Converting a hubmotor to a middrivemotor

[evb]

Does chain touch big sprocket when using rear small sprockets?

 

[SolarRay]

a pic of the new style tensioner.

How many links in the chain? (big and small chain)
How many teeth in the sprocket on the cranks?

 

[spinningmagnets]

Looks good...now I am embarrassed at how slow I am. I will have to work harder. I can sleep when I'm dead!

 

[crossbreak]

Does chain touch big sprocket when using rear small sprockets?

no, but there is only little clearance: 5mm I will add a downhill style roller chain guide to the lower path. also some thin plastic sheet between the chains to avoid touch when jumping down big drops / going through rough terrain. The clearance will become even smaller with the larger BPM motor which needs more distance to the bottom bracket. I'm not sure anymore if this will really work with the short 25mm BPM front motor shaft. I will try that. With the rear motor that has 35mm shaft, another 10mm of clearance could be added using more spacers.

a pic of the new style tensioner.

How many links in the chain? (big and small chain)
How many teeth in the sprocket on the cranks?

The one to the wheel has 124 links, the short one has 54. Maybe i shorten it to 52 links. This will change for the BPM to 128 links / 58 links. Here you can read more about the reduction setup: http://endless-sphere.com/forums/viewtopic.php?f=28&t=45245&p=695163&hilit=links#p695163

Nice work.
Is the motor only attached to the bottom bracket now or does it also attach to the down tube?

thx!! The motor must be attached to the downtube. These where older pics, I can take pics of the final built tonight

 

[SolarRay]

Thanks crossbreak.
I will use xofo xfv. (analog bpm)

----
found 12t, 13t freewheel for 34mm thread:
http://item.taobao.com/item.htm?id=17048475774
http://item.taobao.com/item.htm?id=12247044873

 

[crossbreak]

interesting. i guess that the XOFOs torque sensor wont work anymore after conversion, what do you think?

btw, nice find, the 12t freewheel! Made some more road test, going up the steepest and longest hill here. Always use it as a test track, since there is some easy downhill track with more than 25% gain, ~150m long, which is perfect to drive up for testing. HS3540 in a 26" rim was quite hot @ 2700W input but survived well, just for comparison. It failed the DH track. The SWXB only succeeded in first gear, not so easy since it wheelies frequently. it got me to the top in second gear (29T) with the large 22T sprocket, a total reduction of 1.64 compared to a hub in a 26" wheel. The motor was warm when I came to the top. Ran it full throttle the whole way up. In third gear it was hot (~95°C stator temp) when i reached the top. On the way down I noticed that the planetary freewheel clutch was locked. After temp dropped below ~75°C the clutch worked flawlessly again. All tests where done using a ~47amps /15Fet Greentime controller at 44.4V/12 lipo. I runs only 50kph in both last and next-to-last gears, that tells me that the motor is in saturation. 2000W is just too much, more than 1500W lack of sense with this one

some vid about the chain clearance:
[youtube]8s2-8tpqtgg[/youtube]

 

[crossbreak]

this is what the adapter looks after the first 20km with 2000W input power. Could be easily pulled off by hand, no puller needed. The 14mm length of the tapped payed out...even 100Nm are no problem for this adapter. Hope it also lasts a BPM motor with 4000W input power...that will be 200Nm stall torque lets proceed with converting the BPM2

Note that only 3 of the 6 screws where used for the 2000W test. Hope it proves to be as bullet proof as it is intended to be

 

[speedmd]

I like it. I want to do something like this, only on the jack shaft of a mid drive. Like the idea of the speed geared cranks also. Wondering how efficient it pedals without the motor running.
Good work on this. Thanks for sharing.

cheers

 

[crossbreak]

ok did some quick test using a torque wrench on the left side axle with some counternut. At ~120Nm the axle bend :x still the adapter looks the same, but this time it could not be pulled off by hand, had to use some help of a screw driver.

Wondering how efficient it pedals without the motor running.

this is what all ask. It is like the 2013 bosch motor with built in 2.5 overdrive, I use 3.4 overdrive. You can barely feel the drag, just try it yourself. BUT the motor doesn't see any drag, there is no more freewheel that can fail or crappy and expensive freewheel cranks that bend. BUT what is superior to all other drives: I can push 3000W through the chain due speeding it up, freewheel cranks are limited to ~1200W if you want some reliability, my first freewheel cranks didnt last 600W (cyclone), that is why I developed this drive. Bosch does the same, some proof that it makes a lot of sense

I like it. I want to do something like this, only on the jack shaft of a mid drive.

:? this is actually the best example for a jackshaft drive? Isn't it? What do you want to do different? Plz make some drawing, so we dont have to explain for hours

 

[waynebergman]

Hi Crossbreak. This looks really nice. One issue with my Giant DH will be if I use a cassette and derailer when I shift over to the 11 tooth at the hub my chain will rub on the chain stay. As it is now, I need to run at least a 44 tooth off the crank assembly not to rub on the chain stay when running a 7 speed to 11tooth high gear. I noticed on your Giant your chain stays run much lower than mine because of the way hosrt (or version of) links are configured at your drop outs. If and when I convert my Giant to mid drive like with the "Stiffee" I may just go with your adaptors here and then run it with a Rohloff internal hub. I think this way I can solve the chain stay rubbing issues and also have the clean chain line I have wanted. The only problem now is to find the cash for the Rohloff hub. The more I ride my converted mac (crossbreak-style) I see how its a very solid system but I will for sure want an internal shifting hub for the next build. I am still very content with a 1800 watt system. In my opinion its the good mix of power & weight and reliablility on a mid drive system like this.

Keep the photo's coming. Awesome.

 

[crossbreak]

thx wayne. Some more about chain line:

11-t cogs will induce problems, not only with chain line. They are not made to last much more than 500W. If you push through more than just use 13t and larger. The crossbreak style drive allows virtual chainring sizes of 70T+ so there is no need to use small cogs that eat chains and induce chain line problems.

My bike is tuned so that it is as close to the 50mm "standard MTB-triple" chainline as possible, all Hollowtech2 MTB cranks are made for 50mm chain line. Chain line is measured from the center of the seat tube to the 2nd chain ring. Since I only use the largest chain ring, the chain line to this one is ~56mm. This way the actual output sprocket is at the place where normally the smallest chain ring would be. To gain a better derailleur efficiency, it would be nice to move out the right crank for about 5mm using a spacer on the crankshaft. This then again needs a converted rear motor with long shaft to have enough space on the axle/shaft, here additional spacers are also needed. This way, both a neutral chainline and a bit more clearance would be made possible. More testing...tomorrow

 

[evb]

Does it make sense to make copper Heatbridge?
Copper has better thermal conductivity than aluminum almost twice (203/384)

 

[speedmd]

ok did some quick test using a torque wrench on the left side axle with some counternut. At ~120Nm the axle bend :x still the adapter looks the same, but this time it could not be pulled off by hand, had to use some help of a screw driver.

Wondering how efficient it pedals without the motor running.

this is what all ask. It is like the 2013 bosch motor with built in 2.5 overdrive, I use 3.4 overdrive. You can barely feel the drag, just try it yourself. BUT the motor doesn't see any drag, there is no more freewheel that can fail or crappy and expensive freewheel cranks that bend. BUT what is superior to all other drives: I can push 3000W through the chain due speeding it up, freewheel cranks are limited to ~1200W if you want some reliability, my first freewheel cranks didnt last 600W (cyclone), that is why I developed this drive. Bosch does the same, some proof that it makes a lot of sense

I like it. I want to do something like this, only on the jack shaft of a mid drive.

:? this is actually the best example for a jackshaft drive? Isn't it? What do you want to do different? Plz make some drawing, so we dont have to explain for hours

This puts most of my plans in flux. Need a few days in the garden to think it through before I start drawing / fitting design into frame. I am glad I did not order the trials cranks. Do you like the HAMMERSCHMIDT. Which one if so as I am leaning that way for cranks if funds avail.

 

[SolarRay]

interesting. i guess that the XOFOs torque sensor wont work anymore after conversion, what do you think?

In my motor is not the sensor, so I can not check.
If the sensor measures the force between the cassette and the body, then it will not work.
If the sensor measures the deformation axis, there is a chance that it will give us something to work with.

 

[crossbreak]

If the sensor measures the force between the cassette and the body, then it will not work.

i have no idea. but i'd like to see some pics this torque transducer.

Does it make sense to make copper Heatbridge?

I'd say that thermal transition between rotor, heat-bridge and housing are a thing´s to worry about. And the sink ability of the housing itself. Better get a nice heatsink on the outside, just like bob did. Think of copper when you did all this. Alloy is much cheaper, lightweight, more simple to buy etc etc.

 

[spinningmagnets]

evb, If you have access to a lathe and free chunks of salvaged copper, that would be great for a heat-bridge. But, copper is so expensive, I believe most of the time...aluminum will work "well enough". Green-machines experiments in hill climbing have shown that adding an aluminum heat bridge is a major improvement.

Cost and performance-wise, if an aluminum heat-bridge was barely keeping up (judging from data gathered from an internal temp sensor), I would next go to an external air-fan on the heat-sink side-plate that is attached to the heat bridge...

 

[evb]

Better get a nice heatsink on the outside

It's no problem to place heatsink and fan on the outside. I think maybe the heatbridge is bottleneck of this system.
I guess to know it I need to test both variants with temp sensors inside and outside.

Yes, copper is more expensive than aluminium, but if chunks will cost 3 and 6 usd, it's no problem

 

[crossbreak]

the piece needed is quite thin.... wont be that heavy. copper isn't that expensive. still an accurate shape is much more important. If you calc the heat transfer between stator and the outside you will see that the biggest temp drop is in the pass between the stator->stator holder->heatbridge->housing->heatsink rather than in the conducting material itself.

BTW There can be another path can be directly from the coils to the heatbridge. I'll make some pics of the actual shape and a calculation of the heat transfer through both paths when I have time

 

[evb]

BTW There can be another path can be directly from the coils to the heatbridge.

Good idea. I will use 2 bridges separately, for stator holder and for coils.

 

[crossbreak]

also make sure to use good quality thermal compound. Makes more difference than copper compared to alloy for the bridge. I'd use the inner heatbridge also for fastening, the outer one only as a second thermal path. The thickness of the outer one is critical, it has to be an almost perfect fit in thickness, otherwise you have to use massive amounts of thermal compound, which leads to bad heat transition between copper and heatbridge.

 

[fechter]

Silicone RTV is a pretty good heat conductor for filling the gap between the windings and the heat bridge. It will also prevent chafing of the wire insulation from vibration. A thin layer of silicone grease on one suface will prevent the parts from being permanently glued together.

 

[crossbreak]

Silicone RTV is a pretty good heat conductor for filling the gap between the windings and the heat bridge. It will also prevent chafing of the wire insulation from vibration. A thin layer of silicone grease on one suface will prevent the parts from being permanently glued together.

I want to use Thermal adhesive, with silicon grease as you suggest


my first try to bolt the BPM2 stator to the sideplate. Use 3mm pilot bores. I plan on screwing six M4 bolts into the holder. Only 3 pilot bores yet. The final heatbridge will be consist of two parts as already mentioned:

-Stator holder: 88mm outer /64mm inner dia, 2.5mm thick
-Coil heat conductor: 130mm outer/90mm inner dia, 4mm thick, i'll use alloy rather than copper, since the advance of copper in thermal conductivity is negligible IMO.

note the tiny heatbridge. It is from 2.5mm alloy. Next step is a flange that centers the stator. I'll craft it from epoxy putty.

 

[evb]

Crossbreak, what do you think about way to replace stator holder by sets of washers? Maybe existence of coil heatbridge will enough for remove heat from inside of motor?
How much more efficient to use alloy coil heatbridge than silicone RTV only?

 

[crossbreak]

Crossbreak, what do you think about way to replace stator holder by sets of washers?

Bad thing, stacking up sheets mean bad heat transfer due the gaps between the sheets. A solid piece would be best. One that includes the the stator holder and sideplate with cooling fins would be awesome. Sadly my lathe is too small. Maybe I can make some piece like this on a CNC mill and turn the threads on a large lathe of a friend. Lets see how this first try turns out

Maybe existence of coil heatbridge will enough for remove heat from inside of motor?

hope so.. will add some temp sensor to see difference with and without the coil-heatbridge

How much more efficient to use alloy coil heatbridge than silicone RTV only?

Not more efficient. Better conducting. Just silicone RTV alone will almost do nothing, since The gap is ~4.5mm: too large. BTW That was never my intend.

 

[evb]

Crossbreak, what do you think about way to replace stator holder by sets of washers?

Bad thing
...
hope so.. will add some temp sensor to see difference with and without the coil-heatbridge

I guess you suppose the stator holder is more efficient than coil heatbridge, but I don't realize why do you think so.
The contact areas are different 10 times and coils are open and stator made of steel (I guess), thermoconductivity of steel is less than alluminium several times .
I guess coil heatbridge is more important than stator holder. I will check it, but dont realize how to test without riding.