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From digest.v7.n1375 Tue Mar 24 15:50:03 1998
From: Pete Read <>
Date: Tue, 24 Mar 1998 11:21:41 -0500
Subject: <E28><E24> 750i Bushing Warning (Dinan Bushings are fine)

I don't mean to be overly dramatic, but this could be a safety issue. Many people have modified the stiffer, longer lasting 750i thrust arm bushings to fit their E28 and E24 cars. Unless the bushing is modified correctly, with space for the arm to move laterally, I believe the thrust arm or bracket could ultimately fail from metal fatigue.

This is not an issue with E34 and E32 cars because the bracket is the correct width for the bushings (about 6 mm wider than the E28, E24). The bushings interfere most at full lock on the left front and right front edges of the support brackets -- check these spots carefully.

Last year, Mark Amarandos reported a E28 control arm failure when the lateral movement was restricted by installation of delrin bushings. The 750i bushing doesn't restrict movement as much as the delrin bushing, so I'm not positive about ultimate failure versus some minor damage to the bracket. However, I feel that the bushing should be modified so that there is no chance of contact with the bracket and possible failure over time.

How did I notice this problem? Well, I needed to replace my E28 M5 thrust arms and decided, as preventative maintenance, to replace my three year old Dinan 750i bushings with new 750i bushings while the thrust arms were out. I had read several reports of simply machining down the ends of the 750i bushing aluminum center section, so that was my plan.

I'm always one to measure things, so I checked the old Dinan bushings and found that they machined down both the outer steel shell as well as the center aluminum section. Honestly, Bill Shook caught it before I did -- the significance didn't hit me at first, but he remembered our conversation about Mark Amarandos's failure and how much those arms move laterally.

Thrust Arm and Control Arm Motion

The thrust arm and control arm form the lower "A arm" which locates the front wheel. The very clever double-link front suspension used on our cars (E24, E28, E32, E34) has ball joints at the strut bottom and rubber bushings at the body support brackets. The bushings allow the side-to-side (lateral) movement needed for the double-link suspension geometry.

The side-to-side movement is significant. Last year, I found a diagram of the double-link suspension in an old Autocar five series magazine article. A plan view is shown of the orientation of the control arms when straight ahead and at full lock, both directions. From measuring the diagram, I found the control arms move between seven and eight degrees in either direction (15 degrees total) when turning from left lock to right lock.

The control arms also move laterally during suspension movement. As the suspension compresses, the thrust arm pushes the lower control arm (which isn't quite perpendicular to the direction of travel) forward and out for increased caster and negative camber.

Without the compliance of the rubber bushings (or if the movement is restricted by mechanical interference), the arms would be stressed back and forth until failure. The arms are designed to bend, not break when overloaded (accident), but they are not designed for repeated side-to-side loading which can be caused by restricted movement.

Thrust Arm Bushing Dimensions

Bushings are 58 mm diameter, width varies

  • <--- 48 mm width of steel shell | | [| |] <--- 56 mm width of aluminum center section

    | |

    1. Stock 750i bushing, p/n 31 12 1 136 607
  • <--- 48 mm width of steel shell | | [ ] <--- 50 mm width of aluminum center section

    | |

    2. Typical Modified 750i bushing

    (not enough difference between center section and steel shell for unrestricted movement -- steel shell hits bracket)

    • <--- 43 mm width of steel shell | | [| |] <--- 50 mm width of aluminum center section

      | |

      3. Dinan Modified 750i bushing

      (best design -- difference between center section and steel shell prevents possibility of outer steel shell hitting bracket from side-to-side movements of the arm)

The thrust arms move about 7 degrees each way as the steering is turned from lock to lock (14-15 degrees total). The 7 mm difference of the Dinan bushings (approx 3.5 mm on each side) makes sense when you take the diameter of the bushing, 58 mm, times the tangent of 7 degrees (i.e. 58 x tan(7) = 7.1).

Dinan Does it Right (Steve D' too)

After discovering this potential problem last week, I called Steve D'Gerolamo to see if most vendors were doing the more simple modification. Steve said yes, he was pretty sure most places were making the mod as described above in #2. To Steve's credit, he was very receptive to my concerns and willing to change his bushings to the more conservative Dinan approach.

However, the most credit goes to Dinan for carefully figuring this out in the first place. I continue to be impressed by Dinan's attention to detail and solid engineering.

Pete Read
'88 M5
Arlington, VA

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