80mm Tongxin question

[AndyOfTheSouth]

Agreed. Also, Andrew Hamilton of Freedomebikes has given me very rapid and thoughtful answers by email.

Perhaps someone could spell out authoritatively what rpm for different wheel sizes, given specific objectives.

For example, I am only interested in hill-climbing, so I have been assuming that, for any wheel size, the lower rpm the better. Therefore 160rpm would be what I would want more or less irrespective of the wheel size. It just means that the smaller the wheel, the better the hill-climbing, the lower the top speed with motor assist.

But I don't really know.

 

[HarryB]

I have also had a very quick response from Andrew and I am interesed in the the kit. My concern is that I will need two of the batteries for the 10 miles journey which pushes the price up.

In terms of calculating the right motor for you to use there are several issues. Do you want to stay legal, are you after speed or hill climbing? Also to complicate things the latest Tongxins seem to have been de-tuned, my new 175 rpm motor in a 700c wheel won't really pull up to the legal speed limit, neither is it a particularly good hill climber. I had one of the original Tongxin 190 rpm motors and it was only down a little on the Torq 1 (570 watts peak).

So to calculate what rpm motor to use there is a formula:

rpm=bicycle speed (kph)x1000/60x3.142(Pi)xdiameter(m)

For a 700c wheel I calculate about 189 rpm for 25 kph. In practice these figures are a bit little high and I suspect this is because they are based on a nominal voltage of 36 V and most li-ion batteries kick out a bit more than that.

If you want a better hill climber then go for a lower rpm motor and 160 sounds about right depending on wheel size but it won't pull anything like 25 kph with a 700c wheel. Changing wheel size also changes the gearing in exactly the same way. You just need to play with the formula to work things out. In the end I suspect that if you after a really good hill climbing motor then the Tongxin is not the right choice.

 

[flecc]

Yes, the 160 rpm will be the best Tongxin for hill climbing purposes. Wheel sizes are an important factor though, the smaller the wheel the better the climbing, so a 700c is always a slight disadvantage, This wheel size advantage is distinct from any advantage gained from a lower revs motor so is additive, though it takes a much smaller wheel for the advantage to really be obvious, such as 20" or 16".
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[apshamlton]

Hill climbing and Motor rpm

I've read with interest the postings regarding hill climbing and rpm and was interested in the comment about the 175rpm tonxgin being "detuned."

I think everyone is missing something here which is that the effect of gearing on hub motor efficiency. A 175rpm motor is not detuned, its just less efficient that a 260rpm one because it requires more gearing to change the high rpm of the internal motor down to the required rpm. The greater the change in rpm required the less efficient the gearing will be.

Thus the 260rpm motor is the most efficient because its rpm is closest to that of the motor inside the hub.

I have received printouts of performance figures for the 260rpm motor which shows peak input power of 620 watts producing 20.01 Nm of torque at 193 rpm peak output power of 404.3 watts.

In contrast the printout for the 210rpm motor shows peak input power of 541.7 watts producing 20.05 Nm of torque at 166rpm for a peak output power of 348.4 watts.

The tonxgin website has a graph for what I think is the 190rpm motor which shows peak input power of 400 watts producing 20 Nm of torque at around 167rpm for a peak output power of about 320 watts.

Thus the higher rpm motors are significantly more powerful than the lower rpm ones.
The only reason to go with a lower rpm motor is legality (25km/hr assisted limit).

To go with a lower rpm motor for the purposes of hill climbing is not sensible because your efficiency losses cancel out much of your gains.

This is true for tonxgin but will also be true for all hub motors which use gearing to reduce the rpm of the motor.

In any case I can testify that the 260rpm motor on a Brompton has excellent low end torque and is a great hill climber. I live at the top of a mountain range in a city that is very hilly (Jerusalem) and I haven't met a hill that my Brompton E-Freedom couldn't deal with. This includes hills too steep for cars where the pedestrian path that I ride on goes straight up the hill but the road has to wind around.

The upshot of all this is that smaller wheels are better for geared hub motors and the best option for all purposes, including hill climbing is a Brompton (16" wheels) with a Freedom Ebikes 260rpm kit.
Freedom E-Bikes - Home

 

[HarryB]

I've read with interest the postings regarding hill climbing and rpm and was interested in the comment about the 175rpm tonxgin being "detuned."

I think everyone is missing something here which is that the effect of gearing on hub motor efficiency. A 175rpm motor is not detuned, its just less efficient that a 260rpm one because it requires more gearing to change the high rpm of the internal motor down to the required rpm. The greater the change in rpm required the less efficient the gearing will be.


Freedom E-Bikes - Home

Just to clarify a statement: My NEW 175 rpm motor as a replacement for my OLD 175 rpm lacks power. There are two possible reasons for this. The first is that my old motor was an 190 rpm version incorrectly labelled or the second, as put by Mark at Cytronex, is that they have reduced the power output on the later motors - I call this detuning.

I don't think in engineering terms the gearing changes the efficiency of the motor. Yes it changes the speed at which peak power occurs and multiplies the Torque, but it doesn't add more gears so increase friction which is the only way I can see it being less efficient.

But I am very interested in what you say about the graphs as it may be true that the lower rpm motors have reduced power. It may also be true that they are reducing the power outputs of all the motors to cut down on roller drive and controller failures. At the moment I wouldn't recommend the 175 rpm motor for anybody but the fittest of cyclists who likes to put in a lot of effort.

 

[Erik]

The upshot of all this is that smaller wheels are better for geared hub motors and the best option for all purposes, including hill climbing is a Brompton (16" wheels) with a Freedom Ebikes 260rpm kit.

What are your experiences with 20" wheels, the size favoured by Dahon?

 

[didi28]

Andy I almost purchased one from Frank in Germany. I really want the 100mm one though. If you mail Frank he replies really quickly and can probably comfirm for you.Google Translate He doesn't have any 100mm ones, so I have emailed the manufacture in hope they might sell me a 100mm one but no luck yet after a day or so

I drive a 100mm Mini-FWF from Frank. It is a clone of Tongxin. And you even can mount disc brakes:

 

[jerrysimon]

Thanks for that didi28

There has been some reference to that motor here but also that Frank's stocks of the Mini-FWF are low or all are gone as they are popular in Germany.

I now have a 100mm Tongxin but I had to buy a second hand Cytronex to get it lol

Regards

Jerry

 

[didi28]

There has been some reference to that motor here but also that Frank's stocks of the Mini-FWF are low or all are gone as they are popular in Germany.

He just makes a sell out, because he will move to china in october. I am sure, he will discover a lot of interesting things for us pedelecfans in China.

Bafang, Tongxin, etc. are nothing special in China. Only chance brought it about, that these companies are known in Europe. There are hundreds of motor-companies in China, which produce hub-motors and others. A lot of the motors are similar clones, some are produced better, others worse.

 

[RobinC]

...
Thus the 260rpm motor is the most efficient because its rpm is closest to that of the motor inside the hub.

This makes sense, like the extra losses/diminishing returns of extreme gears on internal hub gears.

Which rpm is optimum for 700C wheels, assuming some hill climbing is required?

 

[HarryB]

This makes sense, like the extra losses/diminishing returns of extreme gears on internal hub gears.

Which rpm is optimum for 700C wheels, assuming some hill climbing is required?

Hub gears have ineffiencies because of multiple gear combinations. I don't think we are talking about extreme gears here as there are the same number of gears involved. Unless of course roller gears are inherently more inefficient than planetary gears and I don't think so. Also the losses are so large that I would expect that to appear as large amounts of heat which doesn't happen. I would like to see some figures for this as I remain unconvinced

 

[flecc]

I also find it very difficult to believe that scale of loss, even with toothed gears. If the Tongxin really does lose that much through reduction gearing (which is in the downward so advantageous direction), it's roller drive is terribly inefficient, which makes no sense.
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[apshamlton]

Perhaps the lower efficiency at lower rpms is only part of the answer.
What's noticable about the performance figures I have is that the 190, 210 and 260rpm motors all have 20 N-m max torque but that the higher rpm motors put out max torque at higher rpm thus have higher power. (Output power = torque (in N-m) x rpm x 2∏ / 60)

Maybe Tonxgin have limited the torque output to 20 N-m to prevent roller slippage or perhaps its an inherent limitation of the roller design.

20 N-m is still pretty good for hill climbing though. The Bionx 500 watt motor, which is twice as heavy (4kg) and puts out twice the nominal (500watt) and max power (1200watt) only puts out 40% more max torque (28 N-m).

 

[flecc]

Yes, I had noticed that torque difference when I saw the Tongxin charts at an earlier date. I'm sure you are right about the limitation to benefit the roller drive, since the slippage was a source of much criticism originally.

It remains a fine motor when suitably used though, and in the 16" wheel Brompton it's difficult to imagine how a motor could be more perfect for an application.
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[HarryB]

Well I am even more confused so perhaps Flecc you can explain some things to me.

As far as I was concerned changing the size of the wheel changes the gearing in exactly the same way as changing it with gears. The end result should surely be identical for say a 16 inch wheel (260 rpm motor) with a top speed of 13/14 mph and a 700c wheel with an identical top speed (say 160 rpm motor). But we are saying that the latter produces less power? How is this done? By varying the number of windings? It isn't done at the controller as they are all identical. Surely the loading on the roller drive is exactly the same for both so no need to limit the power. Of course a 260 rpm motor in a 700c wheel that is a different matter. Confused...

PS the power characteristics of the latest motor I have is quite different to the other tongxins I have experienced. The present motor drifts up to the top speed and any slight hill or headwind dramatically reduces the speed. This is quite at odds with a motor with lower gearing, you would expect the top speed to be firmer. Compare driving a car in 1st and 4th to get an idea of what I mean. A hill would have no effect on the speed in 1st but a dramatic effect in 4th.

 

[flecc]

I don't know how they are doing it, but I feel sure it's within the motor. If those differing power ratings are as given, and they match what I've seen previously, it can't be due internally to the gearing degree of the roller drive since it's efficiency couldn't change that much as you've said Harry.

There's an issue outside of the gearing ratio though as I remarked before, and that's the effect of wheel size. A small wheel like a 16" has a big leverage advantage over that of a large wheel so the likelihood of roller slipping under drive in a given condition of use is much less than with the large wheel. It follows that a greater amount of power can be applied prior to the onset of slipping. This would explain Tongxin's objective in varying the power in this way. The gearing ratio compensation does not compensate for this wheel size leverage difference since the leverage effect applies between the road and the roller drive and is independent of the gearing ratio.

As for why Tongxin feel the need to reduce power for the larger wheel motors, I can only assume it's the slippage knowledge they've gained over time.
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[HarryB]

I was thinking that it was the same as fitting larger wheels on your car and then reducing the final drive to match would be the same, but in this case the clutch (roller drive) is in a different position. I also had assumed that the roller drive itself did the reduction but I could be wrong as I haven't seen any internal pictures.

Reducing the power seems an inelegant solution. I would certainly put up with a larger diameter hub if that was all it took but I doubt they will bother redesigning it. We await the perfect hub motor for ebikes then.

 

[flecc]

I'm sure the roller drive itself does do the reduction Harry, but the wheel size leverage effect is between the output force friction surface of that drive and the road surface, so that's why the ratio of the gearing will have no effect on that.

As you rightly say, power reduction is an inelegant solution, but if the better solution is a larger and heavier motor, they would lose a major part of their sales advantage. In any case, I'm not convinced a friction drive with marginally larger components would make much difference. Friction roller drive limitations appear to have been recognised by other major manufacturers since there's been no rush for the majors like Suzhou to join in, only one minor imitator (FWF mini-motor) to date and that one also small in size and power. It could end up a technological dead end.
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[AndyOfTheSouth]

Very interesting thread, this. I wonder if Didi has any comments from a German perspective or his own experience of the Tongxin clone?

Harry - isn't the onus on Cytronex to sort out your bike? That power reduction must be maddening!

More generally, if Tongxin has reduced the power of the motors intended for 26" and 700c wheels, wouldn't this have implications for Cytronex? Unless they use different versions and perhaps deal with slippage problems differently eg via customised controllers.

 

[flecc]

More generally, if Tongxin has reduced the power of the motors intended for 26" and 700c wheels, wouldn't this have implications for Cytronex? Unless they use different versions and perhaps deal with slippage problems differently eg via customised controllers.

Cytronex don't intend the motor to be used when setting off from a standstill, and from the outset have stressed the bikes should be pedalled to about 5 mph before switching on. In practice on those light bikes this isn't an imposition and it's only a turn or so of the pedals. Since it's when setting off under power that slippage is most likely, they largely avoid the potential for a problem.
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