Split rims – or modular wheels – are wheels which can be separated into 2 or 3 parts. Nuts and bolts (or sometimes only bolts) keep the parts together, but they can be arranged in a number of different ways depending on design.
Split rims gained popularity in the 80s and 90s not only due to their attractive appearance, but also in motorsport where they could be built to specific sizes for a given application and economically repaired when damaged (the centres will normally survive when the lip or barrel takes an impact). More recently there has been a resurgence of split rims on both new and old cars, after market and OEM.
The images below show examples of both 2 and 3 piece wheels in different configurations. The two wheels at the top are 3 piece, with the left wheel “sandwich mounted” and the right wheel “face mounted” (with hidden hardware). The bottom images show a 2 piece wheel with a removable lip and a removable face respectively. More detail on mounting configurations is provided in the following text.
Genuine 2 Piece Wheels
2 piece wheels are constructed in one of two ways:
1) From a single barrel, with a bolt on wheel face.
2) From a single face/barrel , with a separate bolt on lip.
Both are genuine split rims, but the first example is generally used by OEM manufacturers for aesthetic reasons, whereas the second style lends itself better to rebuilding in different sizes and with aftermarket lips. This is because in the first style, the wheel face and barrel have to be designed specifically for each other (although you may still be able to find wider/narrower barrels). A benefit of this style however is that the rim will always be airtight as the barrel is entirely separate to the face of the wheel. In the second style, the “split” between the two faces will be between the tyre beads and therefore need to be sealed in the same way as a 3 piece wheel.
A downside to two piece wheels is offset restrictions when rebuilding. If two piece wheels are built with different centres or lips, their widths can be altered but the offset cannot be adjusted independently to the width – however this problem can be overcome with 3 piece wheels.
Genuine 3 Piece Wheels
3 piece wheels comprise of the same basic parts: a barrel, a face and a lip. This design offers the ultimate flexibility in terms of rebuilding and repairing. Offsets can be finely tuned and wheel sizes can be “stepped up” to larger rolling diameters.
As lips and barrels are available in different sizes, it’s possible to change the width of a split rim by splitting and rebuilding with wider (or narrower) parts. It’s also possible to keep the width the same but change the offset of a wheel by selecting the appropriate components for your build. For example, adding an inch in width to the lip of the wheel while taking one away from the barrel will keep the total width the same but alter the offset – which is not possible with a 2 piece design.
Below are the different ways 3 piece modular wheels can be assembled. No style is “better” than another, but changing the assembly style of a wheel will allow the offset to be changed without having to buy new parts. (Note: It may not be possible/advisable to change the assembly order of certain 3 piece wheels).
- Sandwich Mounted
This is the most common configuration, which as the name suggests, sandwiches the face of the wheel between the lip and barrel. The face of a wheel designed for sandwich mount will be shaped to accept a lip at one side and a barrel at the other. Both the lip and barrel of a three piece wheel sit snugly against a step which is cut out of the front and back of the wheel face. This is a crucial part of split rim wheel design, as if your wheel takes an impact, the lip or barrel will be unable to slide across the face of the wheel and shear the bolts which hold it together.
- Reverse Mounted
A reverse mount wheel is assembled with the face of the wheel behind both the barrel and lip. Wheels which are reverse mounted from new (e.g. OZ Futuras) will only be able to support the lip and barrel from the front side of the wheel, which makes them unsuitable for building as sandwich or face mount unless further machining to the rear of the wheel face is undertaken.
*Note: Sometimes “reverse mount” is a phrase also used to describe building a wheel in a different configuration to the OEM specification.
- Face Mounted
A face mounted wheel is assembled in the order of face – lip – barrel. This puts the face at the outside of the wheel and will increase the offset of a wheel that was originally sandwich or reverse mounted. If you intend to face mount your sandwich or reverse mounted wheels, you MUST make sure that the aforementioned step on the rear of the face is deep enough to accommodate both the barrel and the lip (instead of just the barrel). You may be able to use faces that were previously in a sandwich build, but they are still likely to require further machining. If the face was originally used in a reverse mount set up, you will almost certainly need the back of the wheel face to be machined in order to support both the lip and the barrel.
Besides aesthetics, a major benefit of face mounting wheels is that the lips can be polished without hardware getting in the way!
Rebuilding in Different Styles and the Effect on Wheel Offsets
Below are examples of rebuilding sandwich mount wheels in different configurations and the effect this has on widths and offsets.
- Sandwich mount to face mount
– The total width of the wheel will decrease by the thickness of the face of the wheel.
– The offset will increase (become more positive) by the thickness of the face of the wheel plus the thickness (not width) of the front lip between it and the rear barrel.
- Sandwich mount to reverse mount
– This will also reduce the total width of the wheel by the thickness of the face of the wheel.
– The offset will decrease (become more negative) by the thickness of the face of the wheel plus the thickness (not width) of the rear barrel.
- Reverse mount to face mount
– The total width of the wheel will remain the same.
– The offset will increase (become more positive) by the thickness of the face of the wheel plus the thickness of the rear barrel plus the thickness of the front lip.
By understanding the examples above, you can work out the specification changes from rebuilding your wheels in a different style. Do not forget to take into account any machining which may be necessary as this will also have an effect on the offset.