Noob Question | Where is the torque calculated?

[Dman12]

Hi,
Where do I calculate the torque of the motor? the motor itself has about .8N.m, it has a reduction gearbox 1:9. then from the gear box to the crank another 1:4 reduction, and from crank to wheel the gear is 1:0.5
It is a 26" wheel, 24v motor 250w, 3000rpm. so, 3000 -> ~330 -> ~83 -> 166 RPM.

at which RPM should I calculate the torque? at the crank or at the wheel?

Thanks, sorry for the lame questions. it is my first E-bike and it got me confused

 

[Pete1961]

Hi, To be able to compare with a known hubmotor you need the torque at the back wheel, from the figs you give I estimate you would have 14.4 Nm at your rear wheel hub (not accounting for losses from friction in the reduction drive train) & 12.8 mph at this 166 rpm. Is the torque quoted rated ie. continuous, or peak short term? Another thing to think of is the speed your pedal crank will be driven at, important because the average leisure cyclist pedals @ 60 rpm, professionals @ about 80 rpm (called cadence sometimes) What I am saying is you do not want the crank spinning so fast that you cannot keep up with it when you most need to. If the figs are for light load & the peak torque rpms drop closer to 60 at the crank then that would be more ideal, likewise if the figs are rated torque with a higher peak I would be happier. The whole idea of these crank drives is that you have the motor driving through multiple gearing, either derailleur or hub. I would aim for about 40 Nm @ 6 to 8 mph in lowest gear. Any hill that cannot be climbed @ 6mph well it is time to get off & push

To compare I own this hub motor in a 26'' wheel http://www.tongxin.net.cn/en/down/quxiantu.gif actually at the time mine was sold there were different gear options so I have the 14 mph top speed model this was meant for a 27'' or 700c wheel, but mine is in a 26'' so I probably have about 20 to 22 Nm @ 10.8 mph recorded = max torque & 12A. I would describe this as adequate in the real world but needing 50% pedal assist on anything of a hill. it was thought to be a good all round compromise by my then supplier, I am inclined to agree, but this is a 1 gear machine. I only quote this as a minimum baseline but I weigh 91 Kg, bike & motor is 19 Kg, SLA's a further 12 Kg.
On the flat I can maintain top speed drawing about 5A I would guess about 5-6 Nm but can measure speed & Amps but not Nm

Another way of looking at things is this : A vehicle with rubber tyres on a smooth hard flat road is said to need 2% of total weight to keep it moving (but more to initially accelerate) & a further 1% of this same all up weight for each percent of gradient you wish to climb.
To convert Newton Metres to Lbf times by 0.738. if you were using a 24'' wheel this would be fine as the rad is 12'' but for a 26'' wheel times the Lbf by 12 then divide by 13. This would give you the actual rimpull. This rimpull needs to exceed the amount calculated from the mass + 2% & the max expected gradient percent.
This is theoretical, I would look much more at the real world hub motor graphs charts available, & study the Bafang crank drive. Also the chart at the base of this, red band is wheel off the ground so ignore http://xiongdamotor.en.alibaba.com/product/1582411347-220560510/Great_Power_Energy_Saving_Double_speed_BLDC_E_bicycle_Hub_Motor.html

 

[teklektik]

Hi,
Where do I calculate the torque of the motor?

Not sure why you are looking for motor torque - which varies with rpm and load - you seem a little vague on the circumstances or criteria for the measurement. Try messing with Justin's simulator and see if it offers up the information you are seeking....

 

[Dman12]

Oh ok, I looked at the thing the wrong way, I though the torque is a fixed value.
so basically I need to match the gears to the road condition between max torque and top speed, to get the most out of the motor.

for higher load I need lower speed to get more torque, and on flat I can use higher gear to get more speed.

is that correct?

Thank you.

 

[Pete1961]

Well yes & no, the motor has a continuous torque rating, & a max short term torque rating (I would look at the cont. rating as a basis for traveling on level ground, with peak torque & low gearing for hill climbs) so torque does vary up to a point BUT can be best magnified by multiple gears. This may be of some use, though it has got more complex than the version I used a while ago. It does show the relationship between speed power & gradient
http://www.machinehead-software.co.uk/bike/power/bicycle_power_calculator.html
I have a little 20'' with crank drive & a Sturmey Archer AW 3 speed hub, I am amazed at the gradient it will climb (in gear 1) compared to hubmotors. I will measure the grade, speed & amps drawn once the storms pass but it is about 480 elec watts input max short term, a brushed motor so about 75% efficient in its sweet spot, sadly a worm primary reduction so again this is 75% eff, so maybe only 50% efficiency overall BUT the 240 watts used though gearing just blows me away (I have tried various hub motors but this is just something else. Do you already own a set of freewheel cranks & the motor, if so OK but if not there is the Bafang & another similar crank drive, a rather better set up than I have. Look also at Cyclone, where the motor makes its peak torque, & the reduction train (in my view the cyclone 350 W is more perfect for a 20'') while the bafang 350 suits a 26'' wheel (with the 46t crank wheel) The little 2 speed hubbie is also for a 26'' wheel in its present format

 

[Dman12]

Thank you,
I will be able to understand it better once I will get the chance to go out and try different gears and setups, Now its all just in theory since I cant ride yet.

Thank you for all the help

 

[Pete1961]

Just to firm up I have some real world figs taken from my own bike, in lowest gear I can climb 10% gradient (1 in 9.5) unassisted by pedals @ 5.9 mph. This works out at 33.79 Lb of rimpull at the 10'' wheel radius (= 28.16 Lbf or 38.16 Nm). The wheel rpm in this case is 99.2.
This is at peak torque & the controller drawing 30A @ 24V. On the flat in the same gear speed rises to about 7.4 mph & Amps drop to 7, hence the motor is lightly loaded.

If you went to a 26'' wheel you would use different sprockets to get 76 rpm & the same road speed @ peak torque of the motor, you would then have 48 Nm delivered to the rear wheel centre but the greater wheel radius would make the climbing ability & speed the same.

My second gear (direct drive) gives me 11 mph & my 3rd (30% overdrive) gives me just over 14.4 mph. 2nd can manage moderate hills with light pedal assist, 3rd is for on the flat. In both cases I try to keep the amps under 15 on the move. Up to speed 7 to 8 A is typical

I have a 25:1 worm primary off a brush motor so not ideal, probably about 75% eff for each, so overall 56% eff factor. Later designs use a brushless motor (80% eff) & a planetary or straight cut reduction (again perhaps 80%) so overall about 64% @ max load. this means a bit smaller motor could give the same result. But this would still need the 38 Nm (20'' wheel) or 48 Nm (26'' wheel) to have the same gradient performance. From a riding point of view it is a world away from a hub motor if you live in hilly country