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E46 M3 (2001-2006) Engine: S54 - Max Hp: 333 hp at 7,900 rpm / 262 lb/ft at 4,900 rpm
Total Produced: 45,000+ - Years Produced: 2001 to 2006.


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Old Thu, Jul-19-2018, 09:52:58 PM   #11
rovert604
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Default Re: Do Stiff spring rates have correlation to subframe cracks?

Subframe cracks are because the power of the engine twists the driveshaft which is twisting the rear diff which is mounted in the subframe. That twist from the S54 puts a lot of pressure on the subframe mounting points causing tears in the RACP when hard acceleration pressure is applied.
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Old Thu, Jul-19-2018, 11:17:24 PM   #12
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Default Re: Do Stiff spring rates have correlation to subframe cracks?

OP, i'll go against the flow and suggest that stiff rears ARE doing damage.
in this case, not at the mounting points but where the racp sheetmetal meets the rail.
ill explain:

rear suspension setup on e46 has the spring placed inboard. typically, the primary fulcrum is at the susp arm pivot near the subframe, load is applied at the wheel, and the spring provides the reaction.

if you have stiffer linear springs, that point becomes more of a fulcrum (call it secondary fulcrum) since the relative stiffnes of that point increases versus the pivot.
in effect, this hammers the subframe mounting points down, as the spring compression doesnt absorb as much of the road impact energy.

good news is that this can be addressed by adding the topside reinforcements (vincebar, CMP auto, GTR replica)

i had this happen in my car (only running 11kg rear springs).
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Old Thu, Jul-19-2018, 11:31:46 PM   #13
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Default Re: Do Stiff spring rates have correlation to subframe cracks?

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Originally Posted by M3 dude View Post
OP, i'll go against the flow and suggest that stiff rears ARE doing damage.

in this case, not at the mounting points but where the racp sheetmetal meets the rail.

ill explain:



rear suspension setup on e46 has the spring placed inboard. typically, the primary fulcrum is at the susp arm pivot near the subframe, load is applied at the wheel, and the spring provides the reaction.



if you have stiffer linear springs, that point becomes more of a fulcrum (call it secondary fulcrum) since the relative stiffnes of that point increases versus the pivot.

in effect, this hammers the subframe mounting points down, as the spring compression doesnt absorb as much of the road impact energy.



good news is that this can be addressed by adding the topside reinforcements (vincebar, CMP auto, GTR replica)



i had this happen in my car (only running 11kg rear springs).


That makes sense to me. Seeing as the springs absorb less, wheres the impacts going!? Thatís why I was thinking subframe. I think either way Ill look into a vincebar... but now that we have a contradicting opinion, let the debate begin!
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Old Fri, Jul-20-2018, 07:30:46 AM   #14
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Default Re: Do Stiff spring rates have correlation to subframe cracks?

I can see m3 dude's point. The upper control arm bushings (both inner and outer) will take a hell of a beating too. I don't know shell construction in the back well enough to comment on loads between spring pushing against the top of the shell, while control arms transmitting the forces to/through the carrier mounts and sheet metal.

Maybe Cayn from cmp can chime in?

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Old Sun, Jul-22-2018, 10:58:34 PM   #15
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Default Re: Do Stiff spring rates have correlation to subframe cracks?

basically I ended up doing an independent validation of Vince's findings. and due to the urgent nature of my repair, had my own stuff fabbed.
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Old Sun, Jul-22-2018, 11:21:07 PM   #16
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Default Re: Do Stiff spring rates have correlation to subframe cracks?

I've had 80k miles of h&r lowering springs on my car. Just dropped the rear end and did a subframe reinforcement. One very small crack!
So no, it was normal wear because I had 108k miles.
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Old Mon, Jul-23-2018, 01:49:51 AM   #17
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Default Re: Do Stiff spring rates have correlation to subframe cracks?

I think it's hard to believe that stiffer springs won't add additional stress to the subframe. Is it the determining factor? No. But increasing the load applied has to have some affect, however little.

With a reinforces subframe I wouldn't worry about stiffer springs.

And just because stock cars have subframe issues, means nothing to whether stiffer springs can exacerbate the situation.
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Old Mon, Jul-23-2018, 03:02:24 AM   #18
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Default Re: Do Stiff spring rates have correlation to subframe cracks?

They will not cause it per se, but will definitely contribute to accelerating the damage.

Stiff springs can cause the control arms to crack. Failing RTAB’s will also cause more cracks - once there is a weakness in the rear end of these cars it all starts to tear itself apart pretty quickly. Sort of a chicken v. Egg scenario...
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Old Mon, Jul-23-2018, 05:39:51 AM   #19
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Default Re: Do Stiff spring rates have correlation to subframe cracks?

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They will not cause it per se, but will definitely contribute to accelerating the damage.

Stiff springs can cause the control arms to crack. Failing RTABís will also cause more cracks - once there is a weakness in the rear end of these cars it all starts to tear itself apart pretty quickly. Sort of a chicken v. Egg scenario...


Im aware of the spring perch cracking however I never understood the point of the reinforcement plate because unlike other coilovers the Broadways I have thread through the perch into a nut underneath so the spring its self doesnít even sit on the perch.
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Old Mon, Jul-23-2018, 12:31:41 PM   #20
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Default Re: Do Stiff spring rates have correlation to subframe cracks?

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Im aware of the spring perch cracking however I never understood the point of the reinforcement plate because unlike other coilovers the Broadways I have thread through the perch into a nut underneath so the spring its self doesnít even sit on the perch.
So does the spring float in midair or does the rear height adjuster sit on the perch? More pounds per square inch spread out on a smaller area.
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Discussing Do Stiff spring rates have correlation to subframe cracks? in the E46 M3 (2001-2006) Forum - Engine: S54 - Max Hp: 333 hp at 7,900 rpm / 262 lb/ft at 4,900 rpm
Total Produced: 45,000+ - Years Produced: 2001 to 2006. at BMW M3 Forum.com (E30 M3 | E36 M3 | E46 M3 | E92 M3 | F80/X)