Basic understandings about wheels

[GinoX]

Mester compronde ki ta kiko over di wheels. Ami no a biba pensa nunca cu un wheel tambe cu tempo ta gasta of mane nos na Aruba ta bisa, nan ta "cansa".

Wheel Composition

Alloy Wheels

Alloy metals provide superior strength and dramatic weight reductions over ferrous metals such as steel, and as such they represent the ideal material from which to create a high performance wheel. In fact, today it is hard to imagine a world class racing car or high performance road vehicle that doesn't utilize the benefits of alloy wheels.

The alloy used in the finest road wheels today is a blend of aluminum and other elements. The term "mag wheel" is sometimes incorrectly used to describe alloy wheels. Magnesium is generally considered to be an unsuitable alloy for road usage due to its brittle nature and susceptibility to corrosion (Flammability doesn't help either !)
While many people choose alloy wheels for their beauty, there are equally important performance benefits to be derived including...


Reduced Unsprung Weight: Compared to Steel Wheels: This is one of the most critical factors affecting a vehicle's road holding ability. Unsprung weight is that portion of a vehicle that is not supported by the suspension (i.e. wheels, tires and brakes) and therefore most susceptible to road shock and cornering forces. By reducing unsprung weight, alloy wheels provide more precise steering input and improved "turning in" characteristics.

Improved Acceleration and Braking: By reducing the weight of the vehicle's rotational mass, alloy wheels provide more responsive acceleration and braking.

Added Rigidity: The added strength of a quality alloy wheel can significantly reduce wheel/tire deflection in cornering. This is particularly critical with an automobile equipped with high performance tires where lateral forces may approach 1.0g.

Increased Brake Cooling: The metals in alloy wheels are excellent conductors of heat - improving heat dissipation from the brakes - reducing risk of brake fade under demanding conditions. Additionally, alloy wheels can be designed to allow more cooling air to flow over the brakes.

Wheel Construction

One-Piece Cast Wheels



This is the most common type of aluminum wheel. The casting of wheels is the process of getting molten aluminum inside a mold to form a wheel. There are different ways this can be accomplished and although it sounds simple, this is truly an art when done properly.

Gravity casting: Gravity casting is the most basic process of pouring molten aluminum into a mold utilizing the earth’s gravity to fill the mold. Gravity casting offers a very reasonable production cost and is a good method for casting designs that are more visually oriented or when reducing weight is not a primary concern. Since the process relies on gravity to fill the mold, the aluminum is not as densely packed in the mold as some other casting processes. Often gravity cast wheels will have a higher weight to achieve the required strength.

Low pressure casting: Low pressure casting uses positive pressure to move the molten aluminum into the mold quicker and achieve a finished product that has improved mechanical properties (more dense) over a gravity cast wheel. Low-pressure casting has a slightly higher production cost over gravity casting. Low pressure is the most common process approved for aluminum wheels sold to the O.E.M. market. Low-pressure cast wheels offer a good value for the aftermarket as well. Some companies offer wheels that are produced under a higher pressure in special casting equipment to create a wheel that is lighter and stronger than a wheel produced in low pressure. Once again in the quest for lighter weight, there is a higher cost associated with the process.

Spun-rim or rim rolling technology: This specialized process begins with a low pressure type of casting and uses a special machine that spins the initial casting, heats the outer portion of the casting and then uses steel rollers pressed against the rim area to pull the rim to its final width and shape. The combination of the heat, pressure and spinning create a rim area with the strength similar to a forged wheel without the high cost of the forging. Some of the special wheels produced for the O.E.M. high performance or limited production vehicles utilize this type of technology resulting in a light and strong wheel at a reasonable cost. BBS has used this technology for several years in their production of racing wheels for Formula One and Indy cars. The BBS RC wheel uses this same technology to produce a light and strong wheel for the aftermarket.

Forged or Semi-Solid Forged: The ultimate in one-piece wheels. Forging is the process of forcing a solid billet of aluminum between the forging dies under an extreme amount of pressure. This creates a finished product that is very dense, very strong and therefore can be very light. The costs of tooling, development, equipment, etc., make this type of wheel very exclusive and usually demand a high price in the aftermarket. Semi-solid forging (SSF) is a process that heats a billet of special alloy to an almost liquid state and then the aluminum is forced into a mold at a very high rate. The finished product offers mechanical properties very similar to a forged wheel without the high production and tooling costs of a forged wheel. When low weight and performance are on your priority list, the SSF technology offers an excellent value. Currently only SSR (Speed Star Racing) from Japan is licensed to use this process for the production of wheels.


Multi-Piece Wheels


This type of wheel utilizes two or three components assembled together to produce a finished wheel. Multi-piece wheels can use many different methods of manufacturing. Centers can be cast in various methods or forged. The rim sections for 3-piece wheels are normally spun from disks of aluminum. Generally, spun rim sections offer the ability to custom-tailor wheels for special applications that would not be available otherwise. The rim sections are bolted to the center and normally a sealant is applied in or on the assembly area to seal the wheel. This type of 3-piece construction was originally developed for racing in the early 1970s and has been used on cars ever since. The 3-piece wheels are most popular in the 17â€￾ and larger diameters.

There are now many options for 2-piece wheels in the market. The 2-piece wheel design does not offer as wide a range of application that a 3-piece wheel allows, however they are more common in the market and the prices start well below the average 3-piece wheel. Some 2-piece wheels have the center bolted into a cast or cast/spun rim section and other manufacturers press centers into spun rim sections and weld the unit together. When BBS developed a new 2-piece wheel to replace the previous 3-piece street wheel, they used the special rim-rolling technology (originally developed for racing wheels) to give the rim section the weight and strength advantages similar to a forged rim. On the high-end of the 2-piece wheel market you can find wheels using forged rims and forged centers. Since these are only sold in small volume and due to the high development and production costs associated with the forging process, they tend to be on the high end of the price scale.

Billet


Billets, raw blocks of metal as it is purchased from the manufacturer, are generally significantly cold-worked in manufacture. However, the cold working is done in one direction only, as the material is rolled or extruded into long bars in a continuous process. This means the grain of the metal has only one orientation. A billet wheel is like cutting a part out of an ordinary piece of lumber, whereas a forged wheel is like growing a piece of wood to exactly the shape you want.






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[GinoX]

Composite vs. One-Piece Wheels

Three-piece, or composite, wheels came into vogue in the 1970s, and reached their peak of stylishness for street use in the 1980s. In the beginning, they provided several benefits. At the time, forging a one-piece wheel was not economical. Porsche's factory forged Fuchs alloys, especially in the wider sizes, were and are today considered very special, expensive items. One-piece centers could be forged, however, and bolted to spun aluminum rims, giving a strong, lightweight wheel. Additional benefits included flexibility of fitment and repairability. Rims could be built for nearly any width or offset, so if you needed just 40 or 50, or maybe only eight, for your racing program, tooling up was a piece of cake, and the costs to be amortized quite reasonable. A damaged rim could be replaced separately, making it cheaper to keep going in the rough world of racing. A three-piece wheel's advantages of exact fitment and repairability remain today, as ever, and are significant. Most high-end composite wheel manufacturers deal in low enough volumes that custom sizes and offsets are a regular part of their business. Unfortunately, manufacturing a composite wheel is extremely labor intensive. A human must assemble the pieces`humans are slow, and cost a lot more than machines. A one-piece forged wheel is comparatively more expensive to tool up for. The process is faster, though, so these extra costs can be spread out over a larger number of wheels. In a one-piece forging, all the material is structural. There are no bolts, no flanges to be bolted together, and no extra material for the bolts to bite into, so a one-piece wheel may be a pound or two lighter than an equivalent three-piece wheel.


Have a wheel that looks like this, or worse? It can probably be repaired. We looked into repairing this inexpensive cast wheel, but found it would cost as much as replacement. Repairing more expensive, or perhaps unobtainable, wheels makes a lot of sense.Wheel Repair
As pointed out above, composite wheels are relatively simple to repair. They are simply disassembled, and the bent part can be replaced with a new, straight one. The other parts should be attended to as well, to whatever extent is required to bring them to as-new condition. Repairing any other type of wheel is, in most cases, a rather more delicate art. In fact, we couldn't get any of the companies that do it to talk in detail about their processes. Evidently, the proper techniques are difficult to perfect, and thus become closely guarded secrets once learned. Several choices are available for wheel straightening. We interviewed the proprietors of Wheel Enhancement, one of the most reputable wheel specialists in the country, and learned more than we knew to ask. An ec contributor has also had very good luck with Tru Wheel in the past.


Rim Markings

Wheels are made with markings, usually on the rim, the meanings of some of which are fairly obvious, some less so. Consider a wheel marked "15x7J ET38." We should all know that 15 is the diameter, in inches, of the surface the tire's bead rests on, while 7 is the width, or the distance between the flanges that support the bead as air pressure forces it outward along the axis of the spindle. The J is not obvious, but is simple once you know: It simply refers to the shape of that flange, easier to understand if you imagine a steel wheel on which the lip is rolled over like a J. "ET" may or may not be present. It is an abbreviation of einpress tief, German that translates literally as "pushed in depth," or offset. The 38 is the offset measured in mm.

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Bolt Pattern

That is only a small part of what you need to know about a wheel, however. Most obvious is the bolt pattern. Watercooled Volkswagen wheel bolts or studs are located on a circle of 100mm diameter, with either four or five holes, a standard shared by many other cars. Porsches have four or five holes on a 130mm circle. The Italians do it their way, with Alfa going from four on 4 1/2 in. to four on 98mm in the early 1970s, just close enough to everyone else to cause real problems for someone who doesn't know it's unique. Perhaps the change was a result of coming under the ownership of Fiat, which also used 98mm bolt circles.


Some wheels have a bolt pattern of 4 on 100mm. They use a steel centering ring which, after 5 years of use, is stuck to the aluminum wheel by corrosion.


Centering

The other element that affects directly whether a wheel can be bolted onto a car is hubcentricity. Long ago, in the deep mists of time, wheels were located by the taper of the lug nuts or bolts. This could lead to all sorts of problems, but they can be summarized by saying centering was liable to be less than perfect, and the sheer stress on wheel bolts or studs could be enormous. I am not aware of any passenger car wheels now made that are not hubcentric. Hubcentric wheels have a hole at their center that fits closely over a round feature on the hub, serving to center the wheel on the axis of the spindle, as well as bear the vertical weight of the vehicle. The wheel bolts or studs then serve simply to hold the wheel onto the hub, and are loaded only in tension, where they are strong. If the studs were required to absorb vertical forces, they would be loaded in single shear, the weakest arrangement for any fastener. Factory wheels are all machined to fit their specific application exactly, and some of the better aftermarket wheels are, too. However, many aftermarket wheels rely on centering rings. This means that, instead of machining wheels specifically for each O.E. centering hole diameter, the wheel manufacturer machines all wheels to one size, and then uses inserts to give a centering surface of the diameter required for each application. This is obviously easier to do, and makes inventorying a complete wheel line much simpler and less costly. If you buy wheels that use centering rings, be sure the rings fit snugly in the wheels. If they are loose enough to fall out, how accurately can they be locating your wheel? Some tire shops automatically remove centering rings to balance a wheel, just to make sure there is no slop to make their balancing inaccurate.

The fact that a wheel physically bolts onto a car doesn't necessarily mean it "fits." The centering surface could be too large, in which case there essentially is no centering. Just as importantly, the offset could be wrong.


Offset


Offset is the location of the flat mounting surface of a wheel relative to the wheel's centerline. Negative offset means the mounting surface is toward the center of the car, positive offset means it is toward the outside of the car, or the wheel is "pulled in" toward the center. Offset affects many things other than just whether the wheel has the appearance of "sticking out" past the fender. The wrong offset can cause rubbing problems when the suspension is compressed or the wheel is turned. Offset affects the steering geometry's scrub radius, possibly leading to problems with torque steer or self-centering characteristics. Offset also affects the suspension's motion ratio, which directly determines the effective spring and damper rates. Potentially, in a very heavily loaded vehicle, or with extreme changes in offsets, wheel bearing life can be affected, but this is more often talked about by truck people than by small car enthusiasts. It is very, very important that the proper offset wheels be used.


The formula to find the correct offset = Backspace - (width/2)While not directly a matter of offset, brake caliper clearance is a related issue. If you have, or plan to have big brakes on your car, be sure that your wheels, or the wheels you are going to use, will fit over the calipers. Spacers are available to solve the problem if they don't, but it is best to get a wheel with enough dish to meet your offset specs and still fit your brakes. Consulting the wheel and brake manufacturers ahead of time is wise. Many aftermarket brake companies even have templates of their brakes available that you can easily check against any wheel.


Mounting

Wheel mounting surfaces can vary in thickness, which means that longer or shorter wheel studs or bolts may be required. Fortunately, those items are standard parts and are available in a variety of lengths to fit most cars. The main concern is that there is adequate engagement between the lug nuts and studs, or wheel bolts and the hub. If there is not, the fasteners could seem tight, and even appear to torque down properly, but cause problems down the line. Inadequate engagement could lead to threads stretching or stripping, loosening, and the wheel coming off. We consulted a variety of sources for a recommendation on how much thread engagement is enough. Across the board, there was agreement that it's hard to have too much, but recommendations for a minimum varied. H&R Special Springs said that with their Trak+ wheel spacers (see sidebar) they follow a German standard requiring 6.4 threads to be engaged on 1.5mm pitch steel fasteners, for a total of 9.6mm. Robert Wood, of Wheel Enhancement, said he likes to see at least one diameter of engagement. For example, a 12mm-diameter fastener would have 12mm of threads engaged. The Southern California Timing Association, which governs the racing at Bonneville, requires at least 5/8 in. of thread engagement. It also prohibits the use of closed-end lug nuts, presumably to allow measurement, but also encouraging full engagement. If you have a center cap covering the lug nuts, they'll clear a stud that protrudes from the nut by one thread. It's likely that if you compete, your sanctioning body's rulebook will have something in it about wheel fasteners.

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