Fact or fiction - final drives

[RattyMcClelland]

Surely if both cars are identical in everyway except the final drive then from stationary they accelerate at the same rate. Both reach 4000rpm, 6000rpm etc. If this is right then the longer final drive is reaching a higher speed in every gear.

Basically yes.

But the rate of acceleration will be quicker with the shorter final drive.

As said fd's have no effect on gear ratios so if you have 2 identical cars and one with a shorter final drive even the downshift we will be identical. Gear changes will be exactly the same, rev drop identical but the speed at those revs will be different.

Now if I was to put very large 20" wheels with fat tyres I will undo what my shorter final drives does making the gears longer.

[newkid]

Sorry but from what I'm reading and working out a shorter final drive on a car that's the exact same will be slower in a straight line from a standing start.

Gear ratios haven't changed so the cars are going up the rev range at the exact same rate

Both from a standing start and changing at 7500rpm would result in this if both cars had 205/50/16 tyres

1st

4.266 38mph @7500rpm
4.64 35mph @7500rpm

Now if the ratios haven't been changed then both cars will be changing into second at the exact same time, car 1 already has a 3mph advantage so 2nd would result in if both going to 7500rpm again

2nd

4.266 65mph @7500rpm
4.64 59mph @7500rpm

Same again if ratios haven't been changed then both cars will be changing into 3rd at the exact same time, car 1 now has a 6mph advantage so 3rd would result in if both going to 7500rpm again

4.266 93mph@ 7500rpm
4.64 85mph@7500rpm

Car 1 after 3 gears now has a 8mph advantage!

Now worked out on the tyres me and vanzep used me 205/50/16 with final drive 4.64 and vanzep 205/55/15 with final drive this would result in

1st

4.266 38mph@7500rpm
4.64 35mph@7500rpm

2nd

4.266 64mph@7500rpm
4.64 59mph@7500rpm

3rd

4.266 92mph@7500rpm
4.64 85mph@7500rpm

This results in just the first 3 gears a speed difference of 7mph

Now its impossible that a shorter final drive in these circumstances will accelerate quicker. How can it the gears are the same so both reach 7500rpm at the same time and when you shift to next gear both have the same rpm drop.

So in theory a shorter final drive in a straight line will actually make your car slower if both cars have the same power.

[wurlycorner]

No - although the ratio between each gear in the box is the same (I.e. changing from 1 to 2 is the same change in ratio) the ratio between each engine and the road is different, because the FD is an additional ratio in the gear chain.
So not only does it affect the max Mph you can reach in each gear, it does also affect the rate of acceleration. One of those boxes will accelerate quicker than the other.

[newkid]

I'll use 4th

Driveshaft is turning one revolution per 1.071 turns of the crank

Wheel axle is rotating one revolution every 4.266 or 4.64 (depending on final drive) revolutions of the driveshaft. This means a 4.64 final drive needs to turn more to reach one full revolution of the wheel axle. 4.64 is working harder then to reach the same results.

Great if your on track at x mph in a certain gear but from a standing start the 4.64 has to do alot more work so will always accelerate in mph slower in those circumstances than a 4.266. Its impossible that evenly matched cars with only different final drives from a STANDING start will accelerate in mph at the same rate, the 4.266 is having to do less work (turns) to reach x mph than that of the 4.64. Standing start means 4.266 will reach say 30mph quicker than a 4.64.

[RattyMcClelland]

It's very difficult to explain.
The ratios are the same but it revs through the rev range quicker.

Two cars identical.
Different final drives.
The smaller or shorter final drive will do more revolutions at the same revs as a longer final drive.
The more revolutions means faster spin rate and faster acceleration. Just like smaller diameter wheels and tyres.
The compromise is less mph which is usually where you add a 6th gear to make up for it.

[NCCMUR]

[bennyboy]

Way back when I used to muck about with radio controlled cars, you could really affect the acceleration and speed by changing the pinion gear.
A smaller gear, with fewer teeth, would show faster acceleration but a slower top speed. It's able to spin up quicker because there's less resistance and therfore less force needed to spin it.
The larger pinion would give a higher top speed, but took noticeably longer to get there because it's actually 'harder' for the motor to turn it.
I know I'm massively simplifying it, but if you think about it you can ignore the individual gears as, as wurly says they are the same in that the ratio between them is the unchanged, therfore what you're doing by changing the final drive is a large scale version of the RC pinion gear.
I remember when my brother had his ep3 and I had a my stock UK lude. There was nothing in it from a start, even though on paper you'd think you would separate it. The reality was he would pull just a fraction, but as he had to change gear sooner than me, I would pull level with him, and so it went on up the gears. 5spd vs 6spd.
Theres a lot of variables in your setup, not least of all driver and reaction time on any given run.

[newkid]

It's very difficult to explain.
The ratios are the same but it revs through the rev range quicker.

Impossible, revs are determined by the crank, nothing has changed in the engine so no matter what the engine will still rev at the same rate.

Two cars identical.
Different final drives.
The smaller or shorter final drive will do more revolutions at the same revs as a longer final drive.

The 4.266 needs less revolutions to turn the wheel fully once so surely at the same revs it takes longer hence the slower speed for the shorter final drive to turn the wheel once 4.266 turns off the driveshaft v 4.64

The more revolutions means faster spin rate and faster acceleration. Just like smaller diameter wheels and tyres.

Smaller wheels accelerate at the same rate, standard final drive means still takes 4.266 revolutions to turn that wheel once, this results in a slower top speed
The compromise is less mph which is usually where you add a 6th gear to make up for it.

Above shows that to reach the same speed in each gear a 4.266 v 4.64 final drive the 4.266 needs the engine to reach a full 600rpm less than a 4.64. So if both engines are going up the rpm at the same rate from a standing start then its impossible for the 4.64 to keep up

[K30DPC]

For both cars gear ratios are the same and only fd different. This mean overall ratios for gearboxes are different as it is counted gear ratio times fd.

In very simple words gearbox is changing engine revs into wheels power. For every car it is easy to start going from first gear, possible from second, but not from fifth, bare in mind engine power is NOT related to gears. Also top speed will be different on every gear and from 0 to 35/38 you will be quicker on first then second gear. This is because of different gear ratio. Now think about it this way. You have another gear in your gearbox. This one is fixed and you can't use gear shift to change it. Yours is 4.266 and other one is 4.64. Yours will be slower top speed, but will get you quicker throughout all engine revs. It is like you would compare 1st and 2nd gear.

Also look at serious 4x4. They have fitted something called IIRC overdrive gear. This is something like extra gearbox with second gearshift. It is nothing else like changable final drive. It doesn't affect gear ratios, but giving extra power to your wheels sacrificing top speed. And more wheels power means better acceleration (even if here we are looking for more torque).

Was it worth to put into your car? Only you know the answer. But comparing bb1 and bb4 even with 4.266 fd is like why my h23 is slower than h22. There is much more aspects to look at and personally i would start from weight.

[newkid]

Two identical cars, assume 160lbft of torque, one with 4.266fd, one with 4.64

Car A

Top speeds and wheel torque in each gear for 4.266
1st - 38mph - 2257 lb/ft
2nd - 65mph - 1331 lb/ft
3rd - 93mph - 928 lb/ft
4th - 118mph - 731 lb/ft
5th - 145mph - 594 lb/ft

Car B
Top speeds and wheel torque in each gear for 4.64
1st - 35mph - 2455 lb/ft
2nd - 59mph - 1448 lb/ft
3rd - 85mph - 1009 lb/ft
4th - 108mph - 795 lb/ft
5th - 133mph - 645 lb/ft

0-35mph Both in 1st - Car B has a 9.19% advantage
35-38mph Car B is into 2nd, Car A still in 1st. Car B at a 35.85% disadvantage
38-59mph Both in 2nd - Car B once again has a 9.19% advantage
59-65mph Car B is into 3rd, Car A still in 2nd. Car B at a 24.2% disadvantage
65-85mph Both in 3rd - Car B once again has a 9.19% advantage
85-93mph Car B is into 4th, Car A still in 3rd. Car B at a 14.3% disadvantage
93-108mph Both in 4th - Car B once again has a 9.19% advantage
108-118mph Car B is into 5th, Car A still in 4th. Car B is at a 11.8 disadvantage

All sounds great but the 4.64 needs to turn 9.19% more to achieve full rotation of the wheel.