| Previous notes aren't quite right about what a de dion axle is.
Essentially (in plan):
- "de dion tube" -
______________________________
_ / ____________________________ \ _
| | / / \ \ | |
| |_/ / ____ \ \_| |
| | / / ** \ \ | | ** diff. is fixed
| | |=X========X=< diff >=X========X=| | | to chassis
| |_/ \ / \_| |
| | ^ ^ ____ ^ ^ | |
|_| | | || | | |_|
wheel | | || | | wheel
| | || | |
-------------||----------------------------Universal
|| joints (4)
||
||
prop.shaft ^
|| |
|| [rear of car]
||
Dunno if above is comprehensible. The "de dion tube" is really just like a
live axle in its geometry. It's a rigid tube, with the wheels (hubs) fixed
to it just like a normal live axle. Springing medium can be coils, torsion
bars, leaves, whatever.
As the car goes over bumps the dd tube goes up and down in just the same
way as a live axle. De Dion suspension is *not* independent, because both
wheels are mounted on the same rigid tube: as one is deflected by, say, a
bump in the road, the other must also be affected to some extent.
The following are some of the theoretical pros/cons. of De Dion suspension.
Pros:
1. The road wheels stay vertical to the road surface at all times (ignoring
tyre deflections), regardless of body roll - always maximising tyre tread
contact area, and generally giving predictable handling.
2. Less unsprung weight compared with a live axle. Meaning that you don't
have a great gormless mass of a differential leaping up and down generally
disturbing equaninimity - the differential is fixed to the chassis.
[Unsprung weight means the bits of the car which "directly" jump up and
down over bumps, through being, in effect, fixed to the wheels. The less
unsprung weight the better for good ride and handling]
Cons:
1. Compared with independent rear suspension: usually greater unsprung
weight (the de dion tube itself being fairly massive, even though lighter
than a live axle)
2. Compared with a live axle: greater complexity - four universal joints
(vs none), and the need, with all but one system, for telescopic (splined)
drive shafts.
Re the need for splined drive shafts, think about it - given that the dd
tube is rigid, i.e. fixed in length (= constant track), simple geometry
means that as a wheel rises and falls, that drive shaft must change in
length as its angle changes. This requires either a splined joint somewhere
along the shaft, or that one of the 2 U.Js in each shaft is itself designed
to accomodate length changes - eg. the crude-seeming rubber "doughnut"
coupling beloved of Hillman Imp and original Lotus Elan owners (not that
either of these cars had DD suspension - they were both independent).
The only exception to this (way of dealing with drive shaft length changes)
that I know of is the Rover P5, i.e. 2000/2200/3500, made from 1964 to
1976. Here the drive shafts were of fixed length, but the DD tube itself
was telescopic, incorporating a splined joint. This had the very nominal
disadvantage that wheel track did not stay quite constant as the car went
over bumps/round corners (but nor do most independent systems give constant
track). On the other side of the coin it had the advantage of having no
supposed-to-be-free-sliding drive shaft splines to suffer from "stiction"
when the car was under heavy power in a low gear. This ingenious suspension
was just one of several pieces of lateral thinking in a design which seems
now to have become underrated.
And yes, De Dion cars built around the turn of the century *did* have De
Dion suspension!
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