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UTV's Off Road ( RZR, YXZ, Mini Buggy, Carts,etc.) => UTV Controls and Electrical => Topic started by: Voodoochikin04 on November 02, 2010, 12:51:52 PM
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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.
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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.
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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????
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Give me your tire diameter and I'll try to get to it tonite.
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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.
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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?
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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.
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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.
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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)
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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
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shouldnt matter what speed your going..
If you are driving it off the rear axle. The speed you are going makes all the differance.
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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....
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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.
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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
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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.
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Is there a place near you that does rewinding?
You might be able to get your bike alternator rewound for more amps, and fit an aftermarket diode pack to handle the increased current.
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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
You don't seem to understand that the axle rpms are the only rpm that matters except for the alt rpm needed for whatever output minimum you need. . I don't care if the engine is running at 1000000000 rpm ,if the axle is only turning 375 axle rpm or so you will HAVE to gear that up to somewhere around 2000 alt rpm to get full output. First thing you need to do is choose an alternator you want to use and check it's output at various RPM's OF THE ALTERNATOR ITSELF. Until you do that this is just useless talk.
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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
true but the thing idles at approx 800 rpm. Your axle at 60 is in that range. I don't know about you but I can't average 60 mph offroad unless it's all dirt roads. You'll be LUCKY AS HELL if you average 20 mph truly offroad. You only run the roads?
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the only reason i was talking about engine rpms... is so i could calculate my axle rpms while im driving.. thats all...
most of my driving is gravel and dirt roads..
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I just see it as easier to go from the wheel. Fewer calculations
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the only reason i was talking about engine rpms... is so i could calculate my axle rpms while im driving..
One tiny mistake in the ratios and your final figure won't be right.
If you are driving it off the axle, then you start where the rubber meets the road. It's that simple.
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That's what I've been telling him but it's not sinking in I guess. I'm that way tho at times myself. Can't see the forest for the trees thing ya know. LOL!
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That's what I've been telling him but it's not sinking in I guess.
I know..... I couldn't take it anymore. That's the nicest post I could muster.
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you can do it either fxxking way boost.. the fxxking engine is driving the god damn rear axle... so if i take my normal crusing rpms and go through my primary, and my 6th gear through my rear end.. wha fxxking lah.. theres my damn rear axle rpms.. which i can use to figure what size pulley i need for my alternator... to me that is way easier than figuring how many times my wheel revolves in one mile and such and such mph when i dont have a damn speedo.. so yea its sinking in.. it doesnt seem like how i was doing it was sinking in for you guys.. either way will get you the same damn thing... rear axle rpms.. which is what i needed!!!
so no.. you dont HAVE to start where the rubber meets the road..
just delete this whole thread...i got what i needed from it.
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I'm confused. So how do you determine what size pulley you need?
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take my axle rpms..... and compare to the rpm range for the alternator, and figure what ratio of pulleys i need to run to get the alternator in that range....
but for some reason everyone seems to think its impossible to calulate rear axle rpms from your engine rpms in any given gear... the damn engine drives the rear axle... you can calculate the rear axle rpms using the engien rpms in any gear!!!
and one more time... when im flying down the road in my buggy, at 10,000rpms in 6th gear.....ill know my rear axle is turning 1036 revolutions per minute.... i didnt need to know my mph or tire size to figure that.. thats all ive been saying all this whole thread.. so my alternator with a 2.5" diameter pulley, running on a 10" diemeter axle pulley.... will be turning 4144rpms..... again.. i didnt need tire size or mph... so then ill know... "hey my alternator is putting out (such and such) right now, while im driving"
how in the world is that not correct?? thats all i wanted.. i dont need the alt to put out full power while im in 2nd gear putting around the yard.
60mph with a 22" wheel is gonna give me 842rpms at the rear axle... that does me absolutely no good, unless i figure what rpms my engine is running when im at 60mph so i can make sure im running my alternator within my driving habits... which is exactly why i need to know what my engine rpms are......for everyone that says my engine rpms dont matter... they do matter in this situation.
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You can figure it however you want, but if you are driving anything off the axle then figuring axle RPMs from tire dia and mph is the best way. If you were running the alternator off the crank, then engine RPMs would be the best way.
As Fabr said, "it requires less calculations" and will give you the full picture from dead stop to top speed. Axle RPM and MPH directly correspond to each other. Axle RPM and engine RPMs do not. (that should be reason enough right there)
I personally would figure out how fast you can safely spin the alternator and then so do the calcs so that you achieve that RPM at the buggies top speed. I would do that to prevent damage to the alternator. Maybe even give yourself a little head room. I haven't looked it up, but I have a feeling that would give you a decent charging rate from moderate to slow speeds and up. Plus matching the upper limit of components just make good sense, but don't let good sense interfere with doing things the way you want. Sarcasm
Why is it that you started this thread again? eyes
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gg: I need a beer.
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Voodoo ,it seems like you are just interested in charging at your max speed. That is not the way to look at it. You MUST be able to make some decent alt output at much lower speeds. Whether or not you think so you WILL be spending more time near idle and at lower rpms than you will at redline. In ANY gear. Yes ,of course ,you CAN calc axle rpms from engine rpms. Of course,as always,it's YOUR buggy. Do it however you wish. YOU asked a question and got answers.SORRY ,they don't coincide with your thinking. If you wish to get pissed at our answers ,don't ask .
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You can figure it however you want, but if you are driving anything off the axle then figuring axle RPMs from tire dia and mph is the best way. If you were running the alternator off the crank, then engine RPMs would be the best way.
As Fabr said, "it requires less calculations" and will give you the full picture from dead stop to top speed. Axle RPM and MPH directly correspond to each other. Axle RPM and engine RPMs do not. (that should be reason enough right there)
I personally would figure out how fast you can safely spin the alternator and then so do the calcs so that you achieve that RPM at the buggies top speed. I would do that to prevent damage to the alternator. Maybe even give yourself a little head room. I haven't looked it up, but I have a feeling that would give you a decent charging rate from moderate to slow speeds and up. Plus matching the upper limit of components just make good sense, but don't let good sense interfere with doing things the way you want. Sarcasm
Why is it that you started this thread again? eyes
An alternator will take all the axle rpms it will ever see and then some. Most alts are in the range of 2-3:1 overdriven. Assume 8K redline and the alt will see 16000-24000 rpms at REDLINE. THat axle driven alt will not see that.
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no im not getting pissed.. lol. just figured if i had it putting out good power within the range im normally cruising.. that things would be good.. my axle will never spin faster in any gear than it will in 6th at high rpms.. thats WHY ive been going off my engine rpms...
but ok, i asked for help because at the time i posted the thread I couldnt think of how to do this..
LETS DO IT FROM THE MPH THEN..... (its just easier for me to figure my axle rpms from my engine rpms)
ok so i top out at 99mph according to gearing calc..... now what?
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IMO,what you're not catching is that the LOW rpm is what you should be concerned about. The load on the electrical system is approx the same at idle as redline. You will be running the battery down when at lower speeds.Soooo, you NEED to have it charging even at very low road speeds.
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yea ive gotten that the whole time.. the ONLY reason i want to add an alt.. is to provide some better charging to my battery... duh right? lol.
ill put this thread on hold till i get my alt.
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Is there a place near you that does rewinding?
You might be able to get your bike alternator rewound for more amps, and fit an aftermarket diode pack to handle the increased current.
I agree that that MAY be the best thing to do. I know where you can have a higher output regulator/rectifier built. They built me a couple of 40 amp units for the busa. The busa makes 30 stock. They said the stator could handle the 40 amps stock. BUT they also will rewind your stator if necessary.
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yea ive gotten that the whole time.. the ONLY reason i want to add an alt.. is to provide some better charging to my battery... duh right? lol.
ill put this thread on hold till i get my alt.
yea ive gotten that the whole time.. the ONLY reason i want to add an alt.. is to provide some better charging to my battery... duh right? lol.
ill put this thread on hold till i get my alt.
I assume you are considering a small 1 wire 30 amp approx output alt? If so ,that's a good choice. I know from a fubar that they will withstand approx 26000 rpm for a short time. LOL!!!!! On my dragster I run an electric shifter. FOr some reason it failed to shift on a run. Datalogger showed almost 13000 engine rpm. Alt was at a 2:1 overdrive. Even on a normal run I'll run 8000 shift rpm so for many years it handled 16000 rpm on a regular basis. What I'm getting at is that, IMO ,you don't need to worry about top speed rpm of the alternator and would be better served concerning yourself with getting an adequate rate of charge at LOW speeds most of all. That's where most of your battery discharge will occur due to lower charging rates from the alt at lower rpms.
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sounds like a plan.. im just gonna get a chevy truck alt and configure it for one wire.... did it on my nova..
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I assume you are considering a small 1 wire 30 amp approx output alt? If so ,that's a good choice. I know from a fubar that they will withstand approx 26000 rpm for a short time. LOL!!!!! On my dragster I run an electric shifter. FOr some reason it failed to shift on a run. Datalogger showed almost 13000 engine rpm. Alt was at a 2:1 overdrive. Even on a normal run I'll run 8000 shift rpm so for many years it handled 16000 rpm on a regular basis. What I'm getting at is that, IMO ,you don't need to worry about top speed rpm of the alternator and would be better served concerning yourself with getting an adequate rate of charge at LOW speeds most of all. That's where most of your battery discharge will occur due to lower charging rates from the alt at lower rpms.
If they'll handle that many RPMs then there really is no need to calculate anything. Put the biggest pulley you can fit on the axle and call it good.
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yea really.. lol.. 12" diameter pulley will be max unless i want it below the frame.. (no)
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Sort of what I was saying in my original exaggerated reply.
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so besides tom pro who else sells alternator conversion kits?