Over the corona-cation (specifically the last 3 days), I’ve had lots of time to do CAD, and I’ve used that time to hone my WCD skills in a variety of ways. I’ve created four drivetrains to various degrees of completion, documented below. None of these designs involve chain tensioners, a shortcoming I am aware of and would not include on a competition robot.
8WD
Firstly, I’ve created an 8WD drivetrain with 3.25" wheels, using a custom single-speed 2-NEO gearbox.
10WD
Secondly, a 10WD drivetrain with 4" Colson wheels and a different 2-NEO single-speed gearbox. This design is the most viable for a competition robot and resolves many of the issues I had with our competition robot from this year.
Stealth drive
Then we enter the realm of complete impracticality: a 14WD drivetrain using 2" stealth wheels, including a 3-775pro custom single-speed gearbox. A brainpan is necessary as the chain runs go below the bottom of the 2x1 side rail. The wheels on the ends have just over 0.15" of ground clearance. On the plus side, the bumper rails can mount directly to the top of the end rails since the wheels do not clear the top of the 2x1.
Finally, created completely on a dare by a team member, Camber drive. This design uses the 775pro gearboxes from the Stealth drive, but uses 18 1.625" Colson wheels as the drive wheels, cambered inward at an approximately 45 degree angle from the vertical. I put very little effort into this (it was done entirely between the hours of 11pm and 1am last night) so it is the least complete, and it is the only one where I had to resort to saying “just weld it” because I couldn’t think of another solution to attach the side rails. This is also easily the boatiest-looking drivetrain I’ve ever made.
FWIW 1723 has used an 8 wheel WCD without chain tensioners for the past two years and had absolutely zero issues with the drivetrain. We added .018" to the center distance of the bearing holes. We used #35 chain so that probably had something to do with it working out alright. There was about a half inch of backlash at the wheel furthest from the gearbox (2 chain runs away) but we weren’t doing any odometry so that didn’t really matter.
IMHO chain tensioners aren’t strictly necessary and it’s a design tradeoff to consider if you’re confident in your machining capabilites. That said I certainly wouldn’t fault you for including them, especially if you plan to use 25 chain.
All of these designs look great. I’m particularly a fan of the camber drive
+1 for that. We’ve been using non-tensioned #25 chain on our drivetrains for the last three years, and never had any problems with it. We were inspired from 254 who (AFAIK) do this since 2017 or 2016. It really isn’t hard once you get the CC distance right - which might require some real life tests (we tested this in the offseason of 2017 and found the right amount to add to the CC distance for our mill).
I like how you managed to reach the point where gear drive would be lighter than or comparable to chain/belt, even if you didn’t use it. Maybe an idea for future projects…? I hear banebots has some 1 3/8” wheels.
@Lil_Lavery can probably explain more (I’m pretty sure he’s in the picture with the team around the robot, but I’m not saying which one), or @dlavery can, but as I recall they wanted maximum stability in a full-omni drivetrain, which meant putting the wheels as far to the outside as they possibly could (kind of difficult when your wheels cross the corners). So they flattened the wheels to an absurd degree.
Funny note: The last picture was at IRI or I miss my guess. AFTER the robot had gone through a torrential downpour (OK, by coastal standards) in an open trailer (behind the robot) and had to be dried out.
