TrainerRoad feature idea:
If you could programmatically control an e-bike motor to provide resistance in addition to assistance, you could create an erg mode for outside structured workouts that could help hit/maintain power targets irrespective of terrain (within reason). It would be safer since you would still have control over speed with gear selection, cadence, and breaking while keeping power on target. And while the motor is providing resistance you could recharge the battery to increase range.
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AIRhub
- There is already a solution for that, slightly different than your suggestion (that requires hacking expensive and complex electronics and firmware).
- http://terraindynamics.com.au/airhub
Reviews
It would be interesting to have an electric bicycle capable of doing ERG mode on the road, especially for base season training where we could target a wattage and let the bike adjust the assist.
Is there anything on the market yet? Or has any manufacturer announced something similar? Asking for a friend 
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Nothing out there or planned that I’m aware of. Not entirely sure I understand how this would work - I guess the combination of selected gear and gradient would have to be sufficient that the watts needed to maintain even a low cadence (say 50rpm) exceed your target wattage, so the bike is always providing some assistance, which means it can adjust that assistance to ensure you’re putting out target watts at whatever cadence you choose. Can see a number of practical problems with that. For a start I can’t see how it would work on any kind of significant descent unless the motor actually acts as a brake. Secondly in a lot of (most?) countries, e-bikes are restricted for safety reasons so that they only provide an assist up to a certain speed. So this wouldn’t work for high target watts (again, unless the motor is acting as a brake). Thirdly, I could see there being real handling issues having variable assist based on a plan that has big shifts in power, when combined with real world riding such as corners and traffic lights. E.g. you’re approaching a sharp corner where you need to ease up, just as you get to the end of an interval where your target wattage is dropping from 350W to 150W, which means the motor suddenly kicks in an extra 200W.
Even if all of the above issues could be overcome this seems like a pretty complex/expensive solution to an extremely niche problem, so not sure if there is a good business case for it. There can’t be that many people who are so focused on structured training that they are prepared to pay thousands of dollars for an electric bike that gives them erg mode outside. And if (as seems likely to me) the solution involves that bike actually braking them so that they can hit target watts at safe speeds, then the market just got even smaller. The fun of doing high intensity intervals outside is flying along at high speeds, not artificially low ones…
I merged the 2 posts immediately above from a separate but very related thread. The linked AIRhub ingo above is different, but a solution currently available for ERG outside.
In my original post, my thought was that motors and generators are really the same thing, so an ebike can be programmed to provide both variable assistance and resistance. Trying to maintain 150 watts on a tough climb? Ebike motor provides assistance. Trying to maintain 275 watts on a sketchy descent at a safe speed? Ebike goes in to generator mode and provides resistance, while at the same time capturing the current to recharge the battery. Grossly oversimplified, of course…