Which STS3215 gear ratio do I need?
For any joint that holds up a load (a follower arm, a leg, a mobile base arm), buy the 1/345 version: C001 for 7.4V or the 12V 30 kg.cm version. The lower ratios, 1/191 (C044) and 1/147 (C046), are only for SO-101 leader arms you move by hand, which need 3x C046, 2x C044 and 1x C001.
What matters
- C001: 7.4V, 1/345 gear, 19.5 kg.cm stall. The default for followers and load-bearing joints.
- C044: 7.4V, 1/191 gear. SO-101 leader base pan and elbow; vendor sheets list 27.4 kg.cm, which is higher than you would expect from the lower ratio, so do not size a load joint on that figure.
- C046: 7.4V, 1/147 gear, 14.4 kg.cm. SO-101 leader wrist flex, wrist roll and gripper; the easiest to turn by hand.
- 12V 1/345 (C047 / C018): 30 kg.cm at 12V. SO-101 Pro follower, LeKiwi 12V, XLeRobot and HopeJR arm.
- Most retail listings do not print the ratio; check the C-code in the title or ask the seller.
Our picks
| Pick | Why | Where to buy |
|---|---|---|
| STS3215 C001 (7.4V, 1/345) Followers, legs, any joint under load | Highest gearing in the 7.4V line, so it holds position under load. Six go in the SO-101 follower plus one in the leader's shoulder lift. | Seeed Studio: $20.00 OpenELAB: EUR 23.57 RobotShop: $34.32 |
| STS3215 C044 (7.4V, 1/191) SO-101 leader base and elbow | Lighter gearing that you can move by hand while it still holds the leader's own weight. Two per leader. | Seeed Studio: $21.99 OpenELAB: EUR 23.57 |
| STS3215 C046 (7.4V, 1/147) SO-101 leader wrist and gripper | The easiest to back-drive, so the wrist and trigger feel natural. Three per leader. | Seeed Studio: $20.00 OpenELAB: EUR 23.57 |
| STS3215 12V (1/345, 30 kg.cm) Heavier followers and mobile manipulators | About 50% more stall torque than C001, for when the arm sags or the gripper carries weight. Needs a 12V supply. | Seeed Studio: $22.00 RobotShop: $31.71 |
FAQ
What happens if I build an SO-101 leader with all C001 servos?
It works electrically, but the leader becomes stiff and tiring to move, which makes teleoperation jerky. The mixed ratios exist to fix that.
Can I mix 7.4V and 12V STS3215s?
Not on one supply. SO-101 leaders always use 7.4V servos; if you choose a 12V follower, give that arm its own 12V 5A+ supply.
Is there a bundle with the right leader mix?
Yes. Seeed sells SO-101 servo kits with the leader ratios included, and the official BOM links a six-servo leader bundle.
Which ratio does a legged robot like Open Duck Mini use?
Its BOM lists the 7.4V 1/345 C001 for all fourteen joints, though one open issue on the project questions whether that is the best choice.
Sources: github.com · huggingface.co · www.seeedstudio.com · www.seeedstudio.com · www.seeedstudio.com · www.seeedstudio.com · openelab.io · openelab.io · github.com