Speedway Bike Explained:

WHEEL ADJUSTMENT

To allow a rider adjustment of the slide and traction while racing, two rear wheel adjustable nuts are attached to the centre wheel bolt. This helps align the rear wheel on the bike with equal tension either side and also allows the rider to adjust the distance that the wheel sits in the frame.

The rule of thumb is that the further back the wheel sits in the frame, the easier it is to slide the back wheel. This location is used for beginners and novices.

Experienced riders will have the wheel further forward to have less slip and have more grip and traction on the back wheel.