Engine assemble part 2 Gofo

[vince998]

So if you put a twin motor in a Norton wideline frame would you need to alter the balance factor?
Jim Burgess

I restored an Egli for a friend and had the crank rebuilt to original B shadow specs (balance factor and all). The thing vibrated worse than my mums toploader full of workshop jeans when running above 3500rpm!!!
I criticized myself for months for trusting someone else with the work until Rolf Minkenberg (an avid Egli rider) told me why. All to do with the frame construction.

 

[timetraveller]

I know two people who have seriously fast racing Vins, one in an Egli type frame, the other in a special. Both use a balance factor of 50% but how this figure was arrived at I do not know. Neither bike suffers from serious vibration and both are producing in the region of 100 BHP.

 

[Bill Thomas]

Hello Vince, Any idea what Rolf set the balance to on his ? Or did you change the one you did ?, My standard framed bikes have always been bad, I put it down to big pistons, Not too much of a problem when young, But bad hands with age, Not good. Cheers Bill.

 

[vince998]

Hello Vince, Any idea what Rolf set the balance to on his ? Or did you change the one you did ?, My standard framed bikes have always been bad, I put it down to big pistons, Not too much of a problem when young, But bad hands with age, Not good. Cheers Bill.

Hi Bill,
Unfortunately i´ve forgotton what Rolfs balance factor is but i´ll find out the next time i see him. The Egli i restored (EV 6) was left as it was. the vibration was OK but compared to a well st up standard vin was still excessive.
What i´ve never been able to work out (and what nobody ever tells you) is: if a crank is dynamically balanced with a balance factor with say 8:1 pistons. How can that crank still be correctly balanced with 11:1 pistons (of the same weight) when running in the engine. Surely the increased compression, larger primary (and secondary) forces, faster piston acceleration and de-acceleration rates are going to bugger it all up. Its most probably possible to mathematically calculate whats needed, but i believe much more that the values are based on experience and finding someone thats got it right (planned or luck) and copying?

 

[bmetcalf]

larger primary (and secondary) forces, faster piston acceleration and de-acceleration rates ?

How are these affected by new 11:1 pistons of the same weight? How does increased compression affect the weight being slung around in the crankcase?

 

[davidd]

When I asked Terry Prince what his balance factors were, he said 60% for twins and 74% for singles.

I have been told, but I do not know that it is true, that the Factory did not change the balance factor for higher compression pistons as the weight was not sufficient to warrant it. I assumed this only applied to street machines.

David

 

[vince998]

How are these affected by new 11:1 pistons of the same weight? How does increased compression affect the weight being slung around in the crankcase?

Try and follow my train of thought (sorry, i´m not an expert on internal engine forces )

The static or dynamic balancing will be the same for a given factor with the assembly on the balancing rig (because the mechanical weight being slung around in the crankcase is the same), but surely the forces acting per stroke on the big end in a running engine with increased compression ratio(transmitted through piston crown, gudgeon pin, conrod etc) are going to be higher (higher compression forces, hopefully more energy in the "bang", etc)?

With an engine running on a neutral throttle, i assume the vibration characteristics will be very similar as on an engine with lower compression pistons (The rotating and oscillating masses are identical), but when accelerating (and you´re hopefully getting the max out of your 11:1 pistons), surely the increased forces caused by increased energy requirement for compression and increased energy pulses from the power stroke are going to cause increased vibration? (warranting a different balance factor?)

Maybe the extra forces involved are small enough not to be taken into consideration (or maybe i´m babbling again).

 

[Bill Thomas]

I agree with Vince, If an engine is Soft, It's not the same, I have ridden other peoples Vins Which are very slow, But smooth, Cheers Bill.