Pro connect disc sprocket clarification

[blather]

Hi
I am looking at a number of bikes at the moment, one of which I favour is the pro connect disc. I have read quite a lot about the 11T motor sprocket and 18T rear sprocket changes, maybe too much because i am not really clear on the effect each has
Anyway I wondered is someone might enlighten me as to what each of these changes make individually and if I were to make 1 such change which would it be and why? Also what tools would I need for the job?

Thanks
Conn

 

[HarryB]

The 11t motor sprocket changes the point in your cadence that the motor stops assisting. It raises that cadence to something more comfortable for your knees so you don't have to change up a gear to get more assistance.

The 18t rear sprocket changes the overall gearing so your legs are not working like whisks just to go 20mph. Both to some extent make the bike less legal!

I think that Flecc has an excellent explanation of how the panasonic system works on his website (or you can find similar information here by doing a search).


PS I am sure that Flecc is at this moment typing an excellent response!

 

[flecc]

Best response I can make is that Harry is exactly right!

Increasing the motor sprocket reduces the pedalling speed for a given assist speed.

Decreasing the rear sprocket increases the road speed for a given assist and pedalling rate.

On some units fitted with 35 tooth chainwheels, that can be changed to the 41 tooth which also reduces the pedalling rate while keeping the road and assist speed the same.
.

 

[blather]

The 18t rear sprocket changes the overall gearing so your legs are not working like whisks just to go 20mph


Ok so that make sit pretty clear. Thanks to you both!!
What tools would I need to do this?
Conn

 

[flecc]

See this excellent posted advice from member JamesC, Conn:

Changing the 8 speed rear sprocket
.

 

[blather]

Thanks again

 

[tangent]

One possible downside to changing to an 11T sprocket is that this reduces the maximum assist ratio. So instead of 1:1, you will get 1 to 9/11, or about 1:0.8 and in the higher assist mode insead of 1:2 you will get about 1:1.6. This might not matter to you, both is still worth thinking about. The top assist on the bike that I had was 1:1.3. Fitting a 12T brought this down to 1:0.975 and I definitely noticed it on hills. I still prefered the higher cadence though.

Also worth mentioning is that either upgrade will reduce the range.

 

[tillson]

One possible downside to changing to an 11T sprocket is that this reduces the maximum assist ratio. So instead of 1:1, you will get 1 to 9/11, or about 1:0.8 and in the higher assist mode insead of 1:2 you will get about 1:1.6. This might not matter to you, both is still worth thinking about. The top assist on the bike that I had was 1:1.3. Fitting a 12T brought this down to 1:0.975 and I definitely noticed it on hills. I still prefered the higher cadence though.

Also worth mentioning is that either upgrade will reduce the range.

I'm not sure about the 11 tooth motor sprocket reducing the assist ratio.

P(power) w(angular velocity) T(torque)

P=wT

At a fixed reference road speed, with an 11 tooth sprocket, w will be less than with a 9 tooth, but as a cosequence, won't T increase, thus compensating?

 

[tillson]

One possible downside to changing to an 11T sprocket is that this reduces the maximum assist ratio. So instead of 1:1, you will get 1 to 9/11, or about 1:0.8 and in the higher assist mode insead of 1:2 you will get about 1:1.6. This might not matter to you, both is still worth thinking about. The top assist on the bike that I had was 1:1.3. Fitting a 12T brought this down to 1:0.975 and I definitely noticed it on hills. I still prefered the higher cadence though.

Also worth mentioning is that either upgrade will reduce the range.

I'm not sure about the 11 tooth motor sprocket reducing the assist ratio.

P(power) w(angular velocity) T(torque)

P=wT

At a fixed reference road speed, with an 11 tooth sprocket, w will be less than with a 9 tooth, but as a cosequence, won't T increase, thus compensating?

ie w9>w11 and T9<T11

 

[NRG]

Yes, I was curious about that as well, I thought the assist ratio was programmed in to the Panasonic controller.

 

[flecc]

It is, the assist ratio remains the same with any gearing changes.

But of course the climb ability/lower gear range does reduce if the motor is fitted with a larger sprocket and the rear sprocket not increased to compensate.
.

 

[tillson]

It is, the assist ratio remains the same with any gearing changes.

But of course the climb ability/lower gear range does reduce if the motor is fitted with a larger sprocket and the rear sprocket not increased to compensate.
.

I'm having difficulty with the reduction in climb ability too.

The motor output power surely remains the same regardless of the size of sprocket fitted to its output shaft and the shaft power consists of two components, omega and T. So if omega is reduced by the larger sprocket, then T has to increase to maintain the nominal motor power output. This must maintain the hill climbing ability.The motor may not be working as efficiently under these conditions and hence the range will be reduced, but the bikes perceived climbing ability will remain more or less the same.

 

[tangent]

I'm not sure about the 11 tooth motor sprocket reducing the assist ratio.

P(power) w(angular velocity) T(torque)

P=wT

At a fixed reference road speed, with an 11 tooth sprocket, w will be less than with a 9 tooth, but as a cosequence, won't T increase, thus compensating?

ie w9>w11 and T9<T11

Had a rethink about this and it is complicated. P varies with w, so T will not increase by w11/w9. Need to give this some more thought to what it will do. There is nothing linear here!

I think my original statement was wrong though - far too simplistic. In fact my current thinking is that the assist will be much more adversely affected by changing a 9T to a 11T.

The way I originally thought about this is that for a given power output by the motor, the amount put in by the rider will have to rise when the sprocket size is increased, because the rider will have to pedal faster to maintain the same motor power. I assumed that the rider will have to put in a factor of 11/9 more power, but I think this is now wrong.

For example, assume the rider is putting in 100W and the motor is delivering 100W with the 9T in place, for an assist of 1:1. Now swap the 9T for an 11T. To get the same 100W from the motor, the rider now pedals faster to keep the motor rpm the same as it was before. The rider must increase cadence by a factor of 11/9. Assuming the resisting force does not change (it will of course because wind resistance goes up with square of velocity), that means that the spped of the bike will now be go up by 11/9 hence total power of bike + rider must now be 11/9 of the previous amount. That means the power is now 200*11/9W, or about 244W. Now 100W of this is supplied by the motor (unchanged since w is the same) and 144W provided by the rider. That means the the assist ratio is 100:144 (More accurately 1:13/9).

I have assumed that the motor power does not change. Actually the power output by the motor may be higher because the rider is increasing torque and the power of the motor increases due to the torque sensor! This does however have an upper limit.

 

[flecc]

I'm having difficulty with the reduction in climb ability too.

The motor output power surely remains the same regardless of the size of sprocket fitted to its output shaft and the shaft power consists of two components, omega and T. So if omega is reduced by the larger sprocket, then T has to increase to maintain the nominal motor power output. This must maintain the hill climbing ability.The motor may not be working as efficiently under these conditions and hence the range will be reduced, but the bikes perceived climbing ability will remain more or less the same.

No need to get so complicated!

An increase from 9 to 11 teeth means a revolution of the motor will have to drive the bike 22% further if no other gearing is changed. It is therefore higher geared and that reduces a given climb ability limit.
.

 

[HarryB]

I agree it doesn't have to be so complicated. Presumably it is the same power output but your cadence has to increase to take advantage of this (this is the main point of the conversion). Provided, as flecc says, the gearing is right to keep the motor in the power band, all will be the same. I haven't ridden a Panasonic that doesn't have a low enough gear to keep in that power band (but then again I haven't cycled in Cornwall or anywhere exotic).

 

[tillson]

No need to get so complicated!

An increase from 9 to 11 teeth means a revolution of the motor will have to drive the bike 22% further if no other gearing is changed. It is therefore higher geared and that reduces a given climb ability limit.
.

I was, rightly or wrongly, assuming that the motor output power would be constant regardless of rpm. If it isn't constant, and falls with rpm, then I can see how the larger motor sprocket will compromise hill climbing ability.

 

[tangent]

Yes, I do follow Tillson's argument. If the motor is delivering 100W to the rear wheel then it does not matter what size the motor sprocket is because 100W is 100W. My argument hinges on the assumption that the power will drop as w drops. To get the same motor power, w must stay the same. To keep w of the motor constant, and the torque constant, the chain must move faster so the bike must move faster. For this to happen, the rider must put in more power as no more is coming from the motor.

 

[flecc]

The motor power isn't constant of course, it's only at it's maximum at one point, falling from that as revs fall.

Torque does not compensate adequately, most dramatically illustrated with the maximum torque being at zero revs where there's no power and no hill climbing ability exists!

However, the further complication of the Panasonic system is that the freewheeled motor's revs are rising and falling with each pedal stroke as each crank pressure on the torque sensor is received. The rhythmic "whoosh" of the motor as this happens can sometimes be heard and was more evident on the older unit. So in effect it's a form of servo motor which makes calculations of power and torque over time very difficult.
.