From digest.v7.n1375 Tue Mar 24 15:50:03 1998
From: Pete Read <read_at_arl.bna.boeing.com>
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
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
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
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
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
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
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.
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