Understanding the Basics of Torque Tightening

Putting a wheel on a car is a common operation, but important for passenger safety. To ensure proper assembly, each bolt must be screwed in with a certain amount of torque, known as the “Torque Tightening”. In this post, we’ll go through the details below and in the end, you’ll know how to mount a wheel like a pro.

What Is a Torque Tightening?

A torque tightening occurs when a bolt has to be tightened with a tool.

Bolt Connections

A bolt connection consists of assembling two parts (for example, a car wheel on its hub) in such a way as to make them immobile, using one or more bolts. The assembly can be dismantled.
The bolt may consist of a screw or a stud connected to a nut.

Principle of Bolt Assembly

The assembly of two parts per bolt is based on the application of a force that brings the two parts together. This force, in turn, generates frictional forces between the parts that make them immobile with each other.

Good to know: A bolt joins two parts together by the principle of adhesion by creating a force that brings the two parts together. A bolt is not designed to create an obstacle between the two parts to make them immobile.
The force created by the bolt between the two parts is counteracted by a tension force in the bolt.

Torque Tightening

The tension force applied in the bolt is directly related to the torque tightening applied to the nut.
The torque is a rotational force applied to an axis and is expressed in Newton metres (N.m).

Good to know: to get an idea of this unit of measurement, a torque of 10 N.m is approximately equivalent to a rotational force resulting from a weight of 1 kg applied to a 1-metre lever arm.

The relationship between the tension force in the bolt depends on many parameters:

  • the diameter of the screw or bolt;
  • the diameter of the nut;
  • coefficient of friction at the thread;
  • coefficient of friction at the nut;
  • the pitch of the screw or stud.

In summary, the torque tightening is determined according to the size of the bolt and the materials of the parts to be joined.

Torque Tightening: Correct Assembly

A car wheel consists of a hub attached to the suspension and a removable rim that receives the tyre.

Different Types of Wheels

The wheel hub is usually made of steel, but the rim can be made of several materials:

  • sheet steel rims: inexpensive but quite heavy;
  • aluminium alloy rims: lighter, more aesthetic, but also more expensive;
  • magnesium alloy rims: even lighter, but very expensive, and therefore reserved for expensive cars.

On the other hand, the rim can be made integral with the hub by several assemblies:

  • 4 bolts or 5 bolts.
  • A single bolt actually consists of a hub with a large diameter central axle combined with a nut of the same diameter. Within this configuration by central axle and nut, there are 2 possibilities:
    ◦ the central axle and the rim are equipped with grooves that prevent rotation between the two parts: the central nut is only used to prevent the wheel from coming out by itself ;
    ◦ the central axle is smooth: the central nut is then used to prevent the wheel from rotating on the hub.

Adequate Torque Tightening

Depending on the rim material and the type of fastening, the mounting of a wheel implies different torque tightenings to be applied to the nuts.

Tip: Refer to the car manufacturer’s service manual for the torque to be applied to the nuts.
However, as an indication, you can have an idea of the torque to be applied to a nut according to the diameter of the bolt.
Here are the correspondences for the most commonly used bolt sizes on a steel rim:

  • 10 mm diameter bolt: about 60 N.m ;
  • 12 mm diameter bolt: about 80 N.m;
  • 14 mm diameter bolt: about 110 N.m.

Good to know: for an aluminium alloy rim, apply approximately 20% more torque.
The case of mounting by central axle and nut is different, and this time it is essential to refer to the manufacturer’s instructions:

  • if the axle is equipped with splines, the torque tightening to be applied can be around 150 N.m;
  • if the shaft is smooth, the torque tightening must be higher and can be up to 500 N.m.

How to Mount a Wheel Yourself?

To be able to apply the right torque tightening to the nuts, you must be equipped with a measuring tool.

The torque wrench can be used to tighten a nut to the desired torque. When the torque is reached, a click indicates that it is not necessary to continue tightening the nut.

Good to know: a torque wrench is associated with a certain torque range. For example, a torque wrench can operate between 30 N.m and 200 N.m. Before purchasing a torque wrench, therefore, check that the torque tightening values to be observed is within the operating range of the wrench.

In the case of wheels with 4 or 5 bolts, a “crosswise” tightening sequence must be observed (i.e. alternately tighten bolts opposite to the centre of the wheel), in order to better distribute the mechanical stresses.

In that sense, if you need to buy new tyres or your car need a wheel balancing, your wheels will need to be unmounted then mount again. Ace Motor Care Pty Ltd is located in Sunshine North and they specialise in selling and repairing tyres. They also offer mechanical services, roadworthy certificates, air conditioning system servicing and repair, suspension, brake and clutch repair and servicing, LPG system repair and servicing, car transmission repairs and servicing and car battery services.

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