Gearing or Voltage?

[deronmoped]

I have the ability to gear my bike to go faster, is that just as good as upping the battery voltage? Upping the voltage, from my understanding is changing the characteristics of the motor, a higher voltage will make for a more powerful motor that wants to have a higher top RPM too. I have been looking into a higher then 24V battery, maybe a 36V or 48V battery to achieve a higher speed, it seems to be the way most people go.

The motor I have drives through the crank, it's 400 watt continuous with a 1,000 watt peak. The bike is limited to 20 MPH by the computer in it and the gearing seems to match that well. I would like to up the speed to at least 25 MPH. If I change the gearing for a higher top end will that do the trick?

Thanks, Deron.

 

[Drunkskunk]

The answer is Yes, and No.
Yes. its that simple.
And
No, it isn't.

Confused yet? Wait, it gets better!

By increasing the gearing you effectivly decrease the rpm of the motor at a given speed. The lower the motor's speed the harder it is having to work to maintain that speed, as it's mechanicle advantage is lower.
Another way of saying that is it will take more amps to do the same speed as before.
Your top speed will increase some. the motor is having to work harder, so it won't be a directly proportional ratio. So if tyou figure out a gear ratio that mechanicly gives you 25mph for the same motor rpm as 20mph on your current gearing, you then have to take into account the harder work the moptor has to do at that gearing, plus. the higher wind resistance. 400 watts would have a hard time pushing a bike to 25. wind resistance is nonlinear

The thing with increasing the voltage, is that it also increases the wattage along with the RPM. Double the voltage, double the wattage. that also increases your avalible torque across the whole speed range.

 

[deardancer3]

Sounds like the motor you have might be similar to the power unit off of an old Giant Lafree Sport ebike. A nice brushed motor, but max power downgraded in the stock unit (thru current limiting)to 750watts to make life a little better on the stock SLA. If one is scrapping the stock controller/ batteries, this does not apply.

For your question you may find some insight by looking at a power calculator such as:
http://www.kreuzotter.de/english/espeed.htm

As one experiment input your anticipted ebike frame type and 25 mph to see the anticipated power needed (at the tire). Then change to a little bit more aerodynamics , and then a streamliner.

Look at those power needs at 25mph! One can utilize the lower power needs with a more streamlined bike/posture, Or go with more power thru higher voltage, and or higher power batteries, controller etc.

You mentioned at least 25mph. so punch 30mph into the power calculator and think about the weight/cost of batteries to do this for a 20 mile range.

But to get back to your original issue, one can only get so much from gearing.

But between gearing, aerodynamics AND overvolting, mmmm magic; power, range, speed, hillclimbing and at reasonable battery size.

my input

D

 

[deronmoped]

Hey D

So do you have some experience with these Giant LaFree motors? That is what I have, I though the peak power was a 1,000 watts, you are saying the controller limits that to 750?

I used that calculator and it says I need 500 watts to get to 25 MPH. I figure I will be inputting 100 watts and the motor will need to be putting out 400 watts.

I was looking at upping the voltage to 36 or 48, but that is real expensive. I already have a Ping 24V 20AH battery that gives me more then enough range, I'm now exploring just getting more out of the battery and motor through gearing.

One thing I have going on is the computer is shutting down power to the motor at anything above 20 MPH and on top of that as I hit 20 MPH, the motor seems to be at maximum RPM.

Deron.

 

[deardancer3]

Yeah in a previous life I worked on Giant Sports. I had two & still have a spare motor.

There is a guy in SLO CA that has four of them, he really likes them a lot. He has gotten a stock unit up to about 24mph.

You will be fine over volting to 36v; that will give you 30mph and lots more power. Dont go to 48v. It seems that brushed motors can handle 50% voltage increase but not 100%.

Be sure to keep the chain lubed so as not to degrade the chain drive gears.

The power simulator is talking about power at the wheel; You need to figure in losses due to inefficiencies.
So if you need 400 watts at the wheel, figure on at least 500watts out of the pack.

There are a number of decent brushed controllers that will do 30 amps at 36v. They willl be throttle based rather than pedalec. NBD.

d

 

[totals]

I used to run 26 volts. Nothing beats more voltage. I've got a video in my signature line that will give you a good sense of what more volts can do.

 

[E Moped]

All GOOD info.....Thanks Guys! Tomorrow I pick up a mated pair of Lafree Sports,mens blue/womens red they said they would only sell as a pair. So any help to enhance performance is welcome.....with this decision I'll get a little excersize too .......Thinking Nimh or Lipo 36V-25ah range....I am assuming I need to change controller.......Does anyone know if a Super Rooster will work...(compact)? Suggestions....???

 

[Mark_A_W]

Hullo Deron.


Small world.


Soon we'll have you on 60 or 72v and 35mph+