Single speed e-bike--anybody?

[mhk]

I have been planning to add a 750w Bafang mid drive motor to my Kona Smoke, currently set up as a fixed gear mtb (the rear wheel flips around so that it is also a single speed, allowing freewheel coasting). I was going to add a shifter/derailleur/cassette, but I'm wondering if I really have to. I like the simplicity of the derailleur-less rear (especially in the winter), but I'm wondering if having only one speed will hinder the capability of the 750w motor so much that it's annoying, i.e. getting spun out easily on one end or slow starts that wear the mid-motor drivetrain on the other.

Anyone have a single speed ebike? If so, what gear ratio are you running? Do you wish you had more gears?

I've thought about an IGH (which I think makes a lot of sense for commuters) but then I'd have to get a new wheel, and I'm going for value with this buildout.

 

[rich c]

I have a single speed fat bike. I’ve gotten by for 1700 miles, but would not recommend it. The cadence is rarely at an optimum rate. And definitely a hassle for climbing hills.

 

[Ken M]

The issue with going with a mid-drive for your concept is that fixies/single speeds tend to be optimized for higher speeds. Let say you have a 48T front and an 16T rear which would likely have a decent cadence in the 20-25mph range. That gear ratio only allows 33% of the motor's torque to be transmitted to the rear wheel because of the gear ratio (and that it true at all speeds).

That said a 750W mid drive that may product over 100nM of torque at the crank may be adequate to get over this shortcoming of mid-drives but you may find that a geared hug in the 500-750W range or DD hub around 1000-1500W maybe a better configuration (at least you know that the motor's torque is directly applied at the rear wheel and these motor are just as efficient as mid drives when riding say above 15mph typically).

The good news is that human input is most effective at the lower speeds. A 200lb rider standing on a 170mm crank generates about 160nM of torque at the bottom bracket so the motor isn't that essential to get you to say 5-10mph (probably only a few seconds of effort) and then engage the mid drive (via a throttle would be ideal but it will likely have a torque-bases PAS that you can't turn off at the slower speeds.

 

[mhk]

I was thinking of going 44 x 20 (single speed, not fixed), which would be pretty crazy-legged at 20 mph, but I figure it's about 110 cadence around 18 mph, which is doable, and can be started from a standstill without much trouble. If I don't like it, I can fit a couple more cogs on my SS hub, install an old derailleur and shifter, and run a 4-spd setup. Or splurge on a new IGH rear wheel.

I appreciate the input.

 

[rich c]

The issue with going with a mid-drive for your concept is that fixies tend to optimizes for higher speeds. Let say you have a 48T front and an 16T rear which would likely have a decent cadence in the 20-25mph range. That gear ratio only allows 33% of the motor's torque to be transmitted to the rear wheel because of the gear ratio (and that it true at all speeds).

That said a 750W mid drive that may product over 100nM of torque at the crank may be adequate to get over this shortcoming of mid-drives but you may find that a geared hug in the 500-750W range or DD hub around 1000-1500W maybe a better configuration (at least you know that the motor's torque is directly applied at the rear wheel and these motor are just as efficient as mid drives when riding say above 15mph typically).

The good news is that human input is most effective at the lower speeds. A 200lb rider standing on a 170mm crank generates about 160nM of torque at the bottom bracket so the motor isn't that essential to get you to say 5-10mph (probably only a few seconds of effort) and then engage the mid drive (via a throttle would be ideal but it will likely have a torque-bases PAS that you can't turn off at the slower speeds.

A Fixie has no freewheel. It would be suicide with e assist or throttle.

 

[Ken M]

I really like the idea of a single speed urban mobility ebike but technically I think the best way to accomplish that would be with a rear geared or DD hub motor with enough power/torque such that bias towards higher speed gearing isn't a show stopper at the slower speeds. I tend to think that most riders can still get 48/16 to 10mph where a hub motor starts to become somewhat efficient.

 

[Ken M]

A Fixie has no freewheel. It would be suicide with e assist or throttle.

My fault....I use the term fixie to describe any single speed bike which is not correct nomenclature. I would never ride a single speed without a freewheel/cassette.

 

[sl_duck]

An advantage of mid-drive is the ability to use the bike's gearing. If you aren't using any gearing, a hub motor would be much simpler, less wear on the drivetrain, and likely cheaper. I find I mostly use the 52-13 combo on my cassette and could easily get by with only that gear, with 26" wheels. Motor is a Bafang G310.

 

[ChezCheese:)]

Me, I don't understand the fixie fixation. Gears make riding easier, more efficient. Same with electric assist.

 

[steve mercier]

I have been planning to add a 750w Bafang mid drive motor to my Kona Smoke, currently set up as a fixed gear mtb (the rear wheel flips around so that it is also a single speed, allowing freewheel coasting). I was going to add a shifter/derailleur/cassette, but I'm wondering if I really have to. I like the simplicity of the derailleur-less rear (especially in the winter), but I'm wondering if having only one speed will hinder the capability of the 750w motor so much that it's annoying, i.e. getting spun out easily on one end or slow starts that wear the mid-motor drivetrain on the other.

Anyone have a single speed ebike? If so, what gear ratio are you running? Do you wish you had more gears?

I've thought about an IGH (which I think makes a lot of sense for commuters) but then I'd have to get a new wheel, and I'm going for value with this buildout.

we have a Yunbike C1. It works well and has climbed some large hills and is only 250 watts with a 44 x 11 setup on 20" wheels. I think you could get a satisfactory ride with 750 watt motor with the right gearing so I say why not try it?

 

[Bruce Arnold]

If I were doing a single speed, I think I would go with a front wheel motor. Not sure why. More of a hunch than a reasoned response.

 

[mhk]

Me, I don't understand the fixie fixation. Gears make riding easier, more efficient. Same with electric assist.

Doesn't necessarily have to be a fixation. Some people choose to ride a bike precisely because it's challenging, and some types of bikes present more challenges than others. Some cyclists may prefer the simplicity and value of a track bike.

Me, I happen to have more than a dozen bikes (full susp mtb, hard tail mtb, cross bike, 2 gravel bikes, 2 road bikes, 2 single speed mtbs, single speed road, e-bike, dirt bike), one of which is currently set up fixed. I don't ride it very much any more, which is the main reason why I posted this thread.

 

[mathisbl]

I've tried not shifting on my NCM Moscow Plus and I found myself staying on the 48/13 ratio. I like riding at around 36km/h outputing a rather slow cadence. I'm gonna convert it to single speed with the freewheel spacer sold at ebikekit.com. Glad I found this solution. The smallest single speed I could find for that was 14t on aliexpress and the largest chainring I could find was 52t for 4 bolts 104bcd. I could maybe go higher with a different crank spider but 52/14 is the same as 48/13 and I'm not sure a bigger chainring would fit. I could certainly use a smaller ratio like 50/16 like on the upcoming radmission but I like letting the motor do most of the work and pedal lazily, either that or pedal hard and go really fast so that ratio has worked well for me and I'm sticking to it. Also we should remain mindful that a bigger chainring means less gorund clearance and also a smaller rear cog teeth wear out faster do to increased friction per contact point. So I think 52/14 will be my sweet spot and I'll keep you posted when I receive the parts and do the installation.