Why small bolt pattern?

RustyRatRod said

The theory that lug nuts will loosen or tighten due to wheel rotation is incorrect as long as the nuts tighten into a bevel with the same angle as they have. It is this bevel that keeps the nuts tight. This is why with any other type nut that does not have a bevel, such as a mag wheel lug nut you are recommended to recheck the torque every so often.

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The bevel is not what keeps the nut tight. It's true that a taper fit does offer a significant friction fit (Tip your hat to ball joints and tie-rod ends once in awhile), but lug nuts are held on by the same mechanics that govern all bolts and nuts.

It's the stretching of the fastener that provides the retention. As the nut is tightened, the bolt or stud stretches (pre-loads, in engineering mouth noise) and this preload pulls the nut and joining parts together. Think about a paper clip: the paperclip trying to return to its original shape is what holds the paper together. In order for it to fail, it has to be bent beyond its yield point. Up until then, it's holding tight but once over-bent, it's junk. Same deal on Lugs: If they're stretched enough to hold but not so much they can't pull back to normal, you're golden.

Friction is a function of the force applying the friction: more force equals more friction:
Ff=u x Fn
Where Ff is the friction force,
u is the coefficent of friction
Fn is the normal force

It's a combination of the preload and the friction that provides the clamp load that holds things together. The conical seat serves more to locate the wheel and distribute the load than it does to hold the nut in. (Look in your machinery handbook or engineering guide for taper fits and the taper versus load charts). The 45 degree conical seat on a lugnut is too wide for much of a taper-fit (most machine tapers are about 7 degrees), and the lugnuts aren't hard enough to use it well.

Remember George's '68 Dart 4-door? He changed a tire and put the lugs on backward. He got 'em tight but the traction action eventually slotted the lug holes. The nuts never came loose, but the load wasn't distributed via the conical seat and so something hadda give.

Just like the nut doesn't hold much on the tie-rod end, the taper doesn't hold much on the lug nut.

Mag wheels required retightening because the Aluminum was not as hard as the steel, and because the wheel couldn't deform elastically when tightened as steel wheels do (Look at the back side of an OEM steel mo-rim sometime. The wheel deforms when tightened, and this also contributes to preload). The aluminum deforms plastically and can shift and squirm under load (without returning to its prior shape. It has a lower yield point), and this reduces the preload on the fasteners, thus requiring re-tightening. Modern alloys don't have this issue because the material is tougher and the conical seat allows higher preload but distributed over a greater area so deformation is not a factor.



Why the small pattern versus large?
Simple! Bean counters and engineers, working together, often at odds with each other.

Think: Smaller car, needs less wheel, less stud, and has less profit margin. Not only was the pattern smaller, but so too were the studs. That means cheaper studs, cheaper nuts, and you can pull it off because the car weighs less.
I'm inclined to believe that GGs66GT was partially correct: smaller brakes leaves less room for bigger patterns but the fact of the matter is, the larger the bolt pattern, the more the load is distributed. If you bolt a 15 inch rim at the outer edges, then the leverage you can apply to that rim is minimal; the force goes straight to the studs. If all the bolts were as close as they could be to the middle, the force has to travel through the rim on its way to the studs (which is what's holding supporting the load), first, and it gets a nice boost in leverage from the wheel in the process. The rim sees more load and the load is not applied favorable to the studs.

Think: Trucks. The old Jeep and truck rims still sought a 5-lug pattern but they spread the studs out even further. Why? Because trucks and jeeps see higher loads. Your GVWR isn't increasing dramatically, but you are side-loading the wheels more, and you're more likely to push the load rating of the wheels and tires to the max.
Whereas a passenger car usually doesn't get a bed full of 5/8 OSB and then go around a bunch of corners, or traverse a steep hill crossways whilst chasing cows, the truck or Jeep will. Wide pattern reduces the load against the wheel and studs.
Now, take it to the other end of the spectrum: Small car, small brakes, small wheels, and even less load to worry about. The competition has weak-dick 4-lug rollers. We could get away with it but 5 is oh-so-sexy and it makes nice for the saleman to point out that there's one extra nut on there, just in case (Extra care in engineering, anyone?), when he's selling Grandma a nice new Signet. So for a small price, we add an extra lug, but then we make it up by going to a smaller pattern (Ever seen a 5.5 on 5 on a 13" wheel? ugh.).

Sure, we'd save money by only stocking one set of wheels and all that, but the cost saving on smaller wheels (with thinner material, natch), smaller studs, etc. makes it a winner. There's a reason new cars don't come with monster-truck tires, and even if they worked great, they're $$.

As the cars got bigger and power increased (Compare the shock loads applied to the lugs on a Slant 6 '65 Dart to a Big-block '68 sometime) the need for smaller patterns went away and it became cost effective to share wheels and tires. I'm still amazed that the big block Darts didn't shear studs on a regular basis.

LH vs. RH
In theory, the spin would have little to do with it. The nuts aren't being spun about their own axis when the wheel turns, they're being rotated about an external axis. If the inertia of the nut was a factor, it seems it'd loosen being a LH nut on the driver's side. Lots of big trucks are still this way. I guess the parallel axis theorem might apply, but I don't really know.

I also wondered if it was something like "a little extra", back when materials weren't the best and tightening lugnuts wasn't the art form it is today.

Even still, the LH lug nuts is something I truly do not get.

I will say that I'm sure that it was long-outdated before the changeover, and that Chrysler was just waiting for the tooling to wear out before re-tooling.