BMW Performance Strut Bar - Cracked

[prowlerflyer]

Fantinno has put up some fantastic and spot on info, but keep in mind before we all go running and screaming away from the CF bar, remember it's all in the engineering. Properly engineered CF (particularly in its weave directions and layers, with respect to it's expected load) is stronger than almost any metal equivilent of similar weight. CF also does not alter it's facture mode or point much with temp change, unlike metal, which can do both.

To put it in perspective, F1 cars (and most high end racers for that matter) actually use metal parts as the failure points in most of their suspension components!

While I am under no illusion that BMW is beyond making a mistake, they do, I believe, engineer to a fairly high standard, particularly special parts. With the exeption of F1 other high end racing though, parts are engingeered with cost in mind. Sorry, beginning to sound contrdictory.

[fantinno]

^I agree wholeheartedly with all this.

Just a thought, but I think it's also worth mentioning that the BMW CF bar is probably well engineered to handle the sort of loads it is intended for, i.e. predictable, relatively "smooth" hard cornering loads you'd get at a track or canyon carving.

Shock loading from large potholes and crappy US freeways, especially when coupled with an already stiffened suspension and lower profile tires (your first defense against the road!) are extreme loads. Potholes and such can fracture an aluminum rim, so it's not surprising the could do the same to a strut bar.

[az3579]

Shock loading from large potholes and crappy US freeways, especially when coupled with an already stiffened suspension and lower profile tires (your first defense against the road!) are extreme loads. Potholes and such can fracture an aluminum rim, so it's not surprising the could do the same to a strut bar.

While they may be true, there is absolutely no reason for something to not cause damage to a rim but at the same time be able to destroy a strut bar. That is asinine.

[johnrando]

Great info Fantinno and thanks for adding to it Gary. That is strange BP. I still hate seeing those pics of that bar.

[derbo]

Honestly I am not surprised it did this. I think the biggest problem with carbon fiber is that it is hard to see a failure until it is a huge failure like a snap. A lot of small stress cracks may not be visible until its fair too late. I've seen countless stories of carbon bike frames/forks fail in this area because of the crappy roads. The BMW CF bar looks great but the way it sits, maybe designed to fold just like the M3 Bar in a side collision.

Great info btw Fantinno.

[LivesNearCostco]

Maybe... if the strut towers are flexing up and down in addition to moving towards and away from each other, that might increase the stress on the center piece compared to a 3-part bar without hinges, which would deliver more load to the center than a 3-part bar with hinges. Bending forces from the strut towards should just wiggle the ends of a hinged bar up and down without stressing the middle section.

I don't think the BMW aluminum center section is stronger than the CF performance bar, but under extreme stress it's more likely to bend than crack. (the end plates can crack but are individually replaceable.) And as the engineer pointed out, aluminum might handle compressive forces better than CF.

The BMW Performance bar is all one piece, no hinges or any detachable pieces, so what would explain why this one cracked? I thought I remembered reading another report of a cracked BMW Performance bar cracking as well some time ago.

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[JeffSaysThings]

...I don't think the BMW aluminum center section is stronger than the CF performance bar, but under extreme stress it's more likely to bend than crack. (the end plates can crack but are individually replaceable.) And as the engineer pointed out, aluminum might handle compressive forces better than CF.

Exactly. An aluminum (or steel) bar could definitely handle compressive loads better than carbon. CFRP isn't very efficient in compression, for the reasons Fantinno pointed out earlier.

Shock towers react compression load from the springs. One of the purposes of a strut bar is to react that compression load from one tower to the other. From the pictures shown, the bar appears to be hollow and only a few plies thick in each wall. If the fibers are primarily +/- 45 degrees to the length of the bar, it may have a reasonable amount of compression stiffness (and strength). However in the last picture, the plies look to be primarily perpendicular to the length of the bar. To me, that means the bar is probably not as stiff, nor as strong, as it could be.

I think it was said earlier though, that these parts are designed and built to a meet cost requirements, so I doubt any engineers spent 12 months analyzing the ply stack-up. In the end, the bars are probably adequately stiff to meet their intended purpose, but the tower deflection due to an impact load (pothole, etc) overloaded it.

Lots of products are made from CFRP when they are probably better off being built from aluminum, titanium, fiberglass, etc. Carbon fiber has marketing appeal though, and it looks neat...

P.S. This is my first post, hi guys!

[Oli77]

Hi Jeff, please feel free to make an introduction thread and show us pics of your car!

[johnrando]

+1 Welcome Jeff. We have a new members section.

[fantinno]

Welcome Jeff :-)

It's probably worth mentioning that there's always the chance that the bar was just straight up defective from manufacture. It's not a "flight critical" part, so to speak. I doubt they non-destructive test every single bar for voids and defects. We NDT everything we make at my company, and even still, sometimes (rarely) things just break for unanticipated reasons.