| Author | Message | ||
     Spatten1 |
Kevin Cameron wrote: plain-bearing rods are placed side-by-side on the crankpin, automotive fashion. Does anyone else run both rods on the same crankpin? Will this give the 1125 engine more Harley character? Does it make the engine much slimmer? How does it affect peak HP? Can it ultimately limit RPM compared to a more balanced configuration? Any other thoughts? | ||
| Hughlysses |
AFAIK, almost all other V-twins run a common crankpin (Ducati, Aprilia, Rotax, Suzuki, etc.). The only exceptions I know of are Honda V-twins (including my old Ascot and their modern cruisers) that have something close to a 45 degree V cylinder arrangement with a split crankpin which changes the effective angle between the cylinders and improves balance. I think the one-piece forged crankshaft it allows is a big advance over the pressed-together two or three piece arrangement required by the HD knife-and-fork connecting rod setup. | ||
| Diablobrian |
if the rods are set side by side on the crank it causes a condition called rocking couple because the power is pushing down into the crank on different centers. hence the third balance shaft in the 1125r to cancel out that effect. | ||
| Spatten1 |
close to a 45 degree V cylinder arrangement with a split crankpin which changes the effective angle between the cylinders and improves balance. That is exactly what I was wondering about, if other manufacturers staggered the crankpins in different parts of the crank rotation to balance out the firing order and quell vibration. If no one else does that, I guess having both cylinders on the same degree of crank rotation must be a better engineered design. I wonder what the advantages are? | ||
| Hughlysses |
If no one else does that, I guess having both cylinders on the same degree of crank rotation must be a better engineered design. I wonder what the advantages are? I'd say the main one is it's the easiest way to make the lightest, stiffest possible crankshaft. The split crank used in Hondas is mainly a way to have the engine look like a Harley while having better engine balance. AFAIK they've never used one on a true high-performance motorcycle. | ||
| Hughlysses |
Scott, Here's a page with animations of various 2-cylinder engine configurations: http://www.dinamoto.it/DINAMOTO/on-line%20papers/t win%20motors/twin.html The first animation shown is a Suzuki VX800 which is just like the Hondas I mentioned (45 degree V with split crank throw). It includes various other arrangements and shows the out-of-balance forces for each. Pretty cool. | ||
| Spatten1 |
That is the coolest thing I've ever seen, thanks. | ||
| Hughlysses |
Glad you liked the site. There are lots of other cool motorcycle engineering features at that site. | ||
| Diablobrian |
That was a SUPERB link! | ||
| Blake |
No 45o H-D/Buell engine?  | ||
| Hughlysses |
Yea, that is kind of a glaring omission. | ||
| Anonymous |
This website couldn't do the 1125 72 right because they are only two dimensional. They don't address the rocking couple. Cool little primer though. | ||
| Ducxl |
rocking couple. Huh??? Explain...please? | ||
| M1combat |
The rocking couple is a force generated by having one connecting rod beside the other. As one piston changes direction it puts force into a place on the crank that isn't the center. It "rocks". This generates a "rocking couple". Take a pen in your thumb and index finger... now move your thumb side to side along with the length of the pen... You're thumb isn't generating the rocking couple but imagine that there are conrods on either end and you'll probably get the idea. | ||
| Hughlysses |
Another way to help you visualize a rocking couple is look at the engine from above. An air-cooled Harley or Buell (wow, we have to clarify that now, don't we?) with it's knife-and-fork connecting rod arrangement has no rocking couple because both pistons and rods are perfectly in line with one another and the balancing weights on the crankshaft are perfectly symmetrical as viewed from above. The engine will shake back-and-forth and up-and-down but all the forces are in a straight line. On V-twins with the connecting rods beside one another you can see that things aren't symmetrical as viewed from above. One rod, piston and cylinder is slightly offset from the other. The forces produced when the parts move aren't all in line with one another. The engine tries to "twist" back and forth as viewed from above. | ||
| Blake |
Rockin'! ![: ]](http://www.badweatherbikers.com/buell/clipart/proud.gif) |
