Topic: Lightning Alternator

[MattFreeman]

How much can you take off an cb750 alternator and where? Without compromising charging?

[MCRider]

How much can you take off an cb750 alternator and where? Without compromising charging?

What model/year/K#. The early ones were really heavy and you could take off about 3lbs. From K3 or so forward, Honda did the job for you and only about another 1lb could come off. May not be worth it. There are threads on this with solid numbers if you need them.

If you have APE or CycleX or someone do it they know how far to go.

[Bodi]

These goals are mutually exclusive.
The rotor is an aluminum alloy structure supporting iron magnetic pathway pieces. The iron pieces direct the field coil's magnetic field at the rotor's inner circumference to alternating north/south poles on the outer circumference. These poles sweep across the stator coils and create electricity.
Removing metal from the iron pieces will reduce electrical output.
Removing the alloy matrix supporting the iron pieces will structurally weaken the rotor, risking catastrophic failure at high RPM.
I think there are lighter rotors available but I have never seen one. The stock rotor is made to a budget. If cost is not an problem I'm sure a lighter unit with equivalent performance is possible.
Trying to lighten the stock unit is a risky endeavor, if it fails at high RPM you will destroy the stator at least and large bits at high velocity could fly through the housing.

[754]

Matt , where are you located?

 I can cut them down.

 You can take several pounds off if you are running kick only, less if you are running estart.

  Makes a huge difference in how fast it revs up.

 bodi, its an all iron core on the Honda 4s

[MCRider]

These goals are mutually exclusive.
The rotor is an aluminum alloy structure supporting iron magnetic pathway pieces. The iron pieces direct the field coil's magnetic field at the rotor's inner circumference to alternating north/south poles on the outer circumference. These poles sweep across the stator coils and create electricity.
Removing metal from the iron pieces will reduce electrical output.
Removing the alloy matrix supporting the iron pieces will structurally weaken the rotor, risking catastrophic failure at high RPM.
I think there are lighter rotors available but I have never seen one. The stock rotor is made to a budget. If cost is not an problem I'm sure a lighter unit with equivalent performance is possible.
Trying to lighten the stock unit is a risky endeavor, if it fails at high RPM you will destroy the stator at least and large bits at high velocity could fly through the housing.

I'd have to disagree. YOu may be referriiing to bikes other than the CB750. THe CB750 rotor is a combo of the part that's involved with the making of elecricity and a weight.  THe weight part can be removed with no reduction in generating power. Its done on a lathe or a mill. Period. There is no danger of the thing coming apart. Lightened rotors are used on high rpm racing machines.

Its been done on tens of thousands of early CB750s and Honda did it themselves on the later CB750s. Weight reduction on the early ones is about 3 lbs, Honda removed about 2lbs, leaving about 1lb to be done by owners.

In my opinion, highly recommended.

[wannabridin]

754,

how much can you take off a k3 or later rotor?  and doesn't it sacrifice the charging capabilities regardless of starting options?

[MCRider]

754,

how much can you take off a k3 or later rotor?  and doesn't it sacrifice the charging capabilities regardless of starting options?

There is NO repeat no change in the charging capabilities.

[wannabridin]

fair enough!  so it's worth it on a bike even with e-start??

MC, don't you have yours lightened?

[MCRider]

fair enough!  so it's worth it on a bike even with e-start??

MC, don't you have yours lightened?

I have had several lightened over the course of owning many CB750s. I will use the rotor from Phaedrus I on Phaedrus II. It was lightened more than any I have ever seen. The center was coned, the edges were chamfered, the entire flange supporting the electric starter clutch was removed. THe clutch was just hanging out there in the breeze with nothing holding it in place but its 3 screws. The last 10 years of Phaedrus I life was as a 2 up touring bike. I never kicked it, electric starter only, turning over a 10:1 CR 888cc motor, never failed. I used a 100w H4 bulb, Prestolite ignition with Andrews coils (real power sink) and wore an electric vest whenever I needed it.

It makes the throttle jerky as you have no flywheel effect to smooth things out. Just makes you a better rider, or a jerky rider, I guess.   

But the throttle response is great.

[754]

It should cost around 40 -60.00 plush shipping
 IMO not worth shipping the parts across a border, if you can find someone to do it on your side.

 I dont have exact weight, you can get them down under 4 lbs, you have to leave more meat for the estart parts.
 

 Turning them is easy, checking the balance a bit tougher..

[MRieck]

It should cost around 40 -60.00 plush shipping
 IMO not worth shipping the parts across a border, if you can find someone to do it on your side.

 I dont have exact weight, you can get them down under 4 lbs, you have to leave more meat for the estart parts.
 

 Turning them is easy, checking the balance a bit tougher..

  I can have them balanced.

[754]

I have an engine shop 2 blocks away that can balance them, It would be better doing  several at once.
 The several I did get checked, never did need drilling..

[brandon]

In essance, this modification is like installing a lightened flywheel in an automobile?

Since the bike will rev faster, do the revs drop off just as quick? Can this be done on a 400F?

[MCRider]

In essance, this modification is like installing a lightened flywheel in an automobile?

Since the bike will rev faster, do the revs drop off just as quick? Can this be done on a 400F?

Exactly! Yes revs up faster, nothing holding it back, drops off faster, no flywheel momentum to keep it spinning. THis makes for a jerky ride, some would say unpleasant if that's not what they want to live with.

Don't know about a 400. I don't think so if I were to guess. I think the problems outlined by Bodi may apply to the 400.

[754]

I have a 400 lower end with rotor on it, will try to look at it, it is a lot smaller than a 750 rotor.

 I bet a lot of magazine test bikes had lighter rotors..

[wannabridin]

It should cost around 40 -60.00 plush shipping
 IMO not worth shipping the parts across a border, if you can find someone to do it on your side.

 I dont have exact weight, you can get them down under 4 lbs, you have to leave more meat for the estart parts.
 

 Turning them is easy, checking the balance a bit tougher..

  I can have them balanced.

looks like you'll be balancing my rotor as well as my crank Mike!!


now can just a little be taken off?  just to help it out somewhat but still keep some of the smoothness?  basically, is there a "butter zone"??  i feel that a balanced rotor with a balanced crank would help things out a bit!  also, if you convert the rotor to run kick only, is there anything removed to keep you from re-installing the starter?

[MCRider]

It should cost around 40 -60.00 plush shipping
 IMO not worth shipping the parts across a border, if you can find someone to do it on your side.

 I dont have exact weight, you can get them down under 4 lbs, you have to leave more meat for the estart parts.
 

 Turning them is easy, checking the balance a bit tougher..

  I can have them balanced.

looks like you'll be balancing my rotor as well as my crank Mike!!


now can just a little be taken off?  just to help it out somewhat but still keep some of the smoothness?  basically, is there a "butter zone"??  i feel that a balanced rotor with a balanced crank would help things out a bit!  also, if you convert the rotor to run kick only, is there anything removed to keep you from re-installing the starter?

Unless you actually damage something, no reason you can't put the elec start back on, hardest part is to pull the rotor to get the clutch back on, it screws to the back side of the rotor. Otherwise, piece of cake.

[wannabridin]

sounds like this is going on my to-do list    thanks MC!!!

hopefully Mike will be able to lighten it a bit before it's balanced...

[MRieck]

sounds like this is going on my to-do list    thanks MC!!!

hopefully Mike will be able to lighten it a bit before it's balanced...

I can do that. I have the measurements from a late RC rotor

[brandon]

I have a 400 lower end with rotor on it, will try to look at it, it is a lot smaller than a 750 rotor.

 I bet a lot of magazine test bikes had lighter rotors..

I would be interested in knowing what you find. Thank you.

[Tretnine]

Can you do this on late 550s? I don't know of the relative rotor sizes and functional amounts on them.

[MattFreeman]

Wow, I step out for a minute and you just go on with out me.

So, it's a '76 F1 and I'm going kick only.

754: I really appreciate the offer but I'm in Ohio. I have a friend who's a certified automotive engine machinist who I'd like to have do the work as he owes me a favor or two.

I'm just looking for someone to come off some hard numbers. "A pound or two" sounds awfully vague in a world of hundred thousandths.

Mr Reick: any chance you'd send me the specs from your RC rotor? I'd understand if not as it's how you make $ and all.

I've been told you chamfer the outside edge, I was wondering if you can also take some off the inside circumference. I assume you want to leave the outside alone to keep the gap the same, no?

So really, there are no charging issues with this? I wanna run a ridiculously bright headlight.


Thanks for any and all input.

[MRieck]

Wow, I step out for a minute and you just go on with out me.

So, it's a '76 F1 and I'm going kick only.

754: I really appreciate the offer but I'm in Ohio. I have a friend who's a certified automotive engine machinist who I'd like to have do the work as he owes me a favor or two.

I'm just looking for someone to come off some hard numbers. "A pound or two" sounds awfully vague in a world of hundred thousandths.

Mr Reick: any chance you'd send me the specs from your RC rotor? I'd understand if not as it's how you make $ and all.

I've been told you chamfer the outside edge, I was wondering if you can also take some off the inside circumference. I assume you want to leave the outside alone to keep the gap the same, no?

So really, there are no charging issues with this? I wanna run a ridiculously bright headlight.


Thanks for any and all input.

I'm not going to retire on CB charging rotors. I have the width...the OD is pretty much the same. Chamfering the inside/outside edge and the back edge of the rotor. Center cone edge gets hit a bit...that's about it. It charges fine. I prefer to lighten the crank and run a balanced, late model rotor.

[754]

a few things..

 If you really want a light one.keep in mind a spare is not really pricey..

 If you remove e-start ie kick only, and make it real light you cant go back, just grab another if yoiy want estart. I cut off the lip that locates starter clutch, drill out the mounting holes, plus you can drill 3 more holes for a total of 6   3/4 inch or so holes.

 I made a puller, 2 actually and (finally) got my STUCK one off the 69 crank, and the 400 rotor off.

 The 400 rotor is around 3 lbs.. it looks a lot like lightened 750 one.. no estart provision.. looks like it  can be drilled, and shaved a bit.. maybe 1/2 lb or so.

 I thought I read on here of a 400 rotor being used on a 750.. looks like it is possible if you cut the same taper deeper into the rotor.. The fat center on a 400 rotor is smaller than a 750s. I dont have, but sure would like to see the field coil off a 400. The 750 field coil would fit with slight trimming.. but the doistance to middle would be off..

[seaweb11]

I used a lightened alt from here http://www.cyclexchange.net/Miscellaneous%20Engine%20Parts%20Page.htm on the 836 Tracy.
Had an issue with charging using an aftermarket headlight bulb at 1st, but after installing a smaller bulb, all is well with that now.

Yup it sure gets you going a lot faster ...quicker:D