[Mgs] MGB O/D

Gosling, Richard r.gosling at penspen.com
Fri Sep 21 05:31:08 MDT 2007


An overdrive is an entirely separate gearbox that bolts onto the back of
the main gearbox.  It has two ratios, when it is off the ratio is direct
(speed out = speed in), when engaged the speed out is greater than the
speed in (can't remember the ratio of the top of my head!).

It contains an epicyclic gearbox
(http://en.wikipedia.org/wiki/Epicyclic_gearing), which works on the
same basic principle as the 3-gear systems that were used in bikes when
we were kids, before derailleurs took over.

When you flick the switch, electricity is supplied to a solonoid.  This
solonoid operated a valve, re-directing oil pressure in the O/D (the O/D
includes a small oil pump that uses oil from the gearbox).  The oil
pressure is used to engage a clutch, which changes the gear ratio.

That's it in a nutshell, but I'm sure there's someone here who's had
theirs apart and can tell you much more!


Richard & Sammy ('73 Black Tulip BGT)
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