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Leaf Spring Rear & Panhard Rod

Motorsport Wheel Alignment Fundamentals

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Discussion and questions related to the course Motorsport Wheel Alignment Fundamentals

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The race car I purchased a few years ago and currently going through a rebuild has a live axle rear end on leaf springs, and has had a Panhard Rod added. My concern here is whether having the panhard rod on a leaf spring setup will cause any binding of the suspension?

I expect it will have two potential binding instances - I may be corrected on this.

The leaf springs will move the axle vertically, but the panhard rod moves it through an arc, laterally against the springs, but how much will depend on the suspension travel, the length of the panhard rod, and the angle - the lower, the longer and the flatter the better, respectively.

The other thing is the roll centre of the leaf springs and that of the panhard bar are most unlikely to coincide.

Hi Matt, strictly speaking, the combination of Panhard rod and leaf springs will bind. They are simply both trying to force the suspension through totally different arcs, which is what you're getting at. Practically speaking, it will depend on the lateral stiffness of your leaf springs as to whether this is actually a problem or not. I'm just talking about how much misalignment the shackle bushes will give you without overstressing the mountings.

As Gord says, it will also depend on the angle of your panhard rod and suspension travel as to how much actual lateral movement you'll end up with, so it may or may not be a problem anyway.

Thanks for the quick responses. So in this situation there would be no point having the panhard rod connected. Would a Watts linkage setup work better in this instance or am I just better not running anything?

Hi Matt,

I would say that in this circumstance, if you need to have the axle located laterally, then the Watts linkage is the only good option.

For a track car, you may be looking at only a few cm of suspension travel and, with well implemented 'rod, that could be as little as 1 or 2mm of lateral motion which is really negligible and something spring flex 'should' account for.

A Steven said, a proper Watts linkage would be the normal option, whether axle or chassis mounted.

There are other options, though, such as a Jacob's Ladder or vertical slider.

That said, with leak springs they're prone to roll/bump steer characteristics as the leaf moves the axle through an arc. You may wish to experiment with the front eye height.

Depending on your budget, there are also other options if the rules mean you have to keep leafs rather than going to a coilover setup, they can actually be used with 4/5/6 link linkages

Thanks for adding that extra detail in there Gord. Just to be clear though Matt, you do need something to properly locate the axle laterally. While the leaf springs are adding some resistance to lateral displacement due to the way they are attached, that's not their purpose and they can't be relied on for that. Your application will determine whether a panhard rod will suffice, or if converting to a Watts link or other is a worthwhile exercise for you.

Forgot, a couple of other things that may bias you towards going to a Watt's , or other method, rather than a panhard.

The lateral movement will also affect tyre clearances, and as it moves from the horizontal it will also introduce a lifting, or lowering, force force through the linkage - some have used this to advantage, but generally it's just another complication.

Way back in the day, the racing RS2600 Capri's used a clever method of circumventing the limitations of using leaf springs - stock spring types had to be used, but 'helper' springs were allowed, so they used very low rate leafs and the helper coil-overs as their actual main spring. I can't find a good illustration, but this may give some clues for you - http://www.turbosport.co.uk/showthread.php?t=708463

Ah, if this link works, the RS2600 cutaway shows the leaf 'springs' in yellow, and the rest of the suspension detail also visible - https://cdn1.snaplap.net/wp-content/uploads/2015/05/11083532/FordCapriRS2600_naked.jpg

Thanks for all of the replies. Certainly something to consider as the car will be used mainly for racing but is road legal. I did consider going 4 link rear, but given budget will probably stick with the leaf springs and go about setting up a watts linkage instead.

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