Cytronex lacking "go"

[Barnowl]

While we are on the subject of acceleration and thrust, AtoB have just conducted a review of our Cannondale Capo single speed bike. Whilst David doesn't reveal much before publication, I did get an email to say that he had found the same speed as the Trek last year (which had 24 gears) but 30% greater range.

A 30% incease in range is quite significant - I make that about a 6 mile improvement.
The Cytronex Capo at 13.6Kg is lighter than the Trek. Also the dropped handlebars (streamlining) and 700X25c tyres (rolling resistance) must improve performance making for less effort up the hills and more time at 15mph+ without assisance. Makes sense.
The Synapse though a tad heavier (with gears) must be very similar especially with drops fitted. Another member themutiny has also indicated an impressive range. I think these are great bikes for those of us who like to put some effort in and aren't doing those really long commutes day in day out. I'm still having a great time with the Trek.

 

[HarryB]

Just re-cap and update this thread. I returned my bike to Cytronex a few months ago as something came adrift in the freewheel. A new motor was fitted but the bike was very different and lacked both the power and speed of the original.

Last weekend I was in Winchester so took the bike back to Mark to have a look at it. As it is has the right 175 rpm motor he suggested two possibilities. I may originally have had the 190 rpm motor fitted by mistake (mislabelling) or that some motors have various degrees of current limiting built in and I have one such motor - luck of the draw I suppose.

I will never know which of these possibilities is right. The original motor, even with a fully charged battery, never seemed to be illegally fast but it did squeal of the line in a way that suggests that it was higher geared. The present bike is slower off the line so it suggests some current limiting is going on. I suspect the specification may have changed to increase the reliability of the roller drive but it does leave the bike feeling flat (by the way this is the third Tongxin motor I have tried and by far the least powerful).

I may go for a new wheel at some stage when the rim wears out.

PS why is the 175 rpm motor 'right' for a 700c wheel when according to the equation (supplied by Tongxin)...

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

I think it should be more like 190 rpm - anybody know?

 

[flecc]

That equation just seems to be wrong against reported speeds Harry. The Brompton nano with 16" wheels and the 260 rpm motor produces 13 to 14 mph/20.8 to 22.4 kph with a fully charged battery for example.

The equation says this indicates 217 to 234 rpm at the motor, so clearly the equation must be wrong.
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[HarryB]

That equation just seems to be wrong against reported speeds Harry. The Brompton nano with 16" wheels and the 260 rpm motor produces 13 to 14 mph/20.8 to 22.4 kph with a fully charged battery for example.

The equation says this indicates 217 to 234 rpm at the motor, so clearly the equation must be wrong.
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Tongxin recommend the 260 rpm motor in a 16 inch wheel for a max speed of 20 kph, which is more or less what it does (probably a bit more with a li-ion battery). Assuming 16 inches is about 400mm then the equation is about right.

 

[flecc]

If well over 10% error is about right.
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[HarryB]

When you are dealing with a battery with a nominal voltage of 36V (NiMH) or 37V (Li-ion) that can either kick out 44V when fully charged or 32V when nearing discharge then yup, 10% seems reasonable to me

 

[flecc]

You can't get me with that one Harry.

That 44 volts is only quiescent even if present (most li-ion chargers only run to less than 42 volts), once connected to a load it's about 39 volts briefly but 37 volts most of the time, hence them being rated at that. That Brompton lives nearly all of it's range at 13 mph, making the equation error 14% most of the time.

It's too wrong for me anyway. Maybe the roller drive is slipping slightly all the time.

That would do it.
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[HarryB]

You have trumpt me because I cannot work out how the difference between 20kph and 20.92kph is 14%.

But fitting a motor that only pulls to 13 mph rather than 15.5 mph is just another example of under-rating the Tongxin motor. I don't understand why they are so cautious when fitting this motor - personally I want it to pull to the legal limit even if with a fresh battery it goes half or one mile an hour over.

 

[flecc]

I think I know what is going on here. One is a no load figure, the other a real world under normal light load figure. From your Torq days you'll probably remember how with front wheel lifted the motor would spin the wheel to 26 mph, but on the road the absolute tops was 22 mph. The difference there is 15%, so I think that could be the answer to a discrepancy.

I certainly agree on wanting the real legal speed at least, but I suppose they've suffered so much with controller problems and some spindle problems worldwide that they want to be really cautious now. A pity though, since it's so uniquely good a motor in all other respects.

Some restriction seems to be the trend with the Tongxin motor now though. Schwinn use it on their bikes and have now adopted a low power approach with an SCiB battery on their newest model, and when Sun started using the Tongxin they also adopted the low power approach using a very small handlebar mounted LiFePO4 battery. And of course the Nano-Brompton is geared for a very low top speed, though in that case it's a byproduct of the 16" wheel.
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