A car wheel consists of various elements with different characteristics. Some of these parameters, especially the size of a wheel center bore (or center bore diameter) are commonly ignored.
However, it is worth being aware of what hub diameter and wheel centre bore are. It might be useful especially when buying new car wheels as a wrong diameter can have quite problematic consequences.
The relation between wheel centre bore and hub diameter
Before fitting new wheels, it is necessary to understand the relationship between the hub diameter and the wheel center bore, as well as how spigot rings can be used to relieve the potential wheel diameter problems.
The centre bore of a wheel is the hole in the central part of the back of the wheel where the spigot fits through. Wheel hubs, located between the drive axle and brake drums, connect the wheels to the body of the car. Therefore, in order to fit the new wheels correctly, it is important that the car’s wheel hub size perfectly matches the wheel’s centre bore.
Especially in the aftermarket, however, car wheels come with different hub and wheel center bore sizes. If the diameter of the hub collar is larger than the central bore, it will be impossible to fit the new wheels. On the other hand, if the center bore is larger than the diameter of the hub, the wheel can be fitted, but unwanted diameter differences will be problematic.
Fortunately, many issues related to the difference in hub diameter and wheel centre bore size can be solved with wheel spigot rings.
What are spigot rings?
Spigot rings, also called hub centric rings or center rings, are elements usually made of polycarbonates or aluminum, which can be placed between the car wheel hub and the rim to adapt the size of the centre bore to the hub dimensions.
The spigot ring size is defined by two diameters. The larger number indicates the diameter of the central bore of the wheel and, consequently, the smaller number represents the diameter of the hub’s mounting collar. Such rings will enable you to safely mount the wheel when its center bore is too big for the hub. Spigot ring sizes will be written in a format similar to tyre sizes - 73.1/57.1, for instance.
What are spigot rings used for?
Spigot rings are used in situations where the car wheel centre bore is bigger than the vehicle’s hub. Naturally, this will not be an issue if you are using the OEM (original equipment manufacturer) parts. After all, factory fitted components are designed for use on specific vehicles so spigots are rarely used in this situation.
The need arises, then, when assessing the wider availability of aftermarket car wheels. Because a larger bore can be fitted to a smaller hub, but a larger hub can not be used with smaller bores, it makes sense for aftermarket specialists to make their bores as large as possible.
This way, they outsize more wheels which makes them useful for a larger number of vehicles. As a consequence, spigots are often needed to allow for an accurate, practical fitting.
What to do, however, if the centre bore happens to be too small for the hub? Unfortunately, spigot rings will not help in such situations. What you should do in that case is drill the hole larger. Remember to use a very precise drill or consider having it done at a professional workshop as precision is key to getting properly installed wheels.
The importance of fitting spigot rings
The centre bore should not be ignored as a car wheel parameter. Fitting a wheel when the mounting collar of the wheel does not sit perfectly on the hub is prohibited.
Although, in theory, the wheel can be mounted using only the bolts, the actual role of spigot rings is to press the wheel to the hub. They are not designed to transmit forces and vibrations from the wheels as that is the role of the centre bore.
A lack of a perfect fit causes oscillations and vibrations which can be especially intense at higher speeds of around 60-75 mph. Even with the highest quality car tyres, driving with improperly mounted wheels can result in different forms of instability and may lead to various consequences, from lack of driving comfort and increased tyre noise to damage to other car parts and decreased safety. If you want to avoid such trouble, always remember to use spigot rings to ensure a smoother fit.
Spigot rings are fitted to the inside of a wheel so are not visible on the outside.
Other wheel factors worth considering
While fitting spigot rings is a very simple process, be careful to choose the right ones and have them installed properly. Knowing what spigot rings are and how they work, don’t forget about other aspects of car wheels such as car tyres.
As using a spigot usually indicates you are fitting a larger wheel, you should always keep in mind your choice of summer tyre or winter tyre. Having installed bigger wheels, you may need to change the tyre size of your tyres to ensure a smooth fit. A wider wheel, for instance, may require a tyre with less height as a result.