I while back I remember seeing a pic somewhere of a car that had the alternator mounted to the rear axle, with a pulley mounted to the driveshaft flange to spin the belt, and run the alternator. Obviously this means you're only gonna be charging while you're in motion, but I'm wondering if this setup can be at all practical for a car that actually gets driven regularly. The reason I'm thinkin of this is because I'm planning on running an electric water pump on my next project, and the only other accessory is the alternator. If it can be mounted to the driveline it'll shorten the motor by several inches, and there will be no visible belts under the hood at all. If you were to run a high end AGM battery, with plenty of reserve amps, and your only power draws are a fuel pump, EFI computer system, and lights, would this setup get you by? Also when you're in 1st and 2nd gear the alternator is gonna be spinning slower than the engine, and with an OD tranny at freeway speeds it could see higher speeds than normal. Does anyone know the real RPM range of a GM alternator? More importantly has anyone actually run this setup? How does it work?
I wouldn't worry about it if you were in middle of nowhere Kansas. Some place far and away from traffic jams. But not so sure about San Diego or any major city. Stop and go traffic I would assume would be too much load on battery only.
speedway motors sells this kit they say it will only charge at highway speeds . just low mount it on the driverside of the motor with a short belt off the crank . that how i run mine on my drag car with elc.pump .
i've done it with no problems. this has been covered many times before , if i wasn't so lazy right now i would find the threads
Just a thought / question on this, wouldn't this be best if you used a good quality deep cycle battery like an Optima yellow top ???
I wouldn't call anything with the Optima name on it good quality these days. Their quality has gone WAY downhill in recent years. If I were to do this I'd be using one of the XS Power gel cell batteries. They are about half the size of an optima, and have proven to have far more power and durability in our off road race cars. 36-3window, what kinda driving do you do with your car? Any traffic/city type driving? Most of the time this car will be going for short trips around town, or on the freeway and will be avoiding high traffic times whenever possible. Since I work from home commuting in traffic is not really an issue for me. Obviously I could mount it low, or high, or anywhere, but the whole idea is that I could get rid of all accessories under the hood and make some extra space.
I ran that same set up on my street driven '32 roadster with a sbc for three years. I ran the electric water pump and hung the alternator on a bracket off the differential and a pulley on the rear u-joint. Looked killer.............it also was a perpetual battery killer. The pulley would never spin the alternator fast enough, nor would it spin it long enough in heavy traffic. I had to trickle charge the battery after driving it constantly, always carried a jumper pack, and it would die out at the worst times. It was a total flop for drive-ability but always drew comments at shows and cruise nights. I never could predict when it might not start. I finally smartened up and dumped the whole set-up. DON'T DO IT!
i wouldnt think it would work much for a street driven car because if you think about how many rpms an alternator makes on a motor vs how many rpms while driving, theres a huge deficit
"free up some space under the hood"....for what? A friend of mine has a V6-71 blown SBC in an "A" coupe, not a lot of room. He ended up going with a tiny little alt off some import, runs it off the crank and mounted it down low on the drivers side. I have no idea what the alt is from, but I can tell you it is TINY. Might be better off going with something along this route rather than off the d-shaft.
Here's another discussion on a driveline driven alternator. http://www.jalopyjournal.com/forum/showthread.php?t=382397&highlight=alternator
Had a modern OT truck that had the alternator go bad,and it wouldnt run well enough on the battery to limp home.It took the whole 100 amp"s to make the system function.Installed a rebuilt in the parking lot,and the battery spun the engine over,and it ran fine.If a guy was running a magneto you could likely get away with the drive shaft mount.Saw a nice 34 Ford PU in Rod,and Custom about 50 years ago that had the alternator driven off the back of the drive shaft.No doubt no computer on board to keep happy.
The normal pulley ratio for an alternator is about 3 to 1 (8 inch crank pulley driving a 2.8 inch alternator pulley) so a crankshaft turning at 6000 rpm would be turning the alternator 18000 rpm. I don't think you could turn it too fast, your problem would be turning too slow unless you were at highway speed. Third gear on most older transmissions is 1 to 1 (direct) so at 2500 rpm at 60 mph in third your driveshaft speed would be 2500 rpm, no problem.
Some of the tiny alternators are on forklifts. They seem to put out enough juice too. I wouldn't do it. especially if you're talking about an electric fuel pump and an EFI (SHHHHHH) computer too. I have a small pc680 battery in my t bucket. All I have electrical in that car is lights and 2 heated seats. I have to keep it on the charger at home and carry a jump box with me. If you've got the seats on and the headlights on its gonna groan when you try to start it next time
Those differential drives were designed for race cars that ran wfo all night with like 6.50 gears/7000rpm
Not that handling is a huge concern, but mounting the alternator on the rear axle will increase un-sprung weight. That increase is harder to control. Yeah, not a great deal, but still.
I've been entertaining a line of thought which got me to thinking that on many of our cars you might want ancillaries under the floor boards rather than at the front of the engine — perhaps accessed via a trap door in the floor? It suggests a line shaft running alongside the engine and gearbox, on the passenger side, with as many pulleys turning alternators, compressors, fuel pumps, scavenge pumps, heater fans, or whatever as you want, one behind the other. You'd have no more at the front of the engine than a belt drive similar to those of the cable-drive fuel pumps which have become a thing. Single-V, multi-V, or Gilmer would depend on the total of what you intend to drive. Of course it needn't have this billet look:
NASCAR requires a cable driven fuel pump for their electronically controlled fuel injection on Cup cars, so thatthere won't be fuel fed to a fire in case of a crash and no cutout of an electric fuel pump. Might be something to investigate for driving an alternator. They had trouble the 1st season with some failures of the cable, but that doesn't seem to occur any more. Just a thought...
Electric fuel pump, electric water pump, and EFI? um that's a lot of electric draw for something you would have a very marginal charging system on.
I would think the harshness of the axle going over bumps in the road would be detrimental to the alternator. But I have never done it....... probably won’t ever do it! Bones
There are so many extremely small case alternators available for/on small cars now that put out good amps and could be low mounted where they were pretty well out of sight that hassles that would go with a differential mounted unit don't make putting one back there viable outside of impressing the mouth breathers at rod trots. Back when they came up with mounting them on the rear end there weren't any small case alternators around and not many low mount brackets. Some guys have been using Kubota tractor alternators but I can't find one that puts out over 40 amps and the little bitty one only puts out 14 amps.
I've done two conversions on otherwise stock 1920s/30s cars where a Kubota alternator has been driven by a belt off the driveshaft. The owners were not prepared to have a modern alternator visible on their motors. Both conversions have worked (and are still working AFAIK) fine. The ammeter shows charge from about 20 MPH. But these cars don't have a high electrical demand, nowhere near the nominal 40A output of these Altys; it's also the case that this type of car is more likely to be found pottering around on country lanes than stuck in an urban traffic jam in the dark. Fitting the pulley on the stock driveshaft where the shaft diameter is smaller than either end of the driveshaft was a bit of "ship in a bottle" issue that needed solving:
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