Why do the wheels fall off my '66 Cobra?

I have a 1966 Shelby Cobra kit car that I completed last August. It has a standard 9? Ford rear axle that probably came off an early 80s vintage F-150, and custom aluminum wheels. Each wheel is held on with five ?? lugnuts which came with the wheels.

Two weeks ago, with less than 400 miles on the car, I was doing about 40mph when the left-rear wheel came completely off; all the lug nuts had backed off and were nowhere to be found. Fortunately the wheel well captured the wheel so it didn?t go flying, but I could not figure out how all five lugnuts could back completely out after only 400 miles. My father believed it was sabotage ? that someone had loosened or removed the lugnuts while I had it parked somewhere. It sounded a bit far-fetched, but it was the only explaination that made any sense. I removed a lugnut from each of the remaining three wheels, put the wheel back on, and drove it home (less than 3 miles).

Until losing the wheel the car made no indications that anything was wrong. It handled fine and I heard no weird noises (although the unmuffled 460 cu. in. engine may have drowned out any telltale noise coming from the wheel).

This past weekend I got a new set of lugnuts and went to put them on the car, and it was then I noticed that the three lugnuts I put on the left-rear wheel 2-weeks earlier were nearly backed completely off again. The car had not moved, so these lug nuts backed off after only 3 miles at speeds not exceeding 40mph. How on earth is this possible? And how do I prevent it from happening again? I?m afraid to drive the damn thing now.

Buy a set of DECENT wheels and LUG NUTS before you kill yourself. Most of the “Universal Fit” stuff that use “special” lugs and washers to adapt the wheels to a hundred applications are JUNK.

No explaination from me but I would make replacing the lug studs and nuts as a first attempt to solve the problem. The lug studs stretch every time the wheels are torqued down and perhaps they have met their lifespan.

Make sure the lug studs are clean but don’t use any product like anti-sieze on the threads. Anti-sieze will affect the amount of torque you think you have applied, just make sure the threads are real clean.

Two other ideas, is there any paint on the surface between the rotor and wheel (there should not be)and perhaps some “burnouts” have over stressed the lug studs.

Check your lugs with a go-nogo gage…all of them.

If the lug nuts were previously overtightened, they can stretch at their bases and prevent the lugs from properly seating. The distorted lugs will cause “tightening” via the thread interaction rather than via the seating of the lugs into their proper interface. This can cause the lugs to loosen and work their way off.

Also be sure you’re using the correct lug nuts.

Wheel studs are hardened. They don’t “stretch”, they snap off. The fasteners must be the ones that were MADE FOR those specific wheels and bolt pattern. If the holes in the wheels are hogged out from wobbling around on the loose studs, THEY ARE JUNK and will NEVER be secured properly…

Check out this web site that is devoted to the examination of why wheel studs break. There is a paragraph dealing with repeated torquing/ untorquing of the wheel that can lead to stud failure. My idea about the stud stetching is based upon the idea used with many threaded fastners, that is that the bolt stetches slightly too make torque application or preload possible. I am searching for a description of wheel studs being made of “hardened” material, this idea does need some research to see just what is ment when the term “hardened” is applied to a wheel stud. Strangely I am finding much condemnation to the overuse of anti-sieze on the stud, but much promotion on the use of a “light amount of oil”.


Yes, wheel studs can stretch at their base. They can also fracture and fail as a result of overstretching.

“Hardened” means the metal has been heat treated after cutting to be harder, it does not mean it’s lost all of its maleability. Hardness tests, such as the common Rockwell Hardness Test, work by impressing a shape into the metal under a known pressure. A measurement of the amount of deformity created is converted into a hardness rating. There are other means of hardening materials such as “case hardening”, which hardens only the surface of material, typically by impacting and compressing the material’s surface.

If you doubt that a stud can be stretched, simply bend one. The side on the outside of the bend will be stretched.

mb, If I read you correctly you are saying that wheel studs are not hardened so much that they cannot be stretched and suffer fatigue from this stretching? This is my position. (this question is a result of the “I don’t know whos post you are refering too situation”

Would you express a more exact explaination of the application of the term "hardened’ when it is applied to a wheel stud? Certainly I know what “hardening” is in general.

I think the nuts aren’t properly matching the studs. You know the nextt step.

They match the studs just fine. They don’t match the wheels…

They can be stretch such that they experience breakage. I’m saying they can be stretched enough for thread deformity without catastrophically failing. It’s that thread deformity of the last threads that causes the inability to get the proper compression onto the wheel and allows the nuts to work loose.

“Hardened” for a wheel stud has no different meaning than “hardeened” for any other metal part that utilizes a heat treatment after forming. The metal is typically cut while soft and heat treated to harden after forming.

These guys http://www.reglover.com/Standards.html are the standards for all these issues. They’re also the standards organization used by Department of Transportation (D.O.T.) for all over the road vehicle regulations. It may take some looking to get to the correct standards for wheel studs, but they’ll be in one of the ASME standards.

I recently related a story about a seasonal that put my lug nuts on backwards, the coned side goes towards the rim and with proper torquing, pattern and tourque you should not have a problem, or what caddyman says.

I’m in agreement with Caddyman that it’s time to replace all of the studs and lug nuts; preferably with something not made in China if that’s even possible anymore.

Odds are the stud threads on the wheel in question have been pulled in the past due to some overtightening and when this happens nothing will stay tight.
And not meaning this in a derogatory way, but is it possible that you could have mistakenly failed to tighten the lugs properly on that wheel to begin with and it just happened to take 400 miles for them to work loose?

With a beast of a motor in a car like this it’s definitely no time to toy around with the wheels.

some lug nuts require reverse tightening(left hand threading?). This may have been the case if everything else comes back good

A lot of custom wheel have an extra large hub diameter. You need to have a hub adapter for these to keep the wheel from wobbling around the center. Without this adapter, the wheel becomes lug centric instead of hub centric.

Going lug centric works ok with steel wheels, but aluminum wheels are softer and the aluminum "gives under the lug nut. When that happens, the wheel moves around inside the lugs. Because the outside of the lug and lugnut is on a greater circumference than the inside, the movement will loosen the lug nuts on the left side of the vehicle, unless the left side has left handed threads like the old Chrysler vehicles had.

Vibrations from the rear wheel is not felt anywhere near as great as vibrations from the front wheels.

That’s my theory anyway.

OK4450, I suggested replacing the parts you mention the second thread of the post, take a look.

Gotcha. Sometimes I read all other replies, sometimes scan a few, and sometimes don’t read any of them. In this case I just skimmed Caddyman’s reply and agreed with it.

Heck, it was only about 3 weeks ago a wheel was working loose on my daughter’s late model Mustang and while I haven’t quite understood it yet, the blame has to fall back on me.
I did a complete rear brake job and rotated the tires at the same time. A few days later she said the car had a faint “tick” sound on the LF wheel.

Examination showed all of the lugs were about 1 round loose and since I was the last one with hands on it, this falls back on me. The part I can’t quite figure out is that the lugs sit in narrow, deep wells in the wheels, and are difficult to even start on the threads much less screw them down with the fingers.
This means they must have been run down with a lug wrench and I’ve never run a lug down before and then walked away without tightening it; much less 5 of them.

Threads were good and they’re still tight so this has to be a faux pas on my part somehow, although I’m still trying to understand how it happened.

I did a winter brake job on my 65 GMC and come spring I wanted to drive it in to work, well about 3/4 of the way there I had a lot of vibration in the front,when I got to work I check the lugs, they were loose and I swore for a weeek it was the crazy neighbor down the street that sabatogued me. It was me. The only saving part of the story was that it was terribly cold and dark in the garage, and it is a lot of fun to drink cold beer during a Wisconsin winter.Why is it allowed to sit on a frozen lake looking at a hole you made and drink beer? the entire neighborhood did it,at times someones fishing shanty would fall in the lake and we had another neighbor that liked to dive for them.