Building homemade mid-drive for tadpole trike, need gear selection help please
Hi guys, new user here.
My wife and I are building a new type of vehicle, but to start with we are going to test out some drive systems by modifying one of our 27 speed tadpole bents into electric assist/drive. For various reasons I would like to keep the existing gearing and have a smaller motor (starting with the Kollmorgen 400 watt brushless) operating through the 9 gears on the rear drive wheel. Initial plans are to gear the Kollmorgen down about 5:1 via a timing belt/pulley system and have this ~600rpm shaft connected to both the rear 9 speed cassette and the 3 gear pedal cassette. I would like to freewheel both the motor and the pedals, so either can operate without forcing the other to turn. If you are with me so far, what I would have is a jackshaft turning at about 600 rpm from the Kollmorgen to which everything is connected. So this jackshaft will need a fixed single speed 16 tooth 3/32 sprocket going back to the rear 9 speed cassette/derailer and also a freewheeling 16 tooth 3/32 sprocket going foward to the 3 gears on the pedal crank/derailer. Also either the smaller timing gear pulley on the Kollmorgen motor or the larger timing gear pulley on the jackshaft would need a freewheeling hub so pedaling does not drive the motor when you strictly want human power. (later we may investigate using the motor as a generator, in which case we would want to remove that freewheel).
My background is electrical engineering. I have never done a lot with bikes but I do have a small 7000 pound cnc milling machine and an old manual hardinge lathe in my garage. Obviously I would rather buy as many of the harder to make parts like sprockets and freewheels off the shelf instead of trying to machine those parts from scratch. It would take considerable effort to make a freewheel as small and lightweight as the ones they use in bikes.
Am I missing something easy to use because of my newb status with bike mechanics? I can find 16 tooth 3/32 single speed freewheel sprockets which have a large threaded hole and I can find 16 tooth 3/32 fixed sprockets which also have a large threaded hole. If all I have to machine is various threads on the jackshaft for these that doesn't sound too bad. Is there a type of freewheel that has a smaller hole than the standard? 1.37" x 24 TPI (that makes for an awfully large diameter jackshaft)
Thanks for any advice.
You'll run into a question I posted earlier: http://visforvoltage.org/forum/6205-sprockets-question
Typical sprockets you can get for a regular motor do not use the same sort of chain that is bicycle chain. Yeah they're both "roller chain" but the geometry is different. It sounds like you have more machining experience than I do and can handle creating the conversion.
Hi, thanks for the response.
I do not plan to use a typical (like #35) chain and sprocket for the motor. I do not want to spin a chain at 3000 rpm because it will be quite noisy. I will use a rubber toothed timing belt and aluminum timing gear pulleys to reduce the 3000 rpm top speed of the kollmorgen down to a managable 600 rpm via 5:1 (so, say 60 tooth large gear, 12 tooth pinion on the motor). The shaft that has the 60 tooth timing pulley and turns at 600 rpm is called a jackshaft. It has its own bearings and will also have a fixed 16 tooth sprocket running back to the rear wheel 9 speed cassette and a 16 tooth freewheel sprocket going foward to the 3 gear pedal set.
Anyway, I think I see how to make the off the shelf 16 tooth freewheel and fixed sprockets with the 1.37" 24TPI threads work. I will lathe the jackshaft to the correct size then mount it on my cnc mill and do helical thread interpolation using a sort of modified fly cutter. It would be a lot easier if my lathe had threading, but oh well. I haven't done threading on the cnc mill before, but it shouldn't be *too* bad. Either that or lathe the jackshaft and then beg someone to thread it for me :-)
Ah, that explains a design detail in the Charger bicycle. It has a REVCOR motor and the first stage gear reduction is a toothed rubber belt rather than a chain. It goes through a jackshaft and the second stage is #25 chain. The charger bike however doesn't have enough freewheeling in its drive train and isn't a mid-drive like you're discussing.
Kollmorgen 400 watt brushless - 3500rpm at 5:1 gives 700rpm. 30:1 will give you ~120rpm.
Human - 60-120rpm
Now, if you use a three piece crankset (where the normal pedals go) you can put a drive sprocket on the other side. Then, you can use a freewheel on a flipped disc hub, modify a sprocket to fit the disc attachment points, put the motor on it.
Of course, yours is already a tandem and you'll have a sprocket in the way on that left hand pedal.
What you need is an overrunning clutch, they come in sizes for any shaft, and most are available with #40 chain sprockets already mounted. All you need to do is to thin the teeth of the sprocket on your lovely expensive lathe. (your machinist group could have told you this, and would probably be a better place to ask)
And I had a cheaper idea too, a simple one-way bearing. You'd have to machine a hub to press it into, building that hub to attach a sprocket. You may have problems finding a bearing for your load and shaft size, but it's cheaper for you to use a different size shaft.
Actually both the lathe and the mill were free (I bought them cheap and sold the Fanuc controller on ebay for $500 more than I paid for both the mill and the lathe :-p
I *could* have asked on cnczone (my machinist group) but I figured the electric bike folks might be better informed about off the shelf parts.
The bike is not a tandem, it is a recombent tadpole. But anyway, I didn't quite follow the bike jargon. I will have to look up what you mean by "three piece crankset" "flipped disc hub" and "overrunning clutch".
I was figured 3000 rpm under a moderate load, but if it does 3500 rpm loaded, that is fine. 5:1 = 700 rpm spinning a 16 tooth sprocket driving the rear 9 gear cassette. Selecting the largest gear on the cassette should give me another 5:1 or so (I can't remember if the largest gear on the rear drive wheel is 80 tooth, will have to count it again). So that would be 140rpm on the back 20" wheel. That is 12.2 feet per second bike speed, or 8.3mph. That should be good for hill climbing. Figure a human pedaling at 100rpm would need to gear up 7:1 to the jackshaft to match the motor speed. So at least one gear on the pedal crankshaft should have 112 teeth if it is driving a 16 tooth sprocket on the jackshaft. I don't think the largest gear on the pedal crank has that many teeth, will have to go count it also.
Thanks for the help.