Archive for February, 2009

TECH Stuff # 13 – ET Lug Nuts. What are they, and why?

Friday, February 20th, 2009

Within the Aftermarket Wheel Industry, a controversy began during a meeting of the Wheel & Tire Industry Council meeting at the 2008 SEMA Show. A presentation was being done by some of the members regarding Fitments and Applications. It was suggested that ET lug nuts are recommended in cases where questionable thread engagement was present. A statement came from the audience, that ET lug nuts did nothing for thread engagement and had no usable purpose. I took exception to that statement and argued that ET lug nuts are all about thread engagement. Since that meeting there has been a lot of discussion in regards to the true purpose of ET lug nuts, why they were developed and a lot of other hog wash about technical aspects of lug nuts in general. I’d like to add my 2 cents to this discussion. I’ll start by giving my recollection of why ET lug nuts were developed.

Going back to the 50′s and early 60′s, the Aftermarket Wheel Industry was born because of the desire of car guys that wanted something other than stock painted wheels in stock sizes. These cars guys wanted chrome wheels and they wanted them wider so they could use bigger tires. At that time, the OEM’s didn’t offer aluminum wheels. As a result the stud length was shorter on the vehicles. The average stud length was about 3/4″ to 7/8″. This was adequate for steel wheels and there were no thread engagement issues. Then in the early 60′s, the Aftermarket Wheel Industry started offering aluminum alloy wheels. The developers of these wheels solved the thread engagement problem by coming up with the Mag Type lug nut. Since the mounting pad thickness was about the same thickness as the stud length on the vehicles, which left little or no threads on the stud to attach a standard conical seat lug nut, they came up with a lug nut that had a shank that went into the wheel lug hole enough to accomplish proper thread engagement (see illustration). This system was adequate but left a lot to be desired. In order to assure that the wheel was attached to the vehicle with good alignment and acceptable runout, the diameter of the lug hole had to be drilled with a minimum of clearance for the lug nut. After the vehicle was driven enough to heat up the brakes and wheels, the aluminum wheel expanded, thus shrinking the diameter of the lug nut hole. It was then very difficult to remove the lug nuts when the wheel was hot, in order to change a flat tire or what ever.

Now moving forward in time, the OEM’s began offering aluminum alloy wheels on new vehicles. They determined that the use of Mag Type lug nuts was not the best system. They began to offer their aluminum wheels with conical seat lug nuts. They did however lengthen the studs on the vehicles that were offered with aluminum wheels so there was adequate thread engagement. This prompted the Aftermarket Industry to adjust their offering so they could keep up with the OEM’s. In the late 80′s, the Aftermarket Industry began drilling their lug holes with a conical seat instead of the Mag Type lug hole. It was then discovered that the use of a standard lug nut (Acorn Type) was causing problems with the seating area of the wheel. Steel inserts was a solution, but it was soon discovered that the OEM’s were using a Bulged Type lug nut which eliminated the problem with the seating area. So the Aftermarket lug nut manufacturers started offering Bulged Type lug nuts. At that point it seemed that all was well in the Aftermarket Wheel Industry. Not so.

The problem of thread engagement became an issue when a conversion from steel to alloy wheels was made on vehicles that had short studs and were never intended to have alloy wheels. The Aftermarket again responded by developing a Bulged Type lug nut with an extension that would go into the lug hole enough so that proper thread engagement could be accomplished. This lug nut was called the ET (extended thread). Combined with the bulged seat area, the extended thread provides the best attachment that is currently offered.

I have recently observed another problem. In my illustration I show examples of wheels with mounting pad thicknesses from .390″(10mm) to .630″(16mm). I do not recommend a mounting pad thickness to be greater than .390″(10mm). I have done extensive testing on wheels produced in China since 1993 using a .390″(10mm) mounting pad thickness. I have sold in excess of 750,000 wheels and never experienced any failures. Using a Std. Bulged Type lug nut in a wheel with a .390″(10mm) pad thickness on a vehicle with short studs (.800″) (see illustration) is still under the minimum thread engagement recommendation of SAE. And using an ET Bulged Type lug nut in a wheel with a .630″(16mm) pad thickness on a vehicle with short studs (.800″) (again see illustration) is also under the minimum thread engagement recommendation of SAE. In my opinion, ET lug nuts should be used on all aluminum wheels. In summary, what does an ET Type lug nut do? It provides a method to assure proper thread engagement. What is proper thread engagement? Proper thread engagement is defined as engaging the threads onto the bolt or stud by at least the diameter of the bolt or stud. Prior to mounting a wheel on a vehicle, the length of the stud should be checked against the thickness of the mounting pad below the bottom of the conical or spherical seat. Another way to check is to mount the wheel and check how many turns the fastener makes before it reaches the seating area. An example is if using a ½”-20 nut, then there should be at least 10 complete turns.