First Conversion - Cargo Bike

[nathanfinkel]

Hi all,

Been reading and enjoying the forums here and now ready to start my first conversion!

Bought a handmade donor Cargo bike from Ebay - pics below.

Trying to work out the best way to convert.

Currently thiking a 250W or 500W front - as I think this will be simplest.
Doesn't need to be super quick, but would like about 5mph more than 15.5 and a bit of power for cargo and hills.
Trying to replace a car basically!

Thoughts?

Thanks

 

[Woosh]

Hi Nathanfinkel,

what is the tyre size at the rear?

 

[nathanfinkel]

Hiya,

It’s a 20inch rear. Was worried about the weight of the frame and cargo on the rear wheel, otherwise would probably prefer a rear.

Thanks

 

[vfr400]

It looks like you have steel forks (check with a magnet) so a front motor should be OK. If they're not steel, either get some or use a rear motor.

A rear motor would work pretty well too because you have a 20" wheel, which increases the torque by 30% at the expense of speed, but that decrease in speed will also help efficiency, and you'll still easily get enough speed for what you want.

I think, on balance, I'd go for a rear motor because it won't be too fast to be efficient when you run it at 48v. 48V will get you 33% more torque over 36v.

To explain that, let's consider the same common 36v 260 rpm motor in the front and the back. It'll spin to about 22 mph and give maybe 20Nm torque at 36v. The same motor in the back at 48v will give you a speed of 22 x 20/26 x 48/36 = 22.5 mph, but the torque will be 20 x 26/20 x 48/36 = 34.7Nm. The one in the rear will be slightly more efficient because the substantially higher torque will mean that it'll spend less time struggling at low speed.

If you ran the front one at 48v, it would spin to 29mph which is too fast to be efficient any time your speed is less than 15 mph, which will be most of the time.

If you go with a rear motor, don't get one designed for 20" wheels unless you want to run with 36v because it'll be much faster and inefficient at 48v.

 

[nathanfinkel]

Thanks for the detailed response!

So a common garden 48v 250/500 at the rear?

One question tho, how do I not get one designed for a 20” rear wheel? Isn’t the motor in the wheel and bought as a unit? Or am I missing something.

Sorry if I’m being thick!

 

[Woosh]

a possible solution is a front BPM kit:
http://wooshbikes.co.uk/cart/#/product/uid-183-frontbpm-17ah/front-bpm-kit-with-36v-17ah-battery

 

[vfr400]

No, you need a common or garden 36v geared motor. The power rating isn't important. It's pretty meaningless and has little bearing on how much the motor can give or handle.

There are different versions of the same motor. The difference is the KV, which is how the windings are made that affects the speed. Ebike motor suppliers rarely mention the KV. Instead, they often mention the RPM. A 36V 260 RPM motor has a KV of 260/36 = 7.22 RPM per volt.

Motors fitted to 20" wheels normally have around 330 rpm at 36v, so a KV of 9.1 RPM per volt.

When you increase the voltage from 36v to 48v on a 36v 260 RPM motor, it will spin faster. the KV is the same 7.22 because nothing changed inside the motor, so it will spin up to 48 x 7.22 = 346 RPM.

As a general rule for choosing the right speed of motor, the motor's max speed should be 1.33 times your modal speed when travelling under load. 15 mph is 201 RPM in a 26" wheel or 261 RPM in a 20" wheel.

If you're going to keep the bike to the legal limit of 15 mph, then a 20" wheel would need a maximum speed of 261 x 1.33 = 348 rpm You can see that the 1.33 factor is conveniently the same ratio as going from 36v to 48v, so you get that speed by using a 36v 201 RPM motor at 48V.

All the figures I've used are approximate. You'll see motors of 201, 200, 190 and similar RPMs, which are basically 15 mph max motors, then there are 350, 360 and 270 rpm, which are 20mph max, but will give good power and efficiency at the 15 mph legal maximum, and 320 to 350 RPM motors that give that power and efficiency at 15 mph in a 20" wheel.

It all sounds complicated until you get your head around it. With all motors, power ramps down to zero as the speed approaches and reaches the maximum RPM, so, on the road, it will never reach it's maximum speed given by its RPM.

 

[pn_day]

Interesting bike - nice find!

As a starter, I would get rid of the knobbly tyres unless you are off-roading.

In terms of motors - I agree that having the rear wheel motorised would make sense here - and a smaller wheel will have the advantage of more torque from the same motor when compared with a larger wheel.

Another option would be a crank drive - it looks like there is no front derailleur so it would be relatively easy to just add a crank drive unit (like the Bafang BBSx or TDSZ2, or GSM, or...) This gives the benefit of driving through the gears (so better on the hills), but is more fiddly to use - driven rear wheel is probably less hassle if you are using it for shopping, commuting, and getting about town.

Good luck,
Phil

 

[nathanfinkel]

Thanks for all the responses!

I'll update with what I order