trailer & towing accessories
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Trailer wheel measurements
Trailer wheels are very similar or the same in some cases as a car wheel. There are a lot of different wheels available and if you are building a trailer it is handy to have a bit of an understanding for them. Not all of the information on this page is always required but it may help. Markings found on wheels There can be 4 or 5 pieces of information marked in the centre of a wheel. Wheel size The wheel size cannot be accurately measured from the rim with a tyre fitted. There are special gauges available to do this. Beads for wheels A bead on a wheel is basically a hump in the wheel to stop the tyre being pushed away from the edge of the wheel and letting the air out. Beads are not needed on small wheeled single axle trailers. Most small wheels do not have a bead. Wheel PCD The PCD of a Wheel is very important as it is the measurement used to set the wheel studs / bolts out. There is a lot of information required to calculate some PCD's. This is shown on a seperate page, please click on the link below. Wheel offset The offset is another important measurement this is the measurement from the center line of the rim to the face that bolts to the hub or brake drum. The diagram above shows 2 measurements (A)40mm for the measurement from the front of the wheel to the hub face, and (B)100mm for the rear. These measurements can be taken on any wheel to find the offset.
Below is a general lay out of a wheel.
Wheel manufacture is marked on all wheels.
Part number to get a direct replacement. This is not always possible but part numbers can be cross referenced to get a wheel that will do the same job.
Wheel size this gives an inside width and a nominal diameter.
Wheel sizes are normally followed by a code to give a bead type.
Other information that can be found is:
Offsets
Contry of manufacture
Date or date code.
Occasionally the manufacturer will give the PCD of the mounting holes.
The 3 most important measurements are the rim size, the PCD and the offset, if you get these correct then the wheel will fit.
You will not need to measure a wheel accurately if you have an idea on where to measure.
We do have a few people call and give a tyre size and then say that the rim is 1" larger this is because they are measuring the outer extents of the rim.
The actual rim diameter is measured where the tyre fits and is normally approx 1" smaller than the outer of the wheel.
The width is also meaured on the inside of the wheel.
If the markings are still readable the size is marked as the width in inches followed by a J then the diameter.
for example 4.5J13 means thet the inside width is 4.5" and the Diameter is 13".
Below is a Diagram showing where these measurements are taken.
The diagram below shows a Typical bead on a rim marked H2.
Shown below are the different markings for the different types of beads and a description of the type of bead.
H Denotes 1 rounded bead on the outside of the rim.
H2 Denotes 2 rounded beads as image above.
FH Denotes a flat hump on the outside of the wheel
CH Denotes a combination humps (1 flat on the outside and 1 rounded on the inside.)
SL is used on wide wheels and means Special ledge.
Trailer wheels. As a rule you will get a H, H2 or no bead.
An offset is put on a wheel to distrubute the centre of the load on a wheel to the centre of the bearings in the hub.
Light and heavy duty trailer wheels have what is called a center nave this means that there is no offset. The wheels bolt to a hub with a mounting face central on the bearings. Wheels for middle of the range weights tend to have an offset these go on 160, 200 and 203mm braked suspension as the outer bearing is normally on the outer face of the brake drum. Car wheels also have an offset for the same reason. This means that the unbraked hubs that fit car wheels also have an offset. These hubs are 100mm, 108mm and mini hubs.
The offset is marked on some rims but not all. When it is marked it is marked with the letters ET then a number. The number is a measurement in mm of how far the face is off center.
If the rim is marked ET30 this means that the center plate (face that bolts to the hub) is 30mm outwards of the center line of the wheel.
The diagram below shows a rim with a 30mm offset.
Once you have the measurements you simply take the front measurement from the rear measurement then divide the answer by 2 as shown below.
(B-A)/2
To calculate the wheel above the sum is.
100 - 40 = 60 / 2 = 30mm offset.
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