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Wheel Studs, the Driving Force

by Wayne Scraba
Drag Racing USA, May 2000
 
Remember the old story about a chain being only as strong as its weakest link? That same analogy applies to the drivetrain in your race car. One component that is often ignored is the wheel stud. The loads encountered by the wheel stud might be much larger than you think.
Look at the basic math (not taking traction into consideration): engine torque times torque converter multiplication times transmission first gear ratio times rear axle ratio. Using a typical mild small-block powered bracket car, the loads can exceed 10,000 ft-lb of torque at the axle. Arguably, there are two axles and ten studs to distribute load over, but it's still a bunch.
Because of the aforementioned load factor, a good option for any race car is a "drive stud." These huge studs measure a full 11/16 inch in diameter. Designed to fit the holes in aluminum race-style wheels (Centerline, Cragar, Weld, Monocoque, Bogart, Holeshot, etc. ), the studs incorporate and equally huge 5/8-18 inch axle thread (the portion of the stud that screws into the axle).
A good example of a quality drive stud is manufactured by Mark Williams. These studs have 11/16-inch diameter shoulders for use with racing wheels.
The drive stud is threaded into a 5/8-18 inch thread in the axle flange and secured with a jam nut. Wheels are held on with an open-ended flanged lug nut and aluminum washer. The MW drive studs incorporate a smooth shoulder, which physically drives the wheel. In comparison, a street lug nut for aftermarket wheels has a built in shoulder that drives the wheel. These street oriented lug nuts are designed for use with small diameter studs (1/2 inch being the largest). In simple term, the street models use much smaller diameter studs and the actual lug nut serves to drive the wheel.
Obviously, this isn't the most efficient way to handle large torque loads. What get confusing is the number of different combination of stud lengths and washer thicknesses available (we've seen Pro racers baffled by the variety). The reason for the multitude of stud/washer combinations is the range for wheel center thicknesses on the market, coupled with the actual thickness of the brake drum or disc brake hat and wheel spacer (if used).
According to Mark Williams, the most important factor when choosing the proper stud is that the driving portion of the stud is fully engaged into the wheel. The smooth "drive" segment dimension of the stud needs to be slightly grater than the combined thickness of the brake adapter/drum and the thickness of the wheel. Meanwhile, the washer thickness should be greater that he shoulder extending past the wheel. When ordering a set of drive studs, you must specify the wheel center and brake hat thickness.
In conjunction with the drive stud system, MW has developed a new reduced hex (7/8-inch) "Base Nut" series for use with the drivestud wheel retention system. "Base Nut" are produced in both 17-4 stainless steel and titanium alloy. The aluminum spacer washer attaches with a snap fit over the base nut; it spins freely but won't separate from the nut, which prevents marring the wheel surface. This also eliminates the problem of losing the washers when swapping tires. The washer is available in different thicknesses (1/8 inch to 3/4) inch) to compensate for different wheel and brake hat thicknesses. The washer can be changed if the wheel thickness changes.
By using stainless steel and titanium alloys, rust or corrosion problems are obviously eliminated. These nuts are produced on CNC machines from bar material. This makes the nut a dimensionally (and visually) superior component. Because of the manufacturing method, the thread pitch diameter is extremely square with the flange. This produces an even grip pressure load on both wheel and stud.
You can purchase drive studs and accompanying lug nuts in exotic material (i.e. titanium alloy). These save 1.2 pounds over a set of conventional drive studs, but be prepared to pay for the weight loss. Typically, they cost $290.00 a set, versus $85 for conventional steel drive studs.
As you can see, there's a bunch more to lug nuts and wheel studs than first meets the eye. Never take these components for granted.