IRS vs DeDion Rear suspension

My car uses a deDion rear axle. (photo) It behaves like a live axle in that the two rear wheels are always parallel to each other and perpendicular to the ground. But unlike a traditional live axle, the differential is not carried on the axle and is not part of the unsprung weight. Instead the differential is attached to the frame (or in my case the engine) and power is carried to the hubs via traditional half-shafts. The benefit is a much lighter unsprung assembly than a traditional live axle.

A number of people have written to me in the past concerned about my decision to use a deDion rear suspension for my car. I have been over this so many times it makes my head hurt.

Let me say that although there are definite advantages to a properly designed IRS, the first time builder, or the builder that doesn’t have the resources (read money) to refine and develop an IRS, will almost certainly achieve better results with a deDion. The interaction between the driven wheels and the chassis is an extremely complex one, the car pitches and dives and rolls and with IRS under bump or droop the driven wheels change camber and toe. At the same time, the rubber is expected to provide acceleration and lateral grip. There are so many variables! And to complicate things even more, the IRS needs to be designed in harmony with the front suspension. Without years of design experience or even a computer program to aid simulations I simply couldn’t take the chance that whatever IRS I designed would work correctly the first time out. A Chapman Strut (FYI – in the front of a car it’s called McPherson and it steers, in the rear it’s called Chapman and it’s locked) includes a height penalty that I can’t design around. The beauty of the deDion is it’s ability to maintain the driven wheels perpendicular to the road surface under all conditions. Granted, on a rough road surface the well designed IRS wins hands down, but on a smooth surface there are absolutely no benefits to IRS and in fact the deDion is the winner. In that sense most people are being sold a “bill of goods” with regards to the superiority of IRS. Some of the most agile and best handling cars have been deDion (or) live axle. Alfa Romeo, TVR, Caterham, Lotus. It’s a long list! Because of it’s simplicity, a live axle can be brought up to an excellent state of performance very quickly. Another reason, for me at least, was packaging. In a transverse mid-engine application, the drivetrain sits squarely where the best location for IRS links go. deDion simply goes around it. In the end it was a no-brainer for me to use the deDion on my first scratch built car.

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