127RPM, 26", Huffy gearing.. All the MPHs

[Smurf]

Hi everyone, this is my first post. I apologize if there is a "Hi, I'm me" section somewhere that I skipped , but I figure this might help some people if they are embarking on a similar build.

I've been planning a bicycle that can be pedal-powered as normal, while also having a reasonably-priced DC motor. I finally concluded that I'm better off gearing the motor down to slightly-faster-than-pedal speed with a jackshaft, then using another bike's existing crank & rear gearing 1:1'ed to the left side of the rear wheel.. I started doing some math.

The motor I have targeted is a 24V, 750W that spins 2600RPM. Taking that down to 127RPM with a jackshaft, using the jackshaft's output on the non-sprocket side of the donor's cranks, you could (in theory?) use all the gears of the donor bicycle. To decrease the motor's spin rate, change some of the jackshaft gears while keeping roughly 127RPM output.

My project bike is a plain ol' Huffy 18 speed, with 50T, 39T, and 30T crank sprockets, and 14T, 16T, 18T, 21T, 24T, and 28T rears.

At 127RPM:
50T CRANK
14T==453RPM===35.056MPH
16T==396RPM===30.625MPH
18T==353RPM===27.27MPH
21T==302RPM===23.35MPH
24T==264RPM===20.397MPH
28T==227RPM===17.557MPH

39T CRANK
14T==354RPM===27.329MPH
16T==310RPM===23.977MPH
18T==275RPM===21.25MPH
21T==236RPM===18.238MPH
24T==206RPM===15.909MPH
28T==177RPM===13.69MPH

30T CRANK
14T==272RPM===21.02MPH
16T==238RPM===18.409MPH
18T==211RPM===16.306MPH
21T==181RPM===13.977MPH
24T==158RPM===12.216MPH
28T==136RPM===10.511MPH

 

[llile]

Here are a couple of takes:

First, 127 RPM at the crank is actually a bit fast. Sure, there are plenty of people who ride at that cadence, and if this is you, forget I said anything. A Stokemonkey, a very successful commercial motor that does exactly what you are planning, runs at 90 RPM crank speed. It really feels pretty good to me at that speed.

Now, 750 watts is a whole lotta power. You can use a 350W motor all day at 17-20 mPH average and get a lot of battery range. These speed demons on this board are often using 2000-3000 watt motors for crazed amped up e-bikes, but most of them (me included) just escaped from the loony hatch.

How to control the motor? Planning to use some kind of controller? You can't use on-off control, you'll get whiplash.

You won't need your 30 tooth chainwheel at all. Probably not the 39 either. I am using a single front chainring, 52 teeth, and a wide range rear freewheel, 13 - 36, and I don't miss any of the low geared chainrings. I can haul a big load of camping gear in a trailer up the steepest grades. The stokemonkey ties into the chain drive in the middle, with an extra set of gears. I am running a recumbent which has plenty of room for such things. You are probably stuck with running a second chainring on the left side of your crank, or the non-sprocket side as you called it. Should work OK. Watch out for chain alignment issues, and sprockets that don't run true - that'll throw chains for sure.

Cool build! Keep posting details!

 

[Smurf]

Thanks for the input Llile

First, 127 RPM at the crank is actually a bit fast. Sure, there are plenty of people who ride at that cadence, and if this is you, forget I said anything. A Stokemonkey, a very successful commercial motor that does exactly what you are planning

I realize this is faster than most cadence, however I arrived at 127RPM as the number because I want to be able to blend in traffic at the fastest section of my route to work, and using the gearing on my project bike that's what was required. The Stokemonkey seems close to what I'm trying to achieve, however they openly say they're not trying to replace pedal power. I'm lazier than their target audience

The 35mph desired speed is also the reason for the 750W. I'm still considering a 24V 500W, however being as this is my first project I'm trying to get components that could be reused on "next year's model". Because of that, I keep steering myself away from a simple hub motor.

I'm planning on pedaling up to 10mph before kicking the motor on, and I would be using a common e-bike eBay controller. This may render the entire gearing thing pointless, so I may eliminate the rear gearset and just roll with the 3 chainrings. Think I could get away with a 18T in back, and the 30T and 50T up front? Number-wise, that should 10mph-16mph with the 30T, then up to 27mph with the 50T? That would also reduce my chances of throwing chains.

Or have I lost my mind?

 

[dnmun]

wouldn't it be cheaper and easier just to use a regular bldc hubmotor and controller? you can't do 127 rpm, not humanly possible imo. just one slip on the pedals and if you are not freewheeling it will be really bad too.

 

[Smurf]

The idea I have, I wouldn't have my feet on the motorized pedals, as there wouldn't be any. I'm not completely familiar with precise terms for bike parts, so I'll try to explain the setup a bit better..

Looking at a "donor" bicycle's right side (right hand, sprocket/gear side), remove the horizontal bar from the crank back to rear wheel mount. Remove the pedals, and install the bar onto a separate "project" bicycle. The donor crankhousing with sprockets would be just down and forward of the project's seat post, opposite of the rear red reflector's typical spot. The donor axle mount, with donor bike's rear gears, would be behind the seat about 15", about where the end of a back rack would be. Parallelogram. Mount the motor inside the project bike's frame, about where a motorcycle ICE engine would sit.

Reduce the gearing down to 127RPM at the donor bike's cranks (mounted up near the seat, with chain guards for my future kids' safety of course). You would then have your original project bicycle being a typically operated pedal bicycle with the standard 18 speeds, nothing has changed mechanically to it. But you would also have the donor's drivetrain driven by an electric motor, working with or without your pedaling.

A photoChop of the donor's crank/gears, simplified for your amusement:

Notice that this image is reversed from the description.

 

[amberwolf]

I got your PM; posting the reply in your build thread rather than by PM, so others may also benefit from it later.


I don't know what motors those Jazzy's have, so if you can post some pics of the motors, as well as any data on their nameplates, it will help.

I found a little data online about them, like this page:
http://www.usatechguide.org/itemreview.php?itemid=568
that shows a controller of up to 50A (presumably per-channel, one for each motor).

Wheel diameter is 14", max speed at 4.5MPH for 24V. So you can figure out what the speed would be at your wheel for various wheel sizes, gearing ratios and voltages.

As for what controller you would need for them, well, if you want to run them at speeds up to 35MPH, you're probably going to need to run at least 30-40A at a guess, maybe more. You will need to do testing with a wattmeter or at the least a high-current multimeter to find out what they take at various loads.

Probably tehy are brushed motors, which means that run at higher voltages they will run hotter just because of that, and brush life will be shorter.


These wheelchair motors are not light, either. Typically including the gearbox (which you will want to keep on there) they will weigh 15-20lbs each, if they are like the ones I used. The higher wattage 4-pole motors are better for your purpose, but they will be heavier. I don't know which those Jazzys have. The gearboxes will make it easier to mount the motor to the bike, as well as to mount sprockets to it since you can just bolt or weld them to the wheel collars, like I did.

Some motors can't be used without the gearbox, unless you add new bearings to the end of the motor, or smply cut off the end of the gearbox with the bearings in it for the motor shaft, and then cut the worm gear off the motor shaft so you can mount your sprockets to the shaft. But then you would need a reduction gearing anyway, and that's another good reason to leave the gearboxes, because they usually give about the output RPM you're after to start with.



For 35MPH sustained, you may well need two of those motors, or ventilate and aircool the one. You'll have to do some testing, and I'd recommend using a barbecue thermometer on there--one of the cheap electronic ones that you can remove the probe from, and wire it remotely to the display (which you can then mount on the handlebars), so you can monitor it during rides for testing, and know when to stop before you burn up a motor.

 

[dogman dan]

750w of power will max your speed out at about 25-30 mph depending on wind drag and grade. But of course if you mean you will feed as many watts as it takes into a 750w motor to get er done, then you are among like minded people.

Cheap hubmotor kit is the easy way, but that doesn't mean we don't love to be entertained by a good frankenbike.