Advice: singlespeed torque sensing stealth build

[buddhak]

Hello,

I am an e-bike newbie and I am no electrical engineer.

I have some ideas kicking around in my head for a homemade cargo bike with electric motor assist. Could anyone offer some advice/opinions to help me along? The bike exists only on paper for now, but it will be a Bakfiets/Longjohn cargo bike built largely out of bamboo. I want electric motor assist to help with acceleration from a stop, and to assist with occasional hill climbing. I am leaning hard towards a Cute q100 cst hub with a TDCM torque sensing bottom bracket, and a thumb throttle for additional control. What's more, I'd like to run this with a single speed drivetrain - I am happy to trade top speed for simplicity. My questions are:

1. Is this crazy? I don't see much in the way of single speed ebike builds here. I am a single speed mountain biker + bike commuter, so I fully understand the benefits and drawbacks of running a geared drivetrain. I am more concerned about how intuitively the set up I am describing would work. I want to get up on the pedals from a stop and feel a gentle push, and then I want it to stop automatically when I am at cruising speed or slowing down.
2. What is bare minimum of equipment needed to set up such a system? Do I need to buy the grim CA V3 or do simpler (read: cheaper) devices exist to allow the BB to talk to the motor?
3.Any other suggestions?

Thank you

EDIT: I should probably mention, the hub would power a 26" rear wheel

 

[d8veh]

BMSB have this torque sensor and sine-wave controller to go with the Q100C:

https://bmsbattery.com/ebike-kit/689-b-b-torque-sensor-system-ebike-kit.html?search_query=torque+sensor&results=74

but why so you want the Q100C, which takes a cassette, when you want single speed. Wouldn't the Q100H for freewheel be better.

 

[buddhak]

Cassette bodies allow you fine tune chain line on a singlespeed set up. Moving the chainring in and out relative to the BB shell is a pain, and free wheels have a fixed chain line. Not to mention you have a lot more freedom mixing and matching sprockets and chain rings to get gear ratio and chain length you want. Thanks for the link, I will check it out.

 

[dogman dan]

Well, maybe crazy. But my point of view is an old guy too weak to pedal. My huge cargo bike runs 2000w, though it's rare that I run it using much more than 300w. But loaded down, up the rocky mountains, I need all 2000w to get up a long tough grade without smoking the motor.

If you pedal strong, and don't run the motor full power 100% of the time, you won't kill your motor very soon. The motor may heat up going up the hill, but then it rests going down. My hills are much longer.

To get good PAS, YES, you need the Grin setup, CA v3 and a torque sensor. Cheap hall sensor pas sucks badly, and you'll hate it.

On the other hand, once you get the hang of it, nothing can keep your cadence and foot pressure the same through an entire ride, uphill and down, like a right hand on the throttle. If you can get a kit that has multiple power levels, you get even more flexibility.

 

[buddhak]

Thanks for the great advice, dogman. My hills are nothing like yours (I live in Philly), that's in part why I am considering the Q100. I can see the value of sitting and spinning with a loaded cargo bike, now, feathering the throttle manually. I am not married to the cassette configuration, but I would still prefer a singlespeed option. Maybe a stronger motor without a torque sensor, but a simple handlebar throttle would be the way to go? That way I could stay seated and stable for starts and climbs. This would limit the power I could deliver with my legs (because I could stand and mash), but the benefit would be a more straightforward motor system since I could ditch the torque sensor/PAS. Sound reasoning?

 

[d8veh]

The Xiongda 2-speed motor would be good for single-speed gears. It's normally a bit too wide with 7/8 speed, but with single speed, it would be easy to fit. The 48v version packs a fair punch at full throttle. It comes with a really nice 5-level PAS control with a LCD, so you get the choice of throttle or PAS at any time. The controller uses current control, so you can make the motor as docile as you want. You buy them direct from Xiongda, who advertise on Aliexpress. They're quite cheap too - about $250 for everything except the battery. You have to build your own wheel or get someone to do it for you.

 

[buddhak]

Thanks again!

 

[dogman dan]

True, that's the motor to choose if you do want a small motor.

I'm just really biased to throttles. But if you never rode motorcycles, it can take longer to really get to using them with no thought.

What takes time is getting used to the idea of pedaling constant, and adjusting the throttle, vs just using full throttle, and adjusting the pedaling. Pedaling constant rpm and effort for long rides is the joy of motor assist. But it does not require pas to achieve. just practice.

 

[d8veh]

In Europe, most countries don't allow independent throttles, so we have to pedal all the time. It's not really a problem as long as you have a controller with good PAS function, like the ones with LCDs. You can turn the power up on the LCD, so that you don't have to put any effort into your pedalling. Personally, I think it's easier than a throttle provided that you don't mind moving your legs all the time. If you're not going to pedal, it makes using a bicycle sort of pointless. Wouldn't it be better to get an electric scooter for the road. I can sort of see the point for off-road stuff.

 

[buddhak]

Well your collective input has gotten me reconsidering. The 2-speed motor is interesting - it seems like it can alter it's power sweet spot for 2 different motor rpms. Kind of like having a motor with high rpm winding and low rpm winding at the same time? That advantage would be gutted if I were running a single gear ratio on the bike, especially if I went torque sensing. Also, the inability to back up, may be the real deal breaker. This bike of mine will be over 7' long, and enough of a headache to manuver in the city, so I dread loosing the ability to roll backwards.

I think I will go with the Q100 with a low rpm wind + throttle at first. Practice. Then, if funds match the "need," I can upgrade to a BB torque sensor and CAV3. The Xiongda looks great, but a better match for a geared PAS set up. In the meantime I am going to try to design around the NuVInci N360 +Bafang middrive in case I decide to really upgrade.

I appreciate your advice, everyone.

 

[Drunkskunk]

There are a lot of things the Q100 does well.
-Look cute.
-Be small.
-Power a normal weight human at sedate speeds along reasonably flat roads and paths.

All in all it's a great little motor.
But there are many thing it can't do.
-Look intimidating.
-File your Taxes.
-Move heavy loads up hills.

You have a cargo bike. Cargo has weight. You want a motor to help you up hills. This is not on the list of things the Q100 can do well. In fact it's on the other list. The one of things it can't do well.

In other words, wrong motor for the job. For a cargo bike, look at something in the 350 to 500 watt range. You could easily burn a motor up making it move more weight than it was designed for.

As for torque sensors, I've discovered the best way to get pedal assistance is with the throttle. I let my brain decide how much power I want from the motor, and how much from my legs. There are those moments when a full burst of power from the motor is to my advantage. Stray dogs, near misses with cars, jealous husbands chasing me with kitchen utensils, various things. It's nice to have the full power of the motor instantly at my command.

 

[chas58]

I did one of the first single speed builds here – the big problem was that single speeds have 120mm drop outs and all of the motors I have seen are for 100mm or 135mm+. No one knew if you could fit a motor to a 120mm drop out What drop out do you have (is this an issue for you)?

The Q100 is easy to modify for single speed and 120mm drop out, or of course it is easy enough to use the CST version with a larger drop out.

I’m a strong rider, and the Q100 has about the same power output as my legs. So in my case, its rather like riding a tandem – it’s a nice little push from a stop and adds about 5mph to my top speed. To “feel a gentle push” “to help with acceleration from a stop, and to assist with occasional hill climbing” this motor can work well. Like mentioned above, it’s going to start to get unhappy (i.e. heat up) if you start to spend lots of time at speeds much less than half what it is rated for. I think a 201rpm version is best for single speed work (mostly because the gearing at those speeds can work well with or without the motor).

 

[d8veh]

Kind of like having a motor with high rpm winding and low rpm winding at the same time? That advantage would be gutted if I were running a single gear ratio on the bike, especially if I went torque sensing. Also, the inability to back up, may be the real deal breaker.

I don't know where you get the idea that the motor won't back up. I have both the old 36v version and the new 48v one. Both of them back up just like any other bike.

You can't use a torque sensor with the Xiongda unless you use a CA or similar to change the throttle into a throttle signal because you have to use the special for Xiongda controllers.

I'm not sure what you're saying about the suitability of the Xiongda for a single speed bike, but I think that one would be very suitable. The 36v one has manual and automatic low speed at any time at any power. The 48v one only uses low speed at maximum power for hill-climbing. It will select it automatically if you go too slow on level 5 power or you can select it manually, but it always gives maximum power.

 

[buddhak]

d8veh,

In reading the 2-speed hub motor thread (some, not all), I came away with the understanding that walking the bike with a Xiongda motor backwards was at times not possible depending on which way the hub was spinning before it came to a stop. If I am wrong about the backing up issue, then that's great. In order to manuever the bike, I would need to walk it backwards (into and out of my garage for example) - can I do that with a Xiongda? As for the second point, the one about the benefits of a 2-speed hub motor being diminished on a SS bike, what I meant to convey was that the bike's speed would be limited ultimately by its gearing, not the motor's. I would have to choose either a relatively short gear to make the bike easier to climb and accelerate, or a relatively tall gear to maximize top speed and leave more of the accelerating work to the motor. This is what my imagination tells me, as I have never ridden an e-bike, ever. It seems to me that the Xiongda might pair very nicely with a Schlumpf or Hammerschmidt overdrive bottom bracket.

Drunkskunk, that was hilarious and to the point. On the face of it, putting a tiny motor on a cargo is absurd, but I am planning on sweating when riding. The purpose of the motor would be, as Chas58 put it, to make it feel like a tandem. I may sing a different tune after I get a taste of power. The stronger Bafang CST is still on the short list. The MAC kit probably will not work because I don't want to subject my frame to a 203mm disk brake set up.

 

[d8veh]

I park my Xiongda bike in my porch every day. I have to reverse it to get it out. There is absolutely no problem with reversing.

 

[buddhak]

Right on.