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Wheel Anatomy

Posted in How To: Wheels Tires on February 15, 2018
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Photographers: Trenton McGee

We know that one of the first things you buy for your 4x4 are wheels and tires. We support that decision because we love tires and wheels, and wheel and tire companies help support what we do.

Buying tires is easy. Just get the biggest, gnarliest mud tires that will fit on your rig and go! OK, it’s not that simple, but buying tires is a topic for another article. Here we want to talk about wheels. Wheels, just like tires, must fit what you plan to do with them in order to really excel.

Wheels are more complex than you might think. They come in various diameters, widths, bolt patterns, and more. Do you know about wheel backspacing? Safety beads? Center bore? Don’t worry, we’re here to help and teach. At the end of the day all your questions about which parts make a wheel should be answered, and if we don’t answer a question, don’t worry; it’s probably not that important.

Wheel Diameter
Wheel diameter is one of the most basic dimensions you need to know when thinking about wheels (and tires). In the off-road world wheel diameter is generally tied to brake size, and by that we mean which wheels will clear your vehicle’s brakes, above all else. For cars, larger-diameter wheels also make handling crisper because with the same outside diameter tire there is less sidewall to flex with weight changes and side loads applied to the wheel/tire in a turn.

A wheel’s outside diameter plays a role in many aspects of how a wheel works with a tire. We prefer 15-inch (although they are getting harder to find and are old school), 16-inch, 17-inch, and occasionally all the way up to 20-inch if we’re running a really big tire with a minimum 40-inch diameter. Avoid 16.5s because they have a small and ineffective safety bead and very poor tire availability by this point. What you want is enough tire sidewall (6-7 inches minimum) to absorb big hits off-road without damaging the tire or wheel. Here are a 15-inch OMF beadlock on a forged Weld wheel on the left, a new 17-inch Pro Comp Vapor Pro 2 Competition Beadlock in the middle, and a Trail Ready HD20 20-inch wheel on the right. The first has been modified into a beadlock with beadlock rings welded on, and the other two are cast as beadlocks.

When off-road, larger wheels tend to create more problems than they solve. Off-road vehicles with large-diameter wheels are more prone to tire sidewall damage, wheel rim damage, and tires slipping off a bead when aired down. We prefer 15- to 17-inch wheels for a number of reasons unless you’re running a really large tire. A good rule of thumb is to run the smallest diameter wheel that will clear your brakes.

Wheel Width
Also one of the most basic wheel dimensions is width. There are two schools of thought in the off-road world when it comes to wheel width. Both have their merits—and detractions.

The “wider is better” crew says that wider wheels add track width and thus stability. Wider wheels generally allow a tire to be as wide as possible, aiding in flotation in soft sand or mud. Fans of wide wheels also report that wider wheels allow a tire to be less susceptible to falling into a rut or going offline on rocks.

Wheel width is measured between the insides of the outer rim, or lip, of the wheel. This Pro Comp Vapor Pro 2 Competition Beadlock is 17x9; that is, 17 inches in diameter (at the bead) and 9 inches between the outer rims. If you want to imagine it’s not a beadlock then ignore the silver outer bead ring and the wheel would be a 17x8.

The “narrower is better” crowd believes, among other things, that the tire is less likely to slip off the bead when aired down and wheel rim damage is less likely (because the tire’s bulge helps protect it).

Who’s right? The truth is it boils down to your intended use, style of driving, and experiences. Our personal preference is to run as narrow a wheel as recommended for a given tire width, but honestly it’s nothing more than our preference. Both wide and narrow wheels have benefits and drawbacks and yield a different look to the tire and vehicle.

Bolt pattern
Bolt pattern is simply the diameter and number of wheel studs on your axles. This generally varies by year, make, and model. Common patterns are 5-on-4 1/2, 5-on-5, 5-on 5 1/2, 6-on-5 1/2, and 8-on-6 1/2. The first number tells how many lug holes, and the second number is the diameter of the circle. For example, a 5-on-5 1/2 bolt pattern means there are five lug holes around a 5 1/2-inch-diameter circle. A rule of thumb is that larger and more wheel stud openings are stronger, but bolt pattern is generally dictated by what axles you are running (and that’s usually dictated by tire size, intended use, weight, and horsepower/torque).

Backspacing/Offset
Wheel offset and backspacing are probably two of the more confusing wheel terms you’ll ever run into. They are confusing because they are two different methods of describing the location of the wheel mounting surface relative to the front and back of the wheel (front being toward the curb, and back being toward the differential).

With a straightedge and a tape measure you can measure backspacing on any wheel. It’s a bit harder if there’s a tire on the wheel, but we bet you can figure it out. Offset is a similar measurement, specifically the distance of the wheel mounting surface to the centerline of the wheel. Since the outside dimensions of a wheel are a little bit wider than the distance between the lips of the wheel, an 8-inch wheel with 4 inches of backspacing is not a zero-offset wheel. Instead it’s a -19mm offset wheel. The midline, or center, of an 8-inch wheel is actually about 4 1/2 inches from the back edge. Since backspacing is easier to measure we usually try to stick to using it for determining where the wheel will sit in a 4x4’s wheel opening.

Backspacing is simply the measurement from the wheel mounting surface to the rear edge (rim) of the wheel.

Offset is either a positive value, a neutral value (zero), or a negative value. What it describes is the distance of the wheel mounting surface from the centerline of the wheel. A wheel with zero offset means the wheel mounting surface is directly at the middle of the wheel. A positive number means the wheel mounting surface is closer to the front of the wheel (curb side). A negative offset means the wheel mounting surface is closer to the back of the wheel (differential side).

Hella flush or stance? We get it. Some of you like really ugly wheels, and truth be told wheel styles are all subject to opinion, but the recent fad of huge wheels with little to no backspacing on big trucks is just dumb. What’s more, it’s dangerous. Wheels this big are harder to stop (and harder to get spinning) than smaller wheels because metal is dense (denser than rubber) and these wheels are heavy. They also are hard on steering parts and change the geometry of the steering system on a vehicle. That means that they can cause a vehicle to behave erratically on-road. Also, the super-skinny tire sidewall will ride rough and can easily be damaged on- and off-road. So if you’re into making your vehicle slower, ugly, harder to stop, and more dangerous to you, your family, and those around you, as well as less generally useful as a truck, then please continue this trend. We’ll stick to using what hella works.

A wheel with shallow backspacing has negative offset. This makes the track width of the vehicle greater and may provide more clearance for shocks and other axle components, but can be harder on steering parts. A wheel with deep backspacing and positive offset makes a vehicle narrower and is generally easier to steer, but the wheel can interfere with suspension parts. Once again, neither extreme is necessarily good for your 4x4. Our personal preference if given a choice is to run a wider axle with a wheel with deeper backspacing (positive offset).

Wheels with deep backspacing (or positive offset) like those found on military Humvees make big tires a bit easier to steer and are easier on ball joints, tie-rod ends, and more. But they also narrow the track width of the vehicle, and the wheel can interfere with steering or suspension parts. We like wide axles on a narrow 4x4 with deep backspacing.

Wheel Safety Bead
The safety bead, or bead seat, is a raised lip about an inch inside the wheel’s outer edge or rim where the tire sits. The safety bead, in conjunction with air pressure, helps prevent the bead of the tire from slipping inward. If the tire’s bead does slip over the safety bead then all the air will escape from the tire/wheel. If you’re lucky all you have is a flat. If you’re unlucky you have just destroyed your wheel or rolled your 4x4 when the wheel dug into the ground.

The safety bead (arrow) on these wheels is large and abrupt, and that ‘ what you want in a 4x4 wheel. That will help keep the tire seated on the bead when you’re running low pressure. Low tire pressure increases the footprint of your tire, giving you vastly better traction on rocks, sand, snow, and other off-road surfaces.

One would expect all safety beads to be about the same size and shape, but they are not. Some are larger than others, and some wheels have almost no safety bead to speak of. The worst offenders are going to be wheels designed to be used with inner tubes and, for some reason, 16.5-inch diameter wheels, which seem to lack a decent safety bead.

Older wheels, which are almost certainly going to be steel, may be intended for use with inner tubes. They may lack safety beads altogether. Here’s a trick: You can tell from the outside of a steel wheel whether or not it has a safety bead, even if it has a tire mounted on it. Just look for this indent about an inch inside of the wheel rim.

Outer Rim/Wheel Lip
This is the edge of the wheel either on the front face (curb side) or the back face (differential side) of the wheel. The rim is important because it, along with air pressure and the wheel’s safety bead, holds the tire on the wheel. The wheel rim can be bent or dented off-road, and that’s bad. Generally we look for steel wheels because they can be bent back into shape, or aluminum wheels with a thick outer rim. That allows you to beat them against rocks, logs, and other trail debris without too much worry.

The rim, or lip, of a wheel is one of the most vulnerable parts of a wheel. It is the first part to come into contact with rocks on the trail. For that reason we like to run a wheel with a thick durable rim. These Dick Cepek wheels have seen heavy use on rock trails. The thick cast aluminum lip may be scarred and ugly, but it’s not damaged.
Beadlock wheels are designed to be abused, so it’s good to see the very strong inner rim, or lip, on these Method Beadlock Wheels we got from Summit Racing Equipment. Don’t overlook the inner rim of a wheel just because you don’t really ever see it. These will see rocks and will stand up.

Center Bore/Mounting Style
The hole in the center of the wheel is called the center bore. Gnerally the size and properties of the center bore are determined by the bolt pattern and how the wheel mounts to the hub. Mostly what you will want to know is whether or not the center bore of the wheels you’d like to run will clear your hubs, locking hubs, and or wheel flanges.

Many factory wheels are hub-centric, meaning the wheel indexes on the axle flange or part of the hub. These wheels transmit forces from the tire and wheel to the hub directly and via the lug nuts/wheel studs.

Most aftermarket wheels are lug-centric, meaning the wheel is centered on the hub using the lug nuts alone. This can cause damage to the lug nuts and failure when the wheels and tires take heavy hits. Most aftermarket wheel installers will know whether your 4x4 is an oddity and cannot run lug-centric wheels. We have used and abused lug-centric wheels for decades without any failures, but if it concerns you, know what you need and why. Also, many lug-centric wheels can be made hub-centric with adapter rings.

Here are two older steel wheels that help us show you about a wheel’s center bore. Both wheels are 15 inches in diameter, both have a 5-on-5 1/2 bolt pattern, but the wagon wheel on the left has a large wheel center bore. That means it will clear the locking hub on an old Jeep. The wheel on the right is an old Ford factory wheel for a 2WD car. The bolt pattern is right for an old Jeep, IH, Bronco, Suzuki Samurai, and more, but the smallish center bore is a problem for fitment.
You can tell this wheel is lug-centric and not hub-centric because of the gap between the wheel’s center bore and the hub. If there is no gap, you may have wheels that are hub-centric. That doesn’t mean you cannot use lug-centric wheels, but to be safe you’d better investigate the situation a bit more. Lug-centric wheels are retained by conical lug nuts, while hub-centric wheels will have a lug nut with a straight shoulder and/or a washer under a flat base (with no taper).

Sources

Summit Racing
Akron, OH
800-230-3030
http://www.summitracing.com
Dick Cepek Tires & Wheels
Stow, OH 44224
800-222-9092
www.dickcepek.com
Pro Comp USA
Compton, CA 90220
800-776-0767
http://www.procompusa.com
Champion Wheel Co
Fresno, CA 93722
951-471-2183
http://www.cwibeadlock.com
TrailReady Wheels
425-353-6776
http://www.trbeadlocks.com

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