Rear Solid Sub-Frame Mount?

[rzm3]

Any one have experience with these? Just considering them because of the higher spring rates I am using (750 lb) & less sub-frame deflection.

Apparently on the M3 GTS, "the rear axle subframe is bolted firmly in position" too...

[mastek]

Does the GTS use solid bushings?

Are there BMW part#'s for them?

[lucid]

Do you really want solid mounts on a road car? Things can get harsh. Also, in some cases, you might need to reinforce the mounting points with welded plates (I had the motor mounting points and the rear subframe reinforced in my E30 M3), but I have no idea if that would be necessary in this case or not.

[mastek]

Originally Posted by mastek

Does the GTS use solid bushings?

Are there BMW part#'s for them?

anyone?

[scott@bmwhn]

Originally Posted by mastek

Does the GTS use solid bushings?

Are there BMW part#'s for them?

I imagine that these will be very hard to acquire unless you actually own one....we'll have to wait and see once they start delivering them.

[rzm3]

^we are counting on you for the beautiful GTS parts

[dvpouldar26]

Many ran rear subframe mounts on their e46 m3's. They gave better suspension feedback and feel than the stock rubber pieces. The polyurethane parts can also be greased, but some still made some road noise.
The problem I know that some encountered in the e46's is their rear subframe cracking. Some would get their rear subframes reinforced. A roll cage thats integrated to the rear shock mounts and rear subframe would also help. Someone should try these.

[Radiation Joe]

Originally Posted by mastek

anyone?

You really don't want to know other than for entertainment purposes. These are probably motorsport parts = expensive. The harder motor mounts for the e46 were something like $700 apiece. I'd bet the motorsport subframe mounts will be similarly priced. They try to recoupe some of their engineering costs.

[rzm3]

BMW Motorsport GT4 sub-frame w/ solid bushing is listed just over $3k (vs. stock $1635 retail)

[mastek]

Originally Posted by rldzhao

BMW Motorsport GT4 sub-frame w/ solid bushing is listed just over $3k (vs. stock $1635 retail)

its a different subframe? or is that the only way to get the bushings?

BMW GT4 bushing part#'s?

[rzm3]

3330 8321649 - HA-Träger mit Stahl-Buchsen GT4 - Rear Sub-frame with steel bushes

I don't see any part # for bushings alone.

From my experience stock E92's sub-frame comes with bushings installed, so this is probably the case with GT4's sub-frame as well.

[bigjae1976]

I put urethane subframe bushings in my E46. I haven't noticed any squeaking or increased noise. Maybe my urethane engine mounts and exhaust drown out all of the noise.

Experts have attributed the rear subframe failure in the E46s to faulty subframe bushings. The bushing goes bad, so the subframe twists more which increases the stress on the mounting points.

So I'm hoping that the urethane bushings will last a lot longer...so I don't have to replace the bushings again. I also reinforced the mounting points.

Not sure if E9X will have the same subframe issue.

My theory is that the two poly bushings that will make the biggest difference in NVH is the rear shock mount and the rear trailing arm bushing.

[fullrev]

just for reference

http://www.realoem.com/bmw/showparts...08&hg=33&fg=30

[JAJ]

Originally Posted by rldzhao

Any one have experience with these? Just considering them because of the higher spring rates I am using (750 lb) & less sub-frame deflection.

Apparently on the M3 GTS, "the rear axle subframe is bolted firmly in position" too...

This is not a mod to make without considerable care. BMW's 3-series rear suspensions, both the M and the series production version, are well designed to spread the loads to multiple points in the chassis, avoiding the need for heavy and expensive metal structures in the production vehicles. Changing rear springs or geometry has the effect of concentrating the load on places that may not be strong enough to handle it indefinitely without reinforcement.

The GTS has the same subframe and rear suspension setup (non-coil-over) as a regular M3. In this design, the subframe is subjected to downward pull from the lower control arms when the wheels are deflected up. Replacing the mounting bushings with harder bushings and the springs with stiffer springs will transmit the same average load but more impulse load and shock to the subframe mounts, potentially causing fatigue or stress fractures in the bodywork or the subframe itself around the subframe mounts. I would be very suprised if BMW didn't reinforce the subframe and the chassis around the mounts on the GTS to prevent this.

The GT4 uses coilover rear dampers, which eliminate the downward prying load that the stock geometry produces. The vertical stress on the subrame mounts is considerably less as a result, although that probably didn't stop BMW Motorsports from reinforcing them just the same. They also probably reinforced the spot where the top of the coil-over rear damper attaches. It's the same arrangement of springs and dampers as the front suspension, and if you look at your front suspension, you'll see that the stock M3 has a heavy welded cap that forms the top of the strut tower where all of the vertical loads from the springs and dampers are concentrated.

[rzm3]

Your mechanical concerns definitely make sense. In a perfect world we'd all have 3D models of the M3 and running ANSYS for each change we make to the car... but this isn't a perfect world.

So let's do a back envelope calculation:

Let's say the max load a single rear wheel will see is 3000 lbf (>80% of a car's weight). In stock configuration the rear wheel has a leverage of 1.74 over the spring. Therefore at the spring you will see 5220 lbf. And of course let's assume system is static, so the mounting point on the sub-frame will see 2220 lbf.

Let's further assume, conservatively, that there is only one M12x127mm bolt on the sub-frame. You can run bolt calcs but 2220 lbf on such a bolt and its female threads on the car chassis is not much stress at all.

Of course, in a real design process you'd also have to consider the lateral and transverse loads, fatigue etc., but obviously I don't have the data to do so...

But the point I want to make is that car chassis/suspension components are designed with a pretty good margin because car companies cannot predict to the accuracy of aircrafts what type of service loads/life each car will see. So a lot of times they have to assume the worst case scenario and put in a considerable amount of margin, especially when it comes to critical safety components like the chassis.

Let's just hope that my sub-frame does not get pulled out

(I do not have solid mounts, and I'd be curious to see how GTS's sub-frame looks like in real life. I honestly don't believe BMW reinforced the chassis mounting from all the material I've read.)

[JAJ]

My assessment of the design is based on reading about E30, E36 and then E46 rear suspension problems. The spring layout on all these sequential 3-series chassis is substantially the same as the E92 rear suspension. The springs on all of them are located between the LCA and the body shell part-way out the lower control arm. The arrangement of the other pivots and arms has evolved a lot, but the fundamental spring geometry is the same.

If you go to Turner Motorsports website, they have a well written description of how to weld in the reinforcing plates for the E36 M3 rear suspension. The document goes into some detail explainng why the reinforcing parts are needed. Basically, the stress from the subframe mounting bolts alternately pushing up and down on the body flexes the body sheetmetal. The constant bending eventually causes cracks to form around the mounts. The cracks rust, the bodywork fails and the subframe mounting bolts pull out.

Apparently this problem was so serious on early E46 (non-M) cars that early cars were recalled in the UK to reinforce the mountings. There was a running change on the production line, and by the time the E46 came to the US and Canada, the problem was fixed. It wouldn't surprise me if the E9x M3 chassis is reinforced at the mountings. The E36 and E46 M3 chassis were reinforced from the factory, but not enough for racing or heavy track use.

My original point was just that swapping these suspension parts around can have consequences for the chassis that shouldn't be dismissed. The solution might be as simple as an annual inspection looking for damage or rust around the mounts.

[Dustin@AEp]

We installed the solid rear submounts on Paul Walkers E92 M3, and there is definitely a noticeable difference from the OEM bushings. Once the OEM ones were out I was really surprise with how soft they are versus the new ones we installed. I really expected the M3 to come with stiffer bushings. I guess more people would complain about ride quality and sound.

Regards,


Dustin

[rzm3]

Does that car have any type of reinforcements on the rear mounts?

What brand rear mounts did you use?

[Dustin@AEp]

Originally Posted by rldzhao

Does that car have any type of reinforcements on the rear mounts?

What brand rear mounts did you use?

We do not have any reinforcements, just the Turner solid mounts!

They work great!

[rzm3]

^Keep us posted on any updates!

[Dustin@AEp]

Will do!

[oldmanstyle]

Originally Posted by JAJ

There was a running change on the production line, and by the time the E46 came to the US and Canada, the problem was fixed.

Small point to nit pick, but they had not fixed the problem before it got to the US. The scale of the problem was supposedly reduced at some point with a change, but the failure's have still occurred at least up to 2004 M3's.