Wheel Tech Guide
Everything You Need to Know Before You Buy
By Dan Barnes
Photography: Les Bidrawn
Photographers often say that wheels are a car's jewelry. Perhaps more than anything else, they immediately show the tastes and character of the owner who put them on. It's obvious whether the driver is a poser who just wants flash so people look, or someone who is a little more serious about his or her car. Maybe the wheels are retro, like Panasports, maybe the guy is a racer on a budget and has factory alloys he bought at a junkyard wrapped in competition rubber, or maybe he had the knowledge and money to step up to the latest forged exotica. In any case, wheels are important enough that they must be chosen carefully. More than a few otherwise nice cars have been passed over for magazine features because they had the wrong wheels. The accompanying buyer's guide should help you sort out style and weight of specific wheels you want for your car. This article is to help you understand what other questions you should ask to make sure that the beautiful alloy you bolt onto your car doesn't cause pain down the road.
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.
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.
This Wheel has a bolt pattern of 4 on 100mm. It uses a steel centering ring which, after 5 years of use, is stuck to the aluminum wheel by corrosion.
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 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 offset located above the photo is incorrect. The correct formula is: 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.
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.
We can't talk about wheel fitment without talking about tires, since wheels are mostly just there to connect the tire to the suspension. Overall tire diameter affects speedometer accuracy, as well as acceleration, braking, and suspension geometry. If tires are too tall, they may rub fenders. Overall diameter, if not available from a tire's manufacturer, can be calculated from the sizing information by the following formula:
Diameter (in.) = section width (mm) • aspect ratio • 2 + wheel diameter
It is best to keep this within a few percent of the original tire's diameter. Don't obsess over this number however, as a tire may wear through 1/4 in. of tread in its lifetime, reducing overall diameter by 1/2 in., roughly 2 percent on a 25-in.-diameter tire. As well, tire manufacturers differ, and two tires of the same listed size may not actually be exactly the same size. Section width leads to many fit problems, when people choose a tire that is too wide. Tires that fill a car's wheelwells almost always look better. Certainly, wider tires make more grip in theory, and usually in practice as well. A wider tire has a shorter contact patch, less prone to distortion, and should operate at a lower slip angle for a given side load, a characteristic also influenced, perhaps to an even greater degree, by sidewall stiffness. A wider tire has more material to absorb the heat of cornering, and more surface area to carry the loads, so it may run cooler when driven hard. Wider tires also have greater rolling resistance and aerodynamic drag, so underpowered cars may want to strike a balance with slightly less than monster rubber on a fast track. Wider tires hydroplane easier with water on the road. If your suspension doesn't control camber adequately under cornering loads, the extra width may be wasted as the high edge does little work. Again, different tires are manufactured with different shapes. Some have very square shoulders, some are more rounded, so one tire may fit fine while another tire of the same nominal size may rub. There are different criteria for choosing the width of wheel/tire combinations. One expert we asked says he likes to see wheel width between 85 and 90 percent of section width, while another tells us he likes the section width to be about 1/2 in. wider than the wheel width. Everybody seems to agree that having the flange of the wheel extend past the sidewall is bad, both for tire performance, and because that means the wheel hits a curb before the tire.
While there are DOT standards for wheels that apply to the original automobile manufacturers, and each manufacturer has its own standards in addition, there are none for the aftermarket. There are some industry norms, and the better aftermarket wheel manufacturers follow them, or have some variation they prefer. Kinesis, for example, puts its wheels through fatigue cycle tests twice, rather than just once.
There are no industry-wide standards for aftermarket wheel load rating, but you should definately follow manufacturers' recommendations.
In other countries, notably Germany and Japan, the aftermarket is tightly regulated by their governments. The Specialty Equipment Market Association (SEMA) would like to prevent that situation in the United States, so its Wheel Industry Council (WIC) has undertaken to create a set of standards for the aftermarket wheel industry. Peter Stacey, President of Kinesis Motorsport Corp., a member of WIC, took a few minutes to tell us about the Council's efforts. The WIC's goal is to be able to educate consumers and industry members about wheel safety and quality. It is, in a sense, aimed at effective self-policing of the industry. Low quality products, it is believed, reflect poorly not just on their own manufacturer, but on the entire industry. By publishing standards and following them, the WIC hopes to protect the wheel marketplace from burdensome governmental regulation, as well as from the negative effects of wheels with which quality, and conceivably safety, are sacrificed for price. The WIC is coordinating its efforts with the Society of Automotive Engineers Aftermarket Systems Group and with regulatory agencies in other countries to develop a standard that will have wide acceptance and foster international marketing of quality products. A standard has been drafted by the WIC, but refining and approving it is a long process, and remains a work in progress. More information on the WIC can be found at www.sema.org.
How to Decide
We came up with the following process for choosing wheels and have run it past several experts who said it was basically sound. First, realize that lighter is much, much better. Choose the smallest wheel diameter that will fit over the brakes you plan to eventually use on your car. Don't forget to account for future upgrades, if any are planned. If your car is very light, and you plan to keep small brakes, this may lead you to 13-in. sizes. While these are very light, the handling benefits of low profile tires are real, so you may choose to keep your wheel diameter larger, just to use a 50- or 55-series tire. Second, look at tire availability. What tires are the right size for your car and meet your needs for performance, cost, noise, wear, etc? Once you have made these two choices, look for the wheel that will do the best job of connecting the tire to your car, and that looks right for your car. The path of wisdom here seems to be that of conservatism. Cars vary slightly, especially older vehicles (MKI Volkswagen fanatics, this means you) that were built to looser tolerances, and what just barely works for one may not quite work for another.
Wire wheels were technically obsolete by the time the Jaguar XKE was introduced. XKE race cars used cast alloy wheels.
What Fits Your Car
When you need more information, don't call us. Unfortunately, we don't have a master chart of what size tires fit on each car, adjusted by whether it's lowered 1 1/2 in. or 2 in. If those questions are answered in Tech Letters, you will always find that the editor who wrote the response called either the Tire Rack or Discount Tire and asked one of their sales associates. If you do the same, or consult your local tire and wheel retailer, you'll get your answer much sooner, and can follow it up with more questions immediately. These companies have databases based on their experience fitting wheels and tires on thousands of cars. We have found that their recommendations are conservative, and have never heard of anyone who followed them having to roll a fender or otherwise compromise a vehicle.
The Tire Rack
771 West Chippewa Ave.
South Bend, IN 46614
www.tirerack.com Discount Tire Direct
7333 E. Helm Drive
Scottsdale, AZ 85260
Fax: (480) 483-9230
This Abarth magnesium alloy wheel was rescued, for art's sake, from a 240Z, with which it shares a bolt pattern. It had been bolted onto the Datsun so it could be rolled around in a junkyard. Unfortunate, magnesium is not known for its corrosion resistance, and this wheel has suffered where it contacted iron.
Over the last few years, there have been more companies competing for performance enthusiasts' dollars with lightweight forged wheels. This is a good thing. Lighter wheels are easier for the suspension to control, improving ride and handling, and are easier to accelerate and decelerate. Volkswagen was quicker than others to catch on to this, and twenty years ago, waterpumpers were among the first affordable vehicles to come with lightweight alloy wheels from the factory. As cars have gotten better and more expensive, the benefits to performance and appearance have made the alloy wheel option necessary for an OEM to compete successfully in nearly any segment.
Steel disc wheels, still found on the majority of cars sold, are inexpensive, durable, heavy and flexible. Why did hubcaps fly off cars in old movies (and still do, when old cars are put around a corner hard)? Simple: The wheels flexed. Steel disc wheels were a vast improvement over the wire wheels that preceded them, but today they should be considered unacceptable for any performance use other than an El Camino set up for drag racing.
This is a raw casting at the RW Wheels factory near Ghent, Belgium. Much machining remains to be done.
"Mag" wheels came into use first in the '50s, as European sports car builders began applying aircraft technology to the race track. "Mag" is short for magnesium, and should properly be reserved for wheels actually made from the stuff, such as the Halibrands found on GT40s and Cobras, or the Abarth Cromodoras that rest against the wall in my office. Though magnesium is a lighter metal, most of these wheels represent ancient technology, and are far heavier than modern forged aluminum wheels of the same size.
Almost all modern performance wheels are made from aluminum by one of two processes: casting and forging. Casting is done by pouring molten metal into a mold shaped more or less like what the final object will look like, and letting it cool. Forging is done by heating a chunk of metal just enough to soften it slightly, then slamming it with hundreds of tons of force, squeezing it into the desired shape. It turns out that forged wheels are manufactured in multiple steps. If it seems unlikely that a solid chunk of metal could be mashed into a finished wheel, your intuition is correct. Instead, it is forged as a center with a donut of metal around the outside, which is then split and rolled outward to form the rim halves, essentially forging the metal again. Obviously, one process is a lot easier to do than the other. BBS, for some of its wheels, casts the center and donut, then rolls the rim out like a forged wheel. BBS calls this simply the "rim rolling process."
Porsche was a pioneer, providing one-piece forged Fuchs wheel that, like the car, looked good and performed wheel into the late 1980s. This is a 1967 911S.
We usually think of metal as a simple solid material, without any internal structure, like putty. In reality, it's a jumble of crystals, all grown into each other. Usually, the individual crystals are small enough that we don't see them, but other times we do. If you can find a big, cast bronze door handle, look at the back of it, where thousands of hands polish it smooth, and etch it with their acids. You will probably see a splotchy pattern, where each different shaded region is a different crystal, in a pattern resembling frost on a window. The process of formation is much the same. The grain size in a wheel is much, much finer than that.
This modern, forged aluminum Volk Racing TE37 measures 13x8 in. and weighs a scant 8.25 lb. The Abarth cast magnesium alloy measures 14x6 in. and weighs 12.25 lb. Thirty years of progress have gotten us something.
The quality of cast wheels varies dramatically, depending on process, and sometimes on variables beyond the control of the manufacturer, such as ambient temperature or even humidity. In general, pressure casting, in which the metal is pumped into the mold, is better than just pouring it in. Castings tend to be porous -- some carburetors actually leaked fuel through the metal, with no crack or visible flaw present. Porosity is bad, because it means there are places where the metal isn't in direct contact with more metal on all sides. Voids, which tend to form in the spaces between crystals (a chicken and egg situation), are where cracks begin. Larger, chunky grains may beget larger voids, and cracks along crystal boundaries will have farther to travel. All these points mean that cast wheels must contain more metal to achieve an acceptable strength, and are thus heavier. Still, cast wheels can be made to a high standard with attention to quality processes. The vast majority of alloy wheels are cast, and provide many years of good service.
Forged wheels take advantage of what happens when metal is cold worked. Cold working doesn't necessarily mean you'd want to touch the materials while they're in process, it means the procedures are done at a temperature below the point where the metal starts to melt and regrow a new crystal structure. Just as the spaces between a metal's crystals may hold flaws, the crystals themselves are full of imperfections called discontinuities. They may take a variety of forms, but discontinuities all share one important quality. By traveling through the crystal lattice of the individual grains, they allow the metal to change shape without fracturing like a diamond. When a load is applied to a metal object, it deforms slightly. When the load is removed, it regains its original shape. This happens because discontinuities move a little, and move back. If the load is high enough, the discontinuities will move until they reach the edge of their crystal, or until they run into another discontinuity.
This Fikse FM/10 composite wheel measures 9x18 in., with 45mm offset, and weighs 20 lb. That's a little more than the lightest one-piece forged wheels, but is is custome built for a perfect fit on any car, and is easily repaired if a rim is bent.
Generally, discontinuities move one atom at a time, and their movement is guided by the regular structure of the crystal. If a discontinuity in the structure runs into another, the regularity is interrupted, and they may become tangled, and can't return to their starting position. This has two effects. 1) When the load is removed from the metal, it will not return to its original shape. 2) The metal is more resistant to deformation in the future, because there are fewer discontinuities available to move around. This description of the process is a single case of what is actually happening by the billions.
What we can measure is the average of them all. The idea behind forging is to get, on average, the right number of discontinuities tangled around each other, with crystals oriented in the right direction, so that the metal is very strong and resistant to further deformation. This is a delicate balance, because too much cold working makes the metal brittle, so that it fractures instead of absorbing loads. You can see how this works for yourself: Bend a paper clip back and forth many times until it breaks. It begins soft, then gets stiffer, before finally fracturing.
Fikse installs the fasteners on its wheels from the rear, giving a smooth appearance to the outside of the wheel.
Forging also changes the shape and alignment of the crystal structure. When molten metal solidifies, its grain structure is non-directional, amorphous, grains in the sense of "grains" of sand. As metal is forged, these grains are stretched in the direction of deformation, making them more like the "grain" of wood. The metal is formed so the grain goes in the directions where strength is needed most. Think of particle board versus real wood. One is cheap, heavy, and easily formed into a variety of shapes. The other is strong and light. The forging process, because of the vast pressures involved, also compacts the metal, eliminating porosity and the voids that can be a source of cracks or corrosion. The result is that less metal is required to achieve a given strength, meaning lighter, stronger wheels can be made.
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.
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 piecesÃ‘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.
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.
7312 Laurel Canyon Blvd.
North Hollywood, CA 91605
(818) 765-5577 Wheel Collision Center
7286 Penn Dr.
Bath, PA 18014
Fax: (610) 837-8967