Lee M. Daniels, Texas A&M scribed:
Subject: DeDion info?
> I recall other discussion about the DeDion Rear a few weeks/months ago, and
> Can someone guide me to a good technical description of this design?
Seeing as how I'm now on my second car with DeDion, I guess I should add
to the descriptions we've had so far, as well as relating it to some
British cars. As previously mentioned, a DeDion rear configuration is one
way of achieving some unsprung weight savings of independent suspension,
combined with stability of a solid axle. I beleive this was first
developed by Mssr. DeDion to improve the ride of his carts. The less
unsprung mass you have on a wheel assembly, the easier it is to control
it's deflection, and keep it planted on the road.
The basic layout is a follows: The differential is mounted to the body,
instead of the axle. This makes the diff and driveshaft weight all sprung,
and keeps the diff from rotating under torque. The half-shafts driving
the wheels must now have some give to allow for up/down movement. This
has been acheived variously with sliding splines & U-joints, rubber donuts,
and C-V joints. So far, this is also what goes into a swing axle design.
Now lets add independently sprung wheels, often with coils, but
leaf or torsion bars work as well. OK, so now we have an independant
rear suspension, which allows one wheel to react without disturbing
the other. The problem is, when cornering, the heavily loaded outside
tyre will deflect up and ride on the inside edge, while the inside tyre
will be unloaded and ride on the outside. This reduces traction &
therefore control & speed. It's nice to limit this "feature", and
tie the sides together, to keep the tyres squarely planted, and transfer
weight back to the unloaded tyre.
Enter the DeDion tube- which is a "bar" that solidly connects from one wheel
hub to the other. This transfers loading on one side to the other, which
helps keep the tyres flat on the road. It's usually curved, in order to
pass around the differntial. And it commonly passes behind the diff,
so as not to take up passenger compartment space. A further development
moves the brakes inboard, further reducing the sprung weight. (But then
you gotta have extra robust half-shafts, cuz if one snaps, you got no brake!)
Some examples-
-I beleive Rover used a DeDion in the P3, and possibly later cars.
-Alfa has been using a rear transaxle (complete with clutch!) in the
GTV, GTV6, & Milano, (Spider too?) with inboard disk brakes & coil springs.
-Peerless & Warwick use a Salisbury diff, sliding half-shafts w/U-joints,
outboard drum brakes, and leaf springs. The tube runs *upward*, and
slightly behind the diff, right behind the rear seat area, giving
a bit more space in the boot.
-Gordon-Keeble, which was developed by John Gordon of Peerless, used
a similar set-up, but I think it incorporated disk brakes to help deal
with the Corvette mill up front.
________
/___ _ \ Roger Garnett (Roger_Garnett@cornell.edu)
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