OpenRC F1 Differential with Belt Pulley

OpenRC F1 Differential with Belt Pulley

I wanted to use a differential that reduces shaft sways/wobblings  of the available designs while using a belt for power transmission. After combining several remixes and ideas, I created a  differential that uses three identical bearings same with the ones used in front rims. (This is for original openrc f1)

The power ratio for this differential is as follows:

• Main pulley: 60T
• Shaft pulley: 16T

For silicone molded tires, please visit “OpenRC F1 Silicone Molding for Low Profile Tire Rims” (The photos show the rims for silicone mold tires, as that’s what I’m using.)

Parts List

To Purchase:

• 3 × MR128ZZ bearings (8×12×3.5 mm)
• 140mm(70 teeth) GT2 belt (if you can't source one you can print  in TPU). Aliexpress_link
• 16-tooth GT2 pulley for 5 mm shaft motors (better to purchase since it’s widely available for 3dprinters; an STL file is also provided for 3.17 mm shaft motors).

Test Print:

All parts include a 0.15 mm clearance for the bearings. Since the bearing’s outer diameter is 12 mm and the holes are 12.3 mm, I recommend doing a test print first to ensure the bearing fits snugly into the hole.

• tolerance_test.stl x1 

Tip: Enable “print external perimeters first” for best accuracy with random seam. 

3D Printed Parts:

• Main_pulley.stl x1 → Modified to include a bearing.  You need to pause printing at 5.8mm height (if standart 0.2mm height, beginning of layer 29) to insert the bearing and resume print. If you need to get the bearing out of the pully apply some heat till the plastic softens and take it out. 
• Small_gear.stl x4 → Thingiverse Link
  Tip: Enable “print external perimeters first” for best accuracy with 0.12 layer height.
• Body_gear.stl x1 → Remixed for a tighter fit on the rear shaft
  Tip: Enable “print external perimeters first” for best accuracy with 0.12 layer height.
• 16t_gt2_motor_pulley_3.17_shaft.stl → Requires 2 × M2 insert nuts. The motor pulley is designed for a 3.17 mm shaft (matching my motor). If you have a motor with a 5 mm shaft, you’re in luck—standard 16-teeth GT2 pulleys for 3D printers will fit perfectly.

The system uses a 140 mm GT2 belt (70 teeth). If you can’t source one, you can print your own using TPU.

• 140mm_gt2_belt.stl (70 teeth, 2.5mm thickness, 6mm width)

The  below rims are the remixed original rims. 

• Original-Right-Rear-Rim-With-Gear.stl x1 → I combined the rim and gear since I wasn’t a fan of them being separate and only held together by the small bearing’s contact points.
  Tip: 0.12 layer height, organic supports, random seam
• remixed_axle.stl → Remixed for the new pulley, and left side has a square profile for a non slipping tight fit rim.
• Original-Left-Rim-Remixed-Square-Axle.stl → Square profile hole left rear rim
• square_wheel_lock_nut.stl → 1 piece  for left rear rim 

Below rims are for  silicone molded  tires. For the mold please visit “OpenRC F1 Silicone Molding For Low Profile Tire Rims”

• Right-Rear-Rim-With-Gear_silicone_mold.stl.stl → 
• Left-Rear-Rim-Square_axle_silicone_mold.stl → 

Useful Tools

I used the following OpenSCAD gear and belt generator scripts when designing the parts.
They’re especially useful if you plan to modify the design for different motors, shaft sizes, or belt dimensions.

• OpenSCAD Parametric Belt Generator: Thingiverse Link
• OpenSCAD Parametric Pulley Generator: Printables Link

Extras

For a more professional setup, check out this differential design.
It uses six different bearings—some are harder to find—but it offers a more refined solution overall.

Theoretical Speed Calculation

My setup uses a NEEBRC 540 35T brushed motor with a 3S LiPo battery, GT2 belt drive (16T → 60T), and 55 mm diameter wheels.

• Motor: NEEBRC 540 35T brushed
• Battery: 3S LiPo (11.1 V nominal / 12.6 V full)
• Pulley Ratio: 16 T (motor) → 60 T (wheel) = 3.75 : 1
• Wheel Diameter: 55 mm → Circumference ≈ 0.173 m
• Estimated Motor KV: ~1700–1900 KV (typical for 35T 540 motors)

⚠ Real-world speed is typically 10–25 % lower due to load, drag, and drivetrain losses.

• Added a bearing at the center of the main gear.
• Converted the main gear into a GT2 pulley.