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  1. #1
    Member Joe_Hoffmann's Avatar
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    car alternator on a bike?

    I know this sounds incredibly stupid, but do you think you can use a smaller car alternator for bicycle lighting? (lets just say for the cool factor...for that chopped up cruiser bike you've been building..you know...short trips)
    Spinning an average alternator by hand doesn't seem to produce much drag to me, but this is when its completely disconnected. Maybe you could run one much like a bottle dynamo.
    I suppose you could vary the field coil voltage to get more power on command or less drag, depending on the time, maybe by turning a knob on your bicycle, or to have dim/bright lights. Maybe even a large 6 amp headlight!


    Also also...what if you used some super-capacitors and an average 6 volt bottle/hub dynamo to run high current device for a short period of time on your bicycle?
    ... such as a horn. more specifically, a vintage 6 volt "ahooga" horn.
    slowly charge the capacitor, watch voltage and amps go up on cool vintage guages on your bicycle, and honk the horn to scare people every few minutes!

    anyway...late night ideas.

  2. #2
    Senior Member Looigi's Avatar
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    As a novelty, it might work or be a fun project. I think you'll need a battery to power the field coil. It can power itself once spinning fast enough but every time you start out it will need the battery (or charged capacitors) to provide the field current. I don't know the details, but field currents can be 1/10 the rated output, so quite high if it's a sizable alternator. Providing the field current might take a fair amount of pedaling power.

    Also, in auto applications they spin pretty fast. Spinning them that fast might take some gearing and a lot of effort.
    Ride more. Fret less.

  3. #3
    Old fart JohnDThompson's Avatar
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    Sturmey-Archer's "Dynohub" hub-mounted alternator came on the market in the mid-1940s, Pretty sure the modern offerings from Shimano, etc. are also alternators. What advantage do you see in using an automobile alternator?


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    Don't they weigh like 30-40 pounds?

  5. #5
    "LOGIC!" lopek77's Avatar
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    Quote Originally Posted by njkayaker View Post
    Don't they weigh like 30-40 pounds?
    Good! Muscle building project
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    Dirt Bomb sknhgy's Avatar
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    Any DC motor with permanent magnets will work.
    PM me if you can't find one.
    more cops have been killed by donuts than guns in chicago it is a medical fact ask any doctor.

  7. #7
    coprolite fietsbob's Avatar
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    maybe a stationary bike so you can keep the lights on , like Edward G Robinson in 'Soylent Green'


    Mmm Tuesday is Soylent Green day ..

  8. #8
    Member Joe_Hoffmann's Avatar
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    Quote Originally Posted by JohnDThompson View Post
    Sturmey-Archer's "Dynohub" hub-mounted alternator came on the market in the mid-1940s, Pretty sure the modern offerings from Shimano, etc. are also alternators. What advantage do you see in using an automobile alternator?

    they would be cheap and have a "hacker" look to it. You know, youve seen guys with car steering wheels on their bikes before.

  9. #9
    Member Joe_Hoffmann's Avatar
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    Quote Originally Posted by Looigi View Post
    As a novelty, it might work or be a fun project. I think you'll need a battery to power the field coil. It can power itself once spinning fast enough but every time you start out it will need the battery (or charged capacitors) to provide the field current. I don't know the details, but field currents can be 1/10 the rated output, so quite high if it's a sizable alternator. Providing the field current might take a fair amount of pedaling power.

    Also, in auto applications they spin pretty fast. Spinning them that fast might take some gearing and a lot of effort.
    thats what I thought. I think that friction goes up as the rpm goes up. Those arent exactly high quality bearings in those things.

  10. #10
    Member Joe_Hoffmann's Avatar
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    i find it funny how most consumer food is made of soy nowadays. get it.....soy soy lent green

    school food was always compressed soy bricks with plant filler and "meat" flavoring.

  11. #11
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    Quote Originally Posted by Joe_Hoffmann View Post
    i find it funny how most consumer food is made of soy nowadays. get it.....soy soy lent green.
    "Nowadays"??

    That's why it was called "soylent green" in the first place. The whole "it's people" (rather than made from soy/plants) thing being a "surprise" was the point.
    Last edited by njkayaker; 03-29-14 at 02:05 PM.

  12. #12
    BF Avatar Zombie Hunter Jseis's Avatar
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    Interesting idea though overkill. I mean... Unless you are running 200-300 watts+ of halogen lights (+ AC!) or have an inverter running a blender out back... Probably better wiring wise using a motorcycle AC alternator. If you want lights at a dead stop...that means a battery, voltage controller ='s weight. Find an old street legal moped & take off its lighting "system". Just looking at all the parts might tell you something.
    Amerika, Land of the Very Brief.

  13. #13
    Old fart JohnDThompson's Avatar
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    Quote Originally Posted by njkayaker View Post
    Don't they weigh like 30-40 pounds?
    I never weighed mine; probably 2-3 pounds, I'd guess. I'm not racing when I ride that bike, so I don't really care about the weight. But how much is an automobile alternator and cobbled-up housing, reduction gearing, etc. going to weigh?
    Last edited by JohnDThompson; 03-29-14 at 03:00 PM.

  14. #14
    Old fart JohnDThompson's Avatar
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    Quote Originally Posted by Jseis View Post
    Interesting idea though overkill. I mean... Unless you are running 200-300 watts+ of halogen lights (+ AC!) or have an inverter running a blender out back... Probably better wiring wise using a motorcycle AC alternator. If you want lights at a dead stop...that means a battery, voltage controller ='s weight. Find an old street legal moped & take off its lighting "system". Just looking at all the parts might tell you something.
    The advantage to Sturmey-Archer's Dynohub is that it is designed to work at low rpm. Why re-invent the wheel? Perhaps this thread might be... errr... "illuminating?"

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  15. #15
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    Quote Originally Posted by JohnDThompson View Post
    I never weighed mine; probably 2-3 pounds, I'd guess. I'm not racing when I ride that bike, so I don't really care about the weight. But how much is an automobile alternator and cobbled-up housing, reduction gearing, etc. going to weigh?
    I was talking about an automobile alternator (like the OP was).

    Apparently, an automobile alternator weighs around 10 (or more) pounds or so.

    How much does an alternator weigh
    Last edited by njkayaker; 03-29-14 at 05:01 PM.

  16. #16
    BF Avatar Zombie Hunter Jseis's Avatar
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    I use rechargeable lights to avoid wiring hassle (but my cadence meter is wired...ugh) though once LEDs start dominating lighting systems..might be practical to have super light weight & low output charging systems integrated with a central battery that runs headlights, tail lights, brake lights Di type shifters, auto shift, power brakes, brake lights, etc. Bikes have a long way to go in terms of "electronic" tech.
    Amerika, Land of the Very Brief.

  17. #17
    Old fart JohnDThompson's Avatar
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    Quote Originally Posted by njkayaker View Post
    Apparently, an automobile alternator weighs around 10 (or more) pounds or so.
    Which begs the question: "why?"

    There are plenty of bicycle specific alternators, light weight and designed for the lower rpms found in bicycle application.

  18. #18
    Senior Member dougmc's Avatar
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    The only reason I'd want to use an automotive alternator on a bike would be if I was making a stationary bike generator of some sort.

    Other than that, I'd use a generator designed for a bike.

    If you do use an automotive alternator, you'll need to gear it so the alternator spins a whole lot faster than you pedal. I'm not sure how fast an automotive alternator spins in use ... but I suspect it's geared to run faster than the car engine itself, and that's around 3000 rpms at cruising speeds.

  19. #19
    Gammal cyklist Reynolds's Avatar
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    I don't think it would work, because a 12V car alternator produces around 50A @6-7000RPM, that's 600W. At 50% efficiency, you'd need 1200W to spin it. Pro sprinters can output about 1500W, but only for a few seconds. A commuter probably doesn't output more than 100W.

  20. #20
    Senior Member dougmc's Avatar
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    Quote Originally Posted by Reynolds View Post
    I don't think it would work, because a 12V car alternator produces around 50A @6-7000RPM, that's 600W. At 50% efficiency, you'd need 1200W to spin it. Pro sprinters can output about 1500W, but only for a few seconds. A commuter probably doesn't output more than 100W.
    That's the maximum the alternator can produce, but it can produce a lot less too.

    Certainly, the alternator in your car isn't producing 600 watts all the time. 50 watts during the day seems more like the average, maybe 200 watts at night. But more if your battery is close to dead -- but most of the time it's fully charged.

    An alternator probably isn't ideal, but I imagine it could be made to work for a stationary bike generator with the right gearing. But it's too big to be practical for a moving bike generator for the lights and such.

  21. #21
    Gammal cyklist Reynolds's Avatar
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    ^ You're right, what I meant is you never could use the alternator's potential power. Still, you'd need a lot of power to produce 50W.

  22. #22
    aka Tom Reingold noglider's Avatar
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    Quote Originally Posted by Reynolds View Post
    ^ You're right, what I meant is you never could use the alternator's potential power. Still, you'd need a lot of power to produce 50W.
    And since you'd need a lot of extra leg power to lug the car alternator around, it would be a lot less efficient than a bike dynohub.
    Please email me rather than sending me a private message. My address is noglider@pobox.com

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  23. #23
    another retro grouch Mr IGH's Avatar
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    Quote Originally Posted by JohnDThompson View Post
    Sturmey-Archer's "Dynohub" hub-mounted alternator came on the market in the mid-1940s, Pretty sure the modern offerings from Shimano, etc. are also alternators. What advantage do you see in using an automobile alternator?
    That isn't an alternator by today's definition. An alternator uses electro-magnets to generate the magnetic field, that's why OP think the alternator is easy to spin. No field current means no magnetic field to resist the armature turning. The advantage of an alternator is that its output voltage is regulated (by modulating the field current for load and RPM). Modern cars (since the 50's) require a voltage source that works over a wide output load (HVAC, lights, stereo, etc all are switched independent of engine RPM), that's why generators went away.

    A generator has permanent magnets, there's always some resistance when turning the armature. The output isn't regulated very well, there are usually some extra windings that become energized at higher RPM to limit output voltage (the old Sanyo bb generator was one of the first to use this technique). All the hub and wheel dynos that have ever been sold are generators, Union, Sanyo, Miller, Subetiz, Shimano, SON, SA, SP, et al.
    Last edited by Mr IGH; 04-03-14 at 02:48 PM.

  24. #24
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    Most automotive alternators run 2:1 or 3:1.

    So your cruising rpm in a car can range from 2,000 to 3,000. Multiple this range by 2 or 3 and that's how fast your alternator is spinning.

  25. #25
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    Quote Originally Posted by JohnDThompson View Post
    What advantage do you see in using an automobile alternator?
    Faster downhill rides with the extra weight?

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