Lower than expected top speed

[Bradda]

Hi everyone. I'm new to this sphere but have extensive experience in the push bike world and decent experience with RC hobbies.

I built a conversion kit for my 700c commuter bike.

My question is regarding top speed. It pulls well up to ~16km/h then feels like it cuts power. I expected faster.

The motor/controller/display came as a kit in Australia, seller stated it's for a 16" wheel that should be limited to 25km/h and that because I built a 700c wheel it won't go 25 (my maths says hub RPM x rollout = speed, and 700c>16")

The hub is a Bafang G311 250, external casing says D15, internal spline says D09, windings are 16 pole 18 slot, though I don't know what that means or if it's helpful.

The supplied display has no part number or markings. The controller looks like a generic 6A continuous unit.

The battery is a 36v 10s2p made from Samsung 21700-50S cells with a DALY 20A BMS.
700c wheel with 37mm tyre.


Have I just mismatched components and 16km/h is all I should expect from it or is the controller or display limiting my speed?

 

[MxusMick]

The controller has lower than normal amp rating, I am a shade tree enthusiast but I bet you need a more powerful controller, the battery looks quite small in the picture, if it only has 4 amp hours of range, you won't get far.

 

[neptronix]

If you put a winding intended for a 16" wheel into a 29" wheel, you very likely have too high speed a winding for that wheel. It will want to go faster than the motor can handle.

..as a result of this, this motor is going to be very amp hungry since you have a fast winding. But your controller can't supply the ~2x-3x extra amps.

This looks like yet another random name, non programmable, no support, artificially restricted controller. If you hadn't made the error with the winding choice, and you had a programmable controller, you could overcome this limitation. But your next problem might be heat from the motor going substantially faster than intended.

I would reach out to the vendor about the speed control limitation. You would be very lucky if someone here recognized this controller and knew how to work with it.

 

[Bradda]

The controller has lower than normal amp rating, I am a shade tree enthusiast but I bet you need a more powerful controller, the battery looks quite small in the picture, if it only has 4 amp hours of range, you won't get far.

Hi Mick, thanks for your reply.
The battery is 21700 not 18650 cells, so it's 10Ah, even if it assists for my entire days commute it won't go flat. I intentionally made it "small" as it's a pedal assist bike not a motorbike.

250w÷36v is <7 amps, so the controller rating should be fine.

 

[Bradda]

If you put a winding intended for a 16" wheel into a 29" wheel, you very likely have too high speed a winding for that wheel. It will want to go faster than the motor can handle.

..as a result of this, this motor is going to be very amp hungry since you have a fast winding. But your controller can't supply the ~2x-3x extra amps.

This looks like yet another random name, non programmable, no support, artificially restricted controller. If you hadn't made the error with the winding choice, and you had a programmable controller, you could overcome this limitation. But your next problem might be heat from the motor going substantially faster than intended.

I would reach out to the vendor about the speed control limitation. You would be very lucky if someone here recognized this controller and knew how to work with it.

I can't verify if it's designed for a 16" wheel or not, what I found implied that the D15 marked hubs are for larger wheels.

I understand I'll probably need to buy a new controller and/or display, it's just that it cuts power at 16km/h, which I can exceed with my own two legs for the entire commute.

 

[MxusMick]

Purchase a brushless 36v controller with 20amps continuous or more to achieve around 20mph on 36v, a 10amp hour battery will give about 30 minutes of power.

 

[Bradda]

Purchase a brushless 36v controller with 20amps continuous or more to achieve around 20mph on 36v, a 10amp hour battery will give about 30 minutes of power.

Australian push bike assistance limit is 25km/h and only while pedalling, if I can do 20km/h without a motor it'll only be adding 5km/h. I understand that if it was doing all the work the battery wouldn't last very long at all.

 

[neptronix]

A cheap laser tachometer could help you figure out the RPMs. If the kit is designed for a 16", you very likely have a high speed winding ( 2x the rpm per volt versus a hub motor with the correct winding for an almost 2x bigger wheel )

Digital Tachometer Non Contact Laser Photo RPM Tach Meter Motor Speed Gauge AS | eBay

Purchase a brushless 36v controller with 20amps continuous or more to achieve around 20mph on 36v, a 10amp hour battery will give about 30 minutes of power.

You're missing a big variable: what winding is the motor?
In a given wheel, one motor may do 10mph on 36v 5A, another may do 30mph on 36v at 30A.. the range of variability is too high to make a blanket statement about hub motors.

We have the excellent grin technology motor simulator to help us figure these things out:
Motor Simulator - Web Tools - Resources

 

[Bradda]

A cheap laser tachometer could help you figure out the RPMs. If the kit is designed for a 16", you very likely have a high speed winding ( 2x the rpm per volt versus a hub motor with the correct winding for an almost 2x bigger wheel )

I don't trust the seller to be correct on what they think it is. If I put the bike upside down and pedal it doesn't go any faster than with me on it. Tomorrow I'll see if I can verify the actual no load RPM.

We have the excellent grin technology motor simulator to help us figure these things out:
Motor Simulator - Web Tools - Resources

Thanks for the link, once I work out what my motor actually is it'll be handy. I'm hoping someone here knows lots about the hubs and can identify it from the pictures I put of it's internals.

 

[docw009]

I would agree. Your results at 16 km/hour suggests a motor rpm of 110-120 rpm with a 29" wheel. You're bogged down with effectively very low gearing, so you'll want more than a 13A controller. I originally tried to power my 260 rpm Q100H motor on a 20" folder with a 13A brainpower box. Found it pretty weak and went to a 20A KT box,

Last summer, I built a 13S1P with the 50S and a 10S-1P with the 50E. Intended as spares to be carried on a longer ride, I occasionally use them as the main pack when riding at slower speeds. like 12 mph,

 

[neptronix]

My rule on buying cheap kits:
no specs, no buy

Seller probably doesn't even know and can't support the product. If they made the effort to list relevant specs then that demonstrates they know the product and thus may even support it; otherwise i'm about to waste money on mystery components with a seller that's not detail oriented.

If you don't see faster speed with the wheel in the air then the controller is your limit. An ebike motor will reach ~80% of it's top speed on the ground, in the air will be substantially faster. A controller with a speed limit won't have this difference.

The G311/G310 is sold by ebikes.ca and they have posted a lot of info about it on their site. Even have it on the motor simulator. Their version has dual geared reductions. If yours does too, it's probably a match

 

[Bradda]

Seller probably doesn't even know and can't support the product. If they made the effort to list relevant specs then that demonstrates they know the product and thus may even support it; otherwise i'm about to waste money on mystery components with a seller that's not detail oriented.

This is my suspicion, the kit cost less than the hub does elsewhere and was unused, hence not caring about possibly having to buy a new controller.

 

[Bradda]

I would agree. Your results at 16 km/hour suggests a motor rpm of 110-120 rpm with a 29" wheel. You're bogged down with effectively very low gearing, so you'll want more than a 13A controller. I originally tried to power my 260 rpm Q100H motor on a 20" folder with a 13A brainpower box. Found it pretty weak and went to a 20A KT box,

Last summer, I built a 13S1P with the 50S and a 10S-1P with the 50E. Intended as spares to be carried on a longer ride, I occasionally use them as the main pack when riding at slower speeds. like 12 mph,

The problem with that is going up a steep hill and me deliberately not putting power down it can still do ~15km/h, so it's not power limited. It's either electronically limited or the windings combined with 36v max out their rated RPM.

 

[docw009]

Hmm. You may have a speed limit set in your display, CHeck your manual for the settings and see if there is one.

The other possibility is also related to display settings, Hubmotors usually have internal speed sensors used by the display to calculate speed. There is usually a setting for the number of pulses put out per revolution by the sensor. Bafang motors use 6 pulses per revolution, Most motors use 1 pulse. Again, read your display manual,

 

[Comrade]

This looks like yet another random name, non programmable

They are all programmable. They all run firmware. The only difference is some have documented programming interfaces, and others do not.

 

[Bradda]

Hmm. You may have a speed limit set in your display, CHeck your manual for the settings and see if there is one.

The other possibility is also related to display settings, Hubmotors usually have internal speed sensors used by the display to calculate speed. There is usually a setting for the number of pulses put out per revolution by the sensor. Bafang motors use 6 pulses per revolution, Most motors use 1 pulse. Again, read your display manual,

Thanks, there is no manual for the display.
Do you have a recommendation for a cheap <20A controller/display?

 

[docw009]

Sorry, I did not see your display in your first post. Nothing to change here...



And this is your current kit. Looks decent. Appears to use a 9pin Z91 motor connector that plugs into an extension cable with the typical three bullets and a 6 pin molex at the controller.


Something like this with the cheapo color S866 should work. I don't care for the fell of the buttons on the S866. Feel cheap, You might have to change the connector on the pedal assist sensor. There's no standard.

 

[Zambam]

How's the power? If weak, try a shunt mod on the controller. Easy to do with a soldering iron.

 

[Bradda]

How's the power? If weak, try a shunt mod on the controller. Easy to do with a soldering iron.

Thanks, but I'm not sure if a shunt mod would increase top speed? I thought they were just power.


My current no load RPM is ~125. Rollout is 2140 so almost bang on 16km/h, confirming my suspicion that it's electronically limited, as if it wasn't, we'd see a drop in loaded maximum speed.

 

[Bradda]

Sorry, I did not see your display in your first post. Nothing to change here...

View attachment 386322

And this is your current kit. Looks decent. Appears to use a 9pin Z91 motor connector that plugs into an extension cable with the typical three bullets and a 6 pin molex at the controller.
View attachment 386325

Something like this with the cheapo color S866 should work. I don't care for the fell of the buttons on the S866. Feel cheap, You might have to change the connector on the pedal assist sensor. There's no standard.

Thank you, could I find a similar supplier that sells the display and controller seperately and buy a display first and if that doesn't fix my problem buy the matching controller?

 

[Bradda]

These two are from the same retailer. Only reason for SW900 over S866 is it can show watts or voltage while you're riding.

https://www.aliexpress.com/item/1005010065071965.html?

https://www.aliexpress.com/item/1005008015552477.html?

Any reason you guys can see that would make them not work with my motor/battery?

 

[neptronix]

it's very unlikely you find a compatible screen. Even if you do, like the wires match up, the pinout might be different. Could easily blow the controller by plugging in a wrong combination.

Let's say you get really lucky, the pinout is the same, the screen is of the same lineage;
The speed limit may not be controllable by the screen.

I'd honestly just buy a controller that you know is programmable or at least mostly modifiable by the screen.
Don't buy a random one off aliexpress!

 

[docw009]

KT, SW900. S866. and S900 displays are not interchangeable. They have different communication definitions. Other displays like the 860C have several different firmwares to work with several different ebike makers. Sw900 is like this too,

You should buy displays and controllers together to ensure that they work together.

 

[Bradda]

it's very unlikely you find a compatible screen. Even if you do, like the wires match up, the pinout might be different. Could easily blow the controller by plugging in a wrong combination.

Let's say you get really lucky, the pinout is the same, the screen is of the same lineage;
The speed limit may not be controllable by the screen.

I'd honestly just buy a controller that you know is programmable or at least mostly modifiable by the screen.
Don't buy a random one off aliexpress!

Thank you, it won't be a random one, but I have found AliExpress to be as reliable as eBay when it comes to no name electronics. I'll find a kit they sell that has modifiable settings in the screen and the same pin out for my motor.

 

[Bradda]

KT, SW900. S866. and S900 displays are not interchangeable. They have different communication definitions. Other displays like the 860C have several different firmwares to work with several different ebike makers. Sw900 is like this too,

You should buy displays and controllers together to ensure that they work together.

Thanks. Will do, if it has the right motor pin out, is there a potential it won't like the hall sensor/speed sensor output?