# How to get higher Top Speed on my E-Bike



## Bruce_Kenobi (Dec 30, 2011)

Hi all,

You were all super-helpful when I stumbled upon some problems on my conversion, so now that it is fully operational I have found out that it is not as fast as I thought it would be. I'm running a system based on 48 v (12 v x 4), 55Ah, lead acid AGM battery pack with a Motenergy ME0709 Permanent Magnet brushed motor and an Alltrax 24 - 72 volt 300 amp programmable AXE 7234 controller. The bike has 14-tooth front sprocket on the motor, and 56-tooth on the rear sprocket, for a total ratio of 1:4. My question is: what could I do to get the bike to run faster than 50 km/h at top speed? (aprox 30 mph) I wonder if it's because of the rear sprocket, but I don't want to impact range, should I get more volts? or more amps? thoughts? thank you!!!


----------



## kennybobby (Aug 10, 2012)

The speed and torque are related to the gear ratio for sure. You could drop the rear 1 or 2 teeth and/or raise the front 1 to run at higher top speed, but you would be trading off a bit of torque.

Raising the voltage would increase top speed.

Reducing weight helps increase range.


----------



## Bruce_Kenobi (Dec 30, 2011)

kennybobby said:


> The speed and torque are related to the gear ratio for sure. You could drop the rear 1 or 2 teeth and/or raise the front 1 to run at higher top speed, but you would be trading off a bit of torque.
> 
> Raising the voltage would increase top speed.
> 
> Reducing weight helps increase range.


Thank you very much for that insight. Raising the Voltage would increase it? I though the sprockets had something, but I didn't want to loose torque as I ride in hilly terrain, but your suggestion on more voltage seems like a good one. Would the amps have a say in this?


----------



## kennybobby (Aug 10, 2012)

Increasing the current would increase motor torque.


----------



## dcb (Dec 5, 2009)

there are spec sheets for the ME0709, you can measure your amps and volts and rpm at top speed and see if it is near full power output, or if it can handle lower gear ratio for more wheel rpm.

If you post the running voltage (to account for sag) and amps, and wheel diameter, and gear ratio, and speed at your current top speed (flat course, no wind), you might get a guess what to change for best top speed.

Or you can experiment with different sprocket ratios and see which has the best top speed (which will reduce acceleration accordingly).


keep an eye on your motor temperature. Pretend you are touching an iron at first though.


----------



## Bruce_Kenobi (Dec 30, 2011)

dcb said:


> there are spec sheets for the ME0709, you can measure your amps and volts and rpm at top speed and see if it is near full power output, or if it can handle lower gear ratio for more wheel rpm.


sorry for the noob question but how could I measure the amps and volts at top speed? (I have no gauges on the bike, except for a faulty speedometer  )



dcb said:


> If you post the running voltage (to account for sag) and amps, and wheel diameter, and gear ratio, and speed at your current top speed (flat course, no wind), you might get a guess what to change for best top speed.
> 
> Or you can experiment with different sprocket ratios and see which has the best top speed (which will reduce acceleration accordingly).
> 
> ...


I thought about the different sprocket ratios, but I don't want to loose acceleration, it has a good sprint. Maybe the voltage increase would be the way to go, would have to change from AGM's to something different to keep it light though. Since the motor is exposed I'm not too worried about it, the winds keeps it cool.


----------



## electro wrks (Mar 5, 2012)

The motor (if it doesn't exceed the max recommended RPM - check with the supplier) and the controller can handle 72V. Why are you not using the higher voltage? You already paid for it. You'll have available ~72/48 = ~1.5 X more power, every thing else being equal.

The motor power output is a ~ linear function over a voltage range. The power requirements to go faster, are a function of the cube of the increased speed (am I saying that right?) This means you won't go 1.5X faster if you up the voltage from 48V to 72V. Also, the reduction ratio will have to be increased (numerically, say from 1:4 to 1:5 or 1:6) to take advantage of the increased motor power, because it's available at higher motor RPMs.

All of this can be tough to swallow on your first build. The old saying is: "Speed costs money, how fast do you want to go"

If you don't believe this (I didn't at first) you can experiment with different voltages, battery sizes and types, controllers, and reduction ratios. Just heed dcp's advice and carefully, very carefully, monitor the motor [and controller] temps. Once the magic smoke escapes from the motor and such, you can't put it back in.

Just because the motor is in the "winds" probable won't help. Active cooling, with an electric fan, is usually what is needed for a overheating problem. Some pictures of your build would be great to see.


----------



## Bruce_Kenobi (Dec 30, 2011)

electro wrks said:


> The motor (if it doesn't exceed the max recommended RPM - check with the supplier) and the controller can handle 72V. Why are you not using the higher voltage? You already paid for it. You'll have available ~72/48 = ~1.5 X more power, every thing else being equal.


I originally planned to have 6 batteries, but the weight of it all made the bike slant forward and the handlebars where touching the body, hence I took out 2 batteries and settled for 48....but now I know what I lost! top speed



electro wrks said:


> The motor power output is a ~ linear function over a voltage range. The power requirements to go faster, are a function of the cube of the increased speed (am I saying that right?) This means you won't go 1.5X faster if you up the voltage from 48V to 72V.


I think I got half of that: if I up the voltage to 72 I won't get 1.5x faster, then...? just 1? 0.8?



electro wrks said:


> Also, the reduction ratio will have to be increased (numerically, say from 1:4 to 1:5 or 1:6) to take advantage of the increased motor power, because it's available at higher motor RPMs.


I wanted to go 1:6, but couldn't find a rear sprocket that size, maybe I have to look further.



electro wrks said:


> All of this can be tough to swallow on your first build. The old saying is: "Speed costs money, how fast do you want to go"


No kidding, this being my first build there is a lot of trial and error on this, and sit-of-your-pants testing.



electro wrks said:


> If you don't believe this (I didn't at first) you can experiment with different voltages, battery sizes and types, controllers, and reduction ratios. Just heed dcp's advice and carefully, very carefully, monitor the motor [and controller] temps. Once the magic smoke escapes from the motor and such, you can't put it back in.
> 
> Just because the motor is in the "winds" probable won't help. Active cooling, with an electric fan, is usually what is needed for a overheating problem. Some pictures of your build would be great to see.


So far I haven't seen it over heat, of course it hasn't move that much, but a fan is a good idea. I have a ton of pictures of the build, but I have no hosting and this forum always ask for a URL to upload them, probable photobucket is good place to get started.


----------



## aeroscott (Jan 5, 2008)

With a 48v system at max speed the motor will be putting out a back voltage. This is the reason torque falls as the speed of the motor is increased all motors , anytime a conductor passes a field and the more turns (higher voltage motor) causes higher back e. m. f. Also the faster the wire interacts with the field the higher the back e.m.f. 
So say 48 volt system at max rpm makes 30v. back e.m.f. = 48 - 30= 18 volts at a fixed resistance = less amps then when the motor was going much 
slower. In effect we have a 18 volt motor at higher speed. 
At 72 v. -30v.=42volts will allow a much greater amperage flow then 18 v.
over 2 times greater amps.
If you use a higher ratio you will get a higher back emf at the same speed.
back emf will always increase with Rpm .


----------



## Bruce_Kenobi (Dec 30, 2011)

Thanks for all the input, and sorry for the long delay. As it turns out, the top speed can get up to 60 Kmph (37.5 mph) it only needed a good charge, however it won't go higher than that and as you drive the bike on full speed, you start seeing how top speed decreases as time goes by, proving that the bike as it looses charge it also looses the ability to keep the top speed. Maximum autonomy, with a full charge is 31.5 Km, and the last 9 where at minimum speed. I guess more voltage will increase top speed and autonomy as well.


----------



## Bruce_Kenobi (Dec 30, 2011)

Hi Guys, just to show you a video I made of the construction of the bike, maybe you wanna watch it!  https://www.youtube.com/watch?v=tZdvHIR3Fao


----------

