Saturday, September 23, 2017 Detailed Auto Topics
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The wheel studs and lugs on older vehicles are very basic. Most are 7/16 or half inch diameter and they all work about the same. Modern vehicles use dozens of metric and standard sizes with several thread pitches. They also center the wheel on the vehicle differently and using the wrong application may be dangerous.

A few of the wheel lug styles in common use

 

They design original equipment wheels to be hub centric

The difference in hub centric and non-hub centric wheels

The outside diameter of the hub fits tightly into the inner diameter of the wheel. This very accurately centers the wheel on the vehicle, preventing run out. Some aftermarket wheels are designed to fit multiple applications. The diameter of the center hole in the wheel is large enough to fit over the largest hub they accommodate. These wheels rely on the proper lug nut to center the wheel. If the wrong style lug nut is used, the wheel will not be centered.  This will cause a vibration and the lug nuts may even come loose.  This can cause a wheel type vibration, even after repeated attempts at balancing the wheels

Even with original equipment wheels, they use several designs of lug nuts.  Some have a conical seat, which is slightly rounded where it contacts the wheel.  Other vehicles have wedge based lugs, flat faced lugs and mag style lugs to name a few. With the wedge base, an angular face matches the same type seat in the wheel. These appear similar to a conical seat lug, but Interchanging conical and wedge base lugs can cause the stud to break and the wheel to come off.

Different wheel lug designs in common use

With the flat faced lug, used on Ford products, the lug provides no centering at all. The flat face holds the wheel tightly against the hub. The hub or axle flange centers the wheel.

With the mag style lug, the shoulders of the lug fits down into holes of the same size in the wheel. A thick washer is also used to aid in clamping the wheel down. In non-hub centric applications these type lugs are often used in an attempt to center the wheel.

Matching the proper type lug to the wheel is imperative and mis-matches can cause major safety issues. It is also imperative to match the lug thread diameter and pitch with the stud. Some applications will thread partially on, but will quickly destroy the threads.

Tightening lug nuts

Lug stud permanently deformed from over tightening 

When tightening lugs, regardless of the style, we should always us a torque wrench.  Lug studs operate as very powerful springs. As the lug is tightened, the material of the stud is stretched. This stretching provides the clamping force that holds the wheel tight. Too much torque can permanently stretch, weaken and break the stud.

Lug nuts and wheels are permanently damaged by over tightening

Excessive and improperly applied torque can also warped the hub. When this occurs, the brake rotor, which also fits over the hub can be warped. Over tightening can also collapse the hole for the drive axle, on front wheel drive applications. When this happens, future removal of the axle can become impossible.

Lug torque varies considerably between vehicles. The maximum torque for one vehicle may be insufficient to hold the wheel on another. Professional technicians always consult the manufacturer’s service data.  The owner's manual of your vehicle will usually provide lug torque, listed under specifications.  Lugs should also be tightened in a specific sequence. This allows the load to be spread more evenly. Most technicians also feel tightening lugs in steps is beneficial. For instance, if the specification is 90 foot pounds, they would all be tightened to thirty foot pounds, then sixty and finally ninety.

Wheel Lug Torque Specifications

Wheels lugs have a specific tightening sequence

If no specification is available, the following chart based on lug diameter, is a rough guide. This should never be used to replace manufacturer's specifications and is only an approximation. Lugs should be tightened to proper specification as soon information is available.


 Stud Diameter Lug Torque
 10 MM Stud 50 Foot Pounds 
 12 MM Stud 80 Foot Pounds
 14 MM Stud 90 Foot Pounds
 7/16 Inch Stud 75 Foot Pounds
 1/2 Inch stud 85 Foot Pounds
 9/16 Inch Stud 140 Foot Pounds
 5/8 Inch Stud 160 Foot Pounds

Lastly, it is important to realize the specification for lug torque is a dry specification. Lugs should NOT be lubricated. Torque is a resistance to turning that relates to tightness. Lubricating lugs will drastically reduce this resistance to turning and the same torque will stretch the stud far more.

Like most things on automobiles, lug nuts appear deceptively simple. In fact wrong procedures can result in damage and worse. A few simple precautions can help prevent a very dangerous situation.





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