Whoa Bill, that's a bigger question than you realize!
Three basic types of overdrive:
Older Passenger car and light truck mostly used a planetary
gear type
overdrive similar to the gearsets used in automatic trannys. These were
mostly automatic type that engaged electrically at a certain speed in
high gear. Most were made to replace the tail shaft of a 3speed tranny
so in a way they bolted on but you couldn't just swap it out as internal
parts had to be changed. Having said that I must mention that
Hone-o-drive made an overdrive unit that bolted to the front of the rear
axle, and I have seen an old advertisement for an overdrive unit that
would fit in a car with a torque tube. "Gear Vendors" still makes bolt
on overdrives ($$$$).
medium and heavy trucks and some newer passenger cars used what
I call
"mainshaft" overdrive. In these transmissions what was the 3rd gear (in
a 4 speed) or 4th gear (in a 5speed) has an over 1 to1 ratio by putting
a bigger gear on the countershaft than the mainshaft. In these the gear
shift pattern is reversed from 4th to 5th (or 3rd to 4th in a 4speed).
These transmissions can't be distinguished from the outside except by
model # as the only differences are the gears inside. if you have one
outside a vehicle you can test it by seeing if the output shaft turns
more than the input shaft in any gear.
medium and heavy trucks also used "brownie" transmissions that
went in
the driveshaft where the carrier bearing is. They are built like a small
transmission with u-joint yokes on each end.
If you have a question about a specific unit I'm sure we can
describe
it to you.
Wdonohoe1@aol.com wrote:
>
> Can anyone tell me how an overdrive works? is it in the trans or is it bolted
> on?
>
> Thanks Bill
> oletrucks is devoted to Chevy and GM trucks built between 1941 and 1959
--
John Dorsey
Wauchula FL
http://www.strato.net/~jrdorsey
'49 3800 Chevy Panel
'52 640 GMC Firetruck
oletrucks is devoted to Chevy and GM trucks built between 1941 and 1959
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