Motor & Battery Combination.

PeterPi

Finding my (electric) wheels
Oct 1, 2019
7
0
Hello from me. I hope you can provide some illumination.

I did an E-bike conversion about twenty years ago using a a lead acid battery and a belt friction drive on the rear wheel. I'm sure it was something Clive Sinclair did.

Fast forward and I now want to fit a mid drive Bafang kit but I believe the lowest power motor is 500 Watts and would not comply with UK regs. I assume that 500 Watts is the maximum power for a certain battery voltage. Would the bike comply with the law if the battery voltage was such, say 24 Volts, so that at full output power of the motor did not exceed 250 Watts. The same could be accomplished by modifying the controlor to limit the power.

I say "output power" but I don't know if the regs refer to electrical power in Watts, IE V*A or the output power at the back wheel, which would depend on the efficiency of the motor drivetrain combo.

At first glance it looks as though the legislation may have been written in haste or the information I want is hidden away somewhere.

When I know the answers to the above, I'll be ready to ask for recommended systems.

Thanks in advance

Pete
 

KirstinS

Esteemed Pedelecer
Apr 5, 2011
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You can get a 250w bbs system. I have one

Brighton ebikes and woosh are reputable UK sellers

The law is a mess but breaks down to . Whatever the manufacturer stamps on the side is the motor rating. As long as it says 250w you are fine. It isn't related to actual output wattage. Most 250w stamped motors can put out significantly more
 
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KirstinS

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Nealh

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Aug 7, 2014
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TDSZ mid drive from Woosh is also 250w and more powerful at 48v.
 

PeterPi

Finding my (electric) wheels
Oct 1, 2019
7
0
Thanks for the swift responses.

I'll check them out and come back when I need more info.

One thing that has come to mind is the fact I live on The Gower, west of Swansea (check a couple of profiles below) easy ride into the centre of town, but hard work on the way home! It's pretty challenging for a casual rider but is paradise for the fitness junkies. I expect I'll I need a battery with as much capacity as possible, and will a legit 250w motor cope cope with the terrain?

Thanks in advance.

Home to the centre of town.
profile 1.jpg

Mile and a half to the next village.
Profile 2.jpg
 

Hightechpete

Pedelecer
Jan 20, 2018
100
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west Wales
Having cycled the Gower on a a number of occasions (unpowered), I can put your mind at rest, there are no hills that I am aware of that can't be comfortably handled with a 250 watt motor. You might have a problem on Constitution hill (I'm sure you know it), other than that you're good to go.
 
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Nealh

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Aug 7, 2014
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That's where 48v is a better option then 36v as you will have 33% more torque, battery wise the same ah/capacity will give you 33% more wh of range.
 
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ebiker99

Esteemed Pedelecer
Feb 17, 2019
510
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With car engines, power and torque are the numbers to watch for getting an idea of their performance.
With ebikes things are a bit different, the power rating of the motor isn't the be all and end all by any means. The energy you'll get out of the motor depends on the current flowing through it and this is dependant on the maximum current that the controller can provide, assuming the the battery can provide that current, PLUS the ability of the battery to provide enough volts to get that current through the motor windings.
Don't get too focussed on the power of the motor, a 250W motor will get you up the steepest of hills, the issues to focus on are how long will the battery last for going up those hills and can it and can the controller provide enough current to do so.
 
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vfr400

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Jun 12, 2011
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Those hills aren't too bad. You will use about 3Ah going up them without having to pedal too hard. Even the Gtech , which has about the smallest battery you can get, will do that with loads to spare.
 
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Benjahmin

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Nov 10, 2014
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Hi from Ceredigion.
I can confirm that a '250w' motor will be no problem. As said above, this is the rated power (that which the motor can have flowing through it for 1 hour, without overheating), the actual maximum power consumed is down to the max output of the controller.
So a 20A controller @36v will give you 720w nominal. The trick is to make sure you have a battery that can deliver that current without voltage sag.
 
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Nealh

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720w is the figure the controller can output, once you factor in efficiency loss the figure will be about 550- 570w at the wheel, still a useful push in your back.
 
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PeterPi

Finding my (electric) wheels
Oct 1, 2019
7
0
OK, I'm going to make a purchase over the next week or so. Before I go any further, I've leaned toward a Mid Drive system, but I'd like to hear about the pro's and con's of a rear hub drive. The lower prices suggest that maybe they have become a little obsolete, but I'm an absolute novice so any guidance will be appreciated.

Cheer
Pete
 

Benjahmin

Esteemed Pedelecer
Nov 10, 2014
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Lower prices are because the technology is simpler, in my mind that means less fault liability which seems to be born out by many an anecdote on here.
For me a bike is a machine that is easy to self repair/maintain and understand. Why overcomplicate it with algorythm controlled voodoo that can only be repaired by the manufacturer?
Stories of hub motors failing are few and far between and, even then, are mostly fixable. Otherwise it's controller or wiring/connector issues. Have a look around the forum at the amount of fault enquiries for mid drives, from programming to drive train issues, from water ingress to the cadence range required by the motor not suiting the riders range.
Full disclosure, I'm biased! I get on my bikes, they work every time (maintenance is mostly brakes, lights etc. - normal bike stuff). I turn the peddles and the motor kicks in. What's not to like?
 

vfr400

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Jun 12, 2011
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OK, I'm going to make a purchase over the next week or so. Before I go any further, I've leaned toward a Mid Drive system, but I'd like to hear about the pro's and con's of a rear hub drive. The lower prices suggest that maybe they have become a little obsolete, but I'm an absolute novice so any guidance will be appreciated.
Pros:
  • Quieter
  • A lot more reliable
  • Chains and sprockets last 5 times as long
  • If you have a problem with your gears, pedal, crank or chain, you can still get home (throttle required)
  • More user-friendly, especially in stop-start traffic and in the winter when you have cold hands.
  • Lighter weight variants, so you can make a very light build.
  • Easy to repair. Spare parts are available. Note that there are some exceptions, like Dapu motors.
Cons
  • You have to be more careful in your choice. A crank-drive can work well (powerwise and speedwise) for a wide range of riders and rides (hills). You have to choose the right hub-motor with the characteristics that you need.
  • Not so good for serious off-road rides. Rear suspension doesn't work so well because of the extra mass in the wheel. You get a slight pendulum affect when doing quick turns - no problem for normal cycling.
  • Cheap ones can go rusty inside if you leave them out in the rain and/or install them upside-down.
  • You can cut the motor cable when you fall off and the cable is unprotected. You could argue that that's just a matter of bad installation.
  • You might have to sort out wheel-alignment issues. Things like dishing the wheel, spacing and stretching the frame to get the wheel central. That's normally only a problem with 8-speed or more gears and depends on many factors.
 
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sjpt

Esteemed Pedelecer
Jun 8, 2018
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One advantage of torque sensing (usually but not necessarily associated with mid drive) is that is responds immediately when starting from standstill, which can be very helpful for uphill starts across junctions. Cadence sensors usually need 1/2 turn or so of the pedals before the motor kicks in. On the other hand it is possible to ghost/phantom pedal with cadence sensing when you are feeling very lazy

Mostly I agree with vfr400, but not sure why he says for hubs More user-friendly, especially in stop-start traffic.
 
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vfr400

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Jun 12, 2011
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Mostly I agree with vfr400, but not sure why he says for hubs More user-friendly, especially in stop-start traffic.
You can stop or start in any gear. If you have to stop suddenly with a CD bike in a high gear, you can't get going again. Also, none of that horrible clunking every time you change gear that makes you fear whether your gears will last long enough to get home. No need to keep changing gear with a hub-motor. You can just leave it in a high gear and let the motor do the work.

You can overcome the instant start with a throttle, that's more instant and controllable than any torque sensor. Also, some controllers, like KT, have a very quick response to the pedal sensor, so not too bad. That's a controller issue rather than the motor, but you're right to mention it because the wrong controller choice can make a terrible ebike.
 
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sjpt

Esteemed Pedelecer
Jun 8, 2018
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Good points; our riding is a little atypical. Our (front) hub is not that powerful (XF07) and on a tandem, so we still need the gears quite a bit. Our CD solo has hub gears; partly chosen because my wife got into high gear at the bottom of a hill on a CD derailleur test ride and had difficulty restarting back uphill.
 
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Sturmey

Pedelecer
Jan 26, 2018
82
35
63
Ireland
Hi. I have looked closely at your climbs. On the second chart, coming back from the next village, you will be climbing approx 270 feet (130 -400 feet) over a distance of about .6 mile (3168 feet) which works out at a grade of over 8%.
Now, I personally have difficulties with such grades as it takes a few minutes of puffing and panting to assist my hub motor over such a grade. Then again, I am overweight (90kg) and heading for 64, but I am sure other people have no problems.
 
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vfr400

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Jun 12, 2011
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Hi. I have looked closely at your climbs. On the second chart, coming back from the next village, you will be climbing approx 270 feet (130 -400 feet) over a distance of about .6 mile (3168 feet) which works out at a grade of over 8%.
Now, I personally have difficulties with such grades as it takes a few minutes of puffing and panting to assist my hub motor over such a grade. Then again, I am overweight (90kg) and heading for 64, but I am sure other people have no problems.
If you have 36v, you can solve that by going up to 48v, which will give you 33% more torque. If you have steep hills, want a hub-motor and aren't a club cyclist, 48v is mandatory. I find it really surprising that manufacturers have more or less standardised on 36v, when 48v costs the same, is more efficient and gives more power options.
 
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Sturmey

Pedelecer
Jan 26, 2018
82
35
63
Ireland
If you have 36v, you can solve that by going up to 48v, which will give you 33% more torque. If you have steep hills, want a hub-motor and aren't a club cyclist, 48v is mandatory. I find it really surprising that manufacturers have more or less standardised on 36v, when 48v costs the same, is more efficient and gives more power options.
I had my own solution to the problem. If the trains can have two locomotives, I can have two hubs. I fitted a cyclotricity 250w (xf07 clone?) throttle only to front of an existing e-bike conversion(yose 350 rear hub pas only) with two separate batteries which work great for now. But I am interested in a more legal and robust solution to hill climbing (up to 14%) as I live close to some beautiful hills and I do high miles on sometimes wet and muddy road.
 
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