Can you software limit a bafang motor to 250w to make it road legal?

[mike killay]

Strangely enough, in the 7 years or so that I have been ebiking, the only time that I have even seen a Police Officer is in Belgium and then only when attending the 8pm ceremony at the Menin Gate.
You just don't see real Police in Swansea, occasionally there might be one of their Community Support Officers

 

[Benjahmin]

The only time I've seen or spoken to a copper whilst riding was a local pcso who was on a mountain bike. He showed a great interest as he was trying to persuade his seargent to get him one. He never mentioned laws at all, I think if he hadn't had to shoot of to a traffic incident (a tractor/harvester blocking the high street) he'd probably would have had a go. That would have been interesting 'cos my bike does have a full acting throttle and I have the receipt from November 2014, also my motor is not stamped/labelled at all. Oooer

 

[GLJoe]

Yes of course, if limited, but many hub motor bikes are not like that. My legal one at 36 volts nominal is only limited by voltage. When first charged it runs to 17 mph but by end of charge it's around 14 mph.

Fascinating. I confess I wasn't aware that there were bikes out there that are as unsophisticated as you seem to be describing - not modern ones at least - unless yours is an older type, (but then it seems that's not applicable to the conversation as the OP is talking about current, programmable bafang kits?)

Thinking on this though, it does seem a bit unsatisfactory to rely on the natural power cutoff of the motor design? surely that meant that to be legal, it should not assist past the 25kph (+/- bla bla), but do do this, then the assist must be pretty weak at the higher end? Most people complain bitterly with bikes stopping at the 16.5 odd mph mark. Dropping that to 14mph (pr maybe less in terms of usable assist?) when the battery is getting low seems ...well ... horrible !

That will be what that police officer was thinking of, but misinterpreting as a ban on 48 volts.
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Said as a statement of fact, however something tells me that if the officer didn't even know that you could cycle faster than the cutoff, he's not going to have a clue about any of the above!!

 

[peter.c]

We all know this takes some common sense to interpret the rules but 6 points on your driving licence for no insurance and points for no mot and what ever else is a huge risk . The term must not exceed 48v is pretty clear they use that term for speeding and drink drive just because so far a successful prosecution has not occurred. Some ware there will be someone who has read every part or the standards and we all know this would be hard to prove and even harder to find an independent expert

 

[flecc]

Thinking on this though, it does seem a bit unsatisfactory to rely on the natural power cutoff of the motor design? surely that meant that to be legal, it should not assist past the 25kph (+/- bla bla), but do do this, then the assist must be pretty weak at the higher end? Most people complain bitterly with bikes stopping at the 16.5 odd mph mark. Dropping that to 14mph (pr maybe less in terms of usable assist?) when the battery is getting low seems ...well ... horrible !

Quite the opposite, you're forgetting current. Nominally it has a 20 amp controller but in fact it can deliver nearly 30 amps. Two of us, the other being member Tiberius who is a scientist at one time advising on e-bike technical committees, have measured gross power at 1000 watts.

A to B reviewing the bike said this, "open the throttle and it just takes off at around 16 mph, hills just get flattened".

So although the revs are limited by the voltage and windings, power is there in abundance. The 14 mph is only in the very last stage of battery charge, so in practice rarely reached. Legality doesn't come into it, the DfT has confirmed that the usual speed limits 10% tolerance applies, making 17 mph assist legal.

It's still a current model but I believe the controller current capability isn't quite as high now. Of course none of this applies to most crank units which have been sophisticated to death, losing most of the original simplicity of the e-biking concept.
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[Woosh]

Of course none of this applies to most crank units which have been sophisticated to death, losing most of the original simplicity of the e-biking concept.

they need to justify difference in price.
Take Raleigh for example.
Their current hub drive Raleigh Array E costs £1,100 in the shop. Their cheapest crank drive Raleigh Captus costs £1,350.

 

[GLJoe]

Quite the opposite, you're forgetting current. Nominally it has a 20 amp controller but in fact it can deliver nearly 30 amps.
...
So although the revs are limited by the voltage and windings, power is there in abundance. The 14 mph is only in the very last stage of battery charge, so in practice rarely reached. Legality doesn't come into it, the DfT has confirmed that the usual speed limits 10% tolerance applies, making 17 mph assist legal.
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Ok, I've been thinking on this, but I still don't understand, so help me out if you will.
If there is no programmed cutoff and the bike is 'cutting off' assist by virtue of it naturally reaching its maximum RPM, and the back EMF causing there to be no power delivered from the motor, then if this cutoff (virtually zero assist) is 17mph (at maximum battery capacity), the power curves I've seen from typical motors would suggest that not only is there zero assist at 17mph, there could be a good few mph below this where its very weak. What would its max output be, perhaps 10-12mph? that still doesn't sound very satisfactory to have this decreasing power after this point. And as the battery voltage gets less, those speeds decreases even further.
If at 17mph there is still a good amount of power being delivered, then wouldn't this mean that the bike would keep delivering (albeit decreasing) power ABOVE 17mph, making it illegal - because its assisting above the limit??
And if you design it so that its still assisting at 17mph when the battery is at minimal level, then it would be assisting WAY above the 17mph when the battery is full ?!?

Also, you say I'm forgetting current, but if you're limited by the back emf of the motor opposing the applied voltage of the battery/controller, isn't it a voltage issue and not a current issue? how can you 'increase the current' to overcome this??

 

[flecc]

Ok, I've been thinking on this, but I still don't understand, so help me out if you will.
If there is no programmed cutoff and the bike is 'cutting off' assist by virtue of it naturally reaching its maximum RPM, and the back EMF causing there to be no power delivered from the motor, then if this cutoff (virtually zero assist) is 17mph (at maximum battery capacity), the power curves I've seen from typical motors would suggest that not only is there zero assist at 17mph, there could be a good few mph below this where its very weak. What would its max output be, perhaps 10-12mph? that still doesn't sound very satisfactory to have this decreasing power after this point. And as the battery voltage gets less, those speeds decreases even further.
If at 17mph there is still a good amount of power being delivered, then wouldn't this mean that the bike would keep delivering (albeit decreasing) power ABOVE 17mph, making it illegal - because its assisting above the limit??
And if you design it so that its still assisting at 17mph when the battery is at minimal level, then it would be assisting WAY above the 17mph when the battery is full ?!?

Also, you say I'm forgetting current, but if you're limited by the back emf of the motor opposing the applied voltage of the battery/controller, isn't it a voltage issue and not a current issue? how can you 'increase the current' to overcome this??

Of course there's little power left available at 17 mph (no load spin is just over 19 mph) but why would one want it at the assist limit? It's not supposed to be there then so couldn't be used! All that matters is that there's plenty enough power at 15/16 mph for riding on the flat, even without any pedalling and it accelerates to that very quickly indeed . On a hill the speed drops anyway, pushing the motor further back into the power band. On a 7% near me starting slowly without any pedalling the speed gradually increases and I crest the brow at 11 mph. Climbing the other side 10% without any pedalling is at 9 mph. I can start throttle only without pedalling on 12% at a crawl, and restart with pedalling on a 22% hill. Who would want more? It's a perfectly viable way to set up an e-bike and in the earlier days they were mostly like this without the complication of speed limitation controls. And far nicer than a sudden cutoff of power still present at circa 15 mph.

This is a 20" wheel bike. Exactly the same electrical setup was put onto a 27" wheel model by the makers and which I also owned at one time, the motor unchanged but in the front wheel instead. Also unrestricted it assists to 22 mph at which the power left can't drive it faster, but with the front wheel off the deck it spins to 26 mph. Higher geared like that for high assist speed with lower torque means it has far less hill climb ability of course.
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[GLJoe]

Of course there's little power left available at 17 mph (no load spin is just over 19 mph) but why would one want it at the assist limit? It's not supposed to be there then so couldn't be used! All that matters is that there's plenty enough power at 15/16 mph for riding on the flat, even without any pedalling

Wrong. It might be all that matters to YOU, but that doesn't mean its satisfactory for other people.
For starters, many people find riding on the flat at 15mph achievable WITHOUT needing an e-bike, yet those same people can struggle with inclines but don't want to have to slow down. I'm one of those people (and I know I'm not alone!).
When I commute to work by bike, apart from the obvious places like junctions and traffic lights, my MINIMUM speed is almost always on the cutoff limit, and I want to be able to maintain that even when tackling hills. Its annoying enough cutting off around the 16-17mph mark, not having enough spare power to allow even this when hitting an incline would be frustrating in the extreme.

On a hill the speed drops anyway, pushing the motor further back into the power band. On a 7% near me starting slowly without any pedalling the speed gradually increases and I crest the brow at 11 mph. Climbing the other side 10% without any pedalling is at 9 mph. ....
.... Who would want more?

Me for one, and I suspect a large percentage of e-bike owners!

It's a perfectly viable way to set up an e-bike and in the earlier days they were mostly like this without the complication of speed limitation controls. And far nicer than a sudden cutoff of power still present at circa 15 mph.

This is a 20" wheel bike. Exactly the same electrical setup was put onto a 27" wheel model by the makers and which I also owned at one time, the motor unchanged but in the front wheel instead. Also unrestricted it assists to 22 mph at which the power left can't drive it faster, but with the front wheel off the deck it spins to 26 mph. Higher geared like that for high assist speed with lower torque means it has far less hill climb ability of course.
.

Yes, because you're using an unsophisticated setup. If instead you had the 'complication' of a programmed cutoff limit, you can design a system where the back EMF of the motor doesn't start to significantly interfere with the normal power delivery, yet the controller keeps you within the speed definitions of the law.
You may yearn for the old days when things were simpler, but simpler doesn't always mean better.

 

[flecc]

Wrong. It might be all that matters to YOU, but that doesn't mean its satisfactory for other people.
For starters, many people find riding on the flat at 15mph achievable WITHOUT needing an e-bike, yet those same people can struggle with inclines but don't want to have to slow down. I'm one of those people (and I know I'm not alone!).
When I commute to work by bike, apart from the obvious places like junctions and traffic lights, my MINIMUM speed is almost always on the cutoff limit, and I want to be able to maintain that even when tackling hills. Its annoying enough cutting off around the 16-17mph mark, not having enough spare power to allow even this when hitting an incline would be frustrating in the extreme.


Me for one, and I suspect a large percentage of e-bike owners!





Yes, because you're using an unsophisticated setup. If instead you had the 'complication' of a programmed cutoff limit, you can design a system where the back EMF of the motor doesn't start to significantly interfere with the normal power delivery, yet the controller keeps you within the speed definitions of the law.
You may yearn for the old days when things were simpler, but simpler doesn't always mean better.

I've owned both systems and ridden a number of others so appreciate the advantages either way. Neither are wrong as you put it, they just suit different people with different requirements. Personally I hate a sudden cutoff at circa 15 mph and I'm very far from alone in that in this forum. We've even had whole threads about the nuisance of having to ride a bit below or a bit above cutoff to avoid the power snatching in and out at that point.
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