I have an old Oxygen Ebike with a rear hub motor. The bike is nearly worn out. I bought it second hand before lock-down and have had lots of fun out of it. But so many things needing replacing that I have decided to convert a new mountain bike to electric. I will be using it off road so centre mounted motor will give me more torque for hill climbing. But then it occurred to me that I had no idea what the torque of my existing motor was, so that I could get some sense of what improvement I will get. My existing rear hub motor is a Barfang 8FUN. Marked as 250W and 36V. Can anyone help with an approximate figure please?
Motor Torque Figure
- Thread starter JohnD135
- Start date
Different hubs give different torque, but torque depends on wheel size, gearing current and voltage supplied. Quoted hub motor torques ar 30nm - 60nm so not easy to know.
Simply using 48v vs 36v will increase torque by 30%.
Unlike mid motor that give a given torque figure with hubs motors the torque can be variable dependant on many factors.
I have used both mid drive conversion and hub conversion for off road mud riding and extensive use over the South & North Downs ,Suuer Hills and inbetween.
I can't say one was better in riding use other then reliability with the hub drive I found was far better and both gave me the power and torque needed .
In the end I gave up on the mid drive and constantly used the hub drive with many trips to and fro from home covering many thousands of miles . I never once drove to any location and always rode to and from home sometimes covering 160km a day.
Simply using 48v vs 36v will increase torque by 30%.
Unlike mid motor that give a given torque figure with hubs motors the torque can be variable dependant on many factors.
I have used both mid drive conversion and hub conversion for off road mud riding and extensive use over the South & North Downs ,Suuer Hills and inbetween.
I can't say one was better in riding use other then reliability with the hub drive I found was far better and both gave me the power and torque needed .
In the end I gave up on the mid drive and constantly used the hub drive with many trips to and fro from home covering many thousands of miles . I never once drove to any location and always rode to and from home sometimes covering 160km a day.
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Whichever of the many different motors Oxygen used, the torque is not much determined by the motor. It's determined by the current from the controller, which in your case is probably 15 amps. That's not particularly high, and your motor could have handled a lot more.
A crank motor's torque is sort of unspecified because it has nothing to do with how much torque you get at the wheel, which is determined by the gearing. The torque figures given by the manufacturers is pretty well meaningless, though different values for different motors from the same manufacturer can be used for comparison.
You can get all the torque you need from a hub-motor, even the one you already have. The main advantage of a crank-motor is the more central mass that gives better control on tight windy trails, and it allows suspension to work better because of the lower unsprung mass in the wheel. It's extra torque only becomes useful for exceptionally steep hills, so if you want to do serious off-road stuff, a crank motor is more or less mandatory..
What I'm trying to say is that if you want to do proper off-road stuff, get a crank motor. Any one should be sufficient. you should look at power rather than torque. Power is basically amps times volts, so a 48v motor will give 30% more climbing power than a 36v one with the same current. If you have a lot of power, you can always turn it down, but if you don't have enough, you can't turn it up. on the other hand, if your problem is that your bike is just lacking in power for some hills, you can easily solve that with a new controller and/or a 48v battery. If you went up to 48v and 20A, your torque would double and your bike would still be legal.
Another thing to bear in mind is that the torque quoted for mid drive motors is often at the chainring whereas hub motors are quoted at the wheel. With a middrive you have to adjust the figure by the ratio of the largest working rear sprocket to the motor chainring and allow a few %.chain loss. For example, a middrive with a 80nm torque spec and a 42T chainwheel and a 28T max working rear gives a little over 50 nm torque at the wheel.
You need really low gearing and no chainline problems the get the best torque figure.
You need really low gearing and no chainline problems the get the best torque figure.
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The wheel size of the old and new bikes are the same. 42T to 28T is what I have giving, as you say, around 50nm from an 80nm motor. I am retired now, but from what I remember, most motors are limited at low revs by the dissipation in the windings. My battery has 7 cells in parallel (10 in series) and 2.6AH per cell or 18.2AH for the battery. I gain the impression that 1C is a very safe discharge rate for a battery. So, what level could I risk in increasing the current limit figure on the controller? I am aware of how energy is stored in the windings and then the flux level is maintained via the fly-back diodes. So the whole setup is complicated and I would have thought that experience is our best guide?
I suppose I am disappointed. I thought I would get significantly improved hill climbing performance with a mid drive. But thanks for all the advice everyone. I will try to increase the range of my existing cassette and also reduce the resistance of the current sensing loop in the controller. Fitting an extra battery is possible, since my existing one is mounted vertically in front of the rear wheel. I assume I can parallel connect them, provided I use power diodes in series with each battery output. I am a bit concerned that commutation spikes from the power fets will not be as well suppressed with diodes in series. But I will see if a series C R will help with that.
Best regards, John.
Best regards, John.
My BBS01 250w 36v mid drive is rated at 85nm (set @15a 36v) and my Crossfire-e HESC rear hub motor (250w 36v) is rated at 50nm
My real world experience, my mid-drive pisses all over the my hub drive bike on any hill and especially off road, the difference is like night and day, the mid drive climbs anything, but then I have got a 42t chainwheel and 11-40t cassette on the middrive
My real world experience, my mid-drive pisses all over the my hub drive bike on any hill and especially off road, the difference is like night and day, the mid drive climbs anything, but then I have got a 42t chainwheel and 11-40t cassette on the middrive
What exact motor does your have? The mk1 had Aurora. Mk2 Dapu, then Bafang CST, then Bafang SWX02. If you don't know, show photos. Maybe show us a photo of the bike too. IIRC the 70 cell batteries were only used on the last Emates with the battery behind the seatpost.
That's good gearing for hills, with a 26" wheel, 42T>40T is 27.3 gear inches, which at 90rpm is 7.3mph. My 20" wheeled cadence sensored 36V Bafang BBS01B mid drive conversion also climbs anything with 15A, 52T>32T = 32.5 gear inches, which at 90rpm is 8.7mph. Reminds me, must install that 52T chainwheel I got for free on ebay
With 26" wheels @90rpm your speed with 42T>11T is 26.6mph. 52T>11T on my 20" wheels = 25.3mph at 90rpm. You'll probably beat me in a race on a flat road. I could only manage about 22mph, even when my bike was briefly de-restricted. I'd get closer to top speed with 20A @90rpm, haven't tried.
If throttles become legal, I could make a lighter and smaller folding Dahon 15A 36V BBS01B 16" wheeled hill devouring commuter moped with about 40 mile range, but to get similar low gear as I have on my 20" wheels, I'd need a 64T chainwheel for 8.6mph 64T>32T, top gear would be 64T>32T = 24.9mph.
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Gain the 23% increase in force where the tyre meets hill, by transferring your hub motor etc. to a 20" wheeled bike? Finding a suitable 20" wheeled bike you enjoy riding could be a challenge, must admit mine took a little time to adjust to. Bit squirrely compared to my old 80s Sun racing bike.
I live close to an mtb trail center and the Bosch motor mtb are popular and are good climbers. I often wondered is it that the motors are good or is it that the gearing is good or perhaps both. These bikes have small front chainrings of 34T and large 51T on the back which would give about 50% extra torque. If we could use similar gearing on the BBS01 or TSDZ, they should be able to crawl along with over 120nm torque at the wheel (51 :34 gearing).
PS I am not sure where precisely the torque is measured and quoted for Bosch motor bikes (wheel or chainring or limited to some sort of multiple of crank pressure)???
PS I am not sure where precisely the torque is measured and quoted for Bosch motor bikes (wheel or chainring or limited to some sort of multiple of crank pressure)???
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Wow! Thanks for all the advise guys. You have given me two avenues to investigate. Refurbish the Oxygen or convert the mountain bike. I have understood the, quite complicated, issues with gearing changes and can probably get more torque hill climbing ability by changing both the cassette and the front cog (I am limited by the derailleur on the Oxygen to smaller size cassettes). Also, perhaps an increase in the current limit by 30%. I could also fit a battery from China to compensate for the extra power usage, but i need to find one with recognized cell make. This would be connected in parallel but with power diodes to prevent current flow from one battery to the other. I have attached some pictures to show the bike and motor. Yes the bike is often as dirty!
As to the mountain bike conversion, I can see the logic of a properly balanced bike for use on very challenging terrain with excellent hill climbing ability. I go off road now, but only byways and Forestry Commission tracks. Not really very challenging. I think I will refurbish (it needs front forks, drive train, bottom bracket, etc) the Oxygen and see if it meets my needs. As my fitness levels reduce I planned to use a bike more in the hills. We will see. Thanks again for all the help.
A 48v battery would be about £250 to £300. Decent controller about £80. It looks like you need some new forks too. You can get nice air forks that'll last a lifetime for £50 to £100. That's just over £400 to make your bike really nice for an all-round riding type and last for another 10 years. If anything happens to the motor in the future, you can use just about any replacement motor that takes your fancy. Your system is modular for mix-and-match type upgrades or changes.
The question comes down to what sort of riding you want to do. The extra torque is easily achievable in your present bike, but if you want to start honing your Downhill racing skills, you probably need something different.
The question comes down to what sort of riding you want to do. The extra torque is easily achievable in your present bike, but if you want to start honing your Downhill racing skills, you probably need something different.
Hydraulc brake upgrades are highly recommended .
The hub to me looks like a freewheel so lower rear gearing may be compromised.
The hub to me looks like a freewheel so lower rear gearing may be compromised.
Screw on gear cluster rather then the slide on freehub cassette with splines/key way .
I only say it looks like a freewheel because of the 7 gears and relatively low 28t top /low gear.
You don't have to worry about that too much. The main point is that they often have a 14T top gear that is a bit low for comfortable pedalling at higher speeds. Oxygen covered that bu putting on a larger chainwheel, but then you get a higher bottom gear that prevents you from pedalling efficiently when climbing, which is part of the problem you're experiencing. A solution would be to fit a DNP freewheel 11/30 or 11/34 and a smaller chainwheel, say 42T. That would give you lower low gears and the same high gears - cost about £40.
Starting to change the gearing, etc, as suggested and discovered the motor details under the cassette. It is as follows "Permanent magnet brushless high speed hub motor used for electric bicycles ZL 2003 2 0123400.9"
Does that suggest that I can increase the current by 30% or so, at my own risk of course?
Does that suggest that I can increase the current by 30% or so, at my own risk of course?
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