Overdrive Auto Transmission?

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Sorry, I know i'm a newbie :P So this is probably a newbie question. My new car has an overdrive button on the shifter (Automatic transmission), and i'm not sure what it's for. Could anyone give me a little crash course on how to use it and what it does? Thanks :D (It's a 98 Nissan altima btw)

posted by  ThirdeYe

I think it makes your cornering better at high speed.

posted by  CarEXPERT

ARGHHHH!!! No! No! No! That is not what it does. It lowers the gear ratio for trucks and cars. For cars, it is used to conserve gas mileage while on the freeway or open roads.

posted by  Coffin Type R

What is oversteer and understeer than Coffin? On my dads car there is a button that says sport on it, its an auto. wuts it for?

posted by  CarEXPERT

Understeer and oversteer are different and are not related to the transmission.
Before i answer this question, what does your dad drive again?

posted by  Coffin Type R

HE DRIVES A OLD ACCORD LX. THE ONE THAT KINDA LOOKS LIKE SHIT. HEY I DONT GET WUT UNDERSTEER IS AND OVERSTEER. HOW DO I AVOID IT? HELP ME COFFIN :ticking:

posted by  CarEXPERT

Okay, now to get down to business. Lets take your dads car, it is FF car. Now lets say you are going into a turn where it is recommended that you do 35 mph, but you are doing 45 mph. The car will turn, however, instead of staying along the inside line, it starts to "float" or drift to outside line. For reference, the turn is a left turn and the inside line is the center marker, and the outside line is the shoulder. This is understeer. the car is turning but not enought to stay in the inside line.

Now oversteer can be netter explained using my old car, 1982 Toyota Celica Supra. The Supra is FR car. Lets use the same turn except the entering speed is 55 mph instead of 45 mph. Now it is harder to turn the car because of the higher speed. So I tap my brakes and crank the wheel to make the turn. The will respond, but turning me much faster then the inside line is turn. So my car is now making a sharper turn then is required. This is oversteer.

posted by  Coffin Type R

So FF cars sucks at turning than FR. I heard that AWD cars have understeer cause they have too much grip. Can only FR cars get oversteer cuz they are RWD.? ?WHY DID YOU PUMP THE BRAKE? WUT IF YOU DIDNT, WOULD YOU HAVE GOTTON OVERSTEER STILL? tks.

posted by  CarEXPERT

Can only FR cars get oversteer cuz they're RWD?
-Mostly, unless you have a witchcrafy system like ATTESSA onboard.

Why did you pump the brake?
-Because he was going to fast, and he would've gone off the road had he not

Would you have gotten oversteer still?
-probably, just not as much.

Turn off the caps.

Overdrive is like a mini-transmission that goes onto the end of the actual transmission, correct? Thus, if you have the ability to turn it on/off you can theoretically double your gears?

posted by  Godlaus

Both those examples that I gave actually happened to me, except change the FF to a Tuarus. The reason it is harder to turn faster with a FF is because you are turning with your drive wheels. AWD have a similar problem because the drive wheels a turning also but it is not as bad. All cars can get oversteer, I just changed the cars because like I had said I used a couple of life experiences to explain it better. The reason I had tapped the brake is because the roads were wet and i didn't know how the car would react. The last answer is I dont know.

Here is good post about oversteer in a FF car.
http://www.car-forums.com/talk/showthread.php?t=8185

posted by  Coffin Type R

So if i'm on the highway, I just hit the overdrive button once? How do I turn it off? (or does it click in? i've never used it) I take it I should turn it off when slowing down, correct?

posted by  ThirdeYe

Yeah, push/click it once. If is a newer car a light should turn either saying O/D ON or a light will turn like O/D OFF. To turn it off click it on more time. When you turn it on and you traveling i believe around 45 to 50 mph, the car will surge once(i guess that is an accuate way of describing it), then the rpm will drop. An when travelling up a hill, or slowing down like on an offramp, you should turn it off. :thumbs:

posted by  Coffin Type R

:banghead: :banghead: :banghead: :banghead: :banghead:

posted by  GreekWarrior

indeed... :screwy:

posted by  Coffin Type R

Bahh, any car can understeer or oversteer. Drivetrain has alot to do with it but certain model cars just have a tighter suspension that pulls the car closer in the turn. There are many front-wheel drive cars that have mild or severe understeer. There are alos many rear-wheel drive cars that have bad oversteer. It also has alot to do with the car and its setup in general than just the drivetrain.

posted by  DSMer

I was just changing the car and drivetrain cause i was use a couple of experiances that i had to explain it better. My toyota had bad oversteer cause the shock, struts and springs were bad. So bad I could bounce the front of my car like a basketball. Would have used it but the thing wieght 3500 lbs, and have you ever tried to put that through a hoop?

posted by  Coffin Type R

RWD front wheel slip angle is less than rear wheel slip angle = oversteer
FWD front wheel slip angle is more than rear wheel slip angle = understeer
AWD both slip angles nearly the same = tending to neutral depending on torque split

RWD reduction in understeer with wider tyres (bigger contact patch, less slip)
AWD tractive forces shared by four wheels so less slip

Suspension geometry is a big factor.

Pretty hard to get FWD oversteer in normal conditions.

posted by  Wally

You can also get 4 wheel steer with a RWD like in a Skyline (called HiCas) which improves the cornering. HiCas only offers a small amount of rear wheel steering and only above a certain speed.

Some 4WS systems like I've seen in some Honda's and Mazda's can also operate at low speed to assist parking.

posted by  Who?

:mrgreen:

posted by  GreekWarrior

What is the best wheel drive when your doing fast cornering and want a quick takeoff after the corner?

posted by  CarEXPERT

ATTESSA

posted by  Godlaus

No, I mean rwd, fwd, or awd, without any technology that switch the drive wheels

posted by  CarEXPERT

I don't have any real proof to back this up, but I'm going to say AWD, cuz thats whats logical in my mind.

posted by  Godlaus

How about you drive some cars and conduct some research. OR possibly look the fecking information up. There are millions of sources that will explain realative racing theorys in corespondance to drivetrain set up. You can't possibly be serious if you expect us to cater to your every needs. Get some motivation, go out and buy books, read, experience.

posted by  DSMer

It is an additional gear for efficient highway cruising

posted by  jman

Take it to a mechanic and hand him 2 grand.



Finally, a real response, that is almost right, you seem to be the first in this topic to come close.

Okay, here is an example of gear ratios to boot (as you have to understand this to fully understand overdrive):

Okay, first number signifies the number of input rotations(from engine), 2nd is output (to differential [or just think of it as wheels if you don't know what a diffy is to not confuse you])

1st - 3.35 : 1
2nd - 1.99 : 1
3rd - 1.33 : 1
4th - 1 : 1
5th(aka, overdrive) - 0.68 : 1

Okay. As you can see, the engine is rotating 3.35 times for every one output rotation in 1st gear. In 4th gear they even out (on most cars it's 1 : 1 in 4th gear). Now in Overdrive, the engine is rotating 0.68 times for each output rotation, so the output is now spinning faster than input, and using your own logic I'm sure you can figure out for yourself that that would increase economy.

Now for overdrive's second function. We're talking automatic cars, so, you probably don't know this, but all that's between the engine and transmission is a container called a torque convertor, what happens inside of there is there is transmission fluid inside of it, and it creates a hydraulic pump (it has fins inside of it). The engine shoots transmission fluid to the other side (transmission side) making it turn. That is why you can come to a complete stop in an automatic, there's no solid link between the engine and transmission. So you can imagine that it's somewhat in-efficient, the engine will always be spinning a bit faster than the input shaft of the transmission (Engine may be going 3000RPM but the input shaft of the transmssion may be going 2500RPM) becuase there's no solid link to keep them going an identical speed. I hope I made that easy enough to understand.

Overdrives second function is to actually lock-up (which is why some people refer to overdrive as "lock up") the engine and transmission. So now they are both going identical speeds, like a standard transmission. So with that extra lock-up it increases economy yet again, you get standard economy from an automatic at highways speeds that means.

Just a side note if you want to read it, don't if you're already confused, it's a lot to take in in one sitting. Okay, there's another gearing reduction before the power hits the wheels, in the differential I mentioned earlier. We'll take 4th gear for instance because it's easy to work with due it its 1 : 1 ratio. The differential has a ratio of about 3 : 1 on most cars. The differential lowers the output of the transmission by 3 times that means. So in 4th, the 1 : 1 the trans is making, before it hits the ground it's now 3 : 1, so the engine is always rotating much faster than your wheels. I just thought I'd clear that up for you if you wanted.

Hope everything I said was at least somewhat understandeable :banghead:

posted by  88GrandPrixSE

On FWD transmissions with a transaxle, does it also have a differenctial ratio? Or is the transmission ratio directly to thewheels. Like in a FWD in 4th a ratio of 1:1 in the tranny, will that be to the wheels in a FWD?

posted by  CarEXPERT

Yep, they all have a differential ratio, or the engine wouldn't make any power. Ratio's will be the same as a RWD, for instence I believe the ratio in my 88 GP SE is in the 3.20 : 1 area, forget the exact number. So 4th with a 1:1 ratio on my car would be about 3.20 : 1 to the wheels.

So you indeed still have to take the differential into consideration when gearing the car (which is what 99% of people do before altering the trans gearing itself).

posted by  88GrandPrixSE

How do you change the gear ratios? DO you have to get bigger/smaller gears to change the ratio?

posted by  CarEXPERT

Gears in the trans or the differential? Either way, it's a mechanics job, especially for the trans.

For the differential, you'd need new ring and pinion gears. Usually you want to increase the ratio (ie. from 2.73:1 to 3.73:1), because the higher the ratio, the more power you get.

The gears would be have more teeth than the old ones, not necessarrily bigger, because they may not fit in the housing if they wer bigger. The more teeth you have, the higher the ratio, the less you have, the lower. So you want more teeth most of the time.

Here's a picture of a ring and pinion if you don't know what they are: http://www.usgear.com/tl/Ring__Pinion_Yellow_Fixed.jpg

(The ring is.. well, the ring, and the pinion is the shaft with a gear at the end.)

Changing the ring and pinion gears are very, very easy, just a matter of removing a handful of bolts, but, the reasoning it's a mechanics job is it has to be an exact fit, there's shims that may need to be put on/removed to make it an exact fit. If only changing the ring and pinion you could get away without taking it to a mechanic, but it's just not something you really want to toy with, there's so much you could mess up so easily in a differential.

For a note: The ring gear attaches to what's called a crown, which has smaller gears in it (non-posi) which transfer the power it's recieving from the pinion to the wheels.

posted by  88GrandPrixSE

In a trannsmission gear, having a bigger gear ratio is better for power? With a bigger ratio, the RPMs climb up faster right, is this why its better. But it would effect the top speed if the last gear is big right? Is that how top speed is done? From the transmission gear ratio and the differential gear ratio.
So if you wanted more top speed, you would reduce the gear ratio right?

posted by  CarEXPERT

Right on boy. Good job! It is indeed better for acceleration but hurts top speed. But it's not so easy to increase top speed by making the gearing smaller. The reasoning for that is that the engine has trouble pulling the weight of the car when you lower the ratio, so you can only do so much, and even most over-drive cars the ratio is too high, just good for crusing, but accel/top speed, you could actually get going faster in 4th gear than you could in 5th on lots of cars. Though the more powerful the car, the lower the ratios you can go, because it can pull you more easily. For example:

The ratios I mentioned earlier were from an '05 Mustang, in 2nd gear was 1.99 : 1, but maybe in my car with the little 2.8 motor, 2nd ratio is only 2.30 : 1 because my car just doesn't make enough power to pull the weight of itself.

Hope you understand what I'm tryin' to say here, sorry if it's a little complicated.

posted by  88GrandPrixSE

Yea I understand, So in hondas/acuras, their engines rev very high so it means that they have a higher gear ratio than like a Mustang V8 right? In the V8, there is a lot of torque so wouldnt you want low ratio? Is that why they have a low redline? cuz of gear ratio?

posted by  CarEXPERT

How high the engine can rev doesn't have to do with the gear ratio, it just means they can rev that high, which is mainly on smaller engines like I4s, V6's and Rotary engines. Those ratios for the mustang are lower than normal ones, so yes it does have a low ratio.

As for the redline question, that's just a motor thing, trans doesn't affect the redline. There are a fair amount of factors that can affect the redline of a vehicle, the most common, how much air it can take in. An I4 has no problem getting a lot of air because it only has to power 4 cylinders, so it's easy for them to rev higher, same goes for rotary engines. V6's have 2 more cylinders that it has to feed with air, so there's less to go around, so they can't rev quite as high. V8's+ have a lot more cylinders to feed with air, so the air has to be distributed even more, so they redline even earlier. This problem can be helped with a K&N filter open filter, which is most common, or getting a higher flow intake manifold/better air intake system tubing (ie aluminum which is very smooth, which also desipates heat, so you can get even more air into the cylinders).

Another one may be that the stock fuel pump can't deliver fuel fast enough to keep up, or the injectors can't keep up.

The intake and exhaust valves may not close fast enough, which may require a stronger spring.

All of these an affect when a car redlines, but more often than not, it's how easy/hard it is for it to breathe.

posted by  88GrandPrixSE

So let me see if I understand this... I should have overdrive on, on the highway, but what about cruising the residential/city areas? should I have it off? I'm not sure how it would affect mileage in city areas...

posted by  ThirdeYe

It won't go into overdrive in the city unless you get up to highway speeds, so it doesn't affect the city. But you should always have overdrive on, they only time it should be off is if you're towing something, which I doubt you will be.

posted by  88GrandPrixSE

Awesome, thanks.

posted by  ThirdeYe

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