Tyres & Rims

 
 

Tyres give your trailer more than just a comfortable ride, they add stability and traction to the trailer and ensures that it is responsive to the tow vehicles actions.

Getting the right tyre for your trailer is just as important as all the other components that make up your trailer.

Looking at a tyre sidewall will give most people a sense of bewilderment with all the letters, numbers and codes. Knowing how to read a tyre sidewall can assist you in determining the correct tyres for your new or existing trailer.

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Tyre Sizing

The first designated number on the tyre is the useable width of the tyre tread and is measured in millimetres, the second number is the aspect ratio or profile of the tyre and is a percentage of the tyre width. For example a 185/70 R14 tyre has a tread width of 185mm and 65% of this is 129.5mm (185 divided by 100 = 1.85 x 70 = 129.5mm).

Light Truck tyres or Commercial tyres sometimes do not include the aspect ratio number on the tyre, for example - 185R14C. With these tyres the manufacturers have adapted a standard aspect ratio of 82%, so for 185R14C tyre the side wall height will be 151.7mm


 A -  Tyre Usage Code

The way a tyre is constructed determines the usage code. A passenger vehicle tyre will normally have a lower profile than the other coded tyres (except the Temporary tyre) and will have a more flexible sidewall to give a softer ride to the driver and passengers. Tread patterns will also be vastly different to allow the tyre to operate in most road conditions at high speeds as well as to help reduce road noise. P code tyres have higher speed ratings than all other codes. Depending on the manufacturer, some passenger tyres may not specify a usage code.

P code tyres are suitable for single axle trailers up to 1000kg GVM/ATM as long as the tyre load rating matches the trailers loaded weight.

LT or Light Truck tyres are built for load carrying capacity at lower speeds than the P code and are normally 6 to 8 ply. Some manufacturers call these tyres "Commercial" or "Light Commercial" and will have a "C" branded on the tyre, normally after the tyre size. These are effectively the same as a Light Truck tyre. They have heavier constructed sidewalls and thicker, more pronounced tread patterns. These are suitable to fit to single axle and tandem axle trailers over the 1000kg GVM/ATM as long as the tyre load rating matches the trailers loaded weight.14_inch_tyre_and_rim.png

T or Temporary tyres are specifically for use as space saving spare wheels and should never be used on a trailer.

B - Tyre Construction

Bias or cross ply tyres have almost been relegated to history and the only vehicles to use cross ply tyres these days are the odd agricultural vehicle (tractors and their associated equipment) and by vintage car enthusiasts for their old bangers. Radial tyres are pretty much it! There are plenty of websites which can tell you how they are made if you are really that interested in their construction.

C - Load Capacity Code

Each tyre model has its own maximum load capacity at its recommended tyre pressure. This number is very important in deciding the correct tyres for your trailer. For safety's sake I would be inclined to add at least 10% extra loading on your tyres. For example, if you had a single axle trailer with a GVM of 1000kg, add an extra 10% - 1100kg and find a tyre with an 88 or above load rating (560kg per tyre).

Tyre Pressures

Remember to always keep your tyres at the recommended pressures.Tyres with below the recommended tyre pressure will heat up and get hotter quicker than tyres with the correct pressure (The motion of the tyre sidewalls flexing up/down and side to side create internal friction of which heat is a by-product) Tyre overheating is increased even further if the trailer is overloaded. It is a common occurrence for tyres which get too hot, to rapidly degrade and blow out, and it is one of the most common reasons for tyre failure especially with tyres with a heavier sidewall. The thicker the sidewall, the more heat is generated. 

Reduced tyre pressures also increase the chance of instability of your trailer. If for whatever reason your trailer decides to fishtail or sway excessively,(normally due to an unbalanced load), tyres with low or uneven tyre pressures can exacerbate the situation to the point where it is almost impossible to bring the trailer under control safely.

Tubed tyres will also generate a lot of friction induced heat within the tyre and are not recommended.

The best way to prevent tyre damage and increase their life span is to check and maintain your tyre pressures on a regular basis. A tyre does not need to look flat to be under inflated and only check the pressures when the tyre is cold.

D - Speed Rating

The tyre speed rating is based on the manufacturer machine testing the tyre at the rated speed for 10 consecutive minutes without flying apart, disintegrating and generally staying in one piece. At present the maximum speed limit of a trailer in NZ is 90 km/hr so as long as you have a tyre with a speed rating of J or above, you will have no issues. Australian speed limits for towing a trailer are as per the posted speed limit, except for Western Australia where the maximum towing speed is 100 km/hr.

There is so much information on a tyre sidewall that there are plenty of websites dedicated to deciphering them.

The bottom line with trailer tyres is pick a common sized tyre from a trusted source, get their advice, check that it will carry the trailer and its maximum load comfortably and safely, check that it will fit the rims you want, keep an eye on the tyre pressures on a regular basis and get a spare tyre while you are at it.

RIMS

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It has been common practice for the last 50 years or so, to use old or second hand car rims for use on trailers. As these older rims start to deteriorate or get damaged, it is getting harder to source replacement rims with some styles impossible to find - for example HQ Holden, Cortina, etc.

It is now more commonplace to purchase specific trailer rims to replace these and your local supplier will be able to recommend a replacement rim. In some cases you will need to also replace your hubs to match stud patterns.

4 stud rims are still available in the smaller 12" diameter rims with 5 stud rims being the most common for trailers up to 2500kg GVM/ATM. 6 stud rims should be used on trailers over 2500kg GVM/ATM.

New rims are available painted or fully hot dip galvanized and come in a range of diameters, widths and offsets (see below) and load capacities.

14_inch_alloy.pngThere is also the option of paying a bit extra and fitting alloy wheels especially designed for trailers, just remember to fit then with locking stud nuts to prevent them from being stolen.

Rim Dimensions

Rims are always measured in inches (1 inch = 25.4mm) and there are four measurements you need to know before purchasing.

 - Rim Diameter

This is the diameter where the tyre bead sits on the rim and not the overall diameter. The tyre sits on the first "step" inside the rim and this is the measured diameter. Trailer rims can come in all manner of diameters and standard sizes range from 8", 10.5", 12", 13", 14". 15" up to 16".

 - Rim Width

Rim width is measured across the rim just inside the top lip, again where the tyre will sit on each side. Tyres are pretty flexible when it comes to fitting across rims, but the rim should be matched to the tyre correctly to ensure tyre stability and load carrying capacities are at their best.

Fitting a narrow rim to a tyre that is suited to a wider rim can make the tyre pop off the rim if cornering under a heavy load. If you are fitting a nice fat rim to a narrow tyre, the tyre sidewall will not have enough flex and the trailer will bounce harshly down the road. Talk to your tyre specialist and match the rims you want to the correct tyre.

Trailer rim widths vary between 3.75" up to 9". Alloy rims are normally wider than steel rims and can take a wider tyre.

- Wheel Offset

Wheel offset is the measurement of the inside rim face where the hub bolts onto the rim, to the centre of the rim. A hub mounting face with a zero rim offset is in the dead centre of the rim. An offset of -35 means that the hub mounting face is 35mm back from the rim centre towards the trailer chassis. With a positive offset of +5 the hub mounting face is 5mm towards the outside of the rim from centre. If you haven't build your trailer yet, wheel offset should not matter too much and I would go for a zero offset rim (unless you are after fancy alloys which may have different offsets). If you are replacing older rims, you will need to measure your existing rims to find the offset and fit new rims with the appropriate offset.

If you fit a zero offset rim to a trailer where the old rims have a +35 offset, you will have your tyres and rims sticking out wider from the trailer by 35mm each side which apart from looking a bit dorky, may make the trailer illegal with not enough guard coverage over the tyre.

The opposite will occur if you fit a +18 offset rim on a trailer where it has been set up for zero offset rims. The tyre and rim may rub against the trailer chassis and sides if there is not enough clearance. Ideally there should be at least 20mm clearance between your trailer and the tyres to prevent any binding and to prevent mud buildup.

- Stud Pattern

Many people have bought fancy new rims for their trailer only to find that the stud pattern does not match their existing hubs, or they have bought rims with a 5 hole stud pattern to fit 4 stud hubs. If you are replacing rims, measure your existing stud pattern accurately and count the number of studs. Standard trailer rims come in 4, 5 & 6 hole stud patterns.

To measure the stub pattern you need to find the Pitch Circle Diameter (PCD) of your existing hubs.

The PCD is the diameter of the circle formed if it ran through the centre of each stud.

PCD-on-5-stud-hub.gifOn a 4 and 6 stud hub, this is as simple as measuring from the centre of one stud to the centre of the stud directly opposite.

On a 5 stud hub, a little bit of maths is needed. Measure from the centre of one stud to the centre of the stud next to it. Divide this measurement by 0.58778 and then divide this measurement by 25.4. This will give you the PCD in inches.

For example, if your measurement between stud centres is 67.1mm, divide this by 0.58778 to get 114.15. Divide this by 25.4 and you get a PCD of 4.5".

Final points to consider

Check with your supplier that the rims you want have the appropriate load capacity you require. Trailer rims will have a load capacity rating stamped somewhere on the rim. Double check before purchasing.

If you are buying new alloys for your existing hubs, you will want to check  that the centre spigot of the hub will fit through the rims centre hole. The last thing you want to do is have to machine out your nice new rims to fit your hubs. If your new alloys comes with a fitted cap, make sure that this will fit comfortably over your hubs dust caps.

Check that you have the correct studs and stud nuts for your rim, check with your supplier and get the right ones fitted. Upgrading or replacing studs in your existing hubs is a relatively easy task and while the hubs are off, you can do a bit of hub maintenance.

Lastly, get your new tyres and rims balanced before fitting onto your trailer. If its good enough for your tow vehicles wheels to be balanced, its good enough for your trailer!