A bit more on balancing an RD-350 YPVS crankshaft
In the previous blog post, we tried to reason why we decided to try and re-balance the crankshaft on our RD-350 YPVS engine. In this post, we'll illustrate the process we followed, our findings, and the results of the computer simulations. In a subsequent post, we'll show the reworked crankshaft, and final results from actually running the engine.
So, the first thing we did is measure the actual balance factor of the standard crank. Well, actually it was a "standard" Hotrod +4 mm stroke crankshaft. We didn't bother measuring a stock RD-350 crank, but wouldn't expect it to be far off the Hotrod one in terms balance factor as the flywheels are virtually identical. We found the required weight to balance the crank to be 187 grams. Now, by weighing the reciprocating components, we the following actual balance factors:
| Piston | Weight of piston, rings, small end bearing gudgeon pin, circlips | Resulting balance factor |
| TZ-750 66.35 mm | 265 g | 15.6 % |
| Mitaka 66.5 mm | 288.5 g | 14.6 % |
| Vertex 66.5 mm | 267.6 g | 15.5 % |
As you can see, the balance factor is not even close to the commonly accepted range of 50-60% for 180 degree, parallel twins. It is now wonder that they require the two tie-rod underneath the engine to keep the vibrations at bay. If you look at an RD-400 crank from the previous generation, water-cooled engines, they have lightening holes and a lead counterweight in an attempt to balance the crank. No such attempt seems to have been made on the RD-350 crank. We heard it said that this was due to cost constraints at Yamaha. How true this, we have no idea, of course. Surely, the addition of the tie-rods had a cost as well.
No matter, though, it was clear we wanted to improve on this. Thankfully, Tony Foale has a free crankshaft balancing calculator on this website, here: Freeware from Tony Foale. There isn't an ideal balance factor, and what you eventually choose depends on many factors including frame stiffness, engine mounting, forward tilt angle of the cylinder, etc. However, we went with a fairly conventional 57% based on the lighter weight Vertex pistons we were planning on using. This should be a considerable improvement over the standard one and the out-of-balance forces, according to Tony's calculator are shown below. The plot on the left shows the primary and secondary forces from the 15% balance factor and the one on the right from a 57% balance factor. You can see that the peak force is considerably lower with the 57% balance factor as well as the uniformity of the forces.
In order to achieve the higher balance factor meant meant either adding mass in the counter weight area of the flywheels, or removing mass around the small end pin. To try and predict the best way to accomplish this, we decided to perform a bit of computer modelling. So, we first created a model for the entire crankshaft assembly including conrods with bob weights representing the pistons. And, a second model of just one half of the crankshaft, i.e. a single cylinder. Since this is a 180 degree, parallel twin engine, it is possible simplify the computer simulation to a single cylinder.
Here, first is the entire crankshaft assembly with bob weights replacing the piston, rings, gudgeon pin, and circlips.
We then started to remove material (via the computer modelling) around the small end pin. It quickly became evident that this wasn't going the enough to achieve the balance factor. We also tried a hollow small end pin, which still wasn't enough. So, we needed to consider adding more mass to the counter balance weights, i.e. opposite the small end pin. We opted to use Tungsten dowels rather than lead as it has higher density. The following picture shows the fully balanced assembly with the Tungsten slugs clearly visible.
So, with the balancing requirements worked out, we needed to get the holes cut in the flywheels. In order to create a suitable press fit, we opted for a wire erosion process. We are now awaiting these to come back from the local machine shop. We post the outcome and verification of the balancing in the next post. Stay tuned!