# Increasing efficiency of powertrain by editing gear ratio between motor and transmiss



## DIYguy (Sep 18, 2008)

Ruckeriii said:


> I was thinking of increasing the range of my EV conversion by adding a gear ratio between motor and transmission hub. Something along the lines of a 3 to 1 ratio but I'm not sure which way it should go to increase efficiency, or if I can increase it enough to turn a 40 mile range into an 80 mile range. Has anyone thought of this before?
> 
> Mason


Changing gear ratios will not double your range. You "may" be able to improve efficiency somewhat by using a particular part of your motor's power band efficiently. Power is power, you can multiply torque at the expense of rpm or vice versa but u don't get anything for free. If you are already using a transmission (by the sound of it) all you would have to change (if anything) is the differential ratio. Look at the specs of your motor and figure out the ratios through trans, diff and tire height (there are on-line calculators for this).


----------



## Ruckeriii (Feb 4, 2011)

DIYguy said:


> Changing gear ratios will not double your range. You "may" be able to improve efficiency somewhat by using a particular part of your motor's power band efficiently. Power is power, you can multiply torque at the expense of rpm or vice versa but u don't get anything for free. If you are already using a transmission (by the sound of it) all you would have to change (if anything) is the differential ratio. Look at the specs of your motor and figure out the ratios through trans, diff and tire height (there are on-line calculators for this).



Thanks that is what I needed to know. 

Mason


----------



## Ruckeriii (Feb 4, 2011)

DIYguy said:


> Changing gear ratios will not double your range. You "may" be able to improve efficiency somewhat by using a particular part of your motor's power band efficiently. Power is power, you can multiply torque at the expense of rpm or vice versa but u don't get anything for free. If you are already using a transmission (by the sound of it) all you would have to change (if anything) is the differential ratio. Look at the specs of your motor and figure out the ratios through trans, diff and tire height (there are on-line calculators for this).


Changing the setup so you get two turns of the drive wheels for every turn of the motor instead of one will not increase your range?  that's a bit perplexing. 

Mason


----------



## DIYguy (Sep 18, 2008)

Ruckeriii said:


> Changing the setup so you get two turns of the drive wheels for every turn of the motor instead of one will not increase your range?  that's a bit perplexing.
> 
> Mason


Nope. 
Not really perplexing.

HP= torque x RPM/5252

Try it on your bicycle. 
Go up a hill in first gear. How hard is it to pedal? How many times do you have to turn them to get up the hill? 
easier pedal x many turns = top of hill

Go up the hill in 5th gear. How hard is it to pedal? How many times do you have to turn them to get up the hill?
harder pedal x fewer turns = top of hill.

Both used the same amount of energy.


----------



## Ruckeriii (Feb 4, 2011)

DIYguy said:


> Nope.
> Not really perplexing.
> 
> HP= torque x RPM/5252
> ...


Let's try the same experiment on level ground with the bicycle. As you said same energy expended in first as fifth gear, the difference, greater distance traveled for energy expended in fifth gear. I'm talking about energy expended to maintain a cruising speed not accelerate. Either way I think the difference would be marginal and not worth the effort, I'll do some experiments with smaller DC motors from around work.

I do have one question. Would the lower RPMs save power on the motor or are electric motors more efficient at higher RPMs?

Mason


----------



## Woodsmith (Jun 5, 2008)

I don't get why you would want to put an extra gear ratio between your motor and transmission. You already have a box full of ratios in the transmission and then another ratio in the final drive.

Electric motors are usually at their most efficient when they are close to their maximum speed. At that point the the back EMF opposes the current flowing in and reduces the current right down until there is only enough to keep the car rolling. The amount would depend on the resistance the car experiences at the time.
So, on the flat at x mph there is a combination of air and rolling resistance. That takes a certain amount of torque to overcome. If the motor was spinning fast and the producing only enough torque to keep the car moving then it will be using the least amount of energy to keep going.

It is the opposite of an ICE. In the ICE you would want the RPM as low as possible and still have the torque to keep the car running. With electric you want the motor rpm as fast as it is safe to run at and still move the car.

So, that means for your average car you may do most of your driving in 2nd gear and then using 3rd to reach freeway speeds.
You would figure the highest safe speed for your motor and the road speed you want to cruise at and then find the gear in the transmission that is closest to it. Changing the final drive ratio could help move those transmission ratios closer to the best point for the motor efficency.

However, all that only works well if you do a lot of steady cruising on flat roads. If you don't then you stir the transmission a bit more.

Raising or lowering the overall transmission ratios by 3:1 would either cut your top speed right down as you run out of safe RPMs or have your motor spining really slow and pulling high amps.


Ultimately, in a lossless world it wouldn't gain or lose you anything as the energy to move the car is the same whatever the stuff between the batteries and the wheels were doing.
In our imperfect lossy world you will get losses and then have more losses on top due to running higher current at low RPMs, spinning another gear set, spinning another set of bearings, stiring another quart of oil, etc.


----------



## Duncan (Dec 8, 2008)

_*As you said same energy expended in first as fifth gear, the difference, greater distance traveled for energy expended in fifth gear*_

*NO*

Same distance for same energy - in a higher gear you need to "Push harder"

You are expending energy (yours) against wind resistance - same speed = same energy

The difference is legs going fast - low pedal load
legs going slow - high pedal load

There is a difference as to which is the most efficent for the "human engine" but the output power is the same


----------



## DIYguy (Sep 18, 2008)

Ruckeriii said:


> Let's try the same experiment on level ground with the bicycle. As you said same energy expended in first as fifth gear, the difference, greater distance traveled for energy expended in fifth gear. I'm talking about energy expended to maintain a cruising speed not accelerate. Either way I think the difference would be marginal and not worth the effort, I'll do some experiments with smaller DC motors from around work.
> 
> I do have one question. Would the lower RPMs save power on the motor or are electric motors more efficient at higher RPMs?
> 
> Mason


Sorry Mason, it's the same principal whether going up a hill or on level ground. The sum of all resistance = "the hill" metaphorically speaking. 
Aerodynamics, rolling resistance, gear inefficiencies etc., power train inefficiencies all add up. Gravity is just another one which is extreme enough to get the point across.... kinda... lol 

There are other things to consider with respect to RPM also...for example...when it's really hot in the summer, I tend to use a lower gear to keep the motor spinning faster to help to keep it cool. Unlike the ICE, electic motor will give us power above their rating (which is usually continuous) but for shorter time periods. Look at the extremes. Drag racers pour a few thousand amps through their motors... for 10 or 12 seconds. Many of these motors were originally rated for 20 or so hp.


----------



## Ruckeriii (Feb 4, 2011)

I guess MY EV derams are DOA for now. Thanks for the info.

Mason


----------



## DIYguy (Sep 18, 2008)

Ruckeriii said:


> I guess MY EV derams are DOA for now. Thanks for the info.
> 
> Mason


Why so?...............


----------



## nimblemotors (Oct 1, 2010)

You might try improving the aerodynamics of the car to increase your range if you travel over 30mph. Reduce rolling resistance by increase tire pressure and taller tires.



Ruckeriii said:


> I guess MY EV derams are DOA for now. Thanks for the info.
> 
> Mason


----------



## Ruckeriii (Feb 4, 2011)

DIYguy said:


> Why so?...............


 
Unless you can think of a way for me to put together an EV that will go 100 miles per charge and do so for less than $3000 my EV dreams are going to have to be put on hold.

For me to build a car that will not get me to work and back would be worse than useless, it would be a massive strain on my marriage.

Mason


----------

