Miata Mailing List: March 1996, Message #139
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   From: LANCE K SCHALL <76760.2754@compuserve.com>
Subject: Re: Towing/Tie Downs
   Date: Sun, 3 Mar 1996 16:49:18 -0500

Lester Seal:

You ask a very good question.  What is the strenght of the frame rail?  Good
thing I've got nothing to do Saturday night better than curl up with some
formulas .

Looking up under the front of my Miata reveals that the frame rail from the
front suspension to the front bumper is a surprisingly straight weldment 3 1/8
inches by 3 3/4 inches.  The material is about 0.060 inch thick.  The tie-down
is 2 feet in front of the suspension attach points.  Let me assume that the
material is also good quality hot rolled steel, but nothing too exotic, with a
tensile yield of 60,000 psi.  We can safely take the shear modulus of elasticity
to be 12 x 10E6 psi.  I disregarded any stress concentrations like unfortunately
placed holes, imperfect weld seams and the like.  On the other hand I also
ignore any contributions of the surrounding structure to the strength of the
assembly.  These are fairly significant omissions that fortunately sort of
cancel each other out.  The following results are estimates only.  I think they
have significant utility however and are much better than order-of-magnitude
guesses.

To fail the frame rail in tension would take 50,000 lb.  Not likely to see this
problem.

The front springs have a spring constant of 175 lb/inch each.  To compress them
to close the coils (disregarding the bump stops) would take 800 lbs total on the
front wheels.  This translates to 650 lbs on the front tie-downs, straight down.

If we continue to tighten the front tie-down chains after bottoming the springs,
when we get to 4,000 lbs, the rear wheels leave the ground!  Picture what this
would look like!

Now go to the back of this luckless Miata and attach the rear tie-down chains.
Return to the front, tighten to 5,000 lbs.  Somewhere about here, the frame
rails would buckle at the front suspension attach points.

I can not say if this would be the failure mode observed.  My only point is to
consider the probable kinds of forces and abuse necessary to damage the car.  I
think towing intelligently will not cause damaging forces.  Remember, you do not
tow a car by pulling DOWN on the tie-downs.  They are stronger when pulled
outwards.

All that said, realize that transport truck tie-down chains CAN provide 2,500
lbs per chain damaging force.  It IS possible to bend the car.  If the operator
is used to moving Caterpillar D9s, just hog the chains and go!  However, you
would need to be really creative bend a Miata with a tow rope!

I know many Miata people like to think of their car is something special and
deserving of much respect and cautious maintenance.  Nothing wrong with that!  I
agree!  But it is a sturdy car.  And it is just that, a car.  It does live in
the real world.  Designers think of things like towing, tow trucks, accidents,
enormous potholes, jump starting, push starting, getting stuck in the snow
(Rocking.  It is abuse and Ive done it), trailer towing (Not recommended by
Mazda.  Why is no one jumping up and down about this?), and grinding gears with
a bad shift.  Sometimes living in the real world is abuse.

Lance Schall
Boulder, Colorado