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... > > > > > > To address comments privately to the moderating team, please address: > moderator@xxxx > > To search the archives or view files, log in at http://groups.yahoo.com/group/dmcnews > > Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/ > >