Issue link: https://read.dmtmag.com/i/693447
71 JULY 2016 www.thunderpress.net THUNDER PRESS hell does it mean? Let's take a quick look at options for rods, then get back to stock ones. Tensile Strength/Fatigue Failure For one thing, basically there are three materials to choose from, each with advantages and drawbacks. To oversimplify: OK, there's obviously a point of confusion regarding this bit about durability versus fatigue. Titanium is most durable, which you could take to mean "able to withstand abuse," but a short fatigue life implies it won't take abuse for long. Aluminum handles shock loads very well and will last lon- ger than you might think, unless you abuse it. Steel, in all its forms, oper- ates best between the special extremes of the other two materials. In other words, pretty good stuff after all for most stock, high-mileage propositions. It strikes a good balance between strength and longevity. But, what's that bit about "huge opportunity to improve design" really mean? For that matter how does "fatigue" play into things? Seems to me, "stronger" might even be a vague, useless term, as applied to connecting rods. On the other hand, a lot depends on the concept of tensile strength. "Tensile" means "the capacity of a material or structure to withstand loads tending to elongate elasticity." Now we are getting somewhere! Thing is, that very notion of elasticity is where we often go wrong, because it's most often advertised in terms of the maximum rating. Rods have to stretch (and snap back) to cope with the kinds of stresses even a moderately ridden street bike affords. But, what the rod can handle under "laboratory" condi- tions and what will happen in service are two different things. Yield strength might be the more critical of the specifi cations when it comes to balancing how strong a rod can be absolutely relative to how strong it will likely be for how long. But strength, no matter how measured, changes with time, temperature and use. Fatigue is essentially what that amounts to. Connecting rods get tired! The term most metallurgists like to use for that is "endurance limit" or EL. Another vague criteria comprised of several factors applied to "lab value" of the material to get a rough (real rough sometimes) idea of the limit the useful life of the part. They are as follows: Surface Condition: such as: pol- ished, ground, machined, as-forged, corroded, etc. (Note: Surface condition is perhaps the most important infl uence on fatigue life. Don't nick the rod!) Size: This factor accounts for changes, which occur when the actual size of the part or the cross-section dif- fers from that of the test specimens. Load: This factor accounts for differences in loading (bending, axial, torsional) between the actual part and the test specimens. Temperature: This factor accounts for reductions in fatigue life which occur when the operating temperature of the part differs from room temper- ature (the testing temperature). (Note: Heat = weak.) Reliability: This factor accounts for the "scatter" of test data. For exam- ple, an 8-percent standard deviation in the test data requires a "reliability" value of 0.868 for 95-percent reli- ability, and 0.753 for 99.9-percent reliability. (Note: Geek-tech babble for consistent quality control. You still get what you pay for!) Miscellaneous: This factor accounts for reductions from all other effects, including resid- ual stresses, corrosion, plating, metal spraying, fretting and others. (Note: All the stupid things that can happen to rods in the hands of cretins.) In E nglish what these guys are trying to say is fatigue cycles are cumulative. Suppose a part which has been in ser- vice is removed, magnafl ux/ crack-tested, and passed the inspection. That only proves that there are no detectable problems right now. There's no indication at all and no way to know for sure how many cycles remain until the rod might fail. The thing might run for decades or it could crack in the next 100 cycles of operation and fail in the next 10,000 cycles (which even at 2000 rpm, isn't very long!). Seems even the experts can't really tell you which rod to use for what or how long it will work for you. What they will tell you as fact (and one I agree with) is that almost every time, it's not the rod that breaks the engine, it's something in the engine that breaks the rod! This accounts for David's situation as we've discussed. As for mine, for the time being, I'll just leave you with this account to explain why I'm comfortable with the 45-year-old R.R. 56 Hiduminium con- necting rods in my Trident. Bert Hopwood (engineer)—"Light alloy connecting rods are primarily specifi ed for the purpose of weight reduction and consequent reduction of bearing loading due to inertia forces. Their use also tends, to some extent, to cool the gudgeon pin and piston by providing some capacity for rapid heat exchange." P. Walker (designer/engi- neer)—"R.R.56 has a ultimate strength of 29 tons per square inch, and will never fail under tensile load in an engine, but that the fatigue life of high strength aluminium is particularly affected by the ratio of stress applied to the ultimate strength, this being one of the reasons the rods are so bulky." (Read this one twice…) Doug Hele (development engineer/ race boss)—"The engine would have to be run at full revolutions for about 50 years before that fatigue condition would be reached." (So there you go!) Material Pros Cons Aluminum Flexibility in construction Less durable in some cases Low Cost Steel OEM material Huge opportunity to improve design Titanium Most durable Expensive Lightweight Short fatigue life On the other hand, one essential (and overlooked) difference between the old British twins and the triples was, for the fi rst time, the crankshaft in Tridents was fl ex-free, displacement (by today's standards) moderate, piston speeds (and stroke) well under safe limits and oiling copious and thorough. The combination of a very free-revving pushrod valve train and a solid bottom end meant racing triples were good for 8800–9000 rpm with stock R.R. 56 rods like these. The above illustration speaks for itself and of a time not that long ago! To say there are many reasons why a certain design, made of a certain material, would best suit a certain engine design is to under- state the case considerably! Harley engines throw big pistons a long ways under serious thermal con- ditions and with less-than-optimal lubrication. The unique "knife and fork" rod design with no cap, no less, has served well for generations. (Ultimate Tensile Strength and Yield Tensile Strength) Aluminum rods (7075) = @ 83,000 psi UTS max/73,000 psi yield strength Powdered Metal rods = @ 120,000 psi UTS max/75,000 psi yield strength Titanium (Grd5) rods = @ 145,000 psi UTS max/141,000 psi yield strength Steel (4340) rods = @ 140,000 psi UTS max/68,000 psi yield strength