Worth the Weight

What’s the dif­fer­ence be­tween sprung and un-sprung weight? Which is more im­por­tant?

RC Car Action - - TECH CENTER -

The terms re­fer to the weights of the parts that are sup­ported by the car’s springs, ver­sus those that are not. For ex­am­ple, the chas­sis and all the parts on it are “sprung weight.” The springs hold the chas­sis up. The wheels and sus­pen­sion arms are un-sprung weight, and are not sup­ported by the springs. Rac­ers look for ways to shave weight off of any parts, but trims made to un-sprung parts are par­tic­u­larly at­trac­tive be­cause they al­low the sus­pen­sion to be more re­spon­sive. It’s sim­ple: lighter parts have less in­er­tia, so less force is re­quired to make them change di­rec­tion. Imag­ine wav­ing your arm while hold­ing a ten­nis ball ver­sus a bowl­ing ball—it’s much eas­ier to wave the light­weight ten­nis ball. Like­wise, the lighter a car’s wheels, tires, and mov­ing sus­pen­sion parts are, the more quickly the sus­pen­sion can cy­cle over bumps to main­tain trac­tion.

Wheels, tires, sus­pen­sion arms, and hub car­ri­ers move with the shocks and rep­re­sent un-sprung weight. And the shocks them­selves? The shafts are un-sprung weight, the shock bod­ies are sprung weight.

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