Trailer and Caravan Tyre Load & Pressure Ratings

A table covering popular trailer sizes is given below.

Trailer and Caravan Tyre Speed Symbols:

The tyre SPEED SYMBOL indicates the speed at which the tyre can carry a load corresponding to its Load Index under service conditions specified by the tyre manufacturer. For caravans and light trailers see conditions specified by E.T.R.O 1991, passenger car tyres section 13, reproduced below.

Trailer and Caravan Tyre Load Index

The LOAD INDEX is a numerical code associated with the maximum load a tyre can carry at the speed indicated by its Speed Symbol under service conditions specified by E.T.R.O 1991, passenger car tyres section 13, reproduced below.

Trailer and Caravan Tyre Markings & Condition:

Car, caravan and trailer tyres must have at least 1.6mm of tread over the central 75% of their width for their entire circumference, although a tyre with under 3mm offers little grip in the wet. Tyres must be free from deep cuts and bulges (which indicate they are breaking up) and it is illegal to use cross-ply and radial tyres on the same axle. (Make sure that the tyres you fit to your trailer are of the correct rating.)

Additional Tyre information:

See links above for detailed tables on Tyre Load Index, Tyre Speed Symbols and Tyre Load & Pressure Ratings.

Tyre Sidewall Markings

The illustration below provides an explanation of the markings currently found on a tyre sidewall.

* do not apply in the U.K.

Retreads - must also show either 'RETREAD' or 'REMOULD' on the sidewall, together with 'BSAU 144' in place of ECE Type Approval Mark.

Aspect ratio - ratio between the height of the sidewall and the width of the tread, expressed as a percentage. In the example above, the figure is 70%.

Nominal section width - on radial tyres this is expressed in millimetres, on cross ply tyres, inches are used.

Trailer and Caravan Wheel Bolt Tightening

When refitting a trailers' road wheels it is ESSENTIAL that the wheel nuts/bolts are tightened to the recommended torque figure and in the correct sequence. To neglect this is too run the risk of either the nuts/bolts working loose and the wheel coming off (not tight enough), or damage to and possible shearing of the nut/bolt (over tightened). 

The sequences necessary to correctly carry out this work are as follows:-

Torque figures : Wheel Bolts

It is ESSENTIAL that the torque settings are re-checked after the first 25 miles or 50 km and subsequently before each journey. 

Trailer and Caravan Wheel Information

There are many different types of wheels fitted to trailers, many of them specifically for trailer use. The practice of using car wheels, rather like that of using car brakes, has largely died out.

Wheels, like tyres, have a maximum weight capacity determined by the manufacturer.

Where a wheel/tyre unit maximum weight capacity is declared, if either of the two parts has a different loading capacity, the declared figure will be the lower of the two. Reference should always be made to the supplier if there is any doubt.

The technical terms around a wheel are frequently misused and "rim" and "wheel" are often misused. The main factors to bear in mind, particularly when ordering replacements are as follows:

Rim: The rim serves as the seat for the tyre.

Wheel Disc: The wheel disc connects the rim with the hub.

Wheel: The wheel is the entire assembly, consisting of the rim and disc.

Rim width: The width inside the wheel flanges and is normally expressed in inches e.g. 4.50.

Rim diameter: The diameter of the wheel just inside the rim, where the inner edge of the tyre bead sits and normally expressed in inches e.g. 13"

Offset: The distance between the seating surface of the wheel/hub and the vertical centre line of the wheel/tyre unit. This can be positive offset, as shown above, where the vertical centre line is behind the seating point, when the wheel is fitted to the hub. Negative offset is the opposite and is sometimes known as inset.

Centre disc: There are also centre disc wheels that have neither positive nor negative offset; i.e. The vertical centre line is in exactly the same plane as the seating point.

Centre bore: The hole in the centre of the wheel disc, designed to accommodate the bearing boss and grease or dust cap. Size expressed in millimetres.

Stud or bolt holes: The holes by which either the studs or bolts fasten the wheel to the hub. Normally 4 or 5 in number, the size is determined by the size of the stud or bolt, either metric or imperial. The usual way of expressing the configuration incorporates both these factors, e.g.

4 x M12 = 4 off 12mm studs or bolts.
5 x M14 = 5 off 14mm studs or bolts.
4 x ½" = 4 off ½" studs.

The seating of the stud hole is also important and must correspond with the seating on the stud or wheel bolt. These are either conical (Straight side walls inside the seating) or spherical (curved side walls inside the seating) but there are some continental wheels with flat cylindrical holes.

Pitch circle diameter (P.C.D.): The diameter of the circle around which the studs are arranged. Measurement given in metric or imperial e.g. 100mm, 112mm, 5.5", 4". The measurement of a 4 stud fixing is simply the distance between the centres of two opposite stud holes.

A five stud fixing is calculated by taking the measurement from the centre of a stud hole to the edge of the centre bore x 2 + the diameter of the centre bore.

Studding patterns: The correct way to express a studding pattern, either of a wheel or of a hub is an amalgam of the P.C.D and the Stud/Bolt hiles.
E.g. 4 x M12 on 100mm
4 x 1/2" on 5.5" (or 139.7mm)
5 x M12 on 112mm

Essential information when refitting trailer road wheels: Wheel Bolt Tightening