Changing your final drive ratio

[Roman]

Just a few thoughts down on a page about final drive ratios, and what happens when you change it.

The final drive ratio turns rotations of your driveshaft, into rotations of the wheels.

It also turns power sent to the crankshaft, into power at the wheels obviously.

As a general rule, depending on final drive ratio, for every 3-5 rotations of the driveshaft, the wheels will turn once.

Which isnt too complicated, but what are the advantages of changing your final drive ratio?

Some people say that changing to a higher ratio (say 3.5:1 to 4:1) increases the torque at the wheels.

Although this is kinda true, it means that you are creating more torque, at less speed, which is the trade off.

If you are going 100kph, you are going to have less torque available than 50kph, because it has to share the available torque between say 200 turns of the wheel rather than 100.

This is one thing that the horsepower formula is for, it lets us figure out how much torque you can actually use at a particular speed.

So forgetting gearboxes, final drive ratios, and all the rest of it for now.

Lets say that we had a wheel, which has 200hp, 150hp, and 100hp available to spin it.

How much torque is available, depends on the wheel speed.

As per below, based on Torque = (5252*Horsepower) / RPM, and a 205/50/15 tire

As you can see, when the wheel speed increases, you lose torque. Waah.

Obviously no motor has 200hp through out the whole speed range, as when the motor goes through its own RPM range it picks up or loses power. Also, obviously the car changes gear as the speed increases.

So the amount of torque actually available at the wheels looks more like a sawtooth pattern as we go through each gear:

Some people say that changing the final drive ratio, increases the amount of torque available at the wheels.

What it actually does though, is it changes the speed at which each of your gears work... it moves the sawtooth pattern up or down the graph. So although at some points on the graph you DO gain torque, you are also moving the dips as well. As per the blue line below:

It's impossible to have more torque available at a given speed, without more horsepower. As you can see comparing the blue line to the black one, every time the blue line is higher than the black, more torque is available at that speed. However you also have LESS torque available, at other speeds when the blue line is lower than the black one.

So, when IS it useful to change final drive ratio?

In my opinion the biggest reason is to change the speed at which each gear in your car starts to work.

If your car came with a motor which peaked out at 5000rpm, and now you've got a motor that goes to 9000rpm, 1st gear might have previously gone to 50kph and now goes to 90kph. So you can change the final drive ratio, so you are getting into the powerful part of the powerband earlier in the speed range again... So first gear might top out at 50kph still, but now at 9000rpm instead of 5000.

Also, it can be useful to change the final drive ratio to suit particular corners of a track, for example if your car was getting unsettled because you had to change gear right before the exit of a corner, you could have a lower final drive ratio so you dont need to change gears until you're on the straight.

Here are the pros and cons of going to a higher final drive ratio: (say from 3.9 to 4.5)

Cons:

-Engine RPM while cruising increases... 4500rpm at 100kph is not very comfortable for a daily car!

-Potentially more gear changes to get to a particular speed, which makes you slower/the same

-1st gear can become useless apart from doing skids, if you now have more torque in 1st gear than traction

Pros:

-1st gear can become handy for doing skids, if you now have more torque in 1st gear than traction

-Reduce the speed range of each gear to suit a new motor or gearbox

-Make use of higher gears that were previously only useful at a speed that you cant reach

-Fine tuning speed at which each gear is useful to suit corners of a track

Changing final drive ratio isnt a magical torque booster though!

Better living everyone!

[mjrstar]

This is a great conversation topic, what i found going to a taller ratio (from 4.3 to 3.9) in my starlet was that the car worked well with the increased load and actually made the car a fair bit quicker, the issue i had is that it would not make full boost in 1st or second gear as it would simply rev out too quick. Giving it longer legs was a great upgrade. This would be an isolated occurence and was probably based predomanantly around the current power to weight factor. you could pretty muh ignore this arguement on a N/A car.

Ideally rather than just changing final drives the best thing to do is get a decent gear set, normall comprising of a taller 1st gear and then tighter spaced gears between 2nd and 5th. This provides all the benifits mentioned above, but probably the biggest improvement will be the time it takes to shift.

Clutch mass and speed difference in the shafts are probably the 2 biggest limiting factors for how quick you gearbox will accept a gearchange. (other than general mechanical condition) I noticed a huge difference in my mini when I changed to a close ratio setup, you could feel the synchros were not having to work so hard to "speed Match" the gear selection. it will no swap cogs as fast as you dare wrench on the lever.

Pretty sure most will TL:DR this but Meh..

[Roman]

Hey cheers for the input!

Yeah thats a really good point about turbo engines, didnt think of that.

And yep, with non roadcar transmissions where you can smash through the gears almost instantly, there's more of an advantage to chopping through a lot of gears up to a certain speed.

A lot of aftermarket gearboxes have the 5th or 6th gear as 1:1 as well, so you've got a really tall 1st gear, then a crapload of them within a narrowish speed range. In which case a really high diff ratio might not suit anyway.