Relationship between watts and torque?

[newheights]

I am curious if you can still get the full torque out of a motor if the watts are restricted? For instance, keeping a bike to 750 watts would open up more legal riding options. However, I still want good torque to help get up steep hills. Looking at bikes with bafang ultra or bbshd, and torque ratings in the 160-120 nm range, but also with wattage in the 1,000-1,500 range. Several vendors offer models with wattage restricted to 750, I am assuming because of the legal issues. I don't care about top speed, but don't want to sacrifice hill climbing power.

 

[newheights]

Perfect that's exactly what I was looking for. So at least on paper from a theoretical physics standpoint, you still get the same ability to provide significant force, just at lower RPMs. Sounds great to me.

In practical application, does this tend to work out as well? I suppose the main issue would be if the RPMs dropped too low. Presuming a mid drive motor, they are supposed to be able to handle low RPM and keep up with high force? Any other issues in the wild?

 

[retiredNH]

Torque and watts are two different things. "watts" is a measure of power, which is defined (among other ways) as force times distance. Torque is a measure of force. Since torque is usually used with rotary motion, a motor, power is torque times angular velocity (how far the shaft turns).

In the ebike world, torque gets you up hills at low speeds. Power (watts) controls (limits) how fast you can go up that hill.

Make sense? Good. Now, consider that it's hard to fudge torque numbers, but not hard at all to fudge watt numbers. You have input watts and output watts. You have peak watts and running watts. And with AC motors, you have power factor issues. So take advertised watt levels with a grain of salt.

 

[PedalUma]

I liked the video but what he did not say was, What if you ran the example higher torque motor through drivetrain gears? Would theoretical bike with a smaller lighter motor with less nominal power draw but twice the torque win, if it went through the drivetrain? In the real world this is what I am finding. The battery can then also be smaller and lighter to do the same work per unit time.

 

[retiredNH]

It's not that simple. Gearing can be either a torque multiplier or a speed multiplier. Think of it this way. Torque is what gets you started. Power is what gives you speed. Take cars and trucks as an example. Diesel trucks have motors with very high torque (and lots of gears for torque multiplication) so they can pull an 80,000 pound load from a standstill at a traffic light. Your typical car could not do that. But that same truck is horsepower (watts in a different name) limited so that it crawls up the hill that your car can zoom up.

A high torque low power bike motor will give you lots of oomph at low RPM but run out of assist when you spin faster. A hub motor would be even more interesting - good for getting you started, but soon out of oomph as you start going faster.

Smaller lighter battery? Yes, for a period of time. But you'll get fewer miles out of it. To make things even more complicated, batteries are not even rated in watts. The proper term is watt-hours, a measure of total energy. And energy is power times time. That is to say, how many watts for how long. Think of car batteries that used to be rated in ampere hours. For twelve volt car batteries, 1 ampere hour is 12 watt hours.

No free lunch here. There are many tradeoffs. And many things that affect ebike range, including wind and speed. But that's a separate discussion!

Maybe we should have a sticky on ebike physics?

 

[newheights]

But, at least hypothetically, with the bafang ultra 620 with 160 nm of torque, even with power limited to 750 watts, you could still climb up the same hill you could do with that motor at 1500 or even 2500 watts, just more slowly. It seems that it is not going to be the case that all of a sudden you can't do a 12% grade with the 750 watt setting, that you could do with the 1500 watt setting.

 

[PDoz]

Several vendors offer models with wattage restricted to 750, I am assuming because of the legal issues. I don't care about top speed, but don't want to sacrifice hill climbing power.

W (attevers) = N(ice) M(marketing) x S ( spin)

The advertised wattever figure is a combination of marketing and legal priorities , and if you do the maths it becomes clear that legal wattevers have very little to do with real world performance - especially when you start riding the more sophisticated motors designed to comply with international 250 NOMINAL wattever output, but at the same time compete for maximum marketing performance.

Look beyond the advertising and you will realise torque figures are ( watt) matters - they represent the force available by a motor at the peak of a sine wave , with each pedal stroke punching out a CURVE of torque. Meanwhile, the average wattever figure just reflects the area under that curve - think force from a punch as opposed to force from a push.

Keep that concept in mind whilst watching this advertisement, and you start to realise that by tweaking the timing of that punch manufacturers are starting to optimise both hill climbing ability AND BATTERY efficiency. They are literally offering the same motor ( shimano ep8 in this case) but with a martial art style punch that uses less watts yet still offers more than enough assistance to each pedal strike.

In the emtb world, specialized started this advertising race when they moved to publishing torque figures of their brose motor in 2017. Hmmmm....90 NM at 80 cadence but only 250 wattevers? They even offered soffware so users could tweak the performance and started whispering about concepts like peak power and " natural" riding experience.

My 250 wattever mid drive emtb ( giant , so yamaha pwx - 80 nm) keeps up with a mates 1500 wattever bafang powered mid drive on steep hills. Yet hus bike is illegal in Australia - it's sold for private property use. Yet he was sucked into buying a " more powerful" bike

 

[PedalUma]

I a guy rode one on our group ride last week.
I like your perceptions and explanations. Than you.
Drivetrains do not hold up after about 90Nm. Interestingly my bike could out climb his bike. Because my bike was lighter.

 

[PDoz]

I a guy rode on on our group ride last week.
I like your perceptions and explanations. Than you.
Drivetrains do not hold up after about 90Nm. Interestingly my bike could out climb his bike. Because my bike was lighter.

Your bike is probably more sophisticated as well.

I have the luxury of riding some exceptional examples of improvements in mountain bike efficiency.

My 2009 giant trance represents the era when rear suspension started to isolate power sapping suspension bob.
My daughters 2019 scott genius represents the benefits of variable rear shock dynamics ( bar switch between licked, 90 mm travel and 150 mm travel AND different dampening ), plus modern geometry - the thing pedals better than my emtb in eco!!!

 

[Johnny]

I am curious if you can still get the full torque out of a motor if the watts are restricted? For instance, keeping a bike to 750 watts would open up more legal riding options. However, I still want good torque to help get up steep hills. Looking at bikes with bafang ultra or bbshd, and torque ratings in the 160-120 nm range, but also with wattage in the 1,000-1,500 range. Several vendors offer models with wattage restricted to 750, I am assuming because of the legal issues. I don't care about top speed, but don't want to sacrifice hill climbing power.

Questions like these come a lot, especially in this forum.

Relationship between torque and power is simple. Torque x Angular speed = Power.

Comparing torque numbers without additional information is a big mistake and usually will lead to very erroneous conclusions. Like the one that were made in the posted video.

What makes you move is power. This is a fact. As long as you can transfer that power to the wheels you will move.

For a mid drive what this means is, as long as you have the right gearing to keep the motor at the cadence where it produces its maximum power, the higher the power the faster you will climb.

Likewise this also means that, as long as you have the right gear ratio, you can slow down and climb with similar effort(power-wise) with lower powered motors.
For example, to climb %5 grade, with a total of 250lbs(bike+rider) you need,
~500W to move at 15mph
~145W to move at 5mph.
As long as you have the gear both a 500W and a 150W motor will climb that hill, 150W motor will climb slower.


The problem begins when you can not keep the motor at the rpm that it produces its maximum rated output. That is when you need the power/rpm graph (gives the same information as torque/rpm graph). Most of these motors have a torque/rpm curve which is non-increasing with rpm hence the higher torque numbers are achieved at lower rpms. For mid drives what this means is , If the rider can not keep the cadence high enough, say you are below 60rpm, the higher torque motor is likely to give the higher power output and will move easier...

 

[PedalUma]

Questions like these come a lot, especially in this forum.

Relationship between torque and power is simple. Torque x Angular speed = Power.

Comparing torque numbers without additional information is a big mistake and usually will lead to very erroneous conclusions. Like the one that were made in the posted video.

What makes you move is power. This is a fact. As long as you can transfer that power to the wheels you will move.

For a mid drive what this means is, as long as you have the right gearing to keep the motor at the cadence where it produces its maximum power, the higher the power the faster you will climb.

Likewise this also means that, as long as you have the right gear ratio, you can slow down and climb with similar effort(power-wise) with lower powered motors.
For example, to climb %5 grade, with a total of 250lbs(bike+rider) you need,
~500W to move at 15mph
~145W to move at 5mph.
As long as you have the gear both a 500W and a 150W motor will climb that hill, 150W motor will climb slower.


The problem begins when you can not keep the motor at the rpm that it produces its maximum rated output. That is when you need the power/rpm graph (gives the same information as torque/rpm graph). Most of these motors have a torque/rpm curve which is non-increasing with rpm hence the higher torque numbers are achieved at lower rpms. For mid drives what this means is , If the rider can not keep the cadence high enough, say you are below 60rpm, the higher torque motor is likely to give the higher power output and will move easier...

Johnny,
Look at this: https://support.enviolo.com/hc/en-u...enviolo-Cargo-internal-gear-hub-specification
Timpo has already seen it.
It can work on any frame using a Split Belt.
I would like your insights.

 

[Johnny]

Johnny,
Look at this: https://support.enviolo.com/hc/en-u...enviolo-Cargo-internal-gear-hub-specification
Timpo has already seen it.
It can work on any frame using a Split Belt.
I would like your insights.

I have seen nuvinci specs. Are you planning to used it on a bike?

 

[PDoz]

The problem begins when you can not keep the motor at the rpm that it produces its maximum rated output. That is when you need the power/rpm graph (gives the same information as torque/rpm graph). Most of these motors have a torque/rpm curve which is non-increasing with rpm hence the higher torque numbers are achieved at lower rpms. For mid drives what this means is , If the rider can not keep the cadence high enough, say you are below 60rpm, the higher torque motor is likely to give the higher power output and will move easier...

Most of the big brand mid drives have torque / cadence curves that don't add up to the wattever figures quoted, and most of them have an almost flat torque ADVERTISED curve at normal cadence. The truth is, the motors are not producing constant torque through the pedal stroke, so all these numbers and graphs are little more than the marketing department struggling to represent physics after it has been distorted ( explained?) by engineers and electricians, then adjusted by lawyers.

 

[PDoz]

What makes you move is power. This is a fact. As long as you can transfer that power to the wheels you will move.

I suspect this is why people struggle to understand just how irrelevant advertised power figures are when trying to move a mountain bike up a hill. We don't propel a mountain bike with a constant linear power delivery - it's not a cargo bike. We punch at the pedals, pump the suspension, bounce off obstacles, and desperately try not to smash our cracks against rocks.

For a really good visual example, watch a video of someone riding a pump track - zero pedalling but high speed cycling. Or Danny Macaskill tackling a seemingly impossible climb - massive but brief torque inputs at strategic moments, timed with spectacular gymnastics and just a little insanity.

 

[PedalUma]

I have seen nuvinci specs. Are you planning to used it on a bike?

I am converting my third cargo bike to a mid-drive within a four week period. I am in the SF North Bay where we have hills. Lots of big ones.
I have used motors on earlier builds such as the Bafangs including the HD. I am finding that the weak link is the transmission of power, not power itself. About 80Nm seems right for MTB drivetrains, more than that gives accelerated failure. I mostly do 350W TS conversions, no throttle.
The thought of a Veer belt on a bike with a performance cargo hub that can pull 500lbs and take 100Nm nominal sounds interesting and fun and more interesting and fun than a powerful motor that needs to be down-tuned and hooked up to a weak transmission. People used to think it was cool to have a sleeper car that was an old Volvo with a supped small block Chevy motor levered in. That is until they popped the clutch. Crunch!
I am not quite at the planning stage for a particular project yet. But when it happens I want to have the considerations in line with some peer review.

 

[Johnny]

Most of the big brand mid drives have torque / cadence curves that don't add up to the wattever figures quoted, and most of them have an almost flat torque ADVERTISED curve at normal cadence. The truth is, the motors are not producing constant torque through the pedal stroke, so all these numbers and graphs are little more than the marketing department struggling to represent physics after it has been distorted ( explained?) by engineers and electricians, then adjusted by lawyers.

While I agree that maximum torque number is marketing, I think maximum torque/rpm graph is actually very useful.

You should think about it as the raw maximum, motor output on the bench. The graph actually is consistent with maximum rated power but gives more information since the rpm does not stay constant while riding. It is still not enough though since this is just the graph for peak output not the nominal one.

I agree that per pedal stroke output of a rider is not constant and the response to each stroke is very important on how the bike feels especially when mountain biking through technical sections.

However this graph still gives the maximum support you can get at a given cadence even if the rider's input fluctuates during each revolution. For example you have probably seen a youtuber doing a recent comparison between Bosch, Yamaha, Brose and Shimano. By looking at the graphs, I can have a guess on why he got the best results in his climb with Bosch. He probably stabilized around 70 rpm in that short straight climb and at that rpm gen 4 gives the most amount of power according to the curves I have seen. If he kept around 80 rpm Yamaha pwX2 would probably be the fastest, while around 65 Brose would have led the way. I can also say that since on technical trails you go through the lower cadences frequently , the longer and higher the flat section of the torque curve is the faster you will be. Hence the more you spend time at lower cadences I would speculate that Bosch will have a slight edge on Yamaha pwx2.


Btw, the reason for the flat section of the torque curve is because controller limits the current. If the current was not limited you would see the torque further increasing while rpm decreases. The problem in that case is, lots of heat is generated and manufacturers are probably trying to prevent that every time the rider starts and stops pedaling.

 

[Johnny]

The thought of a Veer belt on a bike with a performance cargo hub that can pull 500lbs and take 100Nm nominal sounds interesting and fun and more interesting and fun than a powerful motor that needs to be down-tuned and hooked up to a weak transmission.

You are much more experienced than I am with cargo bikes.

For Nuvinci hub, in theory, I would be concerned about high powered, low speed climbs. For example let's have a 20" wheel. Pushing 750W while moving around 4mph will put around 110nm to your rear hub which is above the rated 100nm. This can be solved by limiting the power wrt speed or going for smaller wheels (the wheel will be turning faster hence it will decrease the torque).

 

[PedalUma]

The latest Cargo is getting 203mm rotors which arrive Friday. The one last week was purchased for $800 and sold converted for $2800. That is with two 10.5Ah batteries. The one I purchased Monday was for $550 with rim brakes and not many accessories. These bikes are not very useful with out being electric. EBR inspired me with talk of Hunting Bikes to make them useful. A farm/ranch hunting e-cargo bike with a belt and internal gears could get muck shoveled on it's drivetrain all day and still be reliable, quiet and fun.

 

[AHicks]

I am curious if you can still get the full torque out of a motor if the watts are restricted? For instance, keeping a bike to 750 watts would open up more legal riding options. However, I still want good torque to help get up steep hills. Looking at bikes with bafang ultra or bbshd, and torque ratings in the 160-120 nm range, but also with wattage in the 1,000-1,500 range. Several vendors offer models with wattage restricted to 750, I am assuming because of the legal issues. I don't care about top speed, but don't want to sacrifice hill climbing power.

So going back to the original question, if you are riding an Ultra or BBSHD equipped bike that has been capped at 750 watts, and you know this because at wide open throttle the motor is only pulling 750 watts as indicated on your display, there is a LOT of power being left on the table. That motor will NEVER climb as well as one that's getting 1500+ watts (or more)....

My question would be can this power that's left on the table be tapped - and the answer is it depends! All of the BBSxx and Ultra's built to date are able to be hacked pretty easily with an inexpensive cable and some shareware loaded on a laptop and taken up to the rated capacity (1600w). The hitch is, my understanding is this is changing as we sit here. The new Ultra motors are being shipped with CAN bus - which is a fancy way of saying our existing cables and shareware are NOT going to work on them. These CAN bus motors are going to be dealer only changes - and that's assuming you can find a willing dealer, AND they have the CAN bus software and know how to use it.....

 

[Sierratim]

There are several threads that discuss the interrelationship between torque (Nm or ft-lbs), power (watts), and speed (rpm). Here's a sampling;

https://electricbikereview.com/forums/threads/never-too-much-torque.27940/page-2

https://electricbikereview.com/forums/threads/torque-rating-on-e-bikes-marketing-gimmick.2467/page-3

https://electricbikereview.com/forums/threads/50nm-mid-drive-vs-50nm-hub-drive.34726/page-2

Ride On!