Bit confused about the Bosch Drive Unit Performance CX

[PH001]

Obviously quite a few bikes around with this motor such as the Cube Cross Hybrid Race AllRoad 500.

Although I believe this is officially classed as a 250w motor, part of the spec states...

"The highlight of the Cross Race Hybrid is Bosch's CX Drive Unit. With up to 75Nm torque and a maximum of 600W of additional power, it's the most efficient pedal assist system currently available."

Can somebody help explain what that is all about and what are the legal implications?

 

[RobF]

All motors deliver higher peak power than their nominal watt rating.

The Bosch CX is fully road legal, as are the other crank drives from the likes of Yamaha and Shimano, which also peak at higher than 250w.

Bosch has decided to release figures for the CX, but they are meaningless.

None of us know what a watt or Newton metre feels like, but more importantly the measurement method of these figures is not universal, so you cannot use them to compare the Bosch to motors from other makers.

They may quote higher or lower figures, but the other motors could feel stronger or weaker.

Only way is to have a go on a couple of bikes and see which you prefer.

 

[PH001]

Interesting. So 'peak' output of the Bosch system can actually be as high as 850W, with only the average power being 250W in 'cruise' mode as they call it.

From a performance standpoint, that must make a huge difference. Power of course is just torque x rpm, so the quoted 75Nm would produce 850W at a motor speed of 108rpm (1.8 motor revolutions per second). It would depend on the gearing to translate that to actual roadspeed.

I was just interested to understand under what circumstances it produces 75Nm, does it have some kind of dynamic torque curve, or is all this just specmanship and actually always produces 75Nm when full pedal pressure is applied, irrespective of motor speed?

If the latter is the case, then the maximum motor speed @ 75Nm would be just 32rpm (0.5 motor revolutions per second) to stay within the 250W max power envelope. Or perhaps the torque is programmed to drop off as the motor speed increases?

 

[Wisper Bikes]

Hi Phil, according to my notes the Bosch CX peaks at 612W about the same as most good electric bikes on the market in Europe.

 

[PH001]

It seems to me like the 250W limit is taken largely with a pinch of salt by most manufacturers, it is more the 25km/h which is strictly adhered to.

To put this into perspective, you could have a motor which applies a massive amount of torque at very low rpm and still stay within 250W, but at high rpm the torque would have to be much less. This kinds of all lends itself to a limited maximum speed so makes sense.

So I guess what we should really be paying attention to is how much torque each particular motor is capable of producing as this is likely to have the most direct effect on perceived performance. Quite strange then how so few manufacturers specify it.

 

[Wisper Bikes]

I agree, the amount of torque generated is hugely important. Which of course brings us to the next big question, where is the torque measured?

Crank dive bikes publish higher torque figures than hub bikes but the measurement is completely different. A new grey area, just what the industry needs!

 

[PH001]

Comparing one crank drive with another is simple enough as it is simply the torque output of the motor before gearing. Crank vs hub is a different matter altogether!

I guess that's why they went with a power limit, and I am assuming that is average electrical power into the motor as it's much easier to measure volts and amps (whether it's real power or reactive power is another grey area though as the power factor into a typical inductive motor is rarely close to 1). You might have a motor rated at 300 to 400VA but only 250W.

If it is input power that is restricted, the motor efficiency plays a big part as well.

 

[Ray Breen]

Hi Guys,

Just out of interest thought i would post this information the Woosh motors, that i came across while researching what Bikes to go for.

http://wooshbikes.co.uk/?motorcharts

Interesting reading

 

[flecc]

Interesting. So 'peak' output of the Bosch system can actually be as high as 850W, with only the average power being 250W in 'cruise' mode as they call it.

They are exaggerating with the word "additional". Their meaning of additional could be additional to the rider effort.
As David at AMPS says, the 600 watts is about all that's developed.

Or perhaps the torque is programmed to drop off as the motor speed increases?

That will happen anyway without programming. An electric motor's torque is maximum at zero revs and declines thereafter as revs rise.
.

 

[PH001]

That is a very interesting link Ray - thanks for sharing. Can we conclude from that that none of these motors seem to be restricted to anything even close to 250W at any point of their performance curves. I wonder what the s-pedelec gestapo will think of this.

 

[flecc]

That is a very interesting link Ray - thanks for sharing. Can we conclude from that that none of these motors seem to be restricted to anything even close to 250W at any point of their performance curves. I wonder what the s-pedelec gestapo will think of this.

Indeed none are, the lowest power pedelec motor I've ever known of topped at over 300 watts. In general the moderate mainstream motors are around 400 watts.

Most of the "pedelec gestapo" became reconciled to this quite a long time ago, though we occasionally get a new member foaming at the mouth about it, insisting it cannot be. That happened recently, but even that member has accepted the fact now.
.

 

[KTM Bike Industries UK]

That is a very interesting link Ray - thanks for sharing. Can we conclude from that that none of these motors seem to be restricted to anything even close to 250W at any point of their performance curves. I wonder what the s-pedelec gestapo will think of this.

as a proud member of the s-pedelec gestapo I can point out that we, and I'm sure most people don't have a problem with the power or speed at all.

The 250w nominal power is a restriction, as is the assist speed limit, the restriction is not on peak power, or on the speed you can ride without assistance.

The issue is that if you're using a bike that has a 250w nominal power, its legal to use it.... if you use an sPedelec with 350w nominal power, its illegal to use it in 99.9% of cases in the UK. Illegal use of eBikes has lots of implications to the individual and potentially impacts on the use of eBikes of others.

 

[PH001]

We must ensure this does not deteriorate into legal vs illegal rant, however it's quite clear from the spec examples above that none of the motors shown are even remotely limited to 250W nominal.

The original purpose of this post was to aid people in choosing the best motor on spec, instigated by the Bosch claim that the CX can output 600W of additional power, whilst also claiming 250W in 'cruise'.

It does beg the question as to what information the manufacturer has to submit (and to who) to get regulatory approval??

 

[KTM Bike Industries UK]

We must ensure this does not deteriorate into legal vs illegal rant, however it's quite clear from the spec examples above that none of the motors shown are even remotely limited to 250W nominal.

You're getting your nominal and your peak powers confused.

The 250w motors are restricted to 250w nominal power output... that doesn't mean the peak output can't be higher and the motors still be UK legal.

 

[PH001]

You're getting your nominal and your peak powers confused.

I'm an electronics engineer so I can assure you I am not.

Define 'nominal' for me and then explain on any of the graphs posted in Ray Breens link where nominal 250W is.

 

[KTM Bike Industries UK]

I'm sure you are, but I also think Bosch might also employ the odd specialist.

According to the dictionary of engineering, this is the definition of "nominal"

nominal power
A rated or named value stating the power that a component, circuit, device, piece of equipment, or system can produce, consume, dissipate, or otherwise safely handle, when used in a given manner. For instance, the nominal power of a speaker may indicate the RMS power that can be delivered to a dynamic speaker for extended periods, without harming the voice coil or other components. The actual value, or interval of values, may not coincide with this number. Also called nominal power rating, or rated power

http://www.dictionaryofengineering.com/definition/nominal-power.html

But in reality, its more correct I suspect to use the term "continuous power". Now I'm not an electronics expert, its something that 99% of eBike customers don't care or need to know about. The details put them off.

This is quite a good read:

http://www.ebikeschool.com/myth-ebike-wattage/

"How do ebike manufacturers get away with this? One way is to rate the motor for “continuous power” instead of “peak power”. The difference between continuous power and peak power is that continuous power essentially means power a motor can safely handle for an indefinite amount of time without damage or overheating the motor. A “250 watt continuous” motor, theoretically, could run forever at 250 watts without overheating, but any more power would cause it to eventually overheat. If the motor is truly a 250 watt motor by definition, then running this motor at 251 watts would eventually cause it overheat."

When we talk to customers and explain that a 250w motor can put out more at peak. We just use the term average, and say that in order to be legal in the UK the motor has to be rated at 250w nominal power, but this isn't something that joe public can check or even needs to be worried about really.

(edit)

none of the graphs Woosh have posted have an axis for time, so its not possible to look at the nominal power is it? They are just comparing Watts and speed... no mention of how long it can be maintained.

 

[flecc]

It does beg the question as to what information the manufacturer has to submit (and to who) to get regulatory approval??

The law does say "continuous maximum power".

Regulatory approval is by meeting the requirements of EN15194, but not all member states are signed up to that yet, last I heard only the UK and France had.

It appears that at present a manufacturer declaring a 250 watt rating is sufficient. I understand EN15194 does define two alternative power tests, but one of the options is based on an acceleration test, enabling easy circumvention of the maximum power limits.

And of course the expression "continuous maximum power" brings temperature into the equation, another thing a manufacturer can affect by specifying a low peak temperature.

N.B. Post crossed with KTM's above post.
.

 

[PH001]

What the motor is rated for is completely irrelevant. It's what it produces that matters.

All of the motors in the graphs produce in excess of 250W right from above 2-3 mph up to their maximum speed. If that's not 'continuous maximum power' I don't know what is! You can derive time simply enough from that.

If it is a self declaration as Flecc suggests (similar to CE) it doesn't surprise me that they do this in a market so competitive.

 

[KTM Bike Industries UK]

What the motor is rated for is completely irrelevant. It's what it produces that matters. All of the motors in the graphs produce in excess of 250W right from above 2-3 mph up to their maximum speed. You can derive time simply enough from that.

you're again confusing, what you might want to know as an engineer with what the law requires.

What a motor is rated for is what is required, the peak it can produce isn't required to be controlled.

 

[KTM Bike Industries UK]

Its easy for me to think about this as a cyclist because when I train I always use the look at my FTP, which is functional threshold power.

http://home.trainingpeaks.com/blog/article/three-ways-to-improve-functional-threshold-power

This is in my head what Nominal power is on the ebikes. I can put out way more than my FTP, but I can't sustain it for an hour. Now my FTP is more than 250w. So in theory on a flat ride over an hour with no other influence I'd beat a legal eBike, and I would. Now on a long flat hill I couldn't, but in a short sprint I could. So if you compare the watts (and I have power cranks on my road) bike. The numbers Bosch (and others) give out are pretty accurate.