons of fuel, 170# / 77kg driver, and with a measured tire and wheel weight of 45# / 20.5kg reduced from each scaled corner weight, are:

RF – 886# / 403kg
RR – 790# / 359kg
LR – 820# / 373kg
LF – 926# / 421kg.

FWIW, the coilover spring rates are:

Front – 350# / 61296N/m
Rear – 600# / 105079N/m.

With OEM anti-roll bars, I would say that qualitatively the car feels stiff, but not uncomfortably so. I find it to be both usefully trackable and very responsive away from the track.
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      01-03-2019, 04:26 PM   #11
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Quote:
Originally Posted by dradernh View Post
Do you assume that to be the average sprung corner mass for each corner of the car? My corners, with ~4 gallons of fuel, 170# / 77kg driver, and with a measured tire and wheel weight of 45# / 20.5kg reduced from each scaled corner weight, are:

RF – 886# / 403kg
RR – 790# / 359kg
LR – 820# / 373kg
LF – 926# / 421kg.

FWIW, the coilover spring rates are:

Front – 350# / 61296N/m
Rear – 600# / 105079N/m.

With OEM anti-roll bars, I would say that qualitatively the car feels stiff, but not uncomfortably so. I find it to be both usefully trackable and very responsive away from the track.
I assumed an average of 400kg per corner, minus 25kg per wheel and tyre, minus 25kg for brake, hub and suspension arms - for ease of calculation. A variance of +/-25kg at 2.0Hz would cause
a +/-7% variance in the spring rate at resonance i.e. 51,322N/m to 59,218N/m or 293 lb/in to 338 lb/in.
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      01-03-2019, 05:08 PM   #12
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Quote:
Originally Posted by aerobod View Post
I assumed an average of 400kg per corner, minus 25kg per wheel and tyre, minus 25kg for brake, hub and suspension arms - for ease of calculation. A variance of +/-25kg at 2.0Hz would cause
a +/-7% variance in the spring rate at resonance i.e. 51,322N/m to 59,218N/m or 293 lb/in to 338 lb/in.
Thanks - I forgot all about about the other components as far as unsprung weight goes. Regarding the suspension arms, do you use ½ the weight of each arm or the total weight of each arm?
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      01-03-2019, 06:26 PM   #13
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Quote:
Originally Posted by dradernh View Post
Thanks - I forgot all about about the other components as far as unsprung weight goes. Regarding the suspension arms, do you use ½ the weight of each arm or the total weight of each arm?
50% is a good approximation overall, especially for items such as driveshafts, springs and suspension arms that have even weight distribution along their length, less so for other semi-sprung components such as the struts.
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      01-04-2019, 12:22 AM   #14
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Rear spring rates are also typically higher (higher frequency) to produce a "flat ride" effect.

Here's a video explaining it.
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      01-04-2019, 01:27 AM   #15
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Quote:
Originally Posted by FaRKle! View Post
Rear spring rates are also typically higher (higher frequency) to produce a "flat ride" effect.

Here's a video explaining it.
You can get away with softer front springs on a road car, but on the track they can become problematic due to excessive camber change and weight transfer under heavy braking, leading to poor braking performance. Also if there is a decent amount of front camber that is higher than in the rear, soft front springs and firm rear ones will make a RWD car oversteer a lot when trying to put down power on corner exit, especially if the rear anti-roll bar is too stiff.
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      01-05-2019, 02:23 PM   #16
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Quote:
Originally Posted by aerobod View Post
You can get away with softer front springs on a road car, but on the track they can become problematic due to excessive camber change and weight transfer under heavy braking, leading to poor braking performance. Also if there is a decent amount of front camber that is higher than in the rear, soft front springs and firm rear ones will make a RWD car oversteer a lot when trying to put down power on corner exit, especially if the rear anti-roll bar is too stiff.
You just described my car. And I was wondering when someone was going to bring Shaik at FatCat Motor Sport up. He really is quite brilliant at suspension design.
My dalema is finding a aftermarket manufacturer that produces OEM style springs to work with factory dampers, and larger then OEM sway bars.

All I have been able to find are (Beauty Springs) that just lower the car!!!!

I need something suitable for Autocross.....

Does as anyone know the spring rates for the BMW Mperformance (Red) springs
BMW doesn't trust its consumers with that kinda information
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      01-05-2019, 03:44 PM   #17
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Quote:
Originally Posted by FaRKle! View Post
Rear spring rates are also typically higher (higher frequency) to produce a "flat ride" effect.
I wonder if that's car-dependent. An E36 race car I ran had spring rates of:

F – 1,599#
R – 343#.

The front anti-roll bar was hollow and 1½" in diameter w/6 holes of adjustment, while the rear bar was 1" in diameter w/no adjustment. Each one-hole adjustment of the front bar made a significant difference.

I didn't watch the video, but I think that E36 had a very "flat ride".
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      01-06-2019, 12:53 AM   #18
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Quote:
Originally Posted by Shadowfax028 View Post
You just described my car. And I was wondering when someone was going to bring Shaik at FatCat Motor Sport up. He really is quite brilliant at suspension design.
My dalema is finding a aftermarket manufacturer that produces OEM style springs to work with factory dampers, and larger then OEM sway bars.

All I have been able to find are (Beauty Springs) that just lower the car!!!!

I need something suitable for Autocross.....

Does as anyone know the spring rates for the BMW Mperformance (Red) springs
BMW doesn't trust its consumers with that kinda information
I just watched the FatCat video, I think my fun car is generally aligned with his recommendations for spring rates and sway bar settings. The fun car (Caterham R400) is in D-Mod and won the Canadian Nationals in 2015 for the consolidated Mod class when driven by my son, so is setup well for auto-X. It also has a great suspension for sporting road use and for track days too, as far as we know it is the fastest road-legal auto-X car in Canada. In a 60 second auto-X course it is about 5 seconds faster than a Nissan GTR when on slicks and 2 seconds faster than a 911 (991) Turbo S or C7 Corvette on RE-71R tyres. It shines in rapid transition parts of the course such as diminishing distance slaloms.

The key suspension settings that make it rapid in auto-X are:
- Relatively stiff front sway bar for camber control during maximum lateral g
- Minimal rear sway bar (sometimes run disconnected, especially in the wet) to give good traction out of corners and control excessive oversteer
- 2.7 deg front camber
- 1.5 deg rear camber (minimal dynamic change due to deDion rear suspension)
- accurately corner weighted (within 1kg cross weight when necessary ballast to meet minimum class weight is added)
- flat ride at medium to high suspension compression (more about this further on)

Bearing in mind the car is light (650kg / 1433lb with driver, fuel and ballast), the front springs are 170#, but have an effective rate of 120# due to inclination. The rear springs are progressive, 130# at normal ride height, 110# in droop and 200# in medium to high compression. Adjustable coil overs are used without any bump stops. This means flat ride is achieved under rear suspension compression, but is soft enough when extended that the inside wheel doesn’t lift (due to the low droop spring rate and minimal sway bar), as a RWD car with a fairly aggressive carbon plate LSD, it has great traction exiting corners.

The unsprung front corner weights are about 25kg each, rear about 30kg each. The total corner weights are LF=154kg, RF=144kg, LR=181kg, RR=171kg, so the natural frequencies at normal ride height are LF=2.12Hz, RF=2.03Hz, LR=1.95Hz, RR=2.02Hz. When under medium compression the rear frequencies increase to LR=2.42Hz and RR=2.51Hz to give flat ride, this is achieved using the progressive springs on the adjustable coil overs, but engineered bump stops could also be used.

I would say the key things to make an F22 competitive in auto-X (depending on class and restrictions), but let’s say just maximize it for raw time without any structural or bodywork changes and otherwise keep it as a competent road car. I would try to increase front camber, bring the front spring rate up, but still achieve flat ride, ensure front sway bar rates are high compared with the rear and use an LSD if possible. All of these changes are going to be dependent on the class you want to run in, of course. I don’t know if anyone manufactures optimized front bump stops, but I think they have to be changed with the springs to reduce the braking dive, especially if adding camber for front end cornering grip.

Last edited by aerobod; 01-06-2019 at 01:07 AM..
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      01-07-2019, 07:16 PM   #19
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Are people actually testing the springs on a bench or simply running calcs on the coil diameter and number?

Reason I ask is 185 in/lb for the fronts and 415 in/lb for the rears seems off for a 3,400-3,500 lb. RWD performance car. The front rate seems too low and the rear too high.
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      01-07-2019, 11:49 PM   #20
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Quote:
Originally Posted by XutvJet View Post
Are people actually testing the springs on a bench or simply running calcs on the coil diameter and number?
In the case of coilover springs, I've used the manufacturer's data that's stamped on the springs. The Eibach coilover spring below, for example, shows that the spring has a free length of 8.00", an internal diameter of 2.25", and a rate of 500#/inch.

If I had a spring with an unknown rate and needed that information, I'd have it put on a spring tester.

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Last edited by dradernh; 01-08-2019 at 12:41 AM..
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      01-08-2019, 11:01 AM   #21
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Quote:
Originally Posted by dradernh View Post
In the case of coilover springs, I've used the manufacturer's data that's stamped on the springs. The Eibach coilover spring below, for example, shows that the spring has a free length of 8.00", an internal diameter of 2.25", and a rate of 500#/inch.

If I had a spring with an unknown rate and needed that information, I'd have it put on a spring tester.

I'm kind of surprised Eibach would be stating their specs in imperial measurements. Eibach also has TUV documentation on their site with drawings, dimensions, cycling data, and spring rates (in N/mm). Here's one of their documents I used when looking at springs.
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      01-08-2019, 11:50 AM   #22
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Quote:
Originally Posted by FaRKle! View Post
I'm kind of surprised Eibach would be stating their specs in imperial measurements. Eibach also has TUV documentation on their site with drawings, dimensions, cycling data, and spring rates (in N/mm). Here's one of their documents I used when looking at springs.
If I were shopping for Eibach coilover springs, I'd look at the current catalog with its imperial measurements: https://eibach.com/us/file/99-Comple...rt+Catalog.pdf.
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