Wheel Science: How To Buy The Best RimsPosted in How To: Wheels Tires on November 17, 2017
Wheels, rims, donuts, hoops, rollers, or whatever you call them—they are a must on your Jeep, or it’s not going anywhere. More importantly, the right wheels are a must, and we don’t just mean the bolt pattern. Depending upon the tires you expect to run, the manner in which your Jeep has been modified, and the terrain you expect to encounter, there are a host of other things you need to know about the wheels you’re thinking of bolting to your Jeep’s axles.
We’ll discuss the most common types of wheels (construction and material), the pros and cons of each, and give you an overview of how important the diameter, offset, and backspacing can be. We’ll give you the basic information you need to make intelligent choices. From that point on it’s up to you—color and style are a personal thing¬. Unless you’re interested in steelies for that plain-Jane look and bottom-dollar price tag, almost all the wheels you’re probably going to be looking at for your 4x4 are made from aluminum alloy. Aluminum wheels can be forged or cast, and there even different types of casting and forging methods.
Monoblock forged aluminum wheels tend to be lighter and stronger than other wheel build types, when comparing same-size wheels, and are most often used for high performance applications. These are one-piece wheels that are forged (deforming metal into a predetermined shape using forging dies under extreme pressure) from a solid billet of aluminum. These are very sturdy, and on the pricey end of the spectrum.
There are a few types of forging used for wheels. Hot forging (usually in the 300 to 600 degree Celsius range for aluminum alloys) is more common. Cold forging is done below the metal’s recrystallization temperature to retain its microscopic grain structure, allowing for increased dimensional control and better overall surface finish, which means that little or no finish work is needed during final fabrication of the wheel. Heat treating or “tempering” is often done when finishing cold-forged wheels.
Forged two-piece wheels are made of a forged center welded to a forged outer barrel. This construction allows for a very customizable wheel with multiple widths and backspaces available. Forged three-piece wheels are made with a forged center bolted to a forged inner and outer barrel. This wheel style is primarily used on luxury and exotic applications, and allows for the most customization in size, offset, and finishes.
Although the descriptors billet- and monoblock-forged are often used interchangeably, there are wheel makers that offer one-piece wheels CNC-cut from an aluminum log. These are often referred to as billet wheels.
There are a variety of monoblock-forged wheels available, and they are typically more expensive than multi-piece forged wheels. A number of off-road racing and rockcrawling competitors use them, and they have become attractive to hard-core recreational off-roaders too.
This process is also referred to as Spun-Rim, Flow-Forming, and Rim-Rolling. The barrel of a low-pressure cast wheel is heated and spun, while steel rollers under high pressure form the rim. This creates a wheel rim area with strength similar to a forged wheel, but at a lower cost.
One-piece cast aluminum wheels are the most common and affordable option because they can be produced in large quantities. These wheels have set sizes and widths, so they are the least customizable. Some manufacturers stock these as “blanks” with no bolt pattern, allowing them to be drilled based on demand. That keeps inventory manageable and helps control costs.
There are a few different casting methods. The most common are gravity casting, low-pressure casting, and high-pressure casting. Gravity casting is just as it sounds, the molten aluminum is poured directly into the mold. This method creates a casting that is less dense than some other processes, and is frequently a higher weight for desired strength. In low-pressure casting, the molten aluminum is forced into the mold. The pressure fills the mold faster and produces a denser and stronger wheel. Some companies offer high-pressure cast wheels, but that additional density and strength comes with a cost.
Two-piece cast wheels also widely available. These are made with a cast center bolted to a cast outer barrel. This construction allows for custom finishes, like hydro-dipped camouflage. Three-piece cast wheels offer the most diverse offering, but are typically for hot rod and vintage vehicle applications. These are made from a cast center welded to a forged outer barrel. Like two-piece forged wheels, these are very customizable with a huge range of sizes and offsets that can be built.
Bolt pattern, center hub bore diameter, wheel diameter, width, backspacing, and offset are things you need to know when looking at wheels for your Jeep. The bolt pattern and center hole size must match the axle hubs for your specific vehicle. Your choices really come in relation to the rest of these dimensions on a wheel.
Wheel width comes in ranges that the manufacturer suggests for a certain tire width. Go to the wide end of that spectrum and it can help spread out the tire’s contact patch, but also make it easier to peel off the rim at low tire pressures. Running tires on too wide a wheel also means less sidewall flex to soak up bumps.
Most wheels are offered in several choices of backspacing. Greater backspacing than your stock wheel will bring the wheel and tire farther inward, less-than-stock backspacing pushes the wheel farther outside. Too much backspacing can create wheel clearance problems with front suspension components when the steering wheel is turned to full lock, and tires may rub on the inner fenders. Backspacing can be easily measured on a wheel using a tape measure or ruler and a straightedge.
A wheel’s mounting surface centered in the wheel width has zero offset. A positive offset means the mounting surface is closer to the outboard rim of the wheel, making the wheel tuck farther in. A wheel with a negative offset means the mounting surface is closer to the inboard wheel rim, so that the wheel sits farther outside. In most applications, larger (almost always wider than stock) tires need decreased backspacing or less (negative) offset when compared to stock to keep the new tires from having conflicts with components.
The nice thing is that most aftermarket suspension manufacturers can offer recommended wheel sizes, backspacing, and offset for your specific vehicle application when using their products. You will do well talking to the suspension company you’re considering sharing your wealth with before buying new wheels.
If you don’t know the bolt circle on a wheel you’re looking at, or the axle’s bolt circle, you can measure the bolt circle with a tape measure or a ruler. For 4-, 6-, or 8-bolt patterns, measure the distance between a stud or hole on one side of the bolt circle and the farthest stud or bolt on the opposite side of the bolt circle. On five-lug wheels, measure from a stud or hole on one side to either of the studs or holes on the other side of the bolt circle that are slightly off-angle from the lug or hole you started from. A factory JK is 5 inches across a 5-bolt pattern, and referred to as 5-on-5.
Extremely important is the load rating of the wheel you’re looking at. You never want to downgrade when procuring new wheels for your ride. Wheels designed for Jeeps range anywhere from about 2,000 to 3,500 pounds. The more you build up your rig, the more weight gets added to it.
Assuming the stock axles are kept, at some point you will be getting close to the vehicle’s GVWR when it’s fully loaded with people and gear for a trip into the wild. Make sure the wheels you bolt up are capable of handling the chore by multiplying the load rating of the wheel by four. That total load rating number must be greater than the maximum load rating of your vehicle.
A good rule of thumb we use when choosing wheels for off-road use is to look at wheels that are about half the diameter of the tire to be run (20s for 40s, 17s for 35s, and 15s for 30s), although we often run 15s on 35s. This keeps sidewall height tall enough for good flex and soaking up bumps, yet not so tall that the tire is excessively squirmy with too much deflection on the highway.
Wheel width can also play a part in tire performance. It affects the sidewall and tread shape, and how they flex against the terrain. The wheel width will also determine the amount of sidewall bulge beyond the wheel edge. In general, wider wheels will offer more footprint and flotation for the given tire, as opposed to using a narrow wheel. However, narrow wheels typically allow you to run lower air pressure without pushing the tire off the wheel bead.
BeadlocksWe can’t talk about wheels and then walk away without bringing up beadlocks. Really meant for hard-core off-roading like racing and rockcrawling, beadlocks have become a part of the Jeep thing. Or we should say that there are a lot of wheels with simulated beadlock rims on the market, and some with real beadlocks. In our opinion, if you like the look, then bust out and go for the real thing. And all beadlock wheels are not the same. There are some that feature recessed mounting holes to protect the ring’s bolts. Some are cast; some are forged. Some companies offer DIY beadlock conversions. Some beadlocks are considered DOT-compliant (street legal), while others are for off-road use only. Do your research; call the manufacturer and ask.