Spindle motor attached to front sprocket with chain?

[godrek]

Hi,

I want to DIY a non-hub motor drive by mounting a suitable motor in the center of the frame
and attach a sprocket to the spinning part of the motor and use a chain to connect that sprocket
to the smaller sprocket on the front set of sprockets connected to the pedals. This would restrict me
from using that sprocket for pedaling, but that is ok.
This is the plan: Green is motor, Blue would be attached sprocket, yellow is chain, black is some mount from motor to frame
http://imgur.com/IlHd9Do

I have 27" wheels on the bike I'm trying to convert, and I'm looking at trying a spindle motor with controller like this:
http://www.amazon.com/dp/B01DLT24R8?psc=1

I want to control the speed and monitor battery percent, etc. through an Arduino that I have like this:
https://www.youtube.com/watch?v=kYFNUQgE26E

My question is, will that spindle motor give me any usable amount of power? I'm not looking for 0-30mph with no pedaling,
I just want to be able to turn the motor on while I'm pedaling to provide some extra help and ease my effort.

It's 400W of power but says it can be controlled at 60V/10A/600W so I'm a little confused as to all these numbers.

I am choosing to build myself rather than buy a kit because I want to learn and have the satisfaction of doing it myself,
even if the results are subpar. It will also be cheaper this way (besides the fact that it might fail and waste a bunch of $)

Am I totally off track here? There are so many measurements between volts, amps, watts, horsepower, rpm, torque etc. that I'm not sure
how to translate all of it into meaningful data on how well I'll be able to spin a wheel.

I've been doing my best to read and lurk, but I think I will get my questions answered best by just asking them.

 

[Rassy]

I'll share the results of my first e-bike, and a couple of issues I ran into, long before ES was out there. I started with a little 24V motor from a small sit down scooter, along with the controller, throttle, batteries, and even a small drive chain. Made a bracket so the motor was under the down tube and it's sprocket lined up with the largest chain wheel. Needed to get up a steep hill, so wanted the lowest gearing available for the little motor.

Short story, it worked pretty good. I couldn't use the middle chain ring because there wasn't enough clearance between the chain rings to allow the chains to be on adjacent rings. This could have been resolved by adding some spacers between the middle and largest chain rings. Also, there was no freewheel involved so pedaling without using the motor meant the motor had to turn, and when the motor was running the pedals had to turn. The only problem there was if you hit the throttle before pedaling the pedals could be jerked away from your feet.

End of story, the little motor overheated and failed after a couple of dozen trips up the hill. When looking for a replacement motor I found hub motors and never looked back.

You intend to use a more robust motor, so good luck with your project. I'll enjoy seeing your build develop.

 

[godrek]

I'll share the results of my first e-bike, and a couple of issues I ran into, long before ES was out there. I started with a little 24V motor from a small sit down scooter, along with the controller, throttle, batteries, and even a small drive chain. Made a bracket so the motor was under the down tube and it's sprocket lined up with the largest chain wheel. Needed to get up a steep hill, so wanted the lowest gearing available for the little motor.

Short story, it worked pretty good. I couldn't use the middle chain ring because there wasn't enough clearance between the chain rings to allow the chains to be on adjacent rings. This could have been resolved by adding some spacers between the middle and largest chain rings. Also, there was no freewheel involved so pedaling without using the motor meant the motor had to turn, and when the motor was running the pedals had to turn. The only problem there was if you hit the throttle before pedaling the pedals could be jerked away from your feet.

End of story, the little motor overheated and failed after a couple of dozen trips up the hill. When looking for a replacement motor I found hub motors and never looked back.

You intend to use a more robust motor, so good luck with your project. I'll enjoy seeing your build develop.

Thanks for sharing. Funny you mention a 24v scooter motor, I was also looking at this: http://www.amazon.com/Electric-Currie-Technologies-XYD-6B-scooter/dp/B019BM28O8/ref=sr_1_4?ie=UTF8&qid=1461206802&sr=8-4&keywords=dc+motor+500W

It's cheaper, but I'm not sure how the power compares. It's listed as 500W, takes 28amp max at 24v, and 2600rpm. The spindle motor in my original post maxes at 10amps, but lists a 60v/600W control power and 12000rpm. From what I understand, I'm looking for amps to improve horsepower/torque, but I'm not sure how the volts factor in.

I don't really understand how RPM factors into all this, because a small motor will obviously spin very fast with ease, but how much force will be behind it? I'm confused on how to get a real understanding of what kind of torque I can push out given volts, amps, etc.

 

[Miles]

That spindle motor will give you about 0.5Nm of torque. You multiply that by the ratio of the speed reduction from the motor to the chainring. With a single stage, a ratio of more than 6:1 is difficult to achieve. So, that's, at most, 3Nm at the pedals. Not a lot.... 3Nm at 100 rpm gives you just over 30 Watts.

 

[DanGT86]

I am choosing to build myself rather than buy a kit because I want to learn and have the satisfaction of doing it myself,
even if the results are subpar. It will also be cheaper this way (besides the fact that it might fail and waste a bunch of $)

Am I totally off track here? There are so many measurements between volts, amps, watts, horsepower, rpm, torque etc. that I'm not sure
how to translate all of it into meaningful data on how well I'll be able to spin a wheel.

I've been doing my best to read and lurk, but I think I will get my questions answered best by just asking them.

I respect your desire to do it yourself but there is a big difference between learning and doing it the hard way intentionally. There are lots of mid drive options on the market that are optimized for the exact RPM and power requirements of a bike. There are also common RC motors that fall in that range. Trust me there is no shortage of custom work and creativity involved in making one of these attach to a bike and function reliably. You could buy a cheap kit like GNG and throw away everything but the motor. You would still have a giant to to list of things to make and create as well as learn from. Don't mistake using the wrong motor for the job as being creative. If you look for the perfect motor for your application you will probably end up at or close to the motors used in these kits anyway. They use those for a reason. Some of the most creative individuals on this forum use common motors.

When it comes to learning, you learn by having access to information or examples of how something is done effectively. A classroom would have a teacher with experience teaching a group of students without experience. You wouldn't hire an art teacher to teach calculus in an attempt to make the students more creative. Let a kit of proven ebike parts be your teacher. You can always sell it all later and duplicate in your own creative way. No shame in reverse engineering something that is proven to work. I learned more in the first minute of watching the display on my cycle analyst than I did in months of reading before my first build!

It will likely not be cheaper your way unless you have tons of time and access to machinery to make everything you need. The companies selling kits have the price of components down due to high volume production. Don't underestimate this price break. I have several hundred dollars in sprockets, freewheel, and chain for a mid-drive that never happened. That's more than halfway to a BBS or GNG kit.

Just my .02. Don't want to see someone so far off the beaten path that they never arrive at the destination. Best of luck.

 

[fourbanger]

Apologies if it seems I'm trying to hijack this thread, but I'm a little curious as to how you guys know that this particular motor is not appropriate for the job.

 

[DanGT86]

I don't think its a hijack but a relevant question.
Here is why I think it would not be a good choice.

1. Low torque high rpm design
Miles's post covered it pretty well. I will try to expand on that a little with my own less technical words. Small torque with really high RPM can net high power output but 12000rpm for 400watts is very far out of the useful range on a bike. This motor would require a lot of gear reduction to get it down to a speed near human pedal cadence. Gearing from 12k rpm to 100rpm at the cranks would be difficult. You need 120:1 reduction which is impractical since a 10 tooth sprocket driving a 1200 tooth sprocket is not an easy setup for a single reduction. The way I understand motors is the higher RPM you ask it to spin, the faster it will heat up or melt if it can't deliver. I just think of the current trying to pay up on the 12000rpm debt that you have come to collect.

2. General size and shape
Specs list the motor as 186mm long. That's going to interfere with pedaling pretty much anywhere in the triangle. Short large diameter motors are way easier to mount on bikes than long small dia ones.

3. Clues you can infer from its intended use.
It is a spindle motor designed to turn a very small diameter tool very fast at low load. Mounting it on a bike is the absolute opposite of this usage. This leads me to believe it would not cope with quickly changing loads or shock loads common to ebike use. The priorities when designing this motor were for smooth high speed consistent operation. You would be paying for design features that don't help the goal of propelling a bike. I bet it holds very accurate and precise RPM. What does that do for you on a bike? Not much. Same reason you don't tow a camper with an F1 car even though it has the power.

4. Durability in the elements or lack thereof
This may fall under number 3. but if this motor is designed for the controlled environment of a CNC machine in a building then I would question its reliability out in the weather with common wear. Will is survive the bike falling on it? What about kicking it when getting on and off the bike? How about rain? These conditions might not exist in the protected environment of a milling machine. Is that controller made to be bouncing around on a bike frame or mounted safely in a stationary machine cabinet? Vibration and shock shouldn't be underestimated.

I'm far from an expert on motors but these are the obvious things that make it look like a poor choice to me. If someone buys that motor and makes a usable and reliable bike that fits their demands then I guess it was a good choice. I just think this particular motor's specs and design make that scenario very unlikely.

 

[fourbanger]

Good grief, I just realized I was looking at the little Currie motor posted by godrek the whole time and totally missed the original post.

Good answers/explanations though, and I agree; that little motor does not look like an appropriate match for an ebike.

 

[fourbanger]

godrek, there are lot of people on this board with a lot more knowledge and experience than me (by far), but it seems to me you ought to brush up a bit on gearing ratio if nothing else; it's not too complicated once you get your head around it.

For example, let's say you have a a 13tooth gear driving a 52tooth gear. That's a ratio of 4:1 (52/13=4) so for every 4 complete turns of the small driving gear the larger 52t gear will turn once.

Now let's say that 13t gear is attached to a motor that spins at 1000rpm and the 52t gear is attached to the pedals (you). At a 4:1 ratio that means a 1000rpm motor will spin the pedals at.... 1000/4=250rpm. Your legs will fall off or be broken in so many horrible ways I'd rather not imagine or maybe just cheese grate the back of your... anyway it's no good, and this is why people use multiple stages of gear reduction to effectively slow down the motor to something more manageable, as well as implement freewheeling chainrings, etc. etc.

Anyway, there's at least one more thing to ponder on your journey. Apart from that, keep reading.

Good luck!