The clamping force created by fasteners is what allows wheels to function properly. All fasteners are intended to stretch slightly when tightened. This stretch is measured indirectly by the torque (measured in foot-pounds) required to turn the fastener.
If the wheel fasteners are not tightened properly, the wheel may loosen, resulting in wheel damage or, in the worst case, the wheel separating from the vehicle – a wheel-off. The consequences of a wheel-off can be staggering; liability claims have run into the millions.
As a result, some people over-tighten wheel fasteners. Unfortunately, over-tightening is a huge problem and can be as unsafe as under-tightening.
Over-tightening stretches the stud or bolt past its yield level. Once stretched past its yield level, a fastener will not return to its original dimensions. It is permanently weakened and it may break during installation or when the wheel is being removed. Even if it does not break, the weakened fastener may no longer hold the wheel securely.
A proper installation begins with checking the wheels, fasteners and mounting faces of the hubs and wheels for dirt, rust or damage. Use a wire brush to remove debris and rust and then replace any damaged parts. For obvious reasons, avoid cleaners that contain lubricants.
Make sure there is nothing that has the potential of vibrating loose or affecting the clamping force between the wheel and the hub. A thread chaser or tap should be used to remove any burrs or obstructions on the threads. The fastener should be easy to turn by hand until it meets the wheel’s fastener seat.
Wheel fastener torque must be set to the recommended specification for the particular vehicle. Torque specifications are typically for clean threads that are free of dirt, grit and foreign materials, including lubricants (heavy-duty trucks are an exception, 30 weight motor oil is specified for hub-pilot style lugs). Lubricants change the properties of a fastener and it will not torque the way it was designed.
Despite cautions against the use of lubricants, the use of anti-seize compound is popular, especially in areas where corrosion is a problem. If applying anti-seize, it is important that it only be applied to the fasteners, not the stud. The anti-seize compound must not be used on either the seat of the hardware or on the wheel.
The seat of the fastener is the main point of friction where torque is measured, so extreme caution must be used as excess anti-seize compound can either drip or be pushed onto the fastener seat, resulting in inaccurate torque values.
Next install the wheel, being sure to hold it tightly to the hub, and hand-tighten the fasteners. Once the fasteners are all started, the use of a tool with limited torque is recommended for consistency.
Tighten the fasteners following the traditional star pattern sequence shown at the right. This is very important even at the pre-torque stage. At this point, all that’s needed is to get all of the hardware even, with no looseness or wobble in the wheel – 50 to 60 foot-pounds is enough for a good, snug fit.
Brace the wheel to prepare it for final torque. Especially on heavy vehicles with high torque specifications, completely lowering the vehicle and putting all of its weight on the wheels is not recommended. Only lower it down enough to prevent the wheel from rotating during final torque. Wheel chocks can be used if necessary to lock the wheel in place.
Before applying final torque, set the torque wrench to the specifications provided by the wheel or vehicle manufacturer. Torque specifications vary widely, but the manufacturer’s recommendation will always be a safe bet. The star pattern sequence should be used until all of the fasteners are tightened to the specified torque.
It’s best to have a regular procedure that’s used on every vehicle. By doing it the same way every time, all of the wheels on every vehicle will be done exactly the same. It’s important to be consistent; everybody in the shop needs to know how wheels are to be mounted and those procedures must be followed every time. A regular procedure that is followed can be an important shield against liability claims. A procedure that is not followed is a smoking gun for a plaintiff’s attorney.
|Calibrating Torque Tools is Vital|
| According to the experts at Team Torque, implementing a torque program takes little time and is well worth the effort. Tire dealers with a torque program see drastically reduced wheel-offs, reduced concerns withcustomers and reduced company liability exposure – but everything starts with properly calibrated torque wrenches.
To determine the calibration cycle of your shop’s torque wrenches, you must consider the manufacturer’s recommended calibration cycle (generally 5,000 clicks or annually), the number of wheel services performed per day requiring the use of a torque wrench, the total numberof wheel fasteners torqued, and how many torque wrenches your shop is using.
5,000 maximum clicks per wrench, divided by 1,000 clicks per day= every 5 days this wrench should be calibrated.
5 wrenches in one shop = monthly calibration needed for all 5 wrenches.
Ongoing ISO:17025 calibration and repair of torque wrenches is a good start. Other best practices include a documented torquing process and training on proper tool usage.
Resist the temptation of giving the wrench an extra click or two. On some cars, exceeding the torque value by 30-40 foot pounds will ruin the fasteners. That extra push defeats the purpose of the torque wrench. When it clicks (or, if it’s electronic, does whatever it does to signal the proper torque has been achieved), stop.
Many dealers have adopted practices to ensure proper star pattern sequence tightening, the use of suitable sockets and verification of accurate torque. In some shops, a second technician or a supervisor verifies that all of the fasteners have been properly tightened. At an even higher level of sophistication are the fairly new electronic systems that keep a record of the technician, date, time and torque applied.
Especially when installing new wheels, the wheel fasteners should be re-torqued after 25 to 100 miles. The clamping force may have changed since the initial installation due to metal compression or elongation, and thermal stresses can affect the wheels. Parts will seat themselves and fastener torque may drop. Wheels should cool to ambient temperature before rechecking torque. All that is necessary is to loosen and retighten each fastener to the specified torque, in sequence.
Some shops suggest that customers bring their vehicles back for re-torquing whenever the wheels are removed and replaced. They believe that retightening wheels gives the shop an opportunity to build its reputation for thoroughness and safety.
What about torque sticks? A torque stick is used with an air impact wrench to avoid over-tightening fasteners when installing wheels. Torque sticks work by flexing (like a torsion bar) when a torque limit is reached. When the stick flexes, it resists further tightening of the fastener. The thicker the torque sticks, the higher the torque to the fastener.
Torque sticks are useful, but they are no substitute for final torquing using a calibrated torque wrench.
The proper procedure for using a torque stick begins with hand-tightening the fasteners. Then use the proper torque stick on an impact gun to tighten the wheel until there is tension on the fasteners and the wheel pulls up tight. Lightly go over the fasteners again to tighten them a little more. Then tighten each fastener again until it stops moving. Then drop the vehicle to prevent the wheel from turning and use a torque wrench to do the final torquing.
There are some very important cautions when using torque sticks. First, they should not be used to get to anywhere near final torque. Select a torque stick rated for about 2/3 of the final torque and then finish the last 1/3 of the specified torque with a torque wrench. If the fasteners don’t move with a torque wrench before the click for final torque, there is a problem or the fastener has been over-tightened.
Many techs don’t realize that torque sticks are designed for a limited range of torque input. They must be used with an impact wrench, and if an impact tool with too high, or too low, of a setting is used, the torque stick will still over-torque the fasteners and may cause damage.
The impact gun must be set to a specific torque range and the air supply and pressure must remain constant. Also, different torque sticks often will require different impact wrench settings. It is unrealistic to expect a wrench/torque stick combination to be accurate without checking the impact wrench setting.
An impact wrench can be calibrated by first using a torque wrench to tighten a nut to the torque that matches the torque stick. Then, connect the air supply to the impact gun using an air regulator with repeatable settings. While using the torque stick, increase the air pressure to the impact wrench until the nut just moves. Reduce the air pressure slightly, and then tighten another nut. Verify the actual torque on the second nut with a torque wrench.
Air pressure can be fine-tuned to obtain the accuracy desired. By using this particular air pressure, impact wrench setting and torque stick set, consistent results are possible.
Custom wheels pose unique challenges due to the wide range of products available. First, make sure the wheel is structurally sound and that any spacers, centering rings or adapting hardware is in place and functional.
Since the thickness of a custom wheel can differ from original equipment, also verify that the fasteners will properly engage the threads. The chart below illustrates the number of turns necessary for adequate depth of engagement.
|Lug Size||Minimum Engagement Turns|
The importance of following proper torquing procedures cannot be overstated, especially when working with custom wheels.