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Panhard rod effect on chassis alignment

Motorsport Wheel Alignment Fundamentals

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A change in ride height will offset the lateral position of a solid or DeDion rear axle. How do you work around this when setting up the chassis alignment?

Hi Lee, just for everyone's benefit I'll go into the background a little. It depends on the lateral location method you use as to whether and by how much the solid rear axle will move due to changes in ride height. The two obvious examples are a Panhard rod and a Watt's linkage.

The Panhard rod means the whole axle moves through an arc, so you end up with the axle being biased to moving in one direction until the Panhard rod goes horizontal, then it will move back the other way.

The advantage of the Watt's linkage is the axle does not move sideways if it's setup properly.

If you are forced to use a Panhard rod, then you generally want to minimise the lateral movement and the best way to do this is to make it as long as practical. Keeping the installation angle small (relatively flat) also helps to minimise the lateral movement. Ultimatley it's just a compromise between accepting some lateral movement and making sure you don't have any clearance problems with the guards and tyres. When you do the setup, you would normally adjust the panhard rod length to centre the rear axle as well.

The rear roll centre height is also governed by the Panhard rod setup, so generally, I would start with that, then try and keep it as long and flat as possible. Everything I've said here relates to road racing, where we are trying to turn in both directions too!

Minor clarification, the axle will move the same way as it moves from horizontal. Eg, chassis link on left and axle on right the axle will move to the left as it lifts or drops from horizontal.

Another reason to try and keep it flat is the panhard rod will introduce a pull up/pull down into the suspension under cornering if it isn't horizontal, and this may be dignificantly different between left and right turns - it you're really smart, this could be a tuning aid for circle track, maybe to counter the reaction to engine torque through a live axle on corner exit?

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