But you will also see that the spinners on these cars use strategically positioned left-hand and right-hand threads. Generally, the driver-side hubs and spinners are threaded so clockwise rotation of the spinner tightens the wheel to the hub. The passenger-side fasteners employ left-hand threads that are turned counter-clockwise to achieve tightening. With the threaded wheel fasteners oriented this way, there is a natural tightening effect imparted to the spinners as the car travels forward over the road. But keep in mind that the wheels and spinners are rotating at precisely the same speed at all times – unless a careless mechanic hasn’t tightened the spinner all the way down.

Testing shows that finger-tight RH lug nuts quickly unthread and will leave the car with the messy result shown. Left-hand threaded fasteners are far less susceptible to this problem. Chrysler had it right!

Okay, let’s now turn our attention to the use of left-hand lug nuts on cars equipped with conventional star-pattern wheel studs and wheels. Since the nuts are now orbiting (i.e. rotating on a different axis) than the front spindle nut / rear axle shaft centerline as the car rolls down the road, they are not subjected to the same kind of contra-rotational inertia as a center-mount spinner. As such, there shouldn’t be any need to be concerned with right-hand or left-hand threaded wheel fasteners. Just make ‘em all right-hand threads and be done with it, right?

That’s what the vast majority of domestic and foreign automakers choose to do and it seems to work just fine. All of which brings us back to the original mystery of why Chrysler went with left-handed fasteners on the driver-side for so many years.

Some new insight into the left-hand lug nut mystery was discovered recently when I was at a local used book store. I bought a heavy hardbound book called “Passenger Car Safety Dynamics”. Published in 1965 by the Motor Vehicle Research Center of New Hampshire, and authored by one Andrew J. White. This 481 page tome is filled with vintage photos of real world car accidents as well as many intentionally staged vehicle impacts with immovable barriers. An apparently self-funded entity, the Motor Vehicle Research Center of New Hampshire seemed to function as a precursor to the vehicle safety labs of today.

This vintage Passenger Car Safety Dynamics book has sections with titles like “Theoretical Factors in Energy Absorption”, “Vehicles Impacting Bicycles”, “Theoretical Study of Head-On Crash”, “Low Extremity and Knee Injuries” and more. It’s fun reading for the whole family…if you don’t mind graphically brutal photos of actual crash scenes and victims.

Anyhoo, the fourth section is entitled “Investigative Procedures for Accident Cause, Brakes and Wheels”. This, dear reader is where we learn, once and for all, why Chrysler embraced left-hand lug nuts on the driver-side of its passenger cars for so many years. In a nut shell, the Motor Vehicle Research team conducted a series of carefully staged wheel loss tests. Working with a variety of American cars from the major manufacturers, the tests revealed that the use of right-hand-threaded wheel fasteners is perfectly acceptable – “when all the securing nuts on each wheel are properly tightened using a minimum of 20 foot pounds of torque”.

Can’t get enough of mangled Chevy shoe boxes? Here’s a staged roll over test captured with a high speed camera.