Electric Bike Power System

[UrbanPuma]

I was reading an earlier thread which mentioned that the Giant Lafree Twist produced less power from the motor in low gear and more power in high gear. My query is directed at all members. Which mode of operation does your bike use and what are the benefits?

My Powercycle Puma provides more assistance from the motor when in low gear and less in high gear. This would seem to be in-line with a motor car system, which i am happy with. However, i do think it means more battery strain tackling hills and maybe the Lafree system would be more suitable for hill climbing, especially using lithium batteries.

Any comments?

UrbanPuma

 

[flecc]

I was reading an earlier thread which mentioned that the Giant Lafree Twist produced less power from the motor in low gear and more power in high gear. My query is directed at all members. Which mode of operation does your bike use and what are the benefits?

No, that's not correct, the power produced isn't connected in this way on the Lafree Twist, except for power phasedown from 13 mph to comply with the law.

The Lafree's power applied depends on the pedal cadence, the speed at which the pedals are being turned. At low cadences, the software interprets it as the rider needing help and gives more power. At fast cadences, the software interprets this as the rider getting on fine so the motor assistance is reduced.

Because the motor unit drives through the gears, these are true whatever gear the bike is in.

On your bike and most hub motor bikes with pedelec control, the power is sometimes phased into different levels at different pedal rotation speeds, but once again it's not connected with the gears, with which there's no electrical connection. They often have the same power phasedown as the speed rises, and this might be part of what you're observing. If the pedelec sensor is on the rear wheel, it will affect the power level according to road speed, unlike the Lafree Twist.

I think that you may be misinterpreting the effects of gearing on a given power level though, and your mention of cars suggests that might be the case.

Think of this as power over time. Lets say that a vehicle delivers ten times X power for a minute.

In top gear the vehicle might cover 1000 yards in that minute at a given motor speed, so each X unit of power has to drive the vehicle 100 yards.

In bottom gear it might cover 200 yards in that minute, so each X unit of power only has to drive the vehicle 20 yards.

Therefore, in this example, as the distance is only one fifth in the low gear case, the pulling power available in each yard of travel is five times larger, compared with that when it's in top gear. (But the speed is only one fifth of that in top gear).
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[PaulC]

I have a Merida powercycle 550 (from around 2003). This is like the LaFree because the power assistance from the electric motor is via a set of cogs linked to the pedal crank. The rear wheel has a Shimano 4 internal gear hub.
The amount of electric power supplied is controlled by a combined torsion + pedal crank rotation rate sensor.
When I pedal slowly with heavy pressure on the pedals in any gear, I get the maximum amount of assistance from the electric motor. As my pedal cadence speeds up, the amount of assistance drops off and then the electric motor stops once I go over a certain pedal cadence (regardless of the gear I am in). This is the 15mph speed restriction standard to ebikes.
There is an explanation of all this in a patent description available online (merida electric bike patent) but it is difficult to follow.
I find it works OK but there does not seem to be any way to turn off the pedal cadence limit
Paul

 

[flecc]

That's exactly the same operation as the Lafree in every detail Paul, though possibly achieved differently.

There's a description of how that works within the motor repairs page on my Lafree website page here which you might find easier to follow as it includes how it's achieved in this unit.

Scroll down to the Pedal Shaft Assembly and read from there.
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[UrbanPuma]

That seems to make more sense in laymans terms PaulC. I read a recent article about the Schwinn bikes and they have a sensor in the pedal crank too, which i think my powacycle has also.

However, i do find that in low gear you spin wheels faster and high gear slower so surely motor assistance must relate to gears somehow?

 

[flecc]

That seems to make more sense in laymans terms PaulC. I read a recent article about the Schwinn bikes and they have a sensor in the pedal crank too, which i think my powacycle has also.

However, i do find that in low gear you spin wheels faster and high gear slower so surely motor assistance must relate to gears somehow?

No, on your bike it's as I explained before.

The only bikes with a relationship between gears and motor pedelec sensing are those like Pauls and the Lafree, with motors which drive the chain and therefore the gears.

You have a hub motor which drives the wheel directly, while your pedalling drives the wheel directly through the derailleur gears, There is no connection between them, electrical or mechanical, other than the rear wheel hub.

What you probably observe is the effect of torque with changing speed, as I explained above, the torque dropping as speed rises. This is a function of the available power against the rising power needed with speed. You and you alone set the pedalling speed according to your ability and the gear you choose to be in.
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[UrbanPuma]

Ok, now it make sense to me Flecc

Thanks for explaining