Disk drives and seat-stay motors

Disk drive

Skarper, in Britain, sits on the chainstay and drives the rear disc. The published comparison figure is 250 W and about 240 Wh. Your rim, your spokes and your hub stay. The bike needs a rear disc and a stay the clamp can trust. Rim brakes are out. The fit is specialist because the puck and the caliper share a small piece of frame.

Seat-stay and seat-tube

A seat-stay motor clamps under the stays and drives the tyre or a roller from above the wheel. Vivax, historically, put a motor inside the seat tube and drove the crank. Both exist because some frames cannot take a hub or a BBS shell, and because some riders want the crank to look untouched.

• Measure the stay. Round tubing in a photo is not a diameter.
• Clear the brake bridge, the mudguard and your heel.
• A compact case is not hidden horsepower. Read the continuous watts.
• Discontinued seat-tube systems are identification problems first and upgrade problems second.