Help, please with my Oxygen...

[GLJoe]

one important design goals is to minimize battery consumption.
you want the motor to be most efficient where you expect your customers to ride most of his/her miles at (speed-wise).
..
If I build an s-pedelec, the sweetzone will be 28mph.

Interesting. One of the reasons I ask the question is to do with battery consumption as I've been wondering whether there is an optimum (albeit compromised) speed for max spedelec range v reasonable time of journey.
My pondering goes something like this: (I'm assuming a direct drive hub)
If you ride slowly, then you're operating the motor inefficiently, as much of the current is wasted as heat in the motor windings.
The no load speed on an spedelec must be by definition be 28mph or more (if you desire assist at this speed), however wind resistance now becomes a major factor in 'wasting' energy.
So too slow is bad, and too fast is bad.
There is probably some compromise speed in the mid range that gives the best range, but I'm not sure what that would be.

I'm also intrigued at your comment that you'd choose the sweetzone at 28mph. Not having the irritatingly slow 15mph cutoff is one of the appeals of an spedelec, however I would not have thought that most spedelec users would ride at 28mph most of the time. I'd take a guess (and of course this is a guess and it will be different for different people and different bikes), but around 23mph is probably a more comfortable speed to ride most of the time ??

 

[Woosh]

but around 23mph is probably a more comfortable speed to ride most of the time ??

quite possibly for the majority of commuters but the law is the law.
Bikes like the Oxygen and the Woosh Rio can do this quite efficiently if the law allows them.

 

[GLJoe]

quite possibly for the majority of commuters but the law is the law.
Bikes like the Oxygen and the Woosh Rio can do this quite efficiently if the law allows them.

Forget the law for a moment, lets pretend we live in another country ...
You said you'd would choose to design an s-pedelecs sweet spot at 28mph. I'm asking, why would you do that, when that might not be the speed that the majority of cyclist would travel at, the majority of the time ?
Why wouldn't you design for say 23mph?
Wouldn't designing for 28mph mean that if you were travelling slower (at sat 23mph), you'd be wasting more battery power through heat ?

 

[Woosh]

Forget the law for a moment,

Bikes like the Oxygen and the Rio like they are at the moment have a noload speed of 24mph, their maximum efficiency is at 19mph, maximum torque at 18mph.
For 23mph, I would rather swap out the rear hub motor for a crank drive motor and keep the rest like it is. The cost is only £50 extra and I can extend the sweetzone from 10mph to 23mph. It's good for up to 28mph and can climb steep hills at lower than 10mph.

 

[GLJoe]

Bikes like the Oxygen and the Rio like they are at the moment have a noload speed of 24mph, their maximum efficiency is at 19mph, maximum torque at 18mph.
For 23mph, I would rather swap out the rear hub motor for a crank drive motor and keep the rest like it is. The cost is only £50 extra and I can extend the sweetzone from 10mph to 23mph. It's good for up to 28mph and can climb steep hills at lower than 10mph.

Ok, but you haven't answered the question.
Why did you initially say you'd design an s-pedelec with a sweet spot speed of 28mph ?

 

[Woosh]

Ok, but you haven't answered the question.
Why did you initially say you'd design an s-pedelec with a sweet spot speed of 28mph ?

if you design a 28mph commuter bike then you have to meet the customers'
expectation, solid performance at 28mph whatever the weather, headwinds etc. That means a design maximum noload speed at around 35mph. You need a motor capable of 1000W raw output.

 

[anotherkiwi]

The market for the s-pedelec in the EU is split into two categories:

- In the flatter lands they are used for long range or inter city commuting where speed would be the prime reason for buying and they would be used WOT a lot of the time.
- in Austria, Switzerland and parts of Germany more power for better hill climbing so speed is less of a selling point.

Here moped law makes the s-pedelec practically useless, helmet, insurance, top speed limited to 45 km/h... Second 45 km/h is a dangerous speed on a bicycle in town here because you are perceived to be doing much less and cars pull out in front of you all the time. Personal experience from the short period my bike was in s-pedelec mode. The 250 W GSM is capable of s-pedelec speeds on the flat no sweat...

 

[GLJoe]

if you design a 28mph commuter bike then you have to meet the customers'
expectation, solid performance at 28mph whatever the weather, headwinds etc. That means a design maximum noload speed at around 35mph. You need a motor capable of 1000W raw output.

Ok, so if someone did design a bike with those specifications, what would be the optimum speed to ride in terms of maximum distance on a battery charge?
At a constant 28mph, I'd be guessing that the wind resistance is pretty high. However at lower speeds (to minimise the wind drag), because the motor is operating away from its sweet zone/max operating efficiency, then you're now getting more heating losses than usual.
So what speed gives the max mileage. Any ideas?

 

[Woosh]

So what speed gives the max mileage. Any ideas?

the average battery usage per mile is always proportional to the cube of your speed. There are two ways you can reduce WH per mile, higher user contribution and lower your speed. For commuting, I think around 15mph for all bikes. At that speed, the total energy required is 180W or 12WH/mile on a lightweight e-bike. It's very easy to input through pedaling about half (90W, 6WH/mile), the motor tops it up with the other half, average battery consumption 6WH/mile, at that speed, there is not much difference between geared hub and crank drive bikes, even those built for 28mph. If you go up to 20mph (the ideal speed for bikes like the Oxygen and Woosh Rio), energy required is 320W or 16WH/mile on this sort of bikes, your pedaling may start to plafond, strong pedallers may get battery consumption at 8WH/mile, weak pedallers can start to see their battery usage climbs up to 9WH/mile - 10WH/mile.
At 23mph (the ideal speed for derestricted CD bikes like the Bosch CX or the Woosh Krieger), your battery will have to be more than good, energy required goes up to 500W or 21WH/mile, battery consumption shoots up to 13-15WH/mile.
At 28mph, energy required is now 840W or 30WH/mile, battery consumption 22-24WH/mile.

 

[GLJoe]

the average battery usage per mile is always proportional to the cube of your speed. There are two ways you can reduce WH per mile, higher user contribution and lower your speed. For commuting, I think around 15mph for all bikes......

Some interesting wattage figures. Thanks for those.

But another question if I may:
You say two ways to reduce WH/Mile, but what about at the design stage when choosing a motor?
What I mean is this: (Again, assuming a direct drive hub motor)
Bike A has a motor designed so its sweet spot delivery is around 15mph
Bike B has a motor designed so its sweet spot is higher, say 23-28mph
Both bikes are otherwise identical and are ridden at 15mph. How much difference might there be in the WH/mile then?

 

[Woosh]

What I mean is this: (Again, assuming a direct drive hub motor)
Bike A has a motor designed so its sweet spot delivery is around 15mph
Bike B has a motor designed so its sweet spot is higher, say 23-28mph
Both bikes are otherwise identical and are ridden at 15mph. How much difference might there be in the WH/mile then?

10% or less difference for DD motors, assuming that somehow you can find one with sweetzone as low as 15mph.
Most DD motors have their sweetzone around 25mph and efficiency at around 70% for 15mph.
Assuming 50% user input, battery consumption is around 7WH/mile for both motors.

 

[Deleted member 4366]

When you run a motor with less than full power or full throttle, the efficiency curve shifts down the rpm range. A 30 mph motor running at 15 mph on the flat would have similar consumption as a 20 mph one also running at 15 mph. The difference in efficiency would only be noticed during certain circumstances, like acceleration or high power situations.

If you're going to limit the speed to 15 mph, there's no advantage in using a 30 mph motor, and there's plenty of disadvantages.