How a Swing Arm for a Bike Works

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How a Swing Arm for a Bike Works
(Rolling Roscoe)

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Suspension and Swing Arms

A swing arm operates in conjunction with the rear suspension system to maintain the rear wheel's constant contact with the road. As a motorcycle travels down the road, bumps and road surface irregularities send shockwaves through the rear wheel, forcing it upward. These shockwaves push against the swing arm, which moves vertically against its pivot point. The vertical motion is dampened and absorbed by the suspension system, which rebounds and pushes the swing arm back towards the ground to prevent a loss of traction.

Swing Arms and Handling

The length of the swing arm contributes to a motorcycle's steering geometry, affecting the wheel base primarily. A shorter swing arm, which decreases the overall wheelbase, minimises straight-line stability, but provides for easier turn-in when cornering. Conversely, lengthening the swing arm, lengthening the wheelbase as a result, will improve straight-line stability. Turning stability will be reduced and will require more effort to turn and hold a line. Changing the length of the swing arm will also alter the motorcycle's centre of gravity, pushing the balance of weight further forward and preventing the front end from rising.

Swing Arm Vs. Acceleration Squat

The term "squat" refers to the tendency of the rear suspension to compress under acceleration. Weight transfers from the front of the motorcycle to the rear, which combines with the downward force generated by the drive chain. This force pulls back on the rear of the motorcycle and causes the rear end to compress. The angle and placement of the swing arm's pivot point act to oppose this force, pushing forward and upward against the motorcycle in a manner similar to placing a ladder against a wall at an angle. The rear suspension assists to maintain swing arm angle, pushing the rear of the motorcycle forward and upwards, while dampening the pro-squat forces.

Other Functions

Swing arms serve to hold the rear suspension, chain, wheel and axle, but may also be used to hold engine oil, a system used by Buell's XB-9R and XB-12R. Depending on the motorcycle's purpose, lift spools, chain adjusters, and other maintenance items may also be equipped on the swing arm.

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