Need some math help: gearing and wheelspeed
 

there was another thread i had been looking at (http://www.rccrawler.com/forum/gener...ront-axle.html) but figured i'd ask the question in a general forum.

so i'm looking to build a transfer case that will allow me to match wheelspeed between a standard wheeled axle and a tracked axle. the setup is front axle is an scx10 axle with underdrive and standard 1.9 tires and the rear axle would be overdrive with rc4wd predator evo tracks.


this is the problem i was trying to work:

if the front as underdrive (43/13) and a 4.1" tire and the rear has overdrive (36/14) with the 2" drive cog does the math work like the following:

tire to drive cog ratio = 2.05
over to under drive ratio = (36/14) / (43/13) = 2.57 / 3.31 = 0.776

total delta = (tire to drive cog ratio) x (over to under drive ratio)
= 2.05 x 0.776
= 1.591

if that is correct I could get something like a 24 / 15 pinion ratio (1.6) or 19 / 12 (1.583).


suggestions or corrections are welcome!

Re: Need some math help: gearing and wheelspeed
 

Seems you've got the math about right.

I'd just set it up a bit different, going "drive shaft to ground" for each axle.
Front: 13*4.1/43 = 1.24
Rear: 14*2/36 = 0.78

You want these to be closer to each other, though I suspect some overdrive (10-20%) on the front axle is good for tighter turning.

You can simply use smaller front wheels and/or see if there can be more differentiation in the axle gearing.

Re: Need some math help: gearing and wheelspeed
 

thanks for the input. getting confirmation my math was solid helps.

as far as overdrive on the front axle that works ok if both are running wheels but from my experience with tracks on both axles if there is a front overdrive the rear axle tracks end up getting dragged, causing them to tip up so you don't get flat contact with the ground. that is what i'm going to have to be very close on the "drive shaft to ground" balance via the transfer case. i'm probably going to have to make a transfer case with moveable shafts so i can fine tune the pinion sizes.