Torque

[flecc]

flecc,

Thank you for explaining your position but I fear that we are at cross purposes here. In the rating of electric motors continuous means there is no limit of time. One starts the motor against it's rated load and return next week and give the grease caps a wee tweak.

Please excuse me if I take your points out of order and consider your cyclist first. When on the level delivering his rated 100 watts he climbs a hill for 3 minutes at 300 watts then drops to 20 watts down the other side then back to 100 watts on the level, this is repeated over the next 5 hours. And much the same with electric motors.

Dave

I don't really understand the point you are making Dave as it doesn't express maximum power which the law mentions, your cyclist illustration being of an average power over time. My point is simply that the legal 200 watt and 250 watts ratings have no factual relationship with the motors on the market, though they once did many years ago in e-biking's earliest days. Motors have grown in continuous power ability since then while the legal power expressions have remained the same,the two now being completely out of synch.

Your three minute illustration is bit short of reality for many of us. I have the odd 15 minute fairly steep continuous climb and even the odd 1/2 to 3/4 hour climb on which I can be using the motor continuously flat out. The EAPC regulations aren't technical documents and make no mention of what maximum power constitutes and don't specify any time periods so today's motors do not comply.

I always object to anyone saying the 200 and 250 watt ratings are in any way factual since that is clearly misleading, even in real world conditions. As an example, I live in a hilly up and down area and the eZee Quando I bought normally put down an average of 360 watts of net power on my journeys in the area, long or short, so even if the law did speak in average power terms, it still wouldn't have complied.

And of course the "legal" Lynch motor for trikes can reach outputs of over four kilowatts under some circumstances without overheating, an extreme example of the disparity between law and reality.
.

 

[flecc]

I think you and flecc are saying the same thing? You both agree that with an unlimited supply of energy any motor could power itself at its peak power, be it 500w or whatever, indefinately (with proper cooling)?

So with the regulatory speed limit of 15mph, the motor has to reach this speed with a maximum of 250w of power, but is often capable of much more, and is therefore 'capped' after reaching this speed?

Not quite, as my explanation above shows. The EAPC regulations just state maximum power 200 watts (UK), clearly having no relationship to reality, continuous or otherwise for today's motors. The regulations make no connection between the 15 mph limit and the motor power.

Flecc, do you have any links to the Nasa information you quoted? I have access to plenty of journals, but it would cut my search time down significantly if you remember where you found them.

I took them from the cycling technical "bible", Bicycling Science by David Gordon Wilson, third edition, published by the MIT Press, Massachusetts Institute of Technology, Cambridge, Massachusetts, 02142

Hard cover: ISBN 0-262-23237-5

Paperback: ISBN 0-262-73154-1

Also, anyone know the typical torque required to get the wheels moving on a flat, in first gear?

A Mussels said, its a variable figure which even the "bible" referred to above doesn't attempt to tackle, only dealing with acceleration.
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[Orraman]

Flecc,

It appears that the only real difference between us is the way we interpret the power specifications in the legislations and the following was copied from the dft link below .

"Maximum continuous rated power output of the motor shall not exceed Bicycles: 200W."

Rating can be continuous or intermittent. Some old spin drier motors were rated intermittent at 4 minutes in a 10 minute cycle, that is 4 minutes running and 6 minutes idle.
Electric welders are perhaps the worst, some being rated at 10% max output in a 10 minute cycle. Max power for 1 minute in every 10 OR 10% of the max power for 10 minutes in every 10 which is of course Continuous. There is no doubt that such welders will deliver the maximum power on the nameplate but deposit 1 big welding rod then have a fag, chip the slag, look industrious, etc. or else pay.

In the same way with electric motors the max continuous output is determined by long term testing against a load to the point where the internal temperature becomes critical. This is the legal power rating and I think I have seen a figure of about 100W being required for 15 mph on the flat.
In the same way as a welder we can extract much more power than the continuous capability by "overloading" in the short term without damage.
There is no requirement to test in a 15 mph wind so we have this advantage in natural cooling.

A good site for the Prony brake is linked below but where they casually test to the stall, believe me an electric motor would be long gone.

The illustration I gave was using your own figure of a maximum of 10 minutes at 300W for an unassisted FIT adult but you are comparing it with a 15 minute climb which I assume was assisted.
On your 1/2 to 3/4 hour climb, I honestly envy you and wish you joy of it ,even if it is assisted.

James,
I hope the above is of some help but unfortunately I don't think flecc and I are in accord on this one.
Playing with Miles Hellon's calculator in post #8 of this thread and changing Rider 100kg, Wheel 0.7m, Grade 0 shows 100W gives 15.3mph. Do have a go changing metric values.

Department for Transport - Electrically assisted pedal cycles in Great Britain

Horsepower Testing & Pony Brake - Buckley Old Engine Show

Dave

James, try Human-powered transport - Wikipedia, the free encyclopedia

 

[flecc]

Flecc,

It appears that the only real difference between us is the way we interpret the power specifications in the legislations and the following was copied from the dft link below .

"Maximum continuous rated power output of the motor shall not exceed Bicycles: 200W."

Rating can be continuous or intermittent. Some old spin drier motors were rated intermittent at 4 minutes in a 10 minute cycle, that is 4 minutes running and 6 minutes idle.

Agreed on the regulation in inverted commas of course, something I've long been familiar with, but the rest you've added which is your interpretation of the law which may or may not have been the legislators intentions.

I'm not interpreting at all Dave, just sticking to my original point that today's motors do not comply since their maximum continuous power greatly exceeds 200 watts, manufacturer's statements that they do when that occurs being untrue.

The trouble with any insistence on the relevance of 200 watts or 250 watts is that it misleads others into reading that as an absolute which has happened in here far too often and needed subsequent correction. Back in the early e-bike days motors did usually comply which was more acceptable then for a number of reasons, they were limited to 12 mph so the maximum torque/power point coincidence was at lower speeds than todays motors, and a number of early designs drove through the gears anyway. Things have moved on considerably and no amount of arguing about interpretations of continuous maximum power alters the fact of the increased powers that exist now.
.

 

[tillson]

Also, anyone know the typical torque required to get the wheels moving on a flat, in first gear?

This depends on a number of factors, the major three being:

1) Mass of bike plus rider.
2) Rolling resistance of the system.
3) Acceleration of the system from rest.

I would think that you could assume (1) to be in the region of 110Kg, (2) a quick google search suggests 2.9 Newtons (Obviously very variable due to a multitude of different factors). (3) I don't know how quickly a cycle accelerated from rest. Is it reasonable to assume that after 1 second it will be travelling at 5 mph? If so that is 2.23 m/s velocity after 1 second.

Assume that acceleration is constant during the first 1 second. So the Equations of Motion can be used. Therefore rate of acceleration = 2.23 m/s/s

Total forces acting on the bike at it accelerated from rest are: Acceleration force + Rolling resistance

F= (MA) + Rolling Resistance

= (110x 2.23)+2.9

F= 248 Newtons

The above force will be acting about the centre of the bike wheel. Assume 700 wheels. (Does that mean that they are 700 mm diameter? I think it does.)

Therefore the torque about the wheel centre will be 248 x 0.35 = 86.8 NM to accelerate the above bike from rest to 5 mph in one second.

The power output from the motor as the bike passes the 5 mph mark would be (2.23/0.7) x 2 x 86.8= 553 Watts. Of course as the velocity increases drag starts to become more of a factor so the power would have to increase to maintain the same rate of acceleration.

(Rather a boring night at work. Not doing much due to bad weather so I thought I'd try and exercise my memory. I won't be offended if someone shoots big holes in my theory, its many years since I last used it!)

Tom

 

[James1986]

Thanks for the input guys!

Although I am still confused as to how to work out the actual rating of the motors power. I was told to use P=VxI, which tells of the battery supply.

So how does the back emf of a motor calculate the power when the voltage is variable with speed? Especially when you pull current into the equation?

Unless I am being pretty silly, which is quite likely, are you saying that the figures used by the companies have no scientific meaning whatsoever, and are just used so that the legal people can tick the box of 'complies with the 200w regulations'?

Tom, I think, if you were on a 700mm penny farthing you would be right on track? You need to take into account several other factors. The number of teeth on the pedal gear, and on the rear wheel gear are different, thus the ratio of power transferred (torque) will be different. The force required will also be dependant on the position of the pedal from the central axis (of the pedal gear) and so you would need to take moments about this axis.

Although feel free to correct me if this doesn't make a difference

I just wondered if it was a widely known figure? I can do the practical test when I go home for the holidays! (How much weight it takes to move the pedal from the up position to the down position, with me sat on it!

 

[flecc]

are you saying that the figures used by the companies have no scientific meaning whatsoever, and are just used so that the legal people can tick the box of 'complies with the 200w regulations'?

Yes, largely true. Jeremy, a scientist member of this forum was arguing only recently that the continuous maximum power could be whatever the manufacturer specified it to be. Although I was disputing at the time that this is acceptable to the courts where legal considerations were the issue, I fully agree with his contention, and the facts bear that out.

For example, the EU limit is 250 watts, but manufacturers supply the same 250 watt nominal bikes to the 200 watt UK market just by changing the number on the printed specification. In one case a little while ago the manufacturer just changed the label on the motor. That same manufacturer was known to be supplying one of their motors labelled as either 700 watts or 450 watts depending on where it was destined.

There's a widespread feeling that the legal wattage limit is wrong in principle, the law interfering in design where it has no proper place. In keeping the EAPCs close to being bicycles it's perfectly proper for legislation to prescribe speed limits, the requirement to pedal to get power, and maximum weights, but the power necessary for the bikes to achieve those is a matter for designers and no business of lawyers.

Law interfering in design first started to crop up in the 1960s in fiscal legislation and at the time earned the derisory nickname, "Design by Chancellor", but in the last decade in the case of EAPCs, manufacturers just do what is necessary for the market with but a cursory nod towards the wattage limits, that increasingly being the case over the last five years.
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[James1986]

Thats an interesting principle!

I also forgot to thank you personally for the link to the cycling information papers you provided; so uh... Thanks!

Also, thanks to David for passing on his links!

A completely irrelivant point I'd like to rant about is the policy regarding items in your carry on baggage at the airport. My Girlfriend and I are about to go to Germany, and on reading the regulations on what you can and cannot take with you on the plane, were supprised to read that you cannot take a hairdryer, or hair straightners or indeed any other personal grooming device onboard! (Just give me a second to combine my shampoo and conditioner so that I can create some C4...)
They give the reasoning of these objects being used as blunt assault weapons, and sources of heat for burning (though I have no idea how you would plug one in on an aeroplane). You might as well argue the case that they should cut everyones arms and legs off, and inject us all with sedatives to stop us trying to assault them (because everyone wants to kill everyone else) with our last ounce of headbuttable strength! If someone tried to mug me with a hairdryer, I'd probably give them money to stop the hysterical laughter they just induced upon me.

I suppose what I am really saying is that I'm sick of all the political correctness and stupid ways of solving uneccesary problems, like the output power of the motors, when they don't need to. Like you said, who cares how much power it can output, as long as they stick to the speed limit .

 

[Miles]

James,

Pout = (Vin - Iin * Rm) * (Iin - Io)

Io is no-load current
Rm is motor resistance

 

[Mussels]

They give the reasoning of these objects being used as blunt assault weapons, and sources of heat for burning (though I have no idea how you would plug one in on an aeroplane).

Then why are you upset that they can't be carried in hand luggage?

I can understand the point as I like to fly with hand luggage only, but now when I get to the other end it's a big problem when the hotel can't supply
toothpaste or deoderant.

 

[Mussels]

James,

Pout = (Vin - Iin * Rm) * (Iin - Io)

Io is no-load current
Rm is motor resistance

Yes but how much of Pout is heat, and should that count toward the 250W limit?

 

[flecc]

Yes but how much of Pout is heat, and should that count toward the 250W limit?

Surely the heat is part of the efficiency loss and therefore not Pout, in which case it's not part of the 200 watt UK limit, that being the power applied, i.e. net power.

N.B. 250 watts is the EU limit, supposed to be in force here though this disputed by one DoT informal guidance.
.

 

[flecc]

I suppose what I am really saying is that I'm sick of all the political correctness and stupid ways of solving uneccesary problems, like the output power of the motors, when they don't need to. Like you said, who cares how much power it can output, as long as they stick to the speed limit .

A sentiment I entirely share and which I think is increasingly widely shared by our over-governed population. The interference would be more acceptable if the quality of that governance was good, but on present evidence it's been anything but for a very long time.

My lifelong experience is that all governments in any sphere are only capable of making matters worse ultimately, so they should only ever act where action is unavoidably necessary.
.

 

[Miles]

Yes but how much of Pout is heat, and should that count toward the 250W limit?

As flecc said:

None

No

 

[tillson]

Tom, I think, if you were on a 700mm penny farthing you would be right on track? You need to take into account several other factors. The number of teeth on the pedal gear, and on the rear wheel gear are different, thus the ratio of power transferred (torque) will be different. The force required will also be dependant on the position of the pedal from the central axis (of the pedal gear) and so you would need to take moments about this axis.

Although feel free to correct me if this doesn't make a difference

James
I like the idea of a 700 mm Penny Farthing. Sounds like fun I might have to get one.

I thought your original question was, Also, anyone know the typical torque required to get the wheels moving on a flat, in first gear?

I'd assumed you that were using a hub type motor which I suppose applies its power in a similar way to the pedals on a penny farthing, so the torque figures I had arrived at will be correct for the point of application about the wheel centre.

I now get the impression that you were actually wanting to know the torque about the pedal crank. All you need to do if that is the case is to establish the gear ratio between pedal crank and wheel (tip the bike upside down and count the turns) and scale the torque at the wheel by the same ratio.

Obviosly this does not take account of transmision losses, but it will give you a reasonable idea of torque required about the pedal crank to accelerate the bike and rider from rest.

 

[James1986]

Heh, yeah that's what I was on about ! I'm basically using it for technical feasibility, saying that 'Yes' it is worth it to provide assistance, and heres how much the motor would need to provide! (Although your first way still accomplishes this )


Thanks Miles, for the equation. Now I just have to find the information to match!

Mussles, I need to take them with me, but now I'll just have to have terrible hair . Nah, its just the principle... somebody might somehow hide a gun in one , or they could just check it at security to make sure..... nah, better ban them out right and make their jobs easier...

Going back to the torque point, with your first answer of 80Nm, would most direct hub motors be able to provide this without stalling or overheating and melting the coils?

Using the golden motor Pancake printed motors as a reference I came up with 35Nm with severe gearing and a speed of 5mph, which seems not very powerful at all....

Do the hub motors on the market presently use printed motor technology, or am I missing something vital here?

 

[flecc]

Do the hub motors on the market presently use printed motor technology, or am I missing something vital here?

No they dont James, I only know of Goldenmotor who do. Most are like the popular Suzhou Bafang motors, details and photos on this page of my Torq site:

motor

No figures I'm afraid, but you can do some reverse calculation if you wish. This same motor in a 20" wheel bike that isn't restricted and runs to 16 mph on it's 37 volt 10 Ah battery weighs together with me typically 98 kilos. With a freshly charged battery it will just manage a standing start on a 12% (1 in 8) hill without pedal assistance. With a running start without pedal assistance, it will just manage a 14% (1 in 7) hill on a fresh battery.
.

 

[the_killjoy]

Flecc

If you are talking about the law getting in the way of design the worst example of that must be the introduction of the Catalytic convertor. The intent may have been right but legislation was wrong.

The legislation should have merely been a requirement not to exceed specified limits of pollution and leaving it up to the designer to use a converter or produce a lean burn engine ~ it had an adverse effect on engine design.

 

[flecc]

Flecc

If you are talking about the law getting in the way of design the worst example of that must be the introduction of the Catalytic convertor. The intent may have been right but legislation was wrong.

The legislation should have merely been a requirement not to exceed specified limits of pollution and leaving it up to the designer to use a converter or produce a lean burn engine ~ it had an adverse effect on engine design.

Yes, that's certainly another good example killjoy, though it has to be said that some motor manufacturers were their own worst enemy there in actually backing the catalytic converter use originally. That was a case of laziness through being behind some of their rivals in lean burn technology.

An interesting historical point, it was the British Motor Corporation and it's successor British Leyland who led the world in clean burn research originally, and were the first to decide that a three cylinder engine of between 800 and 1500 cc would be the optimum configuration, paving the way for the Japanese and Volkswagen to pick up the information for free with their own three cylinder designs.

Yet another example of Britain leading but failing to capitalise on the effort.
.