[Mk2] [Turbo] Broken a crank? I now know why

[2tmike]

I'm not sure of the logic of some of this thread.....balancing is largely about removing vibration and this as stated, is related to fundamental frequency and harmonics due to the geometry and kinetics of the rotating parts....sure vibration induces stresses but, surely, if we're talking about cranks breaking, these stresses are way,way smaller than the ones imposed by the pistons on the crank from the combustion process...? I'm sure someone could do the maths ......probably Toyota did in the first place..)

[bobhatton]

than the ones imposed by the pistons on the crank from the combustion process...? I'm sure someone could do the maths ......probably Toyota did in the first place..)

The maths are there in post 8 showing the counter weights are 474.55g to light to balance out the weight of the piston and rod etc

[big_joe]

Is breaking a crank a common MR2 problem?

[bobhatton]

Is breaking a crank a common MR2 problem?

No it is not common at all, but there have been some.

[2tmike]

Bob,
Not wishing to turn this into an argument, I just wanted to say that 'out of balance force' is not the only or most significant factor in crank design.
The 'maths' I mean relate to the forces acting on the crank not just in achieving a balance...here an extract from an EPI article on racing engine design which you might have seen...

"FORCES IMPOSED ON A CRANKSHAFT

The obvious source of forces applied to a crankshaft is the product of combustion chamber pressure acting on the top of the piston. High-performance, normally-aspirated Spark-ignition (SI) engines can have combustion pressures in the 100-bar neighborhood (1450 psi), while contemporary high-performance Compression-Ignition (CI) engines can see combustion pressures in excess of 200 bar (2900 psi). A pressure of 100 bar acting on a 4.00 inch diameter piston wil produce a force of 18,221 pounds. A pressure of 200 bar acting on a 4.00 inch diameter piston produces a force of 36,442 pounds. That level of force exerted onto a crankshaft rod journal produces substantial bending and torsional moments and the resulting tensile, compressive and shear stresses.

However, there is another major source of forces imposed on a crankshaft, namely Piston Acceleration. The combined weight of the piston, ring package, wristpin, retainers, the conrod small end and a small amount of oil are being continuously accelerated from rest to very high velocity and back to rest twice each crankshaft revolution. Since the force it takes to accelerate an object is proportional to the weight of the object times the acceleration (as long as the mass of the object is constant), many of the significant forces exerted on those reciprocating components, as well as on the conrod beam and big-end, crankshaft, crankshaft, bearings, and engine block are directly related to piston acceleration"


clearly if an engine builder takes the engine power and torque to, say double the Toyota original design values, then it may well exceed the safety margins they built into the crank design...not to mention the limitations of material inherent in commercial production techniques....and a vibration study carried out by rotating the crank/conrods/pistons does not analyse this situation