Electric assist bicycles

[govman6159]

Originally Posted by puddleglum

govman, nice job. It looks like your motor is driving the crank through a freewheel, right? How do you keep your motor from overspeeding your crank at full throttle? Unless you have a really low speed motor, the gearing doesn't seem low enough. Am I missing something?

Yes, I believe you are missing something. If you look at the close up of the motor/drive sprocket, you will see the crank is attached to a freewheel bearing. When the motor turns (clockwise) it only drives the sprocket and not the crank. This in turn, turns the stock sprockets which turns the chain as if you were pedaling but the pedals do not turn. If you do pedal, the freewheel locks and the crank powers the stock sprocket or if you apply power and pedal, the motor will provide assist. It is truly a hybrid w/ 3 possible power sources (the motor only, pedal only or motor and pedal).

As for the gearing, I can change that to match the terrain just like on a regular bike. The front sprockets are 48/38/28tth and the rear is a 14-34 tth 7 speed rear freewheel cassette.

All this makes for an excellent ebike that is suitable for riding over various terrains. This is really just my proof of concept bike. I am experimenting with various gear ratios to see which one best suites my performance desires. So far I have migrated from a 11-28t rear cassette to a 14-34. The 14-34 is better for all electric riding and I still have high enough gears for normal city riding. My next mod will be switching the front triple spocket to a 24/34/42 setup for even more low end torque. This should allow for better hill climbing capability and faster takeoffs and still allow for full speed operation.

[govman6159]

Originally Posted by Daox

Do you have any form of BMS for your batteries?

I checked prices on the controllers on the electric scooters site. They're way cheaper than other ones I've seen. Is the quality any good? $30 for a controller makes me wonder.

Yes I have a BMS and his name is Govman6159 (lol). IMO, there isn't really a need for a BMS on this bike as long as I monitor cell voltage periodically. What I do is periodically monitor the individual cells as its charging. If I see one getting too high, I will stop charging that cell and continue charging the others. I am using a 24V/1.8A charger to charge the pack. I find I get nearly equal charging up to 27.1V (29.2 is the max for the pack). Once it hits that voltage, I have found it is at this point when cells may become unbalanced. So from this point forward I closely monitor individual cells. This has allowed me to id which cells are prone to overcharging (I only have one set of cells prone to overcharging). 27.1V is about 92% charge for the pack so I usually just stop at this point. I have found that by doing this, I get less than a .08V variance between cells (whereas .3V is considered normal).

As for the controller quality, I'm happy w/ the performance of mine. I've had this controller for 7+ years and its still working (YMMV). IMO it does a fine job.

[sendler]

Originally Posted by govman6159

As for the gearing,

I think they were asking about the gear reduction from the motor. What is the motor rpm rated and what is the crank rpm if you want to pedal to add to the top speed? What is top speed while pedaling on top of a 500 watt motor?

[govman6159]

Originally Posted by sendler

I think they were asking about the gear reduction from the motor. What is the motor rpm rated and what is the crank rpm if you want to pedal to add to the top speed? What is top speed while pedaling on top of a 500 watt motor?

The gear reduction from the motor to the sprocket is 96/11 or 8.73:1. The final reduction ratio to the rear wheel in 1st gear is 9.553:1, 2nd gear is 8.612:1 and so on.

The motor is 0-2500 (variable, obviously) rpm max.

The crank rpm is dependent on the selected gear. In 1st gear, you will not be able to pedal fast enough at top speed to assist but simply shift into 5th and you can begin to add pedal assist and can add more by shifting to a higher gear.

As for top speed, I haven't been able to "maintain" speeds in excess of 30.4 (flat surface) with pedal assist. I can get above that but wind resistance is high and hard to overcome. This is honestly an area I haven't studied much b/c I tend to rely on all electric (I'm lazy). I will do some tests and report back.

Top speed in all electric is 25mph and is limited by wind resistance (little to no wind, flat surface). I'm guessing a 350W motor would get you about 18mph (if your lucky).

[sendler]

So I guess it is just normal that the electric motor will be running at only 800 rpm when you are pedaling.

[govman6159]

Yeah, I'd say that's about normal for all electric mode. If I'm doing 30mph with pedal assist, my calcs put motor rpm @ 1100 (about). This may change once I put the new sprockets on up front. I'm hoping for 0-20 in 6-10 seconds with that swap. Right now its about 8-15 sec's to 15mph depending on terrain. I'm also hoping to be able to maintain 15mph up most hills. Right now I can only maintain about 9mph up the worst hills and 12-13 up other hills (which is painfully slow). All of this is with no pedal assist of course.

[Daox]

What parts are needed to attach a sprocket to the crank with the freewheeling mechanism?

[govman6159]

Originally Posted by Daox

What parts are needed to attach a sprocket to the crank with the freewheeling mechanism?

Here's what you need.
1) one freewheel crank set (Sick Bike Parts)
2) one front freewheel bearing (Sick Bike Parts)
3) one extended length crank spindle (your stock one wont work because you will be extending the attachment point for your crank arms to fit the additional sprocket. You will have to measure for this, each bike varies)(Sick Bike Parts)
4) 90 tooth #25 sprocket (#25 Sprockets for Electric Scooters and Bicycles - ElectricScooterParts.com)
5) bolts and nuts (you pick the size, I went w/ M5)

[puddleglum]

Originally Posted by Daox

I checked prices on the controllers on the electric scooters site. They're way cheaper than other ones I've seen. Is the quality any good? $30 for a controller makes me wonder.

I've tried two different controller/motor combinations with the scooter now, and both seem to work well. I can't speak to their durability, but they both seem to deliver their rated power without getting hot and they cut back power at low voltage. According to my wattmeter, I can get over 800 watts peak from the 35 amp controller and 600 watts from the 25amp one. You will also need a throttle control and a set of brake levers. You could use a Cellog8 as a cheap BMS to cut power to the charger or maybe a simplified version of the one you already designed for your Enginer pack.

[puddleglum]

Originally Posted by govman6159

Yeah, I'd say that's about normal for all electric mode. If I'm doing 30mph with pedal assist, my calcs put motor rpm @ 1100 (about). This may change once I put the new sprockets on up front. I'm hoping for 0-20 in 6-10 seconds with that swap. Right now its about 8-15 sec's to 15mph depending on terrain. I'm also hoping to be able to maintain 15mph up most hills. Right now I can only maintain about 9mph up the worst hills and 12-13 up other hills (which is painfully slow). All of this is with no pedal assist of course.

This was what I was getting at with my question about gearing. If your motor is rated for 2500 rpm and you are pulling it down to 800-1100 rpm that seems like too much to me. I would think that more gear reduction to the motor would give better performance. Thanks for explaining the freewheel mechanism though.