Watts Link Explained



A Watts link is the only type of link, when constructed correctly, that can allow an axle to go through its range of motion without skewing it out of alignment.  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 front to rear in the chassis.  The first example is what is called a perfect watts link.   This means that the front and rear links are the same length which yields a straight axle path as denoted by the red line.

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This is also a perfect watts link.  What has happened here is the links have been shortened.  Because the links are still of equal length the axle path is still straight.  What has changed is the amount of twist introduced to the axle bracket (bird cage in the case of sprint cars) as the axle rises or falls in the chassis.  This effect can be used by chassis designers to obtain a desired effect.

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This watts has an even shorter link length.  Notice the extreme wrapping of the axle brackets but the axle path is still perfectly straight throughout its travel.

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This last one is an imperfect watts link known as a modified watts link.  The front link is far longer than the rear causing the axle path to be arced.   This is representative of the link that is used in all torsion bar sprint cars.   As the axle travels up or down from center, it moves rearward in the car by 0.104" per 4 inches of axle travel.  This setup was adopted as a compromise.   The only way to make the watts link "perfect" would be to either shorten the front link to match the rear or lengthen the rear to match the front.  If the front link were shortened the wrap on the axle brackets would be extreme as in the previous image.  If the rear link were lengthened, some drastic modifications would have to be made to the chassis design.  Either the rear of the chassis would need to be extended or the rear axle would have to be moved about 10" forward in the car.  Sprint cars spend enough time with their front tires dangling in the air as it is.  They definitely do not need the rear axle 10" further forward.  In either case, the torsion bars would need to be much larger, quite likely twice the diameter they are now.  In a sprint car setup, the lower link also serves as the link to the rear torsion bars which holds the car up.  The approximately 0.100" of rearward travel is seen as an acceptable tradeoff to make the car far lighter, to be able to use smaller lighter more responsive torsion bars and to have the axle positioned far enough back in the chassis that the driver gets to see something besides sky.

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