At 02:07 PM 2/21/2000 -0500, Tab Julius wrote:
>
>.... is OD different than 5th gear in ways other than the fact that it
simply doesn't have a stick position? Is pushing/sliding the little OD
button effectively the same as choosing 5th gear or does it do something
very different?
In function the OD is just another gear like any 5th gear, designed to slow
the engine down. In hardware it is very much different than a 5-speed
stick shift. At the time of production of these cars the standard gearbox
was a 4-speed unit, and retooling to make a 5-speed box would have been
quite expensive. The overdrive unit was available off the shelf (from
another supplier I think) and could be bolted right onto the back of the
original gearbox with as little change as different mainshaft for the
gearbox, and maybe a change of propshaft length.
As there would be no direct mechanical shift link from the original gearbox
to the OD unit, the new gear needed to be shiftable by remote means, or
hydraulically in this case, with electrical control switching. Internally
it works much like part of an automatic transmission, with a planetary gear
set for speed reduction and a hydraulicly operated clutch for actuation.
Because of the complexity and expense of the OD unit, they have generally
been replaced in recent times by 5-speed gearboxes.
Barney Gaylord
1958 MGA with an attitude (but no OD)
http://www.ntsource.com/~barneymg
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