Shimming Woes

[The Prophet of Doom]

I haven't done the math

(28 / 21) x (42 / 28) = 2

[Rikugun]

I'm aware of the other gears but only needed the top sprocket count to calculate lobe center shift per tooth.

[injuhneer]

Has anyone noticed that the angle of the cams is different front & rear? (back onto the 550 now)

Obviously on the front they both point in at TDC, and rear they both point out - that's because they are the other way around.  But the actual angle is different (front lobes stick up more at TDC) so the rear will push the valves down earlier in the cycle than the front.

My assumption would be that this makes one cylinder stronger at a particular rev range, and so broadening the power band to make it more rider friendly?

Any thoughts on this? Could judicious cam re-timing get a free HP boost over a tighter rev range?

BTW I'm also inverting one cylinders cams 180 degrees (aka twingle, twumper, big-bang, close firing V)

The reason is far simpler. It is a V twin. That means all events are spaced by the angle of the V. Rear cylinder TDC and front cylinder TDC are separated by the V angle. Therefore all timing events are separated by that same angle in their respective cycles.

These pistons do not reach TDC simultaneously like a twingle or a flat plane crank engine.

[The Prophet of Doom]

The reason is far simpler. It is a V twin. That means all events are spaced by the angle of the V. Rear cylinder TDC and front cylinder TDC are separated by the V angle. Therefore all timing events are separated by that same angle in their respective cycles.

These pistons do not reach TDC simultaneously like a twingle or a flat plane crank engine.

I'm not disputing this, but can't work it out in my head.  where is my thinking going wrong?


There are two TDC marks.  One for the front, one for the rear.  The picture shows the index marks straight upward, but in reality this never happens.  The front index is upwards when the flywheel is at the front TDC mark, the rear when it's at the rear TDC mark.  Each cam's indexing mark corresponds to TDC for that individual cylinder, so they are already 70 degrees out of phase. 


In the picture, the pins (which correspond to the lobes) are splayed differently so the degree before TDC for that cylinder must be different, and the rotation degrees between intake opening and exhaust opening also different.  N'est ce pas?


My engine is in tiny wee pieces, but I think I need to take some accurate measurements as I put it back together again.

[injuhneer]

It would help to have a chart of timing events. I haven't found one yet.

The rear head is reversed in relation to the front of the engine. That means the relative rotation of the cams is opposite. The timing of cam events is the same. If you look at the cams independently then the rearmost exhaust cam @290* is in the same position (pointing rearward) as the front-most exhaust cam was @290* before. The same is true of the intake cam. At 0* the front intake cam is pointing forward. At 290* the rear intake cam is pointing forward.

Does that help? Look at the cams individually rather than pairs (I & I, E & E) and you will see it. Same orientation only 290* (or 70* depending on your perspective) apart in cycle.