Lee M. Daniels writes >
>DeDion Rear ... I DON'T KNOW WHAT IT IS
Live axles, dead axles and DeDion rear ends are all "rigid" axle designs. When
the car is in use, the relationship between the right and left rear wheels is
constant, so (good news) camber change on the outside wheel in a smooth corner
is very slight, (bad news) a bump hit by one wheel instantly changes the camber
on the other.
A live axle is a solid tube having a differential, halfshafts, brakes and the
like all in one package that moves together as unsprung weight.
Live axles located only by semi-elliptic leaf springs are common. Examples:
MGB, '56 Plymouth. This is called a Hotchkiss system. Very cheap to
engineer, lots of unsprung weight and poor location.
Live axles with coil springs and various locating arms and links are also
common. Examples: FIAT 124, RWD Mazda 626, various GM RWD cars. Still lots
of unsprung weight, but location can be much better.
A dead axle is a solid axle having no power stuff (differential, halfshafts),
but having the brakes. It all moves together as unsprung weight. Example, the
leaf-sprung rear axle of FWD Dodge Minivan. Or the coil spring setup with
trailing arms and Panhard rod my on Mighty K-wagon. Less unsprung weight, but
there is no drive stuff.
The DeDion is like a dead axle with drive hubs mounted on it. The "DeDion
tube" is U-shaped, so the hubs fit on the ends and there is room in the middle
of the "U" to put the differential, halfshafts and often, inboard brakes.
These are all fastened to the chassis, so as to be sprung weight. The middle
part of the DeDion tube passes above, below or behind the axle centerlines.
Thus the weight of the differential (and brakes, if they are inboard) is
unsprung. Half the weight of the halfshafts is unsprung.
Normal DeDion tubes are rigid and are located like a live axle or dead axle
would be.
The AWD Dodge Minivan just uses the same leaf springs for the DeDion tube that
the FWD Dodge Minivan uses for the dead axle. (Car and driver said that the
AWD Dodge Minivan has a live axle. They obviously didn't look under one!)
The DeDion tube can also be located by links and arms and use coil springs or
torsion bars. The ALFA Milano uses a Watt's link. A Panhard rod can be used
instead, like on a live axle or dead axle.
There is an interesting variation on the DeDion. The modified DeDion uses
rigid (not splined) halfshafts to laterally locate the hubs. Panhard rods or
Watts links are not needed, just the trailing arms. The DeDion tube must be
made to telescope to allow for track changes. Example: Rover 2000, which also
had inboard disk brakes.
Wide rear tires often work better on a rigid axle. Both dead axles and DeDion
tubes can be made to allow camber adjustment. (Live axles can be adjusted for
camber too, but it is not for the faint of heart. The early Trans-Am cars wore
out differential gears really fast!)
Phil Ethier, THE RIGHT LINE, 672 Orleans Street, Saint Paul, MN 55107-2676
h (612) 224-3105 lotus@pnet51.orb.mn.org (infrequent mail checks)
w (612) 298-5324 phile@pwcs.stpaul.gov (lists go here)
"Gee, I love that kind of talk!" - Tim Conway
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