torque versus high speed motor trade-off

[Jurgen]

Le's say you have 2 different motors both 800W (1000W peak),

but one is configured for torque going max 20KmH (steeper torque/speed curve), the other one for speed going max 30KmH (less steep torque/speed curve):

what would be the difference in behaviour of  the wheel, any other trade-offs?

I guess the one for torque will keep a heavy person more stable, especially during a bump at higher speed, and when it heats up after a longer working period it will still be able to keep it's max speed (all this should be the opposite for a speed biased wheel), or am I missing something?

[Cranium]

With regards to EU's, this isn't really relevant mechanically speaking because they aren't geared.  They are both related to each other as the motor's specs.  Just like batteries, electric motors have a current rating (except in the opposite direction) that will specify the how much constant current and burst current the motor can draw.  At these limits, the motor will produce torque that will cause speed changes as seen in the graph below.  

If you are at the stall torque (max current), then you have no speed.  This could be if you are just staying still on a steep incline.  The motor would have to work very hard just to keep you in place.  If the hill were steeper, it would not be able to.

Max speed can be obtained at the same max current but will depend on how much resistance it will have.  You can't have zero torque because this would mean there is no friction.  So you have to take into account the roughness riding surface, weight of the rider, and angle of the surface.  As you increase any of these three variables, you will decrease your max speed.  But if you decrease the variables, max speed will increase.  For example, if you ride the EU downhill with a very light rider, it can go much faster!

But speed is also controlled by the mainboard to limit it.  So regardless of any other factor, the programming won't let it go faster than it is allowed as a safety feature. 

This link shows some good information on motor speed: http://lancet.mit.edu/motors/motors3.html

 

 

[Jurgen]

I've clarified the Q a bit more:

I'm not referring to the relationship between torque and speed, but about a steeper torque/speed curve versus a less steep torque/speed curve.

I guess the one biased for torque (steep curve) will keep a heavy person more stable, especially during a bump at higher speed, and when it heats up after a longer working period it will still be able to keep it's max speed (all this should be the opposite for a speed biased wheel), or am I missing something?

[dmethvin]

So in a modified graph I think this is what you're saying.  In this case the white line has more torque but a lower top speed. The blue line has less torque but a higher top speed. I have added a red line that represents the least amount of torque the EUC should reserve for safety, which means the actual top speed would be governed by the point where the red line meets the other lines. I probably haven't drawn the two lines to scale though, ideally they should have the same amount of area under the line. 

The only EUC maker I know who offers different torque models is Gotway.

 

I should also mention that this has been discussed a few times, here's an older thread: http://forum.electricunicycle.org/topic/264-speed-vs-torque-margin-of-safety/

One thing discussed there that messes up this theoretical curve is that the batteries have internal resistance, so that the voltage sags when high current is drawn. You can compensate by reserving even more torque and operating further to the left of the graph, but that moves the red line up even further and reduces the top speed. A larger battery with more parallel cells reduces the internal resistance.

[Cranium]

3 hours ago, Jurgen said:

I've clarified the Q a bit more:

I'm not referring to the relationship between torque and speed, but about a steeper torque/speed curve versus a less steep torque/speed curve.

I guess the one biased for torque (steep curve) will keep a heavy person more stable, especially during a bump at higher speed, and when it heats up after a longer working period it will still be able to keep it's max speed (all this should be the opposite for a speed biased wheel), or am I missing something?

Ok...sorry, I misunderstood then.  

The difference between a motor designed for torque vs a motor designed for speed could be just be the magnet size (for motors of the same size).  A motor with larger magnets would be able to produce more torque but would also be heavier than a motor with smaller magnets designed for speed.

You are right in that a motor with more torque would be able to keep a heavy person more stable but it would also require more current as well (less efficient).  And current produces heat.  So it would have to be a heavier EU with a larger battery and better heat dissipation than one designed for just speed if you want the same range and speed.  Or you can get higher torque and give up some speed and range to maintain the same size battery but you would still have to deal with the higher heat in high torque situations.  

 

[Jurgen]

6 hours ago, dmethvin said:

So in a modified graph I think this is what you're saying.  In this case the white line has more torque but a lower top speed. The blue line has less torque but a higher top speed. I have added a red line that represents the least amount of torque the EUC should reserve for safety, which means the actual top speed would be governed by the point where the red line meets the other lines. I probably haven't drawn the two lines to scale though, ideally they should have the same amount of area under the line. 

The only EUC maker I know who offers different torque models is Gotway.

 

I should also mention that this has been discussed a few times, here's an older thread: http://forum.electricunicycle.org/topic/264-speed-vs-torque-margin-of-safety/

One thing discussed there that messes up this theoretical curve is that the batteries have internal resistance, so that the voltage sags when high current is drawn. You can compensate by reserving even more torque and operating further to the left of the graph, but that moves the red line up even further and reduces the top speed. A larger battery with more parallel cells reduces the internal resistance.

6 hours ago, dmethvin said:

So in a modified graph I think this is what you're saying.  In this case the white line has more torque but a lower top speed. The blue line has less torque but a higher top speed. I have added a red line that represents the least amount of torque the EUC should reserve for safety, which means the actual top speed would be governed by the point where the red line meets the other lines. I probably haven't drawn the two lines to scale though, ideally they should have the same amount of area under the line. 

The only EUC maker I know who offers different torque models is Gotway.

 

I should also mention that this has been discussed a few times, here's an older thread: http://forum.electricunicycle.org/topic/264-speed-vs-torque-margin-of-safety/

One thing discussed there that messes up this theoretical curve is that the batteries have internal resistance, so that the voltage sags when high current is drawn. You can compensate by reserving even more torque and operating further to the left of the graph, but that moves the red line up even further and reduces the top speed. A larger battery with more parallel cells reduces the internal resistance.

Sorry, couldn't find it under 'General'.

Hower, the older thread leaves some questions unanswered.

 

I read a lot on the forum about the BMS cutting in, but there are also people taking a face plant for less clear reasons (high speed, steep incline,…)
Maybe there’s more, and it could be related to the torque speed curve, and manufacturers going for faster wheels all the time.

With the high speed wheels it is not clear whether they’ve gone for a quick fix: just modified the software to do away with the speed limiter and upgraded the battery without any changes to the motor.

Some questions that remain unanswered.
A. Torque safety margin: I guess the EUC doesn’t factor in a certain safety ‘margin’, it’s just preset?
B. Weight of the rider: I’m pretty sure the wheel doesn’t measure the weight on the pedals.
C. Temperature: Does it heat up more in high torque situations like Cranium says? I doubt if there’s a temp sensor in the EUC. If I'm not mistaken, when the temp goes up the curve becomes less steep, so torque goes down when the EUC heats up.

A+B+C: at 30KmH with a 100kg rider (like me) on a hot wheel, and a bump in the road (or a pit going downhill), how does this end (figurative way of speech)?

Instead of just locking the max speed of the wheel in the software to a preset level, an app that allows for setting the weight of the rider, and consequently limits speed within a safe torque range, cannot be that difficult?

A higher battery capacity that reserves some of the cells (alternating) to allow for modulation of the current to keep both the torque and the batteries in the safe zone for extremer situations could be in interesting next step, together with a temp sensor in the EUC and detection of damaged cells.

 

 

[Jurgen]

6 hours ago, Cranium said:

Ok...sorry, I misunderstood then.  

The difference between a motor designed for torque vs a motor designed for speed could be just be the magnet size (for motors of the same size).  A motor with larger magnets would be able to produce more torque but would also be heavier than a motor with smaller magnets designed for speed.

You are right in that a motor with more torque would be able to keep a heavy person more stable but it would also require more current as well (less efficient).  And current produces heat.  So it would have to be a heavier EU with a larger battery and better heat dissipation than one designed for just speed if you want the same range and speed.  Or you can get higher torque and give up some speed and range to maintain the same size battery but you would still have to deal with the higher heat in high torque situations.  

 

Thanks for pointing out the temp aspect, it's something that is easily overlooked (see my reply to dmethvin)

[Cranium]

54 minutes ago, Jurgen said:

Instead of just locking the max speed of the wheel in the software to a preset level, an app that allows for setting the weight of the rider, and consequently limits speed within a safe torque range, cannot be that difficult?

I think you are overthinking what the EU control board can do.

The EU controller board is able to measure a few things in real time with regards to this discussion: Current, Voltage, Temperature, & Speed.  

The max speed is set in the software.  The EU could indeed go faster based on the known limitations (with safety factors) for Current, Voltage and Temperature.  Weight doesn't need to be taken into account because is has a direct affect on Current and Temperature and these are already monitored.  Torque is also a spec of the motor for a given amount of current & speed so we don't have to take that into account either.

The problem with doing this is that every test would be different and the published numbers of what it can do would also be different.  The manufacturers would hire Horse Jockeys to test their EU's.   This would create a problem in that a test would not be repeatable without publishing the temperature, weight of the rider, and even the charge with recharge cycles of the battery.  How pissed would you be if your EU wouldn't go as fast as someone else's and you were the same weight?

The second limitation, I'm guessing, is safety and the liability the manufacturer is willing to accept.  Some of us have the need for speed and want to go as fast as possible!  Others, like me, have no business going faster than we can run.  LOL.  But at least I'm aware of my limits and won't go over them (most days).  Increasing speed increases injuries when things do go wrong whether it is from rider error, environmental factors, or a EU failure (mechanical or electrical).  

I would love it if EU manufacturers started building in redundancy of the electrical systems to reduce face plants.  This would at least allow you to safely come to a stop if an issue was detected....much like what the Segway has always done.  But this is partly why the Segway is so expensive.

 

 

 

[dmethvin]

5 hours ago, Jurgen said:

Some questions that remain unanswered.
A. Torque safety margin: I guess the EUC doesn’t factor in a certain safety ‘margin’, it’s just preset?
B. Weight of the rider: I’m pretty sure the wheel doesn’t measure the weight on the pedals.
C. Temperature: Does it heat up more in high torque situations like Cranium says? I doubt if there’s a temp sensor in the EUC. If I'm not mistaken, when the temp goes up the curve becomes less steep, so torque goes down when the EUC heats up.

A+B+C: at 30KmH with a 100kg rider (like me) on a hot wheel, and a bump in the road (or a pit going downhill), how does this end (figurative way of speech)?

Instead of just locking the max speed of the wheel in the software to a preset level, an app that allows for setting the weight of the rider, and consequently limits speed within a safe torque range, cannot be that difficult?

A higher battery capacity that reserves some of the cells (alternating) to allow for modulation of the current to keep both the torque and the batteries in the safe zone for extremer situations could be in interesting next step, together with a temp sensor in the EUC and detection of damaged cells.

If the EUC knew the rider's weight and other things like temperature and internal battery resistance it might be possible to do a better job of determining an appropriate safety margin. Still, it also depends on the terrain. You can imagine going near the max safe speed on flat ground and then starting to go up a steep hill. The EUC needs more torque to climb the hill but is currently at a speed where there is no more torque to be had and therefore no more torque to tilt back or possibly even to keep the pedals level!

The basic problem is this: Once you're traveling at some particular speed, there's no way for the EUC to slow you down without you wanting to slow down. All the stuff with pedal tilt-back is supposed to be a progressively bigger warning that you are soon gonna have a bad time. If the EUC starts going slower without you leaning back, you'll end up falling forward. I know, I've done it a couple of times near max speed and it's not fun!

 

[Jurgen]

7 hours ago, dmethvin said:

If the EUC knew the rider's weight and other things like temperature and internal battery resistance it might be possible to do a better job of determining an appropriate safety margin. Still, it also depends on the terrain. You can imagine going near the max safe speed on flat ground and then starting to go up a steep hill. The EUC needs more torque to climb the hill but is currently at a speed where there is no more torque to be had and therefore no more torque to tilt back or possibly even to keep the pedals level!

The basic problem is this: Once you're traveling at some particular speed, there's no way for the EUC to slow you down without you wanting to slow down. All the stuff with pedal tilt-back is supposed to be a progressively bigger warning that you are soon gonna have a bad time. If the EUC starts going slower without you leaning back, you'll end up falling forward. I know, I've done it a couple of times near max speed and it's not fun!

 

You're right of course about the slowing down of the device. All the more reason to keep a high enough safety margin for having enough torque I guess.

It seems to me that if you want to remain safe, you should factor in an even higher safety margin for torque  when going faster, which is the opposite of what happens if the developer goes for a quick (software) fix on a standard combi batt/motor (instead of upgrading both).

I currently have a LHOTZ 340Kw (older version limited to 20KmH), which copes adequately with my weight (100kg) and riding style.

After the discussion here, I’m not entirely sure whether I should go for the newer version that can up to 30KmH (with the same bat/mot combi?), or look for another make/brand of EUC.

Any suggestions?

 

 

 

[Villac]

@Jurgen , it seems to me that the choice highly depends on what you want to do. If you plan lots of hills, get high torque. If you want max speed/flat course, get a model with high speed allowance. I think the terrain roughness will guide you on the wheel size.

I think the LHOTZ is a great model: big wheel, moderate battery size, wonderful acceleration, wonderful braking. Mine is newer than yours and unlocked to 30 km/h, and I do appreciate that extra speed; it's very hard to go back to below 20!

No matter what you choose, I recommend that you get the biggest battery option for the brand/model/size you can afford.

When I migrated to the Gotway Msuper2 MS 850, I agonized between Middle speed and High speed version. there is only a top speed difference of 6 km/h, but with all the hills in my course I know I was going to appreciate that extra hill climbing ability of the MS. In actual use, I don't approach the top speed but I dig into the torque all the time, especially when I'm off road.

The LHOTZ Xima 30 km/h is a great option; I didn't stay with it because I wanted the bigger 18" wheel for offroad. If you like the 16" wheel for it's maneuverability, portability, tricks, etc., you may want to stick with that size or go to 14". It's another factor that depends on your favorite ride characteristics. But noone ever wants less battery, until they are going through airport security.

[Jurgen]

3 hours ago, Villac said:

@Jurgen , it seems to me that the choice highly depends on what you want to do. If you plan lots of hills, get high torque. If you want max speed/flat course, get a model with high speed allowance. I think the terrain roughness will guide you on the wheel size.

I think the LHOTZ is a great model: big wheel, moderate battery size, wonderful acceleration, wonderful braking. Mine is newer than yours and unlocked to 30 km/h, and I do appreciate that extra speed; it's very hard to go back to below 20!

No matter what you choose, I recommend that you get the biggest battery option for the brand/model/size you can afford.

When I migrated to the Gotway Msuper2 MS 850, I agonized between Middle speed and High speed version. there is only a top speed difference of 6 km/h, but with all the hills in my course I know I was going to appreciate that extra hill climbing ability of the MS. In actual use, I don't approach the top speed but I dig into the torque all the time, especially when I'm off road.

The LHOTZ Xima 30 km/h is a great option; I didn't stay with it because I wanted the bigger 18" wheel for offroad. If you like the 16" wheel for it's maneuverability, portability, tricks, etc., you may want to stick with that size or go to 14". It's another factor that depends on your favorite ride characteristics. But noone ever wants less battery, until they are going through airport security.

thanks for your input, I really like the 16" size, for the reasons you've mentioned (no off-road) but I want >340Wh.

Looked at the King Son website for an alternative, but if there's a way to select the wheel size I didn't find it...

From reading other threads it looks like Gotway is the only company developing diffrent version high torque vs. high speed, or is the just a marketing trick, same batt/motor combi with different software?

[Villac]

9 minutes ago, Jurgen said:

thanks for your input, I really like the 16" size, for the reasons you've mentioned (no off-road) but I want >340Wh.

Looked at the King Son website for an alternative, but if there's a way to select the wheel size I didn't find it...

From reading other threads it looks like Gotway is the only company developing diffrent version high torque vs. high speed, or is the just a marketing trick, same batt/motor combi with different software?

Unless you have a contact at the company (King Song or otherwise), you may be best served working through a reseller. There are many active on this forum. I have never ridden a King Song, but they get high praise from those here who have.

The difference in Gotway MSuper types (I have learned on this forum) is due to the number of windings of wire around the stator, this changes behavior of how electricity is converted into rotational force. This review has a good description of the differences:

http://forum.electricunicycle.org/topic/50-gotway-18-inch-msuper-gw18-review-2014-12-31/#comment-142