Gear Ratios

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Post by Nervous » Fri Feb 23, 2007 6:28 pm

hai1206vn wrote: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.
Ok this actually helped me. So what you're saying, is if my 1st gear has a ratio of 4:1, and my Final Drive Ratio (Which never changes?) is at 4:1, then my Torque would be 4x4=16 times what the engine is giving.

If ^^ is true, then that also means the Wheels are spinning 1/16th the Rev's of what the engine is spinning at?

Edit: Ok, I re-read your post, I think I just made up new #'s and used the same explanation :oops:

Nice Explanation.

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Post by hai1206vn » Fri Feb 23, 2007 9:21 pm

Nervous wrote:and my Final Drive Ratio (Which never changes?) is at 4:1
Correct, final drive ratio can't change (the diff is fixed, unless you install a different gear set for it).

All I wrote was just the theory. For your numbers, in real life you're only guaranteed that the wheels will be spinning at 1/16 of the engine's rev, but the actual torque at the wheel is less than 16 times the engine torque (probably about 12). For automatics without rigid coupling between flywheel and tranny, I'm not sure what that factor will be.

In the case of 4WD with electronic control of the center diff and individual torque sent to each wheel, the situation will be more complicated. Actually I don't know what "final drive" refers to in this case, as 4WD's have 3 differentials. Anyone have an idea?
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Post by black94boost » Sat Feb 24, 2007 2:27 am

Nervous wrote:
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..
I copied and pasted that from another forum :P

Some more information I found about gear ratios.

A five-speed transmission applies one of five different gear ratios to the input shaft to produce a different rpm value at the output shaft. Here are some typical gear ratios:

Gear / Ratio / RPM at Transmission Output Shaft with Engine at 3,000 rpm
1st / 2.315:1 / 1,295
2nd / 1.568:1 / 1,913
3rd / 1.195:1 / 2,510
4th / 1.000:1 / 3,000
5th / 0.915:1 / 3,278
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