Wheel fasteners are the single most important hardware items on any vehicle, yet are often the least understood and appreciated. They can be wheel bolts that thread into hubs, or a combination of hub-mounted studs and removable nuts.
Here, we´ll provide a basic overview of the often-overlooked threaded wheel fastener -- how to identify it and how to handle it. This includes information dealing with fastener thread size, seat styles and torque values.
Our goal is to provide clear and accurate explanations of various wheel fastening systems that will help customers choose the correct type of fastener for a given application. We also want to increase your knowledge of safety-related issues.
Stop using the generic term "lug nuts" when referring to any and all wheel fasteners. Not all vehicles use nuts. Many German vehicles use bolts instead. So, constantly calling all wheel fasteners "lug nuts" simply isn´t accurate.
The term "acorn" nut is a classic example of a term that´s widely misused and misleading. If we refer to a wheel fastener´s seat style, the term "acorn" would translate into a reference of a ball-type or radius-type seat.
Take a look at an acorn. When viewed from the side, the belly of the acorn is somewhat radiused, which makes the term sensible when referring to a ball-type or radiused seat style. However, some folks use the term "acorn" to refer to the head portion of a nut that features a ball-type, rounded, dome-head design. Yet others use the term to refer to a short-bodied nut that features a conical (straight taper) seat.
So, when you hear the word "acorn" in reference to wheels, don´t assume anything. In order to make sure that both you and the customer are on the same page, clarify the term by pointing to a photo, diagram or, better yet, showing him/her an actual sample to make sure that you´re both talking about the same item.
The only proper use of the term "acorn" is when referring to a nut with a rounded top dome. Just remember that the dome shape has nothing to do with the seat style.
The "seat" refers to the contact area between the base of the fastener head and the wheel´s fastener pocket. Although variations exist, three basic seat styles are in common use today: conical, radius and flat-seat types.
Conical wheel nuts (or wheel bolts) feature a straight taper at the seat area. Conical fasteners are also called "tapered" or "cone" types. All three names refer to the same style.
As viewed from the side, you can see that the seat is "chamfered" on each side. This type of taper is created most commonly in a 60-degree angle, although some light truck applications use a 90-degree angle. The angle indicates the degree of separation between the two walls, not the true vertical of the fastener.
Radius seats also are called "ball" or "rounded" seats. As the name implies, the seat features a radiused "ball" shape that nestles into a ball-shaped pocket in the wheel. Many European vehicles such as Porsche and Mercedes use radius seat styles, in either nut or bolt applications.
The flat-seat style is most often called a "mag" style. The term "mag" doesn´t imply anything in terms of geometric shape. It´s simply a slang carryover from the early days of the American performance scene when race wheels were cast magnesium, and commonly used a flat seat because it was easier and cheaper to produce a round hole in a flat hub center area.
Mag style nuts generally feature a smooth shank extension under the head that is used to center the wheel and provide needed thread engagement depth.
The golden rule when discussing seat styles is extremely simple: never mix them! If a wheel is designed for conical 60-degree seats, the only fastener seat style that can be used is a conical type. The same rule applies for radius and flat seat styles -- only use the correct seat style of fastener for the seat style of the given wheel.
Using the incorrect seat style can, and likely will, result in fastener loosening, wheel damage, and potential tragedy due to loss of vehicle control when the wheel wobbles or separates from the vehicle. It´s simply impossible to over emphasize this point. The wheel fasteners are the only connection between the vehicle and the wheels. Without proper fastening, you´re flirting with disaster.
Aside from selecting the proper seat style for the wheels at hand, another consideration is thread engagement, defined as how far the nut travels over the stud, or how far the wheel bolt travels inside the hub´s threaded hole.
The rule of thumb whenever discussing threaded fastener engagement is to achieve a minimum engagement length that is equal to the diameter of the fastener´s threads. For instance, if the stud or bolt shank is 1/2-inch in diameter, the nut (or bolt) must engage its threads by at least 1/2-inch. This provides the minimal theoretical engagement for proper clamping load.
However, the industry usually takes advantage of greater thread engagement depth for added safety; the deeper the threads engage, the longer it takes to actually lose the fastener. And in some cases, the longer studs are used simply to aid in installing the wheel onto the hub.
Thread sizing is not difficult to understand. The problem is that most people won´t take the time to learn this very basic theory.
Whether inch or metric, we need to know the diameter, the thread pitch and the thread length. For example, when dealing in an inch format, a nut identified as a "1/2 x 20 x 1" indicates that:
1. thread diameter (the diameter of the stud it will accept) is one-half inch.
2. thread pitch or "count," is 20 threads per inch. Thread pitch refers to the number of threads that exist per inch of length.
3. thread engagement, indicated by the "1" designation, has an internal length of one inch.
If the fastener is metric, diameter is represented in millimeters, pitch is represented by the distance from one thread to the next thread, and thread or shank length is represented in millimeters.
For example, a "14 x 1.5 x 70" wheel bolt would indicate a 14 mm diameter, a 1.5 mm thread pitch and a thread or shank length of 70 mm.
In terms of thread pitch, it´s really very simple: with inch-format, the larger the number, the "finer" the threads (20 threads per inch would mean more threads within a given area than, say, 16 threads per inch).
If the format is metric, the larger the number, the more "coarse" the threads would be (a distance of 1 mm between threads would result in a greater number of threads in the same area as, say a distance of 1.75 mm between threads).
So, in inch format, smaller pitch numbers mean "coarse" threads, while larger number indicates "finer" threads. In metric format, small numbers indicate finer threads, while larger numbers indicate more "coarse" thread.
Many people tend to make this issue more confusing than it really is. Simply think about what the number means -- number of threads per inch if the fastener is inch-type; or distance between threads, if the fastener is metric type.
If we were referring to a bolt, both diameter and pitch are determined the same way. Length would then refer to the shank length -- the length of the bolt under the head, or in the case of wheel bolts, the length of the bolt from the bottom of the seat to the tip of the threaded end.
Depending on specific models, the bolt´s overall length may be required as well. Overall length would, as the term implies, represent the distance from the top of the head to the tip of the threaded end.
In order to quickly and accurately identify the thread diameter, pitch and length of any wheel fastener, you need the following three readily available tools. A combination bolt/nut hard-plastic sizing card (which covers both inch and metric sizes) and two thread pitch gauges (one for inches, one for metric measurements) will do the trick.
The "head" of a wheel bolt or nut features a specific drive design and appearance approach. The most conventional drive style involves a hex (six-sided) that requires the use of a six-point wrench. Other styles do exist, although they are not as common.
These include female hex drive "tuner" nuts, which feature a relatively small and smooth outer body diameter designed to fit into small-diameter fastener holes in some wheels. Instead of an external (male) hex head, a female hex drive hole is featured instead.
Yet another drive style is the splined head, which features a symmetric pattern of straight-cut external splines instead of hex walls. Naturally, the tuner hex drive nuts require the use of a hex bit as a driver, while the splined fasteners require a dedicated female spline socket.
In terms of appearance, nuts may be either open (the threaded hole runs all the way through the nut) or closed (threads terminate inside the nut). Closed head styles are varied, and include flush tops, raised conical domes, raised flat-top domes, extended hex walls (for easier reach and handling and greater wrench contact area) and radiused domes, which look like acorn bellies and feature a nicely rounded, ball type dome.
All of the wheel nut and wheel bolt styles that we´ve mentioned are available in a wide variety of lengths in terms of drive area or thread engagement area. Drive area height may be dictated by the thickness of the wheel´s center hub or by the design of the wheel (maybe the nut or bolt head needs to project out further for easier wrench access). The necessary thread engagement area will affect length as well, and involve either inboard thread reach or outboard projection based on stud length. Often with lugcentric wheels, a tapered (conical) seat style nut may be used that also features a short, straight shank. The shank may increase thread engagement length and aid wheel centering on the hub.
HUBCENTRIC VS. LUGCENTRIC
These two terms are common and proper labels that indicate how the wheel centers onto the hub. The wheel must be attached to the axle hub as precisely as possible to avoid runout. In order to locate the wheel in a centered position, some method of locating must be used.
* If the wheel centers onto the hub via the fastener locations only, we refer to this as a "lugcentric" method. This means that the location of the wheel studs (or the hub´s bolt holes) must be accurately placed on the hub during vehicle manufacturing.
* If the wheel is centered by means of its center opening fit to the hub´s raised center flange, it´s a "hubcentric" design. This means that the centering approach relies on the center hole of the wheel to the center locating dowel flange of the hub.
WHEEL FASTENER TIGHTENING
Since this handbook focuses primarily on "performance" applications, we can assume that when we discuss wheels, we´re referring primarily to lightweight "custom" alloy wheels.
As a result, when we discuss installing and clamping the wheel onto the hub, we´re interested in both form and function. In terms of form, we don´t want to scratch the wheel. In terms of function, we want to secure the wheel properly in the pursuit of both safety and long-term performance.
We recommend you don´t use an impact gun to remove or install threaded fasteners when dealing with custom alloy wheels because you might scratch the fastener lockets of the wheel. If you insist on removal with a gun, use only a clean thinwall socket and run the gun at a slower speed. But it´s easy to scratch the pockets with the socket or the exiting nut or bolt, even when trying to maintain control of the gun.
Whether we´re dealing with a bargain-basement, impurity-riddled aluminum casting or the finest and strongest billet, forged or investment-cast alloy wheel, proper clamping force is critical for both wheel care and performance. For one thing, over tightening can gall or deform the wheel´s fastener pocket seats. Over tightening, in its extreme, can create a fracture in the alloy, which can lead to an eventual wheel failure. Excessive over or unequal tightening can distort both the wheel center section and the hub.
Considering the lightweight rotor hats featured on many of today´s vehicles, that´s an open invitation to disc brake warping and pedal bounce, which is both annoying and a guaranteed comeback.
When taper or round seat is tightened, an "interference" fit is experienced. That means the male seat of the fastener contacts the female seat of the wheel and creates a small wedge contact when tightened, creating a pressure point that helps to lock the fastener in place.
If either type of fastener is under tightened, they can loosen during operation. If over tightened, the fastener can become fatigued and can deform the material in the wheel´s female seat pocket, which can result in fastener loosening. The shape of the radiused seat reduces the effect of over tightening since contact pressure is more evenly distributed than with a taper seat style.
Flat seat style is used almost exclusively with alloy wheels, since in the early days of alloy aftermarket wheels, the alloy material may not have been strong enough to handle the frictional forces created by tapered or radiused seats.
Over tightening a flat seat nut can deform the wheel, causing the aluminum under the washer to extrude, which displaces the aluminum, causing the nut to loosen.
Over tightening also can stretch the wheel studs or wheel bolt shanks beyond their elastic range.
All bolts or studs are designed to stretch a miniscule amount when optimal clamping load is achieved. This elasticity of the stud or bolt is what helps to secure the wheel on the hub. When torqued to specification, this is referred to as achieving the proper "clamping load."
If the stud or bolt is excessively over tightened, it´s possible that it will stretch beyond its yield point and lose its "rubber band" effect. If stretched beyond the yield point, the stud or bolt becomes so weak that it cannot provide the clamping load needed. The result: The fastener loosens or the stud or bolt shank breaks.
Far too many uninformed do-it-yourselfers approach wheel fastener tightening with the "tighter is always better" attitude. Always follow the torque specifications listed by either the vehicle manufacturer or by the wheel maker.
Don´t guess. Actually take the time to pick up a calibrated torque wrench and tighten all of the wheel´s fasteners, in the proper sequence, in several steps to achieve final (and equal) torque values.
As far as thread preparation is concerned, make sure the threads are clean and free of dirt, grease, grit, etc. As far as wheel fastening is concerned, specifications are generally listed based on dry (no lubricant) threads. Applying oil, grease or moly to the threads will result in inaccurate torque values (you´ll end up over tightening).
Even if you use aluminum wheel nuts (which are popular in some racing situations, Porsche applications, etc.), the advice is the same. Simply make sure the threads are clean and dry. Aluminum wheel nuts are typically made from a very dense, strong 7075 alloy, and will function properly if handled correctly.
Some will disagree with the need to re-tighten wheel fasteners, but our advice is to re-check the value of each fastener after about the first 50-100 miles of operation. Due to metal compression/elongation and thermal stresses, the clamping loads may change during initial use (we´re not saying they will change, but they might).
When rechecking torque value, wait for the wheels to cool to ambient temperature (never torque a hot wheel). Loosen and retighten, to value, in sequence. Again, some will argue that this step is not necessary, but it´s better to be safe than sorry, and it´s better to catch a loose nut early as opposed to too late.
Braving the elements
The three most important elements of wheel fastening are:
1. proper seat style.
2. correct thread size (diameter, pitch, length).
3. proper clamping value.
A few common/potential problems to avoid when dealing with wheel fasteners
1. Make sure the fastener does not bottom-out. In other words, if using a closed-head nut, make sure the tip of the stud does not contact the inside of the nut roof. If the nut is a shanked "mag" type, make sure the shank is not too long. If the shank tip bottoms out against the hub, clamping pressure at the seat will be lost.
2. Make sure that there is no grease, snow, ice, etc., lodged inside a closed nut. When tightened, this can create a hydraulic lock, causing the stud tip to bottom out against this material. If the nut roof bottoms out on its stud, the nut seat will not achieve enough clamping force.
3. Be careful not to mix inch and metric fasteners. Just because it can be threaded into place does not mean that correct thread engagement has taken place. Never guess if you´re using the correct nut or bolt.
4. Never mix seat styles. Proper clamping will be achieved only by using a conical seat fastener with a conical seat style wheel. A radiused fastener can be used only in a radiused seat pocket.
A flat-seat wheel will only accept a flat-seat fastener. Again, just because you can thread the nut or bolt on by hand, don´t think that you can "get by" with merely tightening it more.