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      02-21-2015, 09:08 PM   #1
fe1rx
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Front Suspension Details

The OE front strut top mount looks like this in section:

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The steer bearing is interesting in that it fully packed with balls. More typically in ball bearings the balls are separated by a cage, and it is the ball separation that permits the bearing to be assembled. Without the cage the balls could migrate to one side and the bearing could fall apart. Along the same line, a fully packed bearing is "impossible" to assemble. A close examination of this bearing shows where the outer race is split to permit it to be expanded to complete the assembly.

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Forum member Orb has pointed out that vertical deflection in the top mount is substantial under static loads, and is often not taken into account when considering the stack height in comparison to camber plates. I have been guilty of this omission myself, so I wanted to have a look at exactly how much deflection can be expected. This is actually an easy thing to check when they are installed in a vehicle, but my OE top mounts have been removed and replaced by camber plates, so I rigged up a test fixture, which also allowed me to get an idea of the stiffness characteristics of the mount. Here is the test setup:

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And the results:

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I calculate the static load carried by the top mount to be about 850 lbs when unsprung weight is accounted for, hence the static deflection estimate of 0.2 inches or 5 mm.

Lateral loads on the suspension are reacted by the wishbone, which has a ball joint at the outboard end and a rubber bushing at the inboard end. In contrast, the M3 wishbone has ball joints at both ends. Any deflection in this bushing as a result of cornering loads will result in a tendency of the corresponding front wheel to steer away from the turn (due to the forward-mounted steering rack), which results in a compliance understeer effect. Accordingly the M3 wishbone is a worthwhile modification for a car that sees the track, although the OE rubber bushing appears to be very stiff (I haven't measured it). The bushing is shown below in horizontal and vertical sections, against an M3 bearing:

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"Steady state" aft wheel loads arise from rolling resistance, braking and cornering (the aft component of the tire force due to both steering angle and tire slip angle). In addition transient impact loads from surface irregularities (expansion joints and such) giving rise to NVH issues. To address both considerations the tensions arm reacts these loads in tension as the name would imply. A back of the napkin calculation suggests that apart from severe impact events, the tension loads in this member will be less than about 2000 lbs. The OE tension arm has a ball joint at the outboard end and a large fluid-filled rubber bushing at the inboard end. In the following image, tension loads will shift the inner part of the bushing to the left.

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The bushing contains two separate fluid-filled cavities. The hydraulic fluid permits forces to be transmitted through the body of the bushing without contributing to the bushing stiffness. The fluid also supports the relatively thin wall sections, which would otherwise quickly kink, overheat and fail.

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In the above image, the tension loads are horizontal and the bushing is very soft in this direction. The bushing is very stiff in the un-loaded vertical direction, simply to maintain control over the hydraulic cavities.

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The bushing is soft enough that it is easily deflected by hand over a short range, but then bumpers stiffen up the bushing. The bumper is removed from the right hand side of the above image. The bumpers result in extremely non-linear stiffness characteristics. The initial softness results in a reduction in NVH, and the final stiffening results in appropriate control over wheel position during cornering and braking. In order to characterize the stiffness properties I have measured deflections and loads on an OE tension strut when loaded in the tension direction.

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I have repeated the same test with an M3 tension arm, which uses a much simpler concentric-shell rubber bushing.

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Name:  M3 Tension Arm Test.jpg
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The results are plotted below:

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Clearly the M3 tension arm is more concerned with wheel control than NVH.
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      02-22-2015, 11:48 PM   #2
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      02-23-2015, 11:32 PM   #3
fe1rx
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To complete the picture of the front suspension parts, here are a couple of images of the front outer ball joint construction, which is common to both the 135i and M suspension arms.

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Name:  Front Ball Joint Parts.jpg
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      02-24-2015, 11:26 AM   #4
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Adding M3 tension arms to my list! thanks.
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      02-24-2015, 06:51 PM   #5
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So if I understand correctly the front control arm rear position on the non-m cars have a rubber bush and the m-cars have a ball joint on both ends. Which would be better for tight handling and road feel poly bushes or ball joints, the cost is the same for powerflex poly bushings and the trw complete arms so it's not about price, but I will get a bit more neg camber with the arms.
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      02-24-2015, 07:26 PM   #6
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Quote:
Originally Posted by brocklanders View Post
So if I understand correctly the front control arm rear position on the non-m cars have a rubber bush and the m-cars have a ball joint on both ends. Which would be better for tight handling and road feel poly bushes or ball joints, the cost is the same for powerflex poly bushings and the trw complete arms so it's not about price, but I will get a bit more neg camber with the arms.
You can go cheaper than that, I just fit akg monoballs in my fcabs (200$). Kept the arms from the 135 since I read somewhere the m3 control arms cause more issues than they solve. For wider fronts, since they kind of angle the wheel hub towards the strut hence reducing available space (might be wrong assumption). Plus I did not want to change track, the light leveling rod etc. Steering slops almost gone now the car turns as a whole.
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      02-24-2015, 07:38 PM   #7
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Yes the M3/1M front arms increase negative camber (wheel and tire leans toward the strut) but the increased negative camber helps more than it hinders. The bushings are better quality and deflect less under load. They fix more issues than they create. Not that they create any at all.
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      02-24-2015, 07:49 PM   #8
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Quote:
Originally Posted by aloksatoor View Post
You can go cheaper than that, I just fit akg monoballs in my fcabs (200$). Kept the arms from the 135 since I read somewhere the m3 control arms cause more issues than they solve. For wider fronts, since they kind of angle the wheel hub towards the strut hence reducing available space (might be wrong assumption). Plus I did not want to change track, the light leveling rod etc. Steering slops almost gone now the car turns as a whole.
I have seen monoballs but they are for the tension strut, I currently have poly bushings in there. I asking about the straight wishbones, are these akg's for the straight wishbones? $200 is where I'm at for either the complete arms or poly bushings, I think the white lines may be cheaper though.
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      02-24-2015, 07:53 PM   #9
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Quote:
Originally Posted by HooningB2G View Post
Yes the M3/1M front arms increase negative camber (wheel and tire leans toward the strut) but the increased negative camber helps more than it hinders. The bushings are better quality and deflect less under load. They fix more issues than they create. Not that they create any at all.
They are saying in this post that there aren't any bushings in the arms in question, there are ball joints on both ends. I want the camber, right now I'm at -1* with my springs and pins pulled. So the question remains which is better for steering and road feel hard poly or ball joints?
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      02-24-2015, 08:09 PM   #10
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The M3 arms are a known, an well tested upgrade. Im sure poly bushings would be an improvement as well. Maybe the OP can test the deflection of the poly bushings, and post the results.
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      02-24-2015, 08:09 PM   #11
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Quote:
Originally Posted by brocklanders View Post
I have seen monoballs but they are for the tension strut, I currently have poly bushings in there. I asking about the straight wishbones, are these akg's for the straight wishbones? $200 is where I'm at for either the complete arms or poly bushings, I think the white lines may be cheaper though.
Aah I see . No they are for the fcabs.
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      03-05-2015, 12:06 AM   #12
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As with the rear suspension arms, the windup of the front suspension arm bushings contribute to the total wheel rate (effective spring rate at the wheel location). I have not tested the OE wishbone and OE tension arm to see how much they contribute to the wheel rate, but I have tested the M tension arm. The M wishbone contributes nothing to the wheel rate because both ends have ball joints.

The method of testing is pretty simple. Hold the bushing in a vice and load with barbell weights:

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Measure deflection with a caliper:

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Plot the results:

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Because I am measuring deflection at the ball joint and not the wheel centre, the results need to be adjusted by the motion ratio of the wheel relative to the ball joint (= approx 16/19). Loads decrease by 16/19 and deflections increase by 19/16. The net effect is a wheel rate due to bushing windup of 5 lb/in using the M tension arm. OE arms together are likely to contribute about twice this wheel rate.

CORRECTION: the motion ratio argument is wrong. When the ball joint moves 1" vertically, so does the wheel, so the wheel rate due to bushing windup is actually 7 lb/in, not 5 lb/in.

Last edited by fe1rx; 03-06-2015 at 12:25 PM. Reason: logic error
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      03-06-2015, 10:28 PM   #13
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Thanks for your good work investigating this.
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      03-29-2015, 05:24 PM   #14
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      04-01-2015, 10:46 AM   #15
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Looking at all the pictures and reading about the construction of the front suspension pieces, I am still wondering what could be making the clicking noise in my front suspension. I've taken it to the dealer but they say it's normal. Maybe it is, maybe I'm crazy.

The jist of my issue is that when making a hard right or left turn (used to only be right, hence why I think it may be a worn suspension component), I hear a clicking noise coming from the outside wheel (presumably, could be either). It only becomes noticeable when beyond a certain degree of steering input is generated, as well as only at increases speed (increased load on suspension).

Do you guys have any thoughts on what this might be? I was thinking some sort of bearing in the suspension components, maybe wheel bearing. Maybe a ball joint that' worn, I don't know and the dealer says it is normal.
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      04-01-2015, 10:52 AM   #16
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Could just be a rust ridge on the rotor. The brake rotors flex a surprising amount when cornering.
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      04-04-2015, 03:06 PM   #17
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Could be. The last time my rotors were replaced under BMW maintenance the sound seemed to go away. Any ideas on how to diagnose other than putting new rotors on? Just to be sure that's what it is.
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      01-21-2016, 10:56 AM   #18
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Thant you fe1rx for you efforts. It is definitely appreciated!
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      07-07-2016, 01:08 PM   #19
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Quick question: Can we not just press the factory M3 tension strut bushing into the e82 arm and thereby keep the factory geometry for the 135 while gaining the properties of the M3 suspension?

Besides, it's a pretty cheap part when compared to the options.

31102283579 Rubber mounting f pull rod $45.13
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      07-07-2016, 01:26 PM   #20
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Quote:
Originally Posted by drwillb View Post
Quick question: Can we not just press the factory M3 tension strut bushing into the e82 arm and thereby keep the factory geometry for the 135 while gaining the properties of the M3 suspension?

Besides, it's a pretty cheap part when compared to the options.

31102283579 Rubber mounting f pull rod $45.13
The bushing itself is not offset, therefore you won't gain and caster or ability to adjust caster.

You'll just have a stiffer bushing than prior, which probably isn't a bad thing.
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      07-07-2016, 05:01 PM   #21
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Quote:
Originally Posted by drwillb View Post
Quick question: Can we not just press the factory M3 tension strut bushing into the e82 arm and thereby keep the factory geometry for the 135 while gaining the properties of the M3 suspension?

Besides, it's a pretty cheap part when compared to the options.

31102283579 Rubber mounting f pull rod $45.13
Yes that should be possible with the correct tooling. I have a set of these bushings that were never installed. The bushing has a split casing, so it needs to be compressed on the radial axis while being pressed into the control arm. This needs a special tool to be done correctly and avoid damaging the control arm. My mechanic declined to do this installation on my 135i front arms previously removed from the car. I wanted to install the modified arms on my e90, as my e82 already has the M3 front control arms installed.
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      07-13-2017, 09:19 AM   #22
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Hey Fe1rx, great post, as an Engineer myself I love seeing data about things like this. The one question I have is how much track width does this add?

The reason I ask is I'd like to do this, but I also plan on doing coils/camber plates and want to squeeze as much tire (245 vs 235?) as possible, so my thinking is that camber angles being the same, this mod would actually decrease tire clearance to the fender, as the whole strut assembly would be farther outboard. My other option would be to replace the bushings in the arms and only adjust camber with the plates.

From your Ohlins thread, when using a 235/40 on the ET45 wheels, it looks there is more room between the strut and tire, and less between the tire and fender. So my thinking is a 245 would fit better with the OEM arms?

Secondary question is: Is static camber the only benefit to this mod, or does it help in dynamic conditions as well?
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