TDIMeister
Phd of TDIClub Enthusiast, Moderator at Large
The above begs clarification on a number of thoughts, areas of potential confusion and to preempt potential finger wagging.
1. The above calculation is not to say that the I-beam rod calculated will be ~1.8X "stiffer" or "stronger" or "better" than an H-beam rod. In practise, one would (should) never design an H-beam rod cross section to the same envelope dimensions as an I-beam variant; within the constraints of available space and other limitations, one would adjust the aspect ratio (width vs. breadth) and thicknesses to give the desired Ix/Iy -- if these are even considered at all... An I-beam can certainly be designed so that Iy is closer to Ix with some compromise of the latter and, in the same vein, an H-beam can be designed for maximum values of either Ix or Iy to be higher than an I-beam; the only limitation is how heavy of a part will you tolerate and will your rod interfere with something in crankcase...
2. The calculations done to compare the I- and H-beams themselves are not to be taken as strict gospel and are not themselves the final calculations for the strength of a structure -- they are only to infer quick and dirty insights and trends. If someone took those results as gospel, you can simply rotate each cross section by 90 degrees and it wouldn't make any difference in the calculation results; we know this intuitively to be false - the distinct orientation of the "stiffer" or "less stiff" side makes a definite difference in conrod design.
3. The "best" conrod design may not be a uniform I- or H- cross section after all but some funky geometry. Or maybe an I is best on one part of the rod but an H is better on another so there would need to be a freeform blend to change the cross-section shape as you go from the small-end of the rod to the big-end... Or furthermore the optimal geometry could be a hybrid between an I- and H- or a X-shape. All the while, the envelope breadth, width and material thicknesses could change as well. However, this gets very $$$ to manufacture so it won't be done unless someone has deep pockets (and the need) like in top-flight motorsports, e.g. F1, WEC, etc.
4. There are many other considerations to conrod design than just beam cross section. Equally important, if not more, is the distortion at the small- and big-end bores under the loadings encountered in an engine. These minute distortions can "pinch" the bores or otherwise cause them to go out-of-round and disrupt a proper oil film, which could lead to spun and seized bearings. The cross-section near both ends and the webbing will have a big influence on this. In this same subject, more and more OEMs are using "cracked end caps" to make sure that the rod big end meets diameter and cylindricity specs when both pieces are joined together and don't get mismatched with another rod/cap pair or the caps installed in the wrong direction. Sawtooth joints are one way to achieve a similar objective and is more common for the aftermarket. Even simply tightening the big-end bolts will deform the bore in the assembled condition and steps must be taken to predict and manage this.
1. The above calculation is not to say that the I-beam rod calculated will be ~1.8X "stiffer" or "stronger" or "better" than an H-beam rod. In practise, one would (should) never design an H-beam rod cross section to the same envelope dimensions as an I-beam variant; within the constraints of available space and other limitations, one would adjust the aspect ratio (width vs. breadth) and thicknesses to give the desired Ix/Iy -- if these are even considered at all... An I-beam can certainly be designed so that Iy is closer to Ix with some compromise of the latter and, in the same vein, an H-beam can be designed for maximum values of either Ix or Iy to be higher than an I-beam; the only limitation is how heavy of a part will you tolerate and will your rod interfere with something in crankcase...
2. The calculations done to compare the I- and H-beams themselves are not to be taken as strict gospel and are not themselves the final calculations for the strength of a structure -- they are only to infer quick and dirty insights and trends. If someone took those results as gospel, you can simply rotate each cross section by 90 degrees and it wouldn't make any difference in the calculation results; we know this intuitively to be false - the distinct orientation of the "stiffer" or "less stiff" side makes a definite difference in conrod design.
3. The "best" conrod design may not be a uniform I- or H- cross section after all but some funky geometry. Or maybe an I is best on one part of the rod but an H is better on another so there would need to be a freeform blend to change the cross-section shape as you go from the small-end of the rod to the big-end... Or furthermore the optimal geometry could be a hybrid between an I- and H- or a X-shape. All the while, the envelope breadth, width and material thicknesses could change as well. However, this gets very $$$ to manufacture so it won't be done unless someone has deep pockets (and the need) like in top-flight motorsports, e.g. F1, WEC, etc.
4. There are many other considerations to conrod design than just beam cross section. Equally important, if not more, is the distortion at the small- and big-end bores under the loadings encountered in an engine. These minute distortions can "pinch" the bores or otherwise cause them to go out-of-round and disrupt a proper oil film, which could lead to spun and seized bearings. The cross-section near both ends and the webbing will have a big influence on this. In this same subject, more and more OEMs are using "cracked end caps" to make sure that the rod big end meets diameter and cylindricity specs when both pieces are joined together and don't get mismatched with another rod/cap pair or the caps installed in the wrong direction. Sawtooth joints are one way to achieve a similar objective and is more common for the aftermarket. Even simply tightening the big-end bolts will deform the bore in the assembled condition and steps must be taken to predict and manage this.
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