still not to sure what to make of bosch ebike system

[Deleted member 4366]

... d8veh, as you well know, but others might not, power is torque x revs . In a hub motor the designer decided on an optimum road speed and selected the motor accordingly. In a crank drive, the designer did not have to make that decision, as the gears allow the user to select between cadence , roadspeed and torque.

Power has nothing to do with OP's problem. His problem is that he doesn't have enough torque.

I'm not sure what your point is. If it's that he can get more torque by changing his gearing, that's correct because it will give him more torque. If you're trying to say that he should pedal faster because it will give more power, that's back the argument that if you can't walk up a hill, you should run up it because you make more power when you're running. That's clearly wrong.

 

[mike killay]

If you're trying to say that he should pedal faster because it will give more power, that's back the argument that if you can't walk up a hill, you should run up it because you make more power when you're running. That's clearly wrong.

Is this the Achilles heel of the Bosch system?
Does it mean that the motor cannot assist those who cannot put enough power into the system in the first place? (unlike a twist throttle)

 

[Croxden]

It's for cyclists who want assistance, not for those who occasionally need to assist the motor on hills.

 

[Woosh]

Is this the Achilles heel of the Bosch system?
Does it mean that the motor cannot assist those who cannot put enough power into the system in the first place? (unlike a twist throttle)

I believe so.

To me, Arstu is a typical Bosch rider. His chart going up Winnats Pass shows average motor power of 400W, well within the ability of any CD motor.
As he has to put in 25% of the required energy to climb, his input is about 133W and his average speed is about 11 miles.

TBH, I think the average ebiker may put in as much as that for 5 minutes but not much longer than that.
If they can't, it's better for them to have a system that allows more flexibility.

 

[Deleted member 4366]

Is this the Achilles heel of the Bosch system?
Does it mean that the motor cannot assist those who cannot put enough power into the system in the first place? (unlike a twist throttle)

It's not quite an Achilles heel. It's just that the Bosch system doesn't suit everyone. Yes, it is basically a torque multiplier system, so if you can't put in enough torque, the motor can't give its maximum torque. That's a problem if your legs get tired half way up a hill.

The problem is that everybody judges and makes recommendations based on their own frame of reference. We had a guy say that the Gtech was a fantastic climber. He only weighed 50kg. The Bosch system is great for people that are normally fit, but if you're incapacitated in any way or have to deal with difficult circumstances, it might not be the best choice.

 

[Artstu]

I believe so.

To me, Arstu is a typical Bosch rider. His chart going up Winnats Pass shows average motor power of 400W, well within the ability of any CD motor.
As he has to put in 25% of the required energy to climb, his input is about 133W and his average speed is about 11 miles.

TBH, I think the average ebiker may put in as much as that for 5 minutes but not much longer than that.
If they can't, it's better for them to have a system that allows more flexibility.

You're wrong in your assumptions, those Strava stats are estimated overall rider power, Strava isn't used to seeing those sorts of speeds from a bike and rider that weigh so much, so it's only a guestimate.

I'm not even sure I had my weight set correctly either. This one could be more reflective of my estimated power.

 

[flecc]

Is this the Achilles heel of the Bosch system?
Does it mean that the motor cannot assist those who cannot put enough power into the system in the first place? (unlike a twist throttle)

This dependancy on rider input widely applies to crank drive systems that have torque control of power. Panasonic set the scene long ago with a direct link between rider input and motor output, controlled by their at the time unique pedal shaft torque measuring system. The motor output exactly matched the rider input. When a switched higher power mode was introduced, once again that was a direct ratio of rider input, up to the motor maximum.

Others such as Bosch and Daum (Impulse) have tended to follow this model, sometimes even directly copying the Panasonic pedalshaft torque measuring system. The main difference is in cadences applying, the Japanese like Panasonic and Yamaha preferring to permit maximum powers at lower cadences than the German designs.
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[Woosh]

You're wrong in your assumptions, those Strava stats are estimated overall rider power, Strava isn't used to seeing those sorts of speeds from a bike and rider that weigh so much, so it's only a guestimate.

I'm not even sure I had my weight set correctly either. This one could be more reflective of my estimated power.

View attachment 19608

evenso, the basic reasoning does not change.
As a rider, you put in 25%, the bike 75% of the required energy. Instead of 133W user input, it's now about 200W with your new estimate.
If the rider can't hold on to this level, then he is limited by his level of fitness, his speed will be proportionally lower.
he would be better off with a more flexible system that does not require as much user input.

 

[Danidl]

Power has nothing to do with OP's problem. His problem is that he doesn't have enough torque.

I'm not sure what your point is. If it's that he can get more torque by changing his gearing, that's correct because it will give him more torque. If you're trying to say that he should pedal faster because it will give more power, that's back the argument that if you can't walk up a hill, you should run up it because you make more power when you're running. That's clearly wrong.

... You could say I was being pedantic, but your earlier answer could lead to error by someone less engineering oriented than yourself. To state that torque not power was the problem was less than whole.
Torque just like voltage or current is only part of the equation and needs to be qualified.. just saying. .. Of course you are aware of the following , but others may not...

One can swop \ transform between torque and rotation speed for any motor by using gearing . The gearing can be a fixed internal ratio as in the case of some hub motors, but not in a direct drive motor, or it can be external as in the case of a crank drive. The crank drive is much more flexible in that regard.

Using sufficiently low gearing on a crank drive, irrespective of the weight of the bike and person, progress up a steep hill is possible, subject only to the coefficient of friction of the tyre to road, allowing adhesion to the road surface and preventing the bike falling back. The overall power of the motor and cyclist determines the rate of progress.

 

[Deleted member 4366]

... You could say I was being pedantic, but your earlier answer could lead to error by someone less engineering oriented than yourself. To state that torque not power was the problem was less than whole.
Torque just like voltage or current is only part of the equation and needs to be qualified.. just saying. .. Of course you are aware of the following , but others may not...

One can swop \ transform between torque and rotation speed for any motor by using gearing . The gearing can be a fixed internal ratio as in the case of some hub motors, but not in a direct drive motor, or it can be external as in the case of a crank drive. The crank drive is much more flexible in that regard.

Using sufficiently low gearing on a crank drive, irrespective of the weight of the bike and person, progress up a steep hill is possible, subject only to the coefficient of friction of the tyre to road, allowing adhesion to the road surface and preventing the bike falling back. The overall power of the motor and cyclist determines the rate of progress.

You're making it more complicated than it is. All you need to get up a hill is enough torque. It really is that simple.

 

[Will Tinker]

Sorry to chime in late with this, but for what it's worth I am riding the same bike with same motor.

In Sport/Turbo mode I find it fairly easy to maintain 15-17mph on most inclines.

Normally this has been in 6th/7th gear (Turbo mode).

I find that you do need to pedal harder than on, say, the Impulse 2.0 to maintain that speed as it does have less torque. But still easily enough, at least for my purpose.

If struggling, I'll change down to 5th or 4th. 5th, I can only pedal up to a max of 15mph. 4th, more like 12mph. Both of these, I must pedal quickly but don't need to apply much force.

 

[basicasic]

I may as well chime in late as well. I have a bike with the Bosch CX Motor and there is no doubt in my mind that it performs it's best on hills with a lower gear/higher cadence type of usage. It's almost like a car engine 'coming on cam' in the way that if you increase the cadence it becomes effortless. Often I put in a bit more effort to up the cadence and then the motor 'takes over'; or if I'm tired just drop down a gear.

I'm fit and have never used sport or turbo but the bike just rockets up even the steepest hills in tour mode with (what seems to me) little effort BUT you have to keep the cadence up.

 

[Woosh]

BUT you have to keep the cadence up.

that's the difficulty for less fit riders, cadence is proportional to work and power, keeping up the high cadence is like the difference between walking up the stairs and running up the stairs, regardless how steep the stairs are, running up them is hard for some of us.

 

[Will Tinker]

that's the difficulty for less fit riders, cadence is proportional to work and power, keeping up the high cadence is like the difference between walking up the stairs and running up the stairs, regardless how steep the stairs are, running up them is hard for some of us.

I can pedal slower, and put significantly less of my own force through the pedals and the Active Line still handles that fine (at least in my experience). The difference is a couple of mph speed.

The motor for me that required the least physical effort was the Yamaha Syncdrive. But that tailed-off at a lower speed, so ultimately made me a little slower.

 

[Woosh]

I can pedal slower, and put significantly less of my own force through the pedals and the Active Line still handles that fine (at least in my experience). The difference is a couple of mph speed.

The motor for me that required the least physical effort was the Yamaha Syncdrive. But that tailed-off at a lower speed, so ultimately made me a little slower.

I think the OP wonders how he can get more speed out of the bike on hills. That's no difficulty with a rotational system but with a torque sensing system, you need to be able to hit 90 rpm to get the best out of the bike. His cadence is about 70 rpm, he is stuck more or less at 11mph on third gear (26T) on moderate hills. Shifting down, he can't push up the cadence, shifting up, he has not got enough strength.

 

[Deleted member 4366]

You guys are still missing it. Of course you can go up hills faster when you pedal harder and faster. The problem comes when you can't pedal hard and fast. As an experiment, try pedalling with only one leg, then see how fast you can go up the hills. Also you should think about the difference in your own weight and OP's along with what you mean by a hill compared with what he's trying to get up.

 

[Woosh]

I don't think his road is very steep.
He is limited by both strength and speed. On flat roads, he can upshift and hit 15mph easily but not on that road.

 

[KTM Bike Industries UK]

I think the OP wonders how he can get more speed out of the bike on hills. That's no difficulty with a rotational system but with a torque sensing system, you need to be able to hit 90 rpm to get the best out of the bike. His cadence is about 70 rpm, he is stuck more or less at 11mph on third gear (26T) on moderate hills. Shifting down, he can't push up the cadence, shifting up, he has not got enough strength.

if he's at 70rpm the motor isn't giving maximum assistance.... yes.

if he's at 11mph, and in third gear, the motor speed isn't limiting, and he can change gear.

So if he changes into an easier gear, he can spin his legs and won't be personally putting in any more power, if he spins a bit faster with the same power, the motor will give him more assistance, and he'll go faster, with no extra input from himself.

You need to try a Bosh bike if you don't understand this. It won't take any extra effort.

 

[Woosh]

You need to try a Bosh bike if you don't understand this. It won't take any extra effort.

I test rode a few over the years. The motors are getting more responsive but not different.
The point I tried to make (that you still don't see) is that old people tend to have lower cadence through natural aging. I can still hold on to about 90 rpm for 5 minutes but I will have to slow down after that, even on flat roads. My CD motors have optimum speed at 86 RPM, similar to your Bosch.

 

[Deleted member 4366]

if he's at 70rpm the motor isn't giving maximum assistance.... yes.

So if he changes into an easier gear, he can spin his legs and won't be personally putting in any more power, if he spins a bit faster with the same power, the motor will give him more assistance, and he'll go faster, with no extra input from himself.

The graph you posted earlier is misleading you and some others because it shows output power, not torque.

Your opening statement that the motor isn't giving maximum assistance is sort of wrong, but it depends what you mean by assistance. If you mean power, you're correct, but to get the motor to give it's maximum power, you have to give a greater proportion of the total power yourself than you would at a lower speed, so proportionally, it's giving less assistance.

From the graph:

At 70 rpm the power is 540w
At 85 rpm the power is 570w (maximum)

Rotation x torque = power

we therefore have:
70 x T1 = 540, so T1 = 7.71, where T1 is the torque the motor produces at 70 rpm
85 x T2 = 570, so T2 = 6.71, where T2 is the torque the motor produces at 85 rpm

You can see that the torque is significantly less at a cadence of 85, so the only way to get the motor from 70 rpm to 85 rpm is to put in significantly more torque yourself. That assumes that you're already in bottom gear. You'd have to cover the 13% missing torque from the motor plus 15/70, which is another 21% ,to get to 85 rpm. That means that when hill-climbing in bottom gear, to go from 70 rpm to 85 rpm, you'd have to pedal 34% harder.

If you look at a graph of the power a person gives when walking, trotting, running and sprinting, i.e. speed vs power, it'll show that they produce more power at high speed, but that doesn't mean it's easier to sprint everywhere.