Looking to make simple ebike, need advice.

Discussion in 'Electric Bicycles' started by Gruelius, Jun 13, 2008.

  1. Gruelius

    Gruelius New Member

    Hey all,
    Im looking to make a simple ebike out of:
    2nd hand starter motor
    spare car 12v battery

    Not going to be too complex. Im thinking of welding a rail or rails onto the frame of the bike to allow me to adjust the position and chain tension of the starter motor setup.

    Im thinking of chaining it to the pedals with a seperate cog. That idea would allow me to have use of the full 21 gears but i wouldnt be able to have my feet on the pedals, id prob just weld some foot rests somewhere infront or behind. I want to know tho if anyone has tried that, im a bit worried about stability.

    Also do startermotors have continous power output or it is in large surges? i only ask this because of the noise they make when i start my car :p

    Would i be better off just having a single gear on the rear wheel? Id like to be able to spin the wheel in the wet on full charge but gear it to pull well up hills ( i have some nasty long hills in my suburb lol).

    I understand the safety issues with a car battery but im cool with that :p

    Also for the starter to the front or rear wheel where are the best placements?

    I was thinking if to the front id put it in the centre of the bike and if to the rear id sit it infront of the rear brakes with a few brackets.

    And if i had it above the rear brakes, would i want to go for friction drive or chain drive? I hear that the chains break often.

    thanks for putting up with my lack of understanding! :p

  2. Gruelius

    Gruelius New Member

    Im posting this mainly due to my boredom but i want to see if i have the concepts nailed. I cant find any motors on the internet that give out power and toque stats so lets asume we are using some super dooper electrical motor with max power output of .5kw and a max torque of say 5 n/m (starter motor :D)

    Ive been reading ahead in my first year subjects of UNI so if a engineer could correct me that would be great!

    This all asumes the drivetrains are 100% efficient which they are not, because of heat + sound generated.

    Bike Acceleration

    MotorTorque = ChainTension / RadiusMcog

    ChainTension = MotorTorque * RadiusMcog

    WheelCogTorque = ChainTension / RadiusWcog

    ForceWheelSurface = WheelCogTorque*WheelRadius

    Acceleration of system = [ForceWheel - Friction Forces (road((Air resistance is usually a function of velocity so it wont be included for this simple case)] / Mass (Bike + rider)

    and making it in terms of Radius's

    Acceleration of system = [MotorTorque * RadiusMotorCog * RadiusWheel / RadiusWheelCog - Friction]/Mass

    So you can see that increasing the cog size on the motor and decreasing the cog size on the wheel will affect acceleration (asuming im correct). I think i screwed up because i didnt include a inertial mass thing.

    Top speed due to gearing.
    Im really unsure about this one but here goes. This assume no power is lost in the drivetrain (false because of heat + noise generated)

    Power = Torque * Rotational Speed
    Power = TorqueCog * Rotcog
    Power = TorqueCog * RotWheel
    Speedwheel = Rotwheel*RadiusWheel
    Power = TorqueCog * Speed wheel / RadiusWheel
    Speed Wheel = Power * RadiusWheel/Torque Cog

    This would gives the theoretical speed of the wheel at its rubber surface and of the bike if there were no friction forces.

    I dont know how to calculate top speed with friction in it!

    Anyway i hope someone is bored enough to read and fix all that haha!
    Last edited: Jun 13, 2008
  3. darwin

    darwin Well-Known Member

    Gruelius youll want to use a marine battery thats designed to be discharged instead of a car battery which isnt.
  4. And normally a starter needs a relay to get that torque to the flywheel and if you run a starter motor continuously you'll over heat it.
    So maybe without that relay to give it more juice just the motor directly connected to the battery would spin it enough for your needs.
    Then there's that flywheel gear that engages and disengages via solenoid. You may need to do away with all that entirely.
    Overall without the extra energy you'll need with the relay and solenoid,you may get a good 5 mile range with a deep cycle battery!
    Or if you gear it really high to compensate for the sheer torque of that starter maybe you can go 10.
  5. Gruelius

    Gruelius New Member

    Sorry for the lack of updates just been busy.

    Im after about 20km range as i dont think my uni is going to be helpful with letting me charge my bike there, they would much prefer i continue driving and paying the permit so i will have to make the whole distance from one charge.

    I was just using a starter motor as an example. If there is a cheap and more practical option ill happily use that.

    Ill be attatching it by a chain to the rear wheel. Im not really after extremely fast speeds but 40km/h would be perfect.
    Last edited: Jul 18, 2008
  6. ozzyu812

    ozzyu812 Member

    What's your budget like?
  7. Peter

    Peter New Member

    What were you planning to use as a donor for the starter motor ? What sort of speed control setup ?

    I would avoid using a starter personally. Keep in mind that starters are not designed for continuous operation. Also, being series wound they will continue to accelerate until they self destruct if operated unloaded. So you need to match the torque and RPM characteristics of your motor of choice to your requirements.

    For a simple, cheap (relativley), reasonably capable setup I think you should have a serious look at a brushed, geared motor

    A dire warning though, once you start playing with electric assist, the high torque from low speed and near silent drive (if you go with a brushless, hub motor. Details) is highly addictive. And you will have to beat your friends off with a stick when they won't stop hassling you for a ride :D

    Your other big decision is batteries. Capacity and chemistry. As a starting point and being conservative, budget around 15 Wh / Km with a decent reserve. Just like a car, increase your speed after a certain point (around 30 kph) and expect to pay an exponential energy cost. If you use lead acid batteries plan on only getting around 60% of the labeled capacity. Painfully heavy too. Other alternatives; NiCd, NiMh, Lithium Iron Phosphate (LiFePO4). Check here for a more detailed primer on batteries.

    Avoid lithium cobalt and Lithium manganeese, commonly (and somewhat incorrectly) referred to as LiPo or Lithium Polymer. As used in RC planes and the like. They have a nasty habit of catching fire, violently.

    This may go right off the deep end, but have a skim read if you have the time. Primer

    Hope this is enough to give your design a kick start.
  8. s_beaudry

    s_beaudry Member

    From my experiences of working on and building cars....

    A starter motor (presuming automobile) is designed to run for a VERY short period of time, maybe 3 to 6 seconds with an incredible amount of amp draw to supply the torque to rotate the flywheel and start the car, then it remains dormant until needed again.

    If you were able to weld in all the needed support and hardware to hold the battery and starter, you would not have much running time with this setup.

    Starters have a good bit of weight to them as well, hooking this thing up to a wheel would massively unbalance a bicycle, keep this in mind!

    Also, from the incredible amount of amps it draws to spin, the starter and the wiring will get incredibly hot!

    Think smaller scale for your own well being......