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While climbing a 50-foot or so hill on a 6% grade with an all-in load of of 275 pounds yesterday I was having a heck of a time. I thought, "How can I be this weak after all the road riding and mountain biking I've done lately?!" I decided that the hill wouldn't beat me (as much for pride as for not wanting to try to get that load going again on a grade) so I dug in hard and ground it out. Upon summiting, huffing and puffing, I went to shift up into my second chain ring only to realize I'd been in it all along. Oops! :o

Is that an OOPS or actually a DUHHH!:D

Another valuable lesson learned that will be instilled in your mind for ever. (and your muscles for days.)

Maybe you need a "tachometer" for your bike?

They actually exist. Most cyclocomputers in the mid to upper end (but not the cheapo ones) have a 2nd sensor (1st is for speed/dist) that goes on the pedal crank and measures "cadence" - a fancy name for pedal RPM.

Couldn't it be calculated from wheel RPM if the gear ratios are known?

The cheap way might be to just add a variable reluctance sensor or two (similar to the crankshaft and camshaft sensors in a car engine) that magnetically counts the teeth on the gears and run that to a simple Arduino-based frequency counter calibrated for RPM. Maybe add a servo motor and controls to make a semiautomatic transmission. And a chain tension sensor for accurate power measurements up/downhill with wind compensation and for adjusting the shift points. Maybe it's even possible to use a small brushless DC motor (like one out of an old disk drive) as both a generator to charge a capacitor for power and measure wheel RPM at the same time.

Should give instrumentation similar to what would be possible with an eCVT but without the hybrid components.

On a bike, you're not too worried about how fast the wheels are spinning, you're more worried about how hard the engine is working - YOU. A good, freeform "spinning rate" is 60-90 RPM cadence. Lower than 50-60 and you are cranking too hard and should shift, higher than 85-100 and you are spinning too fast and should shift.

I think some of the super fancy ones use the sensor for speed and calculate wheel RPM as well as having a secondary sensor for cadence.

Once you go so far as to servo-control it, you're adding a LOT of weight to a bike, which will then make it harder to pedal. Awesome idea though for an EV assist bike!

I don't think the servos would add much weight. I'm thinking of the kind used in model aircraft.

And I don't think there's much point to a pure EV bike. It would be very easy to add pedals to make it an IMA-like hybrid to get a longer range and higher top speed and acceleration. Used correctly, regeneration can make it unnecessary to recharge the battery from AC power and even power some electronic devices that would otherwise be powered or recharged by AC power. That would make a bike with a negative net fossil fuel usage!

Unless all your trips are downhill, how do you get excess power from regen?

My idea of electric hybrid bike is to avoid regen and simply conserve momentum (glide, baby!).

Regen while pedaling for a little more exercise on the downhills. You'll be working harder on average but the peaks won't be as bad since you'll have assist on uphills.

No biker I know is going to pedal harder to turn an MG for regen! It takes more than all of the momentum potential of the downhill to make it up the next climb. Brakes just slow you down. Regen is a brake.

I'd take a EV bike with regen if it was offered to me, but I'd avoid the weight, cost and complexity of same if designing my own.

Well, you can say that it technically uses even less fossil fuels than a regular bike since it reduces use elsewhere. And actually, it is possible to keep pedaling at a slow and comfortable rate if the conditions are right. Downhill, it will regen the energy from slow pedaling along with slowing you down if the slope is steep enough to make you go faster than desired. Uphill, you still pedal and use the assist.

Actually implementing regen is very simple with a brushless DC motor. Just operate the inverter as a rectifier. Most low voltage brush DC motors are also easy to use for regen.
http://techno-fandom.org/~hobbit/cars/boost-hack/

And of course, you can also set the bike on a stand to keep it stable and lift the rear wheel and pedal just for generating electricity. With a bike based on eCVT (HSD-like), you don't even need to lift the rear wheel. Just something to keep it stable and then set the eCVT to generator mode.