Light weight flywheel project

[tw1980]

I bought a flywheel off ebay to experiment with.I would like to welcome all suggestions and comments.



sorry the pictures didn't post I'll work on it

[crazy steve]

Uhhh... These motors don't have flywheels...

If you're thinking of the alternator rotor (that's the only large hunk of metal connected to the crank I can think of), cutting this down or lightening it will reduce your alternator output. Seeing how that's already marginal, not a good idea...

[Guy_b_g]

Could be done....if you wanted to do all the work to rig a belt drive alternator.

Worth it? Not in my book..lol..

And you would still be effectively spinning a weight...just at the end of a belt instead of directly.

Matth is rigging a parasitic drive off of a modified middle drive (no longer shaft) if I remember correctly..that would probably be the most effective method of reducing the flywheel effect.

[crazy steve]

Yep, that's the rotor....

[James England]

It looks like a bad idea to me, especially the holes that appear to be drilled in the outside of the rotor. As previously mentioned, it's this rotor that produces current (in conjunction with the other generator components). I'd have thought that drilling holes in it would lessen its efficiency. It may be the case that the rotor is balanced by the manufacturer, or certainly made within tolerances that ensure it will spin at high speed without causing vibration. Drilling holes would, I think, unbalance this component, making the engine like a giant version of a mobile phone on vibrate (that's how they do it on phones.....)

[WMarshy]

It looks like a bad idea to me, especially the holes that appear to be drilled in the outside of the rotor. As previously mentioned, it's this rotor that produces current (in conjunction with the other generator components). I'd have thought that drilling holes in it would lessen its efficiency. It may be the case that the rotor is balanced by the manufacturer, or certainly made within tolerances that ensure it will spin at high speed without causing vibration. Drilling holes would, I think, unbalance this component, making the engine like a giant version of a mobile phone on vibrate (that's how they do it on phones.....)

LMAO great explination! How much weight did you remove by drilling those holes, did you even get an ounce? lol

[bikerphil]

I've heard of people lightening and re-balancing crankshafts to get the motor to spool up quicker. If that is what you are trying to achieve with drilling that rotor, I don't think it will help any, JMHO.

[DiverRay]

Taking the weight off the rotor WILL allow the motor to spin up faster. There are a few things that MUST be done, though....
1. As stated, the rotor MUST be balanced to at least 9K RPM's. This is usually done on a lathe I think.
2. The bike will probably NOT idle as smooth, because of less rotating mass.
3. You MAY cut the charging down a little, but probably not much at all. The windings and wire size have more to do with output than the mass of the rotor.
Let us know how it works!

[crazy steve]

...You MAY cut the charging down a little, but probably not much at all. The windings and wire size have more to do with output than the mass of the rotor...

Ray, I'll have to disagree with you on this one. Increasing the distance between the coils and the rotor or reducing the mass of the rotor will both lower output more than you think. If you remove enough weight off the rotor to actually make a difference in engine response, output will at least drop proportionally, if not more. The XS alternator design is such that you don't actually have a rotating magnetic field, but has a stationary field that is 'stirred' by the mass of the rotor; reduce the 'stirring' and output will drop. Much the same effect as an infinitely-adjustable arc welder; these work by moving the iron core in and out of the transformer coils. Move the core (reducing it's 'size' that the coils see) and output can go from 200 amps down to 50...

[James England]

I don't want to sound rude or anything but, regardless of whether the alternator will still work, or not, or nearly as well.........what's the point? OK, you take half an ounce off one component from an 1101cc engine with loads of torque, masses of power etc etc.... so what? Even if one assumes it's adequately balanced (which I doubt), you'd have to be a computer, rigged up to sensors all over the bike to detect any difference in performance. There's just not enough metal removed to make a difference. I wonder what it is as a %?

[Incubus]

Lightening the rotor (an lightening and balancing the crank) are a couple of things the guys I know on the CB750 forum do when they build a high performance motor, like a road racer. FWIU, this is done not so much so the engine winds up faster, but because a lighter, balanced rotor and crank allows the engine to exceed factory red line without blowing up. Some folks run those engines up to 14,000 or 15,000 rpm and hold it there.

I don't know about you, but I am afraid I would not be able to poop for three days if I ever rode an XS1100 motor at 14,000 rpm in fourth or fifth gear. Small wings should make it flyable.

But it's your bike and your grisly end....

Patrick

[tw1980]

Well it seems as though I've provoked quite a discussion.Thank You for all the responses.It is my intention to balance the rotor and I will weigh both parts to see what the difference is and post the results.I'll also slap an amp meter on it to see how much charging ability is lost. Terry

[James England]

Well it seems as though I've provoked quite a discussion.Thank You for all the responses.It is my intention to balance the rotor and I will weigh both parts to see what the difference is and post the results.I'll also slap an amp meter on it to see how much charging ability is lost. Terry

and presumably let everyone know if it makes any difference to the performance of the engine......

[old_chopper]

If your drilled holes are centered about the centerline of the part and equally spaced you won't have to rebalance the part. We used to drill holes in bolts to reduce weight, every ounce counts in a performance environment. Reducing the spinning mass is a great place to concentrate your efforts. Reducing the diameter of the spinning part is even better but could be a little more involved in this case.

[crazy steve]

Terry, if you're going to try this, here's few ideas/thoughts to help retain alternator output.

Avoid reducing the OD of the rotor. The strength of magnetic flux drops off in inverse proportion to the distance from the source, so if you double the clearance distance between the rotor/field, you halve the flux. The flux is what induces the voltage/current in the field windings; less flux, less output. While the flux is generated in the stator winding (which isn't moved), the rotor 'transfers' that flux to the field so any increases in clearance distance or reductions in rotor mass will reduce the amount of flux the field 'sees'. This may not be directly lineal as you're only changing the 'core' (rotor) and not the actual windings. Those old enough to remember slot cars may recall that one of the 'speed secrets' for the Mabuchi motors was shimming the magnets closer to the armature for more flux. Alternatively, if you could come up with a field winding/core with a smaller ID to maintain or even reduce this distance, that would help.

Increase the ID of the rotor; probably the best place to remove metal. Yamaha did a rather sloppy 'fit' here, and machining this larger will clean it up. The main thing to look out for here not removing so much that you damage the integrity of the welded-in copper ring that connects the two pole halves of the rotor. You could restore (or maybe even improve) alternator output by making a larger-diameter stator for a closer fit inside the rotor. This isn't as hard as it sounds; machine a new tighter-fitting shell (replacing the plastic one) out of non-magnetic aluminum and have it wound at a motor shop, retaining the same total resistance through the coil. This might be the best way to increase alternator output for those wanting more if you left the rotor alone.

I'll throw one more idea out there; changing the entire alternator. Some bikes use a permanent-magnet type rotor that's much lighter (this type rotor for a Sportster weighs about 1/2 or less of what the XS unit does) so there's your rotating-mass reduction. Most consist of a simple steel shell with magnets glued to its inside diameter. The field/output windings fit inside the rotor, so find one with the right OD that'll go inside the XS cover and it's just a matter of fitting the parts. You'll need the matching rectifier/regulator as these operate at 100% all the time and shunt any excess current to ground. You'll probably lose the three-phase output, so you'll need a new tach that takes it's signal from the ignition coils.

Food for thought...