Race tuning and the Yamaha R1 crossplane crank

In the February 4 issue of MCN Rob Mac Racing’s technicians revealed their development plans for the new Yamaha R1.

After reading the feature, an engineer (who asked not to be named but works for a company that specialises in the design and manufacture of crankshaft & connecting rods), offered more insight into the process of tuning the new long-bang engine:

“I noticed the mention of the new crankshaft not needing balancing, because last year’s crank came back from the balancer untouched.

“As the 2008 crank was ‘flat plane’ (and I would expect balanced to a good tolerance at the factory) this doesn’t surprise me.

Assuming that the new design [cross plane] crank being used is taken from a standard road engine, then it will need balancing to match the masses of the lighter race-spec con-rod and piston, as the crank & balance shaft effectively work together as a pair of counter-rotating balance shafts that are designed to compensate for the forces generated by the motion of the rod and piston.

The balance shaft should also be re-balanced for the new masses.

The counterweights on the 2009 crank will more than likely be designed to balance out the primary couple produced by 100% of the rotating and 50% of the reciprocating masses of the rods and pistons used in the engine.

The balance shaft is likely to be designed to balance out the couple produced by 50% of the reciprocating masses.

If the crank is not balanced to the new masses then the counterweights will effectively be too heavy for the new rod and piston and will therefore apply (significantly) higher loads to the main bearings, in particular mains one & five.

The additional loads on the mains will increase friction (and therefore reduce power output) and will likely lead to increased wear on the bearings causing a reduction in their life and potentially lead to a bearing failure. Levels of engine vibration will also increase if the crank balancer aren’t balanced to suit the new masses.

The balancing of these types of cranks is slightly more involved than ‘flat plane’ cranks, requiring ring weights of the correct mass to be attached to each of the big ends prior to balancing.”