Any body got any thoughts on using a motorbike alternator? I know id have to weld a pulley on the shaft but are there any other reasons why it wouldnt work? Thanks.

Motorbike alternators typically work different to car ones - they generally have permanent magnet excitation (car ones have controlled excitation via sliprings to control output voltage) So you will need the corresponding bike rectifier/control module as well - I geuss these have a buck converter in to control output volts.
I assume this means you won't get much out of a bike alternator till it's spinning a bit fast (where a car alternator gives plenty out even at tickover) Pretty much all the above is speculation and prejudice - I look forward to being put right!
Bob C

I think if you're looking to go smaller/lighter a Daihatsu one is the easiest.

Do a search in the archives, I believe a couple of the folks in here use them.

Ian

Thanks, didnt totally understand that, but it sounds like too much trouble. I assumed that they'd work the same if i just put a suitable pulley on so as to get higher rpm. Ill stick to a small car one for now. Thanks, Andrew.

bike ones usually give a peak voltage at certain rpm, and i dont think it is very high.
my hoda vtwin manual says 13.5v - 15.5v at 5000 rpm, (engine redlines at 9000/ dies at 11000), so i would imagine using a pulley system you could work it out and get the speed you need, doesnt matter if it goes too fast cos thats what the rectifier is there for. as long as you dont overload it too much, but you can always mount it on some ally to dissippate heat.



Russ.

All you need to do is find out what loads you have on your car.

Going back to my college days.... and the great explinations of Mr Welsh.

Voltage is the speed that the gremlins run down the wire. Amps is the amount of gremlins that are running.

So in order to get the gremlins to jump across the gap they must be running quickly. This is why the voltage of the high tension side of the ignition circuit is 15,000+ volts.

Amps on the other hand are the amount of gremlins that there are. The biggest drain of amps in your car is the starter motor. This takes upto around 300 amps when cold cranking a decent sized engine. So in order to cramb that many gremlins into the wire, it needs to be a thick one..... This is why starter cables are so heavy.

Endeth lesson..... back to your alternator problem and the reason for my babbling....

All of the loads of the car..... ignition system when running, lights, wipers if fitted, indicators etc etc will need to be measured with an ameter. Once you have measured how much drain there is on the battery, you need to find an alternator that (with some margin of safety) will supply enough amps to charge the battery even when all the loads are on.

That is all.

You dont need a great big alternator and if you have minimum loads you will probably be able to get away with a bike on, providing of course that it is the right voltage.

A pulley arangement to get the alternator to the right speed is obviously esential.

i dont think ive ever seen a belt driven bike altenator.
they are usualy(simplified) a magnet bolted to the end of the crank and some wire coils fixed to a cover that fits over the magnet.

my appreciation of bike alternators is also that they are skeletal, and an integrated part of the engine.....

atb

steve