Lon,
The question you pose; "Is there a down side too this, making the lug snap
easier than it already does while motivating?" is an interesting one that I
found myself mulling over a few years back in related circumstances. For
example, in the case of rod bolts does increasing the torque on the bolts
increase, decrease, or leave the ultimate yield strength unchanged? I think
this is a question any good engineer immediately knows the answer to, but
for the rest of us, we need to think about it a bit, and there's no
guarantee we'll get the right answer either. Although I'm not a good
engineer, I do know one ore two, so I'm pretty sure, at least in this case,
I have the right answer.
Let's pose the problem this way. Suppose we torque two rod bolts to just
under the yield point. What I mean by yield point is the point beyond which
the bolt does not return to it's original length when the load is removed.
We could call it the elastic limit, but that's too self explanatory for
engineers. Anyway, with the bolts at the yield point, what happens when we
pull on the joint? One might think that, since the bolts are already at the
yield point, any additional load will put them beyond the yield point and
on the path to failure. However, this would be wrong. This problem can be
understood, at least it seems logical to me, by thinking in terms of
Young's modulus (law) which is just the linear relationship between stress
(how hard you pull) and strain (how much it stretches). Accordingly, if the
bolts don't stretch, there is no more strain or, consequently stress. Now,
with the bolts torqued to their yield strength, a certain amount of
clamping force is generated between the two pieces (e.g., the rod and rod
cap). This could be several thousand pounds in the case of two 3/8' rod
bolts torqued to 45 lbs-ft. Until the tension on the rod exceeds the
clamping force, the two pieces remain clamped together with no gap and no
additional strain is put on the two rod bolts. When the tensile force on
the rod exceeds the clamping force, the two pieces start to separate and
this adds incrementally to the strain and stress on the bolts.
In the above example, you can see that tightening the bolts to any point
less than their yield strength has no effect on the ultimate strength of
the rod cap, which is always determined by the tensile strength of the
bolts. HOWEVER, if the tensile stress at high rpm is greater than the
clamping force, it will make bolts stretch and create a gap between cap and
rod, and this is BAD, VERY BAD. Thus, you want to torque the rod bolts to
just under their yield strength to get the maximum useable strength out of
them.
In the case of lug bolts, or any bolt for that matter, you want to be sure
not to go beyond the yield strength. If you repeatedly go beyond the yield
strength the bolt will keep stretching until, finally, it fails. On the
other hand, I suspect that the clamping force needed to keep the wheels
firmly on the hubs is rather modest, unless maybe you have big wide wheels
and sticky tires. You could probably calculate it using the weight on the
wheel (say at most 1/2 the weight of the car) and assume a coefficient of
friction of unity and measure what the fulcrum arm is. Everyone on the List
seems to be of the opinion that four 7/16" lug bolts are plenty strong, but
if you use 80 lbs-ft of torque, you should probably be using 1/2" lug bolts.
Well, this was probably a longer answer than you were looking for, but hope
it helps.
Well, TTFN,
Bob
At 12:23 PM 8/29/99 -0700, Lon Walters wrote:
>Saw the question and confirmation, agree that 45-50 lbs for a wheel lug
>seems mighty low to hold a wheel on...have been doing mine at 80 lbs.
>Is that way too tight? Is there a down side too this, making the lug
>snap easier than it already does while motivating? Would really
>appreciate some insight on this...
Robert L. Palmer
UCSD, Dept. of AMES
619-822-1037 (o)
760-599-9927 (h)
rpalmer@ucsd.edu
rpalmer@cts.com
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