What cog ratio for very high torque ebike build?

[duggie]

Hi folks. I think i am seeing the way. I require an ebike which will power me up steep hills without having to pedal, or at least pedalling but with no effort at all. It seems the same idea as using a block and tackle to raise a heavy weight. Thus i need a mid-drive with few teeth, and then a rear cog with many teeth.

I should therefore be able to use a 250w motor.

I could live with 2 or 4 mph.

So, it's just a question of how many teeth for the front cog and how many for the rear cog. There is probably a way of calculating this, but I think start with an educated guess and then trial and error. My feeling is that one revolution of the rear cog should require six revolutions of the front cog.

Has anyone any thoughts about this?

 

[duggie]

Thanks Timpo. The bike I have is a 1000w 18ah 48v step through folding fat tyre rear direct drive hub. This is my first ebike. Whether i can modify this or whether i can buy another and modify it, or use an unelectric bike I have to fit a motor, I don't know.

At this point I am wondering how feasible my thinking is, and what problems i should consider, and what cogs there are and where to get them and that sort of thing.

I think my idea is generally right for my requirement, but this is just a very early un-thought-out stage, so welcome any thinking. I don't know much at all about the whole ebike thing, but i reckon the mechanical idea is right enough. Any input is welcome, and of course one thought leads to another.

 

[tomjasz]

Could you give us more details?

I don't know the make of my ebike. But it is a 20 inch fat tyre, folding, step through, rear drive, 1000w 48v 18ah with an M2 display panel.

 

[AHicks]

Suggest you try riding a bike doing 2 mph before spending a lot of money on an e-bike geared for 2mph.....

I think you're going to struggle keeping your balance at that speed (I know I would!), and would suggest a minimum target of 5-6mph....

 

[indianajo]

I pedal myself +60 lb groceries+94lb bike+tools+supplies+water+racks+stand up 15% grade unpowered @ 1.5 mph sometimes. I have fast reactions. Your result may vary. I have a 32:32 ratio in lowest sprocket ratio. 52-42-32 front & 32 to 11 rear sprocket.
DD motors are unsuited for steep hills. My 500 w geared hub motor can take 330 lb gross up 15% without my help. It will start it from a stop too, which is harder. Of course you can't buy that on a bike in UK. Might try smuggling. Most US states have a 750 w limit here on built bikes, but you can buy more powerful motors as components. My 1000 w geared motor had a sticker I could peel off.
Mid-drive with 44 front & 48 rear does multiply torque, but 48 rear sprockets are pricey. 50's are worse. Then, the mid drive will eat chains 1.5 to 5 times faster than a hub drive. You need mid-drive for roads that climb 1000' in an hour's run. You don't have any such mountains in the UK as far as I know.

 

[duggie]

AHicks..........Yes, 2mph could be tricky. I think 5 sounds right, too.

indianajo.......32:32, interesting, even surprising. One turn of the crank to one turn of the wheel. That sounds like it would be difficult. I'll have to have an experiment and do some tests and measurements. I have an old bike i can use.

My motorbike has a very small front cog and a large rear cog. Its an old dt175. the top speed is only 50, but it handles very well on trails as most of the gears can be used without having to go to fast, so power for every situation. I can't wait to start testing, but alas it is the start of springtime and so I've lots of fair weather things to attend to. I'll start this project in the autumn and post how things go. I'll look into the availability of cog sizes. Maybe I could use motorcycle cogs and chains to cope with the torque loads. I have a workshop.

It seems to me that most ebike stuff is mainly concerned about speed and range rather than torque. I mean, you should be able to have any size motor as long as you was limited to a max speed. I'd be happy with 10mph max. So there doesn't seem to be on the market what i would like, without spending a lot of money, I expect. But I can custom build. As long as I can get a motor which can greatly vary its rpm then, in theory, i could have plenty of torque and also some speed, and all from a 250w motor, so i can research this over winter and hopefully do something practical towards next winter time.

 

[AHicks]

AHicks..........Yes, 2mph could be tricky. I think 5 sounds right, too.

indianajo.......32:32, interesting, even surprising. One turn of the crank to one turn of the wheel. That sounds like it would be difficult. I'll have to have an experiment and do some tests and measurements. I have an old bike i can use.

My motorbike has a very small front cog and a large rear cog. Its an old dt175. the top speed is only 50, but it handles very well on trails as most of the gears can be used without having to go to fast, so power for every situation. I can't wait to start testing, but alas it is the start of springtime and so I've lots of fair weather things to attend to. I'll start this project in the autumn and post how things go. I'll look into the availability of cog sizes. Maybe I could use motorcycle cogs and chains to cope with the torque loads. I have a workshop.

It seems to me that most ebike stuff is mainly concerned about speed and range rather than torque. I mean, you should be able to have any size motor as long as you was limited to a max speed. I'd be happy with 10mph max. So there doesn't seem to be on the market what i would like, without spending a lot of money, I expect. But I can custom build. As long as I can get a motor which can greatly vary its rpm then, in theory, i could have plenty of torque and also some speed, and all from a 250w motor, so i can research this over winter and hopefully do something practical towards next winter time.

I think if you look around enough, you'll see MANY MANY different set of priorities. E-bikes collectively, are all over the map, just like bicycles. Many get into this not realizing that, and it might take a while for them to figure that out. Until then, the way they have figured out to get the job done for THEIR priorities, will be most apparent in their comments.

Point being, the more time spent figuring out your actual priorities, defining your particular mission, can be a big help bringing focus into what it's going to take to get the job done for you. No one bike is ever going to be "best" at everything. They're all a long series of compromises.....

 

[duggie]

AHicks.......that sounds sensible, and it is encouraging to hear the divergence is out there. I'll look. I'm only a month into this stuff. I can tell you that the bike i bought I tested a bit, as it was still a bit chilly, but it performed well for my needs. Steep wet muddy tracks, rough rocky inclines and drops. It folds, too and goes easily into my camper van space under the sleeping platform. The tyres are brilliant, cutting vibration and giving superb grip. I chose well. The only problem is that i wish it had more torque. It says 53Nm, but, yes, will be probably 45Nm. This is just too poor for me. Ok if flat stuff, but I'm in the hills. A 100Nm would probably be fine. More if possible.

I have a vision of a 'monster' that just keeps going on and on over any terrain and incline, 60 degree inclines. But as you say, that may be out there....but i bet it would be well beyond my price range. But.....a 250w throttle-controlled motor with cog ratios found, like I say, a block and tackle. It seems feasible. Who knows, I may be able to post a formula. Haha....but then there have been better brains at this job for a long time, so I'll look. I'll research extreme hill climbing riders/bikes/competitions. Maybe I can cobble something together for a mere £400. Thanks for your advice

 

[indianajo]

The bike I have is a 1000w 18ah 48v step through folding fat tyre rear direct drive hub.

I'll say it again, DD motors are unsuited for hills. DD motors rated torque is produced at near full speed, or 23 mph on a 26" tire bike. The torque falls off fast as the motor slows down. Geared hub motor rated torque is produced at more like 4 mph due to the internal 5:1 gears. Same with mid -drives.
DD motors maintain market share by being very cheap, with only 2 bearings as moving parts, extremely reliable. Some people like to ride 20-28 mph in flat cities. That is apparently not your goal.

 

[duggie]

indianjo........Good info. I assumed the torque would be the same with a dd motor whatever the speed. That explains a lot. Didn't know that. I also assumed that the dd motor would be geared. I never imagined that it wouldn't be. I thought the pa levels ment gears! So it has been a bit of a disappointment. But it is a learning curve i sort of expected. In life i have found that in the very beginning you are on your own and so just to jump in, make a mess, and then you begin to learn. I'm so thankful for the internet and this site and the people here. Maybe I can find a geared hub to fit my bike. But then it would mean a new controller, I suppose. I'm learning, so thank you very much for that advice about hub motors. Lesson learnt.

 

[indianajo]

Maybe I can find a geared hub to fit my bike. But then it would mean a new controller, I suppose.

My $189 DD motor, and the first $221 geared hub motor, used compatible controllers. The rectangular plug (hall effect pickup) were the same. The phase wires (yellow green blue) had a slightly different termination available in dorman crimp connectors at the local auto supply. I got tired of riding the DD, and brought the geared hub to town from the summer camp (27 miles away) without moving all the geared hub controller & battery (defective).

 

[PedalUma]

Hi @duggie, I use https://www.sheldonbrown.com/gear-calc.html for gear calculations. The problem is that your bike has a one-speed motor setup. The external gearing does not impact the internal hub motor on your bike at all. For that you require a mid-drive. Then the torque is run through the gearing by the chain. You can then shift the gears of the power. It is easy enough to take almost any regular bike and make it a mid-drive. I recently took a 267 Sterling ($299) new bike and made it a mid-drive which eats expensive bikes for lunch on climbs. It was a girly girl bike. Here it is. The motor is between the pedals. If I can do this, so can you. A good used bike with full sized wheels will do. I just purchased a good used bike for conversion for $325 US this week. It is the black one. And it will end up looking similar to the classic Union Jack bike.

 

[AHicks]

The right guy could yank just that direct drive motor out by it's roots, and replace it with a gear driven hub, using all the of rest of the parts over again. I know this, because I've done it.... and it made a world of difference in the bike.

 

[duggie]

indianajo, PedalUma, AHicks........Thanks guys, that's really encouraging. I didn't expect to discover an interesting subject when I bought an ebike. I tested the bike again today and it is fantastic for my needs, but alas it still will not climb the ski-jump-like steep hill which is the fly in the ointment to my happy travel to and from my allotment. It certainly helps with me pedaling steadily in 1st, but i can't keep such pedalling up long enough, and it tires me and I have to stop and walk for half of it. It just needs a bit more torque. Well, I have a girly bike too which I can use, so I'll have a go at making it a mid drive, secondly I'll see about sorting the DD to a geared drive, and thirdly I'll just keep at that hill and see if I can get fitter.

It's funny coming to a new subject. I was looking at the bike just now and wondering what would happen if I simply fit a mid drive to it, keeping the DD......I'd probably end up in hospital, haha. But it is all part of learning to think and wonder, i suppose. But I can tell you this, other than that damned steep hill, the rest of the journey is a pure joy, like a liberation, life changing. It's solved a real problem for me. I'm going to have alook for geared hubs right now. Cheers