speed to watt ratio

[mnplus1556]

so i've hear that you can figure out how fast your e-bike can go with a watt to mph ratio. i've hear of 3200 watts to 40 mph. i've also hear 450 watts to 25 mph. these seem like really far apart ratios to me. so does anyone know the correct ratio? i'm just trying to figure out how to gear my bike with a 1000 watt motor (36v)

thanks!!

 

[Tiberius]

Its not quite that simple.

Aerodynamic drag goes as the square of speed. Power goes as the cube of speed.

On a well sorted bike with you tucked down to get minimal frontal area, 1000 W should get you just over 30 mph. With a sit up and beg riding position it will be somewhere well below 30 mph.

Of course, that's working with 1000 W output power from the motor. That's not necessarily the same as the nominal or rated power. Without knowing more about the motor, controller and battery its difficult to be precise, but you could start with gearing for 25 to 30 mph.

Nick

 

[Miles]

http://books.google.co.uk/books?id=0JJo6DlF9iMC&pg=PA187&lpg=PA187&dq=bicycle+drag+coefficients+and+other+data+wilson&source=bl&ots=Tr-DzTbtj9&sig=R3l4QuNuecoTWmDNEPnbCm1PGqw&hl=en&ei=2amYTIn_IYKQjAfl640f&sa=X&oi=book_result&ct=result&resnum=1&ved=0CBgQ6AEwAA#v=onepage&q=bicycle%20drag%20coefficients%20and%20other%20data%20wilson&f=false
http://www.endless-sphere.com/forums/viewtopic.php?f=2&t=6892&p=287797#p287797
http://endless-sphere.com/forums/viewtopic.php?p=312555#p312555

 

[ajtest]

Interested to see this as I've just been out for the first time on my Ross Speed recumbent which now has an x5305 at 20S and was amazed to see only 200W needed to maintain 30mph on the flat.

It just goes to show that it's all about the wind resistance - either that or there's something wrong with my CA!

Andrew

 

[mnplus1556]

wow that seems awesome. 200w to reach 30 mph!! with my gearing that i am setting up i have to lowest gear reaching a max of 16 mph and the highest gear reaching a max of 30 mph. when i have it all together i am going to record it make a new topic on it. do you have any vids on youtube? that seems kind of unbelievable to reach 30 mph with 200w...did you use gears?? oh also yah it probably takes 200w to maintain 30 mph but how many watts to get up to that speed?

 

[amberwolf]

so i've hear that you can figure out how fast your e-bike can go with a watt to mph ratio. i've hear of 3200 watts to 40 mph. i've also hear 450 watts to 25 mph. these seem like really far apart ratios to me. so does anyone know the correct ratio? i'm just trying to figure out how to gear my bike with a 1000 watt motor (36v)

thanks!!

To know how to gear the bike with a particular motor at a particular voltage, you need to know how fast you want to go, how fast the motor spins with a load at that voltage, and then calculate the gear ratios for whatever reduction stages you are going to use between motor and wheel to get the wheel to the speed that equates to the desired road speed.

Now, if you gear for the fastest possible speed from a motor, you may sacrifice startup torque, and then end up having to always pedal up to a certain speed before the motor can safely (or at all) start moving you on it's own. If you are always going to pedal with it anyway, that may not be a problem, If you wnat to run without pedalling, though, you also have to consider what startup torque the motor will end up with at your gearing.

If it is possible, running thru the bike's regular drivetrain will give you the best of both worlds, since you can shift down into granny gears if necessary for hills and startups, and back up into racing gears for speed, just like when you pedal. Some bike's geometries or component layouts make this difficult, depending on teh motor you have.


As for the topic title, there is an interesting thread that is a survey of what people are actually getting for watts at various speeds:
http://endless-sphere.com/forums/viewtopic.php?f=5&t=14786&start=0
which may help you figure out what you want, too (for those that gave build links and stuff).

 

[mnplus1556]

so i've hear that you can figure out how fast your e-bike can go with a watt to mph ratio. i've hear of 3200 watts to 40 mph. i've also hear 450 watts to 25 mph. these seem like really far apart ratios to me. so does anyone know the correct ratio? i'm just trying to figure out how to gear my bike with a 1000 watt motor (36v)

thanks!!

To know how to gear the bike with a particular motor at a particular voltage, you need to know how fast you want to go, how fast the motor spins with a load at that voltage, and then calculate the gear ratios for whatever reduction stages you are going to use between motor and wheel to get the wheel to the speed that equates to the desired road speed.

Now, if you gear for the fastest possible speed from a motor, you may sacrifice startup torque, and then end up having to always pedal up to a certain speed before the motor can safely (or at all) start moving you on it's own. If you are always going to pedal with it anyway, that may not be a problem, If you wnat to run without pedalling, though, you also have to consider what startup torque the motor will end up with at your gearing.

If it is possible, running thru the bike's regular drivetrain will give you the best of both worlds, since you can shift down into granny gears if necessary for hills and startups, and back up into racing gears for speed, just like when you pedal. Some bike's geometries or component layouts make this difficult, depending on teh motor you have.


As for the topic title, there is an interesting thread that is a survey of what people are actually getting for watts at various speeds:
http://endless-sphere.com/forums/viewtopic.php?f=5&t=14786&start=0
which may help you figure out what you want, too (for those that gave build links and stuff).

ok. well i am using a typical dual suspension mountain bike with 5 gears on the back and 3 on the pedals. i am using a non hub motor to turn the largest gear on the 3 ring on the pedals. i will be leaving the main bike chain on the middle sprocket(on the 3 ring pedal sprockets). if it works good and easily reaches 30 mph, i will probably have the motor turn the smaller sprocket on the 3 rings to get more speed, but less torque. i no the formulas too to find out everything. also i am planning on not pedaling at all, the crank arms will be cut off. they have to be cut off because they would hit the motor. oh just a question to...anyone know a good website that sells good speed controllers?? i have this one:
http://www.monsterscooterparts.com/36vocove1.html
my motor can handle 34 amps continuous at 36 volts. i have been looking for a controller that is not really expensive but can handle this amount.

thanks!

 

[gogo]

Here's a link to the power calculator that I use:
http://www.mne.psu.edu/lamancusa/ProdDiss/Bicycle/bikecalc1.htm

 

[amberwolf]

oh just a question to...anyone know a good website that sells good speed controllers?? i have this one:
http://www.monsterscooterparts.com/36vocove1.html
my motor can handle 34 amps continuous at 36 volts. i have been looking for a controller that is not really expensive but can handle this amount.

It doesn't say there (or in your posts) whether that is a brushed or brushless type. So it will depend on what kind of motor it is as to what controller to recommend.

If it's brushed, I suspect most of the ones of that type (scooter parts places) will work fine, although my experiences with them have been only after they were already broken and I got them for parts.

If it's brushless, I'd go either for one of the ones from e-bikekit that Yopappamon is selling in the online market section, or one Lyen is selling. Both should be reliable enough, and there are threads on modding them and/or repairing them if you need to.

In terms of "good", regarding speed controllers, that's a very relative term. There is good, and there is Chinese good, which means that usually most of the parts are present and installed in the correct places and orientation, and even soldered in. :lol: Usually they even work the first time you turn them on.

Most of the time you're going to have to choose between cheap and good (good to me meaning reliable, meaning it will always work if I do not exceed it's specifications and usage conditions; excellent meaning I could probably even exceed those and it'd still always work).

For "good" brushed speed controllers, I'd go with something like http://4qd.co.uk and the 2QD or 4QD, for less expensive ones (but still significantly more than those scooter parts places), and their other series for more features if you need them (including some programmable versions).

 

[icecube57]

After a certain speed the numbers just multiply like crazy. 30mph is manageable on most bikes. I used to do about 24wh mile while going 30mph at 48v. Or 2 miles per AH at 48v going WOT on level ground. For me personally once you get past 35mph it almost doubles. 40mph it doubles again. For me personally to maintain 45mph in the flat takes 80-90wh or 1.25AH a mile WOT at 66v.

 

[jdcburg]

You can get a wider spindle from Cyclone or Sick Bike Parts. 158mm or 206mm. If you use the cranks you'll want to use a freewheel so you can rest your feet on the pedals while the motor is running.

You will want to get the final rpms down to 150 or so at the chainring if you want to be around 30 mph, depending on the chainring and rear cassette ratios. You can use the analyzer at http://www.arachnoid.com/bike/ to determine the final rpms for various speeds for different tire sizes and gear ratios - jd

 

[mnplus1556]

thank you everyone for the help.

this forum is awesome! haha

 

[The Mighty Volt]

thank you everyone for the help.

this forum is awesome! haha

Yes it is. Yes it is.

 

[geetarboy]

After a certain speed the numbers just multiply like crazy. 30mph is manageable on most bikes. I used to do about 24wh mile while going 30mph at 48v. Or 2 miles per AH at 48v going WOT on level ground. For me personally once you get past 35mph it almost doubles.

You got that right Icecube, I try to stay under 40mph. I average 43 watthours per mile in stop & go urban traffic where the posted limit is 35mph.

 

[mnplus1556]

i'm sorry i'm kinda a newbie. can someone please explain watt hours to me?? thanks.

 

[geetarboy]

i'm sorry i'm kinda a newbie. can someone please explain watt hours to me?? thanks.

amps x volts = watts. Watthours is an amount of energy used, the way your power company figures your electric bill. A kilowatt hour is a 1000 watt draw for a period of 1 hour. Hope that makes sense

 

[ajtest]

Interested to see this as I've just been out for the first time on my Ross Speed recumbent which now has an x5305 at 20S and was amazed to see only 200W needed to maintain 30mph on the flat.

It just goes to show that it's all about the wind resistance - either that or there's something wrong with my CA!

OK nothing wrong with the CA, just the shunt value I had in it. When I changed it to something more appropriate for Methods' 100V 100A monster it turns out that cruising at 30mph takes more like 700W.

Andrew