Torque - Wheel Diameter - Dropouts

[steeeve]

I am confused about the issue of torque vs wheel diameter. I would also like to know how wheel diameter relates to stress on the dropouts.

I feel like many qualities are the bike are attributed to torque. Take for example, going from 29er to 20". Does 20" feel more "torque-ie" because of gearing changes or is the motor actually producing more torque in the smaller wheel?

The other thing I am confused about is how dropouts are stressed by various wheel diameters. If motors in 20" rims produce more torque, you would think they would produce more stress on the dropouts. At the same time, I see larger diameter wheels as larger levers. What's the deal with torque vs wheel diameter?

 

[John in CR]

Without changing current the torque of a given motor doesn't change, and that is stated as a unit of force over a given distance. The distance from the center of the axle to the dropout doesn't change and neither does the force, so the forces at the dropout are independent of wheel diameter.

Changes in wheel diameter absolutely change the gearing, so the resulting acceleration from the same motor torque is different. That's why with hubmotors smaller wheels will accelerate better, climb hills better, and run cooler and more efficiently with other variables held constant.

Just don't underestimate the forces in the dropout, since with a motor with 10mm flats, the torque is at a radius of about 5mm. One newton-meter at a radius of only 5mm is 200 newtons of force. One foot-pound is about 45lbs of force, so forces in the dropout can easily get into the thousands of pounds of force at such a small radius.

 

[shock therapy]

Hi Steeeve,

I understand your reasoning regarding wheel diameter/'dropout' torque but remember the force being applied is within the motor's diameter. Like pushing a hinged door in the middle vs it's outer edge.

ST