Torque topic

[Slartibartfast]

It amazes me how frequently EUC riders manage to completely misunderstand torque: Begode Master Pro First Impression! [5:22]
No Hsiang, that is not how torque works!

Torque is a product of the motor and is not affected by the wheel's diameter.
In fact the entire idea of torque is that it is a measure of rotational force that is independent of distance.

 

[mrelwood]

5 hours ago, Slartibartfast said:

It amazes me how frequently EUC riders manage to completely misunderstand torque: Begode Master Pro First Impression! [5:22]
No Hsiang, that is not how torque works!

Torque is a product of the motor and is not affected by the wheel's diameter.
In fact the entire idea of torque is that it is a measure of rotational force that is independent of distance.

"Torque is defined as the product of the magnitude of the force and the perpendicular distance of the line of action of a force from the axis of rotation."

https://en.m.wikipedia.org/wiki/Torque

The problem for you is probably that "torque" means different things in physics and in US mechanical engineering:

"...in US mechanical engineering, where the term torque is used for the closely related "resultant moment of a couple"."

 

But, what bothers me much more and what I see much more often is that people confuse torque with zippyness, ie. the effortlessness to accelerate and brake.

[Slartibartfast]

1 hour ago, mrelwood said:

"The problem for you is probably that "torque" means different things in physics and in US mechanical engineering:

Sorry, what are you saying my problem is?
Are you saying I use the term "too correctly" and I miss what other people actually mean when they use it more loosely?

Perhaps that is the case. It really surprises me though when people say that the same motor will produce more torque if it is used on a wheel of smaller diameter. It simply doesn't.

In fact it's fair to say the very reason torque is used as a measure is to "normalise" the distance and thus eliminate distance as a variable.

Edited October 20, 2022 by Slartibartfast

[mrelwood]

40 minutes ago, Slartibartfast said:

Sorry, what are you saying my problem is?

Just a suggestion, that you might've learned a different meaning to the word "torque" than people who learned it in physics.

40 minutes ago, Slartibartfast said:

Are you saying I use the term "too correctly" and I miss what other people actually mean when they use it more loosely?

No, because on this instance there seems to be two rights instead of right and wrong.

40 minutes ago, Slartibartfast said:

It really surprises me though when people say that the same motor will produce more torque if it is used on a wheel of smaller diameter. It simply doesn't.

They are probably exactly those who explain the word torque to themselves as effortlessness of acceleration.

40 minutes ago, Slartibartfast said:

In fact it's fair to say the very reason torque is used as a measure is to "normalise" the distance and thus eliminate distance as a variable.

I do suggest you have a glimpse in the Wikipedia article I linked to though.

[Slartibartfast]

24 minutes ago, mrelwood said:

you might've learned a different meaning to the word "torque" than people who learned it in physics.

I learnt about torque in physics.

 

24 minutes ago, mrelwood said:

I do suggest you have a glimpse in the Wikipedia article I linked to though.

Yes, I have read it. (well most of it)

 

It seems your suggesting I have a different understanding of what torque is to what you have. What have I said that leads you to think this?

When I speak about torque I am meaning Newton-meters, and I am simply stating that for a given motor the number of Nm remains constant irrespective of the wheel's diameter. It's the straight up definition of what talk is (as a physicist would use it).

How does your understanding differ from what I've said?

Edited October 20, 2022 by Slartibartfast

[The Brahan Seer]

With tractive effort, effective rolling radius and a multitude of other forces acting on the wheel it is very difficult to quantify the characteristics. Other than to say that the smaller diameter wheel pulls up a hill better than a larger diameter one but this isn't due to torque or diameter alone. This is way over my head..

 

https://www.sciencedirect.com/topics/engineering/effective-rolling-radius

 

[techyiam]

7 minutes ago, The Brahan Seer said:

the smaller diameter wheel pulls up a hill better than a larger diameter

The average rider intuitively observes "perceived torque". The torque that allows the wheel to climb a hill, accelerate, or decelerate. 

When a rider leans a certain amount, the wheel has to generate a counter-torque to balance the rider.

But this counter-torque will result in a larger propulsion force at the contact patch if the wheel diameter is smaller.

And hence, riders will generally find 16" wheels accelerate better than 24" wheels for the same amount of leaning effort.

[RagingGrandpa]

11 hours ago, Slartibartfast said:

It amazes me how frequently EUC riders manage to completely misunderstand torque: Begode Master Pro First Impression! [5:22]
No Hsiang, that is not how torque works!

That's nitpicking... @Hsiang has the right idea and is pointing out a real tradeoff. 

Sure, he doesn't use words aligned to scientific conventions, but his message was correct:

• On the same surface at the same speed, a motor with a small tire requires less electrical current, and that same motor used with a larger tire requires more electrical current.
• Or, more practically (and for many additional reasons beyond the motor), small-diameter-tire EUC's will accelerate more easily than larger-diameter models. 

It's good to say these things out loud in a youtube video consumed by people that are new to EUC: an audience that doesn't have practical experience with this effect. For those of us who've personally tried both, it's obvious of course.

Master 19" C38 has a higher maximum acceleration than Master Pro 22" C38. It's true.

 

Edited October 20, 2022 by RagingGrandpa

[Eucner]

59 minutes ago, RagingGrandpa said:

• On the same surface at the same speed, a motor with a small tire requires less electrical current, and that same motor used with a larger tire requires more electrical current.

More nitpicking. Schwalbe about rolling resistance:

Quote

Tires with a smaller diameter have a higher rolling resistance with the same inflation pressure, because tire deformation is proportionally greater. The tire is flattened more and is “less round”.

Larger tire has more wind drag from larger projection area. This will even out the benefits of larger tire, but think a it still has an edge.

[Planemo]

7 hours ago, mrelwood said:

"Torque is defined as the product of the magnitude of the force and the perpendicular distance of the line of action of a force from the axis of rotation."

This.

A torque figure is obtained by combining a rotational force against a perpendicular distance. Change the distance, you change the torque figure. Theres no getting away from that.

Example: put 2" diameter wheels on an RC car and it will accelerate at a given speed. Increase the diameter of the wheels to 12" and I guarantee it will accelerate much more slowly.

Theres other forces at play in an EUC example, but stating that wheel diameter doesnt play a part is simply wrong.

[DragonFZ]

6 hours ago, Slartibartfast said:

I learnt about torque in physics.

Prepare to be schooled. See that's your mistake. A book or class on physics was written many many years ago, like last century. Unlike where I get my up to date information, YouTube and forums. I get the latest information, year to year, sometimes minute to minute. My YouTube education got me to think smart, I'm going to get a smaller Sherman tire to get more torque. boo ya

[mrelwood]

 

6 hours ago, Slartibartfast said:

It seems your suggesting I have a different understanding of what torque is to what you have. What have I said that leads you to think this?

You resigning the distance from the axle in your description of torque.

6 hours ago, Slartibartfast said:

When I speak about torque I am meaning Newton-meters, and I am simply stating that for a given motor the number of Nm remains constant irrespective of the wheel's diameter.

Then where does the m come from in your unit of torque, Nm?

6 hours ago, Slartibartfast said:

It's the straight up definition of what talk is (as a physicist would use it).

To be clear, do you agree or disagree with how Wikipedia defines torque?

[MrMonoWheel]

Idk what happened to the thread, but as long as we are doing physics stuff I have been studying/working in mechanical engineering for the last 5 years and can confirm torque directly relates to the distance from the point of rotation. 

 

[Planemo]

14 hours ago, Slartibartfast said:

In fact it's fair to say the very reason torque is used as a measure is to "normalise" the distance and thus eliminate distance as a variable.

I would suggest it might be worth a mod splitting the thread at this point, but I will answer the above, which I missed before.

I think the better word to use would be 'standardise' instead of 'normalise'. When we talk about torque, like most physics, we need a reference point. The torque reference 'Lb/ft' uses the formula of (surprisingly) 1lb of weight at the end of a 1 foot bar to denote the twisting force at the axis. 'Nm' uses the equation of one Newton at the end of a (surprisingly again) 1 metre bar to denote the twisting force.

Nm was simply introduced as a metric version and 1Nm is equivalent to 0.738Lb/ft. The important thing here is that Nm, Lb/ft or in fact any measurement of torque has to incorporate some mention of length from the axis in its formula otherwise the 'standardisation' goes out of the window, as do the results.

So, lets say we have an electric motor which makes 100Lb/ft. The manufacturer of that motor has tested the unit on a dyno to standardised protocols ie a known weight (lbs) at a known length (1 foot).

If we test that same motor but physically move the stress sensor to 2 feet away from the axis we will get a very different answer to our (measured) torque output. Roughly half I would wager.

Ergo, if we have a 1 foot diameter wheel fitted with a 100lb/ft motor that takes up the full diameter of the wheel we will indeed see (and feel) 100lb/ft being transferred to the floor. If we put the same motor in a 2 foot diameter wheel we will get half the torque and, coincidentally, half the acceleration. I accept that the motor is still putting out the same 100lb/ft but it's certainly not what we would feel (or indeed measure).

Oh look, now we're onto leverages, another physics discussion

 

Edited October 20, 2022 by Planemo

[meepmeepmayer]

(This has been split off the Sherman-S topic, around here.)

[mrelwood]

3 hours ago, Planemo said:

Ergo, if we have a 1 foot diameter wheel fitted with a 100lb/ft motor that takes up the full diameter of the wheel we will indeed see (and feel) 100lb/ft being transferred to the floor.

As always, @Planemo explained all this much better.

Though if I'm allowed to nitpick, in the above quote the wheel's radius (distance from the axle center) is 1 ft, which makes the diameter of the wheel 2 ft.

[Planemo]

I'll nitpick myself too, if the software was fudged so that the bar was deemed double the length, the displayed torque would double not halve as the stress sensors would physically be in the same place

If we were physically moving the sensor by fitting it at the end a 2ft bar, we would indeed see a halving though.

corrected now.

[MrMonoWheel]

Its amazing after hundreds of years of science and research we still have an argument about something as fundamental as torque. 

[OldFartRides]

This is good. I’d have missed it if not split off. I sort of knew what was what, but could not have explained or argued about it.

[KiwiMark]

11 hours ago, Eucner said:

More nitpicking. Schwalbe about rolling resistance:

Larger tire has more wind drag from larger projection area. This will even out the benefits of larger tire, but think a it still has an edge.

I thought that the benefits of a larger tyre were the greater stability and the significantly better handling of surface imperfections in the road/path.  I doubt anyone thinks much about rolling resistance, wind drag or anything of that nature.

Obviously with cars/motorcycles/bicycles/etc there isn't much of a problem going with larger wheels, because you can adjust the gearing to compensate.  With an e-wheel it is different, we have the fantastic simplicity of a hub motor and no gearing required - but that means that we can't easily compensate for a larger wheel diameter.  Some sort of clever gearing system might fix that, but then we end up with something  complicated that can wear out and cause problems.

[Planemo]

1 hour ago, Unicycle Santa said:

Its amazing after hundreds of years of science and research we still have an argument about something as fundamental as torque. 

For me, it's more amazing that people struggle to understand the relationship between horsepower and torque. I've been involved in some golden moments on that one. The funny thing is, it only takes one word to differentiate the two but you'll get people going off on 500 word rants to try and prove their (incorrect) explanations 

Whilst we're talking about torque I will never forget a fascinating fact quoted by a guy who was lecturing us on advanced principles of engine management many years ago, it went something like:

"Well you all know how stout a top fuel crankshaft is. Yep, that mahoosive lump of steel billet which looks like it could be dropped off a cliff and not get damaged. Well, under full power a top fuel engine will actually twist that monster lump out of shape, enough so that it needs to be compensated for in ignition timing".

Blew my mind that one. 

[Slartibartfast]

Oh dear, here we go...

11 hours ago, mrelwood said:

You resigning the distance from the axle in your description of torque.

'resigning'? I'm not sure what you mean.

 

11 hours ago, mrelwood said:

Then where does the m come from in your unit of torque, Nm?

What? It stands for meters, as in newton-meters???

What is you're point?

 

11 hours ago, mrelwood said:

To be clear, do you agree or disagree with how Wikipedia defines torque?

Of course I agree with how Wikipedia defines torque. I mean, that's what torque is!

 

Can I ask again, can you point out what have I said that is different to what you understand?
This discussion can't go anywhere without identifying what you think I have wrong. At the moment were just having an inane back and forth of... "you don't know what you're talking about" which is simply replied with "yes I do". This discussion can't even get off the ground until I know what you're referring to.

Edited October 21, 2022 by Slartibartfast

[Slartibartfast]

14 hours ago, Planemo said:

A torque figure is obtained by combining a rotational force against a perpendicular distance. Change the distance, you change the torque figure. Theres no getting away from that.

Yes, but what you neglect to state is that when you change the distance you also change the force. These two values change in tandem with each other and ultimately leave the torque value unchanged. If you were able to change the distance without changing the force this would change the torque but this is not what happens. The force changes inversely proportional to the distance and the torque remains constant.

 

14 hours ago, Planemo said:

Example: put 2" diameter wheels on an RC car and it will accelerate at a given speed. Increase the diameter of the wheels to 12" and I guarantee it will accelerate much more slowly.

This am not so sure about. It may well be the case that an R/C car with larger wheels will accelerate more slowly but it is not because the torque is reduced. What if you were to both replace the wheels and add a 6:1 gear reduction between the wheels and the motor. Would it now accelerate exactly the same? If so it seems you are implying there is an intrinsic link between RPM and torque which, for an electric motor, really isn't the case.

 

9 hours ago, Planemo said:

So, lets say we have an electric motor which makes 100Lb/ft. The manufacturer of that motor has tested the unit on a dyno to standardised protocols ie a known weight (lbs) at a known length (1 foot).

If we test that same motor but physically move the stress sensor to 2 feet away from the axis we will get a very different answer to our (measured) torque output. Roughly half I would wager.

No you won't, and this is exactly my point.

If you have a motor capable of producing say 100 Nm of torque and you put it in an 18" wheel it will produce 100 Nm of torque, and if you put it in a 22" wheel it will produce (wait for it) 100 Nm of torque. That's what torque is, it's a measure that "normalises" (or "standardises") against a given length so that two different length arms can be compared without the lengths confusing the values as it were. When I say torque is a measure of rotational force that is independent of distance this is what I mean.

 

Now it is fair to say that a given motor that produces a given amount of torque will result in more force (ie. newtons) when applied to a smaller arm than it will to a larger arm –which I think is what people are trying to say. However, it is important to keep in mind that that "greater force" is applied over a "shorter length", and by "shorter length" I'm referring to the length of the arc traced out by the end of the arm (which is of course directly proportional to the arm's length). All this is to say, yes, it takes "more effort" to rotate an arm of greater length but you also "cover more ground" when you do so. As in turning a 22" wheel 1° will require more force than it would on an 18" wheel but you will also have ended up "traveling further" as well. How much further you may ask; well the answer is that you will have travel the exact amount further as the amount of "extra effort" you put in. So, at the end of the day you will have wound up in the same place for the same effort.

Now a smaller wheel will do more rotations than a larger wheel which may lead people to conclude that those smaller rotations are eaiser (which they are) but you will of course need to do more of them and it all comes out in the wash. There will be a difference on what the RPM of the motors is and it's easy to fall into the fallacy of thinking that higher spinning motors produce more torque but that is not really the case for electric motors. Electric motors have a famously "flat" torque curve and, unlike internal combustion engines, will produce seemingly the same amount of torque over a very wide range of speed. You may want to "gear down" your motor by using a smaller wheel but electric motors don't have a "sweet spot" in the way that ICE do and there for don't really require gearing at all. In fact it's remarkable that our EUCs use a hub motor without even a single reduction between the motor and the wheel, which really is remarkable.

 

I hope this all makes since, and please do tell me if there is something you understand differently but please do identify what it is that I have said that you disagree with, you know, so we can actually discuss it: 😉

Thank you.

Edited October 21, 2022 by Slartibartfast

[Planemo]

37 minutes ago, Slartibartfast said:

Oh dear, here we go...

Theres really no need for the downtrodden comment. We're just having a discussion here. Alls good yeah?

37 minutes ago, Slartibartfast said:

Now it is fair to say that a given motor that produces a given amount of torque will result in more force (ie. newtons) when applied to a smaller arm (ie. smaller wheel) than it will to a larger arm –

Well at least we agree on that point.

37 minutes ago, Slartibartfast said:

However, it is important to keep in mind that that "greater force" is applied over a "shorter length", and by "shorter length" I'm referring to the length of the arc traced out by the end of the arm (which is of course directly proportional to the arm's length). All this is to say, yes, it takes "more effort" to rotate an arm of greater length but you also "cover more ground" when you do so.

You do indeed cover more ground with a bigger wheel for X amount of rotations. But that 'extra ground' isn't a free lunch.

37 minutes ago, Slartibartfast said:

There will be a difference on what the RPM of the motors is and it's easy to fall into the fallacy of thinking that higher spinning motors produce more torque but that is not really the case for electric motors.

No one (except you) has fallen into the trap of adding the extra variable you did (rpm). As soon as you add that you are venturing into talking about power (which we are not).

[Planemo]

15 minutes ago, Slartibartfast said:

This am not so sure about.

If you are not sure then I'm not sure of a better way of explaining it. But I will have a think and try.

15 minutes ago, Slartibartfast said:

It may well be the case that an R/C car with larger wheels will accelerate more slowly but it is not because the torque is reduced.

OK what is it then?

15 minutes ago, Slartibartfast said:

What if you were to both replace the wheels and add a 6:1 gear reduction between the wheels and the motor. Would it now accelerate exactly the same? If so it seems you are implying there is an intrinsic link between RPM and torque which, for an electric motor, really isn't the case.

I haven't calculated the reduction you mention but if you are talking about gearing the larger wheeled RC car to match that of the ungeared smaller wheeled RC car then yes, it would accelerate at the same speed (disregarding what would be quite substantial gearing losses for the sake of your example).

Again, I'm not even bringing RPM into the discussion because I have no doubt it will cloud matters even further. Torque measurements are completely devoid of an RPM variable whether it's an electric motor, IC engine or whatever you fancy so it's irrelevant to the discussion.