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Motorhead Memo: Rock ’n’ Rods

By Kip Woodring

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Funny (as in odd) that just a few weeks ago, I would get—almost simultaneously— a comment about a column I wrote three years back about TC crank “scissoring,” and a “restoration project” in the form of a 1974 Triumph Trident T150V. Trains of thought erupt. Discuss?

Well, let’s take them kind of in order, starting with the comment:

David Lindsey on April 30, 2016 6:37 p.m. wrote:
“My 2003 FXDL has a manual primary chain adjuster. It started puking oil into the breather after 40,000 miles. It had cam chain tensioners changed at 28,000 miles worn but still together. Crank is scissored out of spec. Oil pump damaged, not scavenging oil from crank and cam cavity causing excessive oil carried through breather. Cam plate damaged, pinion bushing damaged. The automatic primary tensioner in a 2007 Ultra Glide has taken transmission main shaft bearing and seal, rear primary seal, clutch and crank seal and bearing out at 37,000 miles of a lady’s bike I’m repairing. These same problems exist in the new Twin Cams. Don’t fix what isn’t broken but this has been established as bad design.”

It might sound a little cold… but I wish David had taken some sound advice a while ago. It might very well have kept him from having to write that comment, to say nothing of potentially preventing his engine issues. The advice? Don’t fool around with the older cam plate, oil pump, etc. arrangement on Twinkies and hope for the best. Purchase and install the H-D upgrade kit #25284-11, along with all the other stuff required mentioned in the fine print (#25285-08 spacer kit, #17045-99D cam service kit, #25566-06 drive gear retention kit)! Is he right in saying the stock arrangement for his bike is “established as bad design?” Probably, but he’s been out using it and knew that, all along. So, no sense lamenting after it bit him. Better to deal with it head on, before there’s trouble. Fix what isn’t broken before it breaks if you damn well see it coming!

OEM Harley rods are I-beam style. You can see one popular variation (H-beam) in the above pic, but there are more. Besides materials, lubrication, heat, loads, stresses and basic engine design, this is an important aspect of connecting rod design. In the end, the key is to balance all these elements with a design that stays happy in its intended work for as long as possible.

OEM Harley rods are I-beam style. You can see one popular variation (H-beam) in the above pic, but there are more. Besides materials, lubrication, heat, loads, stresses and basic engine design, this is an important aspect of connecting rod design. In the end, the key is to balance all these elements with a design that stays happy in its intended work for as long as possible.

Some things. First, I think it’s fair to say that the list of troubles his bike is afflicted with, culminating with a scissored crank, comes down to (and originates with) oiling issues, heat-generated stress on the rods and metal fatigue (probably including an oval-ed big end on the rod). Second, the SE upgrade kit deals effectively with preventing all that! Most especially improved oiling. Do not underestimate how important that is! Lastly, it dawns on me, I’m every bit as much a procrastinating cheapskate as he is, or at least I might be. I say that because, while he debated until his motorcycle made the decision for him, I’m doing the same thing as this is written. And it all comes down to connecting rods! Which brings us to a brief rehash of technical stuff regarding H-D oiling and my new, old British Triple. Both machines could be described as archaic in their respective approaches to these critical issues. H-D with its knife and fork rods and low-pressure oiling and the Brit sled with its… well… let’s just get on with it, shall we?

Making the connection

We might as well start with some basics; late-model (2007–on) Harleys use (what I believe to be) a type of (steel) powdered metal rod, with no big-end thrust washers and no small-end bushing. There needs to be that all-important microscopically thin film of oil in there somewhere, doesn’t there? Said lubricant is dispersed (we hope) via a notoriously low-pressured system. All of which is in most peril of malfunction or failure at two points in the engine “use” cycle—idling and wide-friggin’ open! (The fact that the oiling system is better than it was on David’s 2003 model facilitates use of the new-type rods but doesn’t obviate certain realities.) At one extreme you have very little lubricant flowing to critical hot spots like the little end of the rods. At the other, you have massive stress loads on both ends of the rods, at the same time and under conditions when over-heated oil is likely to fail in its dual roles of cooling and lubricating.

In a sharp contrast of design, but working under the same conditions and constraints, we have the Trident. As some of you Anglophiles might know, Edward Turner did something insane back in 1937, when he designed his game-changing parallel twin. He used a material designed by Rolls-Royce for their aircraft program, specifically “R.R. 56,” to make connecting rods. Often erroneously called “aluminum,” these Hiduminium (for “High Duty”) connecting rods do use bolted-on caps (like cars) but the cap itself is made of steel and employs split plain bearing inserts at the big ends. Triumphs use a high-pressure oiling system. The Trident, in particular, needs over 70 lbs. of pressure to live and was the first motorcycle to be issued an oil cooler as standard equipment. In other words, it couldn’t be much more different than the Harley approach! Add to that the state-of-the-art material used in the Harley rods versus the ancient alloy used in the British rods and you have the makings of an entertaining discussion, right?

Once upon a time, this was the way to a high-performance connecting rod! DIY lightning, polishing (hopefully shot-peening) and more really did make a difference. The thing I’m fascinated with is how these rods resisted being turned into shiny pretzels, metallurgy being what it was before World War II. What is known is that these things flexed at high rpm but rarely broke, so maybe that notion of elasticity in the material is what kept them alive. Of course, back then, if you bent ’em you could bang ’em straight again!

Once upon a time, this was the way to a high-performance connecting rod! DIY lightning, polishing (hopefully shot-peening) and more really did make a difference. The thing I’m fascinated with is how these rods resisted being turned into shiny pretzels, metallurgy being what it was before World War II. What is known is that these things flexed at high rpm but rarely broke, so maybe that notion of elasticity in the material is what kept them alive. Of course, back then, if you bent ’em you could bang ’em straight again!

Stronger or longer?

The ’net is full of chat about Harley rods going oval at the big end and the aftermarket “solutions” abound, more than ready to help. Almost all of them involve “stronger” rods. Same thing, on a much smaller scale, applies to replacement rods for Triples. This situation exists because Harley owners fear material weakness in their new factory rods and Brit-wits fear old age and fatigue in theirs. Since “fear,” as the seminars teach, stands for “false expectations appearing real,” we might want a few more facts before reaching an expensive decision, in either case. Trouble is, not being metallurgists, we aren’t really qualified to separate the facts from the crap in most instances. It’s complicated! Puts me in mind of a great old Jimmy Stewart movie No Highway In The Sky. (Available on YouTube, if you haven’t seen it, or need a refresher viewing.) Stewart plays a nerdy scientist who realizes that metal fatigue is going to make the tail of his company’s latest airliner fall off! His experiments prove that, after a certain number of flight hours (1,440, I think it was) the metal will fail with no warning and no clues whatsoever! One moment the structure is sound and the next, catastrophically lethal. Connecting rods are like that, with one important difference. Connecting rods have no real known conditions for longevity and reliability! You don’t get a predictable “life” of X number of hours or dependable parameters for functional failure. Since no one knows when, or how, all you get is a nasty surprise when a rod effs up! So, we hedge our bets and cover our butts with “stronger” rods. As though that was all there was to it. But what the 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:

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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 longer than you might think, unless you abuse it. Steel, in all its forms, operates 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” conditions and what will happen in service are two different things.

(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

Yield strength might be the more critical of the specifications 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: polished, ground, machined, as-forged, corroded, etc. (Note: Surface condition is perhaps the most important influence 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 differs 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 temperature (the testing temperature). (Note: Heat = weak.)

Reliability: This factor accounts for the “scatter” of test data. For example, an 8-percent standard deviation in the test data requires a “reliability” value of 0.868 for 95-percent reliability, 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 residual stresses, corrosion, plating, metal spraying, fretting and others. (Note: All the stupid things that can happen to rods in the hands of cretins.)

The above advertisement 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 understate the case considerably! Harley engines throw big pistons a long ways under serious thermal conditions and with less-than-optimal lubrication. The unique “knife and fork" rod design with no cap, no less, has served well for generations.

The above advertisement 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 understate the case considerably! Harley engines throw big pistons a long ways under serious thermal conditions and with less-than-optimal lubrication. The unique “knife and fork” rod design with no cap, no less, has served well for generations.

In English what these guys are trying to say is fatigue cycles are cumulative. Suppose a part which has been in service is removed, magnaflux/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 connecting rods in my Trident.

On the other hand, one essential (and overlooked) difference between the old British twins and the triples was, for the first time, the crankshaft in Tridents was flex-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.

On the other hand, one essential (and overlooked) difference between the old British twins and the triples was, for the first time, the crankshaft in Tridents was flex-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.

Bert Hopwood (engineer)—”Light alloy connecting rods are primarily specified 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/engineer)—“RR56 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!)

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