Effect of final drive ratio?

[RomanV]

Hi folks.

Okay. I understand what gear ratios do.
However, I have a vague idea of the relationship between final drive ratio and individual gear ratios.
The following is just my assumptions on how things work, so it is very possibly wrong.


I would imagine that the final drive would be the engine RPM/wheel RPM if you were in a 1:1 gear ratio. So it is basically the gearing reduction achieved by the differential, rather than the individual gears. So if you had no gearbox, just an engine connected up to the driveshaft, the ratio of engine RPM/Wheel RPM would be equivilent to the final drive ratio.
Correct?

Here is some random gearbox information.
Gear Ratios:
5th (0.82): 3.53:1
4th (0.97): 4.18:1
3rd (1.31): 5.65:1
2nd (1.90): 8.19:1
1st (3.70): 13.66:1
Final Drive Ratio: 4.13:1

How would I work out the wheel RPM from the engine RPM in a given gear?

Would this be engine RPM/gear ratio/final drive?

There are two numbers listed there for the gear ratios. I would imagine that the one in brackets is the 'adjusted' ratio. (gear ratio/final drive) And the next number is the 'real' gear ratio.

Am I correct? Or am I completely wrong perhaps.

Thanks
Roman

[JT]

You are correct,

Say the engine is doing 5000rpm in top gear and it passes through a 4.3:1 diff. The equation is:

5000/.82(top gear) = 6097.5 driveshaft rpm
6097.5/4.3 (diff) = 1814 axle rpm

Here's a quick read that explains it in a little more detail.

http://home.clear.net.nz/pages/phil.bra ... fferential

Edit: The final drive ratio in that 'random' gearbox info is wrong. It should be a 4.3:1, the last number is the adjusted ratio and is gear x diff. You can take engine rpm and divide it by the adjusted ratio to get the axle rpm

[RomanV]

Cool, thanks JT.

EDIT:

Yeah well I thought I could check if the second set of numbers was the 'adjusted' ratio, but dividing one by the other didnt equal the third, which made me think I was wrong.

That is apparently the gear ratios for an AW11, so it wouldnt surprise me if something was amiss.
Perhaps their gearboxes get rusty too.

[Mr Revhead]

basically the gearing reduction achieved by the differential, rather than the individual gears

correct

There are two numbers listed there for the gear ratios. I would imagine that the one in brackets is the 'adjusted' ratio. (gear ratio/final drive) And the next number is the 'real' gear ratio.

the one in brackets is the gear ratio, as in the change from input shaft to output shaft of the gear box.

the last numbers would appear to be the overall ratio, after the gearbox and diff. however its not quite qorking out right (or i may not be working it out right!)

as to working out the wheel speed....

lets take the first gear above, 3.7:1 that is the input shaft of the gear box turns 3.7 times for every turn of the out put shaft.
so lets take the engine RPM as 5000rpm. divide that by 3.7 = 1351.35
thats the rpm of the drive shaft.
crank divide by ratio = output speed

now the diff or final drive.
4.13:1 that is the input turns 4.13 times every turn of the out put.
input being driveshaft/pinion out put being crown wheel/axle.

so you divide the drive shaft speed to get axle speed.
so 1351.35 divide 4.13 = 327.20

so the wheel speed is 327.2 rpm for the 1st gear given above.

thats how i understand it..... if im wrong feel free to correct me!

[Mr Revhead]

6097.5/4.3 (diff) = 1814 axle rpm

maybe your calculator is rusty?
i work thatout to be 1418.02......

[JT]

the one in brackets is the gear ratio, as in the change from input shaft to output shaft of the gear box.

the last numbers would appear to be the overall ratio, after the gearbox and diff. however its not quite qorking out right (or i may not be working it out right!)

I gave you the answer in my post. It's not 4.13, it should be 4.3

.82 x 4.3 = 3.526, close enough to the 3.53 listed

6097.5/4.3 (diff) = 1814 axle rpm

maybe your calculator is rusty?
i work thatout to be 1418.02......

Pffft, you couldn't work out 4.13 from 4.3

[Mr Revhead]

haha i wrote mine while you were writing yours... and its actually 4.31

[JamesM]

so for u... assuming u got no rev limit.. and u love ur 4age.. 8000rpm in 5th.. assuming wheel circumfrence of around 1.88m ul be going 255km/h... but u should probably test this out to make sure.

[TygerTung]

I don't think a standard 4A-GEU would have enough power and torque to overcome the wind resistance at much above 200 kmh

[wde_bdy]

I don't think a standard 4A-GEU would have enough power and torque to overcome the wind resistance at much above 200 kmh

Easy, just run the car downhill in a vacuum

[Mr Revhead]

hmmm all factory 4age manuals iv been in have been capable of 200, 250 is probably wishful thinking but given enough of a run up i reckon they could do some interesting numbers. i wouldnt be suprised to see 220 from either aw11, ae86, ae101 or ae111.

how they feel at that speed is another thing.....