Flywheel Fun!   I just had the opportunity to lighten two pals' flywheels and thought that I'd get some photos up of the surgery, both before and after. If you're a sporting rider, and you still haven't put your Guzzi flywheel on the Bridgeport diet, then I simply offer my condolences. How many of those poor old sportbikes needlessly suffer from a hefty paunch in the midsection? I suppose the same thing could be said of many of their riders, though, no? :-)

Figure 2: Stock Lemans IV Flywheel (along with my two big feet)

Figure 1: Stock Lemans IV Ring Gear

Figure 3: Lemans IV flywheel after a bit of trimming on a lathe (the shiny surfaces have been trimmed)

Figure 5: Lemans IV flywheel after lathe work and milling

Figure 4: Lemans IV flywheel and ring gear after lathe work and milling

Figure 6: Lemans IV flywheel and ring gear after lathe work and milling

Figure 7: EV flywheel  after lathe work and milling

Figure 8: EV flywheel after lathe work and milling

Figure 9: EV ring gear after lathe work and milling

Figure 10: EV flywheel and ring gear after lathe work and milling

The results (before/after): ’86 Lemans IV   Flywheel: 7.25 lb (before)…4.52 lb (after) ’86 Lemans IV   Ring gear: 4.91 lb (before)…3.95 lb (after)     total weight loss: 3.69 lb (30%) ’98 EV  Flywheel:9.38 lb (before)… 5.05 lb (after) ’98 EV  Ring gear:4.82 lb (before)… 3.68 lb (after)   total weight loss: 5.47 lb (39%)    Keep in mind that much of the removed mass was located at the outer perimeter of the assembly, so the mass moment of inertia of the flywheel assembly has decreased significantly more than just by the percentage of weight lost.  The rotational (mass moment of) inertia of a circular disc is proportional to the 4th power of its radius. In other words, if you double the radius of a circular disc (and use the same material), then you have increased the rotational inertia by 2 to the 4th power, or 16 times!!!! I decreased the weight of the Lemans IV assembly by 30%. The decrease in rotational inertia is somewhat higher, though. It’s probably on the order of 50%. This is a difference that you will definitely feel on the bike.    These flywheels were modified for “daily driver” bikes, so I erred on the side of keeping too much material as I cut these pieces. There are many ways to get them much lighter, for race use. After a point, however, the clutch assembly will begin to lose some rigidity and strength, and you’ll risk failure. Believe me, failure of a flywheel at 6000 RPM can be a fearsome thing to witness, especially when that flywheel is spinning between a rider’s spread legs. Yowza.

2/10/04 Update I've been through a lot of iterations with flywheel lightening patterns. Below is a flywheel and ring gear that are about as light as I'll cut the Guzzi twin plate clutch for a street motor. Flywheel...4.43 lb   Ring gear 3.33 lbs.