QS273 40H 3T Torque? (I got questions, looking for input)

[ns-esk8]

(context)
I am working on my first first "non-mid-drive" bike build for the summer. When it came to the winding, I was trying to decide between the 3.5T and 5T. I wanted something relatively torquey (more below), but since I planned to cruise regularly at 30-40mph, wanted some efficiency and wasn't sure if I was willing to go above 72v, I decided to go w/ the 3.5T (via a partial alibaba enduro bike kit). Motor is mounted on a 19" with 80/100-19 tires.

Good news: my order just shipped early.
(possibly) Bad news: they changed my order to 3T without checking with me. I would have preferred going the other direction (4T/5T), but not going to make a stink about it.

Now, I am not looking to do wheelies on this thing, but I would like it to pull hard from at least 5 mph, if not 0mph (and I am not a light rider, ~115kg). I am new to the hub motor world (outside of skateboards/scooters), but since I was already a bit iffy on going 3.5T, I am slightly concerned about the torque now (even though this sucker is massive). For those with experience with the QS 273 40H @ 3T:
(questions)
- my regular riding will be mostly cruising around, but how much stop and go before I could run into issues w/ heat?
- the hills in my area are not crazy steep, but it's also not very flat around here... assuming I am not trying to start on a very steep hill from a dead stop, should I run into any issues?
- would someone be able give me an idea how much bA/pA I should expect to have on standby for my needs? 200a/400a, 300a/500a, etc
- is there an equivalent motor in I could look at in the simulator?
- am I going to regret the 3T ?
- is throwing more voltage to increase starting, pull-up/out torque a bad idea if I don't plan to ever go above 60 (or regularly above 45)?

(additional details)
Battery pack size: thinking of something in the 4 maybe 5 kWh range. lipo pouch cells or 21700s.
Controller: my preference is usually something on VESC platform, but TBD

Thanks!

EDIT: found speed test with someone roughly my size with same motor and 150a controller. Didn't look bad, but looked like I might want to run at least 200a at 72v.

EDIT: I reposted this over in the technical section, but don't see a way to remove this thread. Mods, feel free to kill this one off.

 

[Tony01]

(context)
I am working on my first first "non-mid-drive" bike build for the summer. When it came to the winding, I was trying to decide between the 3.5T and 5T. I wanted something relatively torquey (more below), but since I planned to cruise regularly at 30-40mph, wanted some efficiency and wasn't sure if I was willing to go above 72v, I decided to go w/ the 3.5T (via a partial alibaba enduro bike kit). Motor is mounted on a 19" with 80/100-19 tires.

Good news: my order just shipped early.
(possibly) Bad news: they changed my order to 3T without checking with me. I would have preferred going the other direction (4T/5T), but not going to make a stink about it.

Now, I am not looking to do wheelies on this thing, but I would like it to pull hard from at least 5 mph, if not 0mph (and I am not a light rider, ~115kg). I am new to the hub motor world (outside of skateboards/scooters), but since I was already a bit iffy on going 3.5T, I am slightly concerned about the torque now (even though this sucker is massive). For those with experience with the QS 273 40H @ 3T:
(questions)
- my regular riding will be mostly cruising around, but how much stop and go before I could run into issues w/ heat?
- the hills in my area are not crazy steep, but it's also not very flat around here... assuming I am not trying to start on a very steep hill from a dead stop, should I run into any issues?
- would someone be able give me an idea how much bA/pA I should expect to have on standby for my needs? 200a/400a, 300a/500a, etc
- is there an equivalent motor in I could look at in the simulator?
- am I going to regret the 3T ?
- is throwing more voltage to increase starting, pull-up/out torque a bad idea if I don't plan to ever go above 60 (or regularly above 45)?

(additional details)
Battery pack size: thinking of something in the 4 maybe 5 kWh range. lipo pouch cells or 21700s.
Controller: my preference is usually something on VESC platform, but TBD

Thanks!

EDIT: found speed test with someone roughly my size with same motor and 150a controller. Didn't look bad, but looked like I might want to run at least 200a at 72v.

EDIT: I reposted this over in the technical section, but don't see a way to remove this thread. Mods, feel free to kill this one off.

The 273-40 is a lot more motor than most need and lends to a very heavy rear wheel. The torque will be fine with the 3t. I push 285/510 amps in the smaller 205-50 with a 3.5t wind. The 273-40 could likely handle 350/650 if not more. You won’t regret it for the turns, but you may for the weight. IMO although it is a popular setup, it is not ideal. I originally got my 205 in a 19 then rebuilt into a 17” going from a 43 to a 35lb rear wheel. What a difference in performance. With the 273 in a 19” that rear is going to weigh 60lbs easily; it will stutter badly in leans and just be an all around straight line only bike. You could rebuild it into a 17” with radial laced spokes which will take off the same 7-8lbs and will be fine as they’d be very short. But still it would be a 53lb rear wheel.

 

[BalorNG]

Amount of torque a motor produces dependant on mass of iron, magnets and copper. Amount of turns is almost completely irreveleant given adequate copper fill - higher turns produce more torque FOR A GIVEN AMPERAGE, but saturation torque is absolutely the same, so is how much waste heat your produce for a given torque... very simply put, given same abstact motor:

Saturation torque - 100nm
Heat loss at saturation - 3kw

Those numbers are fixed.

A 3T motor will require 400A to produce this torque and this heat loss.

A 6T motor will require 200A to produce this torque and result in SAME heat loss. (Because motor resistance will go up 4x)

You will, however, require 2x voltage to achieve same *speed* as a slower motor.

Choose number of turns to match your controller, desired speed and battery voltage. If you want more torque - choose a different motor (preferably a middrive).

 

[ns-esk8]

The 273-40 is a lot more motor than most need and lends to a very heavy rear wheel. The torque will be fine with the 3t. I push 285/510 amps in the smaller 205-50 with a 3.5t wind. The 273-40 could likely handle 350/650 if not more. You won’t regret it for the turns, but you may for the weight. IMO although it is a popular setup, it is not ideal. I originally got my 205 in a 19 then rebuilt into a 17” going from a 43 to a 35lb rear wheel. What a difference in performance. With the 273 in a 19” that rear is going to weigh 60lbs easily; it will stutter badly in leans and just be an all around straight line only bike. You could rebuild it into a 17” with radial laced spokes which will take off the same 7-8lbs and will be fine as they’d be very short. But still it would be a 53lb rear wheel.

Thanks for the reply/feedback.
I also have a couple e-mountain bikes and a sur ron. This will just be for road cruising... do you think the ride quality will still be a problem? I went with QS273 mostly for the improved heat dissipation. I am a bigger rider and have a bunch of hills around me (not too steep, but some long distance ones, etc).
(worst case, I can get a good deal on an 18" QS205 50H 3.5T, which could be a fallback option... but then I have to find someone to buy the QS273 )

 

[ns-esk8]

Amount of torque a motor produces dependant on mass of iron, magnets and copper. Amount of turns is almost completely irreveleant given adequate copper fill - higher turns produce more torque FOR A GIVEN AMPERAGE, but saturation torque is absolutely the same, so is how much waste heat your produce for a given torque... very simply put, given same abstact motor:

Saturation torque - 100nm
Heat loss at saturation - 3kw

Those numbers are fixed.

A 3T motor will require 400A to produce this torque and this heat loss.

A 6T motor will require 200A to produce this torque and result in SAME heat loss. (Because motor resistance will go up 4x)

You will, however, require 2x voltage to achieve same *speed* as a slower motor.

Choose number of turns to match your controller, desired speed and battery voltage. If you want more torque - choose a different motor (preferably a middrive).

Thanks for the info. I knew about the current relationship, but didn't know the saturation torque was the same.

for better or worse, the 3T is on the way ... but the controller and battery voltage are still variable.

I would like to be able to cruise along up to 40-45mph and be within an efficient range for the motor.
Given that, would it be a bad/dumb idea to consider a higher voltage battery with a 3T motor (EG: 96v or 108v) to reduce the current needed? (...if I wouldn't be taking advantage of the increased top speed?)

 

[Tony01]

Thanks for the reply/feedback.
I also have a couple e-mountain bikes and a sur ron. This will just be for road cruising... do you think the ride quality will still be a problem? I went with QS273 mostly for the improved heat dissipation. I am a bigger rider and have a bunch of hills around me (not too steep, but some long distance ones, etc).
(worst case, I can get a good deal on an 18" QS205 50H 3.5T, which could be a fallback option... but then I have to find someone to buy the QS273 )

That would be the better choice for handling. The question is, do you have a controller and batt that can max it out? With the 3.5t 205 18” you’ll have far better tire choices and about 17lbs off the rear. Big difference. Heat dissipation is no problem for the 205 with ferrofluid and hubsinks. You only go 273 if you need to push more than 300 amps.

Some time ago I was riding the local mountain road and actually passed a guy on a ktm motorcycle going uphill. You will not do this with a 273... it felt really good.

 

[BalorNG]

Thanks for the info. I knew about the current relationship, but didn't know the saturation torque was the same.

for better or worse, the 3T is on the way ... but the controller and battery voltage are still variable.

I would like to be able to cruise along up to 40-45mph and be within an efficient range for the motor.
Given that, would it be a bad/dumb idea to consider a higher voltage battery with a 3T motor (EG: 96v or 108v) to reduce the current needed? (...if I wouldn't be taking advantage of the increased top speed?)

Going higher voltage will do absolutely nothing regarding current needed. It will simply increase your top speed.
Current needed depends on your aerodynamics/rolling resistance on 'demand' side and motor mechanical advantage on supply side - since the motor is hub motor, you can only decrease amperage by increase mechanical advantage - by decreasing wheel size.

 

[Adrian_]

I have excatly same setup as you with the only difference being winding count, I have QS 273 40H V3 5T on same wheel/tire combo (80/100-19). My setup is on 72v and on a fully charged battery (84v) my top speed is excatly 40mph, 43mph with 50A of flux weakening but it drops as battery discharges, lowest top speed I seen was 34mph when the battery was close to LVC so since you want to be able to go 40-45mph and still be withing efficiency range of the motor, I would say going 3T/3.5T was the best decision. You could go 96v or higher to get higher top speed on 5T motor but unless you're expecting to be on the highway and want to be able to cruise with traffic, in my opinion it's not worth it.

Another thing to keep in mind, I'm stuck with 80/100-19 because that's the biggest tire that I can fit in my swing arm and I'm having traction issues when pushing above 250A, the bike wants to pop a wheelie and it loses traction all at the same time and it feels like it's going to send you into traffic unless you lift the throttle, it's even worse when the road has any moisture on it. With a 3T motor, I would guess that you'll probably start having traction issues when pushing about 350A depending on your setup on a 80mm tire, going to a 205 motor will be even worse because it's lighter so it will lose traction sooner, probably around 300A. If you can, I would go to a wider tire, maybe 100/90-19 if your swing arm can fit it if you're expecting to push above 350A on a 80mm tire with QS 273 3T.

 

[ns-esk8]

Going higher voltage will do absolutely nothing regarding current needed. It will simply increase your top speed.
Current needed depends on your aerodynamics/rolling resistance on 'demand' side and motor mechanical advantage on supply side - since the motor is hub motor, you can only decrease amperage by increase mechanical advantage - by decreasing wheel size.

Thanks for the reply.

I have seen the sentiment from a few folks on here that increasing voltage does not impact current needed (and/or voltage doesn't increase torque), but this wasn't my understanding and does not seem to align with my anecdotal experience (though I am just a hobbyist ). If I'm dumping 20kW to the ESC at 72v vs 96v, wouldn't the 96v need less current for roughly the same power output on 72v? EG: ESC pulling 21.6kW from battery via 72v @ 300a would be more or less equal to pulling 96v @ 225a, etc? (w/r/t power output via motor, as well)

Things get real murky for me with how this gets 'converted' to phase amps on the motor side though and IIRC ohm's law doesn't work quite the same w/ phase amps, etc (something something inductance/capacitance/etc)... so, that might be where I am missing something... though, I still thought for a given brushless motor, it will increase torque output when voltage is increased (for a given current draw, within the power rating of the motor), not just top speed.

I'm trying to understand if I can size down the BMS/controller/wire current ratings (and size) a bit if I am able to supply more voltage on the battery/controller side and get the same-ish performance or if I am off here. Thanks.

 

[BalorNG]

Oh, I'm talking about PHASE current btw. Going higher voltage will absolutely decrease your *battery* current, but that's not what matters.

I have seen the sentiment from a few folks on here that increasing voltage does not impact current needed (and/or voltage doesn't increase torque), but this wasn't my understanding and does not seem to align with my anecdotal experience (though I am just a hobbyist ). If I'm dumping 20kW to the ESC at 72v vs 96v, wouldn't the 96v need less current for roughly the same power output on 72v? EG: ESC pulling 21.6kW from battery via 72v @ 300a would be more or less equal to pulling 96v @ 225a, etc? (w/r/t power output via motor, as well)

Things get real murky for me with how this gets 'converted' to phase amps on the motor side though and IIRC ohm's law doesn't work quite the same w/ phase amps, etc (something something inductance/capacitance/etc)... so, that might be where I am missing something... though, I still thought for a given brushless motor, it will increase torque output when voltage is increased (for a given current draw, within the power rating of the motor), not just top speed.

I'm trying to understand if I can size down the BMS/controller/wire current ratings (and size) a bit if I am able to supply more voltage on the battery/controller side and get the same-ish performance or if I am off here. Thanks.

 

[ns-esk8]

Thanks for the reply. I know the relationship is not the same (not ohms law, more motor specific variables to consider), but wouldn't increasing the voltage also decrease the motor/phase current needed? (on a given motor for a given power output, etc)

I don't have a great grasp of how an ESC works beyond the basics, but I thought increasing the voltage supply (EG: 96v) increases back EMF (which I don't fully understand) and that would reduce the phase amps needed (to some degree) to reach the same level of torque when compared to the same setup supplied with a lower voltage (EG: 48v/72v/etc).

 

[amberwolf]

BEMF is the voltage generated by the motor as it spins. (this happens whether or not you are powering it to make it spin). This voltage counters some of the incoming voltage from teh controller and lowers the phase current.

This is why spinning a particular motor faster for the same load draws less phase current, such as going full speed up a hill vs crawling up the same hill, the latter of which can overheat the motor for that reason.

 

[ns-esk8]

thanks, that clarification is helpful and does help start to make more sense of things. so, does that mean that starting torque (from 0mph) on a given motor should use roughly the same amount of phase amps whether it is powered by 96v or 48v?

(very) anecdotally, I've had skateboards, etc, that I bumped up the voltage on that felt more torquey/responsive off the line (same motor/ESC)... is that in my head or what might account for that? (I think same battery/phase amps were set in at least one of those examples, but not 100% sure at this point, tbh.)

 

[amberwolf]

BEMF isn't really relevant to off the line (from a stop) torque, since the motor isn't spinning at the start of that, and not usually spinning fast enough to make much BEMF until you get at least partway to max speed.

Phase amps are controlled by the controller software or hardware design, more than the voltage or motor characteristics (though those affect it--by how much depends on the other system limitations; like if the system was power-limited because of insufficient battery watts). Controllers are power converters---they change voltage and current at one side into a different form/amount of voltage and current on the other, but total watts remain the same (minus losses in the conversion process).

When you have a higher battery voltage (either from raising it directly, or using a better battery with less voltage sag), then for the same battery current you then have more power available. If the controller is already setup to use that power, but couldn't get it from the previous battery / voltage, you'll get better motor performance when that power is demanded by the load once you have a battery that can supply that power.

Higher battery voltage gives faster motor speed, all other conditions the same.

Depending on the motor resistance / etc., and the controller's ability to supply phase current, higher voltage can induce higher phase currents, which could create higher torque.

Motor power is created by taking the steady DC battery voltage and current, and chopping it up into small timeslices to feed across the phases in pairs. How the controller does this changes how the motor is affected by a change in system voltage. Some controllers only modulate the average voltage across the motor to control the current, and those generally don't actually measure the motor current, only battery current, and calculate the rest. Better controllers measure the motor current, and control the current itself (by modulating the voltage while measuring the resulting current). So the former type of controller is more likely to be affected by a system voltage change given no other system changes.


If you check out the motor simulator at ebikes.ca, you can learn the relationships between the different parts of the system, playing with changing different parts of a setup to see how the results change. It doesn't directly show you phase current, but it shows motor power vs battery power to see the losses in a system when under the wrong conditions for that system.