Offset & Bolt Circle
The offset of a wheel determines how far in or out it will sit in relation to the hub and also your vehicle's guard / bodywork. Each vehicle has an optimum offset range and the further you deviate from it, the higher the chances are of the wheel not fitting properly.
When compared to the offset of your vehicle's factory OEM wheels, an aftermarket wheel with a negative (higher) offset will stick out further from the side of the car, potentially allowing it to hit the guards whilst cornering or going over bumps. Alternatively, an aftermarket wheel with a positive (lower) offset will sit further inwards from the side of the car, potentially causing it to hit or rub on suspension components or the inner walls of the wheel well.
The diameter is an important factor to consider when replacing your vehicle's original wheels. The distance measured is not from the face of the wheel as commonly thought, but instead from the bead seats where the tyre fits onto it. To ensure your vehicle's speedometer reading is not affected, it is important that the overall rolling diameter of your vehicle's tyres remain the same. So any increase in wheel diameter must result in a reduction of tyre sidewall height to maintain the same overall rolling diameter.
Upgrading to a larger diameter wheel will dramatically improve the look of your vehicle and improve handling responsiveness, whilst also seeing a slight reduction in ride comfort due to the lower profile tyres required to maintain the overall rolling diameter.
Alternatively, selecting a smaller diameter wheel will improve the ride comfort of your vehicle, at the expense of some handling responsiveness and visual aesthetics. It is also important to check whether the smaller diameter wheels will clear your brake calibers.
It is important to consider what width you will choose for your new wheels, compared to the original equipment items fitted to your vehicle. The wheel width must be looked at in conjunction with wheel offset when determining how the wheels will fit under your vehicle's guards and also what tyres you can fit to them.
Running narrower wheels tends to improve vehicle ride comfort and cornering in the wet at the expense of traction in the dry, due to the narrower tyres that will be required. Conversely, choosing wider wheels allows you to fit wider tyres, which will generally improve dry traction (and off-road performance for 4WD vehicles) at the expense of wet cornering and some ride comfort.
As you might have guessed, wheel width directly affects the width of the tyres you should have fitted to your vehicle. Selecting wider wheels with narrower tyres may result in more vibration and a reduction in ride comfort, due to the sidewalls of the tyre being under increased tension.
On the other side of the coin, selecting wheels that are narrower than the tyres may result in poor handling as the tyre distorts sideways more when cornering. As a result, it is vital to get the wheel width correct right from the start to avoid running into these problems.
Wheels traditionally are built from two types of material, alloy (aluminium) or steel. Lower-spec passenger vehicles often come with steel wheels and plastic wheel covers fitted as a means of cost cutting, while higher-spec models will almost always be offered with larger, more attractive looking alloy wheels. The choice then seems clear, but there are pros and cons to both construction types which you must consider:
Cheaper than alloy wheels.
Will never split or crack.
May bend on hard impacts, but can be hammered back into shape as an emergency fix.
Heavier than alloy wheels, decreasing vehicle performance and increasing fuel consumption.
Available in fewer styles and colours.
Stronger than steel wheels.
Available in a wider range of styles, finishes and sizes.
Lighter than steel wheels, improving vehicle performance and fuel economy.
May crack or split from very hard impacts, are often non-repairable.
Are usually more expensive than steel wheels.
SO WHICH IS BEST?
With the above points in mind, alloy wheels are almost always the perfect choice for passenger vehicles. Their stronger construction and lighter weight means your vehicle will see improvements in handling, performance and fuel economy, whilst also looking better visually.
For 4WD vehicles the advantages of steel or alloy wheels can be used to the vehicle owner's advantage, enabling drivers to choose a wheel that will best suit their kind of driving. For people who often drive their 4WDs through harsh, remote conditions, the rugged dependability of steel wheels may be an attractive proposition. Their inherent strength and ability to bend (rather than crack) means there's a reduced chance you'd be left stranded in the event of a hard impact.
On the other hand, 4WD owners who live closer to larger rural centres will find alloy wheels a more attractive option. Aside from a larger range of styles and sizes available (which makes selecting a wheel to suit their vehicle much easier), alloy wheels offer a strong but light construction which pay dividends when it comes to vehicle performance and handling.