New build : Mid-tail ebike with hub motor

[flat_tire_master]

Hello all,

This is my first post here, I have landed on that forum quite a few times in the past years but now I feel a need some advice for a new build.

First a short explanation on why I want to change my actual setup.
I have been riding for 3 years an Orbea Katu frame with a mid-drive BBs02 500w kit (48v battery). I have been very satisfied with this bike : disk brakes, Nexus 8, front hub dynamo lights, 20" wheels, large front panier, enough room in the back to install a rear rack and a child seat.
I used it mainly for my daily commute of about 10km each way : 80% of the commute is with my kid on the back. Typical cruising speed is 30 kph. I was able to maintain 25 kph during the inclined portion of the road (5-6%). If no kid in the back, on the flat, full throttle without pedaling gives about 40 kph (full battery).

Two things have changed now. I have a younger kid who is now old enough to get on my bike, and I tried a hub motor and after that I started to hate my mid-drive
My primary need will slightly change in a few months : my older kid will change school and I will no longer be in charge of the transport. So my primary need will be to commute myself only, with an objective to drive a little faster.
My secondary need will be to transport my 2 kids for any occasions during the weekends : mostly short trips in the city, with no desire to drive fast.

Can we find a setup to deal with these 2 needs ?

To finish this introduction, I can say how I got a taste of a hub motor. To fit my primary need, I test drove a bicycle called Klever X speed. It's a street legal (here in Switzerland) speed bike with a hub motor made by TDCM. It was very pleasant to drive : smooth, silent, and fast. Unfortunately this kind of ebike is not fit at all to transport 2 kids.


For my new build, I intend to use a Benno Carry On. It's a mid-tail bike, the tail is long enough to put 2 kids but no more.
Wheels are 24", 2.6 wide tires, and overall good quality.


I already spent a little bit of time on the ebike.ca motor simulator.
I have a hard time deciding if DD or geared makes more sense in my case. I don't know if the extra weight on the hub is a game changer for this kind of bike.
I have made simulations mainly with Bafang geared G310, 9C DD RH212. I find that both could work.
I ttry to define my working points :
* commuting alone (total mass 100kg) : worst case is 6% incline for about 3 minutes. Ideally I would like the speed to be 30 kph.
* commuting alone : on the flat, I would like to maintain 40 kph for long periods
* with kids (total mass 140 kg) : worst case is still the 6% incline. I would like the speed to keep around 25 kph.

I definitely think I didn't grasp yet all the right parameters to think this problem properly.
I would appreciate some advice, either to converge on a hub setup, or either to tell me my project is completely absurd :lol:

 

[dogman dan]

Well, sounds like your problem is not a too tight budget, as many here have.

Great bike, looks a lot like the one I built, because long cargo bikes are a bit long and clumsy, but mid tails are hard to find and expensive. I also had other criteria, I could not get at any price.

My most emphatic advice would be this. Go with a DD rear hub. Don't worry about the weight, it will ride fine, especially as a mid tail which can be easier on wheels than traditional frames. Overkill it. Get the big direct drive motor.

Forget about legal considerations. Just hide that huge motor with some panniers, or at least some panels that keep the kids legs out of the spokes. Go big man, like a motor that weighs up to 25 pounds including wheel and tire. Then power it with a 48v motor and 40 amps controller. The setup will be 2000w max starting out, or climbing a very very steep hill, but the motor is capable of 5000w easily. It will never overheat because of the bike weight, or cargo. Tow trailers if you want. This way its got lots of power when you need it, and as kids grow. But you can still ride it all day pulling 200-300w, ride it slow and blend in with normal e bikes all day. The big motor does not have to be fast all the time. Its just got the power you need for that couple seconds you need it, on a stop sign on a steep hill.

This is the kind of setup I rode to go touring, overloading the bike with camping gear, and it worked fantastic. I could ride up the rocky mountains fast enough to keep up efficiency despite heavy loads. Yet on the other side of the mountain, I could ride extremely efficiently at slow speeds to increase range.

 

[Voltron]

You're going to love it either way
Mid cargo with powerful motor is just about the perfect car substitute.

From my experience, I would pick direct drive. I really value reliability for this kind of utility bike, so direct drive is nearly indestructible, as long as you don't grind it uphill at low rpm for a long time where loads are high, and air cooling is low. If you go oversized for the flatlands, it will be perfect on hills. Lacing it into a 24" raises the wheel rpm for a given speed, so even better come hill time. Plus they're really quiet, and you'll be able to get regen braking if you want, but that can lead to axle issues etc so is its own thing to think about. Geared motors really shine when you want a lightweight, bike like build, where you might actually pedal it with the power off, so having the clutch so the motor freewheels is important, but you prob won't be pedaling with just your legs a lot? Voluntarily anyway?

I personally failed a gear motor just riding of a curb with the power on... It cut out after the drop, and I thought it was the wires or something, but when I touched the motor shell I could feel it whirring away it there, but turns out it had broken the key stock.
After I read up on it here, I saw the advice to not ride off stuff under power with a gear motor...
I had another one fail when the clutch separated into 2 pieces, and another one shred the gears. That was my fault, maybe, from rampaging it way too hard.

 

[Voltron]

I lucked into this 1st gen Yuba Mundo frame that became my utility monster as I went further and further down the small diameter wheel rabbit hole... I also have a seat from a big scooter that clamps onto the rack for passengers. My kids were older, so less worry, but really really think about ways to keep their feet out of the wheel and the disc brake... They start getting bored back there and trying to stand on the frame and slip etc.

Yours has the same plus of the removable rack. Then you can maybe take it off for pretending you're not an obvious dad while mid week cruising... Like leaving the car seat at home. Maybe that was just me

 

[dogman dan]

Yes, I figured whatever you did that kept the kids feet out of the wheel would also hide the size of the motor.

Btw, there is an easy way to set up a big monster of a dd motor so that it does not exceed legal speeds. A device called a direct plug in cycle analyst, combined with a compatible controller, allows you to set power levels lower, speed levels lower, as well as being the best type of battery gas gauge.

But as needed, a few inputs, and you get that full 2000w for a big hill. But you can ride a legitimately legal enough bike most of the time by using the limiters at the correct settings for your local laws.

 

[flat_tire_master]

@Voltron : that utility monster looks awesome. Reminds me of a hard tail lowered harley davidson.
@dogman dan : It seems really hard to find a mid-tail donor bike. I understand you decided to make frames yourself.

Thank you all for your feedbacks.
First conclusion is that, yes this project makes sense.
Second one is that we have a consensus towards a DD hub.

Despite all my recent reading and searching, I don't have a clear picture of available options for DD hubs.
I investigated via the motor simulator on ebikes.ca mainly on a RH212 Fast and baserunner L10 controller.
Below is a tentative to simulate my worst case use, with some margin :
- 150 kg
- 8 deg slope
- moderate human power (75 watt)

Speed is very decent (even with a lower throttle value), motor starts to overheat after 12 minutes of this severe case (unlikely to happen in reality).
So at least it seems to me that a RH212 would suit my need.

But what are the other options ?
RH205 seems to be basically a RH212 without temperature sensor. What about DD27, what about MXUS, is there any link to help me compare these motors ?

 

[dogman dan]

Hard to go wrong with a grin kit. But I would say get the RH 212 in the standard wind. Fast wind would be chosen for a 20" wheel. That one will be absolutely fine with 300 pound loads or less. (300 pounds including everything, with two kids you could be 400 pounds) More than that, you just have to watch it about bogging down so slow you overheat, and pedal hard on the steeper hills. But again, the motor can do more than that 300 pounds depending on the load and the grade. It can do a LOT more than 400 pounds on the flat. It just all depends on the length of the ride, how hard you pedal, how long are the hills, etc. Don't fear any hill less than a kilometer long, or a mild hill many km long. Pedal 200w on the steep hills, if you can.

This is not as fat a motor (wide stator) as I suggested. But the 24" wheel will help your torque some, and the thermistor inside will help you avoid overheating if you find yourself more overloaded than you thought, on some hill.

What would reallllly work, is the big motor, The DD45 or similarly wide stator motor. Turns a cargo bike into a small truck. But that motor would require wider than standard frame, custom made frame, or at least a steel frame you can bend out to fit.

What I used is so obsolete, the crystalyte 35 mm stator 5304 motor. Seems like last century, 12 years ago. That motor just fit into 135 mm frames.

 

[neptronix]

The RH 212 would be a nice motor for a 24" if high continuous torque is needed due to a slightly higher pole count and diameter than standard DD motors. If the ebikes.ca simulator indicates you need higher power than that, then move up to using a magic pie ( larger diameter DD )
Be prepared to push >2kw if you need to haul 140kg up a 6% grade though.

A g310 would absolutely melt when carrying 140kg up a 6% if the grade is long enough.

If you want a hub motor for a 20" wheel, then a standard diameter DD would be a better choice because as the RPMs climb, a higher diameter/pole count DD will incur higher iron losses, and you will feel them in a 20".

One thing i would recommend for a mid-tail is to add front suspension. That might mean using a 20" front wheel to ensure that the bike geometry does not change significantly. But because the rider sits more in the middle of the bike, the front suspension will soak up some % of hits you'd normally feel in the rear. Note that if you do this, you will want a very strong downhill oriented front suspension fork that can be set very stiff, because you'll have a much higher load on it than it's original application, ie a mountain bike where most of the rider weight is on the rear of the bike.

There also exist 22" BMX rims which could be used for a more fine adjustment of the front wheel height. One could actually fit 18" motorcycle tires to these if you need good flat protection :thumb: