Topic: Twins - a question for the crankshaft experts...?

[lordmoonpie]

I've been doing some thinking and planning for my next project. It's not got a lot of shape to it so don't go asking me "which model" here - I haven't made my mind up! 

However, I got to thinking about crankshafts as you do when the wife is nagging about something I should've done but I haven't and I hit upon a question that I really can't figure out.

If I take a crankshaft out of a 90 degree vee twin engine, would it be different in design if it had come out of a 60 degree vee twin. I can't figure out whether there would need to be any difference just because of the angle of the V or would they essentially be both the same......any ideas? 

[bryanj]

Depends on wether it has a "forked" rod or runs rods side by side.

Balance factor will be way different but should look similar

[lordmoonpie]

Depends on wether it has a "forked" rod or runs rods side by side.

Balance factor will be way different but should look similar

The rods are side by side. I figured that if I got it dynamically balanced that should be enough to allow me to run a 90 degree crank in 60 degree cases. I can't see of any differences other than this. Do you reckon it'd work?! 

[crazypj]

Yes and no.
 Depends on crank configuration.
 If rods share a common pin, same crank.
offset pins (various Honda V-twins) have narrow cylinder angle but fire as though they were 90 degree v-twins because of offset. they also share rocking couple because the flywheel between the pins make crank wider than a shared pin motor (HD style)
 This hasn't come up for a while, the site has developed and has a lot of new stuff.
http://www.dinamoto.it/
 go to online papers at top tab, shaking forces of twin cylinder engines with Union Jack (British) flag (unless you want Italian?) The four cylinder animations no longer work, they are now static drawings showing vibration modes and rocking couples
PJ

[lordmoonpie]

Cool - nice one PJ> I'm thinking of using a Ducati crank so that'll be a common pin then so I guess I should be OK. All still pipe dreams at present but nothing wrong with doing the research. Thanks for the link too - Ill check it out when I get home!