Firstly this is not always the case. Old Harley military bikes and bikes like the flat twin Ural often have the sidecar wheel in line with the bike rear wheel. Although this arrangement is a little less stable it allows for practical rod brakes on the vintage Harleys and the drive arrangement for the Ural sidecar wheel.
However the sidecar wheel is typically positioned further forward than the motorcycle's rear wheel to provide stability and reduce the tipping over effect while turning away from the sidecar (this means turning right if the sidecar is mounted on the left). It creates the triangle shape like the milking stool with three legs.
When a motorcycle turns, it leans into the turn to maintain balance. However, this leaning motion is not possible with a sidecar attached because the sidecar adds weight and stability to the bike, making it impossible to lean. With a left side sidecar, if the sidecar wheel were positioned in line with the motorcycle's rear wheel, when turning right the bike would be more prone to lifting the rear motorcycle wheel off the ground, which could result in a loss of control and a potential accident.
To minimise this effect, the sidecar wheel is placed further forward than the motorcycle's rear wheel. This configuration creates a wider base and a lower center of gravity, providing more stability and reducing the risk of tipping over during turns. The forward positioning of the sidecar wheel also helps to counteract the forces generated by the motorcycle's engine when the power is increased.
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