Do solid/poly subframe bushings help or make worse the cracking issue.

[E46m3zcp]

Hi, I know this is a debatable topic. I want to see what are the theories out there. I'm particularly interested in diving deeper into the topic and hearing different perspectives from this knowledgeable community. Let's focus on thoughtful discussions and reasoned arguments to enhance our understanding of this subject.

The question is whether putting more rigid subframe bushings helps prevent RACP cracking or makes it more likely to crack. I had this conversation many times at track events. I heard people say it helps, I heard people say it makes it worse. I've rarely heard well-articulated explanations in engineering terms so I'm putting this here for brainstorming/debate.

To kick things off, I'd like to present one theory. Please keep in mind that I'm not an expert on this topic, so I'm open to being corrected or challenged. Here's the theory:

The impact of using more rigid bushings on the rear axle carrier panel (RACP) of the E46 depends on the type of forces involved in the cracking. If the cracking is caused by fatigue due to pushing or pulling forces on the subframe mounting points, softer rubber bushings provide energy absorption, potentially reducing fatigue. Solid bushings, lacking this energy absorption, could worsen the issue. However, in cases of shearing forces (side-to-side, back-to-rear), solid bushings might be advantageous. They distribute sideways forces more evenly (forces at one mounting point are distributed across the remaining 3 because lateral displacement is limited by the rigidity of the bushings), potentially reducing stress concentrations on the RACP.

According to one video posted by Practical Performance RnD, failure of the right front mig welds inside the cavity is mainly due to sheer motion. So according to my arguments, solid/rigid bushings would help in this case.

So, I'm curious to see what theories there are out there.

[Tbonem3]

We had this discussion a few eyars ago when alloy bushings got popular. I think the takeaway was that the movement allowed by rubber was thought to lead to stress. Plus we have newer models that have alloy bushings from factory.

The issue really isn't any of this though, imo, it's the RACP itself I believe or the lack of reinforcement to frame rails, more specifically. Bushing choice isn't an issue when you "fix" the RACP.

[bmwfnatic]

The front right mount is always dented because the internal 'base' of the threaded receiver has a ridge/cavity where the bush contacts the car, and due to the rotational forces that one is hammered into the car over and over again.
When I did my reinforcement, I opened that part up a bit and pooled up some weld there prior to fitting the reinforcement plates.

For the rear left mount, the (excessively worn) rubber bush basically starts to act like a slide hammer.

But as Tbone said, if the RACP is actually connected to the frame rails, it really doesn't matter any more.