ROCHESTER HILLS, Mich. (July 7, 2003)—Proper torque is not something that's essential to only commercial vehicle maintenance.
In the April 28 issue of Tire Business, I talked about the importance of maintaining wheel system studs and nuts to achieve proper torque on commercial vehicles.
Then TB re-ceived a request from a reader for similar information dealing with wheel systems on passenger cars and light trucks. So in this issue I'm departing from my normal focus on commercial vehicles to deal with these smaller conveyances.
I must confess, I'm no expert when it comes to passenger and light truck tires and wheels.
So I turned to my good friend Kevin Rohlwing, senior vice president of education and technical services with the Tire Industry Association (TIA), for his technical assistance.
Some of the information presented here comes directly from TIA's Automotive Tire Service (ATS) Program, which is scheduled to be introduced this summer.
If you are a commercial truck tire dealer and still reading this column, you're probably wondering, “What's the big difference between wheel systems? Don't the same principles apply to both? Isn't one just bigger than the other?”
Well, basically yes. But it gets more complicated when you are dealing with automobiles as opposed to trucks.
A nightmare revisited
In the commercial vehicle arena, truck manufacturers do not feel the need to design a wheel system for each and every truck model they build.
They use one of three wheel systems—spoke wheel/demountable rim; hub piloted disc wheel; and stud piloted disc wheel—and then encourage fleets to spec their standard system. (Hub piloted is the standard now for most truck OEMs.)
The automobile companies, however, look at things differently. Instead of having only three systems that have specific ranges of torque that should be attained on the whole range of heavy-duty vehicles, each and every car/light truck has its own specific torque that has to be achieved.
This is virtually a nightmare!
Also, instead of having eight- or 10-bolt-hole systems on disc wheels as are employed on trucks, auto technicians are forced to deal with four-, five-, six-, eight- and 10-bolt-hole wheels.
Trying to figure out the crisscross pattern on these wheels can really make you see stars.
But let's start at the beginning. Passenger and light truck wheel systems operate under the same laws of physics and torque-tension as wheels for larger vehicles. Torque is a turning or twisting force that is used to create tension. It is the resistance of the nut to turn and is expressed in foot pounds (ft. lbs.).
Tension is the stretching of a bolt or stud when a fastener is applied to create clamping force.
Clamping force is the tendency for a fastener to remain in compression and is expressed in pounds of tension. Clamp force is what actually keeps the wheels on.
To put this information in the proper context for you, we could say, “It takes 100 ft. lbs. of torque to produce the 6,000 pounds of tension or clamp load needed to keep the wheels from flying off the car.”
Tension can only be measured in a lab with very sensitive gauges by engineers in white lab coats, of course. Torque—the only force you and I can calculate in the real world on a vehicle—is measured with a torque wrench. So that is why torque is important to anyone actually involved with changing tires.
When torque is insufficient, the wheel becomes loose and begins to wobble.
This elongates the bolt holes and puts enormous stress on the wheel studs or lug bolts.
Ultimately, the lug nuts will either fall off or the studs will break and the wheel will come off. This can be very disconcerting to the driver.
When too much torque is applied to a wheel, really bad things happen as well. Studs actually stretch a bit as they are put into tension when the nuts are installed.
When the nuts are removed, they spring back to their original length. However, when they are overstretched due to overtorquing, they “yield,” meaning they do not spring back and subsequently lose their ability to retain tension.
As a result, you get no clamp force at all. In addition, the bolt diameter is reduced as the stud is stretched and breakage of the stud is imminent. Even if the stud doesn't actually break, though, once it has “yielded,” it has failed.
Overtorqued lugs typically result in broken studs. They're a common cause of vehicle vibration problems since they can distort the wheels, brake discs and/or drums.
In fact, when customers complain of a brake pulsation immediately following a tire change, it is usually the result of overtightened lugs. At that point the affected rotor or drum must be replaced. Improper torque also can cause fatigue cracks and alignment problems.
As you can see, tightening lugs to the recommended torque setting is extremely important.
To ensure that the proper amount of clamping force is applied to each wheel lug, follow these guidelines:
* Inspect all parts including wheels, studs and mounting faces of hubs and wheels for dirt, rust or damage.
* Make sure the area around the bolt holes, as well as the wheel and hub mating surfaces, are clean and dry.
* Use a wire brush to remove corrosion and other debris. Then wipe the areas clean using a shop rag.
* Inspect the bolt holes for damage. Do not put a wheel back in service if the bolt holes are deformed.
* Inspect the wheel studs and lug nuts or lug bolts and bolt holes (depending on the vehicle) for damage.
* Replace or repair any damaged or worn components. Use only the specified size and types of studs and nuts for the vehicle being worked on.
* Determine the proper torque for the wheel nuts on the vehicle.
* Always follow a star or crisscross pattern when tightening lugs.
All of the above steps are equally important to achieving the proper torque and in-stalling the wheel correctly on the vehicle. But determining proper torque is the one that makes my head spin and is a point many people ignore.
Auto companies design wheels for almost every model car and light truck they produce.
Each wheel uses different grades of material and degrees of heat treatment for the fasteners and is designed to carry different loads.
Thus, distinct amounts of torque are required for every vehicle. Therefore, technicians must look up the specified wheel lug torque for each vehicle they service.
These specifications can be found in the “Tire and Wheel” section of most vehicle service manuals and also can be found in Lug Nut Torque Charts that will be available soon from TIA.
You can't just use a standard torque like 450-500 ft. lbs. used on all commercial truck disc wheels. The torque specifications listed on the 2003 Lug Nut Torque Chart range from a low of 43 ft. lbs. on 1989-1990 Chevrolet Metros to 468 ft. lbs. on the 1990 Ferrari F40.
Unbelievably, some torque specifications are in tenths of pounds, such as 72.3 ft. lbs. on the 1991 Isuzu Amigo with aluminum wheels and 47.5 ft. lbs. on 2000-2001 Suzuki Swifts.
About half of the specifications listed on the chart are for one torque value.
The others specify a range of torque that is acceptable. When specifications are indicated as a range, you should choose a value in the middle of the range to compensate for normal variations in tool calibration.
Although proper wheel lug torque is important, it does not guarantee that the required clamping force will be achieved.
Rust, dirt and debris on stud/bolt and nut threads increase friction, which is the resistance to motion when a nut is installed on a stud/bolt. This additional friction sucks up clamp load.
As a result, instead of getting 6,000 pounds of clamp load when 100 pounds of torque is applied, only 3,000 pounds is attained—which may not be enough to keep the wheel on, even though 100 pounds of torque is measured on the torque wrench.
That's why cleaning fasteners is so important. To improve the chances of achieving the precise clamping force between the wheel and hub, proper torque must be applied to clean components that are in good condition.
When wheel lugs are tightened using an impact wrench alone, an overtorque condition is almost certain to result.
Modern impact wrenches have torque outputs of 300-600 ft. lbs.—or more than three times the average torque required for a typical automobile, sport-utility vehicle or light truck.
Although most impact wrenches provide variable power settings, these tools are not effective torque control devices. Many people use torque sticks to install wheels.
A torque stick is essentially a drive extension with a built-in wheel lug socket.
Unlike a conventional extension, however, a torque stick is designed to twist, similar to a torsion bar in a suspension system. The adapter attaches to an impact wrench.
Once the amount of force applied to the adapter exceeds its maximum torque rating, the bar begins to twist.
This limits the amount of torque applied to the lug. Torque sticks come in a variety of sizes and strengths to accommodate a wide range of vehicles.
The torque rating is stamped on the drive-end of the tool, and the sticks are color-coded according to lug size. To ensure the lugs are tightened properly when using a torque stick, follow these guidelines:
* Make sure the running air pressure at the impact wrench is at least 90 psi. Low pressure will result in an undertorqued lug.
* Never wrap your hand around the torque stick while tightening a lug.
* Keep the impact gun running until the torque stick stops turning.
* Always tighten the lugs in a star or crisscross pattern.
Torque wrench is best
Whether you're tightening truck wheels or automobile wheels, using a torque wrench is still the best way to ensure you have attained the proper torque level. It is the most time-consuming method for tightening wheel lugs, but it's also the most reliable.
When using a torque wrench to tighten the lugs, the process can be speeded up by snugging the lugs in a star pattern using an impact wrench on the lowest power setting.
If the impact wrench does not have a variable power setting, the lugs should be secured by hand.
Always use the crisscross or star pattern to install wheels. This is very important. Failure to tighten nuts in the crisscross sequence will cause a misalignment of the wheel.
Once the lugs are secure, the vehicle should be lowered so the tire contacts the ground but the hydraulic lift is still supporting most of the weight. At that point, the lugs can be retorqued in a star pattern.
When using a clicker-type torque wrench, apply force to the end of the tool until the audible click is heard.
Stop applying pressure as soon as you hear the click, since additional pressure will cause the lugs to become overtorqued.
As with truck wheels, the vehicle should be driven for a short distance and the nuts then rechecked for tightness.
Parts will usually seat naturally and torque on nuts will drop. As a result, it is necessary to retighten all nuts to the specified torque. Otherwise, you'll once again have a very disconcerted driver.