Crank speed/power output relationship .... questions.

Electrically assisted bikes, trikes, etc.
Geoff.D
Posts: 1942
Joined: 12 Mar 2010, 9:20pm

Crank speed/power output relationship .... questions.

Postby Geoff.D » 12 Aug 2018, 5:43pm

As a complete novice to electric assists I embarked on a conversion project. I bought a new (but cheap on eBay) front hub motor, and associated wiring. I built it into a rim I already had. I found a suitable battery, also on eBay. It works fine. I have two modes - twist throttle or pedelec. The throttle is very straight forward, and I can give myself whatever lever of power output I want.

My understanding of crank speed sensors was that they stimulate a power output in relation to crank speed. The faster you pedal the more output. And this creates the conundrum that when pedalling slowly uphill ( and needing greatest assist) you don't get that maximum assist. However, my pedelec seems to only have on/off modes. Once the sensor has detected movement, it appears to kick in and give me the same power (measured in my sense of acceleration) whether I'm pedalling fast or slow. In other words, I have no control over the level of power assist.

Was my understanding wrong? Might my electronic box be faulty? Might the box be configured in this way as a cheap alternative?

This isn't a crisis, and I'll soon be managing the new experience OK. But I'm curious.

hemo
Posts: 201
Joined: 16 Nov 2017, 5:40pm
Location: West Sussex

Re: Crank speed/power output relationship .... questions.

Postby hemo » 12 Aug 2018, 11:54pm

There are two pedal sensor types cadence or torque sensor.

Cadence is for relaxed riding and is the most common assist for hub bikes, you can basically ghost pedal and the system will whoosh you along or pedal harder for a little more speed. Not very many hub bikes use torque sensing though there are one or two Bottom brackets kits to adapt hubs to torque sensing.

Torque systems are different and most often appear on oem Bosch, Yamaha etc, mid drive bikes they are torque multipliers so giving power accordingly to the riders input.

The better ebike kits have lcd with five pas levels(cadence pedal assist sensor) and intelligent controllers, cheaper kits have basic controllers and pas systems usually 1 or 3 speed functions so none or very limited power control under pas.

With my 5 level pas sytem I get more control over the power under pedal assist from controllers.

20a controller 17a controller
PAS 1 = 13% power or 2.6a load. 2.2a load.
PAS 2 = 20% power or 4a load. 3.4a load.
PAS 3 = 33% power or 6.6a load. 5.6a load.
PAS 4 = 50% power or 10a load. 8.5a load.
PAS 5 = 100% power or 20a load. 17a load.

Range is also affected by the pas level used, I have a pair of 3+ year old batteries if I use PAS 3, I get about 18 - 24 miles. Using only PAS 2, I see much improved range and 32 - 40 miles . Top speed is the same of about 19.8mph but aceleration is a bit slower.

Marcus Aurelius
Posts: 124
Joined: 1 Feb 2018, 10:20am

Re: Crank speed/power output relationship .... questions.

Postby Marcus Aurelius » 13 Aug 2018, 5:50am

Power is simply the product of Cadence and torque. If you wish to maintain a given power, you need to maintain the balance. Lower the cadence, increase the torque. Lower the torque increase the cadence.

Geoff.D
Posts: 1942
Joined: 12 Mar 2010, 9:20pm

Re: Crank speed/power output relationship .... questions.

Postby Geoff.D » 13 Aug 2018, 9:12am

hemo wrote:..............

The better ebike kits have lcd with five pas levels(cadence pedal assist sensor) and intelligent controllers, cheaper kits have basic controllers and pas systems usually 1 or 3 speed functions so none or very limited power control under pas.



Thanks for the reply, hemo. Very informative.
It's as I suspected. My cheap pas controller is unsophisticated. It's just "on" if I pedal, until I reach 15mph. I don't have the facility to dictate the level of power, unless I override it by using the twist throttle.
At least I know that I haven't screwed up in making the wiring loom,