This article assumes some existing knowledge. Learn more about the basics and wheel terminology here.
If you’ve got your eyes on a new set of wheels, the big question is always if they’ll fit. And not only that, but how well will they fit? The answer to this depends largely on the following:
– The total rolling diameter of your wheel and tyre
– Wheel width and offset
– The offset of the wheel
– Brake clearance
– Centre bore size
– Stud pattern
When choosing wheels, these factors need to be considered as a whole. With a little experience it’s easy to tell if certain wheels will fit your car without having to resort to a tape measure or making any calculations, but when clearances are reduced or you’re going for a tighter fitment you’ll need to take careful measurements and possibly test fit wheels before driving on them. As long as your car already has wheels which can be used for comparison (or you know the OEM specifications), it’s normally easy to determine if different wheels will physically fit.
Dealing with different specifications
Wheel/tyre rolling diameter
- Increase in diameter
Generally this isn’t a cause for concern unless you are significantly increasing the diameter of the wheel. If you are increasing the diameter of the wheel, then there is potential for the tyre to rub on the arch (all other things being equal). It’s difficult to say exactly how much room you’ve got to play with due to suspension travel but the total rolling diameter of a wheel can be calculated for comparison with the formula below. Before you start, ensure that the wheel diameter and tyre width have both been measured in millimetres and that the tyre profile is represented as a percentage. E.g. a 45 profile tyre would become 0.45.
Total Rolling Diameter = [(Tyre width x Tyre profile) x 2] + Wheel diameter
This calculation assumes a vertical tyre wall, i.e. not stretched or ballooned
- Reduction in diameter
If the wheel size is reduced, the concern becomes brake caliper clearance. As some wheels have more clearance than others, the only way to find out is to test fit the new wheels (although a visual inspection of your original wheels should give a good indication).
Wheel width and offset
- Change in width
If the width of a wheel is increased, it has the potential to make contact with either the suspension assembly at the rear of the wheel, or the arch at the front. Physically measuring clearances with the existing wheels fitted will show the amount of available space. If the width is reduced, the new wheel may not fit due to reduced brake caliper clearance and the only realistic way to find this out is by test fitting.
Put simply, the offset determines exactly where a wheel sits within a wheel arch. Offsets are related to wheel widths, as if a wheel becomes wider the offset needs to change for the outer face of the wheel to remain in the same place. Learn more about offsets on our basic wheel information page or try out our wheel offset calculator tool.
An unknown offset can be calculated by following the instructions below:
– Lay the wheel face down (it makes no difference if a tyre is fitted or not)
– Put a flat edge across the wheel from one side to to the other and make a note of the measurement from the floor to the the flat edge. We will call this “Measurement A”.
– Next measure the distance from the back of the wheel to the flat edge and make a note of this. We will call this “Measurement B”.
– To calculate the offset, divide the value of “Measurement A” by 2 and subtract this from “Measurement B”. For clarification we can express offset as: (Measurement A/2) – Measurement B.
– The calculated offset may be a positive or a negative value.
After you have determined the offset of the new wheels or already know them, you can use our wheel offset comparison tool to easily compare different wheels.
As the offset is part of each wheel’s physical design, it can’t be increased (made more positive), but it can be effectively decreased with the use of wheel spacers. Wheel spacers may be used to provide greater inner clearance or to increase the vehicle’s track for grip or aesthetic reasons. A 5mm spacer will effectively decrease the offset of your wheel by exactly 5mm; increasing inner clearance while reducing outer clearance by the same amount.
Wheel spacers and PCD adaptors
When using spacers or adaptors, always use wheel bolts that are long enough while ensuring that nothing is fouled by bolts that are too long.
Spacers come in two different forms: hubcentric and non-hubcentric:
Non-hubcentric spacers are the most simple in design and typically come in widths less than 10mm. These spacers are just flat pieces of metal, with a centre bore and holes for wheel bolts. A benefit of these spacers is that some of them can be used with a number of different stud patterns and as they are generally thinner than hubcentric spacers (useful for dealing with tight clearances). The downside of these spacers is that the centre bore of your wheel can no longer locate itself on the hub of the car if the spacer is thicker than the length of hub’s own seat.
- Hubcentric spacers
Hubcentric spacers are designed to fit on to your hub with a centre bore that matches that of the OEM wheels. The outer (wheel) side of the spacer replicates the face of the hub itself, which means that your wheel will be perfectly centred when the wheel bolts are tightened. This design is superior to non-hubcentric spacers.
- PCD adapters
For wheels with completely different stud patterns, e.g. Porsche > VW, you’re going to need adaptors. These adaptors are essentially hubcentric spacers with two different PCD patterns. One stud pattern is used to bolt the adaptor to the hub of the car, while the other stud pattern matches that of the wheel itself.
The images below show a non-hubcentric spacer which has the correct centre bore to locate on the car’s hub. As the hub still protrudes through the bore of this slim spacer it would not be possible (or necessary) to use a hubcentric spacer. A wheel with the correct centre bore for the hub could be bolted straight to the car, but a spigot ring is pushed onto the hub to allow the wheels shown in this article to be fitted.
Centre bores and spigot rings
The centre bore of a wheel is the hole use to locate the wheel on the hub of the car. If the centre bores of your new wheels are smaller than your current wheels, the wheels can not fit as they will not be able to locate on the hub of the car. If the centre bore is larger on the new wheels, spigot rings should be used to effectively reduce the size of the bore. These rings are just inserts – usually made of plastic – that allow the wheel to locate in the correct way.
Selecting Correct Wheel Nuts/Bolts
When replacing wheel bolts, you can generally get away with slightly different lengths but you can’t use bolts of a different diameter or thread pitch. As a rule of thumb, there needs to be at least seven full rotations for a nut/bolt to properly secure a wheel. Do not use grease of any kind on your wheel bolts as the manufacturer’s torque values are for dry assembly, unless stated otherwise.
Read more about wheel nuts and bolt specifications in our basic wheel information article.
- Ball Seat and Cone Seat
Wheel bolts most commonly come in “ball seat” and “cone seat” styles and it is vitally important that you match the seat type to that of your wheels. Ball seat nuts and bolts have a curved seat, whereas cone seat nuts and bolts most commonly have a straight 60° taper. You CAN NOT use ball seat bolts with wheels designed for cone seat bolts, and vice versa. The contact area between bolt and wheel would be massively reduced, resulting in undue stress on both the wheel and the bolt. If the wrong style for the wheel is used there is a very real chance that they may fail or work loose.
- Wobble bolts
Wobble bolts allow for slight variances in PCDs. They normally offer up to a couple of mm allowance, which means you can fit e.g. 4×98 wheels on a 4×100 car without the use of adapters. The seat and shaft are two different pieces of metal which allows the threaded part of the bolt to be tightened slightly off-centre.