Where to measure sprocket radius

[Abasz]

I was wondering where should the sprocket radius measured? Is it measured up to the top of the teeth, or the bottom of the pitch or in the middle (I would assume the latter)?

[JaapvanEkris]

I think middle would be best, as that would be the point where the force would be positioned against the teeth.

[klamp169]

For chain-driven flywheels like those used in Concept2 rowers the equation for the sprocket radius $r_s$ is:

$$r_s = \frac{L_c N_s}{2\pi}$$

where $L_c$ equals the chain pitch (1/4 inch for Concept2 rowers) and $N_s$ equals the number of gear teeth (usually 14) on the sprocket. This leads to $r_s$ = 1.415 [cm] for Concept2 rowers. Only when using this equation the following equation relates handle speed $v_h$ in meters per second (during the drive phase) to flywheel angular speed $\omega_f$ in radians per second:

$$v_h = \omega_f r_s.$$

[JaapvanEkris]

You realize you are resurrecting a three year old thread on an abandoned repository, where the answer is marked?

[klamp169]

Not really, no. Sorry for that. Yet it is noticed very quickly 😏. I came here via the Q&A. Any suggestion where to post it? Taking the guessing out of the determination of the sprocket radius (for chain driven rowers) is quite valuable for many ORM users, I think. See for instance Merach R50?, where I posted something similar, btw.