Sprocket wear
- Thread starter HughMorris
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I disagree with above and would like to clarify this matter.
There is a difference between 'torque' and 'thrust'. Strictly speaking, torque is the rotational force and when quoted in Nm (Newton Meters) assumes a radius of 1 meter. Torque equal to radius times force (thrust), so with a wheel, to convert torque to the forward (tangential) force that actually pushes the bike forward, we must divide this figure by the wheel radius.
This makes sense with a hub motor as the torque is a product of the magnetic forces caused by the motor current and its internal gears and has nothing to do with wheel size.
Grin Technologies try to explain this when people use the simulator but it is often overlooked.
Motor Simulator - Tools
Our ebike motor simulator allows you to easily simulate the different performance characteristics of different ebike setups - with a wide selection of hub motors modeled, and the ability to add custom batteries and controllers and set a wide variety of vehicle parameters you'll be able to see...
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Thanks for clearing up the "semantics"I disagree with above and would to clarify this matter.
There is a difference between torque and thrust. Strictly speaking, torque is the rotational force and when quoted in Nm (Newton Meters) assumes a radius of 1 meter. Torque equal to radius times force, so with a wheel, to convert torque to the forward (tangential) force that actually pushes the bike forward, we must divide this figure by the wheel radius.
This makes sense with a hub motor as the torque is a product of the magnetic forces caused by the motor current and its internal gears and has nothing to do with wheel size.
Grin Technologies try to explain this when people use the simulator but it is often overlooked.
Motor Simulator - Tools
Our ebike motor simulator allows you to easily simulate the different performance characteristics of different ebike setups - with a wide selection of hub motors modeled, and the ability to add custom batteries and controllers and set a wide variety of vehicle parameters you'll be able to see...ebikes.ca
The point I was trying to make was that motor "torque" needs to be quoted independently of wheel size in order to be able to compare one motor with another.No, Its not just semantics. The torque quoted is always independent of wheel size. But wheel size affects the thrust. So a 20 inch wheel with a 40Nm hub motor has the same 'thrust' as a 28 inch wheel with a 56Nm hub motor.Thanks for clearing up the "semantics"
That's why the smaller wheel appears to have more 'torque' whereas in actual fact it has more 'thrust' or force due to the smaller wheel size rather than more motor torque.
If you read the Grin quote, you will see its a common mistake but it can cause confusion.
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Hence the quote marks on semantics
People get terminology wrong in common usage all the time, like saying weight, when they mean mass.What we need though is a consistent way of comparing one motor with another, independent of wheel size, so a motor torque value, not a number related to the wheel it happens to be in.
But we do already have a consistent way of comparing one motor with another in terms of torque and rpm. Both the motor rpm/volt and torque is normally quoted measured at the shaft (or the hub motor body in our case). The wheel size affects the load and speed, but not the motor torque or rpm (its the same with pulley size with an industrial motor).
Of course, there is a general problem of some motor manufacturers not supplying this info. But of course, a hub motor works in conjunction with a controller which is really part of the motor system ( and a certain voltage battery,) so actual maximum torque is a combination of all these.
What we don't have and whats difficult to compare is the continuous rating/durability of different motors. Grin has attempted to do this by inserting temperature sensors into test motors but there is still unknowns in particular models. e.g. at what temperature is motor unsafe/liable to overheat/damage.
I suppose that why you need something like Grins motor simulator to see how all these factors combine.
Anyhow, I'm not trying to be pedantic. I am trying to explain why fitting a smaller wheel does not increase the motor potential maximum 'torque' but increases the bikes forward 'thrust'. (like changing down a gear)
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PS Some additional info on how 'torque' , wheel radius and climbing ability relate.
A full understanding of ‘torque’ is useful when considering what a motor can do. For example, lets say myself and my bike together weight 110kg and I want to climb a 1 in 10 hill (10% hill). Approximation the value G (gravity) is 10, I need a horizontal force of 1100 Newtons (110G) multiplied by 10% grade which works out at 110 Newtons. (with G=10) To convert this Force to torque, I need to multiply 110N by the wheel radius.
So our formula is Torque equal radius times force for a 20 inch wheel, the radius is approx .25 meter so we need an additional 110 X .25 = 27.5Nm more than we normally would to overcome the hill ‘gravity’.
With a 700c wheel, we need 110N X .35 =38.5 Nm in addition to what we would normally use on a flat road.
In practice, things are more complicated. The motor needs to deliver these torque figures towards the middle of the power curve or above for efficiency reasons if this climbing is to be sustained for a long time, so we need to take figures above as minimum additional torque needed over and above whats normally needed for a flat road. In practice, for example, for continuous climbing, you may need a motor with a maximum capability of double these figures above for plenty of headroom.
Anyhow, very few people (other than designers or students) work these things out from first principles and I'm not sure if this has any practical value but its always great to understand the physics and how it all ties together.
A full understanding of ‘torque’ is useful when considering what a motor can do. For example, lets say myself and my bike together weight 110kg and I want to climb a 1 in 10 hill (10% hill). Approximation the value G (gravity) is 10, I need a horizontal force of 1100 Newtons (110G) multiplied by 10% grade which works out at 110 Newtons. (with G=10) To convert this Force to torque, I need to multiply 110N by the wheel radius.
So our formula is Torque equal radius times force for a 20 inch wheel, the radius is approx .25 meter so we need an additional 110 X .25 = 27.5Nm more than we normally would to overcome the hill ‘gravity’.
With a 700c wheel, we need 110N X .35 =38.5 Nm in addition to what we would normally use on a flat road.
In practice, things are more complicated. The motor needs to deliver these torque figures towards the middle of the power curve or above for efficiency reasons if this climbing is to be sustained for a long time, so we need to take figures above as minimum additional torque needed over and above whats normally needed for a flat road. In practice, for example, for continuous climbing, you may need a motor with a maximum capability of double these figures above for plenty of headroom.
Anyhow, very few people (other than designers or students) work these things out from first principles and I'm not sure if this has any practical value but its always great to understand the physics and how it all ties together.
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I didn't venture into my dead controller's potting - there was some burn-like discolouration on the surface which may have been an indication of heat damage further down. Not at all helpful, and I can't think of any way to avoid it, other than to replace the potting material before using the controller, with some other potting material which doesn't become unremovable when it's exposed to high temperatures, if such alternative potting material exists. Damn you Bafang!The potting in the controller is melted to the pcb its glue like and i could not remove it
Do we know if the potting material used by Bafang is thermally conductive?The potting in the controller is melted to the pcb its glue like and i could not remove it
Yes, the potting compound will be reasonably thermally conductive. It will either be an epoxy or a polyurethane based potting compound. Probably polyurethane based on castor oil, as that is the cheapest. It will be filled with things like chalk etc to pad it out.
Very interesting... if oil based, would oil burned onto @peter.c 's Bafang controller be dissolved or softened for easier removal, after a good long soak? Or would that kill other compments? Trying to dissolve the potting material with acetone, would be going too far.
I found myself looking at a Montague folding bike on ebay the other day, remembered this thread, and wondered if this new shorter format "Petrol tank" style battery would fit on the short downtube above the motor. It'd need a mud shield. I'll have to find a Montague Paratrooper to measure. Perhaps a rivnut and a bodge would be required.
52V 17.5AH~ 29Ah fuel tank e-bike battery -GreenBikeKit BBS, ebike batteries, Bafang M620, Bafang M600, Bafang M500, Bafang M510, KT controller with display-GreenBikeKit.com
52V 17.5AH~ 29Ah fuel tank e-bike battery for e-motorcycle.
www.greenbikekit.com
[QUOTE="guerney, post: 681204, member:
[/QUOTE]
I like this. It could go on the downtube of my Radrunner plus. Best if there was a domed cover for the back, house the controller, look better and keep out spray n muck!
(If it would clear my big feet, hehe)
[/QUOTE]
I like this. It could go on the downtube of my Radrunner plus. Best if there was a domed cover for the back, house the controller, look better and keep out spray n muck!
(If it would clear my big feet, hehe)
Attachments
There's also a slightly smaller size:
...neither are shorter than the type I have lol. I'll keep looking...
fr.aliexpress.com
...neither are shorter than the type I have lol. I'll keep looking...
21.28￡ 10% de réduction|Jsaluting Boîtier de batterie vide pour cadre de vélo électrique, décodage, tube, 36V, 48V, 52V, 80 pièces, 91 pièces, 18650 | AliExpress
Achetez malin, vivez mieux! Aliexpress.com
fr.aliexpress.com
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