Trailing Arm Bolt Concerns

[tobyp@xxxx]

Hello List - 
I've got some information relating to trailing arm bolts that I think 
would be of interest. First, let me give you a personal profile so 
that you know who I am. My name is Toby Peterson, and I am, and have 
been, a Principal Structural Engineer at Boeing for almost 20 years. 
My responsibilities include the engines and engine pylons for the 
entire 747-400 fleet. I have owned my DeLorean, VIN 2248, since 1988. 
I have developed many connections within the aerospace industry and 
some of the best aerospace manufacturers in the world. Now ... on 
with the story - 

The trailing arm bolts (TA) have a great deal of work to do. They 
react almost all engine torque and braking torque at the rear wheels, 
establish rear wheel alignment, and transmit all "thrust" from the 
drive wheels into the frame of the car. They are a "critical load 
path" item with no significant redundancy. If a bolt fails during 
certain driving scenarios, directional control could be lost, and the 
event could be non-recoverable. The importance of the TA bolts has 
always been a concern of mine.

At a club-sponsored tech session last year, I did a complete 
inspection of the suspension components, as usual, and also re-torqued 
my TA bolts. The drivers' side bolt took a very small torque, and 
then became free-spinning ... not a good thing. As some other people 
went off in search of a replacement bolt, I removed both halves of the 
fractured bolt, and kept them for further inspection. The other bolt 
was clearly bent, as well. After the replacement, I took the bolts to 
a metallurgical lab for analysis of the fracture. The bolt had 
cracked 80% through in slow crack growth, due to fatigue, with another 
10% in fast growth. The drive up to the session had been "spirited", 
and resulted in the last three crack striations. The remaining 10% 
failed during the torqueing procedure. Scanning electron microscope 
views of the fracture surface revealed that the crack had started at 
several small corrosion pits in the area of the first thread, and 
propagated through the bolt due to fatigue from bending stresses. The 
material tested out as alloy steel with cadmium plating, and had a 
tensile strength of 136,000 psi. That's about right for a bolt with a 
metric rating of 10.9. After I explained where the bolt was 
installed, and what it did, the lab technician asked me a very simple 
question ... "Why did they use such a crappy bolt for this critical 
function?" Good question. The alloy steel is subject to rust and 
corrosion, the plating deteriorates over time and can be damaged 
during installation or use, and the material strength is not adequate 
to prevent bending in a single shear application under high loads. As 
mentioned in other messages, the washers are showing signs of crushing 
and wear, which will reduce the preload on the bolts. This will 
increase the induced bending stresses during driving, resulting in 
faster fatigue damage to the bolt.

At a subsequent tech session, we looked at the TA bolts in seven cars 
by completely removing the bolts and examining them visually. Several 
were bent, and several others were corroded and rusty. A couple were 
quite loose, while others needed to be pounded out with a hammer. 
Only two cars had bolts in what I would call "good condition". As I 
said earlier, I have been very concerned about this situation, and the 
apparent lack of understanding about this issue, as evidenced in other 
entries on the list. The main issue with the TA bolts is not that 
they can cause a clunk ... the main issue is that a failed bolt can be 
catastrophic under some driving conditions.

I will post a second entry tomorrow with details about what I did to 
solve this problem for myself. I will be asking for an idea of the 
level of interest in making my solution available to the rest of the 
DeLorean fleet. Please consider what I have shared here, and be ready 
to give me some feedback when I share my solution with you. 'Til 
then...