The rear axle is one of the more complex aspects of building a CycleKart. Not only does it have to somehow mount the wheels, but it also needs to take drive from the engine and torque converter as well as provide braking and a parking brake. All of this whilst also being mounted into the chassis in some kind of bearing set up.
Traditionally GoKart rear axles have been used. The original Stevenson specifications listed a 1" solid rear axle, but you can also get hollow axles which have also been used quite successfully. Axle width is generally about 1200mm which will keep the wheel track within the guidelines (no more than 1000mm) and be able to pass all the way through the wheel hubs. When buying your axle you need to make sure that it is of course long enough as most of the GoKart axles for sale are in the 700-800mm range. Ideally the axle passes all the way through the wheel hub so that it provides additional support to the hub (more on this below).
Disc brake and sprocket mounting-hubs hubs are readily available to suit all axle styles, so procurement of those items is usually just a case of ensuring that they match the chosen axle, but wheel hubs are a different matter. Regardless of what axle type you decide to use, the key point in both cases is working out how mount the wheels to the axle. Generally off-the-shelf GoKart hubs can be adapted to work with both Postie Bike wheels and Pit Bike wheels but there is nothing that works out of the box available in Australia. If you don't have the skills or resources to be able to make some kind of hub adaptor, you can buy custom hubs or even full axle kits from Renegade CycleKarts in the US, but you will need to factor in shipping, import duty and GST.
If you are a bit more handy or willing to have a go, you can make adaptors that will allow you to fit your wheels to a standard GoKart hub. Generally this consists of an adaptor plate that bolts to the GoKart hub that is the correct dimension to suit your wheel. The important thing to consider is that any adaptor you make needs to be perfectly concentric. In the case of Postie Bike wheels the adaptor can be made so that it its inside the outer flange of the wheel-hub helping to centralise the wheel. You will note in the photo above that the Renegade Cyclekart adaptors have an additional smaller plate that serves this purpose. There is also a spreader plate that sandwiches the wheel-hub from the outside, this spreads the clamping force from the mounting bolts across the entire wheel-hub, which prevents them from pulling through the aluminium under force.
The Pit Bike wheels are a slightly different design. They are supplied with four threaded mounting 'ears' that would normally be where the disc brake mounts to. Generally these are used to bolt the adaptor plate directly to. However it is generally regarded that these ears are not strong enough to stand up to rough handling and so it is advised to run the axle through the inside of the hub such that the hub is a snug fit. This provides additional support to the hub and takes some of the strain off of the mounting 'ears'.
To prevent hubs from sliding along the axle they can be held on to the axle using locking collars.