Let's talk torquey

[JCG]

Ok, so one thing we see a lot on non-hun motor type bikes are large sprockets on rear wheels, put there to drop the often dizzying rev rate of the motor spindle to a manageable one for wheel spin rates, and just as importantly, to multiply the torque available to that drive wheel.

We hear a lot about power and speed, but not much about torque. I've scoured the previous posts in this particular forum and haven't found a defining range of "target torques..." What is necessary, as a function of bike mass, to provide great acceleration from a dead stop? To provide a good torque boost for acceleration at speed?

My NC BLDC 500 W motor gave around 90 Nm from a dead stop at 48 V... what have those who have build a non-hub motor bike used as a guide for how many teeth (i.e., what ratio) to use on that rear sprocket? Or, has speed reduction been the only idea in mind, and whatever torque boost appeared was just a bonus? For a typical tire size, as a function bike weight, how little torque can you get away with before the bike becomes to "laggy?"

I'm curious to hear what design considerations there have been in past projects.

 

[gunthn]

with a 58t spr. on the bb I get incredible mutlitples, but my only way to measure is what;'s the steepest hill will the bike climb.

My bike will take practically any hill at reasonable speeds with the only problem being that battery is constantly pulling the bike front end up .

What method can be used to measure torque in a bike?

 

[swbluto]

Ok, so one thing we see a lot on non-hun motor type bikes are large sprockets on rear wheels, put there to drop the often dizzying rev rate of the motor spindle to a manageable one for wheel spin rates, and just as importantly, to multiply the torque available to that drive wheel.

We hear a lot about power and speed, but not much about torque. I've scoured the previous posts in this particular forum and haven't found a defining range of "target torques..." What is necessary, as a function of bike mass, to provide great acceleration from a dead stop? To provide a good torque boost for acceleration at speed?

My NC BLDC 500 W motor gave around 90 Nm from a dead stop at 48 V... what have those who have build a non-hub motor bike used as a guide for how many teeth (i.e., what ratio) to use on that rear sprocket? Or, has speed reduction been the only idea in mind, and whatever torque boost appeared was just a bonus? For a typical tire size, as a function bike weight, how little torque can you get away with before the bike becomes to "laggy?"

I'm curious to hear what design considerations there have been in past projects.

Seriously, I just plug the numbers into my e-bike/e-scooter simulator and see if I like the acceleration and speed profile. If I do, I go with that sprocket tooth-count.

(There's a nifty auto-optimization feature that'll optimize a parameter(Say, the tooth-count) to optimize whatever desired output (Say, the speed). I use that for a 7% hill and on flat land to get an idea of what tooth-counts I'd like to use.)

 

[amberwolf]

Since mine was built from parts at hand, I was a little limited in what I could do, but the basic concept is to use the regular bike drivetrain to do all the torque conversions for me, and just get the motor input to that down to around sane pedalling speeds (80-100RPM).

So in my case, I can't really worry about torque specifically, but rather simply about slowing down the motor's output to something manageable.

Fortunately, the wheelchair/powerchair motor, at 24V, is made to go about 120RPM. At 36V, it comes out to about 180RPM. A 24T on the output shaft and a 52T receiver on the drivetrain input gives about 0.46:1 reduction, or about 83RPM at nominal voltage.

At fullly charged pack voltage, around 40V, that's more like 200RPM geared down to about 92RPM at the drivetrain input.

Unfortunately I don't have anything to measure actual torque, but it's a 350W motor meant to directly drive a 10" wheel to move (with an identical twin motor on the other wheel) up to 350 pounds of person *plus* around 100-150 pounds of wheelchair/powerchair at around 4-8MPH.

So being able to move 150 pounds of bike plus 150 pounds of me at up to 20-something MPH in the highest gear, and able to start me easily and quickly from a complete stop in the lowest rear gear (of 7) and highest front gear (of 3) is good enough. I do think I need a more powerful motor for hills, though. It'd probably smoke this one to try going up 7th Street or Cave Creek Road on North Mountain here in Phoenix.

 

[coln72]

I currently have a 14t motor gear driving a 63 tooth gear at the wheel with a 20" rear rim. I have a feeling that this may not be geared down enough for my 300w brushed motor. Hoping it will be able to take off from a dead stop (it does now but half of it is missing). Once rolling will be kicking in a petrol motor to help acceleration and dropping it out once the desired speed is reached.

So the problem is do I gear it for acceleration from rest or to the desired speed once it is rolling :?

 

[dontsendbubbamail]

The only time Bubba thinks about torque is when I hit the throttle too hard and the front wheel comes off the ground. This is bad because I am not adept at ridding on one wheel and have solid front forks, so the landing is harsh. I am not breaking parts, so I don't think about it.

Bubba

 

[JCG]

Good stuff all. Looks like torque is a tough thing to target as well as to quantify.