Trailing Arm Bolts - Engineering

[tobyp@xxxx]

Hello List - This is Toby Peterson ... checking in.

I thought that I'd take a little time and give you some addtional 
background behind the engineering considerations that go into 
resolving issues like the trailing arm bolts (TAB). I will try to be 
as brief as possible, and will also try to make it "value added" for 
everyone's learning. The following terms need to be defined because I 
will use them a little later: "Ftu" = allowable ultimate tensile 
stress; "Fty" = allowable tensile stress at which the material starts 
to yield in tension; "Fcy" = allowable compressive stress at which 
the material starts to yield in compression; 1,000 PSI = 1 KSI 
(reduces the number of zeros in an equation). "Yielding" means that 
the material is beginning to deform and deflect under load. When a 
material is stressed beyond the allowable yield values, it takes a 
permanent "set". If it's a bolt, the bolt becomes bent. If it's 
stressed beyond the ultimate tensile values, it breaks or ruptures. 
Okay, are you still with me? I think that this is important when 
discussing the various options, as well as the ultimate solution.

The original TAB are made from 4130 steel (probably), with Ftu = 125 
KSI - 145 KSI. My fractured bolt checked out at 136 KSI using the 
Rockwell hardness method. For this strength range, Fty = 103 KSI, and 
Fcy = 113 KSI. This material is also highly susceptible to corrosion, 
so it must be cadmium plated for protection. The downside to cadmium 
plating is that it can be damaged by wear, and installation, and it's 
protection becomes compromised. It's also sacrificial, which means 
that it dissipates over time.

The critical loading condition for the DeLorean TAB is bending. We 
have a long, slender bolt in a single-shear joint. We don't have 
significant tension loads applied during any driving scenarios, so the 
Ftu values don't really mean much. The important numbers are Fty and 
Fcy, which define how resistant the bolts will be to bending stresses. 
We have all either seen or read about bent TAB's. However, there are 
many people who have never experienced this problem (yet). That means 
that the applied bending loads in our application are hovering in the 
range of the capabilities of the stock TAB. Aggressive drivers have a 
very real concern that they will overload their TAB's, while the 
'Sunday drivers' may never exceed the capabilities of their TAB's. 
The way this bending phenomenon works is that the bending loads 
increase until the material in the bolt either meets the Fty or Fcy 
values. Then, the bolt begins to yield in whichever manner is 
critical for the material. This increases the other stress 
dramatically, which causes the bolt to yield in both ways ... it will 
crush on the compression side, and stretch on the tension side. If 
you exceed the maximum allowables, the bolt will be permanently bent. 
The highest stresses will almost always be in the first few threads 
after the bolt shank. If there are any corrosion pits or other damage 
such as galling of the threads due to installation of the nut, a crack 
may start at that point of maximum stress, and propagate through the 
thickness of the bolt. Crack growth may be slow at first, because 
most of the bolt is still intact. But, as the crack spreads, the 
stresses go up, and the crack speeds up. It will eventually fail, 
just like mine did.

I have received a suggestion to use type 316 CRES for an alternate 
bolt material. The numbers for 1/2 hard 316 are as follows: Ftu = 
141 KSI; Fty = 93 KSI; and Fcy = 61 KSI. As you can see, for a given 
applied load, the value for Fcy is about 46% lower. At a strength 
range of "full hard", it's still only Fcy = 83 KSI. Not necessarily a 
good solution if bending is our primary concern. I will say that 316 
is very good for corrosion resistance, but ...

Is everybody still awake? Okay, now for a glimpse of what I decided 
to do. I selected the very best material that money could buy. It's 
called Inconel 718. This is a nickle-based super-alloy with the 
following numbers: Ftu = 220 KSI; Fty = 200 KSI; and Fcy = 200 KSI. 
Inconel 718 also has a very high fracture toughness, which means that 
it is very difficult to initiate and propagate a crack. It's 
virtually corrosion proof, non-magnetic, and is used in the aerospace 
industry whenever a failure is absolutely not acceptable (engine 
mounts, landing gear, and wing attachments, to name a few). I have 
developed a business relationship with the Vice President of Product 
Development at a world-class manufacturer of specialty fasteners for 
the aerospace industry. This company is headquartered in Torrance, 
California, and supplies fasteners to Boeing, NASA, Airbus, and many 
others. I selected a high-performance nut made from another 
super-alloy called A286, with an Ftu = 180 KSI. The washers are made 
from hardened steel with a zinc dichromate finish.

I obtained a small number of bolts from this company, and have 
installed them on seven cars in our club. The fit is perfect. Due to 
the length of this post, stay tuned for what we need to do next to 
make these bolts available to concerned DeLorean owners everywhere.