Brompton M2L-X (Titanium) - Super Lightweight Pedelec Conversion

[jerrysimon]

When using the larger Tongxin I have peaked at 36V*11A amps i.e. 396W. So allowing for 48V and keeping the same power it would have to be restricted to smaller Tongxin to 396/48 = 8.25A.

The smaller KeyDe Goldants would need to be run lower at around 6A at 48V.


Regards

Jerry

 

[patpatbut]

When using the larger Tongxin I have peaked at 36V*11A amps i.e. 396W. So allowing for 48V and keeping the same power it would have to be restricted to smaller Tongxin to 396/48 = 8.25A.

The smaller KeyDe Goldants would need to be run lower at around 6A at 48V.


Regards

Jerry

Hi Jerry

What speed will you get for 48v 6A on keyde motor for brompton? Is it safe to run this kind of setup?

Thanks
Pat

 

[jerrysimon]

Interestingly I have just been learning how to work this out. This is my method which I think is correct.

At 48V it will turn a Brompton 1340mm circumference wheel (this is fitted with a stock Brompton Kevlar tyre) at over 17mph. In practice a 48V battery will be just over 57V hot off a charger which knocks that up to around 20mph.

I feel a little unstable on my Brompton once it goes over about 22-23mph.

Of course that would be on the flat which would drop significantly on a hill given the limited current setting.

The method I use for RPM to MPH calculation can be found here

To find RPM its just a case of substitution given you know the Tongxin turns at 260RPM when supplied with 36V.

i.e. for 48V RPM = (260*48)/36 = 347RPM

Regards

Jerry

 

[cwah]

Ok thanks for this nice advice Dave.

So I need the Cute100 rather than the larger Tongxin. Although with the larger Tongxin I'd have the advantage to be able to ask for 28h drilling.

Or as Jerry sugggested, 48V at 6A is 288W power and would allow me to go to 20mph. Is the little motor about to do that?

 

[Deleted member 4366]

Think back to the first ebike you had - the one you sent back. That would have had a controller of minimum 12 amps. The next one that you kept for a bit was 15 amps and 36v (actually 40v), so was 600w. 48v and 6 amps is half the power. The Q100 can run up to 17 amps with 12S lipos if you're careful. That's 800 watts. 288watts is OK if the roads are flat and you want to pedal hard.

 

[jerrysimon]

Excellent points Dave. These little motors are really optimised for light assist up to 15mph. If you want faster AND more power buy a bigger motor

6A up a hill would not give you a lot of assistance. At 8A they do ok but then you have to run them at 36V.

As with all things ebikes its a trade off.

Regards

Jerry

 

[Deleted member 4366]

If it's flat and you ride at about 2/3 the max RPM of the motor, you could get the full 288w out of that Tongxin, which is enough to make cycling on the flat easy, but on a hill, the motor will slow down and lose efficiency, so the power will rapidly reduce, which will soon have the motor stalling out.

 

[cwah]

Can the q100 handle 800W constant?

That's about 25mph

Ps: my first one was soooo slow and assist up to 10mph. No way it was at 12A

 

[Deleted member 4366]

It's rated at 350w, so the answer has to be no. You're back to your MAC if that's what you want. You can't have small, lightweight and high constant power. If you want small and light-weight, the Q100 is probably the most powrful. After that, it's the MXUS, and then the bigger motors like Ezee, BPM, CST, MAC, etc.

 

[cwah]

ok when you mentioned 800W you mean peak power?

What's the maximum constant power it can handle? maybe 500W?

 

[Deleted member 4366]

It's not really a question of power. It's the speed. A hub-motor only draws high power when it's going slowly. As you speed up, the motor generates a back EMF that cancels out some of the voltage. The faster you go, the higher the back EMF until you reach the maximum RPM of the motor when back EMF = battery voltage, so net power is zero. If you fit a wattmeter, you can see the current going down as the speed rises, so riding the bike fast on a flat road doesn't heat the motor nor draw as much power as it does when climbing a hill. Another consideration is the controller, which also can overheat if you run at maximum current all the time which is what happens if your motor's too fast a winding for the speed it can go, like into a wind, up a hill, normal acceleration etc.

 

[cwah]

I understand during acceleration and hills it's much less efficient, and generate heat. I'm always careful during long hills with my Mac, although it never gets barely warm most of the time.
Especially now that I can only go up to 23mph since I had to remove 3 cells from my A123 pack.

If it's too much power for the motor, I can limit the peak power. Maybe say to 10A on 48V battery? So even if constant power equal to peak it shouldn't overheat?

 

[Deleted member 4366]

I think you should try it, and let us know how it goes. I have the 24v 201 rpm motor at 44v and 17 amps on my Dahon, and it'll hold my weight at over 20mph on a flat road. I don't often go that speed, and I've never done it for more than a mile, but you're a lot lighter than me. I think it should be OK at 15 amps with a 48v battery to sustain about 20mph. To go faster than that (sustained) would require a bigger motor.

 

[cwah]

Ok I'm going to get the cute100 then. There is only 2 problem with this motor which are the 100mm dropout and 36 holes drilling. Not sure if anyone managed to go through them

Ps: also apparently the q100 24V might have been the older model of motor?

 

[jerrysimon]

Wouldn't you be better with a Q85 ?

36H means you have to use a Sun CR18 rim.

Ah I see the Q85 is rated less than the Q100.

Also you may have to spread the forks more than 100mm if the sides don't clear the Brompton fork sides as you will need to insert spacer washers.

Regards

Jerry

 

[Deleted member 4366]

The Q85 isn't as powerful as the Q100 though. That's the problem. Nothing is easy on those Bromptons.

 

[jerrysimon]

That's the problem. Nothing is easy on those Bromptons.

Don't I know it lol

Jerry

 

[cwah]

Which members put a 100mm motor on their brompton?

 

[Deleted member 4366]

Here's one with a Bafang QSWXk5. It went pretty fast with 48v.

 

[cwah]

Ok it doesn't fold completely but it still fold no problem.

Cute100 then

Will use it with Zippy compact