Don Terrill’s Speed-Talk

I am thinking about my Car's rear suspension. By the way my car is a 71 Plymouth road-runner. the car currently has super-stock springs in the rear. the Car also has Sub-frame connectors, a full Cage. the Car also has a Magnum-Force Coil-over front suspension.

My question I want to ask you Guys is: Does anyone have a thought on my idea of adding a Watts Linkage to the car's rear-end? i mean, would it help the rear stay true in it's vertical travel? I am just thinking out load here.

OK, this is all:

Good By The BIG Guy

You need to keep the RC of the Watts linkage as close as possible to that of the existing leaf springs otherwise you'll have two competing RCs trying to bind up the rear suspension. The effective spring rate could as a result shoot to the moon, and you would now have a very loose (oversteering) car.

Personally I'd go with an axle mounted Watts, the RC will follow the axle as it already does with the leaf springs. JMO.

What is the R-C?

Roll Centre.

You need to map out your existing RC with the leaves first, and get the RC of the WL as close to that as you can. Plenty of info out there without repeating it here, just be careful as suspension detail can be a lot more subtle than it seems, and you can end up doing more harm than good.

Once you've got an handle on the RC you need to determine to what extent (if any) you have axle wrap on acceleration (and maybe deceleration) and again make sure any WL chosen can accomodate this, again avoiding bind.

In the long run a 3 or 4 link plus WL will always offer more adjustability, and probably less unsprung weight, but it's a lot more work, and obvious.

I have a 71 road-runner. I have been thinking that if I add a Watts link to my leaf-spring rear-end set-up. The watts link would prevent any side to side shifting of the rear-end. But things became ( For Me that is) when my R-c was brought up on a reply.

As of now, I'm not sure what I will do.

Short answer, no. BTDTNA.

A real road race suspension designer looked at my leaf springs to which I'd added over-axle traction bars, anti-roll bar, Watt's linkage and said, "Well, you've done a wonderful job of polishing garbage."

Go with poly bushings and good shocks and leave the Watt's in the theory section of the book.

If you are going to be running the Silver State open road race or other very high speed road courses, a correctly located Panhard bar will give most of the benefits of a Watt's with 90% less hassle

Ok, I'll break cover......

Doesn't matter if you have a Watts Linkage or a Panhard Rod, with leaves, unless you know what you're doing, it's a bad idea.....

I might be all wet here, but I figure that if my leaf springs are "sideways" solid in both the front hangers and the rear shackles that I wouldn't need a panhard bar or a WL.

But all this said, I figure that my leaf springs would also need to be sideways solid too in their leaf stacks. Especially at the spring wraps..

I guess what I am poorly trying to say is that the leaf springs should be able to move independently up and down, BUT not be able to twist in the front hangers or rear shackles/sliders.

And isn't this addressed when guys use sliders instead of shackles??

pdq67

Paul yes, to some extent, but it is still a spring laterally, if a very stiff one.

Firstly tightening up what's there; the spring wraps, front eyes, rear shackles, maybe consider sliders. Further; installing wider springs (of similar rate if it's currently balanced) and possibly going to single leaf (though you may loose some rising rate) might be needed. All will help lateral stiffness and might make more sense for the OP. It will all need to be dialled in.

JMO and ultimately it depends on what he's already got and where he want's to be........

In " Tune to Win" Carroll Smith devotes an entire chapter to the "Peculiar case of the large sedan" for competition making left and right turns.

He covers Lateral location of beam axle pretty thoroughly. He favors leaf springs slightly over coil springs, and lists several details to improve shackles etc.
" Regardless of the springing medium, the sedan that is to be raced is going to require some sort of lateral axle
location-leaf springs by themselves won't get it done......
I do not think the theoretical advantages of the Watts Link over the Panhard Rod are worth the extra structure and complexity...."

Dan Timberlake wrote:In " Tune to Win" Carroll Smith devotes an entire chapter to the "Peculiar case of the large sedan" for competition making left and right turns...
I do not think the theoretical advantages of the Watts Link over the Panhard Rod are worth the extra structure and complexity...."

Of course when used in conjunction with leaf springs a Panhard rod has the same binding potential as a Watts link. (Ask me how I know this..) My solution was to modify the front spring mounts to allow lateral float/compliance (the shackles were already sufficiently 'willowy' to do the same), thus allowing the roll center to be defined solely by the bar height.

MadBill wrote:

Dan Timberlake wrote:In " Tune to Win" Carroll Smith devotes an entire chapter to the "Peculiar case of the large sedan" for competition making left and right turns...
I do not think the theoretical advantages of the Watts Link over the Panhard Rod are worth the extra structure and complexity...."

Of course when used in conjunction with leaf springs a Panhard rod has the same binding potential as a Watts link. (Ask me how I know this..) My solution was to modify the front spring mounts to allow lateral float/compliance (the shackles were already sufficiently 'willowy' to do the same), thus allowing the roll center to be defined solely by the bar height.

Or just make it all stiffer. The production class axiom comes to mind, "Any suspension design can be made to work, as long as you don't let it."

Another option would be axle location like used in Tickford Capri. Mostly unknown in US leaf spring cars, but quite popular with British Fords. There are many variations of the theme if you google "Capri axle location kit". It's simple and takes little space from over axle exhaust.

Had not seen this configuration before! Apart from the difficulty of adjusting the RC height as desired, it's simple, lightweight, takes up little real estate, etc.