Consequence of sprocket size change?

[danfoto]

OK ... my Kalkhoff Agattu which had a 35T chainwheel and an 11T motor sprocket now has a 41T chainwheel and a 12T motor sprocket. Everything else stays the same.

The higher overall gearing due to the chainwheel change is straightforward enough, but what about the bigger motor sprocket? Less motor revs for a given roadspeed is the obvious consequence, but is there in fact anything else? I don't know enough about the control gear to work this one out ...

(BTW, I know that in theory bigger sprockets in a chain drive mean greater efficiency, but I'm talking real world here)

 

[Deleted member 4366]

You get less maximum torque at the back-wheel, but more speed. You probably won't notice the lower torque except in first gear climbing a steep hill, because the power profile is complicated and depends on cadence, speed and pedal power. You might get more torque in the higher gears because of the change to cut-off and ramp-down speeds. Ask Flecc to explain: It's too complicated for me - too many things changing at once.

 

[Fordulike]

Apologies for hijacking this thread slightly, but can anyone point me to an online calculator to show final rear wheel rpm, when inputting the front chainring and rear gear size?
Hope you get what I mean

 

[D8ve]

RPM is simple cadence * chainring/cogteeth
ie cadence 60 *36 chainring/12 cog = 60*3= 191 rpm.

To OP if origonal gave motor cut out at 60 rpm then old wheel did 3 2/11 times 60 (about 191 rpm) then new Sprocket is 12 and rotates at 191 rpm and old wheel is still the same then the RPM will be 208 rpm
now if the old did 25KPH at 191 rpm then at 208 rpm the speed will be 27.3 KPH but a cadence of 61.

I havent done these sums for years so any errors are mine and corrections welcome.(except spelling as dyslexic so tought)

 

[tillson]

OK ... my Kalkhoff Agattu which had a 35T chainwheel and an 11T motor sprocket now has a 41T chainwheel and a 12T motor sprocket. Everything else stays the same.

The higher overall gearing due to the chainwheel change is straightforward enough, but what about the bigger motor sprocket? Less motor revs for a given roadspeed is the obvious consequence, but is there in fact anything else? I don't know enough about the control gear to work this one out ...

(BTW, I know that in theory bigger sprockets in a chain drive mean greater efficiency, but I'm talking real world here)

Firstly, you will have a lower pedal cadence for any given road speed in each gear. That is the effect of switching from a 35 to 41 tooth chain wheel.

The increase in motor sprocket size means that it will rotate more slowly for any given road speed in each gear than it did when you used an 11 tooth sprocket. The motor cut off (and power phase down if your model still has this feature) is determined by the motor RPM. So, your motor sprocket will now be turning more slowly at, say 15 MPH, in top gear than it did when you were using the smaller sprocket. If your motor used to cut out at 15 MPH with the smaller sprocket, it won't any longer. The motor will continue to deliver power until you reach a higher road speed than before. If the 15 MPH cut of is an important feature for you, you could fit a larger rear sprocket to rein it in a bit.

I'm not sure how the larger motor sprocket's effect would translate into perceived motor assistance. I suspect it would make the motor feel slightly weaker, but it would assist to higher speeds. It too hot to think about it in too much detail.

 

[flecc]

There's no simple way to say this.

The relationship of the motor sprocket and chainwheel is little changed at 7% slower chainwheel speed at a given motor sprocket speed, so of little consequence.

As Dave (d8veh) has said the motor drive to rear wheel gearing has increased slightly with the slight loss of torque that he mentions, but again the change there is small at only 9%, more than easily compensated for by changing down one gear. With any Nexus gear hub that will reduce gearing by around 14%, depending on the gears changed between, so a slight around 5% overcompensation.

Whether you notice any loss of torque in any existing choice of gears though depends on the year of your Agattu and your preferred cadence when climbing. However, paradoxically the change in chainwheel cadence that I mention at the outset won't affect that since the unit measures the cadence at the motor sprocket output speed, making an assumption of a standard chainwheel to motor sprocket ratio. So the following will determine if there is a loss of torque.

The Agattu's Panasonic units roughly pre 2011 had the declining power with cadence as it exceeded 40 rpm, so if your motor sprocket speed was typically at a higher than 40 rpm cadence before, the fall in motor sprocket speed relative to the rear wheel at any given speed will be compensated for by the increase in power that the unit gives as the motor sprocket speed falls towards the 40 cadence equivalent of the pre sprockets change speed.

Calculation shows that the 9% increase of your motor to rear wheel gearing from the 41/12 change will produce a power increase of 29% at speeds between 10 and 14.5 mph, so that's the even bigger paradox with those earlier models. The loss of 9% is compensated at a given speed by the 29% power increase, leaving you at a given speed with 20% more climb power now after the change!

However, remember that this doesn't apply to the later and current models that retain full power to nearly the cutoff speed. With those there is the 9% power loss due to the sprockets change, leaving you with the choice of you supplying that much extra to maintain a given speed or changing down and you supplying 5% less effort at the road speed reduction corresponding to the hub gear ratio change down.

Phew!
.

 

[danfoto]

Phew indeed! Many thanks chaps. I had a feeling there was far more to it than met the eye!

Whatever, the main reason for my question (apart from natural curiosity) is that so far, all I can really tell is that it's a different bike now - but on the road, it's not obvious to me (yet?) how it's different, apart from the expected higher overall gearing.

I'm quite happy with the change so far, but I still need to check hill-climbing with a loaded trailer ...

P.S. We bought the bikes discounted in Jan/Feb 2012, so presumably they're 2011 models. Is there a simple way for me to determine whether they're "earlier" or "later"?

 

[flecc]

I don't know how you can tell the manufacture date on Kalkhoffs, but 50cycles do their best by discounting to clear stock at the end of every season. So as it's likely to be a 2011 model, I'm certain yours will have the full power to almost 15 mph when standard geared. The fact that all you've noticed with the change is the higher gearing supports that.

 

[danfoto]

Thank you mister.

 

[Weldinpatt]

Firstly, you will have a lower pedal cadence for any given road speed in each gear. That is the effect of switching from a 35 to 41 tooth chain wheel.

The increase in motor sprocket size means that it will rotate more slowly for any given road speed in each gear than it did when you used an 11 tooth sprocket. The motor cut off (and power phase down if your model still has this feature) is determined by the motor RPM. So, your motor sprocket will now be turning more slowly at, say 15 MPH, in top gear than it did when you were using the smaller sprocket. If your motor used to cut out at 15 MPH with the smaller sprocket, it won't any longer. The motor will continue to deliver power until you reach a higher road speed than before. If the 15 MPH cut of is an important feature for you, you could fit a larger rear sprocket to rein it in a bit.

I'm not sure how the larger motor sprocket's effect would translate into perceived motor assistance. I suspect it would make the motor feel slightly weaker, but it would assist to higher speeds. It too hot to think about it in too much detail.

Details are what we need to obtain optimum performance.I was ill and now equate to an old engine with worn rings and a lower compression ratio no longer can i pull original factory gear ratios. Because my rings are obsolete and unavailable, the only option left is to lower the gearing (so i can get up and down hilly Snowdonia). I love my N380se but could do with moving its internal ratio to the Left a bit

 

[tillson]

Details are what we need to obtain optimum performance.I was ill and now equate to an old engine with worn rings and a lower compression ratio no longer can i pull original factory gear ratios. Because my rings are obsolete and unavailable, the only option left is to lower the gearing (so i can get up and down hilly Snowdonia). I love my N380se but could do with moving its internal ratio to the Left a bit

I would say you need a 35 tooth chain wheel, a larger motor sprocket and larger rear sprocket. The perceived affect would be a higher pedal cadence, but lighter pedal effort.