[DML] Re: TA Bolt questions

[tobyp@xxxx]

John, and "The List" - 

After reading post # 21251 from John, I felt that I had to help 
clarify a couple of points. Also, I have had several requests for a 
technical comparison of several of the bolts that have been discussed. 
I have the data, and will be doing that piece shortly.

John has suggested that bending of the bolt is not a bad thing, and 
could almost be considered somewhat beneficial. I will dare to repeat 
a statement that I made earlier, "Bolt bending in this joint is not 
acceptable". When the bolt bends, there is motion between the sleeves 
in the arm and bushing, and the washers and shims that occupy the rest 
of the space in the joint. This motion causes wear, which leads to 
loosening of the joint clamp-up. In our case, the bolt is also 
yielding (permanently stretching) in many cars, due to applied loads 
from the car, combined with induced loads from the bolt torqueing. As 
the joint becomes loose, the bending stresses in the bolt increase, 
which results in more yielding. The eventual result will almost 
always be fatigue failure of the bolt, usually at the first thread 
next to the shank of the bolt. If there is rust or corrosion on the 
bolt, then the stress concentrations caused by the corrosion pits will 
create a starting point for any cracks (My failed bolt had the crack 
start in small corrosion pits). I can support all of this with facts 
and data. This is not my opinion ... this involves physics, 
engineering principles, and the mechanical properties of materials.

The second point I wanted to clarify is in regards to the subjects of 
"fail safety and structural redundancy". A "fail-safe joint" means 
that if any one component fails, there are other load paths that will 
pick up the loads, and keep the joint together. If the rubber bushing 
fails, the trailing arm will remain in place (although it will rattle 
and clunk) because of the large flat washers on one side of the frame, 
and the arm itself on the other. The same is true if the attachment 
bolts for the bushing assembly to the frame fail (I had this happen a 
couple of years ago ... made some noise, but no big deal). If the 
frame or the trailing arm start to fail from buckling or cracking, 
visual inspection will find it long before there is a complete failure 
(has this ever happened ? ... I doubt it). However, if the trailing 
arm bolt fails, the joint is completely lost. There is no redundancy, 
no alternate load path. A critical component with no back up, whose 
loss could result in catastrophic loss of control or major damage to 
the car. This situation is not allowed anywhere in the aviation or 
aerospace industries. I don't think that we should accept it here 
either. Colin Chapman was a sharp guy, but he dropped the ball in 
this case (IMHO). We can't necessarily redesign the entire joint, but 
we can improve the components in order to minimize the risk.

Toby Peterson VIN 2248
Winged1


--- In dmcnews@xxxx, dherv10@xxxx wrote:
> Walt and others, Don't get me wrong, The application engineer told 
me this.