I have a very superficial knowledge of the laws and equations which
govern suspension systems, but I think I can describe a DeDion and what the
main reason for using it is. A solid rear axle has great wheel locating
qualities. Unfortunately in a rear wheel drive car it also has a lot of
unsprung weight. Unsprung weight is the weight associated with components
in direct contact with the road. This is opposed to sprung weight which is
connected to the road via the suspension. For example, a wheel is unsprung and
a seat is sprung. Unsprung weight is bad if you want a responsive suspension
(weight in general is bad for performance). An alternative is to use an
independent suspension which has most of the driveline components bolted to
to car body. Sometimes they even put the brakes inboard too. This gives
very little unsprung weight, but as people who install a rear sway bars can
tell you, completely independent suspensions have problems locating the wheels.
The DeDion is a trade-off. It has the mechanicals of the independent
suspension, but the rear wheels are rigidly connected by a 'axle' tube. Thus,
the main purpose is to get the low unsprung weight of the independent suspension
and the wheel locating properties of the solid rear axle.
I guess I should add on a big "I think".
woodruff@caen.engin.umich.edu
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