Designed by our GP engineer Wayne Wright, these race-proven crankshafts are built with Ignition tie-ins .. other tie-ins ... BF piston mass notes Precise Crankshaft Assembly Balancing: More power, reliability, smoothness, longevity, comfort, allows lighter construction, less interference, stresses, The Bearings - roller bearing TZ type - s/cut + g'box bearings FLOATING CRANK GP FEATURES
The LC and YPVS cranks alike were only ever designed to be used in a 250cc incarnation - as befitted the Japanese 250cc learner licence laws of the time. The problem is that Yamaha simply decided to put bigger pistons/barrels on an already underbalanced crank, and then relied on the under-engine torsion bars to keep the vibes and rocking-couple forces under control. This was apparently a cheaper option than balancing the crank correctly with tungsten alloy counterweights, as per the Hoeckle TZ cranks and indeed the 'proper' RD400 crank. The out-of-balance crank scenario was then made worse when Hot Rods et al decided to make stroker cranks by simply offsetting the crankpins on what is an exact replica of the RD/RZ250 crank web, and of course this throws the balance factor out even more, and then in adding the 5mm longer rod, threw the balance out still further. We at TSS disagree with this cynical, penny-pinching approach, which basically abandons good engineering in favour of lowest-cost bodge solutions. Therefore we have had 100 cranks manufactured to our specs, that ARE correctly balanced for the 190 gram-plus pistons of 350cc+ engines. The main problem with the RD/RZ cranks is that they are made cheaply, with the webs made from all the same material. As is explained in the diagram below, there is neither room to add more material on the opposite side of the web to the crankpin because of conrod clearance issues; nor can any significant amount of material be removed from either side of the pin for fear of weakening the critical crankpin supporting area. The only answer, then, is to insert counterweights opposite the pin that are of a significantly higher density than the surrounding metal - exactly what we have done:
In the next two pics you can clearly see the tungsten alloy counterweight on the opposite side of the web to the crankpin. Naturally, the weights are welded in place.
See this diagram, it shows what I mean about adding counterweights, as well as some of the other advantageous features of TSS cranks:
We do also make stock 54mm stroke, 110mm rod YPVS/Banshee cranks, firing at 180 degrees, as per the pic below. These are appreciably less pricey than the balanced 58mm stroke TSS cranks, as nowhere near as much work goes into them, i.e. the extra steps of crank balancing via insertion of tungsten alloy counterweights are not undertaken for the stock 54mm stroke cranks, and the main bearings are standard Yamaha OEM spec 7-ball - as per stock Yamaha/ Hot Rods/ Wiseco cranks. All our cranks are guaranteed to be assembled to within 0.02mm runout.
Crankshaft Balancing: Yamaha YPVS/Banshee and Big-Bore Variants It doesn't 'feel' like the Banshee engine they were so used to. Yet the engine is properly balanced, and the crank under far less strain than before, and therefore can rev higher with greater reliability ... so why might it feel there was something odd about riding the thing? Some explanation is in order. This is an interesting story lads, so pull up a chair and pour yerself a mug of finest char. Steve's Campfire is crackling away nicely ... I don't want to lapse into too much tech-babble here; suffice to say, a crank that is 'under-balanced', such as the YPVS/Banshee crank, tends to create oscillations/vibrations in the vertical plane. This, for anyone who has ever converted an LC or YPVS 250 to a 350, is exactly the type of stronger vibrations you feel when switching from the 250 to the 350. What's happening is that the crank, which was originally intended for the smaller, lighter pistons of the 250, has effectively become more unbalanced with the addition of the heavier 350 pistons. The crank's balance factor has not been corrected for the greater loads being put upon it. Still, this unbalanced state is no great drama - unless of course revs are sustained beyond 10,000rpm - in which case the crank will be toast in no time. The truth is that if YPVS/Banshee cranks were balanced properly, they would be able to spin to far higher revs with a greater margin of reliability. But the OEM Yamaha cranks are NOT balanced anything like they should be. Very few OEM cranks are balanced with any degree of accuracy. "Awww it costs too much", they would whinge. So, short of buying a Merc or high-end BMW, your automotive cash is buying compromised cranks. It just happens that in the case of the LC350 and 350 YPVS, the crank balance error factor is compounded by the fitment of heavier pistons for which the assembly was never designed. It gets worse. It gets worse, because the typical stroker cranks used in big-bore Banshee-based engines (+4mm, +7mm stroke etc) use webs that are simply lifted from the YPVS design - which of course means more stress on the crank assy anyway, PLUS the cranks run a heavier 115mm rod rather than the stock crank's 110mm rod, AND of course a much bigger gudgeon pin AND a much heavier piston assembly. Just how bad can this out-of-balance crank scenario be? In fact, and this will shock some people, even the 250LC and 250 YPVS cranks aren't balanced anywhere near right. And since the +4mm stroker cranks only run YPVS webs with a 2mm offset pin anyway, the figures make for sobering reading.
To be exact, the static "out of balance" mass of a +4mm HR crank is 54g measured on a digital scale perpindicularly tied to the centre of the crank pin. But the total reciprocating mass of, say, a 73mm Wiseco piston assy is 342g. A crank's Balance Factor is typically expressed as a percentage figure. Normally the balance factor on a twin such as the RD is set at 50% to 55% of the total reciprocating mass - in this case 342g - so this would need 171g for the 50% case and 188g for the 55% case. The crank's balance factor, therefore, should be set at or around those figures of 171 to 188g. BUT, the actual balance factor is only 54g. Even in the case of a stock YPVS crank fitted with 250 pistons - total reciprocating weight of around 190g - 55% of that is 104.5g .... and we have only 54g. That makes for a balance factor of only 28.5%.
So, the YPVS, Banshee and aftermarket stroker cranks are hopelessly out of balance. This partly explains why these cranks go bang with sustained high-rpm running. It explains the stronger vibes felt when converting a 250 to a 350, and by extension the sometimes intolerable vibes of a stroker crank/big-bore motor, whose crankshaft is, after all, only based on the original OEM Yamaha crank webs. We are in a world of trouble. Naturally, we at The Two-Stroke Shop decided to remedy this slipshod situation, and therefore set about balancing our cranks professionally.
Please note that the Rocking-Couple Peak Forces are independent of the crank’s Balance Factor. The Balance Factor is all about balancing the crank assembly precisely to match the reciprocating components, i.e. the piston/pin/rod etc, whereas the Rocking Couple relates to, in the case of a 180° parallel twin, the violent oscillation of the engine to the left and then to the right as opposing pistons reach TDC and BDC simultaneously.
The rocking couple effects a sort of see-sawing motion to the engine, as both pistons reach the end of their travel. This is not something that can be fixed really, it is an inherent trait of 180° parallel twins. However there is a fix, even for the rocking couple. In order to combat these forces, we need to re-phase the crank firing order - and in the case of parallel twins the most effective setup is a 90° firing order. What happens with a 90° configuration is that a higher inertial momentum (flywheel effect) can be maintained about the crankshaft by having one piston always at or near maximum velocity while the other one comes to a ‘stop’ and then changes direction. The pistons are never both ‘stationary’ at the same time. Therefore a 90° layout will always give the crank an easier life, and accordingly crank life will be extended. This is particularly important for racing applications.
However, one cannot simply re-phase an RD crank to the 90° layout and retain the OEM Wasted-Spark ignition system, because this system is not able to differentiate between cylinders, and in any case it fires both cylinders simultaneously; one before TDC and the other below BDC. This is why it is called 'wasted spark' - because the system puts out twice as much spark energy as it needs. That is why we developed a new replacement 100mm stator and flywheel with two ignition triggers and one flywheel trigger bar. This setup, which runs extra charging coils for road use, has a flywheel that is half the weight of the stock Yamaha item. More importantly, we moved over to a non wasted-spark ignition setup, using a custom-designed, fully-programmable twin CDI ignition unit by Ignitech with programmable parameters for ignition timing; power-valve control; electronic power-jet control and throttle position system (TPS.) This is a twin ignition coil system that does away with the dreaded 'wasted spark' system, enabling each cylinder’s firing event to be timed independently of the other; freeing-up ignition timing options beyond the stock Yamaha 180 degree timing layout; making possible 90 degree crank firing layouts.
This brings me back to the original point of the Banshee lads saying their engine felt strange. Yes it did, because although the crank's Balance Factor had been corrected, the peak Rocking Couple forces started to become more apparent. Not horribly intrusive, but apparent. It's almost like a musical scenario whereby two drummers hammer out a tune together, and suddenly the bloke on the small drums stops drumming altogether, leaving only his mate pumping away on a big bass drum. All of a sudden, the bass drum becomes the sole point of focus. So to fix this, we need to reduce the Rocking Couple forces - and we do that by the aforementioned process of re-phasing the crank to a 90 degree firing order. This reduces peak rocking couple forces by an average of 31% - right through the rev range. That's a lot of relief from crank grief. So to recap, what we have done is to correct the crank Balance Factor, and then further carried on to ameliorate the Rocking Couple forces. These two fixes combined represent a major improvement to the running of the engine, and directly lead to: (1): A smoother-running engine at all revs, with no intrusive resonant vibes Thanks, for anyone who made it this far! Cheers, |
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Banshee/RZ350 Crankshaft, +4mm Stroke
(58mm), 180°
Banshee/RZ350 Crankshaft, Stock 
This brings us back to the YPVS/Banshee chaps who said the balanced crank setup felt unusual. It felt unusual because although the crank was correctly balanced, and consequently the forces that were previously trying to tear the cases apart had been tamed, nevertheless the eerily smooth-running motor then made apparent a different set of forces - and these forces, known as the 'Rocking Couple' were being felt.