Topic: 750f - anyone used a different (smaller) rotor - generator setup

[Mark M]

If you see the 'trimming the fat thread' you will see where this started.
I have a CR750 roadbike project that I'm working on. It is kickstart only so only needs to run a battery for the legal niceties of Tail and Head lamps, oh and the instuments light up, along with Oil and Neutral lights, and of corse a horn. So I only have a small battery under the tail unit to keep fed (it will run flat too quickly on a total loss system). I have seen some nice replica sandcast generator covers from M3 racing but they are a total loss setup and also V' expensive. What I'm looking for is some advice on a smaller/thinner generating system so that I can cut back the HUGE OE cover.
I do have a rudimentry understanding of how, rather than why it all works, good enough to rewire several bikes anyway. What I have been thinking is there must be something from a smaller bike that would suit my purpose. Even if you cant suggest an actual fitment some advice on what I should be looking for or be considering when setting it all up would be good.

     

[bryanj]

All the fours from 350 to 750 run the same type of generator and i think the "taper" of the crank is the same--but dont shout if i'm wrong, Now the 500/550 gave less output than the 750 SO it ought to be smaller and i have no idea about the 350/400. What you could do is make up a cover with a centre seal and machine a "V" belt pulley to fit the crank and use a car generator. Dont laugh I seen it done, admitedly on a big Kwaka for endurance racing but it worked

[Terry in Australia]

Now that would actually look pretty cool if you skipped the car generator thing (ugh!) and adapted a small motorcycle generator, ala modern sports bike, running a toothed belt from a pulley on your crank, and mount it about where your redundant (in the name of light weight and small battery) starter motor would normally sit.

It'd be a piece of cake to turn up the sprockets, you'd just have to hunt around for a toothed belt of the appropriate length, and you could just cut a blanking panel to cover the area where the alternator assembly used to sit like they have on drag bikes, imagine the cornering clearance, woohoo! Cheers, Terry.  

[cben750f0]

is there not a stator that you could bore to match the crank?... you know like a CB250 or something, and bore it to match the taper on the crank, then cut don the original cover,make your adaptor plate, and rewire?.... surely it cant be that hard....(am a fitter though so dont seem that hard to me)..peace

[Mark M]

What I'm after is some suggestions on a smaller setup, something that I can adapt to fit easily on the end of the crankshaft, that is small enough for me to then cut down the old cover so that it looks like a racing one.

thanks for the suggestion Terry but I removed the starter to save weight, now you want me to put more on.......

If we are going heath Robinson then parhaps a bycycle dynamo and torch bulbs would be legal?    

[Dennis]

Just thinking out loud here, don't know what specific bike you could steal this from ....
how about a permanent magnet alternator from a late model bike. I'm thinking about my Yamaha RZ (maybe you call it RD350YPVS) with it's fairly small (permanent magnet) rotor. Of course you will not need the trigger or ignition coils built into a unit like that. Maybe you can find something else that has only the 3 charging coils. You will have to fabricate something to mount the stator and match the rotor to the crank. Harness mod is easy, use a 5 wire rectifier regulator, 3 ac coil wires, batt and ground.
Unfortunately I have no idea what bike may have a unit like that.

[Sam Green Racing]

Mark, we was going to have a self generating system useing a rotor from a CB350 twin with the taper moded to suit the 750 crank.
We allready have the factory cover machined to take this set up, but the guy that was going to do it started to become unreliable, so we opted for total loss.
I am not sure if this would work on yours with lights and all the rest of the road crap, but if you are interested in a cover I would try to get a price.

Regards Sam.

[Terry in Australia]

thanks for the suggestion Terry but I removed the starter to save weight, now you want me to put more on.......

No Mark, I doubt if a modern motorcycle generator would weigh anywhere near the boat anchor currently hanging off our bikes, and moving it up to where the starter was previously located will chop about 3 inches in width from your engine.

Rex Wolfendon, Australia's premier CB750 race bike builder built the bike in the pic below as a daily commuter, note that he hasn't bothered messing with his alternator, but then again when you've got an 1100cc engine putting out 120 reliable Bhp at the rear wheel and a fully braced frame with plenty of ground clearance, you probably don't need it! Cheers, Terry.

[MRieck]

You could cut down the stock rotor OD & length a bit to take some weight off. I remember the 4 cylinder Kawasaki 750 from about 1987 thru 1990 having the belt driven generator set up.

[Mark M]

Terry,
Your absolutly right - if it ain't broke then why fix it!
It must be a great cumfort being so wize, especially at your time in life

[eldar]

The only thing I could really say would be to get the o.d. and lenth you want to go with and see if you can find parts that may fit. It would be a BIG stroke of luck to have someone say "sure, just bolt on this unit and thats it". I dont think this is going to happen.

So going with that, I suggest you find out what your current draw is and find a unit the will exceed it. Any small motor can be used as an alternator, you just have to zero out the timing on the motor you use as some are timed to operate better in a certain direction. A motor turned by an outside source is a generator.

If this motor is small enough, you may then cram it in the starter box and just have to seal off the old housing and make a drive of some sort for the motor.

The rectifier is probably not a big issue but you may then need to do some regulator tinkering.

[Terry in Australia]

Terry,
Your absolutly right - if it ain't broke then why fix it!
It must be a great cumfort being so wize, especially at your time in life

Ah grasshopper, at last you see: "Besides the noble art of getting things done, there is a a nobler art of leaving things undone...........the wisdom of life consists in the elimination of non-essentials." Lin Yutang. There you go Mark, Mr Yutang has probably never owned a CR750 replica, yet he understands my point entirely.

As much as a smaller alternator cover/slightly lighter bike would be nice, skipping your weekly ration of pork pie and chips or black pudding and fried bread (yes,for the non-British readers, it is as bad as it sounds!) will probably have the same end result, without all the hard work and expense, remembering of course, that the bike will still be expected to perform as a relable road bike?

I appreciate your praise as always Mark, keep up the good work! Cheers, Terry.

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