Final Drive Ratios.....making my head hurt.

[Nitroholic]

I am trying to work out how to calculate the final drive ratio of a car.....and EXACTLY which parts are used to calculate it. To make matters worse....the end product is harder to measure as I have not put it together yet. I am trying to work out if it is feasable and whether I could gear it correctly.

 

So lets take...as an example...the HPI Savage.

I know that 360 rotation of the spur gear ( 1 complete turn ) results in roughly 60 degrees of axle rotation. That's in first gear. Which makes it harder.

I plan on replacing the 2 speed stock box with a Hyper 7 46T centre diff. I have BP diffs with 9T:29T gears.

 

So the diffs have a 3.2:1 reduction. If I used a 23T  pinion gear to drive the spur...that would be a further 2:1 reduction.....giving a 6.4: 1 reduction.

 

So ..if I have thought this out correctly, 1 turn of the motor ( 360' ) would give 56' of axle turn.. ( 360/6.4 )

 

If I then chose to mount another engine in the same driveline....if I knew the same details from that other engine and the vehicle it was meant for...I could work out what one turn of the motor gives in terms of axle rotation...I could work out the pinion/spur ratio needed to give the same overall axle rotation in, for example, the Savage chassis above. That would ensure the final drive suits the motor and the revs it was intended to operate at. 

[Tug]

I have as excel sheet that might help, pm me if you want it. 

[Nitroholic]

Sure Need to get my head around this to see if a project I have in mind is really possible or if I need to put in extra effort to get what I want.

[dazp1976]

Aren't you over thinking it? Don't you just divide the ring by the pinion in a drive diff & the spur by the pinion, Then multiply?

Say you used a 15T pinion don't you just go:

 

29/9= 3.22222:1

46/15= 3.0667:1

Then times the 2 together giving a final drive of:  9.881:1

 

Bit quick for a MT like that I feel.

I burned a couple of motors in the past on a MGT with a CD conversion.

Why do you think they come with reduction gear boxes.

Edited August 5, 2018 by dazp1976

[Fly In My Soup]

You can gear down on your crown and pinion when swapping gearbox for CD to avoid heat/draw issues......or just get a overspecced ESC ....or alternatively get a lower kV motor.....if that helps

[Nitroholic]

Heat draw. ..nope.   Nothing electric going on here ?