Pedal resistance

[jollyDodger]

Hello Pedelec world,

Do people know what sort of extra resistance e-bike motors generate when over the top-speed cut off please? I know from experience that the extra weight slows one down. But it's difficult to tell when riding if its the weight or the pedal resistance.

I've read that the direct-drive hub motors turn into dynamos, and some bike dealers are claiming particular brands of mid-drive hubs have this advantage over others:

looking at these mid's?
Bosch Active (2.9kg)
Bosch Active Plus (3.2kg)
Bosch Performance (4.0 kg)
Bosch Performance CX (4.0kg)
Shimano Steps E6000 (3.2kg)
Shimano Steps E8000 (2.8kg)
Brose (3.4kg)
Yamaha

 

[Woosh]

all the crank drive motors have internal clutch that isolates the motor after the motor has its electricity cut off. The residual resistance is very small, you can just about feel it. To experience how it feels, put the rear wheel of the bike on a stand, pedal at speed under and over the cutoff speed.
The difference (in resistance) between motors is an order of magnitude smaller, so don't worry about it.

 

[Emo Rider]

The above cut off resistance of crank drives is negligible. Having proper tyre pressurse and cycling in the correct gear will affect resisitance much more.

 

[Deleted member 4366]

I don't agree. Many CDs have resistance when you turn the pedals without power. It's just that it's not obvious. The only way you can test it is to remove the chain then turn the pedals by hand with the power off. You will see that it takes some effort to maintain 60 rpm or more.

The present Bosch motors ans some BSSXXs are amongst the worst. That's why Bosch redesigned their motors for 2018.

 

[jollyDodger]

Thanks good to know..

Also its the Shimano Steps motor that a reseller was claiming had less crank resistance than any other current mid-drive.. Has anyone compared this to the Brose in this respect?

...also little wierd that on the shimano site the ,more powerful Shimano Steps E8000 is listed than lighter than the less powerful E6000.

 

[Woosh]

I don't agree. Many CDs have resistance when you turn the pedals without power. It's just that it's not obvious. The only way you can test it is to remove the chain then turn the pedals by hand with the power off. You will see that it takes some effort to maintain 60 rpm or more.

The present Bosch motors ans some BSSXXs are amongst the worst. That's why Bosch redesigned their motors for 2018.

Let's watch Hatti spinning up the GSM CD motor:

 

[Deleted member 4366]

The crank arm is quite heavy, and even then, it's weight is not enough to pull all the way down until it's vertical, and she has to give it a start, even though it's at the 2 0'clock position. On a normal bike, that crank wouldn't stay in that start position. it would drop, then oscillate backwards and forwards at the bottom.

That BB is consuming something like 5w to 10w when pedalling at a normal cadence, which would be 10% of the pedal power that most casual ebikers give.

 

[topographer]

Let's watch Hatti spinning up the GSM CD motor:

That's horrendous.

 

[Woosh]

and she has to give it a start, even though it's at the 2 0'clock position.

she only moved it passed 2 o'clock, did not push it forward.

That BB is consuming something like 5w to 10w when pedalling at a normal cadence, which would be 10% of the pedal power that most casual ebikers give.

I think it is much less than that. That crank arm in the video weighs about 250grs and is 175mm long. The centre of gravity of the crank arm is about 90mm from the bottom bracket.
at 2 o'clock, the torque generated by gravity is approximately:

R*F*sin(360 degrees *2 /12) = 0.25 * 10 * 0.09 * 0.866 = 0.195NM

At 60 RPM (one rev per second), the resistance is thus 0.195NM * 2 * PI() = 1.22W.

 

[Woosh]

That's horrendous.

yes, if you compare its resistance to that of a GXP bottom bracket that I fit to the Karoo and Rio MTB.
An ordinary BB has two ballbearings, CD motors' BB has 5.
The extra ballbearings cause a noticeable resistance.

 

[Deleted member 4366]

she only moved it passed 2 o'clock, did not push it forward.



I think it is much less than that. That crank arm in the video weighs about 250grs and is 175mm long. The centre of gravity of the crank arm is about 90mm from the bottom bracket.
at 2 o'clock, the torque generated by gravity is approximately:

R*F*sin(360 degrees *2 /12) = 0.25 * 10 * 0.09 * 0.866 = 0.195NM

At 60 RPM (one rev per second), the resistance is thus 0.195NM * 2 * PI() = 1.22W.

Good calculation, but that torque is at zero RPM. Resistance due to friction and viscous forces will increase as speed goes up, so torque at 60 rpm will be much higher. There's inertia affects as well to accelerate the additional mass, which is also felt as resistance.

 

[Danidl]

Good calculation, but that torque is at zero RPM. Resistance due to friction and viscous forces will increase as speed goes up, so torque at 60 rpm will be much higher. There's inertia affects as well to accelerate the additional mass, which is also felt as resistance.

Actually I was going to suggest that the viscosity will go down with speed. There is stiction in the static case and as rotation begins a film of lubricant adheres to the moving surfaces. Likewise as the grease heats up, it's viscosity falls. There is of course rotational inertia but that is not a resistance ,at a steady cadence it becomes zero. What will not go down will be any magnetic reluctance if the rotor in the motor rotates, or any resistance in a clutch or pawls needed to isolate the motor. But probably sloshes calculation includes that.

 

[Deleted member 4366]

We can make all the theories we like, but if you want to know the actual pedal resistance on your CD, all you need to do is slip off the chain and turn the pedals at normal cadence with your hand.

 

[anotherkiwi]

I'll give it a try - I'm betting on the 5-10 W bracket but my bike is such a tank I can't guess what is weight and what is motor drag.

I'm having fun moving between two completely different bikes - one that is a tank and another that as soon as it gets to the top of a slope takes off, I have never in my life used the brakes on downhills so often... The aerodynamic wall seems to have shifted from around 45 km/h to about 60 km/h, the spokes start whistling, aero spokes added to shopping list

 

[jollyDodger]

Wow lots of geekery. Although most interesting and impressive, I'm hoping to find out which mid-drives have the least pedal resistance pls?

 

[Deleted member 4366]

Wow lots of geekery. Although most interesting and impressive, I'm hoping to find out which mid-drives have the least pedal resistance pls?

Nobody likes to admit that they have it because they all think that they have the best bike in the world. You'll have to test yourself by slipping off the chains and turning the cranks by hand.

 

[Jonah]

This article has some info. I can't vouch for its accuracy.

It says the Brose motor has least resistance followed by Shimano and Yamaha. Bosch is worst. Of course there are new Bosch and Brose motors out so they might be different.

http://ebike-mtb.com/en/the-best-emtb-motor/