Very new to electric bikes/conversions but am thinking of buying a used motor hub and having a wheel builder fit it to a 20" rim for a folding bike conversion. The issue I'm a bit uncertain over is whether hub motors are built specifically for particular wheel sizes? Apparently this particular motor was originally used on a 29" MTB wheel so does it follow that it can just as easily be used on a 20" rim? Or is that ignorant thinking?!
Hub Motor and Wheel Size/Diameter
- Thread starter charltonguy
- Start date
It should be fine as most controllers (be careful what you buy) can be adjusted for wheel size, just so they remain legal speed wise, while being ridden.
Are you good as a DIYer and part time electrician, you need both these skills if you are doing the work yourself. Not forgetting a reasonable full tool set and a place to work.
Did you look on ebay for such a wheel size with a motor already mounted? That will save both time and money if there is one. Maybe a controller will be part of the deal and a PAS as well.....all needed!
Finished bikes that fold are also available for under 1000 UK pounds, though I have not tried any myself (I hate folders and small wheels, too unstable!)....Maybe someone here can help you better....
With rear hub motors, and a quick release for the front wheel, you can make a normal size e-bike relatively small for transport or storage, if that helps.
I am an avid DIYer and Electrical engineer, plus I worked for many years in Electronics but I would not even dream of doing it all myself, too much work and no success guarantee! And definitely more expensive than a finished article....
Best wishes and regards
Andy
The hub for 29er wheel won't be suitable for a 20" wheel as the rpm will be too slow.
Is the motor rpm/winding known ?
One needs a hub wound to suit a 20" wheel so typically needs to be about 328rpm which would allow it to spin at approx. 19.5mph no load speed or approx. 15.5mph on the road.
Alternately if the hub is 36v one can over volt it at 48v to increase the rpm by 33%, how ever it is better to have the motor rated at the correct voltage and correct rpm/winding.
Is the motor rpm/winding known ?
One needs a hub wound to suit a 20" wheel so typically needs to be about 328rpm which would allow it to spin at approx. 19.5mph no load speed or approx. 15.5mph on the road.
Alternately if the hub is 36v one can over volt it at 48v to increase the rpm by 33%, how ever it is better to have the motor rated at the correct voltage and correct rpm/winding.
Thanks for the advice Andy, very useful. No, I'm not a part time electrician and although not scared of tackling most DIY stuff, you may have said enough to make me re-think this one!
Thanks Neal for that advice, I have very little info on the motor so it sounds a bit of a gamble to pursue this option at the moment!
Of the few controllers I have had, all were capable of being programmed for wheel size, though I would guess some are not of course! So it would be important to find that out before purchase.
Surely any controller sold say on ebay, should have this facility.... Plus, buying such a wheel and a controller from a single supplier, should make that a no problem purchase!
Best wishes
Andy
I am MOST happy if I have put you off DIY, as we get regularly people with bad self made problems and they are difficult to assist, because they don't know the "lingo", and we cannot see the fault(s).
Having been on Pedelec for some times, such heart rending "plights" have put me completely off trying, plus my back would not allow it!
regards
Andy
Yes displays can set the wheel size but it can't change the motor rpm which you are failing to take in to account, one can change the wheel size as much as you like to no avail. All it does is trick the motor to run faster on the road but is quite different to real world motor rpm.
You have never yet converted a bike so really have no idea, all you do is spout rubbish on hear say.
Folks mess up because they are not practically minded, buy the wrong stuff and then try to marry stuff up that is incompatible.
I have not yet had any issue marrying up hub motors and KT they are pretty easy to get going if one buys the correct stuff. Many shed /garage builders have over the years the forum has been running built many successful working kits.
I have use a lishui kit without issue but prefer the KT controllers and the current control input of them, I have used Bafang motors, aikema & mxus hubs with them all with no issue.
The bit most have issue with is the pedal sensor arrangement as it involves practical bike mechanic to fit them to the BB and msot have never held a bike spanner or ever removed a BB so throw a bit of a paddy when they don't get it right first time.
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The forum isn't about poo pooing or putting people off on hear say , for diy builds it is about practical advice and giving good advice to achieve the main aim of a working bike.
I'm probably have an above competency of some on here, one only has to look at my Cotic Road Rat build thread to see that with a bike built from just a bought bare frame.
It is the likes of I and a good other half dozen or so equally highly skilled guys and girls who show that it is possible.
The forum has lost a few good members or should I say they of late no longer post which is a great loss to others because of nay sayers as your self who distort things.
I'm probably have an above competency of some on here, one only has to look at my Cotic Road Rat build thread to see that with a bike built from just a bought bare frame.
It is the likes of I and a good other half dozen or so equally highly skilled guys and girls who show that it is possible.
The forum has lost a few good members or should I say they of late no longer post which is a great loss to others because of nay sayers as your self who distort things.
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You are not understanding how this all works. Your explanation is only 50% correct!
What would be the point of a controller needing a wheel diameter as a parameter, but not changing the rpm?
Answer, no point at all. It is so the RPM can be increased to give the correct legal road speed, with each wheel size. Its that simple!.
Its also an old trick to get an e-bike to go faster than the legal limit, by selecting a smaller wheel size.......Well known and fully understood by most people on Pedelec!
But obviously not all!!
regards
Andy
Sorry but you are totally wrong Andy and until you ever own an ebike you will never find out, all you are doing is trolling rubbish..
The manufactures wind the motors for a certain rpm, this is set and can't be changed one can't increase or decrease the copper winding strands in a motor by comm's or signals, the display in just communicates with the controller neither it nor the controller can change the rpm winding.
The wheel size setting in a display uses the wheel speed magnet and signals to the controller to adjust the approx. speed for any given size wheel, it will use algorithms set in the pcb software to roughly calculate the speed via the rpm of the wheel rotating and not from any internal motor rpm winding.
Lcd displays use a setting in most it is parameter P1 which has a numeral value used to to adjust the real time speed, (this is done using the algorithm) one has to use a GPS to adjust the P1 numeral value up or down so that the display reading matches the GPS as near as possible. It doesn't and can't change or alter any copper windings in the motor which are set to achieve a certain rpm for that particular motor it was manufactured as.
The manufactures wind the motors for a certain rpm, this is set and can't be changed one can't increase or decrease the copper winding strands in a motor by comm's or signals, the display in just communicates with the controller neither it nor the controller can change the rpm winding.
The wheel size setting in a display uses the wheel speed magnet and signals to the controller to adjust the approx. speed for any given size wheel, it will use algorithms set in the pcb software to roughly calculate the speed via the rpm of the wheel rotating and not from any internal motor rpm winding.
Lcd displays use a setting in most it is parameter P1 which has a numeral value used to to adjust the real time speed, (this is done using the algorithm) one has to use a GPS to adjust the P1 numeral value up or down so that the display reading matches the GPS as near as possible. It doesn't and can't change or alter any copper windings in the motor which are set to achieve a certain rpm for that particular motor it was manufactured as.
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Lets go back to the OP's #1.
The hub for the 29er wheel lets say it is wound for 260rpm, in a 29" wheel that would give it a NLS speed of approx. 22.4mph or a OTR speed of approx.18.1 mph, If the same hub is used in a 20" wheel the rpm stays the same but the NLS speed will change and will be slower, the max one would see is 15.46 mph or a 12.5mph OTR speed.
A display with the option of parameter P1 setting changing would simply use the algorithm to calculate the speed by changing the P1 value.
The only way to physically change the motor rpm is by Kv increase, at 36v to put more voltage through the motor one would overvolt to 48v then one would see an increase of 33%. This would increase the kV winding to 345rpm and would then give the 20" wheel a NLS of approx. 20.5mph or an OTR speed of approx.16.6mph.
This topic has been done to death many times over the years here was one of the previous simple explanations.
https://www.pedelecs.co.uk/forum/threads/swx02-36v-vs-48v.42003/
The hub for the 29er wheel lets say it is wound for 260rpm, in a 29" wheel that would give it a NLS speed of approx. 22.4mph or a OTR speed of approx.18.1 mph, If the same hub is used in a 20" wheel the rpm stays the same but the NLS speed will change and will be slower, the max one would see is 15.46 mph or a 12.5mph OTR speed.
A display with the option of parameter P1 setting changing would simply use the algorithm to calculate the speed by changing the P1 value.
The only way to physically change the motor rpm is by Kv increase, at 36v to put more voltage through the motor one would overvolt to 48v then one would see an increase of 33%. This would increase the kV winding to 345rpm and would then give the 20" wheel a NLS of approx. 20.5mph or an OTR speed of approx.16.6mph.
This topic has been done to death many times over the years here was one of the previous simple explanations.
https://www.pedelecs.co.uk/forum/threads/swx02-36v-vs-48v.42003/
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Your misunderstanding is misleading you......badly!
Unless a motor has a gear system in it, (possible but I myself have never seen one), what you say cannot work for a range of wheel sizes.
The motor will be wound for a certain RANGE of RPMs, not a fixed RPM as you imply.
Having owned 2 e-bikes since 2010, and ridden a great many more, none of them worked as you seem to think they do. Maybe somewhere there is such a bike, but its not a mainstream one, as it will basically only run at one speed, according to you!
There are different methods of control, but basically of the bikes I have ridden, they all allowed me to ride at the speed I wished to, up to the legal limit.
To do this, the controller controlled the RPM of the motor. Really simple.
Just so you know, I have never ridden a middle motor e-bike, as they are generally too expensive and plagued with reliability problems, not the least being chain wear, and if the chain breaks (can happen on any bike), you are stuck with walking home (unless its all downhill to where you live!).....But not on any hub motor bike that I have ridden....I could fool the PAS on the first bike I owned (after removing the broken chain) by simply pedaling, and it would take me home at the legal limit.
My current bike has a throttle, and I can drive anywhere, up to the capacity of the battery, without using the pedals, but still controlling the speed (RPM of the motor), at any speed I require, up to the legal limit...with or without the chain!
I am sorry to have to break your bubble, but your explanation is full of holes and is not how e-bike motors work. Sadly, I do expect others here to misunderstand how motors work, simply because it is outside of their area of knowledge.
Do try and find a reliable source, properly documented, that follows your line of reasoning, and post the link for us all here.
Thanks in advance.
I looked around for you and found a few links that may help you to further better understand how e-bike motors work.
Enjoy:-
Hub-motor or Mid-drive Motor For My E-Bike? - Keego Mobility
Keego Mobility makes E-Bikes with both Front and Rear-Hub and Mid-Drive motors. What are some of the benefits with different motors?
keegomobility.com
THE MOST POWERFUL ELECTRIC BIKE MOTORS – LithDrives
The most powerful electric bike motors are equipped with top-notch technology to give riders high-quality riding experiences.
lithdrives.com
Regards
Andy
Your misunderstanding is misleading you......badly!
Unless a motor has a gear system in it, (possible but I myself have never seen one), what you say cannot work for a range of wheel sizes.
The motor will be wound for a certain RANGE of RPMs, not a fixed RPM as you imply.
Having owned 2 e-bikes since 2010, and ridden a great many more, none of them worked as you seem to think they do. Maybe somewhere there is such a bike, but its not a mainstream one, as it will basically only run at one speed, according to you!
There are different methods of control, but basically of the bikes I have ridden, they all allowed me to ride at the speed I wished to, up to the legal limit.
To do this, the controller controlled the RPM of the motor. Really simple.
Just so you know, I have never ridden a middle motor e-bike, as they are generally too expensive and plagued with reliability problems, not the least being chain wear, and if the chain breaks (can happen on any bike), you are stuck with walking home (unless its all downhill to where you live!).....But not on any hub motor bike that I have ridden....I could fool the PAS on the first bike I owned (after removing the broken chain) by simply pedaling, and it would take me home at the legal limit.
My current bike has a throttle, and I can drive anywhere, up to the capacity of the battery, without using the pedals, but still controlling the speed (RPM of the motor), at any speed I require, up to the legal limit...with or without the chain!
I am sorry to have to break your bubble, but your explanation is full of holes and is not how e-bike motors work. Sadly, I do expect others here to misunderstand how motors, work, as it is outside of their area of knowledge.
Try and find a reliable source documented, that follows your line of reasoning, and post the link for us all here.
Thanks in advance.
I looked around for you and found a few links that may help you to further better understand how motors work. Enjoy:-
Hub-motor or Mid-drive Motor For My E-Bike? - Keego Mobility
Keego Mobility makes E-Bikes with both Front and Rear-Hub and Mid-Drive motors. What are some of the benefits with different motors?
keegomobility.com
THE MOST POWERFUL ELECTRIC BIKE MOTORS – LithDrives
The most powerful electric bike motors are equipped with top-notch technology to give riders high-quality riding experiences.
lithdrives.com
Regards
Andy
I see now where you are making your mistakes, you are talking about brushed DC motors, WITHOUT a controller to basically pulse the amount of power to the motor!!!!
THERE ARE NO CURRENT MODERN MODELS TODAY THAT DO THAT!!!!
The models of motor in use today, use what basically look like a 3 phase AC motor, and the controller switches power to the 3 field windings, controlling the time, from short to long pulses, for each field, to achieve speed control, in a smooth fashion.
Generally there are 3 Hall effect sensors to send positioning information to the controller, though some do not use there and rely on tables, inside the controller chips, to achieve speed control.
A motor designed as you feel, would try always to make sort of "RACING STARTS", and would be MOST uncomfortable to ride, even possibly dangerous.......
I now see that your thinking is back in the 19 Century, since when things have progressed remarkably!
My first motor of this modern type used on e-bikes, was in the 1980's on some computer equipment, that required variable speed motors. I was entranced.
Since the 1960's as transistors have improved, quite dramatically, such speed control has become normal.
Before MOSFETs and the like, I remember working on Cranes, onboard HMS Triumph, in the '60s, before all these MOSFETs and the like were even dreamed of, where we controlled speed/RPM with a so called "Ward- Leonard" System....... See here:-
Ward Leonard control - Wikipedia
en.wikipedia.org
You are basically back in those early days of speed/RPM control with your thinking, it would appear!!
I trust that I have managed to bring your thinking into the 21st Century now!!!
Andy
Any given motor will have a naturally occurring RPM limit (no load speed) that is dictated by both its specific motor winding and the operating voltage. The only ways to increase this RPM limit are by either increasing the operating voltage, or changing the winding of the motor.
Controller limits can only be used to reduce the speed of a motor, not increase it past its naturally occurring RPM limit.
Controller limits can only be used to reduce the speed of a motor, not increase it past its naturally occurring RPM limit.
...and the purpose of telling the controller the wheel size is simply to allow it to calculate rpm at the pedelec 25km/h threshold so that it knows when to stop assisting.
A motor is wound with a certain number of magnetic poles with a certain number of copper windings on each. These are calculated to match the desired RPM of the motor. When the motor reaches this RPM then magnetic saturation occurs. That is to say that the Back EMF produced equates to the supply voltage (or nearly so) and the current drawn decreases, limiting the RPM. The laws of physics dictate that it is IMPOSSIBLE for a motor to spin faster than this unless the supply voltage is increaed.
A motor for a smaller wheel needs a higher RPM, to reach desired speed (15.5mph),as the circumference of the wheel is smaller and, therefore, the wheel travels less road distance with each revolution.
Ergo, a hub motor designed for a 29" wheel will have too low an RPM for a 20" wheel and would end up with a slow assist speed.
The wheel size settings in the display are merely to match display parameters with the actual wheel size so that correct mph will be displayed and to allow correct assist cut off speed.
A motor for a smaller wheel needs a higher RPM, to reach desired speed (15.5mph),as the circumference of the wheel is smaller and, therefore, the wheel travels less road distance with each revolution.
Ergo, a hub motor designed for a 29" wheel will have too low an RPM for a 20" wheel and would end up with a slow assist speed.
The wheel size settings in the display are merely to match display parameters with the actual wheel size so that correct mph will be displayed and to allow correct assist cut off speed.
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Hi Andy. The problem with most ebike motors is that they use permanent magnets to create the magnetic field, so very little can be done to control the field strength as used with Ward Leonard above. There is a timing method (field weakening) that can be used but its not very efficient. Its possible to switch from star to delta wind but this is beyond most users.Your misunderstanding is misleading you......badly!
Unless a motor has a gear system in it, (possible but I myself have never seen one), what you say cannot work for a range of wheel sizes.
The motor will be wound for a certain RANGE of RPMs, not a fixed RPM as you imply.
Having owned 2 e-bikes since 2010, and ridden a great many more, none of them worked as you seem to think they do. Maybe somewhere there is such a bike, but its not a mainstream one, as it will basically only run at one speed, according to you!
There are different methods of control, but basically of the bikes I have ridden, they all allowed me to ride at the speed I wished to, up to the legal limit.
To do this, the controller controlled the RPM of the motor. Really simple.
Just so you know, I have never ridden a middle motor e-bike, as they are generally too expensive and plagued with reliability problems, not the least being chain wear, and if the chain breaks (can happen on any bike), you are stuck with walking home (unless its all downhill to where you live!).....But not on any hub motor bike that I have ridden....I could fool the PAS on the first bike I owned (after removing the broken chain) by simply pedaling, and it would take me home at the legal limit.
My current bike has a throttle, and I can drive anywhere, up to the capacity of the battery, without using the pedals, but still controlling the speed (RPM of the motor), at any speed I require, up to the legal limit...with or without the chain!
I am sorry to have to break your bubble, but your explanation is full of holes and is not how e-bike motors work. Sadly, I do expect others here to misunderstand how motors, work, as it is outside of their area of knowledge.
Try and find a reliable source documented, that follows your line of reasoning, and post the link for us all here.
Thanks in advance.
I looked around for you and found a few links that may help you to further better understand how motors work. Enjoy:-
Hub-motor or Mid-drive Motor For My E-Bike? - Keego Mobility
Keego Mobility makes E-Bikes with both Front and Rear-Hub and Mid-Drive motors. What are some of the benefits with different motors?THE MOST POWERFUL ELECTRIC BIKE MOTORS – LithDrives
The most powerful electric bike motors are equipped with top-notch technology to give riders high-quality riding experiences.
Regards
Andy
I see now where you are making your mistakes, you are talking about brushed DC motors, WITHOUT a controller to basically pulse the amount of power to the motor!!!!
THERE ARE NO CURRENT MODERN MODELS TODAY THAT DO THAT!!!!
The models of motor in use today, use what basically look like a 3 phase AC motor, and the controller switches power to the 3 field windings, controlling the time, from short to long pulses, for each field, to achieve speed control, in a smooth fashion.
Generally there are 3 Hall effect sensors to send positioning information to the controller, though some do not use there and rely on tables, inside the controller chips, to achieve speed control.
A motor designed as you feel, would try always to make sort of "RACING STARTS", and would be MOST uncomfortable to ride, even possibly dangerous.......
I now see that your thinking is back in the 19 Century, since when things have progressed remarkably!
My first motor of this modern type used on e-bikes, was in the 1980's on some computer equipment, that required variable speed motors. I was entranced.
Since the 1960's as transistors have improved, quite dramatically, such speed control has become normal.
Before MOSFETs and the like, I remember working on Cranes, onboard HMS Triumph, in the '60s, before all these MOSFETs and the like were even dreamed of, where we controlled speed/RPM with a so called "Ward- Leonard" System....... See here:-
.Ward Leonard control - Wikipedia
You are basically back in those early days of speed/RPM control with your thinking, it would appear!!
I trust that I have managed to bring your thinking into the 21st Century now!!!
Andy
It is very simple Andy after all that has been said if you really knew your stuff, the geared hubs we use can only have the RPM changed via voltage despite your so called electronic expertise.
You state you are controlling motor rpm by throttle , this is not so. All you are doing is controlling the bike wheel speed this is very much different to being able to regulate or change the internal motor winding speed.
You state you are controlling motor rpm by throttle , this is not so. All you are doing is controlling the bike wheel speed this is very much different to being able to regulate or change the internal motor winding speed.
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