A test on final drives

[Dylanamus]

Please answer if you are certain and can justify/explain your answer with math/facts:

Take a hypothetical car that has a final drive of 4.266

1. If all the gears were left unchanged and the final drive was changed to 4.785, would this make the existing gear ratios

A. closer together or;
B. further apart


2. If the answer to 1. was closer together, would this

A. improve acceleration and lower top speed or;
B. increase top speed only
C. closer together is wrong

3. If the answer to 1. was further apart, would this

A. improve acceleration and lower top speed or;
B. increase top speed only
C. further apart is wrong.

Thank you for participating in my questionaire. :thumbsup:

[ZeForce]

A shorter final drive will result in more torque at the wheels, due to torque multiplication, which will improve acceleration but reduce top speed

This link might be worth reading....

http://www.team-integra.net/sections/articles/showArticle.asp?ArticleID=707

[Dylanamus]

That is a good article. Thanks.

Now I fully understand the math that explains why a 4.785 final drive in place of a 4.266 would result in faster acceleration.

[kyle]

A 4.785 final drive would make your accelration times faster and lower top speed, but it wouldn't make your gear ratio's closer together. To my understanding the clossness of the ratio's is the space between 2 gears.

Example:
Y1 Gearbox
1st = 3.166
2nd = 2.052
Difference = 1.54

Y21 Gearbox
1st = 3.230
2nd = 2.105
Difference = 1.125

Therefor between 1st and 2nd the Y21 has a closer gear ratio.

The final drive is just the ultimate final gearing. Weather it was 4.2, 4.4 or 4.7 doesn't change how close the gear is just its final output.

Please someone correct me if im wrong as im eager to learn.

- Kyle

[Dylanamus]

Kyle I agree with the point you are making. This explains why a cheap alternative to changing the FD is just bringing the most used gears closer together - ie just changing the amount of teeth/size of 3rd and 4th.

However, while increasing the ratio of the final drive won't change the ratio of difference between all 5 gears, it should technically bring their relative "closeness" together EVENLY. I mean changing the FD won't make the ratio between First and Second or Second and THird etc any different, but will make them all evenly closer. Does that make sense? :/

[kyle]

yes i think i get it
*.*

[SeverAMV]

well for simplicity, a 10% larger final drive will reduce the top speed of every gear by approximately 10%, thus bringing them closer together.
this improves acceleration between gears as well, as usually in close ratio gear boxes, when you upshift, you'll be higher up in the torque/power band.
when deciding on a final gear ratio, focus on what you will use the car for. higher final drive ratio will boost fuel consumption as your motor will be running at higher revs than normal. (ie. 100km/h in fifth might go from 3000rpm to 3300rpm). if you're tinkering with the gear ratios for track use, then decide which gear you use the most and how long you want it, and adjust the final drive ratio accordingly. ie. if you take corners at 100km/h normally, probably better to lengthen second gear to about 110km/h.