Mid-drive Regen Brainstorming

Hello,

I built 2 ebikes so far with ready made after market kits, got already addicted, and moving to the next project. As I live near mountains, the feature I miss the most is a good Regeneration solution. Since questions about regen pop up on regular basis, I figured it would be nice to sum it all up in one thread. I'm far from an expert, so feel free to correct / expand this.

I. To regen or not to regen ?

Probably one of the more controversial qestions about this topic. Whithout going too much into details, I'd propose the following answer to that question:
a. On flat or relatively flat roads with a lot of commuting stops, regeneration is probably not worth the extra weight, drag and complexity.
b. Things are quite different in hilly or mountaneous aeras with long steep climbs, and equally long downhills. There Regen can make quite a difference. Though recoverable energy varies a lot, mostly between 10 and 20%. See this famous analysis: https://endless-sphere.com/forums/viewtopic.php?t=7891

One thing often forgotten about regen ist the amount of current your battery pack is able to use. Most cells are asymetrical, and can only take 1-2C charge-current. This means that more than 20% regen is probably not suitable anyway.

II. Hub regen

1. Direct Drive
The only market ready option if you want regen, are heavy and inefficient Direct Drive motors. While those certainly shine when it comes to reliability and silence, they come with some serious drawbacks:
- heavy weight, and perhaps even more importantly for mid to high power solutions: heavy unsprung weight
- drag
- low torque per Kg - difficult climbing
- poor efficiency (see torque ripple and cogging)

2. Geared Hub
Using a geared hub motor instead of a direct drive one would certainly allow for higher rpms and thus more torque and better efficiency. Unfortunately, there is no such option on the market. Probably because the increased efficiency is not enough to justify the drag, noise and overheating of a geared hub motor without freewheel, in comparison to a direct drive one.

III. Mid drive regen

This is where is gets interesting: mid drive e-motor offer some sizeable advantage over hub motors when it comes to mountaneous regions: since your are not limited by the size of a wheel hub, and you can use regular drivetrain gears, it is possible to achieve more powerful (torque) setups in comparison to hub motors with a similar weight. A notable exeption to that rule is the infamous 2 speed xiongda motor, which is a great climber though top speed is not his strength. The optimized weight distribution of a mid drive near the gravity center of the bike is also better suited to hilly off-road use. In any case: since mid drive work well in the mountains, and so does regen, why not combine both ?

Of course, there would still be the increased drag and the heat issue to be dealt with, but the real problem (to me) lays somewhere else: since all mid drive options use one or more freewheels that allow power transfer in one direction only, the main difficulty of this task is figuring a way to capture the brake power.

1. Mid drive without gears

a. Bi chain
The most obvious option would probably be adding a second chain on the non drive side of the bicycle, by modding a disc rotor mount to also fit a sprocket, which could be directly coupled to a geared mid drive motor. While this certainly works (there are examples on this forum), it discards one of the main advantages of a mid drive motor: the ability to use your bike's gears. This translates into a loss of efficiency, since the optimal rpm of the motor is more difficult to reaches. In that case: why not go back to a simple direct drive hub motor ?

b. Single chain option
In this hypothetical case, a motor would be fitted somewhere between crank-set and rear hub, using only one (probably a bit elongated) chain. The only doable configuration I can see in this case would imply using an expensive crank-gear like pinion's. This configuration would still bear the same problems as the previous one... on top of being rather pricey!

2. Mid drive with gears

a. Bi chain
Moving to the more suitable configuration, where the motor uses some or all of your bicycle gears, the first option would also be adding a second chain on the non drive side of the bicycle. The base setup would still require fitting the motor somewhere between the crankset and the rear hub in a simple bi chain config (or somewhere within the triangle in a tri chain config). It also requires at least 2 freewheels running in opposite directions: one forward to transmit motor power to the rear wheel, and one backwards to transmit brake power to the motor.
This configuration has the advantage of allowing all sorts of classic bicycle gears: IGH and Derailleur. Even pinion's is possible, if you want to go fancy. Drawbacsk include increased weight and complexity, more chains to maintain and the difficulty to properly tension both chains simultaneously.

b. Single chain
The golden solution: the motor is fitted somewhere between the crankset and the rear hub and both motor and brake power are transmitted through the same chain. While this setup looks simpler and saves weight and chain-wear it proves quite difficult to construct, due to the lack of suitable parts. Nearly all gearing options available to cyclist include one or more freewheels which are great to reduce drag, but make it impossible to harvest braking power. The only exception to that rule would be the sturmey archer S3X - a fixie 3 speed IGH - which seem to be plagued by reliability issues on top of being very expensive. Also, 3 Speed is not nearly enough for realistic touring in the mountains when you use motors with less than 1KW of power.

I don't know if anyone tried to mod another IGH, like the nexus 8 for an instance, to block all freewheels. I asked Rohloff, and they told me that it was impossible: even if the freewheels could be blocked (which is not the case), their entire drive shaft has been designed to sustain heavy loads in one rotating direction, and would probably fail if used as a braking device.

The last option I would like to investigate: is it possible to mod a derailleur to work in both directions ? This would certainly be an elegant solution to the tricky problem of mid drive regen.

I've been working on a 2-direction derailleur concept to collect braking power, and came up with the following idea:

Rear derailleur come with a system to block it rotating backwards. This blocking mechanism is divided in two parts:

1. Hanger bolt blocking
A small metal arm located on the hanger plate is bent towards the upper arm and hits a "blocker" cut into the metal housing of the upper arm.


2. Lower arm blocking
A metal dowel fixed onto the side of the lower arm hits a "blocker" consisting of a protruding metal part of the middle arm.


To make it work backwards, the simplest thing to do would be to add a second "blocker" in each case thus limiting the movement of the lower arm when pulled towards the crank. I'm thinking about drilling a hole an brazing another metal dowel into it.
Yet there are two limitations of this solutions:

1. The spacing of the two blockers must be large enough to allow for enough chain tensioning for all sprockets. This means that if you're not in the lowest gear (largest sprocket), there will always be a "floating" moment when actioning the drive-train brake, where the lower arm of the derailleur must first travel to the stopping point, before actual power transfer can take place. This means a small loss every time. Increasing the gear ratio (size difference between smalles sprocket and largest sprocket) increases that loss.

2. The entire rear derailleur must bear the full load of the regen-friction. Yet those parts have been designed to work in the opposite direction, so I doubt that - if it works - it will last long. Maybe some serious beefing up of the lower arm should be undertaken first, to avoid bending it.

What do you think ?

I also ride in very mountainous areas.
I would like to have regen braking mainly for the ability to ease off the brakes while going downhill.
I wonder if a smallish front hub motor could do this?
Going downhill ,the front is where all the action is, the back will skid out if braked too much.

espresso said:

I also ride in very mountainous areas.
I would like to have regen braking mainly for the ability to ease off the brakes while going downhill.
I wonder if a smallish front hub motor could do this?
Going downhill ,the front is where all the action is, the back will skid out if braked too much.

Click to expand...

This has come up on regular basis. The problem is "smallish": this automatically implies a geared motor, which will have a hard time shedding off the brake heat. Moreover, even if you go for a tiny Q100 motor, it's going to be an additional 2Kg - more than the weight of a 80100! If the target is efficiency (low weight, high power), it is kind of absurd in my opinion use 2 motors on a single bicycle. If you add both weights, you get the same than a small direct drive motor, which will work better in regen mode.

Also, my next project will probably be a tadpole trike. Difficult to fit a front hub motor there!

Perhaps a friction drive off the front wheel?
Will spin up a small motor nicely then.

The answer is surprisingly simple. The motor is in a constant connection with a fixie rear chainring and instead of the rear casette being freewheeled - as with a normal bike - it is the front crankset that is being freewheeled. Ta da!

For a long time I thought that these mid drives are shite relative to hubs mostly because of the lack of regen and limited power due to the weak bicycle chain. I just looked into it recently and these are such delicate things the whole makeup of such a mid drive is so good with the ipmsm motor and all. I am a great fan of these now and now that the regen question is also solved it has basically no drawbacks. Maybe a stronger chain should be developed so more power can be pushed through the system and I think it is the perfect setup. I think I notify bafang about my idea so they start manufacturing the regen capable stuff.

This will only work if there isn't a derailleur on the back. Don't forget that the part of the chain that does the driving has to be the lower part for regen as opposed to the upper chain for normal driving.

Sattva Ram said:

The answer is surprisingly simple. The motor is in a constant connection with a fixie rear chainring and instead of the rear casette being freewheeled - as with a normal bike - it is the front crankset that is being freewheeled. Ta da!

For a long time I thought that these mid drives are shite relative to hubs mostly because of the lack of regen and limited power due to the weak bicycle chain. I just looked into it recently and these are such delicate things the whole makeup of such a mid drive is so good with the ipmsm motor and all. I am a great fan of these now and now that the regen question is also solved it has basically no drawbacks. Maybe a stronger chain should be developed so more power can be pushed through the system and I think it is the perfect setup. I think I notify bafang about my idea so they start manufacturing the regen capable stuff.

Click to expand...

"The answer is surprisingly simple. The motor is in a constant connection with a fixie rear chainring and instead of the rear casette being freewheeled - as with a normal bike - it is the front crankset that is being freewheeled. Ta da!"

It doesnt have to be constant connection, this will add drag when you are not using regenerative braking. You fix it with a discbrake and then it starts spinning the mid-drive engine in the oposite direction with one gear spinning the other gear connected to the engine in reverse, to generate power.