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: |
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1 ) |
Rim: The
rim serves as the seat for the tyre. |
| (
2 ) |
Wheel Disc:
The wheel disc connects the rim with the hub. |
| (
3 ) |
Wheel:
The wheel is the entire assembly, consisting of the rim and
disc. |
| (
4 ) |
Rim width:
The width inside the wheel flanges and is normally expressed
in inches e.g. 4.50.
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| (
5 ) |
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" |
| (
6 ) |
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. |
| (
7 ) |
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. |
| (
8 ) |
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. |
| (
9 ) |
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. |
| (
10 ) |
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. |
| (
11 ) |
Studding patterns:
The correct way to express a studding pattern, either of
a wheel or of a hub is an amalgam of 9) and 10),
E.g. 4 x M12 on 100mm
4 x 1/2" on 5.5" (or 139.7mm)
5 x M12 on 112mm |
| (
12 ) |
Two piece
wheels: Although rarely used as
original equipment in today's light trailer market, a number of
older trailers may be fitted with two piece wheels, particularly
those using 600 x 9 tyres. It is vitally important that the nuts
holding the two sections together are NOT loosened whilst the tyre
is still inflated.
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Essential information
when refitting trailer road wheels: "Wheel
Bolt Tightening"
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