Converting a hubmotor to a middrivemotor

[crossbreak]

no. the newer gears (the blue upgrade ones) can take a lot of abuse as long as they stay cool. The plastic gears wear most/ collapse at high temperature, which is a big problem when used in a hub drive that can not be properly cooled. In a well cooled converted hub they can do much better...and even twist the shaft

The white gears are weaker... i only use the blue ones now..

There where a few upgrades in the past years: Better balanced rotor.. which makes a huge difference (noise) and said blue gears that appear that they are made from better plastic. The motors i bought recently had still had the white ones. The CST version comes with the blue ones always. The CST also has thinner lamination, but due the Cassette flange it is very hard to convert. Also it is quite expnesive. So i stay with BPM2 which does just fine.

 

[crossbreak]

i got a twist shaft.. 120A phase current

I don't realize, the axle is twisted, but plastic gears are ok? Don't you think it strange?

You have to realize that it is the tension/force in the material that makes something collapse. At the shaft, tension in twist direction is very high, since the diameter is so small <12mm. The torque is high ~100Nm. Force=2*Torque/diameter so there is a lot of force/tension!

In case of the plastic planetary gears this look different: The sun gear of the motor stresses the gears the most, since it is small and there are only few teeth engaged (about 6 or 9 teeth). But the torque of the motor is only 1/6 (~17Nm ) of the torque at the shaft Also the diameter of the sun gear is much larger than the shaft, it is ~25mm so about double. As a result, material tension in the plastic teeth is much much lower.

So now it should be absolutely obvious that for instant torque, the shaft suffers the most. Still there is one big difference: The gears wear over time, which the shaft doesn't if it does not get overstressed.

Edit: you can buy the blue gears as spare part from bafang. For example here http://www.bmsbattery.com/accessory/634-gear-set-for-bpm-bpm-cst-motor.html

 

[evb]

Edit: you can buy the blue gears as spare part from bafang. For example here http://www.bmsbattery.com/accessory/634-gear-set-for-bpm-bpm-cst-motor.html

Thanks, I bought several Bafang BPMII 10T from bmsbattery, all motors have same gears.
If your arguments about the reliability of the gears are true, I'll be very glad.

 

[crossbreak]

you're welcome. my explanation may be scientifically not 100% accurate, but i hope that i could give you an idea what actually happens with few words/limited english

guess i melt enough gears to know when they start to

Heat transfer from stator to the coverplate /heatsink is the key here. use plenty of thermal compound between stator/heatbridge/heatsink.. and make a most direct heat path from the coils to heatsink. Then it stays very cool and you get a LOT less problems.

 

[madin88]

guess this was just bad luck. Of course the hollow shaft of the MAC suffers from less material..still it can take the torque+bike /~500N) weight in a hub so why should it not take chain force (~2500N) and the same torque in a middrive?

you forgot in a hub you have also the other side of the axle which is not hollow so it can hold much more torque^^

 

[cal3thousand]

You guys have tricks or tips for making a tensioner pulley for your systems? I'm planning the IGH out and it seems that I will need some tensioner device.

 

[crossbreak]

you mean for the chain to the wheels? with my design the Jackshaft is already used for tensioning the pedal chain (vary distance between BB and Jackshaft). So you'd need a tensioner at the wheel (vary distance between wheel and Jackshaft). If you would combine a normal idler tensioner with my design, you could use the Jackshaft mount for tensioning the wheel drive chain. But that will only work for hardtails.

Tensioning both chains by varying distance may sound tricky..but it's not. Simply tension the short chain first and then the long one to the wheel. Never tried that but it will work for sure. Simply use a frame that was made for IGH+single speed that has a tensioner dropout. So you wont need any idler at all. Will look very clean

 

[cal3thousand]

you mean for the chain to the wheels? with my design the Jackshaft is already used for tensioning the pedal chain (vary distance between BB and Jackshaft). So you'd need a tensioner at the wheel (vary distance between wheel and Jackshaft). If you would combine a normal idler tensioner with my design, you could use the Jackshaft mount for tensioning the wheel drive chain. But that will only work for hardtails.

Tensioning both chains by varying distance may sound tricky..but it's not. Simply tension the short chain first and then the long one to the wheel. Never tried that but it will work for sure. Simply use a frame that was made for IGH+single speed that has a tensioner dropout. So you wont need any idler at all. Will look very clean

I'm going to be using an Xtracycle Edgerunner, so it will be a long run from motor to wheel, but hardtail, so no suspension issues.

So to vary the distance between BB and jackshaft, you design some eccentricity in the mounting to allow variance? And I could use that to also tension the motor to wheel run?

I think I should do some research on how to incorporate a spring mechanism for an idler pulley. Any good places to start beside El Goog?



EDIT:

Did some searching and found this:

http://www.ebay.com/itm/Gusset-Bachelor-chain-Tension-SS-tensioner-/390833418201?_trksid=p2054897.l5672

Since my frame was designed with a derailleur in mind, I could install this on the now-defunct derailleur hanger. Thanks for the idea!

 

[albystarvation]

Hi from Italy!

One question relevant to the subject of this thread: if I should buy an hub to (later ) modify into a mid-drive, what would be best? BPM, BPMII, CST os SWXH?
I have to state the I have no access to big shops or sophisticated machinery..... so ease of conversion would be an important issue.
The application is a low power, street legal ( 250W here in Italy, no more than 25Kmh) custom bycicle.

thanks to all a nd great work on this forum!!!

alberto.

 

[crossbreak]

SWXH isn't so easy ..problem is to center the motor in the housing. Not trival if you never did it. I know some guy who did it after my "how-to" and it runs great.
BPM is not possible to convert without parts made on a lathe you could buy from me. You need at least some thread taps.
For the MAC you can do all steps without a lathe, have a look at spinningsmagnets tutorial.

You can' make a sprocket adapter without a lathe, you could buy it from me or someone else.

 

[evb]

Today I had some tests of my BPMII 10T. Sprocket 19T. Controller GreenTime 100v 45A. Rear sprocket 14T, wheels 26".

LiPo 12S, min.voltage 44.1v, max.current 42.1A, max.speed 50.4km/h (I guess sprocket 19T is small, for 55km/h need 20-21T)
LiPo 16S, min.voltage 58.4v, max.current 41.7A, max.speed 63km/h
LiPo 20S, min.voltage 72-73v (calculated), max.current 41.2A, max.speed 71km/h

and I got twisted broken axle after last race

 

[crossbreak]

sorry for your loss.. as said you need to limit phase current. we have no possibility of porgramming Greentime controllers ATM. I'm not suprised that your adapter did not fail as it was from steel Go for a Kelly. they are expensive but worth the price IMO.

Guess it must have been fun though

 

[evb]

as said you need to limit phase current. we have no possibility of porgramming Greentime controllers ATM.

I ordered some GT controllers and said Leo to limit phase current 65A, he said "ok" but I realized, Leo can say "ok", but don't do it

Go for a Kelly. they are expensive but worth the price IMO.

I think about Kelly, but it's double price. I look to KBS72051X

Guess it must have been fun though

sure

I also plan try to make a new axis integral with the adapter.

 

[crossbreak]

the kelly is double the price, but half the size It can do 7000W, which only the 24Fet greentimes can do.. but they are 4-times the size.
IMO the good throttle control of the kellys is what makes them really desirable..what is a "controller" worth if you have no good controll over it???

Leo tells you a lot. I guess he did not really understand the whole programming software. If you did buy something that was not 100% programmed, the porduct is worthless. I have some controllers striped since they where programmed so badly by Leo that they were useless.. just recycled the Fets and Caps.

I also plan try to make a new axis integral with the adapter.

That's a good plan. It would be nice to extend the 4-spline that takes the clutch to the adapter so it can take a lot more torque. Still, your effort may be worthless in the end if you realize that efficiency is horrible at high torque levels...or maybe the gears just stripe in the end.. but i think as long as they stay cool they could even break the improved shaft

 

[crossbreak]

One last thing: i just noticed you did not use counter sunk screws here. That is a big fault. you want the sprocket as close to the motor as possible to minimize bending load. Just by doing this you can push it somewhat further

 

[izeman]

my adapter is 17mm off. or in other words the inner gear is 17mm away from the cover. i hope this can be handled by the stronger 42crmo4 axle.

 

[evb]

One last thing: i just noticed you did not use counter sunk screws here. That is a big fault. you want the sprocket as close to the motor as possible to minimize bending load. Just by doing this you can push it somewhat further

I don't agree. My adaper fit maximum near to motor (less 1mm from sprocket to mount ), because caps of bolts planted inside the hole.

 

[crossbreak]

I see...your mount limits the distance.That's different with mine. Anyway, it helps to get it as close as possible.. as you did within the limitations of your mount. If there is only 1mm left, distance is ok.

if you dont use countersunk screws at the adapter, you got no defined surface for the adapter->bearing spacer. If there is no defined contact, your adapter + the end of shaft can bend within each rotation. The shaft will break very quickly. The bending loads shall go through the spacer, not the shaft itself.

using countersunk screws is more effort...but I had good reason to design it that way

 

[evb]

you got no defined surface for the adapter->bearing spacer

Why do you think 6 points of bolt caps are not enough to define surface?

 

[crossbreak]

6 point are not a defined surface. it will only really fit on one. In German we call this a "double fit" (Doppelpassung), which must always be avoided. What we need here is a plain surface.

 

[evb]

Now I chose new way to convert BPM motor with more strength with native axle assembled with Loctite 638

 

[crossbreak]

looks really good! this solves the issues you had with your last version the elegant way. A slight drawback is that you will now need a freewheel remover tool for disassembly.

Suggestion: Instead of using one thick spacer between sprocket and freewheel, you could use multiple, thinner ones so you can play/adjust chain line.

How is torque transferred from the shaft to the adapter? I can't see a spline or key?

 

[evb]

A slight drawback is that you will now need a freewheel remover tool for disassembly.

yes, it's price for this way

Suggestion: Instead of using one thick spacer between sprocket and freewheel, you could use multiple, thinner ones so you can play/adjust chain line.

sure, it's spacer for my config - 9mm, I will make 5mm and 3mm spacers for customizing.

How is torque transferred from the shaft to the adapter? I can't see a spline or key?

There is Henkel Loctite 638. It's very strong shaft-hub connection compound.
http://krayden.com/tds/henk_loctite_638_tds.pdf

 

[crossbreak]

So you cant disassemble it and more? What do you do if you want a New planetry assembly
?

 

[evb]

So you cant disassemble it and more? What do you do if you want a New planetry assembly
?

Sure. The axle consists of two parts connected by thread.
If you need to remove adapter from axle you must to heat it 200C