Which Bafang to get?

[marshy]

Does anyone happen to have corresponding manufacturer's performance data for model (SWXH http://www.szbaf.com/en/product_detail.asp?ProductID=513).

I'd be very grateful for someone to do a sanity check on my reasoning of which Bafang SWXH motor configuration to get. Here's my situation:
- I intend to use a SWXH rear motor. I haven't been able to find model performance data specifically for the SWXH but I suspect they are similar to the SWXB range (covered well in this thread).
- need a genuine assistance motor (I want to do lots of peddling, but need more speed in headwinds - common in my neck of the woods)
- I like to cruise between 25-32 kmh, typically ~30 so I want useful assistance up to and beyond low 30's. Stock Bafang configuration seems to be designed around 25km/h max
- I'd like the motor to work efficiently in my normal cruise speed range
- I intend to comply with the Oz 200W limit. In practice I intend to select a "200W" motor from the range, plus run at motor's rated voltage (36V = my battery pack voltage).
- 700c wheel

So, 700c wheel, >32km/h max assisted speed and assuming the top 10% of the motor speed range isn't usable, then I need to aim for say 35km/h no load speed, i.e. or 265rpm for 700c wheel.

Looking at the data for the SWXB, the 36V, 200W, 20inch wheel version has no load speed of 262rpm. Would that be a sensible choice, or am I violating the natural of things in some way by running a "20inch" wheel version on 700c??

(a second choice might be the 22inch/36V/200W -> 240rpm version. This would give me a little less top speed (maybe 29km/h in practice?) but perhaps lead to a little less stress on gears? Not sure how available the 22" version would be)

All the other 36V/200W versions are too slow or too fast (rpm).

Would be grateful for any opinions. Hoping to order a motor early next week.

 

[zukster]

I'm not sure you can achieve those numbers with a Bafang. The ones I use are

Model SWXH (rear with disk mount holes)
Voltage: 36VDC (clear sticker)
Output: 250W (same clear sticker)
26 inch (green sticker)

I went through a radar check at full speed with a 36V Keywin tweaked controller
and a 60V battery pack on a light bike and it was about 40km/hour.

A 200W motor run at 36V is going to get less than you want I think.

 

[marshy]

I'm not sure you can achieve those numbers with a Bafang. The ones I use are

Model SWXH (rear with disk mount holes)
Voltage: 36VDC (clear sticker)
Output: 250W (same clear sticker)
26 inch (green sticker)

I went through a radar check at full speed with a 36V Keywin tweaked controller
and a 60V battery pack on a light bike and it was about 40km/hour.

A 200W motor run at 36V is going to get less than you want I think.

thanks zukster, yes, without peddling there's no way that I'd get 30km/h+ with 36V/200W. But an important consideration in my case is that I will always be peddling. The motor will just be there to supplement my power input. E.g. In a headwind I may have to peddle hard to go 20km/h. With the motor I may be able to go say 25km/h with the same amount of effort. Alternatively, on a still day I may be able to cruise at 32 km/h, putting in the effort I would normally use to go 25km/h without the motor. But I'll always be putting in a moderate amount of effort.

The fundamental problem using a "stock" configuration for me is that the assistance will run out before the no load speed (approx 25 km/h). I want assistance at higher speeds, so I'll either need to choose a motor with a higher no load speed (i.e. one intended for a smaller wheel) or overvolt. I'd prefer not to overvolt, to avoid compliance complications.

So the question really is - is it a silly idea to run a 36V/200W/20inch SWXH in a 700C wheel for higher speed assistance - or would I be better off say using a 24V/200W/26inch and overvolting to 36V to get more speed?

 

[Russell]

I own a 250W 36V front Bafang model QSWXB which I purchased in a kit. The motor was laced to a 28" rim so I re-laced it to a 700C rim. My GUESS is the charts above show the NO-LOAD RPM at the NOMINAL voltages, i.e. 36.0V for a 36V motor, but without more detailed specs it's not entirely clear.

This is the kind of data you really need if you want to make an informed decision, it's from the goldenmotor site for their Bafang look-a-like motor;

http://www.goldenmotor.com/hubmotors/Mini Rear 36V250W Performance.pdf

This motor's performance seems to match mine since if I work backwards from my no-load speed I calculate my motor would spin at 230 RPM at 36.0V. In practice however a 36V battery pack starts out higher, my LiMn pack fully charged is 41.3V, so the RPM and therefore the speed is higher than the manufacturer's spec.

Anyway my assumed 230 RPM motor spins at a no-load speed of 21.8 mph (34.9 kph) at 41.1V using a 700x47C tire. On the road the maximum speed on a flat road with a 255 pound total weight is about 18 mph (28.8 kph). I also shortened the cable by two-thirds which helped my speed a little. Some useful assistance is felt up to a bit over 20 mph (32 kph) but obviously the motor will slow as the battery voltage falls. My average trip speeds on a 36V pack were generally in the 16-17 mph range (26-27 kph) with continuous moderate pedaling with a best trip average of 18 mph (28.8 kph) with a heavier throttle. I've since added an SLA booster for 54V peak and while the no-load speed climbed to 28.7 mph (45.9 kph) I use the throttle more conservatively so my trips are in the 17-18 mph range (27-29 kph). Using a heavy throttle I increased that to 19.4 mph (31.0 kph).

In all my examples I must stress that the speeds I quote are the AVERAGE MPH my computer displays after a ride. While the speeds may seem low that is somewhat deceiving because I'm often moving at quite a good clip it's just that slowing and stopping reduces average speed by more than you'd think.

I'd suggest if you want useful assist over 19 mph (30 kph) on a 700C bike with a 36V pack you should look for a motor with a rated no-load RPM at least 230 RPM and use a LiFePO4 battery (~43.8V peak). I would not go higher than 250 RPM since you will lose low end torque and hill-climbing ability. The little motor sounds pretty angry if I should go below 10 mph (16 kph) on a steep grade with my big 700x47C tire.

FWIW

-R

 

[zukster]

I didn't know what a 700c was but a quick google shows it as a 29 inch wheel. The SWXH
is for a 26" wheel. Its wound and geared for that. You'd loose all the torque in a 29 and it
would prob really suck. Bafang makes motors for 28 inch wheels in other models, but Keywin/
Ecrazyman was not selling those ones, so I did not investigate further. If your set on a
700c, I'd look into that.

Without crunching numbers and considering a 26inch wheel bike with a SWXH, here's my
take. On the 250W motor and at 40V, on the flats the motor will take my cross country
mountain bike up to about as fast as I can peddle by itself. The 60V takes it farther.
My impression is
that the 40V setup supplies about the equivalent power as I can deliver to the bike myself.
My guess is that you'd be okay with this setup too. 40V is close to 36V. I peddle as much
as I'm into the throttle. I use my bike to go further and up more hills than I would normally
dare to go. I'm about 160lbs. edit: after re-reading your wanting the assist at the higher
speeds - I wonder if the bigger wheel may work for you. You'd need more of a road bike
too I guess where you can peddle up to those higher speeds.

Also, I'm not sure what the torque/winding/gearing characteristics are of the 200W/24V
compared to a 250W/36V. Its prob on the form here somewhere. Maybe they're the same.
We're you worried about that au max?

I wouldn't worry too much about mildly overvolting a Bafang. I've run mine at 40V and 60V
hard and up hills with extra weight and the motor/controller has only got mildly warm with
the Keywin 36V controller. 60v Still makes me a little leery re longevity. I've been thinking
of trying to get 48V or 50V out of my battery setup somehow. That would also give me a bit
of the needed hill climbing power that I see at the 60V setup.

 

[Russell]

I didn't know what a 700c was but a quick google shows it as a 29 inch wheel. The SWXH
is for a 26" wheel. Its wound and geared for that. You'd loose all the torque in a 29 and it
would prob really suck. Bafang makes motors for 28 inch wheels in other models, but Keywin/
Ecrazyman was not selling those ones, so I did not investigate further. If your set on a
700c, I'd look into that.

Didn't know what a 700C is? No wonder, the kit I got was from Canada and had a goofy 28" rim For the record though a 700C rim is pretty well known and has an ISO diameter of 622mm. In Europe they often refer to a 700C as a 28" rim and those are ISO 622mm too. There is however the other lesser known 28" rim like I got which has an ISO diameter of 635mm. Tires for these rims are quite rare in the States. Then of course to make matters worse there are the big-tired mountain bikes which are equipped with 29" wheels and tires which are in fact just wide...you guessed it...ISO 622mm rims. The bike I currently have my Bafang installed on is a Kona Smoke 2-9 which is supposed to be a "29'er" though the tires aren't quite large enough to officially qualify.

In any event the 250W Bafang I have perfoms well even when turning the big 700x47C tires, and I even use a 15A controller. Of course I use it for assistance and in that capacity it works very well. As long as I pedal I have been able to climb any hill I've encountered, up to (a short) 12% grade so far, and not bog down. As I said though it's a trade-off so if some assist is wanted at higher speeds then the low end will invariably suffer, it depends what a person is looking for.


-R

 

[marshy]

I'd suggest if you want useful assist over 19 mph (30 kph) on a 700C bike with a 36V pack you should look for a motor with a rated no-load RPM at least 230 RPM and use a LiFePO4 battery (~43.8V peak). I would not go higher than 250 RPM since you will lose low end torque and hill-climbing ability. The little motor sounds pretty angry if I should go below 10 mph (16 kph) on a steep grade with my big 700x47C tire.

Thanks for the info guys, exactly the type of info I was after. I had done some basic calculations but was looking for some real-world experiences to correlate against.

Russell, your current setup is very similar to what I'm aiming for, except I plan to run at slightly lower power ("200W" motor). I think you are spot-on with your suggestion i.e. 230-250-ish rpm no load, LiFePO4 (which I have, 36V).

I'm waiting on keywin for info on what's available in the SWXH range but the basic selection principle I'm using is:
- peak power is delivered at half of unloaded rpm (in theory, plus roughly speaking in practice). So a bafang in the stock wheel configuration will deliver peak power at around 13km/h (their no load rpm typically corresponds to 25km/h)
- I'm riding mostly on flat ground, and willing to sacrifice low speed torque for more assistance at higher speeds. So I'll aim for a "higher than stock" no load speed to achieve peak power at higher speed - e.g. 18km/h instead of 13. That would correspond to a no load speed of 2 x 18 = 36km/h = 270rpm in a 700c wheel. The bafang 36V/200W/20" SWXB has a no load speed of 260rpm so that looks like a reasonable choice. Just need to find out closest match in SWXH range. Does anyone have the selection table for the Bafang SWXH series please?

I often hit moderate to strong headwinds, in the strongest ones I can still manage 18-20km/h with my legs but it's far from pleasant. I'm hoping a motor set up for an 18km/h "sweet spot" would make that experience much more enjoyable, plus still provide a useful boost at 30+ km/h on still days. Ultimately the help from a little bafang will allow me commute 4 or 5 days a week instead of the current 1 to 3.

 

[zukster]

Not sure where the specs are for SWXH but you'd think Keywin would have them. Also re B vs H,

From: Keywin Ge [ecrazyman@gmail.com]
Sent: Sunday, March 15, 2009 10:49 PM
Subject: Re: Bafang Motors

Hello Jeff,
the best is SWXH (2007 designing ,resist 180 Celsius degree) ,it suited
install disc brake and 7 speed freewheel(please refer attached file),SWXB is
2004 designing ,resist 120 Celsius degree,i have SWXH for 26" wheel in stock
now ,the price is 189 usd included shipping by UPS(3~5 working days arrive to
your door),thank you.

Regards
Keywin

Keywin only sells some of the motors anyway, so you're selection will be limited.

 

[marshy]

Keywin only sells some of the motors anyway, so you're selection will be limited.

I did contact Keywin - he replied quite fast initially but hasn't responded to my question about availability of SWXH 36V/200W/20". I suspect he doesn't deal with the 200W variant (which is fair enough, I just need to know!)

So I've ordered a Bafang SWXH 36V/200W/20" + "100 box" (350W) controller through http://www.bmsbattery.com. I was leaning towards "known quantity" Keywin to avoid another Brett White experience. However I have been quite impressed with the service so far from bmsbattery and it seems like others have reported good things too. Fingers crossed...will certainly report on my experience.