# Why Do Some Cars Have Lugs With Left-Handed Threads?

As I understand it, some older GM cars have lug studs that utilize left-handed threads on the driver’s side and right-handed threads on the passenger’s side. Is this accurate?

What was the reason for doing this?

Thanks.

Thanks.

Tom and Ray’s explanation doesn’t appear to make any sense, though. I don’t understand how the forward motion of the car could act to loosen right-handed threads on the driver’s side.

And even if it did, since the rear wheel supplies torque while the front wheel receives torque, the forces acting on the lugs of the front and rear wheels on the driver’s side act in opposite directions.

It is to stop them from self-loosening, but I think it’s more to do with the fact that, at any one moment, the forces acting on the lug are uneven (because it is supporting the weight of the car against gravity) - as the wheel rotates, the uneven force rotates around the lug (at least from the POV of the lug).

That’s an interesting point, Mangetout. I had forgotten that lug nuts have a conical base, and therefore are actually bearing the weight after they are tightened.
I suppose that as the pressure is transferred around the perimeter of a lug nut, there might be some slight remote chance of it gently walking its way off.

After reading the linked article, I was assuming that it was to counteract possible unlugging from the vanishingly-small inertia of the lug nuts as the stud bearing them rotates. In this case, it would seem that you would need a frictionless stud/lug nut combo, well oiled, in order for this inertia to actually turn a nut. Of course, it ain’t gonna happen with a cranked-down lug nut.

I believe that it was actually Chysler that did this. The reason for it? I gross misunderstanding of the way in which fasteners work (and the way in which people don’t) lead some engineering genius to conclude that the lug nuts should turn against the rotation direction of the wheel. This might make some small amount of since from the viewpoint of any individual lug nut, if the nut were large (in mass) in comparison to the wheel, such that there is some possibility that the intertia of the nut might create a significant D’Alembert torque to loosen the nut, but in fact the nut is so light in comparison to the wheel, and the forces are sufficiently symetrically balanced that this is not an issue.

What is an issue, however, is some gorilla with a cheater bar or some mechanic with an impact wrench applying a force the “wrong” way, and instead of unscrewing the nut ripping the entire lug right out or stripping the threads off. It’s only one of the many “features” Dodge introducted during its illustrious early Seventies period…you know, right before it fell into bankruptcy. :rolleyes: I don’t think GM ever did this, but then, they weren’t exactly producing the finest quality of their products during that era either, so it could have happened, but nobody noticed 'cause the car didn’t last long enough to have to change a tire.

BTW, lugs (and other threaded-nut fasteners) work by translating their torque load into an axial load down the bolt/screw/lug which (slightly) stretches the screw, like a spring, in what’s called a pre-load. The preload generates friction between the mating surfaces which accepts all shear loads (between the lug nut and the wheel and the hub, in this case). If properly designed, the bolt/screw/lug never sees any shear or bending load which is a good thing, because with all those notches in it (from the thread) it would have a hell of a stress concentration in bending. If you need a fastener to take a shear load (as in bridge design, for instance) then you use a rivet or a shoulder bolt, where the shear is taken over a non-threaded portion of the shaft.

So…make sure your lug nuts are properly tightened (and not overtightened). And if you own one of Chrysler’s fine products from that era…beware. :eek:

Stranger

I believe it was mainly Chrysler, too. But at one time I drove an International Scout that had left-threaded threads on one side. So, at least International Harvester (now Navistar International) used to believe it as well.

Chrysler products began using left handed threads in the late 1930’s and phased out their use during the 1971 model year. I raced Mopars for many years and tossing the left hand threads was one of first thing I would do when building my race cars.

I own a '36 Dodge touring Sedan and the front left and rear lug nuts are left threaded. Chrysler did this to assure the owner the lug nut would be tight all the time. They are marked with an L (left thread) and R (right). Hope this answer helps to prevent broken knuckles. LOL

Don’t forget that some vehicles had the wheels held on by only one large nut at the center of the wheel. Those are definitely threaded differently on the right and left sides.

I had a '65 Chrysler Windsor (Canadian model) with left-thread nuts on each wheel of the driver’s side. Unlike your '36 Dodge, the right-rear thread on my '65 was normal. (But I bought the thing when it was 28 years old, so who knows what was done to it.)

I always assumed they did it so they could sell more replacement wheel studs and provide mechanics with the frustrating job of removing the sheared ones. Well that seems to be the net effect anyway.

Left hand lugs, an answer without a question. I forget if my 71 Valiant had them.

Boy howdy, was this fun to find out on a Dodge Dart at 11:30 at night in the pouring rain by the side of the freeway.