Years of evolution have removed a lot of instinctive behavior from human beings, but there is one impulse that hasn't been drummed out of us over the eons. No, we're not talking about our hooter fixation. We mean adding hoops, shoes, rollers, or, well, wheels and tires to your ride. It's the first thing on your to-do list when you buy a car, isn't it? It's an urge that's just too powerful to ignore.
Have you ever asked yourself why? What drives us to remove perfectly functional rims and rubber from a car and drop hundreds-if not thousands-of dollars to replace them? The look, obviously. There isn't a single bolt-on in the automotive world that can personalize a car faster than a new set of wheels and tires. But the beauty of new rims and rubber goes way beyond the aesthetic. Slap on the right set and you'll see huge gains in your car's performance without making any other changes. Carve corners like you're on rails! Launch from stoplights like Kirstie Alley on her way to a Hometown Buffet! All of that, and more, can be yours with the right choice.
Improved safety is also a bonus. Not to kill your buzz, but safety should be as important as style and performance. Consider this: A tire's contact patch-the amount of rubber on the road at any given time-is only about as big as your palm. Or look at it this way: The total contact patch area of your four tires is about the size of an 81/21/4x11 sheet of paper. That's all there is on the ground to control your ton-and-a-half car, with you and your loved ones inside, as it hurtles down the road. Don't you want to make sure that small amount of rubber is as good as it can be?
The tires that apply to our kinds of cars fall into three broad categories: mass market, high-performance, and ultra-high-performance. How a tire is classified depends on its design parameters and how it balances certain trade-offs.
Tires are one area where you really can't have it all. A tire designed to grab like GI Joe with a kung fu grip is going to wear out quickly. Likewise, a tire designed for hardcore cornering will give your car a stiff ride-we'll go into why later. So you should choose a class of tire with the traits you want, while understanding that those traits will have consequences.
Mass-market tires are the kind of tires that the factories put on low-end coupes and sedans. Also called all-season tires, these are designed for ride comfort and long tread wear, so they aren't going to stick or corner very well.
On the other end of the spectrum are ultra-high-performance tires. These are built to withstand extreme top speeds (149-mph plus) and provide quick, responsive handling, usually at the expense of tread life and ride comfort. But for the ultra-high-performance buyer, tread life and comfort don't mean squat compared to the feeling of sticking like glue to a twisting mountain road. As you'd probably guess, ultra-high-performance tires are pricey. All that road-grabbing technology is expensive, and you can expect to pay between $100 and $200 per tire or more for this level of performance.
Not all of us can afford that kind of investment, and many of us don't really need outer-limits speed capability and handling. So high-performance tires fill the gap between ultra-high and mass-market. High-performance tires aren't rated for ultimate speed ("only" 125-130 mph or so), but their construction gives them crisper handling response and better traction than the mass-market tires. They'll ride a little softer than the ultra-highs too, so they won't be as harsh over curbs and potholes. You can still pay dearly-maybe $150 or so per tire, depending on size-but you'll find tires in the $60-$70 range too.
All tires are made up of similar components, starting with the innerliner, which keeps the air inside the tire. Surrounding the innerliner is the tire's body, which is made up of fabric plies. The terms "radial" and "bias ply" refer to the orientation of these body plies. A radial tire's plies are perpendicular to the tread, while the plies in a bias-ply tire run diagonally ("on the bias"). You've heard of "steel-belted radials," right? That refers to belts that run under the tread area to strengthen and stabilize the tread. Around the outside of the tire are the sidewall, which gives the tire its lateral stiffness and protects the side of the tire from damage, and the tread, which is where the rubber meets the road. The bead, where the tire meets the wheel, consists of steel-wire hoops built into the sidewall.
As a tire's performance envelope increases, various components are beefed up to withstand the forces acting on it. For example, the centrifugal force at high road speeds can make the tire's belt package want to pull away from the rest of the tire. So companies reinforce their high-speed-rated tires with cap plies. Bridgestone, for example, uses nylon for its cap plies, as nylon shrinks when hot (speed generates heat) to fight those centrifugal forces.
High speeds also create bulges in a radial tire's sidewall. If that bulge happens too fast, or if a tire develops multiple sidewall bulges, the heat generated by the bulging can tear a tire apart. Tires with high speed ratings have extra reinforcement in the sidewalls to stiffen them and prevent high-speed bulges.
Now, both of those technical aspects have a benefit, even if you're not going 150 mph. Cap plies under a tire's tread help keep it flatter on the ground, which gives it a larger footprint and adds stability. A stiffer sidewall translates directly to crisper handling, since more of the cornering forces are actually changing the car's direction and not causing the sidewall to roll over. This is a difference you can feel even at 30 mph. Even if you're planning to keep your car at legal (or near-legal speeds), you can still benefit from a tire designed to go 130 mph or more.
We know most of you are going to supersize your rims when you upgrade. To avoid as many fitment hassles as you can, try to keep the overall outside diameter of your tire the same as the wheel grows inside it. That's a concept called "plus sizing." For each inch in diameter the wheel grows, the sidewall has to shrink a corresponding inch to keep the overall diameter the same. Why is that so important? There are a lot of computerized engine and driveline functions that are based around how far your tires travel when they make one revolution. When a tire guy talks about rpm, he means revolutions the tire makes per mile, not engine speed per minute. Changing the overall diameter will change the tire's rpm, which can throw off your speedometer and antilock brake system and also affect how quickly the car accelerates.
Going from a 15- to 16-inch rim is considered plus-1. From a 15- to a 17-inch rim is plus-2, and so on. Here's an example of how you can plus-size tires straight from a popular tire retailer's Web site. Say you're starting with a stock combo of 205/65R15 tire on a 15x7 rim. A plus-1 conversion would up the wheel size to 16x7.5, and the tire would change to a 225/55R16. A plus-2 conversion would mean going to a 17x8 wheel and a 235/45R17 tire.
Note that tire sidewall height isn't the only thing affected by plus-sizing. The tire's width grew, too. When sidewalls get shorter, the tire needs to get wider in order for it to maintain the same amount of air volume, and therefore maintain its load carrying capacity. If you don't keep the air chamber the same size, load capacity is reduced. That means the tire will have to work harder to hold up your car, which can generate excess heat, the leading cause of tire failure.
In some cases there's fudge room with a tire's load capacity. If you have a small car, or one that you've lightened for racing, you may be able to get away with using a tire that has a lighter load rating. But be very careful if you go down in load rating, and double-check your intent with a tire pro before you make the change.
Reading a Sidewall
There's a ton of information about your tires printed on the sidewall. Size, sure, but also speed rating, load-carrying capacity, treadwear, traction and temperature grades, recommended inflation pressure, and more.
Here's what the size numbers mean. For a 205/55R16, the 205 designates the tire's section width (sidewall-to-sidewall) in millimeters; 55 is its aspect ratio, or the ratio of its height to its width (the lower the number, the shorter the sidewall); and 16 is the wheel diameter that will fit inside the tire.
These days the tire's load index and speed ratings follow the size numbers. The load index is expressed by a two-digit number and the speed rating is indicated by a letter-we've included charts in this story to translate what those numbers and letters mean. Then there's the Uniform Tire Quality Grade, or UTQG. These are ratings, required by the U.S. Department of Transportation, of the tire's treadwear, traction, and temperature resistance.
Treadwear is expressed by a number that's relative to a baseline wear number of 100. So a tire with a treadwear rating of 200 will wear twice as long as the baseline, a 600 rating would wear six times as long, and so on. As a rule, high-performance and ultra-high-performance tires have lower treadwear numbers than mass-market tires due to their softer tread compounds. A max-performance Bridgestone Potenza S-03 Pole Position has a treadwear rating of 220, while a high-performance Potenza RE950 is rated at 400, and an all-season BT70S carries a 460 treadwear rating.
Traction grades the tire's ability to stop on wet pavement and is expressed by a letter grade, with A being best and C being acceptable.
The temperature grade describes the tire's ability to resist heat generation during laboratory testing. It, too, is expressed by a letter grade, with A being best and C acceptable. The government won't let a company sell a tire in the U.S. that receives less than a C grade.
After that, the data on the sidewall isn't all that interesting. One thing to note, though: The inflation pressure listed on the tire is the maximum inflation pressure, not necessarily the inflation pressure that's recommended for that tire on your car. That info can usually be found on a sticker in the doorjamb, or in your owner's manual. Recommended inflation pressure is, by the way, for the tire when it's cold. So you should always check the air in your tires when they're cold. What's "cold" according to tire manufacturers? Do it first thing in the morning, before you start driving. Or wait three hours after you stop driving. Otherwise heat in the tire will raise the air pressure, and you won't get a true reading.
There are tons of different tread patterns out there, but they generally fall into four different types: directional, nondirectional, symmetrical, and asymmetrical. The first two terms have to do with the pattern as it goes around the tire. A directional tire has a tread pattern that is designed to grip best with the tire rotating in one direction only. These tires are pretty easy to spot; they're the ones with a tread pattern in a V or arrow shape. These have to be oriented a certain way when mounted so that the pattern is pointing in the right direction. Just in case there's any doubt as to which way that is, there are usually arrows printed on the sidewall that point in the direction of proper rotation.
A nondirectional tire has a tread with grip characteristics that work with the tire rotating in either direction. There's no need for special mount orientation for this type of tire.
The last two terms have to do with the design of the pattern as it moves from the inside to the outside of the tire. A symmetrical pattern, as you'd guess, is the same sidewall to sidewall. An asymmetrical pattern, on the other hand, will look different as you move across its face. For high-performance road applications, an asymmetrical tire often has large tread elements on the outside. This puts big blocks of solid rubber in contact with the road while cornering. The inside of the tread is more cut up, with lots of voids and channels for water removal. As with a directional tire, an asymmetrical tire has to be oriented when mounted so that the proper side of the tread faces outward. And as with directionals, there are usually instructions printed on the sidewall as to which side should face outward.
Don't confuse tread patterns with tread compounds. The pattern is the design of the blocks on the tread surface; the compound is the chemical makeup of the rubber used to make the tread. Patterns are designed in such a way as to provide biting edges for grip and to channel water between the tire and the road. Compounds are made up so as to provide certain traction and treadwear qualities. A soft or sticky compound will provide excellent traction but will wear out quickly. Treadwear and traction ratings can be found on your tire's sidewall in the UTQG section.
As with tires, wheels fall into some basic categories, all of which have to do with their construction. Almost all of the high-performance and aftermarket wheels in the import tuner segment are made from aluminum alloy, although back in the day people used to call them mags. Mag was short for magnesium wheels, which were popular with racers in the 1960s for their lightweight character. Mag wheels burned when they got too hot though, so their use was short-lived.
Why aluminum? It's lighter and stronger than steel, and weight is a big issue with wheels. Along with the tires and brakes, wheels are considered "unsprung" weight, meaning they're not supported by the vehicle's suspension. If you can reduce the weight of your wheels, you'll find your handling to be crisper and more responsive, as the steering system won't require as much effort to push them around. Plus, taking off weight in general helps you to go faster. So an alloy wheel is a win-win situation.
How that alloy is made into a wheel makes up the next category. Two different processes are used to make wheels or wheel components: casting and forging. To cast a wheel, molten aluminum is poured into a mold and allowed to cool, which forms the wheel's basic shape. When it comes out of the mold it's machined to remove excess aluminum, cut in the bolt pattern and hub circle, and sized to the mount pad. Then the wheel is finished by polishing, painting, plating, or some combination of the three. With forging, aluminum billets are pressed into dies under extreme pressure, which shape the billet into the wheel. That wheel then goes through a machining/finishing process like the cast wheel.
In general terms, forged wheels are stronger than cast, since forging doesn't alter the aluminum's molecular structure like casting does. Since the forged aluminum is stronger, less of it can be used without sacrificing structural integrity, which can ultimately result in a lighter wheel. Forging is not as design-friendly, however, as it's hard to create curved spokes and intricate wheel-center designs in the forging process. So you'll notice that most forged wheels have straight, flat spokes. Forged wheels generally cost more too, as it is more expensive than casting.
This doesn't mean a cast wheel isn't strong or can't be lightweight. It's just that the casting itself requires more aluminum, to make sure the metal is dense enough throughout the mold's nooks and crannies. Using more metal adds weight, though some companies, like RO_JA, are experimenting with new casting techniques to shave metal from the wheel and achieve forging-like weights. RO_JA's new 17x7 Neo LT 5 wheel, for example, tips the scales at a svelte 15 pounds, whereas cast wheels of that size typically weigh 20 pounds and more. Casting also allows more design freedom, as the molten aluminum will follow the contours of intricate centersections.
Some wheels bring together both worlds, as they combine a forged outer hoop with a cast center. Which brings us to the topic of one- and two-piece wheels. A one-piece wheel is just that: a single unit, with its center and outer rim created together. One-piece wheels can be cast or forged. Two-piece wheels are made from outer rims and center sections that are separately produced, and the two pieces are welded or riveted together to form the wheel. With two-piece wheels, the rims are generally spun or forged, and the centers are cast or forged.
Each wheel has its advantages. One-piece wheels are generally less expensive to make and are produced for high-volume applications (with an offset that will fit many different cars, for example), so they typically cost less than a comparable two-piece wheel. But a two-piece offers greater fitment flexibility, as the centersection can be welded in a number of places within the rim to create a variety of offsets. This is especially helpful when trying to fit a car where the front and rear offsets don't match, like an S2000, or when you need an ultradeep offset to fill a widebody kit's fender well.
Talking about offset brings us to the subject of wheel sizing. There are several dimensions to consider when upgrading your wheels: rim diameter, rim width, bolt pattern, and offset. Rim diameter and width are easy to understand. The bigger the better, right? Well, sort of. There is a finite amount of space in your wheelwell to fit new rims and rubber, and if you've slammed the car there's even less. You don't want to go so big that the tire or wheel makes contact with the fenderwell, steering, or suspension components. Remember, too, from our discussion of plus-sizing that wheels need to get wider as they get taller to maintain the tire's load capacity, and that extra width can interfere with the chassis.
Something else to consider: As the wheels get bigger and the tire sidewalls get shorter, the car's ride is going to get stiffer. A tiny band of a tire can't protect those precious rims from road hazards-potholes, especially. So keep your real-world driving circumstances in mind when choosing a big rim, or you could find yourself buying replacement wheels at an alarming rate.
When choosing a wheel, make sure it fits your car's bolt pattern. That pattern is expressed as an equation of two numbers: the number of wheel lugs or bolts on the hub times the diameter of an imaginary circle drawn through the center of those lugs. So a 4x100mm bolt pattern means you have four lugs that span a distance of 100 mm.
Offset describes the relationship of the wheel's centerline to the location of the mount pad, which is where the wheel bolts to the hub. This is typically expressed in millimeters. Front-wheel-drive (and newer rear-wheel-drive) cars generally have wheels with positive offset, which means the mount pad is towards the outside of the wheel. A wheel with negative offset means the mount pad is positioned inboard from the centerline.
Proper offset is critical when fitting a new set of wheels, as it affects everything from the car's handling to wheel and tire clearance of fenders, brake calipers, and chassis components. All wheel companies have fitment charts that list offset specs to match wheels with a particular car. Or if you're facing a unique situation, like if you've installed a widebody kit, many manufacturers can customize a set of two-piece wheels with the right offset. This will take time and cost money, but the ultimate result will be well worth the investment.
All of the major tire manufacturers have good technical information about their products on their Web sites. But we found one that goes a step further. Tire Safety.com (www.tiresafety.com), from the folks at Bridgestone-Firestone, offers thorough tire tech and maintenance info, without the ad banners you'd expect for its own tire brands. Plus you can sign up for regular e-mail reminders about keeping your tires properly inflated. So, when was the last time you checked your tires?
Have we reached the outer limits of wheel diameter and low-profile tires? Not yet. This 305/35ZR24 BFGoodrich g-force T/A KDW was the edge of the envelope not long ago, but there are even bigger rims available now. These humongous sizes are primarily for SUVs, but it probably won't be too long before some kink tries to put a deuce deuce on his Civic.
What's Hot in Wheels?
Hey, if we had the answer to that question we'd be in the wheel business, not the magazine business. But when we asked some of the folks who are in the wheel business about trends, this is what we heard:
· Wheel diameters keep getting bigger. Wheels for SUVs in particular are going haywire, with diameters running as big as 28, even over 30 inches. The folks at Motegi and RO_JA wheels say they're getting lots of calls from Civic owners for 19s these days.
• As body kit shapes change, so will wheels. Widebody kits need extra-deep wheels to fill the fenders, and the wheel companies are stepping up to the challenge.
• There are new wheel finish options available. New paint and machining techniques are allowing manufacturers to offer different colors than before-gun-metal gray is real popular - and also the combination of painted and machined surfaces on the same wheel.
Because of the way they're produced, cast wheels are generally heavier than forged wheels, but not always. RO_JA is experimenting with new casting processes for its Neo LT 5 wheel to make it as light as a similar-sized forged rim. By reducing the rim's wall thickness and taking metal out of the cap tower (the wheel is designed to run without a cap), a 17-inch LT 5 weighs just 15 pounds.
But you know, by the time you read this, there may be new trends at work that will yet again change what wheels look like. Wheels are like fashion in that way-subject to changing tastes. Check out our wheel guide in this issue to see what we mean.
A tire earns its speed rating based on lab testing, during which it is pressed against a drum (to approximate a load) and then run at increasing speeds until its required speed has been met. That required speed is expressed as a letter (see below).A few years ago it was believed that a Z rating (149-mph plus) was the highest a tire could achieve. But with the recent introduction of supercars that can surpass that number, new W and Y ratings were developed.
This is a number assigned to a tire that represents its ability to carry a certain amount of weight. The number has no relation to the actual load capability, it's merely a number that's relative to others in the index. There are too many load index numbers to list them all here, so if you're interested you should be able to find this data on any tire maker's site.
Note that a particular brand, type, and size of tire can have different load ratings. For example, a 205/55R16 Goodyear Eagle RS-A high-performance tire comes with either an 89 rating (1,279 pounds) or a 91 rating (1,356 pounds). When choosing a replacement tire, make sure you either equal or go above the OE load rating index, or you risk overheating your tires by making them work too hard to carry the weight of your vehicle.
Wheel Manufacturing: An Inside Look
During our research for this story, we were lucky to get an inside tour of the manufacturing facilities that make Motegi Racing and RO_JA wheels. While the processes are pretty complex (and some of them are so proprietary that we weren't allowed to photograph them), here are a few images that'll give you an idea of how one- and two-piece wheels are made.
A. These huge Borli machines cast aluminum wheels. Molten aluminum is siphoned up from tanks at ground level and fed into molds above, where it then cools to form a wheel. The large gray tank that the hard-hatted worker is standing next to is feeding aluminum into one of the molding station's tanks. In its liquid state, the aluminum's temperature is about 1,300 degrees F; when a wheel comes out of the mold it has dropped to 900 degrees. It takes about seven minutes for a large (20- to 22-inch) wheel to form in the mold, about four minutes for a smaller (15-inch) wheel.
B. Here's a rack of Motegi MRM wheels as they've come out of the mold. Note that there's extra aluminum-called casting flash-in the spokes and on the rim, and the center has yet to be machined.
C. Compare this finished Motegi MRM wheel with the raw casting. The MRM is distinctive for its center finish, which is both machined and painted for a nice 3-D effect.
D. Making a two-piece wheel actually starts with the spinning of these hoops, but that is done elsewhere, so we had to begin here. These will become RO_JA Furious 5 wheels.
E. The hoops are put in this machine, where they're spun and shaped to the spec.
F. Once the hoops are shaped, they're matched with the forged centers and await mating. How romantic!
G. The hoops are heated (so they'll expand a bit) and then the centers are put in place. When the hoops cool they'll hold the centers as they're welded in place. In between all these steps the wheels are checked for proper runout to make sure they're true both laterally and radially.
H. Here's a welding station shown just before the curtain closes and the wire feed begins. The solid plate on the inside of the wheel is a spatter shield, to keep the welding spatter from marring the wheel center. After welding, the wheel goes off to be machined and finished, just as the cast wheels do.
I. And here's what the finished Furious 5 rim looks like after final machining and finishing.