full counterweighted crankshafts???

[Dave Koehler]

Geoff,
in this case no.

[Ken_Parkman]

I have always wondered how a sprint car can get away with no damper. No flywheel would change the frequencies, but the fundamental modes and exciting orders are still there. Hard to imaging a water pump could be that good a damper, But if it's not normal to break cranks all the time it works at least to some degree, so who am I to argue.

But description sounds like a fatigue problem, and that is usually driven by vibration in the crank (not the engine), and that is something a damper is supposed to control. So there is a fatigue exciting force greater than this crank can deal with. The different firing order can change the exciting order criticality.

Engine balance is a different topic.

Can you post a close up of the fracture? Fatigue starts a crack somewhere and it propagates until the remaining metal is not strong enough and it shears. You can see it in the failure pattern.

[Kevin Johnson]

I have always wondered how a sprint car can get away with no damper. ...

Probably because of the direct drive, so the tires and their loading characteristics on the track act as the damper. Downside is that when this changes dramatically the crank receives torsional shockloads.

[MadBill]

..Can you post a close up of the fracture? Fatigue starts a crack somewhere and it propagates until the remaining metal is not strong enough and it shears. You can see it in the failure pattern.

Usually from some tiny stress riser in a journal fillet..

[Circlotron]

For those not familiar with a sprint engine there is no crank hub or dampner, gear drive to turn cam, no flywheel just a yoke with splines that weigh about 3 or 4 pounds. titainium driveshaft etc.

If there is no flywheel, what is to stop the back end of the crank torsionally vibrating as it goes through it's resonant frequency? I'm guessing having no flywheel or for that matter a torque converter would invite all manner of problems for stuff driven off that end of the crank, as well as the crank itself.

It would be interesting to compare at what position along it's length a crank might break with no flywheel, compared with having a flywheel but no harmonic balancer.

Just thinking about it further, I get the gut feeling that if you had a big flywheel and no HB the crank would resonate at a certain fundamental frequency, with the point right next to the flywheel being the point of maximum stress but zero oscillation, and the front of the crank going nuts, but if you had nothing at either end then the halfway point of the crank might be the node point of maximum stress but zero oscillation. It would maybe resonate at twice the frequency and half the peak to peak amplitude, similar to a 4 cyl crank because of being half the effective length and so perhaps not be an issue. Just a guess on my part.

[mag2555]

How much resonance do you invision cylinder pressure might suppresses at some points in the cycles of certain cylinders?

[Rick!]

Take some really good pix of both break faces and post them up. There should be a few beech marks showing how many cycles after the crack it took to completely fail.
There is a difference between torsional bending and rotating bending that can be identified by the direction of the crack propagation.
Breaking forward of the center main means the direct drive stuff has minimal spinning mass so I'm not surprised it broke there.
The flip events may have contributed but with so little inertial mass in the drivetrain, stopping everything behind the center main and then the 4 front cylinders still wanting to twist is a remote possibility but landing outside the cushion is pretty squishy.
What RPM does this engine turn? Lightened crank or knife edged counterweights?