Mid-drive, Hubs and Drivetrain Efficiency

[qwerkus]

One thing I'm not sure to understand is the point of reducing a mid drive to crank speed, only to overdrive it afterwards through the regular bicycle gears. Isn't this a pure waste of energy ? A more efficient way would be the 2 chain / 2 Belt option, where the mid drive motor reduction is directly linked to the rear wheel. One could still set the motor power to match his pedaling cadence. This of course adds some unwanted weight ( 2nd drivetrain ) and the need to care for a 2nd chain.

Assuming all things equal (which can hardly be the case since those motors will be wound differently), and defining efficiency as the ratio between input energy (Wattage) and output energy (torque rpm), isn't the most efficient motor a highly reduced hub motor ? No drivetrain loss, some gear loss, but most trip time spend in his optimal rpm range. Sadly the hub size will always be the limiting factor here. Second comes a highly reduced mid drive with its added gear-drivetrain losses, and third a direct drive hub motor, which spends least time in its optimal rpm range.

So theoretically at least, the geared hub motor is the one that should give the most battery-range. Has this been verified experimentally ?

 

[dustNbone]

Bicycle chain drivetrains are pretty efficient, I don't think you're really looking at much efficiency loss.

Some mid drive setups do work the way you specified, separate sprocket on the left side of the wheel driven by a mid mounted motor, but you lose the variable gearing that you get by running it through the right hand side.

The thing that makes small BB mounted mid drives as efficient as they are (or at least can be) is their ability to keep the motor running at an efficient RPM using the rear gear cluster.

Take that away and the only thing you have left over a hub motor is weight distribution and a lighter wheel.

 

[-dg]

I had a mid-drive bike with two chains. The problem is that you need to keep two chains adjusted and removing the wheel is a giant pain. Think about fixing a flat on the road in the dark. It was noisy although only part of that was the extra chain. But the main problem was that there is a very limited selection of hubs that can be adapted to work with two chains.

 

[dogman dan]

If you want to ruin a mid drives efficiency, or even damage it, just put it in too high a gear for starts, steep hills, towing, etc.

It will suffer just like an overloaded hub motor does then. This was a big problem with early brushed mid drives, that were often comically underpowered. They would have run reliably, except the riders get dumb. Modern mid drives are more powerful, and more reliable brushless motors that stand a short period of overheat better than brushes did in brushed motors.

But you can, still, ride dumb and put your mid drive in just as inefficient rpm under heavy load, if you choose to.

 

[spinningmagnets]

The biggest benefit of a mid drive is the ability to shift gears that the motor is using, when you are climbing a steep hill.

On flat land, the ability to have 3 gears means you can get "adequate" performance from a smaller and lighter motor.

There are many scenarios where a hubmotor is a good choice.

 

[ScooterMan101]

qwerkus ,

I have been looking at mid-drive motor designs for a couple of years now, waiting for the newer, U.S. West Coast designed kits to become available .

From what I have read there advantages, and disadvantages to each design

Example :

For a higher power and torque and travel speed mid-drive the left side with the kart 219 chain would be best. Higher Power just destroy's a typical bicycle chain and chainrings and cogs. In the case of the left side you just power your way up hills.

For lower power set ups , like 2k and under and for someone who is easy on the gears the right side using the bicycles gearing/drivetrain is better , going slower and using less energy to go the distance. Frequent changes of the chain and chainrings and cogs is the downside of that design though.

The best design for me, However it has not been designed yet , is a longer swing arm where a motor can fit on top of the swingarm right in front of the rear tire, where the sprocket of the motor is in line with the center cog of the rear cogset, and the cog of the motor and the rear cassette is made for a 219 chain . And ... where a newer designed rear derailleur is also made for shifting a 219 chain.

This design will have the advantages of both systems .

Now I am waiting on someone here in the U.S. or Europe to make all this happen.

 

[ScooterMan101]

Mike has pointed out a few considerations / possible problems with making a shifting # 219 drivetrain,
So
Below I have copied and pasted here-from his thread my response to another possible solution to having a mid-drive motor on the swingarm that will allow some gear changes on the rear , so that we can have the best of both worlds.

...

ScooterMan101
100 kW
100 kW
Re: LightningRods mid drive kit
Post Sat Jun 09, 2018 1:26 pm

Interesting, I should have mentioned that when I was thinking about a custom made # 219 chain compatible rear cog set , that I was thinking about about 5 speeds , and a derailleur that had a few machined parts that replaced regular parts that would handle the issues you talk about here.

If it is too much work to make / have made front sprockets and rear cogset that is # 219 chain compatible then ...

Another option is to use the new 8 speed chainring/chain/rear cog , that Sram made for E-Bikes.
And
Design a rubberized crush drive , like that used on the rear wheel of motorcycles , in order to soften the force put though the drivetrain .
For those who do not ride motorcycles here is a youtube video that shows what the crush drive rubber parts look like ... https://www.youtube.com/watch?v=aNTtmp-N1-E
I have been thinking about this for a few years now since I work on my motorcycle and when seeing mine thought of how a good idea it is to have one .
design a small version that is directly on the shaft of the motor. If it would be better somewhere along the drivetrain other than the motor then put it at the location that works out best.

I still would prefer a mid drive where the motor is located on the rear swing arm in front of the rear tire, and that will shift along 5-8 cogs on the rear cassette.
So then instead of a # 219 drivetrain, the E-Bike Specific drivetrain that Sram makes , along with a crush drive in the system.
And do not even have a 11 tooth rear cog, more like 13 t or 15 tooth as the smallest.

With a crush drive perhaps then a regular 8 speed cassette with the 11/12 and 13 tooth cogs removed and two larger cogs near the hub located , ( like what Wolf Tooth Designs makes and sells )
https://www.wolftoothcomponents.com/col ... o-11-speed

( Made it 8/9 speed compatible though )

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LightningRods wrote: ↑ I don’t want to sound like one of those expert naysayers that say things can’t be done. Here are some of the challenges in making a #219 cassette (I’m assuming with derailleur):
- In order for a derailleur to work the chain has to have a fair amount of side flex. From the reading I’ve done the chain also needs to twist on downshifts to larger sprockets. Sort of like lifting your inside butt cheek to get up into a tall truck. 219 has tight tolerances and thick side plates. It doesn’t side flex much and it twists pretty much not at all.
- A derailleur by design needs a chain that will tolerate a lot of cross chaining, or not running straight. If you get 219 even a little out of a straight chain line it runs super noisy and wants to derail automatically.
- The pitch of 219 is .303” instead of .500” like the rest of the derailleurs on the planet. This means making all new jockey wheels and cassettes.
- Even in the world of bicycle chain if you compare single speed chain and sprockets to derailleur chain and sprockets you’ll see that derailleur chain is weaker and more flexible, and that the sprockets have much shorter teeth that penetrates less in between the rollers. This contributes to chain skipping under power.
- A built in problem with derailleurs is that on the smallest sprockets, where the fewest teeth are engaged in the chain and loads are the highest, the cage spring is also the most slack. This combination of factors is why derailleurs fail hard when you’re trying to push a lot of power for high speed on an 11t rear cassette sprocket.
My personal OPINION is that the inherent design of derailleurs makes them unsuited to strong chains and sprockets, and therefore to powerful motors. Most internally geared hubs also can not take power too far beyond what a human can produce. The exception is the very expensive Rohloff Speedhub. I have several customers running 3000 watts through them with no issues.
Running a high powered single speed mid drive is a step towards being like a geared hub motor. High amps provide the torque and high voltage provides the speed. Low powered mid drives are way more energy efficient and more bicycle like. A high powered single speed mid drive or big hub motor is just going to murder the little shifter mid drives in terms of acceleration.