Regarding Q100CST vs Q100H, you say in your messages that with a 7 speed freewheel it ends up being ~134mm, BMS says the same. But TopBikeKit says its 138m (!). See their diagram here: http://www.topbikekit.com/AKM-100H%20Rear%20motor.png
I sent an email and they confirmed its 138mm at their end for the AKM ... 138 mm it's definitely too large for me and my 130 mm alu frame.
Are you sure your drop-out width is 130 mm, all the ones I have used were 135 MM. Anyhow it doesn't matter.
First of all, all chain stays are different, they are built on jigs and they vary to a degree.
The H with the 7 sp DNP drops right in, the DNP 9 sp usually requires pulling the chain stay apart slightly by hand and the CST w/ the 9 sp needs some gentle prying w/ a screwdriver. You will be suprised at how easily the ends of the chain stay can be spread apart.
The problem with the CST fitment of the wide-bodied Q100's is not getting them between the drop-outs, it's getting the rim centered. The spline to fit the cassette is longer than the free whl.s and this moves the hub center-line to the left (when viewed from the rear). What then is required is "dishing", adjusting the spokes to pull the rim into an off-set to get back on the center-line and it varies from bike to bike how much.
All my bikes have been quality alum framed mountain bikes, Idrive, Rocky Mountain and my current Motobecane. For the CST, the Idive and the Motobecane required little dishing while the Rocky Mountain required extreem dishing, to the point I gave up and used an H. How the CST/cassette would fit on your bike I can't say. The free whl units, even with a 9 sp all dropped in.
Especially that I'll need steel washer to prevent marks in the aluminium dropouts
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You do understand that you will likely have to enlarge the drop-outs w/ a round file to accept the motor axle? We all strive to prevent scratching our beautiful bikes, but it does happen. You can use rubber strips for mounting things like the torque arm clamps, but you need to accept that converting your bike runs a risk of it getting scratched somewhere..
You will need one sm. flat washer supplied w/ the motor kit, it goes between the cassette spline/free whl. and the inside of the drop-out to prevent rubbing. Actually, I keep a selection of flat washers that I stack to get perfect alignment of the torque arms (yes, two).
Use the C-washers that come w/ the TA's first, it goes in the recess formed by the Lawyer's lips and provides a flat surface to stack on, then some variation of washers to get the TA aligned to a flat surface of the chain stay to clamp it to. Then the nuts w/ thread locker.
Re controller I am thinking of a simple KT15A,
This where things always get complicated.
That tiny KT only comes in 24V/36V and that's problem. If you use a 36V batt., with the 260, that's about 18 MPH. The 328 is not relevant. Actually, I don't care for any of TOP Kit's controllers (or batt.s for that matter).
The next step up is the 22 Amp 9-FET square wave (What they call Torque Simulation), which would be ok with the 260H. I've run them up to 25 Amps before they start to hammer the motor w/ shock waves, great fun! But another thing I don't like about TK's off'ings is the displays they offer are only LCD, there goes the stealth, they are dead give-a-ways.
I always try to get everything from one vendor to save on shipping, both my builds and the ones I've try to help with here, but have never succeded. The combo seems to want something from at least two vendors.
An example, here's the controller I prefer;
https://www.pswpower.com/products/36v-48v-350w-brushless-dc-square-wave-controller-ebike-electric-bicycle-hub-motor-controller-with-right-output-31
w/ a simple LED display, 17 Amps is perfect for the Q100.
A comment on stealth;
When I started, I was crazy about it, dark bike, all blk. wires, homemade water bottle style batteries (the round brushed alum. kind), I even put blk. tape over over the sm. LED lites on the display. Eventually, I moved away from it because where I live and ride, nobody cares about ebikes. But if you are serious about it, you need to think about how to hide the batt.
Anyhow, back to reality. You could get the KT sine wave that goes into the battery case, they work with 48 Volts. That's probably your best bet if you want to get everything from TK.
Final question, what's your bafang model? Is it a SWX02 rated for 36v 250w? I thought it was more than 2.5kg.
You are right! It's 3.3 Kg. This is the exact one I got;
https://hilleater.ca/bafang-swx02-rear-cassette-motor-fast-11-turn-winding-fits-juiced-ccs/
Notice that is an 11 turn, most SWXO2's would be 13 turn 270 or so speed range. I had one and it did 27 mph, faster than I nedd or want to go. The 11T is around a 250 sp.-range. And as I said, it accepts the cassette and it's blk. ticked all the boxes for me. I'm using the 17 Amp controller from the Q100 system and it could easily take a controller in the 20 to 25 Amp range, but it's ok. Forces me to pedal a little more.