Why Re-balance the Crank?
You could rightly ask why we bothered trying to re-balance the crank on a well-proven RD-350 YPVS engine. Well, it's the classic tale of one thing leads to another, I'm afraid. To recap this project, we wanted to create a hybrid utilising the venerable RD-350 YPVS engine and put into a more modern chassis. It provides the platform to a vast array of "hop up" parts courtesy to our US cousins who go wild with their Banshee quad bikes. So, using this base engine in a modern 250 sized chassis meant to us a 2-stroke bike that could be developed as we go.
The YPVS engine is fairly long, so the TZR-250 3MA chassis is a perfect combination since the 3MA frame is fairly long when compared to 250 chassis that use a V-twin engine, e.g. the 3XV. The 3MA chassis uses engine mounts at the rear and top of the engine, i.e. there is no engine cradle like on the RD-350. So, rather than adding weight, we decided to utilise the top engine mounts and suspend the engine without an underslung engine cradle. This meant making special brackets that picked up the outer cylinder base mounts and located the top mounts near the extreme left and right side, and mounting the engine much more solidly. It also meant there was no point trying to put the standard Yamaha engine tie-rods back on.
We track tested the bike in this configuration and it behaved very well indeed. It was a successful first step even though the engine setup was far from perfect. Although not excessive, the engine vibration made themselves felt. This is something we wanted to try to address in a subsequent engine rebuild. Well, that opportunity arose sooner than we expected when the right cylinder lunched its piston.
This provided us the opportunity to make a number of upgrades which are detailed in the preceding blog post. The key one for this article was the 58 mm (+4 mm) stroker . We chose the one sold under the Hotrod brand, which is a Taiwanese product.
As we said at the beginning of this post, one thing led to another...but we are where we are. Our next post will go into the analysis and engineering of the actual balancing work.
Anyone still awake?