# Should I lower my gearing?



## Duxuk (Jul 11, 2009)

My EV trike can reach 56mph! That's a genuine 56, not just what an optimistic speedo shows. I don't intend to go that fast. My motor (ME0709) never gets hotter than about 56 Celcius so I don't have a problem there. My Alltrax 7234 is set for a maximum 180A output current and gives adequate performance. My batteries are a 72V 110Ah LA pack.

My question is would there be any advantage in going from a 12 to a 10 tooth motor sprocket?

I know that the motor current would be lower at a given speed but would there be any effect on battery current and hence on range?

I have driven 26.16 miles in one go, averaging 25mph, cruising at 30 but this will exhaust the batteries. In a headwind or hillier country I wouldn't get that far. 527 miles so far and going well!

Any opinions would be welcome.

Andrew


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## kek_63 (Apr 20, 2008)

Why fix it if it ain't broke?

Seriously though, if you are using 50 series chain (530, 525), 12 teeth is the minimum for acceptable chain noise and life. It would be wiser (although much more expensive) to look at a larger rear sprocket - but you appear to have lots of room with your controller for more amps if you want better acceleration.

Later,
Keith


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## TX_Dj (Jul 25, 2008)

The answer to your question comes from another question:

Do you feel you have enough torque as it sits now, or would it be better with more?

If you feel it has ample torque, and everything is running cool now without sapping too many amps from the battery, I'd say leave it.

If you feel it doesn't have as much torque as you'd like it to, or if it's pulling too many amps from the battery, I'd say change it.


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## frodus (Apr 12, 2008)

11/12 is about as SMALL as I'd go, even with smaller chain. The bend radius could be a little too tight for RPM's that high, and even so, the chain WILL wear quicker.

You could experement, but be ready to buy new chain if it binds.


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## Duxuk (Jul 11, 2009)

Thanks for the replies. I think if I won't gain any range through reducing the gear ratio I will leave it alone. Initial acceleration is poor, especially up a slope, but it picks up and goes well above 10 mph. I've adjusted the tracking of the front wheels to reduce friction and inspected the brakes to make sure they're not dragging. Apart from this I can't think of anything that will give me more range.

I use about 115Wh/mile going up to 30mph being very careful with the throttle and up to 150 if I drive quicker.

Andrew.


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## dmac257 (Jun 30, 2010)

Duxuk said:


> My question is would there be any advantage in going from a 12 to a 10 tooth motor sprocket?
> 
> I know that the motor current would be lower at a given speed but would there be any effect on battery current and hence on range?


I am new to the CONCEPT of electric propulsion and these questions raise another question. I read elsewhere that you can calculate (assuming you know a bunch of numbers of coefficient of drag etc) the required power to move a vehicle at a given speed. Lets say for this "example" 15kW to go at 50mph on a flat road. Does the gearing actually change the power consumed at speed?

I know it makes a difference in drive torque. But what is the normal state of the drive motor and traction pack/controller? If you build a vehicle that CAN go 70mph .. does that mean that at 70mph the controller has full pack voltage to the motor and if it is rated at 3600rpm it can't go any faster because no more available voltage? Assuming that your normal commute speed is closer to 50mph the controller holds motor voltage below pack voltage and the motor is running slower than 3600rpm? These different speeds mean different amps draw from pack right? But if the speed stays at 50mph and you have gear shifting transmission you also can controll the motor to drive shaft ratio and make the motor turn slower or faster. How would this effect the controller and the motor? If the speed is the same would the power consumed be different at all if the motor is turning faster or slower and why?

(I hope these aren't stupid questions)
dmac257


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## Posthumane (Feb 21, 2010)

dmac257 - A vehicle going at a constant speed will require a constant power, regardless of the gearing of the motor. If you need 15kW to go 50mph, then your motor will have to supply the same 15kW irrespective of how fast it's turning. If you gear it to turn slower (taller gearing) you will need more torque to make that power, and if you gear it shorter and have it spinning faster, you will need less torque. More torque = more current draw, but lower voltage due to lower speed. In general, power = torque * angular velocity (rotational speed) for rotating machines. In metric units, this is P(kW) = T(N-m) * w(rad/s) and in traditional automotive units it is P(HP) = T(ft-lbs) * RPM/5252.

That being said, a motor's efficiency varies over it's rpm range. A motor is 0% efficient if it is stalled (at a stand still with current flowing through it to create torque, but not enough to overcome the load), and when it is in a no load condition (spinning freely and producing no useful torque, but still drawing some finite current). It will have a peak efficiency somewhere in between those two points, as well as a peak power at some different point. The primary loss in energy at the lower end of the motor's speed is due to copper (resistive) losses, and in the upper end it is a combination of things such as friction and windage losses, magnetic hysteresis, back emf, etc. Maybe more information than you wanted...

Duxuk - if you have the power and efficiency curves for your motor, then your best bet would be to set the gearing so that your motor is turning at its most efficient speed at the vehicle's desired cruising speed. This is assuming that you spend most of your time in a steady state cruise, rather than stop/go driving. If it is mostly stop/go, then having shorter gearing would put you in a more efficient regime during acceleration.


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## dmac257 (Jun 30, 2010)

Posthumane said:


> dmac257 - A vehicle going at a constant speed will require a constant power, regardless of the gearing of the motor. If you need 15kW to go 50mph, then your motor will have to supply the same 15kW irrespective of how fast it's turning. If you gear it to turn slower (taller gearing) you will need more torque to make that power, and if you gear it shorter and have it spinning faster, you will need less torque. More torque = more current draw, but lower voltage due to lower speed. In general, power = torque * angular velocity (rotational speed) for rotating machines. In metric units, this is P(kW) = T(N-m) * w(rad/s) and in traditional automotive units it is P(HP) = T(ft-lbs) * RPM/5252.
> 
> That being said, a motor's efficiency varies over it's rpm range. A motor is 0% efficient if it is stalled (at a stand still with current flowing through it to create torque, but not enough to overcome the load), and when it is in a no load condition (spinning freely and producing no useful torque, but still drawing some finite current). It will have a peak efficiency somewhere in between those two points, as well as a peak power at some different point. [snip]
> 
> Duxuk - if you have the power and efficiency curves for your motor, then your best bet would be to set the gearing so that your motor is turning at its most efficient speed at the vehicle's desired cruising speed. This is assuming that you spend most of your time in a steady state cruise, rather than stop/go driving. If it is mostly stop/go, then having shorter gearing would put you in a more efficient regime during acceleration.


So, if I understand correctly, if the motor is most efficient at 3600RPM you should use whatever gearing will make motor run at 3600RPM no matter what speed you are going on the road?

And IF I have a motor speed sensor I could shift transmission from 4th to 3rd and adjust throttle to get motor speed back to efficient RPM = (some lower road speed) but least current draw from batteries if road conditions dont change?


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## rillip3 (Jun 19, 2009)

dmac257 said:


> So, if I understand correctly, if the motor is most efficient at 3600RPM you should use whatever gearing will make motor run at 3600RPM no matter what speed you are going on the road?
> 
> And IF I have a motor speed sensor I could shift transmission from 4th to 3rd and adjust throttle to get motor speed back to efficient RPM = (some lower road speed) but least current draw from batteries if road conditions dont change?


Sounds like you have the idea. Keeping the gearing such that you are keeping the RPMs at the motor ideal will lead to the best performance. If the current length of gearing makes it difficult to keep the RPMs at peak, changing the length of gearing will make it more efficient. If you can keep it at peak RPMs as is, then you'll just change what gear you need to be in to get peak RPMs.


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## dmac257 (Jun 30, 2010)

rillip3 said:


> Sounds like you have the idea. Keeping the gearing such that you are keeping the RPMs at the motor ideal will lead to the best performance. If the current length of gearing makes it difficult to keep the RPMs at peak, changing the length of gearing will make it more efficient. If you can keep it at peak RPMs as is, then you'll just change what gear you need to be in to get peak RPMs.


So for a bicycle, motor RPM would be like cadence. You swap gears to keep crank speed as close to the same as you can. I am thinking of making electric bicycle conversion as first EV project .. trying to figure out how to add the motor without buying a hub motor "kit" as then I would be able to use the gearing when I hit a hill to keep motor speed closer to ideal.

Don


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## GerhardRP (Nov 17, 2009)

Posthumane said:


> The primary loss in energy at the lower end of the motor's speed is due to copper (resistive) losses, and in the upper end it is a combination of things such as friction and windage losses, magnetic hysteresis, back emf, etc. Maybe more information than you wanted....


 Actually BACK EMF is not a LOSS mechanism... 
The power delivered by a motor is the armature current times the back emf.
Gerhard


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## rillip3 (Jun 19, 2009)

dmac257 said:


> So for a bicycle, motor RPM would be like cadence. You swap gears to keep crank speed as close to the same as you can. I am thinking of making electric bicycle conversion as first EV project .. trying to figure out how to add the motor without buying a hub motor "kit" as then I would be able to use the gearing when I hit a hill to keep motor speed closer to ideal.
> 
> Don


I know nothing about what the gears on a bicycle do, what they are for, etc. I am clueless when it comes to bikes. But the principle should be the same; the gearing is there to keep the wheels turning while keeping the input as consistent as possible, i.e. high torque at low speeds, lower torque at the upper end. Now how this applies to a human-electric hybrid, I have no clue. If it's going to be an all electric bike, then the motor matching I described still works. Might want to ask about it on the Bike section of the forum for a human-electric hybrid.


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