I have been designing a three gear system for a while now, but I have been always stuck on what gear ratio to use, since there are so many conflicted opinions here.
Here are some pictures of what I have designed a couple weeks back
Working with @uigiroux
@pixelsilva since you have seen some of this design before, you might have something to say
@MonkeyM since you have a mecheng background
Currently thinking of gears being 16-27-38 with ratio of 2.375:1.
The middle idler gear is 11 teeth from both the motor gear and the wheel gear so it should have equal torque from both sides… That’s the idea at least.
… any particular reason for three gears?.. two points of friction vs 1 with only 2 gears? :o
Yes, specifically for clearance. It was found out in the fatboy prototype for TB trucks and PU wheels that there wasn’t enough clearance from the base plate and king pin when running motor larger than I think it was 6364’s. We want to be able to run any motor, up to 6384 and have no issue with clearance of any kind. This is really the only way we can do it while keeping the wheel gear small enough that the drives clearance from the ground is able to be a sufficient amount without worrying about the drive case scrapping the ground.
This drive is specifically meant for PU wheels, so the gears need to be small and 2 gears doesn’t place the motor far enough away from the truck base plate. 3 gears does.
Remember for the gear ratio, that only the motor gear and the wheel gear will affect the final ratio 
I guess you already know this, but a reminder can never hurt anyone 
I have just finished my 3D printed 3 gear drive system like this, and I would strongly advice you to support the middle gear on both sides (hard to tell if it is already) Not only for better alignment, but also because it is subjected to alot of stress.
It will be, this is just an opened version so the internals can be seen.
Will this be of plastic or cnc`d metal?
I run 2 gears on my carver :o with aliens. they sit good where they are.
Gears will be POM, the case will be aluminum.
That’s a totally different type of truck though, we’re making this for normal trucks.
What is POM? is that a 3d print filament?
I’ve designed a shaft where the middle gear can spin. It also makes for great allignment
Is this what you meant?
POM is a type of plastic I think… It’s CNC’d and is self lubricated. It’s what a lot of the gears are made of in the drives currently offered.
I mean like, you should support the axle that the gear spin on, on both sides.
Gear ratio is very much personal preference, motor and battery specific. Sorry, it isn’t much help!
When you know what speed/torque you want use an online eskate calculator to work out the ratio and ensure you have that overall ratio.
Best to make the design such that you can adjust the ratio after building by a simple modification, ie change one gear. Then you can fine tune it later without having to start from scratch.
The gear profiles (module etc) have pros and cons, like noise, strength and efficiency. Best to have a look around if you haven’t already. Bear in mind you could buy gears off the shelf in metal. Might be cheaper and less problematic than 3D printing; just thinking out loud. Daval gears is a supplier in UK that comes to mind, use the knowledge on their site/catalogue but they probably wont sell 1s and 2s.
Ok will do that, thanks!
For the production drives we won’t be 3d printing anything, they’ll be either bought or CNC’d.
Yes of course, no one has the same build 
Noted
That is exactly whats been done in the design. Although the wheel gear and middle gear are both customised but based on module 1.5 POM gears (not 3d printed) . So in the future ratio could be altered.
What are your thoughts on the current ratio, 16-27-38 Teeth with ratio of 2.375:1
Initially we went with module 1.5 gears, but when we realised 40T was not going to have any clearance at all we decided to drop down to module 1, then when I 3d printed it all it didnt seem right. So gone back to module 1.5
3 or higher imo.