Flywheel, how light shoul you go.

griff851

Just been mucking around with the blowbag bevel engine and I decided to look the flywheel as I have the ubeaut straight cut primary's and an alloy basket ( yes I know its not going to last long, but if I get 20,000km on it before I die I'll be lucky). Any how years ago the old fly wheel was machined down to being just a spacer, yes accelation increased, but at a price, a knackered ubeaut german pin and minor damage to the side of a corrello rod on the alternator side after 60,000km of serious flogging. So it got me thinking, how light is too light. Well I found this last night. http://www.w8ji.com/rotating_mass_acceleration.htm
After some thought I decided that the rotating mass of the fly wheel and ignition timing dorgon has got to be at least the same as the rotating mass of the alternator to get balance. I'm just waiting for some figures of 4V standard flywheels and ducati performace ones so I can see what numbers our Italian boffins use. I have a sneaking suspicion that when I match them against standard bevel ones, certain percentages are going to appear before my eyes. Anyone else looked into this black magic?

Mr.R

Hi Griff,

I doubt that your lightened or missing flywheel caused the bigend failure/damage to your rod.
My experience is that well thrashed BEVEL twins eat bigends for a pass time. 😏
I run a lightened flywheel on my "Old Nail" with the OEM Bosch ignition rotor replaced with a alloy spacer, very narrow/lightened straight cut primaries and a alloy basket/hub dry clutch conversion.
The bigend life certainly hasn't been any worse with this set up IME.

I'm not convinced that there's a corralation between the differing weights of what's fitted to each end of the cranks??
On the 851/888 four valve motors all of them have similar primary gears on one end, but have differing alternator/flywheel weights on the other. Both ends being supported by out rigger bearings.
The trick is to have all of the ancillaries fitted to the crank when dynamically balancing it.

Steve R

brad black

std ones are 1900g across nearly all models, except st2, st3, maybe st4. dp ones are just aluminium replicas, so 650g ish.

i don't believe there's much science to it.

Mr.R

Brad's correct here the ST3 motor that a mate had tuned and then fitted into his 900SL (he got 125 rwbhp from it), had the biggest/heaviest flywheel that I've seen fitted to a modern motor.
...and lets face it here the ST range apart from the bore/stroke are pretty much the same motors as our 851/888's.
I think that you're over thinking this situation Griff.

Steve R

griff851

Quite possibly I am, you know how it is, sitting in the shed having a think about things. Flywheels dampen out pulsing and surging. No flywheel damaged pin on opposite side... mmmm? I'm thinking I might as well while I'm there bung in a flywheel of some weight. So I figure I'll use the calculator in the website to determine how much I machine off an original flywheel.

BABA

Here is mine.
From the 1875g original flywheel, it remains 1063g

I didn't lightened the other parts on the letf side (which I regret a little now😁)
As I have extra parts from my engine donor for my 851R, I think I will lighten all left side parts and permute them.
Flywheel and hub will be machined like the 89 Factory version...