Gear Ratios

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Gear Ratios

Post by Audiphile » Sat Oct 22, 2005 7:54 pm

Can someone explain to me the practical aspect of what those gear ratios mean? How about final drive gear ratio -- what does it mean if your final g.r. is 4.1 compared to something like 4.7?
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Post by LS1Leader » Sat Oct 22, 2005 8:34 pm

Shorter gear ratios (higher numerically) result in more torque to the wheels and better acceleration (to a point). They also result in lower top speeds. Riding a multi-speed bicycle should make it easy to understand. :)

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Post by Audiphile » Sat Oct 22, 2005 8:39 pm

Do you know how they calculate the final gear ratios. Doesn't seem like they just add up all the ratios for the separate gears together...
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Post by LS1Leader » Sat Oct 22, 2005 9:08 pm

It's not a calculation. It's an actual gear reduces the driving speed by an even lower ratio after the transmission. On my car, it's done by a differential in the back of the car connected to the tranny by a driveshaft.then the axles connect to the differential. On FWD cars, the mechanism will be up front, but I'm not sure about the details. That 4.10 gear in my sig is my final drive ratio.

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Post by IMBoring25 » Sat Oct 22, 2005 9:45 pm

Gear ratios mean the number of turns of the upstream component per turn of the downstream component.

You have ratios for all the gears in the transmission and one between the transmission and the wheels (Usually at the differential). Traditional 4WD vehicles will also have a low-range ratio in the transfer case.

Final drive ratio is the product of all the ratios in the line.

If you have a 5.31 first gear and a 4.30 axle ratio on a 2WD or AWD vehicle (or in High range in a 4WD vehicle), your final drive ratio (Engine turns per wheel turn) would be 22.8 while in first. There's a calculated final drive ratio (Called crawl ratio by off-roaders) for every combination of gear and High or Low range selection in a particular vehicle.

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Post by VTECaddict » Sat Oct 22, 2005 10:09 pm

no. the final drive is a gear. the final drive ratio is the ratio of the final drive gear, liks LS1 said.

lets say your first gear has a ratio of 3.27 and your final drive ratio is 4.77. that means that the engine must turn 15.5979 times to make the wheel make one full revolution when the clutch is fully engaged.
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Post by IMBoring25 » Sun Oct 23, 2005 12:15 am

There seems to be some dispute on this matter. I find sources that support your interpretation and those that support the one I presented (I try not to say things authoritatively without collaboration). For instance...
An automobile uses gear ratios in both the transmission and the drive axle to multiply power. The two ratios multiplied together equal the final drive ratio.
http://chevyhiperformance.com/techartic ... 0208_gear/

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Post by Audiphile » Sun Oct 23, 2005 1:22 am

Ah it's starting to make more and more sense. Thanks guys. By the way, when you say that an engine turns "x" amount of times, does that mean revolutions of the engine?
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Post by LS1Leader » Mon Oct 24, 2005 1:36 am

Perhaps we're talking about two different things. I'm talking about the final drive ratio that are often included in a list of the car's gear ratios. Y'know, the ones that usually list the individual gear ratios, and then a final drive ratio at the end. Another question is whether that axle ratio really is the final drive ratio, or whether it's a product of the lowest gear ratio and the axle ratio. I believe that when I say I have a 4.10 final drive ratio, my axle ratio is actually 4.10, not 0.5 (6th gear ratio) X 8.2. I'll see if I can dig up info on the number of teeth on my ring and pinion gears to corroborate this.

That said, I could definitely see there being an alternate definition of the final drive ratio representing the gear X axle ratio for any given gear, like IMB said.

Also, can anyone provide info on the reducer gear used in a FWD vehicle? I know this is done by the rear differential on a RWD car, so I'm assuming the front diff does the work on a FWD vehicle?

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Post by SouthernBoy » Sat Feb 17, 2007 9:21 am

The final drive (ratio) is the ratio of the ring and pinion gear set. If you have a RWD machine, this is the gear set found in the differential (also called the "pumpkin").

There are a few things, terms for example, that some people find confusing and/or they use incorrectly. A low final drive, or rear end, vs a high final drive. A low rear has a high number so a 4.10 or a 4.56 are low and very low gears respectively. Conversely, a 2.73 or a 3.08 are high rears. As mentioned above, a lower rear end (gear) will increase torque multiplication at the expense of top end speed. Incidently, all of this also applies to FWD machines.

Another thing that people find confusing is the term "close-ratio" transmission. You'd be surprised how many people think this is a description of a short throw gear shift. Close ratio is a term which describes the gear ratio spread between gears in a manual transmission. The reason a close ratio is preferrable over a wide ratio, with certain conditions which I will explain, is that there is less RPM difference between gears. This is important when drag racing since it allows the engine to remain in its power curve where it will produce the most torque and horsepower. Also it lessens the shock to the drive train during shifts. Now the conditions. Close ratio trannies work best with lower final drive ratios whereas wide ratio trannies favor the higher final drives.
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Post by Tinton » Sat Feb 17, 2007 3:07 pm

http://www.f-body.org/gears/

Everything that I would say has been said. Except for that link. Its a gear ratio calculator. Put in your gear ratios, and it'll give you information for speeds in each gear, etc. Its pretty handy, and its much faster than calculating it all by hand.

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Post by black94boost » Fri Feb 23, 2007 5:15 pm

t provides a link between the engine and the drive train so power from the engine can get to the wheels. It does this by the use of gears. A trans has to have a clutch, forward gears, a neutral position, and a reverse gear. The clutch is a switch that lets driver disengage the power from the engine; this will let driver change gears based on the speed of the car. The gears provide either a torque increase or a speed increase. At low speed you more torque to get the car to move. At high speed you don't need as much torque. lets look at the D15B7 in 92-95 dx coupe.

P20/A000 Trans
1st 3.250 - large torque increase to get moving
2nd 1.762 - lower torque increase for speed
3rd 1.172 - lower torque increase for speed
4th 0.909 - very little torque increase for speed
5th 0.702 - very little torque increase for speed
Reverse 3.153 - large torque increase to get moving backwards
Final 4.058 - this is a direct drive for cruising, the trans output will turn faster than the engine crankshaft giving the highest speed increase
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Post by Nervous » Fri Feb 23, 2007 5:29 pm

black94boost wrote:
t provides a link between the engine and the drive train so power from the engine can get to the wheels. It does this by the use of gears. A trans has to have a clutch, forward gears, a neutral position, and a reverse gear. The clutch is a switch that lets driver disengage the power from the engine; this will let driver change gears based on the speed of the car. The gears provide either a torque increase or a speed increase. At low speed you more torque to get the car to move. At high speed you don't need as much torque. lets look at the D15B7 in 92-95 dx coupe.

P20/A000 Trans
1st 3.250 - large torque increase to get moving
2nd 1.762 - lower torque increase for speed
3rd 1.172 - lower torque increase for speed
4th 0.909 - very little torque increase for speed
5th 0.702 - very little torque increase for speed
Reverse 3.153 - large torque increase to get moving backwards
Final 4.058 - this is a direct drive for cruising, the trans output will turn faster than the engine crankshaft giving the highest speed increase
Does this make sense to anyone? Torque for 1st is Higher than Reverse? I thought usually Reverse has the most Torque..

Reading it again, I lost it...Should Final Drive have a goal of 1:1? I just don't see how they get that number from! I'll try to read up on it..

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Post by hai1206vn » Fri Feb 23, 2007 5:58 pm

SouthernBoy wrote:Close ratio trannies work best with lower final drive ratios whereas wide ratio trannies favor the higher final drives.
The reason for that is the engine output, not because tranny gear ratios have to match final drive ratios.

Normally weaker engines like Honda's have peaky power and narrow torque band at top rpm. So tranny gears are closer. And also because of the weak engine, final drive has to be lower to multiply the torque sent to the wheel.

Bigger engines have a lot more torque, so the final drive need not be as low (you don't want too much torque at the wheels). Wider tranny gear ratios is not a big problem for big engines as they have enough torque in the next gear to keep the car moving.
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Post by hai1206vn » Fri Feb 23, 2007 6:18 pm

I thought I'd add another explanation for final drive. Think of the RWD drivetrain - the easiest to understand.

engine ---> tranny ----> differential ---> wheels

(By 'differential' I'm referring to the pumpkin block at the rear, which is not exactly what the term means.)

Engine drives tranny, which multiplies engine speed by a certain factor in each gear. (usually the output rev is less than input rev, but means more output torque). Let say your 1st gear ratio is 3:1. That means at the tranny output, your torque is tripled (but spinning at 1/3 the input rev)

What comes out of the tranny goes into the diff. In the diff, there one set of gear (ring & pinion) that again reduces rev and increases torque. The teeth ratio of the ring vs pinion gear is your final drive ratio. If your final drive ratio is 4:1, your input torque into the diff has been multiplied by 4. In total, your engine torque is multiplied by 3 x 4 = 12.

Finally whatever comes out of the diff drives your wheels directly. So in 1st gear the wheel is spinning at 1/12 the engine rev, but has 12 times the engine torque.
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