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After the rush of the holidays and new year I was able to get more time in on the engine.  I did some long block assembly tests to check fit of all the reciprocating parts. All the parts fit properly and go up and down as the crank goes round and round.

Main bearing/conrod journal assembly

The 180 degree crankshaft was chosen to minimize the fluctuations in reflected crankshaft inertia torque.  This is similar to the idea used in Yamaha’s M1 GP bike and R1 street bike.  Normal I4s have a flat crank which means that when the outer 2 pistons are at TDC the inner 2 pistons are at BDC.  This results in the reflected torque of the entire piston/conrod assembly varying greatly.  At high engine speeds variation can be up to 20 times the combustion torque.  Because this variation is both positive and negative this variation of course does not add to the engine’s output torque. What is does do is overlay a large amplitude noise signal over the small amplitude torque signal, muddying the rider’s connection between the throttle and rear tire.  This Youtube video from Yamaha does a good job explaining the difference in the 2 configurations.  At around 2:30 the details of the inertia torque variations are explained.  Yamaha’s Furusawa wrote a technical paper analyzing the signal to noise ration of various crank configurations which backs this up.

Where I am departing from the status quo is my choice for a screamer firing order.  I am proposing that the minimized crankshaft inertia torque variations is the real reason that Ducati twins seem so much more driveable than the other I4s in WSB.  Single crankpin vee twins by default have a much better crank inertia situation than the normal I4 because of the piston/cylinder bank angle relationship.  I think the whole ‘big bang’ theory is rubbish and therefore am trying to get the best of both worlds- low crank torque inertia variation for improved driveability but a screamer firing order for maximum power potential.  There is a small penalty of having to add counterweights to the crank to reduce the rocking moment produced by the 180 degree crank arrangement but I think the benefits will far outweigh the costs.

You can see the details of this in the following 2 pictures:

The front bank cylinders are at TDC and BDC while…………….

The rear bank cylinders are both at midstroke.   This configuration results in a minimal variation in crank inertial torque and I hope a clean signal from the rider’s wrist to the crank.

You can see the interplay of the pistons in this short video:

Next up is checking out the camchain runs and doing a longblock assembly test with the cylinder heads and cams.  Once this is done I can send both crankcase top parts to Millennium Technologies for nikasil plating and final honing.  While the parts are being plated and honed I have plenty of smaller parts to make, various shafts and both engine side covers.

That’s all for now.

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  • Emmet

    very impressive! Can you elaborate on why you think the vtwin Big Bang Theory is garbage?

  • coseng

    Thanks. It feels nice to have an assembly that moves as opposed to a pile of parts.

    On the big bang topic, I am extrapolating from the sometimes vague comments posted online from tire manufacturer techs. It seems to be a universal consensus from them that you can't tell what bike a given used race tire came off of. They can usually tell what rider was using it (at the very highest levels) but that a V-twin and I4 don't leave markedly different wear patterns. I think if big bang was effective at managing traction on the order of 2-4" of tire circumference, which is the approx distance between firings at speed, there would be visible artifacts in the wear patterns. This feeling is reinforced with the use of traction control systems. These work on reducing power to individual cylinders on the same 2-4" of tire travel yet we still have talk about big bang. If a traction control system has the same effect big bang is supposed to have then there would not be a difference between a big bang engine and a even firing order with traction control. Yet we still hear how 'big bang' gives more feel. Maybe it is not 'big bang' but another inherent property of a v-twin, which happens to be consistent crankshaft inertia torque. Yamaha's paper on this gives a clear technical explanation for the effect, greatly improved combustion torque signal to inertia torque noise ratio, something that big bang theory has yet to provide.

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