When driving your car, you’re not going to get very far without the wheels. Wheels are a crucial component of any vehicle. Not only do nice wheels make your car look great, but they also play a very important function in attaching the car to the tyres and moving it forward. Wheels are also one of the easiest and most popular modifications or upgrades that you can make to your car. Getting new wheels for your car can be one of the best ways to give it a whole new look. However, before you go ahead and change your wheels for some new and shiny ones, you need to consider the handling of its wheel offset.
How Do Rim Offsets Work?
The wheel offset or rim offset is a huge factor to consider when you replace the wheels on a car. However, it is quite a complicated concept that isn’t always easy to understand. Even people who work in the tyre industry and understand a lot about tyres find the wheel offset a challenging thing to understand or explain to customers.
In simple terms, the wheel offset refers to how your tyres are mounted on the wheel and where they are in relation to the wheel wells of the car. It is the distance between the wheel’s centre line and its hub mounting surface or mounting plate.
How to Measure Offset on Rims – Different Types
There are some different wheel offset types to be aware of. These are:
This is when the centre line of your wheel is in line or in sync with the mounting plate or the mounting surface of the wheel hub.
A positive offset or inset is when there is a larger distance between the mounting surface and the centre line of the wheel. This causes the tyre to sit deeper under the wheel arch, on the axle. It results in the wheels on either side of the car sitting closer together.
A negative offset or outset occurs when the top surface of the mounting surface is sitting on the other outer side of the centre line of the wheel. This causes the wheels on either side of the car to sit further apart on the axle facing toward the suspension. The result is wheels that slightly protrude from the wheel well.
How to Measure Offset of a Rim – Why It’s Important
If you are replacing your wheels or need to put the wheels back on your car, it’s important to get the offset right. This is because the correct wheel offset places your wheel and tyre assembly in the best location in line with the rest of the car. If the wheel offset is wrong, you will run the risk of damaging your car’s suspension along with further damage to any replacement tyres or wheels you plan to get in the future. And, getting the wheel offset wrong can increase the risk of a tyre blowout while you are driving. If the wheel offset isn’t right, then the wheel and tyres might end up sticking out towards the road or facing too far into the wheel well.
How Do You Know the Offset of a Rim
You can usually find the offset specifications in these two places:
Back of Your Wheel
In most cars, the offset measurement will be embossed at the back of the wheel. Usually, this can be found on the rear of the spoke or on the mounting plate. Typically, it will begin or end with the letters ‘ET’.
Online Tyre Guides
You can check the wheel brand’s website to find most offset data online. All you will usually need to do is perform a search for your wheel’s model number or name to bring up the correct offset for the various sizes.
How to Measure a Rim Offset
To measure the offset of a wheel, you will need to get the backspacing measurement. Backspacing is a concept that is closely related to the offset, but the two are not the same thing. Backspacing is the distance between the wheel’s inner edge and the mounting plate. It’s usually a positive number and is measured in inches. When combined with the offset, the backspacing amount can change where the wheel should be positioned on the car.
Problems With Incorrect Offset and Backspacing
If the offset and the backspacing are not correct, there is a high risk of both wheel and tyre rub as the wheel will be sitting too far inside the wheel well. When the wheel is sitting too close to the car, specifically too close to the wheel well, this can lead to the inner edge of the tyre and wheel rubbing against the suspension. Wheel rub on its own is often a bad enough problem to deal with, but over time, it can cause significant damage to the inner edge of the wheel and the suspension components on your car. If left unchecked, it can cause serious damage to the inside wall of the tyre, which is hidden from sight, so the damage often goes completely unnoticed until the tyre blows out when driving.
How to Measure the Offset of a Rim
You can start figuring out the correct wheel offset if you’ve got the measurements for the wheel or rim width and the backspace. For example, if your wheel is fourteen inches wide, then the centreline should be half of the width at seven inches. However, if your backspace is eight inches, then you have one inch more backspace than the centre line, so you have one inch of positive offset. If the idea of doing all of this manually confuses you, like many others, then you will be glad to hear that there are some handy tools that you can use to make your life easier.
Wheel Offset Calculators
All you need to do is a quick online search to find a reputable wheel offset calculator. This will immediately inform you as to whether or not the new set of wheels that you want are going to be compatible with your car. You will usually need to provide information on your current wheel width and offset, and the wheel width and offset of the set that you want to replace them with.
Wheel Offset Charts
Once you have your wheel width and backspace, you can avoid doing all the maths and use a wheel offset chart instead for a quick, easy, and visual method of determining whether or not the new wheels are the right fit for your car.
Other Factors to Consider
- Legal Requirements
It’s important to bear in mind that each state has its own legal requirements to adhere to when wheels outside of the recommended range from the car manufacturer are fitted to a car. These requirements are mainly in relation to wheel rim widths, how nuts and studs are fitted and any track measurement increase. Any modifications to your wheels outside of these legal guidelines are not permitted, so it’s important to ensure that your wheels and tyres are correctly matched to your car to ensure that it is legal on the road and can be operated safely.
For most states and territories, including Western Australia, Victoria, Queensland, and the Australian Capital Territory, the wheel track of a passenger vehicle should not be increased by any more than 25mm further than the maximum specified for the car by the manufacturer. Legally, you are not permitted to change the offset by over 12.5mm per wheel.
In South Australia, the rules are slightly different, as here the wheel track measurement is not permitted to be more or less than 15mm further than the maximum given by the manufacturer. And if you are in New South Wales, you are not permitted to change the offset further than the maximum specified on the placard. The offset limit is 25mm for each wheel on commercial or off-road vehicles.
Before changing your wheels, it’s important to check with your insurance company to ensure that your car is still eligible to be insured by them with the new wheels. In most cases, this will not be an issue if you are using manufacturer-recommended wheels. However, bear in mind that other wheels may need you to report a modification to the vehicle on your insurance, which could increase your premium in some cases.
- Care and Maintenance
Consider the care and maintenance required before changing your alloy wheels. To keep them in the best condition for longer, it is recommended that you regularly clean them or have them cleaned with soap and water. If you use any detergents or cleaning products that are designed for cars, check the ingredients first as some are not designed for use on the alloy metal and might corrode it.
Wheel offset can be a complicated concept to understand, but it’s important to ensure that yours is correct if you are changing your car’s wheels.