How many threads should be engaged on lugnuts?

Wes Eargle · **Joined:** Mon Oct 27, 2003 9:47 am **Posts:** 3683

My 15x7 wheels that I'm currently running on the Miata have an offset that only allows 7 threads to be engaged. I've heard of people using Mazda MPV studs because they allow an extra 3/8" of length whenever wheel spacers are used, but is the current arrangement safe?

Aaron Buckley · **Posted:** Thu Sep 23, 2004 1:10 pm

I talked with Tirerack about this, and their site says 7 turns at a minimum, but the consultants state that 4 turns will suffice on a 5 bolt setup, and 4.5 on a 4 bolt setup, even on the track. - AB

Wes Eargle · **Joined:** Mon Oct 27, 2003 9:47 am **Posts:** 3683

I'll just make sure that they stay torqued and not worry about it then. Methinks the seven turn minimum is CYAspeak by lawyers.

Adam Ligon · **Posted:** Thu Sep 23, 2004 1:32 pm

Wes Eargle wrote:

:thumbsup:

I'll just make sure that they stay torqued and not worry about it then. Methinks the seven turn minimum is CYAspeak by lawyers.

I have always heard they need to go in the same depth as the width of the bolt. ie if they are 12mm then they should enagage 12mm worth

DickRasmussen · **Posted:** Thu Sep 23, 2004 4:36 pm

The rule of thumb that I've heard/used is enough threads to equal the stud diameter. I've also actually seen wheels come off an autocross car when there wasn't enough lugnut engagement . . .

Another approach that I used when I needed really thick spacers for tire clearance many years ago was to get lug nuts with long shanks that would go through the wheel and into larger diameter holes in the spacers. Took some shopping to find the appropriate lug nuts.

Dick

Brad Mackey · **Posted:** Sat Sep 25, 2004 3:58 pm

How about just get longer wheelstuds and press 'em in. Problem solved and big-time piece of mind.

DickRasmussen · **Posted:** Sat Sep 25, 2004 7:05 pm

Brad,

In my case it was a combination of lack of availability of class legal longer studs (per various rules at the time) and/or my desire to be able to continue to use closed end lug nuts with my normal street wheels without running spacers. I already had slightly longer than original studs in the car.

Karl Shultz · **Posted:** Sat Sep 25, 2004 7:58 pm

Wes, FWIW. I've been running my Integra with 6 threads worth of engagement in the back for years now. Never had a problem.

--Karl, whose H&R long-length wheel studs are sitting in the garage, same shelf they've been on since 2001...

DickRasmussen · **Posted:** Sat Sep 25, 2004 8:31 pm

FYI and just for the "heck of it" I did some surfing to see if anyone sells the type of long shank lug nut I used years ago to solve a problem.

http://www.gorilla-auto.com/products/?s ... e=5326.jpg

Looks like they still exist.

Wes Eargle · **Joined:** Mon Oct 27, 2003 9:47 am **Posts:** 3683

There are longer studs available that the Spec Miata guys run, I might do it this winter for peace of mind when I have the car apart.

Donnie Barnes · **Posted:** Mon Sep 27, 2004 11:08 am

Mechanical Engineer types have told me that *three* threads is all you "need" in any application. Personally, I wouldn't drive on anything less than twice that. I tend to like the rule of using the diameter of the bolt as a minimum.

As for the lugnuts that reach into the wheel, those won't work on wheels that require the conical type of lugnut unless you want to drill them out a bit. You'd then need to make sure you got the proper lugnut so that the shoulder still grabbed the wheel before the lugnut bottoms out.

The Spyder came with the "reach through" lugnuts on the stock rims, but all the aftermarket rims and the Miata rims we used on the Spyder wanted conical. We had a thread problem so we put longer studs on the car from TRD (expensive! There are others that will work on that car that are cheaper, but you have to do some modification to the studs to get them to work). I have the notes somewhere at home, but the '99 Miata I bought has some longer Mazda studs on one end and some Corvette studs on the other. Don't remember any more details than that. Steve found those way back when, so there may be Mazda parts available for both ends now, I don't know. I do know we have longer studs for the Spec Miata, but I'm not sure if those are Mazda parts or not.

--Donnie

MikeWhitney · **Joined:** Wed Aug 20, 2003 9:15 pm **Posts:** 4724

Wes Eargle wrote:

Guess what? I have a fever, and the only prescription is more cowbell.

Scott Holthausen · **Joined:** Wed Sep 03, 2003 11:43 am **Posts:** 23

Paraphrased rom the Machinery's Handbook Edition 24:
Formulas for Stress Areas and Length of Engagement of Screw Threads.

The critical areas of stress of mating screw threads are: (1) The effective cross sectional area, or tensile stress area, of the external thread; (2) the shear area of the external thread, which depends principally on the minor diameter of the tapped hole; and (3) the shear area of the internal thread, which depends principally on the major diameter of the external thread. The relation of these three stress areas to each other is an important factor in determining how a threaded connection will fail, whether by breakage in the threaded section of the screw (or bolt) or by stripping of either the external or internal thread.

If failure of a threaded assembly should occur, it is preferable for the screw to break rather than have either the external or internal thread strip. In other words, the length of engagement of mating threads should be sufficient to carry the full load necessary to break the screw without the threads stripping.

If mating internal and external threads are manufactured of materials having equal tensile strengths, then to prevent stripping of the the external thread, the length of engagement should be not less than that given by Formula 1:

Le = 2 X At / 3.1416 Knmax {1/2 + 0.5773n (E3min - Knmax)}

In the formula, the factor of 2 means that it is assumed that the area in shear of the screw must be twice the tensile stress area to develop the full strength of the screw (this value is slightly larger than required and thus provides a small factor of safety against stripping); Le = length of engagement, in inches; n = number of threads per inch; Knmax = maximum minor diameter of internal thread; E3min = minimum pitch diameter of external thread for the class of thread specified; and At = tensile stress area of screw thread given by formula (2a) or (2b) or the thread tables on pages 1530 to 1539 for Unified threads which are based on Formula (2a):

At = 0.7854 (D - 0.9743 / n) squared

Where D = basic major diameter of threads.

Have fun with the math 'yall.

:twisted:

Wes Eargle · **Joined:** Mon Oct 27, 2003 9:47 am **Posts:** 3683

So, like 4 or 5 then? Math in the am makes the baby Jesus cry.

Scott Holthausen · **Joined:** Wed Sep 03, 2003 11:43 am **Posts:** 23

Wes Eargle wrote:

So, like 4 or 5 then? Math in the am makes the baby Jesus cry.

Hahah!!

It's not about number of threads, it's about the length of engagement, which is a variable depending on threads per inch, and bolt diameter.

Adam's 1 to 1 ratio of diameter to length of engagement is a safe bet when in a pinch, but it's less than that usually.

Example:

M12 x 1.5 thread (typical wheel bolt)

At = 0.7854 (0.47244 - 0.9743 / 16.9333) squared

At = 0.7854 (0.1721436696)

At = 0.13520163811

so...

Le = 0.2704 / 1.3205 {1/2 + 9.7756 (0.4341 - 0.4203)

Le = 0.2704 / 1.3205 {1/2 + 9.7756 (0.0138)

Le = 0.2704 / 1.3205 {0.6349}

Le = 0.32252 inches, or 8 mm.

Where's your god NOW moses!

:twisted:

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How many threads should be engaged on lugnuts?

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