Can you explain how an overdrive automatic transmission works?

Question

My father told me long ago that an overdrive in a car or truck transmission was very different than just adding another, bigger, gear, like 5th or 6th or 7th gear, but he wasn't able to explain it to me. Now I'm trying to explain it to my son, with the same lack of success that my father had with me. Please, no links unless you can, in your own words, tell me how it is different than just a bigger gear. I know it lowers RPM of the engine and saves gas, but the question I have is how is it mechanically different than just adding another, bigger gear?

Answer 1

There are sure a lot of people here just making stuff up as they go.

Your dad was right, in a way - it is a bigger gear.  But you aren't really ADDING a bigger gear, you are reconfiguring the existing transmission to lock up parts of it so that there are more revs on the output shaft than the input provides.

The thing that will help your son understand this (and will have you saying aha!, too) would be a primer on how planetary gears work.  That is the basic building block that will open it all up for him.

The magic of this really are the planetary gears.  These gears have 3 opportunities to either multiply or divide the input for different drive ratios.  You have the sun gear, the planetary gears and the outer ring gear.  Most 4-speed transmissions have 2 SETS of these gears to provide lots of gearing possibilities.  These are all used in the transmission to provide different combinations of outputs to inputs.

Answer 2

It really is to hard to explain in layman's terms on here. The best web page you'll find is here http://auto.howstuffworks.com/automatic-...

from the page "In this transmission, when overdrive is engaged, a shaft that is attached to the housing of the torque converter (which is bolted to the flywheel of the engine) is connected by clutch to the planet carrier. The small sun gear freewheels, and the larger sun gear is held by the overdrive band. Nothing is connected to the turbine; the only input comes from the converter housing. Let's go back to our chart again, this time with the planet carrier for input, the sun gear fixed and the ring gear for output.

Ratio = 1 / (1 + S/R) = 1 / ( 1 + 36/72) = 0.67:1

So the output spins once for every two-thirds of a rotation of the engine. If the engine is turning at 2000 rotations per minute (RPM), the output speed is 3000 RPM. This allows cars to drive at freeway speed while the engine speed stays nice and slow."

Take a look at the page. The pictures really will help you understand.

Answer 3

It is the exact opposite, the overdrive in a transmission is actually a smaller gear which is driven by a bigger gear. The overdrive drops your gear ratio to .75 to 1 where the high gear is 1 to 1.

Answer 4

It is just bands in the transmission, the faster you go, the tighter they get, then another set of bands kick in when you drive at normal highway speeds, smaller, less engine power, makes the rpm drop, kind of like a four-wheeler with low and high range, but fully automatic. The clutch packs are two different size packs, one expands out a lot faster, thus giving a lot of power at low end, but no speed. The other takes longer, is smaller, and creates more speed. Note (high range) It is a pully with a belt around it and a clutch pack. That part isnt easy, just go to an auto store, get a book about transmissions, and go over it with him. God Bless and good luck

Answer 5

I'll do better than that - It is not different.  Your father was simply making c**p up, and now you're doing it.  Stop.

I'll give you an example.  The Ford AOD was the first automatic overdrive to come out here in the USA.  It was based on the old Ford FMX transmission from the 60's, and like the FMX, it used a SINGLE set of planetary gears!  It's amazing!  Four forward speeds, including overdrive, plus reverse from one planetary arrangement.  

The GM overdrives were more complicated.  They tended to have 2 planetaries to begin with, and when they wanted overdrive usually they added a third one, right in the front.

Manual overdrives are so simple that I won't bother explaining them here.  If you just find one cutaway picture on the internet showing how any manual works, you'll see what I mean.

So anyway, your dad was wrong, and so your problem is solved.