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Electric bike motor help! Answered

Helli guys.. after a long time racing motocross, I would like to experiment into doing an electric dirt bike for myself.. My plan is to find an 85 or 125cc frame with no engine, and build from that.. the biggest question is, what motor could I use that is no less than 15kw, and on the affordable side? I was looking at the Turnigy Rotomax 150cc eqiv. And it has a max power output at 9000 something watts, would that be any good for a motocross bike? Would it burn out quickly since its pretty small? Notes: i am strictly looking for powerful bikes, a 2kw bike is not for me since I am used to overpowered 250cc two stroke hell-born suicide machines, thanks..

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Downunder35m

9 months ago

Before you think like Nicola Tesla you should think like the Tesla brand.
I mean batteries...
For a dirt bike weight and being agile is a big draw card.
If your desired motor and controller need 200 ampere to perform at max load then of course you batteries need to supply this.
This is the first factor.
The second is the desired run time before you need to recharge or change the battery pack for a fresh one.
Together this means weight.
Ideally this weight would be located to perfectly balance the bike.
This limits the size and capacity/weight unless you lower the handling performance.
A higher center of balance and gravity is something you know well enough.
Once you go to high it becomes impossible to swing the bike around fast enough without loosing the balance.

Take you frame and racing experience and check how the added motor would change your ideal system.
As the orignal engine is gone it will be way off.
Now try to add the imagined weight and size in terms of pysical replacements for motor and battery.
Once you know where and how much batteries you support without causing an imbalanced system you know what sized motor you can actually use.

If you are really serious about going green than consider starting from scratch.
Take some wheels and fake electric motor for the weight and real feel with the corresponding gear system.
Add suspension components as required in the same imagined way or with replacements.
Now forget what you know about good frames and start drawing some battery boxes that hold it all in the right place.
Using commercial battery packs as the comparison and actual source means you can use the available dimensions and weights for them as templates for your mock ups.
Draw the "frame" around the packs and components, try to make it "fluid" and round instead of having too many straight and sharp angles.
Keep the batteries low and placed so they won't affect the handling in a negative way.
If you can 3D print then start now to create some 3D models to play with.
Assuming all works out start building the real thing and if it satisfied you when riding it hard you might have a business opportunity waiting for you ;)

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dsiroticDownunder35m

Reply 9 months ago

Thanks for the elaborate reply.
BUT, i'm used to weight differences and center of balance since i've driven 450cc four stroke bikes also for a pretty long time, which are less powerful that a 250cc 2 stroke and also has a much higher center of gravity, but its just a thing to get used to. After three or four sessions I already have a good feeling of the bike, and I basically learn around the bike.
Also, the frame i got is basically a KTM SX 85 with no engine, and with the engine inside the whole bike weighs 66kg dry.
My plan is to have most of the weight well distributed on the bike, but no frame building from scratch..
Riding times are not a problem, I need one batt pack to last at least two hours, and thats pretty doable with li-ions i think?
Also one more, i still don't know if I could trust that Turnigy Rotomax 150 to power something like this? Maybe run two rotomax connected with a belt and to an output shaft, would that be too much stress on the motors?

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Downunder35mdsirotic

Reply 9 months ago

As said, start with the batteries.
Check the datasheets for your motor(s) and calculate what you need in Ah from the batteries.
Two hours of hard run time might be really on the upper limit then.
30 to 60 minutes more realistic.

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dsiroticDownunder35m

Reply 9 months ago

What should I do to get at least 90 mins of run time? More batteries? Also, what do you think about the twin rotomax 150cc setup? Could they supply enough power? Obviously i'd put support bearing on the end and some cooling features

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Downunder35mdsirotic

Reply 9 months ago

Lets just go for one rotomax:
51. something volt from a 14S setup, lets just say 50 to keep it easy.
190 amps at max output.
That makes 9500 Watt.
I just ignore little things like required gearing and controller here.
For a full 90 minutes of usable run time I guesstimate an average consumption of 100 instead of 190A - after all you won't be on full throttle for 90 mins dead ;)
And it makes the math much easier too ;)
The capacity in Ah for a battery and if taken dead simple means:
A 50Ah battery can provide a max of 50 amps for one hour before it is depleted.
Or 25 amps for two hours and so on.
You would need a 14S lipo with at least 150Ah to get about 90 min out of it.
Going only full throttle and you need close to 300Ah !

Lipo systems can provide the amps you need if the right combination and amount of cells is used - but be aware of the insane amps you are dealing with.
Wouldn't be a problem to do some TIG welding with that power!
Wiring needs to be up for it, same for all connectors.
A tiny problem in this area and you can end up with a little fire on board.
That is why I suggested to go for commercial replacement battery packs like used for scooters, E-bikes and so on.
They will come with the required protection and safety stuff to prevent the worst things.

What kind of power, from just enough to what the motor can handle, you get only depends on the controller, batteries and wiring.
How fast you can go with this depends on the gear system you use.
At 150kV and the 14S voltage of around 50V the motor delivers 7500rpm - at no load.
The higher the load, the lower the RPM.
How much the motor can deliver in torque depends on how much it will get in amps and the winding of the motor.
If you consider that 7500rpm are well within what a good dirt bike provides on petrol you see that the motor is more designed for light weights and demands.
Like a big sized plane model of 20 odd kg or a small scooter.
The ratios between motor, gear system and wheels must match what you need in torque to the wheel and desired top speed.
Using two motors certainly doubles the output torque but also the battery power required.

Last but not least you need to address the back-current problem and fact that you have no engine break.
If the wheels turn the motor then it acts like an alternator - the controller needs to be able to handle this or it will go up in smoke.
Best of course would be one capable of feeding this energy back into the batteries.
Without such a feature or in the advanced version with an active system to use this waste energy your motor won't provide any resistance when going off the throttle.
Would be like you pull the clutch instead and is something you will learn to deal with while riding.

You see, there was a reason after all I suggested to design a frame that acts as a battery compartment ;)
To get real fun out of it you bike might be more a battery with wheels than a bike frame with batteries.

As an alternative have a look what the modern E-bikes can offer and use.
Most are limited one way or the other for the top speed but people out there already have workarounds to some extent.
The main difference to your approch however is the direct drive system.
More direct means less losses and more control plus less hassles in terms of complexity and service.
Of course something for a bike axle won't fit on a motorbike axle or work with the thicker sprockets...
But it might give you some ideas how to use something like a brushless motor more directly than by using chains or belts.

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dsiroticDownunder35m

Reply 9 months ago

Thanks again for the awesome explanation. Anyways i've done some extensive reseatch on it and I will probably ditch the rotomax(since its also out of stock for a while now) and go with the alienpower motors, also twins. They are either 90kv or 150kv, both 7kw peak. I was thinking making a CNC plate that holds them both, with two big pulleys and a much smaller one for the output shaft, and make some kind of freewheel on it so when the tire is spinning the motors dont. I'm used to have no engine brake since two stroke bikes have very little if none of it, so that wont be a problem. Alsi if i go with two ESC's that have regenerative braking, i might be able to squeeze a bit more runtime out of it? I've come to the conclusion that building a battery pack from panasonic cells would cost about $100ish each, so i might settle at building a smaller capacity battery, one that lasts for about 30 mins, but making three of them, with an easy-ish system to swap them at the track. Also, i think the gearing on the pulleys would be eough since its then also geared from the output shaft to the wheel with a chain system. The alien motors are also really popylar in high power e-bikes builds, so it should do the trick













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Downunder35mdsirotic

Reply 9 months ago

In that case go for the 90kV motors.
Less rpm per volt means higher torque.

Even considered the head on approach?
Make a coupler for the two motor shafts that holds your gear or sprocket.
As you need some sort of gearing anyway it should be easier to match it on a double motor than for two indepently mounted ones.

Regnerative breaking is great if you have a mix of up and downhill, but far less effictive for normal riding conditions on a track unless you really do a lot of hard breaking as well.
In comparison and with a trip that includes some downhill the savings can be as high as 30%.
All depends though as in most cases you will going flat out anyways and only break if the is no other option LOL

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dsiroticdsirotic

Reply 9 months ago

Also, the alien motors are almost half the price of the rotomax, and tested in the e-bike world