I've never fully understood what a final drive is, and what advantages you can see by replacing a final drive gear with another. I've been looking at Mista Bone's tranny page and there's good useable information there, but I still have that question. Can anyone explain this for me? This may be a good future source for tranny information.

 

Not to get too techish on you, putting in a "lower" FD is like shifting to a lower gear on a mountain bike. You can accelerate more effectively with a given amount of power - in essence - at the expense of ultimate top speed. NOTE: A "lower" final drive ratio is typically referred to as a HIGHER number (e.g., 4.25 vs. 3.25).

The math gets complex because aerodynamic drag increases as the square of velocity. You can get to redline in 1st, 2nd, 3rd, and maybe 4th but never in 5th - typically - because you just don't have the power to overcome the drag. By putting in a lower gear, you shift the whole range downward.

If you have a Win machine, try a copy of AutoInfo from http://www.davesport.com and play around with some combinations on the gear chart.

Kirk

 

From the little I know, what Kirk said makes sense. Think of a short final drive as going from the big ring to the small ring on a bicycle. The gears get proportionally shorter.

 

So, the final drive affects "each gear", not just 5th?

Example/Question: If my DX 3rd gear is 1.172 with a final drive of 4.058 and I swap out the final drive for a EX/Si 4.250, does this "change" the ratio of the 1.172 3rd gear?

John -- who maybe just isn't thinking clearly after chiseling tar off the floorboard all evening, and thinks he almost understands it

 

Well, your 3rd gear would still be a "1.172 gear"... but, at redline in 3rd gear you would be running a slower mph with the 4.250 than with the 4.058.

 

Good analogy. To keep it clear, say that the gears on your rear bike sproket are your 1-5 gears in the car, and the FD would be the gear at the crank. When you change to the smaller crank gear, your sproket gears don't physically change, but the overall gear ratio for all of the sproket gears has changed.

-231, who likes bikes 'cause you can see what's happening.



[Modified by ITR#231, 11:55 PM 5/29/2002]

 

JohnG...

I never understood it either until I tore apart the tranny.

I saw 2 shafts that had gears on it... a counter & a main.

I saw that when the shafts spin, the size of the gear dictates how fast the shafts turn for what gear you are in. This speed gets transferred to the final drive gear.

The final drive is attached to the differential that holds your axles. The size of the final drive dictates how fast the differential is turned. Wheels turn... go fast unless its a Geo.

Pete

Just adding to the confusion & sense... go buy a messed up tranny & rip it apart

 

gear reduction..........

with a Honda tranny you have 2 sets.

the gear you are in + the FD reduction

It is hard for me to explain, because it is so simple for me.

OK lets try this...........
Redline in first gear........
9,000 / 3.23 = 2786.4, at 9000 engine rpm in first gear, the countershaft will be turning 2786.4 rpm.
Now the final drive.....
2786.4 / 4.40 = 633.3 rpm, this is the rpms of the diff/axles (output shaft)

Not to sound like TOO/Larry, but I can "see" this very easy, just hard to explain. This is one reason I understood his ideas. Just don't agree with some of the business practices.

You can add a thirg reduction by using smaller tires. Figure using 20" slicks on a Civic which uses 22.7" stock, would be another 1.135 ratio reduction.

 

BTW here is the link of a speedcalc that "nonsense" wrote. It is based on the same thing I've been using for 20 years now.

http://nonsense.netfirms.com/speed.html

I hope I've helped.

 

wow.....great thread. ive always wondered this myself and now i feel as if i understnad pretty well. so then.....the ultimate goal in using a different final drive (4.250 vs. 4.058, for example) would be quicker acceleration while sacrificing top speed in any particular gear? so this is kind of like messing with gears in gran turismo (to put things in a video game sort of way). thanks for all the info....

CHRIS

 

Changing the FD will allow either quicker acceleration in all gears or a higher top speed if maxed out, without affecting the optimal ratio spacing of the gears.

 

Fact be known, GT1 is great for playing with gearing. You have to have similar mods done to your PS1 car as real car.

GT1 said my 1.5L would be .3 faster with closer ratio gearsets. It was close, I was .35 faster in the 1/4.

 

so where would one get a new final drive for say a stock tranny for a d16z6? im not really looking for one, more curious than anything else.....i like to learn as much as i can. i have seen new final drives for the b-series trannies but never for the d-series trannies. do you just use ones from different models and make hybrid trannies or do companies out there sell these? thanks again....

CHRIS

 

The effect of your entire gearbox is a function of both the ratio of the gear you are in and the final drive ratio. The short answer to your question is yes, although the actual ratios of your gears (1,2,3, etc) do not change. Since we are getting techier by the minute...

Your gearbox allows your engine and wheels to spin at different rates, for any given road speed, remembering that your engine is most powerful/efficient in a particular rev range and just won't work at all in others (say, over 10,000rpm!). Think how limited your performance would be if you only had ONE gear...

Your engine turns one shaft in your gearbox (input), which turns another shaft (output) via a pair of gears that you can select with your shifter. There is a ratio (both gear and rotational speed) between these shafts, different for each gear. The output shaft then spins the final drive gear set, so there is a ratio (the "final drive ratio") between the output shaft and the axles as well.

A 1:1 ratio between two shafts would mean that both gears have the same number of teeth, so the shafts will spin at the same rate. This is the dang near the case with the CX 3rd gear (thanks Mista Bone!) that has a ratio of 1.066 - the input and output shafts are spinning damn near the same speed.

In that same gearbox, 2st is a "1.761" - this means that the input shaft (and the engine, since it splines right onto it) are spinning almost 1.8 times as fast as the output shaft (or the output shaft is spinning 1/1.8th of the speed of the input/engine - a crappy fraction but think something like output spinning 1/2 of the speed of the input shaft & engine).

Follow so far? Now, if this car has a stock 3.25 final drive, the output shaft is spinning 3.2 times as fast as the axle. The ratios multiply as you go so the engine is now spinning at a rate that is 1.761 x 3.25 or approximately 5.7 times that of the axle. If your tach said 5700rpm, your axle is spinning 1000rpm, to pick easy numbers...

Note here that your tire diameter establishes yet another ratio, between the axle rotational speed and your road speed. That is a different lesson...

Technically, these ratios are actually 1:3.25 or whatever and the REAL thinking goes the direction that I started with that crappy 1/1.8 fraction (equal to .555555...) but I think it is easier to visualize this way.

Gotta go. Hope this helps a little and I didn't dick anything up - didn't have time to re-read all of this - so feel free to catch my errors.

Kirk

(Who used to be a mediocre junior high science teacher)

 

Yes. And what you really need to look at is the OVERALL final drive ratio for any gear which is (gear ratio) x (final drive) = overall final drive. To get the effective torque at the driveshafts you multiply the flywheel torque by the overall final drive ratio. So if you use a numerically higher FD, you are effectively increasing the torque multiplication at the wheels.

 

I may be late with this, but this is how I explain it...

Its the FRONT sprocket on a 10 speed. All the rear sprockets are 1-5th gear.

Change the front from big to little.... thats the effect of changing the FD

 

Here's a little Excel spreadsheet I put together to calculate speed in each gear at various RPMs.

Just change the <FONT COLOR="blue">blue</FONT> numbers (tire size, gear ratios and FD) to the numbers you want.

ratios.xls

 

FILE --&gt;SAVE AS......

that spreadsheet is awesome.....nice work...thanks for posting it up....

CHRIS

 

Cool spread sheet. Now where do I find out the gear #'s for a D16Z6 tranny? What model # is that one anyway? Thanks!

 

the easist way to think of a final drive set is to think of a 21 speed moutain bike.. the 7 speeds in the back each represents a different set of final gear

 

gear ratios for d16z6.....these should be right...i found them in another post on this forum.....

92-95 ex
1st: 3.25
2nd: 1.9
3rd: 1.25
4th: 0.909
5th: 0.702
Final Drive: 4.25

i like the idea of a 4.7 final drive in that tranny....keeps you in VTEC nicely....4.9 is even better....but highway driving would be at really high rpm's

CHRIS


[Modified by deviant94ex, 2:10 PM 5/30/2002]


[Modified by deviant94ex, 2:13 PM 5/30/2002]

 

One important thing to keep in mind about changing the Final Drive, is that it affects the top speed in each gear, but it does NOT affect RPM drop during shifts. This means that even with short tires and a higher (bigger number) final drive, you'll still be dropping off the power band on all your shifts.

Also, with the higher (bigger number) final drive, you'll have to shift a lot more, which might actually end up costing time in the long run, especially depending on the track and how fast you need to be going in various places.

It seems to me that if you could somehow extend the power band of the DX motor out to about 7500rpm (limiter is at 6800 stock, I think) or maybe even 8000rpm, and get the motor to safely rev that high (prolly not happening with stock rods, though) then the stock gear ratios wouldn't hurt quite so much, and the extra RPMs would put your top speed in each gear, with a higher (bigger number) final drive, at about the same place they were before.

Final drive modifications are hotly-debated in the NSX community. While they're a great modification for *feeling* like you're going faster, their effect on actual quarter-mile times is very small, especially compared to the amount of money it costs to change it out on the NSX. In most cases, with the NSX at least, the money would be better spent increasing horsepower. But I don't know what you can do to your engine in your class, so maybe that's simply not an option.

-Mike

 

In an ideal world, you would start by figuring out what the top speed of your car would be (given HP, and the "negative horsepower" of aero drag and frontal area), considering the nature of the course on which you are racing. This really isn't all that tough - given the Bonneville salt flats, an '95 DX in IT trim is probably drag-limited to 120mph or thereabouts and this probably doesn't happen in 5th!

Given this, plop in the final drive that gets you to redline at the end of the longest straight that you expect to see. With stock gear ratios, this makes the low gears obscenely low - you could pull from a dead start in 2nd or 3rd

"Real" racing cars do the same at the bottom end of the gearbox as well, raising the ratio (numerically lower) of the lowest useful gear so that they work in the slowest corner on the course, and splitting the remaining ratios approximately evenly - this last point is an overgeneralization but work with me.

Kirk

 

???

I dont see how that is so. A shorter final gear would mean that the matching engine speed for a given speed would be higher than w/ the factory final drive gear. So in the next higher gear, wouldn't the matching engine speed for the given speed (in this case the redline speed of one gear) would be higher than stock, thus minimizing the RPM drop when upshifting?

 

You are correct that the engine speed for a given CAR speed in a given gear would be different with the different final drive. However, the change in RPMs when shifting from one gear to the next, would be the same. As an example, in the stock 91 NSX transmission (4.06 final drive), 1st gear runs out at 45mph (8000rpm), and when I shift to second the rpms drop to roughly 4500rpms. If I were to put in a Comptech 4.55 final drive, 1st gear would run out at 40mph, but when I shifted to second, the RPMs would still drop to about 4500. Now, if I were to put in the JDM NSX-R gears, which have a shorter 2nd, 3rd, and 4th, then when I shifted to second, the RPMs would only drop to about 5000, because the difference in ratio between 1st and 2nd gear is different.

So a really simplified view would be to say that the gear ratios (and the difference between them more importantly) controls the RPM range within which you operate the car, making sure that you're always on the power band. Then the final drive is selected to give you the desired top speed in your highest gear.

It's also interesting to note with a particular engine, no matter how much you mess with the transmission, you're still going to end up in the same general RPM range when accellerating in a given speed range. Going back to the NSX example, the stock transmission has me in 4th gear from 115-140mph. Now, I could put in a REALLY short final-drive, like 5.0 or something, so that I'd be in 5th gear from 115-140mph, or I could leave the final drive and make all my gears shorter so that I needed to be in 5th gear from 115-140... But no matter what I do with the gears, I'm still accellerating from 115-140, and I'm still running the engine from about 5000-8000rpms, and the overall ratio of the drivetrain will still be the same. Granted, this is a big simplification, but it's an important note. At the end of the day, it's still your engine and it's torque/horsepower that has to accellerate you, and all the gearbox and final drive can really do is help make the most of it, by keeping you in the power band, and shifting the speed ranges of each gear around so as to best match the track you're on. But the example is still important when thinking about race track performance. If you just completely forget about the existance of the gearbox for a moment, and imagine that you are exiting the final turn onto the straight at 5000rpms at 80mph, and will be going at 7000rpms at 112mph at the end of the straight... It really doesn't matter whether that gear is 4th gear or 5th gear, and what combinations of gear and final drive and tires brought you to that speed/rpm ratio. As long as that's your speed/rpm ratio, you will accellerate down the straight at the same rate.

-Mike