Steering, Suspension, & Brakes

We're probably all familiar, at least conceptually, with Panhard rods and Watts links for laterally locating rear axles. I'm curious about a third alternative. On the Lister Jaguar chassis, the DeDion tube is laterally located by a "sliding block". The concept is so very simple: a stout pin sticking horizontally off the back end of the rear axle (or the DeDion tube in this case) is restrained in a vertically-slotted chassis bracket. If you have a copy of Racing and Sports Car Chassis Design (Costin&Phipps), this is illustrated in Figure 49 / page 87. Clearly, the pin's location defines the rear roll center, right?

I'm looking for answers to two questions:
1) What are the disadvantages?
2) What other cars (besides Listers) have used this approach?

Let's hear your thoughts!

I don't know why someone couldn't use the sliding block but I really fail to see the advantage of using a De Dion style rear suspension over a well designed IRS. I would think that the downsides of the De Dion when used with modern radial tires would be enough to make most builders shy away. It was a great design for old bias ply racing tires designed to keep the tread as flat against the track surface as possible while reducing unsprung weight, but with modern tires which can roll and still keep the tread surface relatively flat on the track I'd think an IRS would be much better handling. I suspect that is much of the reason the DeDion went out of favor many years ago.
As to the advantages and disadvantages of the sliding block arrangement, I wonder how it would affect the roll axis depending on the position relative to the wheel axle centerline. We know that panhard rod position has a big effect there, would the sliding block be the same? Getting a low roll center and still maintaing ground clearance might be one problem.

A stout needle bearing type cam follower should eliminate any sticking problem, but I suspect Bill is right and the instantaneous roll center would move around a lot.

Jim

Upon further research, it seems that the sliding block concept was pioneered by Mercedes Benz, and later used by BRM. (I'm still looking for more info. Probably other people have used the idea.) BRM did something a little different than Lister. They designed the "slot" right into their differential housing. Their "pin" is coming off the FRONT side of their DeDion tube. It's a neat approach because the differential housing needed to be robustly mounted anyhow. Here's a photo of the BRM (V16) installation:


(copyright Tony Matthews, 2009).

If you were building a DeDion suspension anyhow, it's probably best to mount the pin facing forward... but honestly, when I asked the question I was thinking about live axles. If you were installing a Ford 8" axle, for example, it would probably be more convenient to mount the pin off the BACK of the axle housing.

Obviously a Panhard rod is the cheap and cheerful way to go... but some people want something better. A Watts link is usually considered a step up over a Panhard rod. In these next two photos, you see that a Watts link has been installed on an MG Midget. The center pin of the Watts link is mounted off the chassis, and two radius rods run outboard to brackets on the axle. It's a very neat installation, but not exactly simple:

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With regard to IRS vs. DeDion... I would agree that a decently designed IRS should definitely be superior on bumpy roads. However, roads have never been smoother than they are today. Furthermore, it's going to be pretty tricky for the home hobbiest to design or fabricate a really good IRS to suit one of the lighter British sports cars. The Jag IRS will work okay on something like the Roadmaster... but it ain't especially light! For a Super 7 replica or even for a Spitfire or a Spridget, it's just ridiculous overkill. I really think a DeDion makes a lot of sense for these cars, especially if they're running wide tires.

Food for thought:


Why did DeDion tubes fall out-of-favor for racing cars? Surely one reason had to be that engines moved rearward, and with the engine behind the driver there's just not much room to package a DeDion tube.

I chimed in on this in the "Watts" thread. What your calling the "sliding block", as I was taught, described as "pickle fork", based on shape of verticle track roller would glide in. DeDion, I just don't see the point. No dynamic steering potential. No de-camber potential,(which helps bigger tires clear outer curve of fender).Probably heavier than comparable stength ,smart IRS. So many good IRS, modular set-ups available, on the cheep, like RX 7 , Toyota, Nissan and Infinity.RX 7 is lite,strong and racers claim will hold approx. 500 hp. Probably NOT in drag racing. I wish we had coverage of more mid-engined Brit. cars or conversions. roverman.