DIY ebike Cog sizing
 

Hi,

I'm thinking of making a very basic electrically assisted bike. The idea is just to bolt a small battery drill type motor (havent got one yet but I imagine 12v to make it easy) onto the underside of the bottom bracket, with a cog on it, using a short chain up and around the inner chain-ring on a 3 ring mtb chainset. This will sacrifice my inner ring. Then a sealed lead acid battery jubilee-clipped to the top tube (did this before for old school big bike lights years ago and it works well) with a couple of wires linked to a push button on the bars.

I'm not bothered about LCD displays and computers and throttles and flashy stuff. I just want a motor I can switch on when I hit the hill that my house happens to be at the top of!

I want it to help me out, not propel me without pedalling. When the charge runs out, it runs out and I'll have to pedal again! (Although I realise full drain could be a problem for the battery). I need to do about 3km uphill every day. I'd expect to do maybe 10kmph, as now I can get up there at a similar speed but some help would be great!

So, firstly, is it a goer? Secondly, what size of cog should I use on the motor? Of course, it needs to be cheap! Any other advice?

Thanks for your help!

So basically you just want a hill helper only motor, preferably using a cheap brushed motor with just a simple switch/button activation.

I totally understand what you are trying to do. Problem is that there is no off the shelf kit for that application and those of us desiring to build a hill helper only set-up especially a lower powered simple one have to just make do with whatever we can come up with ourselves.

Using a cordless drill has been done before and is a decent approach to the problem especially since it offers a ready to go gear reduction on the motor. Attaching via. the smallest chain-ring on the crank although a possible approach is among the least desired approaches if you are trying to gear the motor down quite a bit for strength for hill climbing.

On my build for this kind of application where I used a different kind of motor set-up then what you are considering I sacrificed the largest #1 granny gear 34t sprocket on the rear wheel and put a second chain on that rather then trying to hook up to the crank in the front. Realize that with chain spacing you don't just loose the sprocket you put the second chain on but also the one next to it as well since the spacing is tight enough that you can't run two chains on two sprockets right next to each other. So in my build on a seven speed rear sprocket spool I lost both the #1 and #2 position and have only the upper five gears on the rear. If you put a second chain on the smallest #1 position chain-wheel up front on the crank I think your going to loose the middle #2 position chain wheel as well because I don't think the spacing is wide enough to run two chains right next to each other even up front where the spacing is more generous. I could be wrong but short of some very clever routing of the second chain loop to keep it tucked in very tight behind the crank to prevent interference I think if you try to hook up front your going to end up loosing all your front gears and being stuck on the big #3 up front only for your pedal power and as I said it gives you less gear down capabilities since the crank ratios are built to gear up not down.

Long story short, I think your best bet is probably to run your second chain on the big #1 granny gear on the rear wheel like I did and mount your cordless drill gear head motor behind the seat (where you have more width to work with anyway with no pedals spinning or legs pumping back there).

As to the actual size of sprocket to use on the drill, that is going to depend how fast the drill spins at the voltage you are going to run it on and how fast you normally climb that hill and how big your #1 position rear sprocket is and how big your wheel size is and even then your going to need to experiment a little to get it just right for you.

1st determine your cadence (pedal rate-rpm). 60rpm?
2nd - inner chainring tooth count. 32T?
3rd - drill rpm. 800rpm? 600rpm for good torque?
Would require 3t cog!

12V drill with top speed of 300 rpm, (200rpm = good torque), with 9T cog would work.
Also, be aware ...
Drill will be too wide to allow crank-foot clearance.

Simpler to add wheel to drill chuck and go friction drive?