I want to haul my 250lb ass 25kph up long hills without sweating, in the EU

[Cyclotron]

The constraints:
- It's Germany, so the motor needs to have a 250watt sticker on it that isn't obviously lying
- Pedal assist only; whether cadence or torque based
- It needs to be demonstrably unable to exceed 25kph under motor assistance. (is EggRider still the state of the art in this area? I'm not handy enough to hide a reed switch)

The optimization target:
- My 12km commute includes a 3km hill with 400m of rise.
- I'm 6'2", 250lbs, and in my 40's; and I don't want to arrive at work sweaty
- I want to go at least the max legal motor-assisted speed--25kph--the whole way
- I'm not experienced building bikes--even acoustic bikes--so the more pre-built the solution is, the better.

After reading the articles on ebikes.ca, I think I need more amperage and less voltage; and that the "250 watt" sticker has a very fuzzy meaning. So I could probably do this with a 25Amp-35Amp motor controller, some overbuilt "250 watt" hub motor which is compatible with statorade and heat sinks. But I don't know how to calculate just how much torque I need at which rpm to do 25kph on a 20% grade.

 

[Chalo]

Hills tend to be either long or steep, but not both. 20% grade is rather rare on paved routes, and I'm skeptical that you have such a steep incline on your commute. If you really do, then you probably need either a high turn count rear hub or a mid drive coupled to generous gear range.

I use Bafang G020 front hub motors on some of my bikes. They are surplus from dockless rental e-bikes, and they're engraved "250W", but I feed them 48V x 22A (which is nominally 1056W) and they never have the slightest trouble with that. Their peak output power is a little over 750W.

That's what I would do in your situation. Use a "250W" (but really much more capable) front hub, feed it 48V x 22A, and then use the controller's display to impose an artificial speed limiter if you think you need one. In my own personal case I'd rather use an unrestricted controller without a display, and do the speed limiting myself as appropriate. But if you restrict to 25 kph and the motor clearly says 250W, then there's really nothing to hide from the law.

I would use a front hub motor because that leaves the stronger conventional bike wheel in the harder working rear position. It also gives your 250W rating label conspicuous placement.

EDIT:
If you really do have a 20% grade to ride up, my rough calculation suggests it will take 2300W or so at the wheel to maintain 25 kph on such a grade. That's a high performance e-bike that won't easily pass for a legal 250W model.

 

[Cyclotron]

thanks for the thorough advice! And, yeah, it’s actually a 180m rise over 2km, but that’s at the endpoint of a 6km hill that does have a few 20% parts, and a few prebuilt e-bikes I’ve test written were shockingly weak on hills, so I wanted to overbuild.

It sounds like around 1kw might be plenty, especially if the motor's heat dissipation can keep up. But I don't think I can source surplus rental e-bikes; does anybody sell those new, especially built into a wheel, or even a frame with power routing already done? (I don't have a donor bike, yet).

 

[neptronix]

If you have a lot of hills and a ridiculous power restriction you've decided to follow, i would really recommend a mid drive.

You might want to consider a lightest.bike mid drive.. they are expensive.. but they are super compact.. the 250w rated model is the same size as the 1000w rated unit.

..other alternative is to use two geared hub motors.. but.. you should check the law there... and also.. on 20% they are vey likely to overheat still.. that's a huge grade.

To do 25kph up a 20% grade is going to require multiple kilowatts of power.. that would require a bulky motor.. in order to not have a bulky motor, you need a very high RPM RC motor with 2-3 stages of reduction.. but that's a lot of fabrication and the end result is not a stealthy kit that looks like it's 250w rated..

..basically you are asking a lot of an ebike with a super constrained motor size!

 

[Cyclotron]

If you have a lot of hills and a ridiculous power restriction you've decided to follow, i would really recommend a mid drive.

To do 25kph up a 20% grade is going to require multiple kilowatts of power

Thanks for the advice and sanity check! I haven't ruled out mid drives, but I'm already heavy enough to round off cogwheels pretty quickly, and I'd rather not add to that--plus hub motors have far more options for aftermarket cooling.

Lemme ask a different, but related question:

Constrained by an engine claimed as 250w by the manufacturer, what's the best I can do with a 250lb rider on a 6km, 5%-10% hill?
According to the trip simulator, the Bafang G01 would get hot enough to ignite the atmosphere and end all life on Earth. Since the simulator doesn't care about claimed wattage, though, I'm not sure what might go well in there. Maybe some direct-drive motor with a slow wind, statorade, cooling fins, even more cooling fins, and an upside-down Canned Air to spray on it whenever the temp exceeds 110C?

 

[Chalo]

Lemme ask a different, but related question:

Constrained by an engine claimed as 250w by the manufacturer, what's the best I can do with a 250lb rider on a 6km, 5%-10% hill?

I've seen plenty of motors nominally rated at about half of their electrical input power to about half of their peak output power. So if we assume you can put down 500W at the speed that 500W would allow you to climb a 10% grade, the answer is: about 11.5 km/h. That's pretty much the best case for a true 250W rated system, and it would require very close matching of peak power to peak climbing demand (which would probably constrain top speed to less than 25 kph, if the bike's gears aren't used to give adaptable ratios).

You can play with this calculator while you decide how to make a "250W" e-bike that really isn't:
Bicycle Speed (Velocity) And Power Calculator

Set the bike type to "roadster" before changing weights and other values. That's the best approximation of an e-bike's qualities.

E-bikes.ca has a really good motor simulator that will predict bike performance and more if you give it enough data.

 

[neptronix]

Here's a medium size DD that can come labeled as a 250w motor.. with statorade ( ferrofluid ) & has a ton of thermal mass t o handle the abuse ( compared to a geared motor ) :

Simulation of said motor:
Motor Simulator - Tools



If you want to go with the 'on a technicality' approach then i'd go this way. But there's nothing stealthy about a DD.
If you want to be stealth, run a pair of 250w rated dual gear reduction hubs and hide one in the rear behind some panniers.

Also.. use as small of a wheel as you can get. 26" or smaller is ideal. 20" is awesome.

 

[Papa]

Ebikes.ca will laser etch "250w" on most any gear hub they sell. Last I checked.... $15 US.

 

[Cyclotron]

Sounds like, with my sparsely limited skills and lack of workspace, I want that “250 watt” all-axle kit, on the non-driven wheel, and I want to cover it with panniers.

I guess I’ll be keeping an eye out on eBay for Zox bikes, Cruzbikes, and any other front-wheel-drive bicycles I can find.

 

[neptronix]

You should be able to get the all axle in a front or rear configuration.

Cruzbike would be a lot of fun with a motor wheel due to the aerodynamics.