Hub (Tongxin) power delivery

[andyh2]

I think I've got a reasonable understanding (thanks to information on here) of how my Agattu gradually phases down power as it approaches particular (65) rpm. Which on standard gearing equates to legal 15mph maximum.

What happens with the Tongxin motor though. Take for example a 175rpm motor in a 700c wheel. Does the 175rpm represent the maximum revs that motor will produce power at and if the wheel turns at 176rpm all power is stopped and the motor is freewheeling. Or is there a gradual reduction in power approaching the max motor rpm?

 

[flecc]

If the controller hasn't got interfering software to modify the responses with speed, the 175 rpm is normally the maximum speed and therefore the maximum speed at which you get full power, the fall off almost instantaneous above that. That speed on a 700c wheel and tyre is roughly 18 mph if Tongxin's information concerning the RPM relationships to EU law is correct. I think in fact it could be a bit higher which will proportionally influence the later figures I'm giving.

From the point of coincidence of maximum torque and maximum power, on that version in a 700c wheel being a bit over 11 mph, the motor efficiency is around the maximum until a little before the end of maximum power, in this case at around 16 mph. That means if you run at 16 mph you will achieve the best economy to power ratio. However, there is no roll off in power unless the controller provides for it, though rising natural effects such as from rolling and air resistances do give some natural roll off in acceleration rate as speed increases.

Tongxin users will probably have noted whether their controllers do give any roll off.
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[frank9755]

I've not noticed any falll-off approaching top speed, although I'm not really the best person to comment because on my motor top speed is around 24mph so it doesn't get exceeded too often, and my controller is a third party one, not a Tongxin original.

 

[andyh2]

So there is the similarity with the Agattu that once you get past the max rpm (wheel for Tongxin, crank for Agattu) the motor is off and drawing nothing from the battery regardless of whether or not the throttle is open.

And if that same 175 rpm motor was in a 20" wheel the same thing would be happening, but with the max power to energy use at around 12mph and outright max assist to around 13mph.

I'm looking at getting a Tongxin kit and might fit to a 26" wheel crankforward bike or 20" front wheel long wheelbase recumbent. So if I choose the latter with a 20" wheel with the power being delivered lower down the speed range I could expect better assistance at lower hill climbing speeds, but no assistance at normal cruising speeds above 13mph.

 

[flecc]

So there is the similarity with the Agattu that once you get past the max rpm (wheel for Tongxin, crank for Agattu) the motor is off and drawing nothing from the battery regardless of whether or not the throttle is open.

Yes, though the Agattu is cutting off the power with it's software as determined by the unit's measured output shaft speed rather than at motor maximum rpm, since as you know, it doesn't run constantly, just operating on the pedal inputs.

or 20" front wheel long wheelbase recumbent. So if I choose the latter with a 20" wheel with the power being delivered lower down the speed range I could expect better assistance at lower hill climbing speeds, but no assistance at normal cruising speeds above 13mph.

In fact you'd be repeating the Nano-Brompton performance characteristic, also a good hill climber but runs out of assist power at 13/14 mph, the exact point depending on the battery charge state remaining.
.

 

[frank9755]

You may want to read this thread on a successful implementation in a 20" wheel recumbent

 

[andyh2]

I was thinking a lower max assist speed could work on a Tongxin conversion as it can be kept reasonably light. I really like the ride on my Agattu, but the weight means some cruising speed assistance is welcome.

I've seen that thread thanks Frank, Jeremy did such a fine job on it. He should market those battery packs and control consoles! I think he chose a motor speed to give him assist at cruising as well as climbing speeds.

I guess an advantage of lower max assist speed is conservative battery use.

If I have to choose one it will probably be the crankforward bike as it is more of an all rounder and can be used by a wide range of different height folk so it can fit me at 6' and my wife at 5'3". Of course it would be nice to do both just for the fun of it. And then again there's always the mad notion of a 'Super Agattu' with a Tongxin front hub added, perhaps a 260rpm version to assist at the upper end. Not legal of course, but it'd be a fun way to get to work

 

[Orraman]

andyh2,

You may want to research the traction that is available on front wheel drive, crank forward and particularly long wheelbase recumbents when climbing steep hills.

Dave

 

[andyh2]

That is worth considering Dave. Thinking about it I found the LWB recumbent pretty squirrily going up steep hills and fitted a steering damper to reduce the front end wandering. So it probably would be better suited to a rear drive solution. I think there'd be no problem with the crank forward as the weight distribution is a bit lighter at the front, but not extreme. I've seen a report of a Crytalyte motored Rans Dynamik (same base bike as mine) that apparantly worked very well.