2209 Stewards Proposal #22: Lug bolt length


Well-known member
This proposal offers a minimum safe length for lug bolts on cars so equipped. The proposal is intended to assure that wheels do not shear from their mountings, causing a situation dangerous to drivers and bystanders, alike.
After a first reading of this proposal, I find it to be flawed, as it does not specify the
LENGTH of the lug nut. From Carroll Smith (Engineer to Win) the rule of thumb is that you should
have thread engagement of 1 1/2x the diameter of the fastener...

but the length of commercially- available
lug nuts varies wildly- I have some that are about 18mm (correct for a 12mm stud)
some that are 20mm, (from a reputable supplier, the ones I race on)
and some that are well over 30mm with closed ends.
And that's not counting the 'shoulder nuts' for the old- style aluminum wheels...
The 'double- ended' Nascar nuts are another example of this- they have extra
material added so that you can install them either way 'round.

The proposed rule change specifies one complete exposed thread, but not the number of engaged threads.

As much as I agree with the intent, I feel that the wording
isn't correct to achieve the intended goal, and that the intent could be
circumvented by some fool cutting their nuts down to meet the letter
of the law, but removing that last marginal thread that would have
saved their bacon...

Humbly submitted,

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Toby, you are correct in the amount of thread engagement recommended. The rule is for a quick check during technical inspection. It would be very difficult to have each technical inspector measure the bolt diameter and the nut length on each and every wheel assembly.

"Mall Cop"

I read this as lug BOLTS. No mention of the nut. Are we confusing the stud/nut mounting that is on most cars with the fewer, less common bolt system?
Nope. I'm not. The rule uses the nut as a measure of length for the stud that sticks out,
and that's what concerns me. An overzealous/underthought enforcement of
the rule would boot someone who had over- length, but properly engaged, nuts over
her or his proper- length studs.
And per my own example, I can have a thread of NUT exposed (not stud) and still have
the (very conservative) 1 1/2 x diameter of thread engagement....

I may be the culprit who started this. While completing my novice requirements, I was working in the pit at Portland. I saw a car where the stud ended well inside of the nut. I pointed this out to one of the experienced folks, who took the matter upstream, and the answer that came back was "they passed tech inspection". Not satisfactory either. I would not want to be next to or behind this car if it bumped a curb at speed.
Mike -- How bout a little clarification to whether this proposal is for lug bolts or wheel studs/lug nuts?

The rear lug nuts on my car stick out about a 1/4" past the studs. BUT that is because when I when converted to longer wheel studs I had to get open type lugnuts. The lugnuts I purchased were about .625" longer than the OE nuts, and the wheel studs are only .375" longer than OE.

but your wording of the proposal sounds like it has nothing to do with my situation...
Dave wrote this one, but I believe it's meant to say that studs should show at least one thread past the open end of the wheel nut.
The situation that prompted this was several cars came through tech with the wheel stud recessed inside the lug nut. The cars were using the standard lug nuts and stock stud but aftermarket wheels. When I measured the actual threads holding the wheel to the hub, it was less than one bolt diameter. This is a dangerous situation but the current rules do not address this adequately. Therefore the rule change was submitted.
The wheel studs on my Pro-7 car are 12 mm diameter (0.472 in) 1.5 mm/thread. Standard nuts (same material as stud/bold) for a 12mm-1.5 bolt are 0.389 -0.398 inches tall. The rear axle flange on my Rx-7 is 0.410 thick. Most wheel studs I have been able to find have a tap end length that is 1 times the stud diameter. On the basis of this I believe that, for similar material strengths, as long as threads engage for one stud diameter less one thread, they are strong enough. If I were to lengthen the stud and wheel nut, no additional strength is gained, since the strength of the assembly is limited by the threads at the rear axle flange. In the case where the tap end is in a weaker material (example?; cast brake rotor/hub assembly?) additional engagement in that material should be used. There are standard stud designs with tap ends of 1.0D, 1.25D, 2.0D and 2.5D, though they can be hard to find. (Try Maryland Metrics.) The strength also depends on sthength of materials and thread fit tightness - tighter is better. I have found 12 mm - 1.5 wheel nuts with heights of 16mm, 18mm, 22mm and 1 inch (25.4mm). I believe the wheel nuts are taller than standard nuts not because they need to engage more stud threads to provide strength, but so they can have a conical tip to mate with the corresponding area in the wheel and so enough nut is left sticking out to use a lug wrench or impact wrench.
I don't think this rule should be passed because it takes a good thumb rule - nuts should have 1 thread protruding to ensure full engagement on a bolt/stud - and applies it to a situation where nut height is determined by other factors.
I agree. You shouldn't be penalized for upgrading your lug nuts to the point where the entire stud is covered. Also, many lug nuts don't have threads going all the way out to the outermost edge; they may be bored larger at the end or have acorn caps, but that doesn't mean there aren't enough engaging threads.

I'd support a well-worded rule about lug thread engagement, but I haven't seen one yet. It would be nice to have a rule that could be checked easily in tech with a glance, but it's not that simple.
I support Dave B.'s proposed rule change. When I was maintaining Army helicopters (UH-60s), the SOP for my maintenance platoon was 2 threads showing past the open end of the nut.

This is from the Industrial Fasteners Institute FAQ (http://www.industrial-fasteners.org/faqs.html), under the "What is the rotational capacity test?" subsection:
"How many threads should protrude through the nut for full thread engagement? A minimum of two thread pitches should protrude beyond the nut face following tightening to assure full thread engagement."

Having said that, I can see the need for accomodating situations that apply to Steve, Steve, and Bryan's long nut cases.
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Ha!!! Dang, that made spit spit out beer from laughing.... Not fittin I say, just not fittin.

Beer before high noon? Hmmm, put another turn on your lug nuts :D

Unless of course you're using it to brush your teeth. That's okay to do before noon when camping at the race track IF it's from a can that went flat over night. Just don't breath on any Stewards :cool:
I would STRONGLY support a rule that said
'1.y x d engagement'
where you can make the y any number from 0 to 9.

And I understand what Dave's up against with enforcability.

I just fear that
basing engaged thread on lugnut dimension isn't good science....

in most cases
it's good, but it can be 'fudged' with in a manner that could be unsafe.

That's my opinion.

You folks from Orygone and Worshington what use beer for brushing teeth are nuts ... flat beer is for they stew and such. Nope I use it right outa tha bottle for brushing teeth, the foamy suds help scour.... oh wait, thats scrubbing bubbles bathroom cleaner.....No wonder it burns..