Light Aircraft Association

In the April magazine the article on engines is interesting but in the section on 90 deg V twins it says a heavy flywheel is needed for the primary and secondary imbalance. As an enthusiast of Italian motorcycles I thought the advantage of a single crank pin 90 deg V twin is perfect primary balance. Am I wrong in thinking this?

This is at the expense of secondary balance but a heavy flywheel is not needed so you get a responsive motor. And as the article says, a downside is the uneven firing intervals, as seen by the rate at which my Ducati stretches chains!

I'm glad somebody else has raised this.

My understanding agrees with yours in that a 90 degree V twin has perfect primary balance.

It's the 60 degree ones like the Aprilia Mille engine that require a balance shaft.
Harley Ds (air cooled ones) with their 55 degree V just rattle themselves to bits.

And I am glad someone else thinks the same.

Has anyone ever done anything to improve the secondary balance? As it is a force at 90deg to a line bisecting the cylinders a rotating balance shaft does not seem suitable.

Alan George wrote:Has anyone ever done anything to improve the secondary balance? As it is a force at 90deg to a line bisecting the cylinders a rotating balance shaft does not seem suitable.

In theory it's possible to use a forked con rod that allows both cylinders to sit on the same plane and thus eliminate the secondary ("rocking couple" I believe) balance problem. For some reason Harley engines (I believe) use this arrangement but I'm not an engineer enough to decide if this actually helps balance or has been done for other reasons (e.g. will make the engine slightly narrower across the bike).

Apparently, these are also known as "fork and blade" con rods, an example is here http://www.tpub.com/engine3/en3-51.htm. I'm pretty sure that the Merlin used this arrangement which is why the two banks line up.

This technique is used to keep the engine narrow or shorter ie multis. The rocking couple is reduced as the plane width is reduced. BMW twins rock like mad at tickover especially if the carbs are unevenly set up, the engine trying to twist laterally. all boxer engines want do this but fours are cancelling the forces out.
Graham

As I understand it the rocking couple and the secondary imbalance I mentioned are 2 different things. The rocking couple is as described above, especially noticeable on a BMW boxer twin.

On a 90 deg v-twin their is a rocking couple but the secondary imbalance mainly comes from the 2 pistons and the connecting rods. The CG of this system moves along a line on a Ducati from under your backside to the ground near the front wheel.

Thanks for your comments, Alan.