I'm working on developing an automatic bicycle (mostly microcontroller
based) and I'd presume I'd be shifting in a similar way to a car.
Well, you're in for a shock: it's going to be pretty different.
So my question is, what factors does the transmission take into
consideration when shifting?
So is it concerned more with speed, rpm, some other factors,or all of
them? Which ones take precedence in its decision to shift?
It depends. The ruleset is going to be situation dependent and somewhat non-deterministic (also known as using fuzzy logic). A car ruleset is also going to be very different from one that you'd use on a bicycle.
Here are some examples of the conflicting priorities that you face in a car's automatic transmission:
- Fuel economy
- Throttle response (is there a shifting delay between stomping on the gas and feeling the go?)
- Fuel economy (what, again?)
- Annoyance (oscillating shifting is annoying: e.g., 3-4-3-4-3-4).
- Fuel economy (yes, yes, we get it!)
The point being, the transmission is trying to optimize a bunch of conflicting metrics looking for a local maxima.
So for instance, my main concern is riding the bike downhill. Now if
you're riding a regular bike like me, you would stop pedaling and let
gravity do the work for you. Does an automatic transmission do this as
well or does it shift up to match the speed or down to match the low
On a bicycle, you have a complete different set of problems. For example, I like to pedal at a fixed pace and I'll fiddle with the gears regularly to match my road speed to my pedaling pace. Your automatic transmission would need to gear up and down according to road speed. It would also need to anticipate the upcoming hill - I do, since I can see the slope and gear down. Waiting until my pace slows down will be annoying. It would be better to detect the situation (e.g., using a level or GPS data).
On a downhill slope, I gear up so that I can build up momentum. I also like knowing that I'm in the right gear for my road speed even if I'm coasting for a bit: I don't like the overspeed situation where I can end up running over my own foot! In an automobile transmission, the engine almost decouples in a coasting situation (even though the engine is still "pedaling").
Finally, an automatic bicycle transmission can't oscillate between gears. In a car, it's annoying to hear the engine jump from 2000 RPMS to 3000 RPMS and back. On a bicycle, where there's a distinct shock to the system with every shift, the rider would quickly take a hammer to the transmission if it were switching every second or so.