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Old 03-29-14, 01:31 AM   #1
Joe_Hoffmann
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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.
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Old 03-29-14, 07:41 AM   #2
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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.
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Old 03-29-14, 09:30 AM   #3
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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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Old 03-29-14, 10:24 AM   #4
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Don't they weigh like 30-40 pounds?
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Old 03-29-14, 11:23 AM   #5
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Don't they weigh like 30-40 pounds?
Good! Muscle building project
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Old 03-29-14, 12:30 PM   #6
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Any DC motor with permanent magnets will work.
PM me if you can't find one.
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Old 03-29-14, 12:45 PM   #7
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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 ..
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Old 03-29-14, 01:20 PM   #8
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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?

they would be cheap and have a "hacker" look to it. You know, youve seen guys with car steering wheels on their bikes before.
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Old 03-29-14, 01:22 PM   #9
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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.
thats what I thought. I think that friction goes up as the rpm goes up. Those arent exactly high quality bearings in those things.
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Old 03-29-14, 01:23 PM   #10
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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.
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Old 03-29-14, 01:59 PM   #11
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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.

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Old 03-29-14, 01:59 PM   #12
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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.
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Old 03-29-14, 02:52 PM   #13
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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?

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Old 03-29-14, 02:58 PM   #14
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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.
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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Old 03-29-14, 04:58 PM   #15
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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

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Old 03-29-14, 04:59 PM   #16
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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.
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Old 03-29-14, 05:16 PM   #17
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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.
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Old 03-30-14, 03:19 PM   #18
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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.
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Old 04-02-14, 07:27 PM   #19
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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.
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Old 04-02-14, 07:51 PM   #20
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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.
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.
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Old 04-02-14, 09:19 PM   #21
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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.
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Old 04-03-14, 11:11 AM   #22
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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.
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.
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Old 04-03-14, 01:33 PM   #23
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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?
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.

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Old 04-04-14, 02:22 PM   #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.
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Old 04-04-14, 05:32 PM   #25
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What advantage do you see in using an automobile alternator?
Faster downhill rides with the extra weight?
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