Internal work, with a bit more relevant detail.
For FG and SS, both internal and external work are identical when you are maintaining the same speed in the same gear.
If you stop doing any external work of pushing the pedals and just spin at exactly the speed you are going, you are still doing the internal work and it will be the same for both.
Where it gets different is when you slack off below doing the internal work of moving the pedals at the current speed.
On a freewheel, you can take the work all the way to zero, your feet stop and you coast. In fact, any level of work less than the current internal work makes no difference to the motion of the bike and it is just a matter of how much you are coasting.
On a fixed gear, you can take the work to zero but the pedals continue to turn and the bike will use its kinetic energy to do the equivalent of the internal work and keep your feet moving at the same speed. Of course, this has a bit of a braking effect.
For example, it takes me 180 W of external work to sustain 18 mph on gravel, which requires 90 rpm on my 67.5 inch gear.
Using Fornenti’s data, I can estimate about 0.7 W/kg at 90 rpm times my weight of 80kg or about 56 W of internal work.
So if my wheel had a fixed gear and I stopped pedaling and let the bike turn my feet, I’d be initially pulling 56 W from the bike. Of course, this quickly lessens as the bike slows because the internal work depends on cadence.
With a fixed gear, the level of effort always matters to the motion of the bike, whether it’s just a little bit less than the internal work, all the way down to zero or if you start adding resistance to brake even faster.
This behavior is all too familiar for FG riders I’m sure, but i find it helpful to have a more quantitative idea of the energy involved.
Otto
Last edited by ofajen; 07-08-21 at 06:43 AM.