How much weight can I take off the flywheel?

[chevy43]

I got a Lombardini 1120cc 2cyl that has a pretty heavy flywheel. I'm going to put it in the lathe and start taking meat off of it to lighten it. The flywheel is blank and the cooling fan is on the otherside.
The question is how much flywheel to I need to have it run well?

I guess I can go by trial and error but if anyone has imput on this I'd like to hear it.

[oldbmw]

I got a Lombardini 1120cc 2cyl that has a pretty heavy flywheel. I'm going to put it in the lathe and start taking meat off of it to lighten it. The flywheel is blank and the cooling fan is on the otherside.
The question is how much flywheel to I need to have it run well?

I guess I can go by trial and error but if anyone has imput on this I'd like to hear it.

I suspect Lombardini have already trialled few to see how little they could make the flywheel. no sense in producing more than is needed.

If I were you I would test run the motor up and down the rev range and see how much it shakes.

[chevy43]

Mine seems extra heavy (16+kilos) - probably to make it run as smooth as possible. Extra weight isn't ususally a problem for industrial use but I don't want extra on my bike. It also take the engine a while to accelerate all that iron. I guess I will start turing some off and see how it runs...

[tappy]

I suspect the heavy flywheels are for two reasons:
The first is to make it run smoothly
The second is to put plenty of lag in the system so the throttle control has an easier time when it's trying to make a generator run at a constant speed.
Aside from running roughly and knocking at low rpm, I'd expect you can remove some weight.
Most sensible place to remove it is from the centre, as the stuff near the outer diameter contributes more to the angular momentum. Trouble is, if you remove too much from the centre then the outer part might be left unsupported and have a tendency to burst at high rpm. Especially if it's cast. My money would go on making a new one out of a steel ring, or possibly laminated steel plates...

[mark_in_manchester]

Hi all -
On my bike (Daihatsu-Ural) I used the Ural flywheel and clutch, machined to fit Daihatsu crank. The car flywheel was at least 30% heavier than the bike one, with a lot more mass towards the rim. Not fancying the derivation of some nasty integral equation to calculate the relative moments of inertia, I stuck the Ural one on...bike ticks over at 800rpm, even at 0°C, no problem. It's a triple (more bangs per rev than your twin) and also employs a balancer shaft and fairly significant crank-web counterbalancing, all of which will add to internal rotational inertia - but it seemed I could lose a whole load of mass without worrying too much. Could you make a small flange / taper whatever to fit crank (or chop down radically a spare standard flywheel), and then bolt rings to it until it ticks over OK? Maybe then weld it all up for safety at the end...

My guess is tickover is all here - rotating inertia will only increase with frequency, meaning if it works at tickover then it should be OK at higher revs. Think race engines - tiny flywheel, won't tick over, but smooths out OK when you rev it.
Mark, Manchester, UK

[Diesel Dave]

Most stationary diesel flywheels are cast iron, for this reason I wouldn't take any metal away from the inner parts - machine from the outside in unless taking off the cooling fins.

The flywheel will be heavy for may reasons including being capable of starting under full load with all sorts off industrial mechanical loads.

You are only going to find out how light you can go when you take too much off and need to go back a little

Dave