Connecting rod design
Connecting rod design
(OP)
Hi,
I'm looking for a good book on connecting rod design, i.e. forces, fatigue stress, optimisation, etc. Does anyone know if there are any out there? some guidance would be appreciated
Many thanks,
I'm looking for a good book on connecting rod design, i.e. forces, fatigue stress, optimisation, etc. Does anyone know if there are any out there? some guidance would be appreciated
Many thanks,





RE: Connecting rod design
MIT Press, Taylor, "internal combustion engine" I forget whether volume 1 or 2, has a section on con rod design. It's kind of an industry survey type book, last updated a few decades ago, but has some theory and examples, and usually many bibliographical references.
If you are considering PM (pressed metal) designs the SAE transactions from the 90s and 00s have a lot to say.
RE: Connecting rod design
RE: Connecting rod design
All papers I have seen describe the design and verification of standard I-beam rods. It appears no OEMs or serious design houses consider any of the common aftermarket types, like H-beam, X-beam, etc.
RE: Connecting rod design
Some successful airplanes have been built of wood. They don't look much like airplanes made of steel tubing or aluminum or XXX
There was a time when there were articles in Machine Design and Hot Rod magazine and all over the place about connecting rods and other engine components made of epoxy/whatever fiber composite materials. Google can only find an occassional dreamy patent for one now.
I bet successful "cutting edge" connecting rods made of steel or titanium could and have been made using H and I beams, and even tubular shanks (old Offenhausers).
I'm not sure the beam choice has one answer, or that we really get to see what is currently in vogue, or even that con rod beam design is being scrutinized as a frontier limiting engine design in any class of racing.
I happened to be in a position to MagnaFlux inspect lots of car and bike connecting rods in the period 1973 thru ~1990. Some vintage designs, and some genuine race engines derived from production cars-n-bikes and also some purpose built, but not Formula 1.
Cracks (indications!) generally appeared in the transition between beam and wrist pin eye, and especially in the area around bolts at the big end transition. In the rare case when a crack developed in the beam itself, there was always a significant excuse to initiate in the form of a stress riser. Might have been a knick, scratch, stamped number (ouch), filed notch, or once in a great while an original manufacturing flaw like a forging seam or lap.
My conclusion is the beam shape with "best" bending strength should and does lose to the shape that permits the "best" (lowest stress, most manufacturable) transition to pin and big ends.
In 2000 Ferrari may have chosen an I-beam derivative for reasons of their own.
http://w
But here's a fairly serious engine (Cosworth TJ V10 ca 2003) with a sort-of H-beam con rod.
h
RE: Connecting rod design
From what I've read, rod design begins with Euler column theory for pinned columns and works from the pinned column's 4 times the bending moment in the plane of rotation vs the axial plane to arrive at an I-beam with its strong axis toward the plane of rotation. To vary from that (except for hollow beams) would seem to inevitably result in a rod with inferior strength to weight. As you say, there are few racing applications where extra weight in the rod is a critical disadvantage, but F1 is one of those areas. Consequently, you see only I-beam rods in F1 (except that some I-beam rods have been made with H-beam like ribs, including Ferrari rods -later than the one you show, as well as the Cosworth rod you show).
However, in the FEA stress analysis images I have seen, the stress distribution in I-beam rods looks complicated where the big and little ends join the beam. Stress in the same areas in an H-beam rod looks a lot simpler as you might guess from the smooth shape.
I noticed that CF Taylor talks about stress risers as one reason to resort to the H-beam, but none of the aftermarket makers talk about it nor anybody else.
The other popular beam type is the X-beam, which seems to have nothing to recommend it vs the I-beam, but which is probably stronger in the plane of rotation than the H-beam.
RE: Connecting rod design
ht
Modern pistons don't have much skirt and length on the sides, so I wonder how much lateral rod guidance they can offer.
It's a little hard to see how spindly the I-beam is, and how stout the big-end is on stock Honda RSX rods.
http://
RE: Connecting rod design
I know first hand the I beams in boosted OEM Honda D series engines tend to bend in compression at around the 220ish hp area. They don't break, just bend.
The same rods when used in a short stroke high rpm NA engine tend to fatigue and break as the failure mode.
I think a lot of favour was placed with H beams with the advent of the Carrillo brand rods which became very famous for increased durability at much higher loads than OEM rods. Initially virtually all serious after market racing duty rods where Carrillo or Carrillo copies and it was widely believed that the H beam design was the reason for their superiority where in fact it was more just extra metal where required, precision of manufacture and the use of superior materials and treatments.
I was always of the unsubstantiated belief that the reason Carrillo chose H beam and OEM chose I beam is that in the day and age, I beam suited manufacture by forging and H beam suited manufacture by milling and that rather than strength or durability inherent in either design was the reasons behind the different choices.
Regards
Pat
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RE: Connecting rod design
RE: Connecting rod design
I also suspect that Fred Carrillo believed the H-beam form was more trouble free as a design compared to the complexity of the I-beam. All one had to do was to look at the wide variety of I-beam designs to realize that the engineering was ambiguous. Carrillo made a rod that was much stronger than OEM rods and was of superb quality. I believe the H-beam's reputation was made by this and its inherent freedom from fatigue failure associated with stress risers. This last point, reliability, is essential.
H-beam rods are probably a safer choice for racers except when compared only to I-beam rods from the very top makers, like Pankl and Carrillo themselves or the OEMs when they offer a truly strong rod.
In fact, it is telling to see that Carrillo recommends their light I-beam rods (called A-beam, a marketing name) for high RPM applications where weight is critical. One would expect they would sell you a light weight H-beam, but by recommending the I-beam they are confirming that the I-beam is stronger by weight.
RE: Connecting rod design
http://www.inamar.cl/ingles/e_levante.html
The problem I see is, you never seem to see an I beam rod, formed similar to the H beam rod, that is viewing it from front or back inline with the crankshaft centerline. The I beam rods are always much narrower than an H beam rod. If designed correctly the I beam rod is much stronger than an H beam rod.
RE: Connecting rod design
Where rod choice is free, one heck of a lot of high budget endurance races have been won with H beam rods
Regards
Pat
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RE: Connecting rod design
For OEMs using H-beam rods, I know of two examples. Ford bought an off-the-shelf Crower H-beam rod for one of their turbo engines a few years back. Audi used an H-beam rod in their recent (low rpm) turbo-diesel LMP car. This rod is steel with a steel piston. Also, the piston lacks a skirt section in the axial direction.
Furthermore, H-beam rods were once common in F1 before the extreme RPM era. According to Ilmore's Mario Illian, the perfection of pneumatic valve springs around 1994 took valves out of the equation as an rpm limiter in F1 and made pistons (and rods) the the power limiting parts. According to BMW's Dr. Theissen, H-beams are "...much too heavy...", so the preference has been for "...I rather than H...", according to Illian.
But, in classes of racing where metal valve springs are used, the weight of H-beam rods is less critical and their virtues are either known or reputed. Which is it, known or only reputed? That is what I'd like to know.
RE: Connecting rod design
I agree, however I disagree that a crane is a good comparison and that I beams are much stronger. A bit stronger sure, but much stronger as a blank statement with zero supporting data, give me a break.
Regards
Pat
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RE: Connecting rod design
RE: Connecting rod design
Regards
Pat
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RE: Connecting rod design
http://www.pauter.com/acura_honda.htm
No failures in either rod type so far. The Carillo rods are in an engine developing 0.13+ hp/cc at 9500 rpm and the Pauter are in a smaller engine @ 0.10+ hp/cc and 8000 rpm. Apples and oranges...vintage race engines, not F1---
Rod
RE: Connecting rod design
http://w
Brian,
RE: Connecting rod design
As far as ANY alternative design compares to conventional "stock" rods, if they are heavy enough and made of 4340 or some other superior steel, they can be stronger than the stock parts. I note that Pauter X-beams tend to be the heaviest of all rods which could be taken to infer they are the weakest DESIGN. Ironically, engine builders often measure the stiffness of a rod in stretch and crush apparently not understanding that they may be measuring ONLY the average quantity of material in the rod's cross section (proportional to its weight) and not the merit of the design. They don't test the rod to buckling in the proper pinned column fixture.
RE: Connecting rod design
What sort of environment do you want them in just so that Ill know whether to go to S for steam trains, or H for high performance.
It all depends as you know. I tend to sit near the F1 end of things.
For me, H beams will always win, one major point is the support they offer around the big end - and how that helps to keep it round under extremes. The H beam surface area does tend to be higher(oil weight) but, the over-all mass about the big end is lower than others while offering better big end bore support.
Brian.
RE: Connecting rod design
Back in the 70s Chevrolet produced a semi-finished rod ( K rod) based on the big block forging for small block endurance racing purposes. It's bulk required a lot of clearancing to miss engine innards. It was made of some mundane 104X steel I think. Good Geometry can trump exotic materials. Even exotic materials can not compensate for a botched design or manufacturing details in a cool looking design.
Seems like modern rods recognize the "footprint" around the rod bolts at the parting face needs to be big to make the bolts' life less bendy, and the housing bore needs to be bulky in the right way to keep the hole round at high rpm if the bearings are to survive. Checking off I/H/X on the Design sheet does not go far accounting for that.
In street use Harley big twin forked rods were almost certain to slowly develop cracks inches away from the I-beam, where the thickened rib meets the side panels where the races are located. http://www.indianpartseurope.com/xlrodkit.jpg
Some subtle re-shaping combined with shot-peening pretty much eliminated the problem.
RE: Connecting rod design
Steam engines and F1 use I-beam rods. It's the in-between racing applications where there is disagreement.
Tmoose, the forked rod looks like the hardest rod to design and avoid stress risers. I like this solution by Daimler-Benz: <http://
You can see that rather than have a fork, they split the I-beam in half and kept them separate right up to the piston pin. What is not seen is the webbing that joins the I-beam halves, turning the cross section into an H with feet. Don't know if this DB601 rod inspired Taylor at Wright Aero to try an H-beam master rod after the war, but it might have.
RE: Connecting rod design
The general feeling I have about both, and I have studied them lots, is depending on how much time and money you spend on either I or H the better that rod will be. There is no superior one as such.
I feel that many of the big names have just spent more time with one design and have taken them to the limits, for them to change now would involve starting over - more money.
I think you'll also find it hard to see an I beam in any current F1 engine now, but I could be wrong.
Below are what I call in-between rods - when you have them in your hand its even harder to decide,
http://sto
http://s
http://www.flickr.com/photos/auto123/4427262335/
Brian.
RE: Connecting rod design
To assign a type to a rod with characteristics of both types, look at the web and flanges. The Ferrari rod you show is an I-beam rod with its web aligned with the plane of rotation, but with small ribs along the edges of the flanges. This Cosworth rod (from Tmoose) is more ambiguous: h
I believe that the H-beam rod excels at distributing stress throughout the rod while the I-beam requires more careful design to avoid stress concentrations.
RE: Connecting rod design
Brian.
RE: Connecting rod design
http://www.eng-tips.com/viewthread.cfm?qid=246316
http://www.eng-tips.com/viewthread.cfm?qid=84032 > 140airpower this should interest you too.
http://www.eng-tips.com/viewthread.cfm?qid=67425
http://www.eng-tips.com/viewthread.cfm?qid=115058
http://www.eng-tips.com/viewthread.cfm?qid=19615
https
h
Brian.
RE: Connecting rod design
I characterize the Cosworth rod and any rod by the distribution of of mass. The I-beam/H-beam separates mass into flanges connected by a thin web. Ideally, the flanges contain most of the mass and the web contains the minimum amount of mass possible. It is THIN. The H-beam rod web is a thin span running straight down the middle of the rod when viewed flat. The I-beam's web is a thin span running down the middle of the rod when viewed from the edge. The Cosworth rod has a web in two parts, under the piston pin eye and above the crank pin eye, that fits the I-beam description. There are features in the Cosworth rod and some other F1 I-beams that are H-beamish. Also, the Cosworth rod is the most solid I've seen. I already mentioned that the strongest I-beam rods have big ends that were originally more typical of H-beams. Also, the Ferrari I-beam rod has ribs along the edges of its flanges (BTW, do you think it is an H-beam?). There are more pictures of recent F1 rods out there, -ALL I-beams.
In this SAE paper by Mario Illien of Ilmore-Mercedes he makes it clear that Ilmore doesn't use H-beam rods in F1 (I think they do use them in other areas): <ht
Illien's words are no surprise in view of Euler column theory (check out column theory for pinned columns). Also, column theory is for the static case, zero rpm. As rpms increase, additional inertial forces come into play that increase the bending moments in the plane of rotation and makes the I-beam even more preferred. The universal use of I-beams for connecting rods makes all the sense in the world... or maybe not.
The beam portion of short rods is less than 1/2 the rod's mass and the transitions from the I-beam to the pin eyes are areas of stress concentrations and differing opinions by designers. The FEA stress analysis I have seen all show the H-beam to be a more stress-free shape. I conclude that the justification for H-beam rods lies in the way the rod actually performs with respect to durability (in applications that can tolerate some extra weight) and not in column theory or strength-to-weight.
I am looking for more and especially differing analysis.
RE: Connecting rod design
FWIW... Less of an engineering and more of a practical over abuse point of view: My dyno guy does lot of nitrous motors with high cylinder pressures. His rule of thumb recommendation based on broken parts is preferably full round skirt pistons and I-beam rod, at least one or the other. He has sideways bent H-beams used as wall decoration. Sideways meaning toward the unsupported side of slipper skirt piston. He said they failed when spraying ~500hp, which may be a little theoretical in many cases.
RE: Connecting rod design
Calypso, very interesting info. I also think a full round skirt is called for in high boost or nitrous situation. Don't know if I understand about the H-beams bending sideways. I expect them to bend in the plane of rotation with the flanges wavy, if that is sideways. They look extremely strong the other way, the flat way.
RE: Connecting rod design
The advantage of a full round skirt is that it dissipates hat better and spreads cylinder wall loads a bit better. Really important features in a very high cylinder pressure application.
However I really don't see it influencing rod bending and actually strains bolts and big end bore more due to increased weight.
A heavier piston shifts the favour more toward the H beam as they typically support the big end better and reduce stress concentrations near the bolt holes,
Regards
Pat
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RE: Connecting rod design
Okay, enough about these piddly little Carillo rods. Now THIS is what I call an engine. 108,000+ hp at 102 rpm.
http://www.emma-maersk.com/engine/Wartsila_Sulzer_RTA96-C.htm
I've worked on some big engines, but nothing with a 38" bore and 92" stroke. Awesome.
Rod
RE: Connecting rod design
Interesting. I guess Doug needs to do some more work. In order to see the link, you must highlight the url and click "open link". Then it opens fine.
Rod
RE: Connecting rod design
Pat, I agree that the extra weight is not good, but the concern was about out of column forces, which occurs when the piston has no stability along the pin axis. Ordinarily, the rod can take whatever side loads exist, but Taylor cited this in recommending the extra strength along the pin axis of the H-beam. A boosted engine and/or one experiencing detonation would tax the rod worst. A full-round skirt should take all out-of-column compressive loads off the rod. The question would arise, which increases reciprocating weight the most, a heavier rod or a piston with added skirt area? I notice that Top Fuel and other top drag racing classes seem to use full-round-skirted pistons. F1 does not.
RE: Connecting rod design
I certainly agree that cars like blown alky or nitro or nitrous benefit from full round skirts. I always put it down to heat dissipation.
My point was that the desirable full round skirt weighs more. This extra weight comes into play over TDC and stretches bolts more and pulls big ends into an oval shape more. H beams better resist that.
Top fuel uses aluminium rods anyway so the point becomes mute as aluminium rods are almost a solid rectangle beam.
Regards
Pat
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RE: Connecting rod design
For the same peak tensile stress under tensile load the I-Beam is a lighter more effiecint design by a noticeable amount. This mean if you want to avoid peak stress for fatigue an I-Beam is better due to the stress concentrations being lower. For high rpm repeated tensile applications not involving complete overload (yield) the I-Beam will be lighter and therefore better suited.
For the same mass both an H and X have more beam CSA which means ultimate strnegth driven by plastic crippling will be higher than an I. So the I has lower peak stress but global stresses in beam are higher.
I would say the X-beam is suited for extreme tensile loads where you wont see too many cycles
Assuming rods of same mass when you use Euler buckling of the I-Beam about its best axis the force levels seem to be high enough that the beam would have gone plastic so elastic buckling is not a big concern . For the weak axis the end conditions are not pinned and so the same applies for the rods it wont be elastic that i looked at. The same can be said for H-beam while its strong axis is less strong it seems plenty and its weak axis is stronger than the I weak axis. For the X-Beam about its strong axis is much same as H-Beam about its axis and about its weak axis is much same as I-beam about its weak axis,
When comparing rods it is quite hard to speak generally but i think what i have mentioned is a pretty good comparison. To go further one would need specific geometry, application and loading to compare.
RE: Connecting rod design
"For the same mass both an H and X have more beam CSA which means ultimate strnegth driven by plastic crippling will be higher than an I. So the I has lower peak stress but global stresses in beam are higher."
Assumptions I have used are that when machined from solid forging, the contours of the big and little ends would be the same for all types. The only difference would be the shapes of the beam and the transition regions. I also assumed either the beam cross sectional area (CSA) must be identical in all types for the same tensile strength OR the bending strength in the rotational plane must be identical for the same buckling resistance. Then the comparison would be the strength to weight ratios.
When you say the H and X have more CSA at a given weight you imply that the parts of the rods other than the beams are heavier in the I design. If the ends are identical, then the differences would have to be in the transition regions. Is that what you mean?
Also, an unfortunate fact is that I-beam rods vary quite a bit in the transition regions with some much heavier than others in those areas. Did you model a particular I-beam rod?
Lastly, did you make any comparison in the torsional stiffness? This factor escapes my simple analysis beyond noting that the I has the highest overall area moment while it appears the X has the lowest. But, I believe details of the structures should come into play, like the length/thickness of the web in the cross section and the filet radii.
RE: Connecting rod design
http://weldingdesign.com/blodgett/wdf_79843/
http://weldingmag.com/images/archive/79843dont5jpg_00000052332.jpg
If for some reason you want great torsional stiffness, "boxed rods" may be the way to go.
They were used in various hot-rodded car and bike engine decades ago.
Fuel burning harley dragsters needed some help in regards rod buckling, and boxing DID offer some relief
half way down the pager here, right hand example hard to see.
http://midstateantiquestockcarclub.com/flat_heads6.html
http://image.carcraft.com/f/editorials/boxed-connecting-rods-antique-engine-tech/31910310/no-subject.jpg
I'd consider Welding on a con rod anywhere, but especially at the transitions, a really bad idea.
RE: Connecting rod design
Yes, boxing was used before there were so many good aftermarket pieces. And, a boxed or oval section is indeed much better in torsion than an open section. Twist is important because it appears to sometimes be a factor in the buckling process. A column with asymmetric strength can fail prematurely if twisting can partially expose the weaker direction to the bending load. The I-beam is the most asymmetric of the three we were discussing.
inline6, what you say about the X-beam being like the H-beam in the strong direction (weaker than the I-beam) and like the I-beam in the weak direction (weaker than the H-beam) is what I expected. This follows if you look at the concentration of mass in the X-beam straight down the center of the rod where, except for tensile strength, it is dead weight. It therefore is the weakest design of the three for the weight, ...except that I believe an X-beam is actually stronger ultimately than the H-beam in the strong direction because, IMHO, its thick blade is not subject to the flange buckling of the two thin H-beam blades.
Your analysis is very helpful.
RE: Connecting rod design
with further development i think i could design an I-Beam rod to have the same min CSA in the beam area for the same mass which would mean the I's would have it.
It is quite difficult to compare them you could do
1) Same mass what is the strength comparison
2) Same strength what is the mass comparison
but what does stength mean? fatigue, bending, axial, proof load, gross collapse, fracture........)
but the most noticeable thing that came out was the I-Beam transition was far superior than the other two which is import in tension. I did not look at doing a pin contact analysis to see whther the loads are carried closer to the edge of the bearing i used a sinusoid pressure around a suitable arc uniform through thickness
RE: Connecting rod design
That is for or aft installed in an engine. I beam rods are most always a smaller profile than an H. Lighter yet stronger as well.
RE: Connecting rod design
The H-beam rod obviously has other advantages not explained in this analysis otherwise it would not be the preferred design in so many areas of racing. From what I have read, it is only in F1 where H-beams are definitely not considered suitable.
RE: Connecting rod design
I still think the initial popularity with "H" beams was relative simplicity of machining from billet at the time they became popular.
Regards
Pat
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RE: Connecting rod design
I think that serendipitously they learned that the disadvantage of the H-beam (being not as strong as an I-beam) was not so important in practice. In any case, H-beam rods have been popular in high buck racing except for F1 since the days of the extreme rpm challenge.
BTW, although some F1 rods are weird, others are straightforward I-beams with added reinforcing ribs between the big-end bearing eye and the beam flanges. Back in the '80s, when rpms were not much higher than 12,000, many F1 engines used H-beam rods.
RE: Connecting rod design
Even if I really knew "what" this year's F1 con rod looks like, I'm not real confident in my speculative powers about the "why" behind the design decisions without input from the designers. And even then, I'd want to compare their techno-confessions made when sober, drunk, and under the influence of truth serums.
Suzuki GSX R600 has been rated max power at 13,500 rpm or so for a few years.
At Suzuki.com Suzuki says this "The GSX-R600 engine revs to 15,500 rpm, the highest figure among production Suzuki bikes, and similar to racing machines."
http://www.suzukimotorcycles.com.au/bikes/road/sup...
They come with a 12 month limited warrantee.
Their rods reportedly look like this.
http://static.zigwheels.com/media/content/2011/Mar...
RE: Connecting rod design
The Suzuki shows what can be done with steel valve springs. The rods look like "ordinary" I-beams.
The main bearings look small for lower friction, but looks like there is little pin overlap. This is an F1 characteristic.
RE: Connecting rod design
agreed. They cannot be called ''I'' beams as I mentioned before. A recent chat with Sauber guys in Paris led me to believe they call them A beams. Not for cross-section form, but the Elevation outline. The cross-section is a mishmash of different forms as you mentioned.
I sectioned a ti rod not so long ago to get a feel for material position. Ill post some pictures once I find all the bits.
Brian,
RE: Connecting rod design
"A-beam", if referring to a straight tapered rod, is not related to the beam cross section at all. There can be A formed H-beams, I-beams, X-beams, hollow-beams, etc. All F1 rods I've seen over many years are wide, tapered rods in either H or I beams. They all have a curved taper except for a BMW H-beam from the early '80s. There are a lot of straight-taper aftermarket rods.
I wonder if the Sauber guys have anything to do with Ferrari engine design? Can you ask them what are the engineering reasons F1 stopped using H-beam rods since H-beams are very popular in other elite classes of racing?
RE: Connecting rod design
Fair enough if you want to label them I's but thats just a terminology row then, and pointless.
I believe, and will settle on that they are a combined design, with some.
I will dig out the pictures in a week or two. They are on a dead drive that is currently with data recovery solutions in Dublin.
If they dont come out ok, Ill take more. Hopefully I still have all bits, as I gave a box of stuff/cutaways to a local school recently as learning tools.
peace,
Brian,
ps, Ill ask a few questions the next time Im with some engine guys,
pps, I dont think there is any 'best' design. The best design comes from thousands of euros, a lot of lads staring at screens, best materials, and a whole load of testing. The final shape is just a by-product of all the 'trimming'. Keep in mind too that all that has to tally with the working targets/endurance/lifespan in hand.
RE: Connecting rod design
RE: Connecting rod design
Here is an example of a nice con rod design.
Personally I can not understand the love affair with the H design. Who in a right mind would want 2 skinny edges with not much skin area as the beam? Does anyone know stress and strain anymore?
http://www.youtube.com/watch?v=uTpRfREYa9Q
This shows the dynamic bending moments setup in operation. And how many overhead crane systems use an H beam, ie the beam twisted 90 degress? I think H beam is a cool factor marketing tool.
And yes look at something like a GSXR motorcycle, just a good old I beam rod, not fancy at all. 13K rpms and very high power density.