Fw: [DML] Trailing Arm Bolt Concerns

["Chris Parnham" <chrisparnham@xxxx>]

This man seems to know what he's talking about"!
Chris P

----- Original Message -----
From: <tobyp@xxxx>
To: <dmcnews@xxxxxxxxxxxxxxx>
Sent: Tuesday, November 06, 2001 1:11 AM
Subject: [DML] Trailing Arm Bolt Concerns


> 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...
>
>
>
>
>
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