Torque topic

[Slartibartfast]

1 minute ago, techyiam said:

Would you agree that a greater force applied directly on the axle would accelerate the wheel quicker?

I will agree that more torque would accelerate the wheel more quickly.

[Paul A]

22 minutes ago, Slartibartfast said:

The main issue I have is how people are insistent of the fact that "bigger is slower" but if you believe that than you must also believe that "smaller is quicker", and that's the part that gets me.

The video explains why a smaller wheel has greater acceleration.

 

23 minutes ago, Slartibartfast said:

If you could just make smaller and smaller wheels and get more and more acceleration accordingly than why even make wheels as large as they are now?

Acceleration is not the one and only desired outcome.

Larger wheels have benefits.

[Slartibartfast]

3 minutes ago, Paul A said:

Acceleration is not the one and only desired outcome.

Larger wheels have benefits.

Sure, but I take it you are of the belief that the smaller a wheel gets the faster acceleration will be, yeah? This is the part that is the sticking point for me.

 

Edited October 30, 2022 by Slartibartfast

[Paul A]

2 minutes ago, Slartibartfast said:

Sure, but I take it you are of the belief that the smaller a wheel gets the faster acceleration will be, yeah? This is the part that is the sticking point for me.

 

The video does a great explanation of why a smaller wheel has greater acceleration.

 

 

[techyiam]

6 minutes ago, Paul A said:

The video explains why a smaller wheel has greater acceleration.

You right, it does, since rolling friction is equivalent to having gears in contact. In rolling friction, there is no relative movement at the contact point.

[techyiam]

22 minutes ago, Slartibartfast said:

I will agree that more torque would accelerate the wheel more quickly.

OK.

For a minute, forget about torque at the wheels driving a car.

Consider the case where a car is in neutral, and a force is pushing against the back of the car. Would you agree that if the force is greater, then the car would accelerate quicker?

[Slartibartfast]

5 minutes ago, techyiam said:

You right, it does, since rolling friction is equivalent to having gears in contact. In rolling friction, there is no relative movement at the contact point.

Can this difference in wheel size be compensated for with different gearing?

1 minute ago, techyiam said:

Consider the case where a car is in neutral, and a force is pushing against the back of the car. Would you agree that if the force is greater, then the car would accelerate quicker?

Yes, this I would agree with.

[techyiam]

2 minutes ago, Slartibartfast said:

Can this difference in wheel size be compensated for with different gearing?

Absolutely. You just have to use lower gearing if you use a larger diameter wheel.

 

[Paul A]

A practical experiment with a bicycle might help.

 

When stationary, it is going to be easier for a person to accelerate faster using the smaller crank wheel, than with a large crank wheel.

 

[Slartibartfast]

9 minutes ago, Paul A said:

A practical experiment with a bicycle might help.

Yeah, I know.

I'm just saying it's not the "wheel size" that is the issue in the "Cybertruck" example above, it's more a matter of gearing. Basically it seems one should be able to achieve the same advantages of smaller wheels but simply gearing the drive-train to a higher ratio, yeah?

 

It must there for follow that or those of the belief that acceleration is bound to wheel size then the smaller the wheel gets the faster the acceleration becomes and consequently that the same performance can be achieved with higher and higher gearing, no?

 

I know EUCs' don't have gearing ratios we can mess around with, I'm just saying expecting an EUC to accelerate faster and faster when you fit smaller and smaller wheels is equivalent to expecting a car to gain more and more performance by simply gearing the drive train higher and higher.

[Slartibartfast]

When considering questions about force/work/power I always find it useful to think of things in relation to raising a bucket from a well:

If we consider the bucket, filled with water, to be at the bottom of a 10m well and that filled bucket to be applying a force of 100N (about 10kg) to a rope, then it would require 1000Nm of work to lift the bucket to the surface. This can be related to the work an EUC does to either pull a rider up an incline or even just to overcome resistance at speed.

As mentioned before power is a measure of how much work can be done in a given amount of time, and is expressed as Watts and is equivalent to 1 Nm/s. In this instance the amount of power governs how quickly the bucket can be raised to the surface. As stated before lifting the bucket requires 1000 Nm of work, and lifting that bucket in 10 seconds will require 100 Watts of power.

If the rope were attached to the wheel such that turning the wheel will pull the rope one can lift the bucket by rotating the wheel.

If this rope were attached to a 20" wheel it will require a given amount of "effort" to turn the wheel 1°. If the rope were instead attached to a 10" wheel it will require less effort to turn the wheel but it will need to be turned twice as far to lift the bucket by the same amount. My point being that it will take the same amount of work to turn the 20" wheel 1° as it will take to turn the 10" wheel 2°.

The question is, would it be faster to turn the 10" wheel 2° than it would be to turn the 20" wheel 1°. I'm assuming that most in this discussion here would assert that turning the 10" wheel 2° would be done more quickly. However, when I see this scenario I see both doing the same work and there for requiring the same amount of power to do that work in the same amount of time. Can anyone tell me why this wouldn't be the case?

 

Edited October 30, 2022 by Slartibartfast

[NSFW]

11 hours ago, Planemo said:

Dynos only ever measure torque and then calculate the power, depending on how quickly the load they apply is accelerated.

Some dynos do that. Others measure the time it takes the car to accelerate the rollers (whose moment of inertia is known) from one RPM to another, and then calculate the power required to achieve that change over that time. 

...all of which are just distinctions with a difference. Fundamentally, if you can use arithmetic to calculate X from other parameters, then measuring those other parameters and doing arithmetic is equivalent to measuring X. 

[NSFW]

40 minutes ago, Slartibartfast said:

The question is, would it be faster to turn the 10" wheel 2° that it would be to turn the 20" wheel 1°. I'm assuming that most in this discussion here would assert that turning the 10" wheel 2° would be done more quickly. However, when I see this scenario I see both doing the same work and there for requiring the same amount of power to do that work in the same amount of time. Can anyone tell me why this wouldn't be the case? 

Bravo.

In this thread a lot of people are getting torque and power mixed up. Hopefully that analogy clarifies it for them.

Torque a is much less interesting parameter than most people think. For most performance questions that come up in the real world, the answer depends on power, not torque. Like I mentioned earlier, there's a reason that drag race calculators have places to type in the power and weight, but not torque or RPM.

[Slartibartfast]

11 hours ago, NSFW said:

Bravo.

Hehe, why thank you @NSFW. I'm glad at least you appreciate my efforts: 

Must say I've been feeling rather "unpopular" in this thread otherwise: 😅

[Planemo]

14 hours ago, Slartibartfast said:

I know EUCs' don't have gearing ratios we can mess around with

And thats exactly the problem.

14 hours ago, Slartibartfast said:

When considering questions about force/work/power I always find it useful to think of things in relation to raising a bucket from a well:

The question is, would it be faster to turn the 10" wheel 2° than it would be to turn the 20" wheel 1°. I'm assuming that most in this discussion here would assert that turning the 10" wheel 2° would be done more quickly. However, when I see this scenario I see both doing the same work and there for requiring the same amount of power to do that work in the same amount of time. Can anyone tell me why this wouldn't be the case?

Your bucket and well example is good. I like it. But you're making the assumption that whatever power source you're using (lets say a BLDC motor) to lift the bucket has the same efficiency at the rpm it's using for the 20" 'wheel' as the 10" wheel.

The best efficiency (which is also usually the max power point) of a BLDC motor is around 50% of it's max rpm speed. Whatever load you apply to that wheel that keeps the motor from operating further from that 50% point will reduce the power it can apply. And I do see where you're coming from with your example but again you're making the assumption that the motor will be just as happy at half the rpm when using the 20" wheel. It won't. Not unless it has a power graph which is flatlined throughout it's RPM band (it won't).

This is why it's easier to overlean an EUC at very low speeds than higher speeds. Hell, many people can overlean a Sherman by ankle flicking at a standstill. I've done it myself. Try doing it at half it's max speed (say around 20~23mph) and it will take a LOT more effort. All because the motor is producing it's highest power at 50% rpm, not at 1% rpm. The further you take the motor away from it's 'goldilocks' rpm, the less power it will produce.

In just the same way that using a 20" 'wheel' on your BLDC motor for your bucket experiment will take the motor further away from it's goldilocks zone than the 10" one, using a wheel diameter thats twice the size on an EUC will also do the same thing.

Of course, if we added gearing to your bucket example to provide the same operating rpm for both the 10" and 20" options then yes the total time would be the same (not accounting for gearing losses). But as I say, we can't do that for EUC's.

13 hours ago, NSFW said:

Some dynos do that. Others measure the time it takes the car to accelerate the rollers (whose moment of inertia is known) from one RPM to another, and then calculate the power required to achieve that change over that time. 

Well yes, if you're referring to something like a Dynojet which is nothing more than a heavy roller. Regularly referred to as 'grossly inadequate' in the professional tuning world as they can't hold a constant load over a decent enough amount of time to allow proper mapping. You can of course do a power run for every single data point in your map but thats an awful lot of runs...

13 hours ago, NSFW said:

...all of which are just distinctions with a difference. Fundamentally, if you can use arithmetic to calculate X from other parameters, then measuring those other parameters and doing arithmetic is equivalent to measuring X. 

Agreed.

Edited October 31, 2022 by Planemo

[Chriull]

3 hours ago, Planemo said:

The best efficiency (which is also usually the max power point) of a BLDC motor is around 50% of it's max rpm speed.

No. Simplified one could say, the higher the rpm, the better the efficiency. Almost up to max speed. But burden (torque) makes this an "3d" plot...

from https://www.tytorobotics.com/blogs/articles/brushless-motor-power-and-efficiency-analysis

However, disregarding magnetic and mechanical losses at maximum power output, which is at 50% maximum rpm one has an efficiency of exactly 50%. 

3 hours ago, Planemo said:

Whatever load you apply to that wheel that keeps the motor from operating further from that 50% point will reduce the power it can apply. And I do see where you're coming from with your example but again you're making the assumption that the motor will be just as happy at half the rpm when using the 20" wheel. It won't. Not unless it has a power graph which is flatlined throughout it's RPM band (it won't).

Maybe one should distinguish between motor output power and motor input power?

3 hours ago, Planemo said:

This is why it's easier to overlean an EUC at very low speeds than higher speeds. Hell, many people can overlean a Sherman by ankle flicking at a standstill.

Bldc are extremely inefficient at low speeds and especially standstill. There is much limiting and other special handlings in firmware so electronics and wires can survive.

So this special case is not really great to show general bldc properties.

Ankle flicking reactions could show a problem in the "power delivery design" too and be not really related to some "simple bldc mechanics"

3 hours ago, Planemo said:

I've done it myself. Try doing it at half it's max speed (say around 20~23mph) and it will take a LOT more effort. All because the motor is producing it's highest power at 50% rpm, not at 1% rpm. The further you take the motor away from it's 'goldilocks' rpm, the less power it will produce.

Power is has no correleation with ease of overlean, besides that one has to accelerate harder and/or drive faster to reach the overlean limit.

One will overlean "harder/faster" with a more powerfull wheel, as maximum torque at some speed is generally higher. And overlean happens by applying more torque to the wheel as it can provide.

But other than this overleaning needs "the same effort" at each speed. 

If one does not regard the firmware limits at lower speeds one could argue that overleaning is easier at higher speeds as maximim available torque becomes lower and adfitionally "general burden" as air drag gets higher. So less "effort" (leaning) is needed ...

3 hours ago, Planemo said:

In just the same way that using a 20" 'wheel' on your BLDC motor for your bucket experiment will take the motor further away from it's goldilocks zone than the 10" one, using a wheel diameter thats twice the size on an EUC will also do the same thing.

As seen in the link above one main influence for efficiency is burden ~ torque ~ motor current.

For the same force  the 20" wheel would need twice the motor current and by this create four times the motor losses (electric motor losses are mainly coil resistance time the square of motor current)

As written above for such examples in general it makes more sense to regard output power.

Regarding input power with an bldc gives no "intuitive" outcome to learn from this example as there is no simple relation between radius and power anymore.

3 hours ago, Planemo said:

Of course, if we added gearing to your bucket example to provide the same operating rpm for both the 10" and 20" options then yes the total time would be the same (not accounting for gearing losses). But as I say, we can't do that for EUC's.

Don't know if that's a goal in this topic, but to arrive to an answer/solution one would need to define the question/general conditions first

Edited October 31, 2022 by Chriull

[Planemo]

9 minutes ago, Chriull said:

No. Simplified one could say, the higher the rpm, the better the efficiency. Almost up to max speed. But burden (torque) makes this an "3d" plot...

The point I am trying to make is that a motor has a 'sweet spot'. A 'goldilocks zone'. A 'maximum power output RPM'. Whatever you want to call it. Move away from that zone and you are reducing power. It's that simple.

Here is a graph which simplifies things I feel:

 

9 minutes ago, Chriull said:

Maybe one should distinguish between motor output power and motor input power?

We don't need to, it just complicates things. Lets stick with the bucket and well example, 1x 10" drive wheel, 1x 20" drive wheel and assume that the available current, voltage and motor are the same.

9 minutes ago, Chriull said:

Bldc are extremely inefficient at low speeds and especially standstill.

Indeed. And the slower you make them turn the more inefficient (and less powerful) they become.

9 minutes ago, Chriull said:

There is much limiting and other special handlings in firmware so electronics and wires can survive.

We're trying to avoid involving firmware/wiring size etc to keep it simple.

9 minutes ago, Chriull said:

For the same force  the 20" wheel would need twice the motor current and by this create four times the motor losses (electric motor losses are mainly coil resistance time the square of motor current)

In that case I think you're agreeing with me. Eg if you don't have twice the motor current available then your bigger wheel will be struggling (and therefore slower).

9 minutes ago, Chriull said:

Don't know if that's a goal in this topic

The goal in this topic is take Slarti's bucket and well example because that's something he can relate to, as can I.

The wheel at the top has a motor. The motor, available voltage and current are the same for all iterations.

Will the bigger 20" wheel take longer to pull the bucket up than the 10" wheel or not?

[Planemo]

17 hours ago, Slartibartfast said:

I know EUCs' don't have gearing ratios we can mess around with, I'm just saying expecting an EUC to accelerate faster and faster when you fit smaller and smaller wheels is equivalent to expecting a car to gain more and more performance acceleration by simply gearing the drive train higher and higher.

I missed this one.

Exactly right, yes. Thats why a car accelerates quicker in 1st gear than in 4th. The gearing allows the engine to reach a decent operating RPM far quicker than sticking it in 4th.

We don't get to gear EUC's, so, to give you an analogy, it's like we are left in 4th. Thankfully the motors are strong enough to do this without gearing but the physics of accelerating a load with a power source that produces max power at a certain RPM remain.

I have corrected your use of the word 'performance' as although I knew what you meant its not really the correct term for the basis of our discussion.

[Chriull]

14 minutes ago, Planemo said:

The goal in this topic is take Slarti's bucket and well example because that's something he can relate to, as can I.

The wheel at the top has a motor. The motor, available voltage and current are the same for all iterations.

Will the bigger 20" wheel take longer to pull the bucket up than the 10" wheel or not?

In 

17 hours ago, Slartibartfast said:

If the rope were attached to the wheel such that turning the wheel will pull the rope one can lift the bucket by rotating the wheel.

If this rope were attached to a 20" wheel it will require a given amount of "effort" to turn the wheel 1°. If the rope were instead attached to a 10" wheel it will require less effort to turn the wheel but it will need to be turned twice as far to lift the bucket by the same amount. My point being that it will take the same amount of work to turn the 20" wheel 1° as it will take to turn the 10" wheel 2°.

The question is, would it be faster to turn the 10" wheel 2° than it would be to turn the 20" wheel 1°. I'm assuming that most in this discussion here would assert that turning the 10" wheel 2° would be done more quickly. However, when I see this scenario I see both doing the same work and there for requiring the same amount of power to do that work in the same amount of time. Can anyone tell me why this wouldn't be the case?

@Slartibartfast always uses the expression "effort to turn the wheel" which i would translate only to output power.

As it's the same work(energy) needed to get the bucket up some height it's the same power needed to get the bucket up some height in some time.

Wheel radius, rope or whatever else in involved in this example are irrevelevant...

... unless we start to ask for the used battery energy/power driving an bldc motor lifting some weight some height (in some time)

As written before this is no simple, intuitive linear proportional relation anymore.

With your (very different) question:

26 minutes ago, Planemo said:

The wheel at the top has a motor. The motor, available voltage and current are the same for all iterations.

The same motor (with spokes to reach twice the radius) will generate the same torque with the same current in both cases.

So without the spokes (10") it will apply two times the force on the rope as the 20" construction. So one has "uniformly accelerated movement" F=m*a.

With twice the force the smaller wheel will bring the bucket up faster. If the bldc can spin fast enough and does not reach it's max torque over speed limit inbetween.

Times and their relation should be easy to be calculated with formula sets for uniform acceleration.

Used electrical power/energy should be some more effort ... If they are of interest, anyhow...

[Planemo]

30 minutes ago, Chriull said:

The same motor (with spokes to reach twice the radius) will generate the same torque with the same current in both cases.

OK...

30 minutes ago, Chriull said:

So without the spokes (10") it will apply two times the force on the rope as the 20" construction.

OK...

30 minutes ago, Chriull said:

With twice the force the smaller wheel will bring the bucket up faster.

So if we are agreeing that we are generating the same torque with the same current with the same load AND also agree it's coming up faster then we are doing the same amount of work in a shorter time. For this to occur there has to be an increase in power (work done over time) with the smaller wheel.

Whether we apply the circumstances to the bucket and well, the RC car with the bigger vs smaller wheels, the ebike with a hub motor, the bicycle Vs the Penny Farthing or lord knows how many similar examples, all else being the same, a larger wheel will accelerate a load slower than a smaller wheel.

If indeed this isn't the case I will consider myself a total failure because either I haven't been able to explain myself or I have failed to grasp what Slarti is saying. Either way, I call that a fail.

*goes off to sit in misery and despair*

[RagingGrandpa]

2 hours ago, Planemo said:

I like diagrams  

The one above could be improved by acknowledging controller current limits, which are a big part of the story.

Here's my favorite example:

The Blue and Green lines are most relevant.
The flat sections of the torque curve are where we operate EUC's most of the time. So, putting the motor into a higher speed range by using a smaller tire, in this "flat" part of the speed range, implies no change in torque limits.

In this example graph, compare a large tire that does 20mph at 3000RPM, to a small tire that does 20mph at 5000RPM.
The result is: the small tire option has the same torque limit (90, in this graph), and a higher max acceleration (due to tire radius). 
(And also a higher power limit... but power is just a distraction if we're interested in acceleration.)
 

Efficiency is interesting sometimes too, but in general it's "good" for these BLDC motors: >80% everywhere.
And efficiency has a very minor impact on output force, when considering gear ratios, tire sizes, etc.

[mrelwood]

 

On 10/30/2022 at 5:57 PM, Slartibartfast said:

So really, when the article is talking about swapping out 15" rims for 19" all it's really talking about is changing the drive ratios, right?

Yes. Which isn't very practical to do in a car. Tire change otoh is a very common modification.

On 10/30/2022 at 5:57 PM, Slartibartfast said:

I'm also curious as to how the 460:360 Newtons is calculated. By my thinking 15:19 is more 460:390.

It might have something to do with the tire heights and aspect ratios for each rim size. 15” and 19” are only rim diameters.

 

On 10/30/2022 at 6:22 PM, Slartibartfast said:

Yes, I grant that a smaller wheel will produce more force at it's circumference (this I have now said several times myself). The thing I am not convinced of is that this extra force necessarily translates into greater acceleration.

Acceleration = force / mass. That should be quite clear.

 

On 10/30/2022 at 6:22 PM, Slartibartfast said:

If you could just make smaller and smaller wheels and get more and more acceleration accordingly than why even make wheels as large as they are now?

Because there is no need for more acceleration. Overleans during acceleration are mostly a thing of the past already, so we don’t need to have a higher maximum acceleration from our wheels.

Again I want to bring up the reason this whole thread started. It wasn’t about the maximum acceleration the wheel can provide. It was about the effort it takes to accelerate.

 

On 10/30/2022 at 7:35 PM, Slartibartfast said:

It must there for follow that or those of the belief that acceleration is bound to wheel size then the smaller the wheel gets the faster the acceleration becomes and consequently that the same performance can be achieved with higher and higher gearing, no?

Yes. But if we are back to talking about EUCs, here’s a conundrum:

 Rockwheel used to make an EUC model GR16 with a geared motor that allowed the rotator to spin faster than on “direct drive” EUCs. Everything else being equal, which of the following applied:

A: They accelerated faster.

B: They were easier to accelerate fast.

Edited October 31, 2022 by mrelwood

[Slartibartfast]

Hehe, seems we have gone from "RPM has nothing to do with it" to "it is nothing but RPM": 

 

On 10/31/2022 at 10:27 PM, Planemo said:

...Thats why a car accelerates quicker in 1st gear than in 4th. The gearing allows the engine to reach a decent operating RPM far quicker than sticking it in 4th.

We don't get to gear EUC's, so, to give you an analogy, it's like we are left in 4th. Thankfully the motors are strong enough to do this without gearing but the physics of accelerating a load with a power source that produces max power at a certain RPM remain.

I had thought the reason car's require gearing is because internal combustion engines have a very narrow range where they provide workable power/torque so one needs to continually keep shifting gears to stay in this "sweet spot" as it were. For electric motors I was of the understanding that this "sweet spot" was so broad that one was largely immune from the need to change gears and I had thought this "famously flat" torque curve meant that it didn't much matter where in the rev range the motor operated.

It appears, however, you're saying these motors are far more sensitive to operating speed than I had previously understood. I know typically electric cars will have a fixed reduction between the motors and wheels to move this very broad curve higher up the RPM range where as our hub motors have no reduction what so ever and what you're saying is that a gearing reduction would enable the motor to operate in a more favorable range (at least for low speed acceleration).

So at the end of all this we have converged on the understanding that the apparent "zippyness" of different sized wheels is not related to torque as such but is related to the RPM range at which the motor most happily operates. Is that right?

Edited November 3, 2022 by Slartibartfast

[Tawpie]

1 hour ago, Slartibartfast said:

So at the end of all this we have converged on the understanding that the apparent "zippyness" of different sized wheels is not related to torque as such but is related to the RPM range at which the motor most happily operates. Is that right?

I didn't get there... at least not for an electric motor. Internal combustion, absolutely but not electric.

[NSFW]

If you think that torque translates to "zippiness" consider that you can generate 1000 ft-lb of torque all by yourself, for hours, without breaking a sweat.

No fuel or batteries required,

Just sit on an appropriately long lever. And remain seated.

How zippy is that?

Power matters. Torque is a distraction. It's a useful ingredient for power, but a torque figure all by itself doesn't really tell you much.