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      01-14-2015, 09:03 PM   #1
fe1rx
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Exclamation OE vs M3 Rear Suspension Arms

Questions that have been plaguing me:

1) What exactly are the benefits of the M3 forged aluminum rear suspension arms vs. the 135i pressed steel arms?
2) Given that the upper, guide and toe arms function structurally as struts, which is to say they react only compression and tension loads (with some structurally insignificant bending due to torque in the rubber bushings), why are the M3 forged aluminum arms curved? This is not required for installation clearance and is structurally weaker than a straight arm. (The corresponding 135i steel parts are straight.)

Perhaps some of the answers can be found below:

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M3 Rear Suspension Arms

1 – Wheel Carrier (Upright)
4 – Camber Arm
10 – (Semi-)Trailing Arm
13 – Steering Arm (Toe Control Arm)
17 – Wishbone (Upper Arm)
18 – Guide Rod

These members provide the functional equivalent of a double A-arm suspension, with the triangles formed by the two upper arms (green) and the two lower arms (purple) forming two virtual A-arms.

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135i Rear Suspension Arms (Looking Outboard)


RUBBER BUSHINGS

The rear suspension is rubber-bushed at the following locations:

- Camber Arm inboard end (pressed into subframe, M3 and 135i)
- Trailing Arm both ends (pressed into subframe and wheel carrier, M3 and 135i)
- Toe Control Arm both ends (135i only)
- Upper Arm outboard end (M3 and 135i)
- Guide Rod (both ends on 135i, outboard end only on M3)

The rubber bushings in the Upper Arm and Guide Rod consist of a steel bushing core and two concentric metal shells, isolated by rubber. The M3 and 135i bushings are functionally identical and stiffness measurements indicate a radial stiffness of approximately 10,000 N/mm. The stiffness is linear in the (assumed) working range of ±3000 lbs (±13000 N), corresponding to a deflection of ±1.3 mm, but becomes progressively stiffer beyond that. Suspension travel causes rotation of the inner core relative to the outer shell (in the axis of the attach bolt), shearing the rubber and contributing to the total spring rate of the suspension. To minimize stress on the rubber core, it is essential that rubber bushed joints are torqued at static ride height. Rotation of the bushing in the other two axes is possible, preventing any binding in the joint.

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Bushing Compliance Test (note dial indicator)

Inspection and measurement of 2008 rear suspension arm bushings with 100,000 km service found no sign of deterioration in the rubber bushings; however, the used bushings were approximately 14% stiffer than new M3 rubber bushings, presumably due to aging of the rubber.

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Inboard Rubber Bushing / Outboard Rubber Bushing / Inboard Ball Joint

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The other rubber bushings used in the suspension at the Camber Arm (inboard end) and the Trailing Arm (both ends) are similar construction, but were not sectioned or measured for stiffness.


BALL JOINTS (Spherical Bearings)

The rear suspension is ball jointed at the following locations:

- Camber Arm outboard end (pressed into wheel carrier, M3 and 135i)
- Toe Control Arm both ends (M3 only)
- Upper Arm inboard end (M3 and 135i)
- M3 Guide Rod inboard end (pressed into arm)

The ball joints consist of a spherical steel inner race and a plastic outer race housed in a mating outer metal shell. In the 135i Upper Arm, the ball joint is a separate assembly, pressed into the arm weldment. Where ball joints are incorporated into the M3 suspension arms, the aluminum forms the housing for the plastic outer race without a separate steel shell. The bearings are permanently assembled by swaging the shell (or aluminum arm) to affix metal end plates that fix the plastic outer race in place. The bearings are permanently lubricated and sealed by rubber boots. The bearing is free to rotate in all axes, so it is not necessary to torque ball-jointed connections at ride height. This construction provides virtually no radial compliance (large radial stiffness).

Inspection and measurement of 2008 rear suspension arm ball joints with 100,000 km service found no sign of wear on the plastic outer race and no detectable radial play in the bearing assemblies.


UPPER ARM

The Upper Arm and the Camber Arm together establish the camber of the rear tire. Under static loading, the Upper Arm is in compression (less than 1000 lbs). Lateral weight transfer and bumps increase the compressive force in this member, but cornering loads on the more highly loaded outer wheel reduce the compressive force in this member.

The OE Upper Arm is pressed steel with inboard ball joint and outboard rubber bushing. LH and RH parts are identical as the pressed steel body is straight. Asymmetrical section ensures that that compressive failure will be by crippling. Ultimate compressive strength of the arm is approximately 9000 lbs, based on static testing. At this load the arm begins to cripple. The failure is progressive and benign.

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Upper Arm Test

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Upper Arm Crippling Failure

The M3 Upper Arm is forged 6082T6 aluminum with an inboard ball joint and an outboard rubber bushing. The forged bodies of the LH and RH arms are identical but installation of the bushings makes them handed. The bodies are curved (upward), not for installation clearance issues, but to ensure that the compressive failure of this member will be by progressive bending. The ball joint and bushing on this part are nominally equivalent to those in the OE Upper Arm, so installation of the M3 Upper Arm in place of the 135i Upper Arm is unlikely to have any handling benefit (e.g. reduction of compliance steer effects).


GUIDE ROD

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The Guide Rod and Trailing Arm react thrust and braking loads. A simple analysis of the in-service load levels is impractical due to the sophisticated geometry of the multi-link suspension.

The OE Guide Rod is a pair of pressed steel halves welded together with inboard and outboard rubber bushings. LH and RH parts are identical as the welded assembly is straight. The two halves of the weldment form two parallel columns that fail by buckling away from each other at a load in excess of 14,000 lbs, based on testing. The unit would be stronger if it were fully welded, but the partially welded design provides for controlled failure.

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The M3 Guide Rod is forged aluminum with inboard ball joint and outboard rubber bushing. The forged bodies of the LH and RH arms are identical but installation of the bushings makes them handed. The bodies are curved (downward), not for installation clearance issues, but to ensure that the compressive failure of this member will be by progressive bending. The ball joint used at the inboard location effectively doubles the stiffness and halves the deflection under axial loading versus the OE Guide Rod, which will alter (and presumably decrease) compliance steer effects.

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TOE CONTROL ARM

The Toe Control Arm maintains the toe angle and minimizes bump steer over the range of suspension travel. A simple analysis of the in-service load levels is impractical due to the sophisticated geometry of the multi-link suspension, however loads in this member are obviously less than those in the Upper Arm. This member is also designed for progressive crippling failure under compressive overload.

The OE Toe Control Arm is pressed steel with inboard and outboard rubber bushings. LH and RH parts are identical as the body is straight. A reduced section depth at the center of the arm ensures that that compressive failure will be by crippling at this location, and that the failure will take the arm away from conflict with the suspension spring. I have tested the OE toe arm to a compressive load of 2250 lbs without failure or permanent deformation but have not tested this member to failure. Because Toe Control Arm loads are believed to be relatively modest (in comparison to the other suspension arms), deflection in these bushings is likely small, but it will result in dynamic toe changes. The Toe Arm bushings slightly different construction to the rubber bushings in the Upper and Guide members, and they are about 30% stiffer.

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The M3 Toe Control Arm is forged aluminum with inboard and outboard ball joints. T LH and RH arms are identical. The bodies are curved (downward), not for installation clearance issues, but to ensure that the compressive failure of this member will be by progressive bending. The ball joints used at both ends of this member virtually eliminate compliance at the bolted connections. It should be noted that the M3 Toe Control Arm is not dimensionally interchangeable with the OE Toe Control Arm due to different overall pin-center lengths. To say it eliminates all compliance in this member is incorrect though, because the curved design turns the member into a spring. Provided the bending stresses are below the yield stress for the material, the compliance is elastic and reasonably linear.

I have tested an M3 Toe Control Arm to compressive failure to verify its strength and failure mode. Ultimate strength approximately 7600 lbs:

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Bending in the member is quite obvious as load increases. Beyond this load the deformation becomes permanent, but the bending is evenly distribute along the length of the member, showing that very large deflections will be tolerated before catastrophic failure. The failed arm is sitting on top of an untested arm for comparison.

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This testing was motivated by the desire to design and fabricate a ball-jointed toe arm using the same design philosophy as the M3 arm, but with the correct pin-centre geometry for the 135i. I tested a prototype article with some care to accurately measure deflections vs. load.

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Testing proceeded to ultimate then the load was removed. The load was then re-applied to produce significant permanent deformation and the load removed. The resulting load vs. deflection graph shows the classic distinction between the areas of elastic and plastic deformation.

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This test shows that the prototype toe arm is structurally equivalent to the M3 toe arm.

Aftermarket Toe Control Arms like the Rogue Engineering that use straight members will provide greater stiffness, but they do not provide the same provision for progressive failure under compressive overload and will fail suddenly if they are overloaded.

(SEMI-)TRAILING ARM

The OE and M3 Trailing Arms are identical and ride in identical rubber bushings pressed into the rear subframe and wheel carrier respectively. These parts are pressed steel and are designed to cripple under compressive overload. As BMW describes it, “the semi-trailing arm features crash beading to ensure the fuel tank is not damaged.” Under compressive overload the arm will buckle down and away from the fuel tank. Rigid, symmetrical trailing arms, like those offered by ECS Tuning do not appear to have this feature.

BUSHING WINDUP

All rubber bushing connections should be torqued at ride height so that the bushings are relaxed (unwound) at ride height. Ball joint connections can be torqued at any height as they are free to rotate and do not wind up. Rubber bushing windup contributes to the overall spring rate of the vehicle. With the OE rear suspension configuration, the wheel rate contributed by the rubber bushings is 35 lb/in. This equivalent to a spring rate of 109 lb/in or 19 N/mm (at the rear spring location), so is quite significant to the overall wheel rate of the vehicle.

Changing the Upper Arm from OE to M3 has no effect on wheel rate as both configurations use similar rubber bushings at both ends.

Changing the Guide Rod from OE to M3 removes one rubber bushing and replaces it with a ball joint, resulting in a softening of the suspension.

Installation of a ball-jointed Toe Control Arm (like the Rogue Engineering part) removes two rubber bushings, resulting in a further softening of the suspension.

Assuming the latter two changes are made, the wheel rate contributed by the rubber bushings is reduced to 23 lb/in. This is equivalent to a spring rate of 72 lb/in or 13 N/mm (at the rear spring location). A fully ball-jointed suspension would remove all this wheel rate and would need to be compensated for by a corresponding increase in spring rate.
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      01-14-2015, 09:28 PM   #2
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Science rocks! Now I know I can go stiffer spring in the rear.
I love my rear arms - I think they dont get enough credit on here with everyone doing front arm updates.

Have tou loked at the F3x, F2x TOE CONTROL ARM? Is it the same length as the 135i/e8x e9x toe arm 33326792533?
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      01-15-2015, 04:50 AM   #3
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Having OE rear springs with Koni Yellows, and M3 Upper Arm/Guide Rods, what is the best settings for the Yellows?
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      01-15-2015, 06:15 AM   #4
gap
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Great research, thank you for sharing this. I was under the impression that the M3 parts firmed the suspension but you are saying it softens it. Not sure I understand this.

"Changing the Guide Rod from OE to M3 removes one rubber bushing and replaces it with a ball joint, resulting in a softening of the suspension.

Installation of a ball-jointed Toe Control Arm (like the Rogue Engineering part) removes two rubber bushings, resulting in a further softening of the suspension."

Good info on the trailing arm crash safety design.
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      01-15-2015, 08:05 AM   #5
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Quote:
Originally Posted by gap View Post
Great research, thank you for sharing this. I was under the impression that the M3 parts firmed the suspension but you are saying it softens it. Not sure I understand this.

"Changing the Guide Rod from OE to M3 removes one rubber bushing and replaces it with a ball joint, resulting in a softening of the suspension.

Installation of a ball-jointed Toe Control Arm (like the Rogue Engineering part) removes two rubber bushings, resulting in a further softening of the suspension."

Good info on the trailing arm crash safety design.
Basically since there is no bushing wind up (which contributes to spring rate) with a ball joint, you are decreasing the spring rate, thus softening the suspension.
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      01-15-2015, 08:07 AM   #6
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Quote:
Originally Posted by andrey_gta View Post
Science rocks! Now I know I can go stiffer spring in the rear.
I love my rear arms - I think they dont get enough credit on here with everyone doing front arm updates.

Have tou loked at the F3x, F2x TOE CONTROL ARM? Is it the same length as the 135i/e8x e9x toe arm 33326792533?
Interesting. The slender design of this and the 135i part points to the relatively low operating loads on the toe control arm. I assume the non-visible top of the part is either open or welded only at the ends to destabilize the section and control its failure mode.

Possibly the bushings in this part are stiffer than in the 135i part because they do appear to have wider rubber elements.
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      01-15-2015, 08:11 AM   #7
fe1rx
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Quote:
Originally Posted by TS135i View Post
Having OE rear springs with Koni Yellows, and M3 Upper Arm/Guide Rods, what is the best settings for the Yellows?
To be glib, the one that gives you the best combination of ride quality and grip.

To be serious, I have no idea. My experimenting is about finding out how much I don't know, no showing how much I do. I figure having gone to the effort, it is worth sharing.
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      01-15-2015, 11:01 AM   #8
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Quote:
Originally Posted by chris82 View Post
Basically since there is no bushing wind up (which contributes to spring rate) with a ball joint, you are decreasing the spring rate, thus softening the suspension.
That explains it. I was only thinking about the firmness that would come
from the arms/bushings not flexing and didn't think about the reduced friction
when the arms pivot.

What are your finds about the difference in the flexing of the OE arms/bushings vs the M3 arms/bushings?
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      01-15-2015, 02:26 PM   #9
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Quote:
Originally Posted by chris82 View Post
Basically since there is no bushing wind up (which contributes to spring rate) with a ball joint, you are decreasing the spring rate, thus softening the suspension.
Wouldn't it be more correct to say the ball joint has an extremely large spring rate? There is no metal fabrication with a zero spring rate, but some are extremely large.
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      01-15-2015, 08:56 PM   #10
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Quote:
Originally Posted by 128Convertibleguy View Post
Wouldn't it be more correct to say the ball joint has an extremely large spring rate? There is no metal fabrication with a zero spring rate, but some are extremely large.
Apparently Sir Henry Royce (of Rolls-Royce fame) said "In the final analysis, every engineering material is rubber." That is the significance of Young's Modulus.

So you have a point, but you are confusing the torsional spring rate of the bushing/ball joint with the axial spring. A rubber bushing acts like a torsional spring, resisting vertical deflection of the suspension and trying to restore the suspension to its normal ride height. A ball joint offers a small amount of friction but no restoring force, so it has zero torsional spring rate. I didn't show you how I measured the torsional spring rates of all the suspension bushings relative to wheel travel, but it is relatively simple once you remove your springs and shocks.

As far as axial spring rate goes, both ball joints and straight arms have very large axial stiffness. Curved arms and rubber bushings have less axial stiffness.

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Last edited by fe1rx; 01-15-2015 at 09:02 PM..
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      01-18-2015, 10:55 AM   #11
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I wish I had the resources to do all this years ago. Made a M3 toe arm by using FEA. The end results as the arm starts to yield at 4600 lbf. The goal was to meet 4x corner load. I used Hirschmann scp20 bearing rs2 (size 22) bellow seals.
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      01-18-2015, 06:57 PM   #12
fe1rx
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Quote:
Originally Posted by Orb View Post
I wish I had the resources to do all this years ago. Made a M3 toe arm by using FEA. The end results as the arm starts to yield at 4600 lbf. The goal was to meet 4x corner load. I used Hirschmann scp20 bearing rs2 (size 22) bellow seals.
Very nice. I am really impressed by how long an OE spherical bearing lasts provided the seal remains intact, so I wanted to use a similar approach. I have reverse engineered the bearing cavity on the M3 toe arm and figured out how to swage the thing together, so I will use this approach on my prototype arms as I bought a couple of M3 toe arms to a) test and b) get the bearings from.

This wouldn't be a good approach for a commercialized solution (which I am not intending to do). I had assumed the M3 bearing cavity would have been cylindrical, but it is actually hemispherical and thus much more critical with respect to tolerances. Your approach is a good one. Does Hirschmann make the bellows seals? I briefly considered using a standard Aurora spherical bearing with turned spacers pressed in from each side, but I did not find a convenient source for appropriate seals so stuck with the M3 bearing approach.

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      01-19-2015, 10:20 AM   #13
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Quote:
Originally Posted by fe1rx View Post
Very nice. I am really impressed by how long an OE spherical bearing lasts provided the seal remains intact, so I wanted to use a similar approach. I have reverse engineered the bearing cavity on the M3 toe arm and figured out how to swage the thing together, so I will use this approach on my prototype arms as I bought a couple of M3 toe arms to a) test and b) get the bearings from.

This wouldn't be a good approach for a commercialized solution (which I am not intending to do). I had assumed the M3 bearing cavity would have been cylindrical, but it is actually hemispherical and thus much more critical with respect to tolerances. Your approach is a good one. Does Hirschmann make the bellows seals? I briefly considered using a standard Aurora spherical bearing with turned spacers pressed in from each side, but I did not find a convenient source for appropriate seals so stuck with the M3 bearing approach.

Attachment 1144975

I notice the same issue with reproducing the M3 bearing cavity. I though about this but the swaging was a deal breaker for me. Getting the swaged area material to flow without cracking and preload tolerance is a bit of pain with this design. Hirschmann does make bellow seal for many sizes. They are made of Viton and do not require a clip. The cost about $10 each so not cheap.

Your certainly welcome to my CAD files (STEP or Pro/e) and drawing which you could modify to your liking. The drawing have all the details and part numbers. I would do a few thing differently and change a few tolerances if I were to make these again. Drop my PM with an email I can send you the files for review.The cost for seals and bearings is about 240. 00 US.

Last edited by Orb; 01-19-2015 at 05:12 PM..
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      01-27-2015, 06:40 PM   #14
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is this the entire kit?

http://www.ecstuning.com/BMW-E82-135...Arm/ES2622633/

or are more pieces needed?
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      01-27-2015, 07:13 PM   #15
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This is the TRW kit, BMW M factory seconds. They seem to have a significantly higher failure rate...

Quote:
Originally Posted by mike082802 View Post
is this the entire kit?

http://www.ecstuning.com/BMW-E82-135...Arm/ES2622633/

or are more pieces needed?
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      01-28-2015, 01:12 PM   #16
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Love the analysis.

Had a question, why are you fabbing the parts, won't using the RE or an akg racing part work just as well? Since they are adjustible?
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      01-28-2015, 11:08 PM   #17
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Quote:
Originally Posted by aloksatoor View Post
Love the analysis.

Had a question, why are you fabbing the parts, won't using the RE or an akg racing part work just as well? Since they are adjustible?
I am trying to achieve the refinement, durability and compliance of the M3 toe arm. I am not building a race car. A turnbuckle-adjustable toe arm is appealing for ease of alignment, but it lacks the progressive benign compressive failure mode of the OE components.

Also, and probably most important, I enjoy the process.
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      01-28-2015, 11:17 PM   #18
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Quote:
Originally Posted by physh View Post
This is the TRW kit, BMW M factory seconds. They seem to have a significantly higher failure rate...
Based on what? I haven't heard of any failures on this board, and even then ,it would be purely anecdotal. Even the assertion that TRW's kit are "factory seconds," implying that the parts were rejected by BMW, is unsubstantiated.
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      01-29-2015, 12:53 PM   #19
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Quote:
Originally Posted by fe1rx View Post
I am trying to achieve the refinement, durability and compliance of the M3 toe arm. I am not building a race car. A turnbuckle-adjustable toe arm is appealing for ease of alignment, but it lacks the progressive benign compressive failure mode of the OE components.

Also, and probably most important, I enjoy the process.
totally understand the last part!
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      02-12-2015, 03:22 PM   #20
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Sorry for dumbing down and hijacking this thread somewhat, but what is which makes the most difference between replacing stock rear guide rods versus rear upper links with M3 equivalents?

I have M3 rear subframe bushings, Quaife LSD with 3.46 final drive, and running Extreme Performance tires. There is a fair bit more torque getting transferred to the suspension arms now especially with stage 2 agressive tune. Perhaps not as much as would be the case with R-comp tires though. Just trying to prevent some bending back there, and toe out during hard accelerations ...

Thanks for any insight ...
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      02-12-2015, 05:02 PM   #21
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Quote:
Originally Posted by dcaron9999 View Post
Sorry for dumbing down and hijacking this thread somewhat, but what is which makes the most difference between replacing stock rear guide rods versus rear upper links with M3 equivalents?

I have M3 rear subframe bushings, Quaife LSD with 3.46 final drive, and running Extreme Performance tires. There is a fair bit more torque getting transferred to the suspension arms now especially with stage 2 agressive tune. Perhaps not as much as would be the case with R-comp tires though. Just trying to prevent some bending back there, and toe out during hard accelerations ...

Thanks for any insight ...
Here is the marketing mumbo-jumbo that tells you you need rear M guide rods and upper links:

"Wanting more control and improved handling from your non-M 1 or 3-Series? This is not only an extremely common handling upgrade, but one of the best upgrades you can do. These M3 control arms are one piece of solid aluminum instead of your factory control arms which are weak sheet-metal. If your BMW has power upgrades or sportier tires the factory arms flex and twist, causing lack of stability and unpredictable handling.
That's where the M3 control arms really shine. They feature a much sturdier design with a sealed monoball bushing that allows the arms to rotate instead of twisting. You can expect much more predicable and precise handling, without losing your ride quality. There are absolutely zero drawbacks to this upgrade."

Analysis:

"best upgrades" - very dubious claim

"weak sheet metal" - absolutely false as my testing has shown

"flex and twist" - misleading. Rubber bushings "flex and twist" but the steel arms themselves to do not flex or twist significantly.

"causing lack of stability and unpredictable handling" - no realistic evidence to this effect. The aluminum arms actually have more compliance than the steel ones (ignoring the bushings). Total compliance is not vastly different between the OE and M parts. I haven't tried them yet but when the placebo effect is removed, I expect the benefit to be subtle. As I have noted, only the guide rod is actually stiffer.

"sealed monoball" - only the guide rod gains a monoball and loses a rubber bushing. The OE upper arm already had one monoball and one rubber bushing.

"arms to rotate instead of twisting" - the implication that the OE steel arms twist is misleading. What actually "twists", which is the same thing as "rotates" is the rubber bushing. Functionally there is no difference.

"zero drawbacks" - aside from the expense, I agree.
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      02-12-2015, 05:15 PM   #22
ShocknAwe
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So basically nearly zero point. Also, aren't the bushings the same anyways?
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