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Wheels & Tires
Construction Of Wheels & Rims
Some are made from steel. They are pressed in 2 sections - the wheel center, with a flange or disc that is drilled for the wheel fasteners, and the rim. They are then welded together.
Others are made from cast aluminum alloy.
Alloy wheels are lighter than similar steel wheels, and since aluminum is a better heat conductor than steel, alloy wheels dissipate heat from brakes and tires more quickly than steel wheels.
The wheel center must accurately locate the wheel rim centrally on the axle. It must also provide the required distance from the centerline of the wheel, to the face of the mounting flange. This is called offset.
Offset is important because it brings the tire centerline into close alignment with the larger inner hub bearing, and reduces load on the stub axle. This allows the inside of the wheel center to be shaped to provide space for the brake assembly, usually located inside the wheel.
The offset, measured in millimeters, can be negative or positive, and is the distance from the hub-mounting surface to the rim's true centerline. A positive offset means the hub-mounting surface is closer to the outside edge of the wheel; a negative offset means the hub-mounting surface is closer to the inside edge of the wheel.
Offset affects the scrub radius of the steering and it is advisable to stay within the limits allowed by the vehicle manufacturer. If tires are fitted which are significantly wider than those specified by the manufacturer, a compromise may have to be adopted whereby a wheel with less offset is used to prevent the tire rubbing on the suspension.
Ventilation slots allow air to circulate around the brakes.
In some cases wheels are directional to assist the airflow.
The rim must be accurately shaped, and dimensioned, and strong enough to support the tire under the load of the vehicle and the forces generated by the motion of the vehicle.
Passenger cars normally use rims which are of well based, or drop-center design. The drop-center is used for mounting and demounting the tire onto the rim.
When inflated, the tire is locked to the rim by tapering the bead seat towards the flange, or by safety ridges or humps, close to the flange.
In the event of sudden deflation, or blowout, safety ridges prevent the tire moving down into the well. This helps maintain control of the vehicle while it is being braked.
The rims are referred to as 15-degree drop-center rims, because the bead seats are inclined at 15 degrees towards the flange.
The taper gives a good grip, and an airtight seal between the tire beads, and the rim.
The low flanges and drop-center allow the special size, flexible, tubeless truck tires to be mounted and demounted in a similar manner to that used on smaller passenger car tires.
The stiff sidewalls of larger cross-ply tires mean they cannot be mounted and demounted in this way, and many 4-wheel-drive and commercial vehicles use a flat-base, demountable flange rim.
When all of the air is removed from the tire, one flange can be removed so the tire can be demounted.
Wheels are fastened to the hubs by wheel studs and nuts.
They’re highly stressed by loads from the weight of the vehicles, and the forces generated by its motion, and they’re made from heat-treated, high-grade alloy-steel.
Bolt pattern
The bolt pattern is merely the number of bolts on the wheel. As the bolts will be evenly spaced, the number determines the bolt pattern. For example, most U.S. passenger cars have 5 bolts while compact models sometimes have only 4 and pick-up trucks can have as many as 6 or 8.
Bolt circle
4 Hole Pattern
4 Hole Pattern
5 Hole Pattern
5 Hole Pattern
The bolt circle is the circle determined by the positions of the bolts; the center of every bolt lies on the circumference of the bolt circle.
The important measurement is the "pitch circle diameter" (PCD), usually expressed in millimeters, although inches are sometimes used. For a 4- or 6-bolt car, this measurement is merely the distance between the center of two diametrically opposite bolts. In the 4-bolt picture below, this would be the distance between holes #1 and #4. Some basic geometry is needed to find the center of a 5-bolt pattern: draw a line between any two neighboring bolts, and draw a line from the midpoint of this line to the opposite bolt. Repeat with a different set of three bolts, and the two long lines will cross in the center, thereby making the distance between this intersection and the center of a bolt the radius of the bolt circle.
Wheel size
The wheel size is the diameter of the wheel, in inches, not counting the tire. Modern tires in North America have several other measurements associated with their size. A tire designated as a "225/70R14" denotes a tire with the following dimensions: width of tread: 225 mm; ratio of tire height to width: 70 percent; rim diameter: 14". Therefore a 185/70R14 has the same rim diameter and aspect ratio, but a smaller overall diameter and narrower tread face than the 225/70R14.
Centerbore
The centerbore of a wheel is the size of the hole in the back of the wheel that centers it over the mounting hub of the car. Factory wheels have a centerbore that matches exactly with the hub to reduce vibration by keeping the wheel centered. Wheels with the correct centerbore to the car they will be mounted on are known as hubcentric. Hubcentric wheels take the stress off the lug nuts, reducing the job of the lug nuts to holding the wheel to the car. Wheels that are not hubcentric are known as lugcentric, as the job of centering is done by the lug nuts assuming they are properly torqued down. Centerbore on aftermarket wheels must be equal to or greater than that of the hub or the wheel cannot be mounted on the car. Many aftermarket wheels come with "hubcentric rings" that lock into the back of the wheel to adapt a wheel with a larger centerbore to a smaller hub. These adapters are usually made of plastic but also in aluminium.
The threads between the studs and nuts are close fitting and accurately-sized.
All wheel nuts must be tightened to the correct torque, otherwise the wheel could break free from the hub.
Alloy wheels
wheels which are made from an alloy of aluminum or magnesium metals (or sometimes a mixture of both). Alloy wheels differ from normal steel wheels because of their lighter weight, which improves the steering and the speed of the car, however some alloy wheels are heavier than the equivalent size steel wheel.
Alloy wheels are also better heat conductors than steel wheels, improving heat dissipation from the brakes, which reduces the chance of brake failure in more demanding driving conditions.
Alloy wheels are also considered more visually attractive than hubcaps.
Alloy wheels are more expensive than standard wheels, and thus even though they are becoming more available and are standard on higher-priced models, they are not standard on many cars.
Alloy wheels are prone to galvanic corrosion if appropriate preventive measures are not taken.
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The tire assembly is mounted on a wheel and rim assembly. Wheels must have enough strength to carry the weight of the vehicle, transfer driving and braking forces, and withstand side thrusts. Passenger cars are driven at high speeds, which means that they must be set close to the ground.
Therefore, they use perfectly balanced wheels of small diameter. Even a slightly unbalanced wheel and tire assembly will cause steering problems and rapid tire wear at high speeds. Small balance weights can be attached to the wheel to offset an unbalanced condition.
Passenger car wheels are usually of the steel-disk type. The disk may be solid, or it may have holes in it to reduce weight and to provide ventilation for cooling. The disk is welded or riveted to the rim and bolted to the brake drum and axle. Some passenger cars use wire wheels, which are light and easy on tires. They consist of a pressed steel hub and rim connected by spokes that are welded at each end. In both types of wheels, the wheel is dished to bring the point of ground contact under the large wheel bearing.
The purpose of the wheel rim is to complete the enclosure for the tube, hold the tire beads in place, and connect the tire to the wheel. Several different types of rims are used. For correct mounting, demounting, and tire fit, you should be familiar with the difference in rim types.
The drop-center rim is made in one piece and is permanently fastened to the wheel disk. Its important feature is a deep well. On some wheels, the well is to one side of the rim center, and the tire must be removed and replaced from that side. Drop-center rims are generally used on smaller vehicles, such as passenger cars and 1/4-ton, 4x4 trucks. Some passenger cars and light trucks are equipped with safety-type drop-center rims. Safety rims have a slight hump at the inside edge of the bead ledges. This holds the tire beads in place when the tire goes flat.
The semidrop-center rim has a shallow well in the rim. It has tapered bead seats to fit the shape of the beads on the tire. A demountable rim flange or side ring fits into a groove (gutter) on the outside edge of the rim to hold the tire in place. You must remove this side ring to remove and replace the tire.
The flat-base rim has a flat seat for the bead, and the tires must also have flat beads. This rim is flat with no well. This type rim has a demountable side ring to permit mounting and demounting the tire. Flaps are required on the flat-base rim to protect that part of the tube not protected by the tire. Flat-base rims are no longer used on vehicles.
The advanced rim is similar to the flat-base rim except that it has a slight taper (5º) on each bead seat. The taper provides a squeeze fit of the tire beads to prevent slippage between the rim and tire. Different manufacturers make the side rings and grooves (gutters) in different shapes, which prevents interchangeability. |
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Article Pages
1. An Introduction to Wheels and Tires
2. Tires
3. Learning To Read Your Tires
4. Manipulating The System
5. Valves
6. Tire Pressure Monitoring Systems (TPMS)
7. Tire Inspection
8. Tire Wear Guide
9. Tire Rotation
10. Wheel Assembly
Hot Topics
-> 4 Things You Can Start Doing Today To Extend The Life Of Your Tires
-> New Tires Guide
-> Tire Safety Tips
-> What Does It Mean When My Wheels Are Imbalanced?
-> Wheel Balancing
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