samatman
10 µW
- Joined
- Sep 3, 2022
- Messages
- 5
Hi! Long-time lurker, first time poster.
A month ago I purchased a Class 3 cargo bike, rear hub with a cassette and
derailleur. Overall I'm very happy with this choice.
The bike, a Blix Packa Genie, has a cadence sensor and thumb throttle. I'll
end up replacing the throttle with a half-twist for comfort, but the real
problem is with pedaling.
The issue is that the bike is geared for torque, making it usable (ish...)
without power. The result is that at reasonable speeds, not even crusing
speed, there's no resistance on the chain at all. It's uncomfortable to spin
air for any length of time, and safe handling of a bike is much reduced when
the pedals have no bite, anyone who has done long downhill rides knows that
handling suffers when frame angle can't be adjusted with pedaling.
I could add a larger front cassette, maybe a front derailleur to go with it,
and I grant that this is the easier way to go, but long chain runs and two
derailleurs is a recipe for dropped chains in my previous Xtracycle
experience.
So I had an idea, and I'm looking for pointers to prior art, or to get talked
out of it. The idea is to have a single-speed freewheeling gear on the hub,
and one on the crank, with an electromagnet armature surrounding the rear
gear on both sides. There would be a full chain case to keep grime out, so
tolerances could be appropriately tight.
The gear ratio would be that at which I can actually move the bike forward
while pedaling briskly, the lowest practical in other words. When the bike
is moving faster than that, which is almost always, magnetic resistance is
applied to the rear cassette and the resulting current is sent to the
engine.
Instead of a gear shift, I would put in a selector which sets the target
resistance for the generator. It seems plausible to get 80% recovery of
effort as torque, which is terrible in comparison to 98.5% for a good chain,
but I'm thinking about it like this: if I'm putting 150W into a chain and
getting 120W out, that means I need 30W from a battery for it to feel like a
natural bike. If I'm having a full electrical failure then I'm getting picked
up, but rate-limiting the last 100Wh in the batteries will get me as far as I
need to go. The packs are rated for 1200kWh, so I figure that gets me 1kWh of
full range and a long tail of getting places at pushbike speed.
Measuring the transient power output, and putting a photodiode sensor between
the gear teeth, should give me a high-quality measurement of both torque
and cadence, at any speed high enough that the gear isn't turning the wheel
mechanically.
I have access to a makerspace and community, I can wind armatures and write
firmware, but I can't help asking myself why I've never seen this before.
There's no show-stoppers in theory, the Schaefller Free Drive claims 85%
efficiency, although I know that hasn't been tested in the wild.
For my purposes, I'd get most of what I wanted just dumping the power as heat,
I want the bike to handle like a bike, and I like pedaling fast and working up
a sweat sometimes. Maximum efficiency doesn't matter here, because my stoker
puts out 750W continuous, and I don't.
It seems like a promising design, because it just drops into the space a full
cassette would take. None of the mechanical strain is on the custom parts,
which can probably be 3-d printed, making this a practical DIY project for a
lot of people.
Am I missing something obvious? I looked for threads on the forum but any
plausible search terms overlap with things which one can buy, so if the needle
is in the haystack, I didn't find it.
A month ago I purchased a Class 3 cargo bike, rear hub with a cassette and
derailleur. Overall I'm very happy with this choice.
The bike, a Blix Packa Genie, has a cadence sensor and thumb throttle. I'll
end up replacing the throttle with a half-twist for comfort, but the real
problem is with pedaling.
The issue is that the bike is geared for torque, making it usable (ish...)
without power. The result is that at reasonable speeds, not even crusing
speed, there's no resistance on the chain at all. It's uncomfortable to spin
air for any length of time, and safe handling of a bike is much reduced when
the pedals have no bite, anyone who has done long downhill rides knows that
handling suffers when frame angle can't be adjusted with pedaling.
I could add a larger front cassette, maybe a front derailleur to go with it,
and I grant that this is the easier way to go, but long chain runs and two
derailleurs is a recipe for dropped chains in my previous Xtracycle
experience.
So I had an idea, and I'm looking for pointers to prior art, or to get talked
out of it. The idea is to have a single-speed freewheeling gear on the hub,
and one on the crank, with an electromagnet armature surrounding the rear
gear on both sides. There would be a full chain case to keep grime out, so
tolerances could be appropriately tight.
The gear ratio would be that at which I can actually move the bike forward
while pedaling briskly, the lowest practical in other words. When the bike
is moving faster than that, which is almost always, magnetic resistance is
applied to the rear cassette and the resulting current is sent to the
engine.
Instead of a gear shift, I would put in a selector which sets the target
resistance for the generator. It seems plausible to get 80% recovery of
effort as torque, which is terrible in comparison to 98.5% for a good chain,
but I'm thinking about it like this: if I'm putting 150W into a chain and
getting 120W out, that means I need 30W from a battery for it to feel like a
natural bike. If I'm having a full electrical failure then I'm getting picked
up, but rate-limiting the last 100Wh in the batteries will get me as far as I
need to go. The packs are rated for 1200kWh, so I figure that gets me 1kWh of
full range and a long tail of getting places at pushbike speed.
Measuring the transient power output, and putting a photodiode sensor between
the gear teeth, should give me a high-quality measurement of both torque
and cadence, at any speed high enough that the gear isn't turning the wheel
mechanically.
I have access to a makerspace and community, I can wind armatures and write
firmware, but I can't help asking myself why I've never seen this before.
There's no show-stoppers in theory, the Schaefller Free Drive claims 85%
efficiency, although I know that hasn't been tested in the wild.
For my purposes, I'd get most of what I wanted just dumping the power as heat,
I want the bike to handle like a bike, and I like pedaling fast and working up
a sweat sometimes. Maximum efficiency doesn't matter here, because my stoker
puts out 750W continuous, and I don't.
It seems like a promising design, because it just drops into the space a full
cassette would take. None of the mechanical strain is on the custom parts,
which can probably be 3-d printed, making this a practical DIY project for a
lot of people.
Am I missing something obvious? I looked for threads on the forum but any
plausible search terms overlap with things which one can buy, so if the needle
is in the haystack, I didn't find it.