Help! Gt all terra conversion

[Backflips]

Hi All,
I am new to the forum and the ebike world
I recently inherited a front hub bafang motor FM G31 D DC 250w and 36v batteries. Missing the controller and the bottom bracket torque sensor has a stripped cable. I 've digested all the info I could possibly find but as all newby I now have more questions and doubt. The ride ideally would be a city commuter with no need of hill climbing or speed but rather range and PAS only. On the back of the motor there is a sticker "20", is this the rated wheel size for the motor? If so , considering the commute use , would I be able to fit it to let's say a 26 inch? As for the controller I understand that the KT is very versatile is there a specific model I should look for for PAS only. Thank you in advance for any help you can share

 

[saneagle]

The 20" relates directly to the wheel size and indirectly to the speed. A 20" wheel has to spin faster to get the same speed as a 26" one. Hub-motors need to run in their efficient zones considering RPM, which is at about 75% of their maximum speed, so a 15 mph bike needs a 20 mph motor, which translates to 260 rpm for a 26" wheel and 328 rpm for a 20" one.

If you run the wrong speed motor (too fast), it won't run as efficiently, so will use too more battery for the same journey. The efficiency goes down, the slower the motor is spinning. You don't really want to go much slower than 50% of maximum speed, which would be 10 mph for a 260 rpm motor in a 26" wheel and 12.5 mph for a 328 rpm one. The main problem is when the motor slows right down for hill-climbing, where the inefficiency can cause the motor to overheat.

Another consideration is the way the back emf from the motor regulates the power from the controller. It's the reverse voltage the motor generates that's proportional to speed. At higher speed the back emf prevents the maximum current that the controller would otherwise allow. A 260 rpm 36v motor generates 36v at 260 rpm and 27v at 15 mph. That leaves a net voltage of 9v to power the motor, which isn't enough to drive maximum current. A 328 rpm one would be generating 22v at 15 mph, which leaves 18v to power the motor, and that is enough to drive the maximum controller's current into the motor. The controller's maximum current is normally set at twice its rated current. The rated current is the current it can run at continuously without overheating. In other words, without the motorbike emf preventing the current, the controller will be giving its naximum current more of the time, which can cause it to overheat and shutdown.

All these things depend on the weight of the rider, the speed that the bike's being ridden at and the hills. From this info, you have to decide whether any motor is suitable for your needs. Ideally, you choose a motor with a maximum rpm equal to 1.3 times your modal riding speed.

Putting it all together, if you're light, don't have serious hills and you want to spend most of your time pedalling around at 15 mph, a 20" wheel would work. If you're heavy and have hills, it wouldn't be a good idea.

One more thing. Front motors generally give you a worse riding experience than rear ones, but their main drawback is the safety aspects. They're fine in forks designed for them or any steel forks, but they can make the drop-outs of MTB type aluminium forks pop, which is very dangerous. If you're going to use that type of fork make sure that you fit at least one torque arm to keep the motor in when they pop and/or reduce the chance of them popping.

 

[matthewslack]

A picture of labels, stickers and engravings will help identification.

If it is a motor intended for a 20 inch wheel, it will have a nominal rpm of about 328 at 36V, which is a bit fast for legal bikes with bigger wheels. There is a long recent thread about another 328rpm motor in a bigger wheel and some quirks that arose.

Help! - My next project

My current ride is approaching 20kmiles. It's a front hub Ezee and I think the whole ensemble is getting a bit old. Not only that but the increasing number of confiscations make me think I need to get fully legal. I have a KT kit with an AKM rear hub which I've taken off my recumbent ( a...

www.pedelecs.co.uk

You would be better off with a motor intended for your wheel size, but no harm in gaining experience by building your motor into a wheel, adding the KT stuff and seeing what happens. All the rest of the system will happily transfer to a different 250W nominal motor. You only lose out if you pay someone to build the wheel for you.

 

[Benjahmin]

The above linked thread was mine.
I tried putting a 328rpm motor into a 700c (28") wheel. I had a lot of control problems , overheating, hall sensor issues and apparent sychronisation difficulties. I eventually bought a 260rpm motor and have now solved all issues.
If your intended use is for commuting why not build your 20" motor into a folding bike with 20" wheels. Such a bike can possibly be taken on trains and buses.