Adding an alternator

[Voodoochikin04]

i need some help figuring out what size pulley i will need to run an alternator off my rear axle...  for some reason i just cant get my brain to work.. please help..  what info do i need to produce?

gear ratios for my engine.

primary--1.926 (79/41)
first------2.785 (39/14)
second---2.000 (32/16)
thrid----1.600 (32/20)
fourth---1.363 (30/22)
fifth----1.208 (29/24)
sixth----1.086 (25/23)

13tooth front  60rear sprockets.

[fabr]

All you need to calc is axle rpm. It's gonna be a pretty slow average. You will need to gear UP the alternator to get to the rpm speced for the alt you plan on using to get full alternator output. I would use a slow cruising axle speed for my calcs if i were you.

[Voodoochikin04]

we usually cruise down the roads at 60 to 80mph....


i have no clue how to do this..    my math got me at 1035 axle rpms at 10,000 tach rpms.    divided tach rpms by the primary, got 5192rpms before the tranny..  in 6th gear thats 4781rpms at the front sprocket, reduced to my rear axle gives me 1035 rear axle rpms..  average alt. puts out max power at 4000 rpms.  so at 10,000rpms my rear axle is at 1035... i need a 4:1 pulley ratio.....   so lets say 9,00rpms since majority of my driving is at or above 9grand.   = 932 axle rpms..   

I would need an 12" pulley (assuming the stock pulley is 3" in diameter for 4:1) that means at 9,000rpms the alternator would be spinning at 3728rpms...  is this ok?

[fabr]

Give me your tire diameter and I'll try to get to it tonite.

[fabr]

At 60 mph you are traveling 1 mile a minute. That means your tire travels 5280 feet times 12"=63360 inches. DIvide that by your tire circumference. I think you said 27" tire so it is approx 85 inches in circumference. Divide the 63360 by 85 and you find you have an axle rpm of 745. IMO,I feel you should actually use 30 mph as the highest average speed you will see and that is what counts. That would make you only approx 375. Also,just an opinion, I think you need to turn the alternator quite a bit faster than that to get you full output.Most car alts run at about a 2:1 overdrive of the engine  most alts reach full output capability at about 2500 engine rpms or 5000 alt rpms. Sooooooooo,IMO you'd need to have approx 13:1 pulley ratios. Even with a very small 3"alt pulley you'd need a 37" axle pulley. Need to check out where the small alternators make their full output. I may need to revise the 13:1 thing if they make full output at substantially lower rpms but the reality is that even if I'm off by 50% the axle pully will still need to be HUGE! Now if you only need a few more amps we can also look at lower ratios .

 I ran thru that really quick without much thought. Some one correct me if I missed something.

[Voodoochikin04]

what does tire diameter got to do with it.. no matter if my tire diameter was 10" or 30"   my engine rpms, reduced through tranny and final ratio will give the same axle rpms  at a certain tach rpms, no matter what the tire size is.. only difference is the speed of the buggy then...

but my tires are 22"


reading your last post..  why is it based on how fast your going, as opposed to the engine rpm reduced to axle rpm?  it should matter what size tire you got and how fast your going.. as long as you base it off of your average engine rpms...  if my average engine rpms are 6000.... then i should work it from there.. tire size should'nt matter huh?

[fabr]

Tire dia has EVERYTHING to do with it. If you're using the axle to drive the alt the tire diameter determines the axle rpm for any given road speed.

[fabr]

what does tire diameter got to do with it.. no matter if my tire diameter was 10" or 30"   my engine rpms, reduced through tranny and final ratio will give the same axle rpms  at a certain tach rpms, no matter what the tire size is.. only difference is the speed of the buggy then...

but my tires are 22"


reading your last post..  why is it based on how fast your going, as opposed to the engine rpm reduced to axle rpm?  it should matter what size tire you got and how fast your going.. as long as you base it off of your average engine rpms...  if my average engine rpms are 6000.... then i should work it from there.. tire size should'nt matter huh?

Sorry but you're not thinking this through right. Gotta get to shop now. Think about it,maybe someone else will chime in before I get back.

[Voodoochikin04]

no i totally understand that tire size determines the axle speed at any given road speed...  but whats the difference in taking your average cruising rpms.. and doing it that way.. not even using what your mph are.. you dont need to know your mph to figure your axle speed... you can use the engine rpms... thus matching your engine rpms to the rpms needed for the alternator, shouldnt matter what speed your going..  if you got it matched to your engine rpms, then no matter what tire size your using, or swapping around... your alternator will still be matched to the enging.. instead of a certain size of tire..    set the alt for full output at 10,000engine rpms, and it doesnt matter if you have a 20" tire or a 27" tire, because at 10,000engine rpms your alt will be putting out full power...

put the buggy up on jack stands and eliminate road speed..  mph shouldnt matter one bit.. as long as you set your alternator to match along with your rpms... to produce max output around your average engine rpms..    (not arguing or fighting..just understanding)

[fabr]

You need to step away for a bit. Forget what you're thinking and start over. Just effing with you but I have to go for real. Back L8tr

[Nutz4sand]

shouldnt matter what speed your going.. 

If you are driving it off the rear axle. The speed you are going makes all the differance.

[Voodoochikin04]

well yes.. but since i dont really give a hoot what mph speed im going when im crusing at 10,000rpms... i always pay attention to the tach.. i dont have a spedometer..   so as long as i have my alt pumping out maximum power within my average driving "tach rpms" my tire size makes no difference......  because no matter what size of tires one uses, as long as the alt is working in the range of average rpms according to the tach.. then all is good....

most of my crusing is from 9000 to 12500rpms.. down the long long roads and trails i drive on...  so as long as my alt i want to add is pumping out full power within that range... then GOOD.... and since we can easily figure what my axle rpms are from my engine rpms... why even bother with mph speeds and tire sizes?


nutz- i meant it shouldnt matter what speed your going, because if your geared to do 30mph at 10,000rpms... or 60mph at 10,000 rpms..  if your alternator is pumping out full power at that rpm range then tire size and speed DONT matter....

[Nutz4sand]

If you are going to worry about the RPMS of the motor then you need to find a way to drive alternator off of it before the tranny and the chain and all.

Driving it off the axle does worry about the speed you are going and does not give a hoot about engine rpms. Each gear will change the speed of the rear axle and thus the buggy and the alternator and even if you do drive mostly in one gear most of the time at a certain rpm you can optimize it to spin the alternator for max output at that speed but it still matters.

The alternators have a decently broad range they will perform in RPM wise and it does not take an alternator to be cranked to the max rpm for it to make good power. It would be eaiser for you to simply use a tad bigger alternator than to try to max out a tiny one. It will last longer and less drag too most of the time is a good bet. 

IF... if you are gonna drive it off the rear axle look at the car it came off of. What size crank pulley did it have? Use that to figure how many rpms the car would spin the alternator at common car crankshaft rpms.

Then figure out how many RPMS your rear axle is spinning at the speed you drive the most. With those two numbers in hand you will get an idea of how much you will need to have for a lower pulley diameter to get it close.

It does not need to be smoking with rpms to make plenty of power. Good rpm's yes// But not turbine speeds.

[Voodoochikin04]

lol turbine speeds.   well all i was saying is that 90% of my driving is in 6th gear going fast..   atleast this whole last year was..    but ok lets say.  1980 chevy truck alternator..the alt pulley is 2.5" diameter, and the crank pulley is 7.75".....

considering alot of people are running axle driven alternators.. i find it hard to believe i would need a 30+ inch pulley.. lol

[Nutz4sand]

I do not think you would need nor want such a large pulley either.

Even if you said "Fudge it" and just put a pulley on it that did not go as tall as the sprocket and just ran a belt it would likely make all the juice you need unless you add like five more light sets. ESPECIALLY if you still got the bike system adding juice to the fray.

Iffin you was wanting to you could put an empty rim on the wheel studs and drive a belt off it with a quickie bracket holding the alternator just to see what it puts out. Likely the rim center is smaller than the sprocket or close.

Jack the back end in the air (safely and securely) and spin it in the gear you drive in and the rpms you hit and put a load to the alternator and whatch a voltage guage and an amp meter to even.

Of course with your rear spool be real easy to just get the hub and pulley on there and test it there too. 3/4 the size of your sprocket would likely do you plenty of good.