Where are the rims on a tire?

Outer part of a wheel on which the tire is mounted

For other uses, see Rim

Cutaway diagram of a rim and tire from an ATV Cross section of a bicycle rim A wooden bicycle rim with tubular tire

The rim is the "outer edge of a wheel, holding the tire".[1] It makes up the outer circular design of the wheel on which the inside edge of the tire is mounted on vehicles such as automobiles.[2] For example, on a bicycle wheel the rim is a large hoop attached to the outer ends of the spokes of the wheel that holds the tire and tube.[3] In cross-section, the rim is deep in the center and shallow at the outer edges, thus forming a "U" shape that supports the bead of the tire casing.[4]

In the 1st millennium BC, an iron rim was introduced around the wooden wheels of chariots to improve longevity on rough surfaces.[5]

Characteristics

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Scratched rim on a one-piece alloy wheel. The black residue remains from where the tire was seated on the "safety profile" rim.

Design

The first pneumatic tires for bicycles were simple tubes in shape secured to the wooden outer concave surfaced circumference of the wheel by glue and air pressure pressing them against it.[6] The surface for receiving the tube was not very secure thus causing the tires to sometimes come off the rims.[6] Bicycle manufacturer and inventor Thomas B. Jeffery developed an improved tire that had a wire embedded in the rubber of the tire; that wire could be tightened onto the rim.[7] His 1882 patent became the ancestor of all clincher tires, the design found on modern bikes and cars.[7] Modern clincher tires have wires embedded on both beads of the tire so the wires fit inside the edges of the rim to hold the tire in place when it is fully inflated.[7]

Diameter (effective)

A distance between the bead seats (for the tire), as measured in the plane of the rim and through the axis of the hub which is or will be attached, or which is integral with the rim.

Width (effective)

A separation distance between opposed rim flanges. The flange-to-flange width of a rim should be a minimum of three-quarters of the tire section width. And the maximum rim width should be equal to the width of the tire tread.

Type

Depends on the type of vehicle and tire. There are various rim profiles, as well as several rim components.

Modern passenger vehicles and tubeless tires typically use one-piece rims with a "safety" rim profile. The safety feature helps keep the tire bead held to the rim under adverse conditions by having a pair of safety humps extending inwardly of the rim toward the other tire bead seat from an outer contoured surface of the rim.[8]

Heavy vehicles and some trucks may have a removable multi-piece rim assembly consisting of a base that mounts to the wheel and axle. They then have either a side ring or a side and lock ring combination. These parts are removable from one side for tire mounting, while the opposite side attached to the base has a fixed flange.

Low tire pressure applications such as off-roading and drag racing use a beadlock that clamps or physically attaches the bead of the tire to the rim of the wheel. This reduces the chance of the tire separating from the rim, causing a sudden deflation.[9]

Material

Various metals can be used for the rim. Commonly seen are alloy (magnesium and aluminum), mag (magnesium), aluminum, and chrome. Teflon coatings are sometimes also applied for an extra layer of protection.

Vehicle performance

Because the rim is where the tire resides on the wheel and the rim supports the tire shape, the dimensions of the rims are a factor in the handling characteristics of a vehicle. For example:

Overly wide rims in relation to the tire width for a particular car may result in more vibration and a less comfortable ride because the sidewalls of the tire have an insufficient curvature to flex properly over rough driving surfaces. Oversized rims may cause the tire to rub on the body or suspension components while turning.

Overly narrow rims in relation to the tire width may cause poor handling, as the tire may distort sideways under fast cornering. On motorcycles, a narrow rim will alter the tire profile, concentrating tire wear in a very small area during cornering, with a smaller contact patch during braking.[10]

On bicycles, the optimum tire width is about 1.8 times the rim's internal width.[11] An example is a 35 mm tire on a rim with an ETRTO 17 mm internal width, or one-and-a-half times the rim's external width. Considerable variation outside this range is safe, but very wide tires on a narrow rim can overstress the rim and damage the tire sidewalls, whereas very narrow tires on a wide rim give a hard ride and can result in a high-pressure tire blowing off.[11] However, wider wheels and wider tires are increasingly popular, making prrievious guidelines obsolete because manufacturers are focused on providing a more comfortable and faster riding experience.[12]

Production

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Damage to the rim can cause vibration and cause a tubeless tire to fail to hold pressure

A standard automotive steel wheel rim is made from rectangular sheet metal. The metal plate is bent to produce a cylindrical sleeve, and then the two free edges of the sleeve are welded together. At least one cylindrical flow spinning operation is carried out to obtain the desired thickness profile of the sleeve—and the desired angle of inclination relative to the axial direction in the zone for the outer seat. The sleeve is then shaped to obtain the rims on each side with a radially inner cylindrical wall in the zone of the outer seat and with a radially outer frustoconical wall inclined at an angle corresponding to the standard inclination of the rim seats. The rim is then calibrated.[13]

To support the cylindrical rim structure, a disc is made by stamping a metal plate. It has to have appropriate holes for the center hub and lug nuts. The radial outer surface of the wheel disk has a cylindrical geometry to fit inside the rim. The rim and wheel disk are assembled by fitting together under the outer seat of the rim and then being welded together.[13] The disk is welded in place such that the center of the wheel is equal to the center of the hub. The distance between the centerline of the rim and the mounting plane of the wheel is called the "offset" and can be positive, negative, or zero.[14]

One-piece rim and wheel assemblies (see image) may be obtained by casting or forging.

Meaning

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Used broadly, or used figuratively, the word rim can mean the outer edge of any circular object.[15][16]

On a bicycle wheel, the rim is clearly just one component of the assembly, and it can be purchased separately and replaced if damaged or if the sidewalls have been eroded by rim brakes.[17]

In discussions of automobiles, however, the terms wheel and rim are often incorrectly used synonymously, as in decorative wheels being called rims. One engineering text says, "alloy wheels [are] often incorrectly called aluminum rims".[18]

Some authors are careful to use rim literally for only the outer portion of a wheel, where the tire mounts,[19] just as the rim of a coffee cup or a meteor crater does not refer to the entire object. Others use rim to mean the entire metal part to which the tire mounts,[20] because the rim and the wheel are often cast or stamped from a single piece of metal instead of being distinct as with wire wheels. At the same time, "wheel" may refer to the entire rotating assembly, including the tire.[21][22]

Railroad usage

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In railroad usage, the conical running surface of the wheels may be called a rim, a wheel tread, or a tire.

Historical development

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Early wheels of motor vehicles started as bicycle wheels, with the rims attached to the central axle by spokes. As vehicles became heavier, wood-spoked wagon wheels with steel rims were used. Later, solid rubber tires were mounted on the rims of those wooden wheels. Some wooden automobile wheels had a demountable steel rim that was bolted onto the outer circumference of the wooden wheel. Wheels that were completely made of metal (single or multiple pieces) gradually became widespread around the 1930s.[23]

See also

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References

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“Hey, check out my new rims!” quips your car enthusiast friend. But what part of the vehicle are they referring to? If your mind jumps to car wheels, you’re not alone. When it comes to the question of rims vs. wheels, the terms are often used interchangeably. However, the rim is just one part of the wheel — and an important one at that.

What are wheels?

To understand the difference between wheels and rims, it helps to know how automobile wheels work. Let’s start with the basics. While sometimes people think of tires when they talk about wheels, technically your wheels and tires are separate entities that ultimately combine to form the tire-wheel assembly.

Rims play an important role in this setup. As the outer edges of the wheel, your rims secure the tire on to the wheel by forming an airtight seal. But rims are only one part of the wheel equation. In addition to rims, wheels are comprised of other important features:

• Wheel bearing: This is the part of the wheel assembly that connects the wheel to the axle.
• Lug nuts: Located around a wheel’s center, these nuts secure the wheel to the vehicle. 
• Center cap: These caps help keep dirt and grime away from the wheel bearings and lug nuts. They often sport a manufacturer’s logo.
• Hubcap: Larger than center caps, these disks are hammered or clipped on the hub of the wheel, covering the lug nuts. Hubcaps used to be more common, helping protect steel wheels from the corrosive effects of winter weather, road salt and other harsh chemicals. Nowadays, wheels are increasingly made of aluminum or metal alloys, which tend to better protect against corrosion, making hubcaps less of a fixture.
• Spokes: These are the metal arms that connect a wheel’s center to its outer reaches, such as the rim and the barrel.
• Barrel: While your rims are the outward-facing edge, the curved portion of the wheel outside of the spokes is known as the barrel.
• Wheel cover: A wheel cover helps protect the entire surface of the wheel. Wheel covers are often designed to make the wheel look more appealing and may also be used as a cosmetic fix for minor scratches.

What are rims?

As we mentioned, the main difference between wheels and rims is not technically a difference at all: rims are simply one important part of the wheel. Of course, in popular culture the phrase “Nice rims!” has long been used as shorthand for complimenting someone’s wheels, particularly in reference to a sharply designed or intricate wheel cover. But what we’re talking about here is what rims actually are: the two outer edges of each side of your wheel.

What do rims do?

As the edge points of contact between the tire and the wheel, rims help hold tires properly in place. As stated, they also help maintain an airtight seal between the tire and the wheel.

For these reasons, your rims play a crucial role in supporting the entire wheel assembly and its place in your car’s suspension system. If you drive over a bump or pothole, the rim is the part of the wheel that receives the initial impact after it passes through the tire.

You may think of your tires and rims as the front line of defense against the rigors of the road. If your rims get banged or dented, it may affect the performance of your vehicle, from flat tires and reduced handling to distracting vibrations inside the cabin.

What do rims look like?

If you look at a wheel from the top without a tire on it, you’ll see that it dips down in a “U” shape, which continues along its circumference. The rims are the upturned stems of this U-shaped design. The outermost edges of the tire, known as “tire beads,” press against the rim when inflated to form the tight seal.

What are rims made of?

Your rims are made from the same alloy or metal as the rest of your wheel. They’re also often specially treated with a protective coating during the manufacturing process to help extend their longevity.

How to know what size rims you have?

Knowing how to determine rim size can be helpful, because wheel size and tire size depend in part on the diameter of the rim.

Your rim size can be found in several places. Firstly, you can check the sidewall of the tires that came with the wheel. If you no longer have those tires, the information may also be found in various places such as the inside frame of the driver’s door, on the gas tank hatch, on the glove box door or stamped on the back of the wheel itself. When in doubt, checking your owner's manual is always a good idea.

If you spot a numeric sequence like “16x6 5-110,” the initial two dimensions give you the diameter of the rims and the width between them (16 inches and six inches in this case). The other two dimensions refer to the number of bolt holes and their diameter (five lug nuts in a circular formation measuring 110 millimeters at its widest point in this case).

Do rims come with new tires?

Generally, when you replace your tires you don’t also need to buy new rims, unless you’re investing in a wheel-tire combo, possibly due to damage to your rims.

How do rims become damaged?

Rims can become dented by curbs or potholes and corroded by road salt. Similar to tires, they also tend to wear with age. Signs of a damaged rim may include sudden changes to handling, vibration when steering, problems with your wheel alignment and tires that repeatedly deflate. It’s generally best not to leave any concerns with your car’s suspension to chance. Consider asking a trusted mechanic for advice.

The difference between wheels and rims

Let’s return to where we started: the difference between wheels and rims. As explained above, it comes down to a matter of detail. The wheel is the metal unit that attaches the wheel hub to an automobile via an axle, while rims are the outermost part of the wheel assembly. Perhaps think of it this way: “rim” means “edge,” and rims are located on the edges of your wheels.

In the more colloquial sense, “rims” may refer to the wheel covers you might sometimes see on a car, typically added for aesthetic reasons. Though somewhat of a misnomer, this correlation might possibly have come about because the rim size does dictate the size of wheel cover required.

In summary

The difference between wheels and rims is more than simply semantic. If it weren’t for your rims, your tires wouldn’t be properly secured to your wheels, leading to a ride that is shaky and potentially dangerous. Understanding rims vs. wheels may also make it easier to explain any issues with your wheel assembly to your mechanic. Although actual rims may not have the decorative flair of modern-day wheel covers, this crucial wheel component still deserves its chance to shine.