"Significant Motor Resistance While Pedalling With Motor Off" Myth Busted: Physical Proof

Stefan Mikes

Well-Known Member
Region
Europe
City
Brwinów (PL)
"I experience significant motor resistance while pedalling with assistance OFF or with the motor switched off": There exist several threads discussing the behaviour of, say, Bosch or Shimano mid-drive motors. True or false? Such discussions remind me of the "holy war" between audiophiles and electronics engineers: The former can only rely on their senses (and beliefs) while the latter have instruments such as oscillograph and can prove the facts.

Let me make a physical proof:

Thesis
"Motor resistance (if any) is insignificant. The perceived
resistance is the outcome of the heavyweight e-bike and of big difference between rider's own leg power output against e-assisted ride".

Too long; didn't read [the proof]:
Given:
  • Rider's mass = 80 [kg]
  • Traditional bike mass = 10 [kg]
  • E-bike mass = 25 [kg]
  • Rider's leg power output = 100 [W]
  • Motor assistance power output = 100 [W]
Let us analyse accelerating time from 0 to 32 km/h (8.88 m/s) in three cases:
  1. The rider rides the traditional lightweight bike
  2. The rider rides unpowered e-bike
  3. The rider rides assisted by e-bike motor.
Kinetic Energy Gain
ΔEk =1/2 * m * Δv^2 where,
ΔEk - kinetic energy gain
m - total mass of rider and bike
Δv^2 - bike speed gain squared (starting from 0 m/s).

Theoretically, it will take the rider time t to achieve the speed of 8.88 m/s from standstill in absence of any resistances (the time is the kinetic energy gain divided by the power supplied):
  1. Traditional lightweight bike: ΔEk = 0.5 * 90 * 8.88 * 8.88 = 3548 J. t = 3548 J / 100 W (rider's leg power) = 35.5 seconds
  2. E-bike, unassisted ride: ΔEk = 0.5 * 105 * 8.88 * 8.88 = 4139 J. t = 4139 J / 100 W = 41.4 s.
  3. E-bike, assisted ride: ΔEk = 0.5 * 105 * 8.88 * 8.88 = 4139 J. t = 4139 J / (100+100) W = 20.7 s.
Observe:
  • The same rider on a lightweight traditional bike would hit 32 km/h by almost 6 seconds faster than on heavy e-bike with the motor OFF;
  • The same rider on assisted (with only 100 W!) heavy e-bike will hit the target speed by 14.8 seconds faster than a lightweight traditional bike, and by 20.7 seconds faster than the same bike with assistance OFF.
Conclusion

The difference on acceleration between the assisted and unassisted e-bike ride is so tremendous that the rider will think something resists his/her pedalling. But it is not the motor resistance: it is gaining kinetic energy that requires so much more leg power unassisted that the cyclist thinks there is some resistance in the bottom bracket (read: the mid-motor).

Afterthoughts
  • Potential energy gain: The amount of energy input in the cranks/chainring to rise a body from elevation h1 to h2 is directly proportional to the body (read: cyclist + bike) mass m.
    ΔEp = m * g * Δh, where
    g - gravity
    Δh - elevation gain.
    Without any complex calculations, we can say you need to input 105 kg / 90 kg =1.1666, or 16 2/3% more energy to just climb the same route on a heavy e-bike compared to a lightweight trad bike for the same rider. Again: assisted climb feels easier than traditional climb, and unassisted climbing on e-bike is very hard (giving the impression of some resistance at the crank).
  • Some of us weighed 70 kg when riding traditional bikes, and now they weigh perhaps 100 kg or more... :D
Disclaimer

Discuss. If you find a weak point in my reasoning, I'm ready to retract my views.

Experiment by @fooferdoggie:
 
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Haystacks

Active Member
Way to go wasting your time on nonsense. Simple experiment; take an analogue bike and an ebike, put them on stands/turn analogue upside down. Turn cranks, compare.

There is one scenario where that thesis comes into play and that is hubs, there is no resistance at the BB, the perceived (actual) resistance is from the load from driving the hub.
 

fooferdoggie

Well-Known Member
Way to go wasting your time on nonsense. Simple experiment; take an analogue bike and an ebike, put them on stands/turn analogue upside down. Turn cranks, compare.

There is one scenario where that thesis comes into play and that is hubs, there is no resistance at the BB, the perceived (actual) resistance is from the load from driving the hub.
thats too much like work just pop the chain off. then turn the cranks.
 

PDoz

Well-Known Member
Stefan, if only life was that easy!

Consider some of the other features (compromises ) ebike designers can hide behind the motors power. Then reflect on how they reduce pedalling efficiency.

Start with that lovely plush suspension. Pedal bob sucks power - try locking out your shock / forks for a ride and see how much range increases.

Now park those fat, chunky , under inflated but comfortable wheels next to a skimpy road bike. Yes, there is a reason they are called tyres!

Now look from the side at the ergos - that upright sitting position, rearward seat tube, even crank length will compromise performance.

Then have a critical look at your service schedule - that dirty chain / derailleur cogs / gritty wheel bearings / stiff pedals - all those things suck power. If you were relying on 100 w legs, those things would have been noticed / fixed.

ps - I swap between my giant and the kids manual bikes regularly. Sprogette 1's scott genius 940 feels like my giant with 25% assistance - especially when I flick the twinlink into stiff suspension mode. I often ride the giant with the motor off , then hopping directly onto ANY of their bikes feels like the first time I rode an ebike - so much easier to pedal. Older bosch powered bikes felt like pushing through treacle compared to the giant. Imagine how surprised I was the first time I rode my levo sl and it jumped forward without the motor on - definitely a LOT easier to pedal motor off compared with the giant.
 

Mike_V

Active Member
Hi Stefan,
I always read your interesting, detailed and often productive posts.
I did read this one and I think you have the correct conclusion - I think you are right however:
Consider that you've only calculated one instant in time while the example describes an accelerating rider
You have applied mechanical 'statics' physics arithmetic to a 'dynamics' example requiring calculus.
Or measured best by test with a bike dynomometer seen here at the 50 minute mark
Good reading - makes you think
Mike
 
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Stefan Mikes

Well-Known Member
Region
Europe
City
Brwinów (PL)
Stefan, if only life was that easy!

Consider some of the other features (compromises ) ebike designers can hide behind the motors power. Then reflect on how they reduce pedalling efficiency.

Start with that lovely plush suspension. Pedal bob sucks power - try locking out your shock / forks for a ride and see how much range increases.

Now park those fat, chunky , under inflated but comfortable wheels next to a skimpy road bike. Yes, there is a reason they are called tyres!

Now look from the side at the ergos - that upright sitting position, rearward seat tube, even crank length will compromise performance.

Then have a critical look at your service schedule - that dirty chain / derailleur cogs / gritty wheel bearings / stiff pedals - all those things suck power. If you were relying on 100 w legs, those things would have been noticed / fixed.

ps - I swap between my giant and the kids manual bikes regularly. Sprogette 1's scott genius 940 feels like my giant with 25% assistance - especially when I flick the twinlink into stiff suspension mode. I often ride the giant with the motor off , then hopping directly onto ANY of their bikes feels like the first time I rode an ebike - so much easier to pedal. Older bosch powered bikes felt like pushing through treacle compared to the giant. Imagine how surprised I was the first time I rode my levo sl and it jumped forward without the motor on - definitely a LOT easier to pedal motor off compared with the giant.
Your remarks have been duly noted PDoz. I said "theoretical, no resistance" while there is at least the rolling resistance and the air drag.
You might believe some motors create a drag. As @fooferdoggie has remarked, take your chain off the bike and spin the crank. How much of resistance do you feel?
 

fooferdoggie

Well-Known Member
Your remarks have been duly noted PDoz. I said "theoretical, no resistance" while there is at least the rolling resistance and the air drag.
You might believe some motors create a drag. As @fooferdoggie has remarked, take your chain off the bike and spin the crank. How much of resistance do you feel?
you will get a lot more because there is no resistance on a regular bike. but in life its too little to notice.
 

PDoz

Well-Known Member
Your remarks have been duly noted PDoz. I said "theoretical, no resistance" while there is at least the rolling resistance and the air drag.
You might believe some motors create a drag. As @fooferdoggie has remarked, take your chain off the bike and spin the crank. How much of resistance do you feel?

it all adds up, but spinning an old bosch at 90 cadence is very different to spinning your specialised / giant.

Even that video confirms this - the cranks didn't keep spinning , they didn't even get to 6 oclock.

BUT, imho the REAL difference pedalling an emtb vs memtb is in the bikes design - the manufacturers didn't build an ulta efficient pedalling machine because they didn't need to. At least, not until they started chasing liw weight / low powered / small battery exotics
 
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fooferdoggie

Well-Known Member
it all adds up, but spinning an old bosch at 90 cadence is very different to spinning your specialised / giant.

Even that video confirms this - the cranks didn't keep spinning , they didn't even get to 6 oclock.

BUT, imho the REAL difference pedalling an emtb vs memtb is in the bikes design - the manufacturers didn't build an ulta efficient pedalling machine because they didn't need to. At least, not until they started chasing liw weight / low powered / small battery exotics
true but we are taking a tiny bit of resistance I doubt you could ever feel it.
 

PDoz

Well-Known Member
true but we are taking a tiny bit of resistance I doubt you could ever feel it.
That's P D(antic) - I've had days where the bike just doesn't feel right, a bit sluggish or slow to bounce off an obstacle . Someting as simple as a couple of psi in the tyre or a gritty suspension bush can be noticeable.

Hopping from a giant to an old bosch - powered off - the bosch drag is horrible.

Hopping from my levo sl to my giant feels almost as bad , although I suspect most of that is bike dynamics.
 

harryS

Well-Known Member
This is a Bafang BBS02 with chain slipped off the sprockets. I thought it showed there was little resistance. You can see it ghost pedalling when PAS is turned on.

Then I tried it with a regular bike. The pedals just went round and round for almost a minute. There's some forces, slight as there are, to overcome.

 

Stefan Mikes

Well-Known Member
Region
Europe
City
Brwinów (PL)
This is a Bafang BBS02 with chain slipped off the sprockets. I thought it showed there was little resistance. You can see it ghost pedalling when PAS is turned on.

Then I tried it with a regular bike. The pedals just went round and round for almost a minute. There's some forces, slight as there are, to overcome.

[snip]
I did a similar thing on my Vado with the chain off. In my case, the cranks were rotating the same: unpowered or OFF. (I can't remember what was happening with assistance on but I don't think the motor there would work as it required proper signals from more than just the torque sensor). Anyway, the crank spinning easily with the motor OFF indicates the motor resistance is rather symbolic.

PDoz: I cannot really speak of Bosch Gen 2 motor as I have never owned one. With your Levo SL, there are two factors making it easily rideable unpowered: lower mass (especially noticeable if you are a lightweight person yourself), and the clutch design, totally disconnecting the crank from the motor when it doesn't work.

My point is, many people believe in the "motor drag" of the modern motors such as Bosch CX, new Shimano 6100, Giant/Yamaha SyncDrive Pro/PW-X2, Specialized/Brose motors where there is hardly any motor drag. People are confused how it comes the agile e-bike suddenly becomes so sluggish with the power off. And they look for the explanation in the wrong place.

I own a 250 W hub-drive motor e-bike that -- of course -- has the same pedalling resistance as any traditional bike. Honestly, I can hardly ride it unpowered even if the brushless hub motor offers no noticeable resistance.
 

steve mercier

Well-Known Member
The best strategy is to lose body mass :D
I did a similar thing on my Vado with the chain off. In my case, the cranks were rotating the same: unpowered or OFF. (I can't remember what was happening with assistance on but I don't think the motor there would work as it required proper signals from more than just the torque sensor). Anyway, the crank spinning easily with the motor OFF indicates the motor resistance is rather symbolic.

PDoz: I cannot really speak of Bosch Gen 2 motor as I have never owned one. With your Levo SL, there are two factors making it easily rideable unpowered: lower mass (especially noticeable if you are a lightweight person yourself), and the clutch design, totally disconnecting the crank from the motor when it doesn't work.

My point is, many people believe in the "motor drag" of the modern motors such as Bosch CX, new Shimano 6100, Giant/Yamaha SyncDrive Pro/PW-X2, Specialized/Brose motors where there is hardly any motor drag. People are confused how it comes the agile e-bike suddenly becomes so sluggish with the power off. And they look for the explanation in the wrong place.

I own a 250 W hub-drive motor e-bike that -- of course -- has the same pedalling resistance as any traditional bike. Honestly, I can hardly ride it unpowered even if the brushless hub motor offers no noticeable resistance.
I once had my Intuvia battery die when I was far from home. If I ever again had to ride 35 km with no power on an ebike I would consider it a cruel joke on me , but since it has never happened again in over 40,000 km on 3 bikes I do not worry too much about it and care less about motor drag.
 

kmccune

Active Member
I do not really know-one thing I noticed that Mid-drive TDZ on the Buzz trike seemed to make the Bike have absolutely no resistance that I could feel,had a hub drive "Fattie" that seemed like the brakes were half on unpowered.( my legs do not have a lot of "watts" in them so every little bit counts)