Aluminum VS. steel control arms


It's not the aluminum control arm itself that fails; it's the steel ball joint shaft that suffers a fatigue crack. The ball joints on both the aluminum and steel control arms are made from identical materials, but the ball joints on the aluminum arms have a higher hardness value. Roy Hopkins did a failure analysis on control arms for Jim Dresser in 1997 and came up with the following data:

The material of both ball joints were tested with a Texas Instrument X-ray Fluorescence Analyzer and Rockwell Hardness tester.

Steel arm ball joint : 4100 series steel, 19 HRC
Alum. arm ball joint : 4100 series steel, 26 HRC

The materials are the same, (carbon analysis not done, the steel may be 4130 or 4140), but the hardness difference is significant. This steel at 19 HRC is probably annealed, with a tensile strength of 95,000 psi. When the steel is 26 HRC, it is hardened and tempered and could have a tensile strength of 136,000 psi.

As you can see, the ball joints on the aluminum control arms are more resistant to fatigue failure due to their much higher tensile strength. For more info on the E30 M3 control arm failures, see the BMWCCA Club Racing tech bulletin:

Bob Stommel
88 M3 race
97 M3 street


Bay Area ///Motorsport

Last updated on 29 January, 2003 12:14