Jacobs Ladder Explained



A Jacobs Ladder is a link used in sprint cars to locate the axle in the center of the car.  It was chosen due to its compact size and more importantly the size of the arc that the axle will go through.  When talking about the arc that a link locating an axle in the center of a car will produce, the bigger the better.  As the axle goes over a bump you want it to move side to side as little as possible.  For the purposes of this explanation, the pictures below are color coded as follows:  The green image is the suspension starting point,  the blue images are at 4 inches either side of center as the suspension goes through its travel and the red line represents the path of the axle side to side in the car.  The first image represents what is used in most sprint cars today.   Notice the very large arc produced at the axle hook up point (point "B").  The distance from point "B" to the centerline of point "C" is 14".  The "straps", from point "A" to point "C" are 7".  The arc produced by this arrangement is 87".   As the axle travels up or down from center, it moves the axle to the left in the car by 0.095" per 4 inches of axle travel.

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The more common type of link to use for centering an axle is a pan hard bar, sometimes called a sway bar or track bar.   If we take a panhard bar the same length as a jacobs ladder, the arc produced is much smaller causing the axle to move 0.584 to the left per 4 inches of vertical axle movement.  This is an unacceptable amount of movement that would cause the suspension to bind.

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As explained at the beginning, the arc produced by a 14" jacobs ladder is 87".  In order to duplicate this and thus produce the approximately 0.100" of movement per 4" of travel, you would need an 87" pan hard bar.   A sprint car chassis is only 30" wide.

87_inch_panhard.gif (3750 bytes)