Connecting rod shape
Connecting rod shape
(OP)
I cannot find any analysis of a I-shaped connecting rod vs. a H-shaped. The H-shaped rods seemed to be used in the more expensive applications, but I cannot figure out why it should be preferable.





RE: Connecting rod shape
H-Beam design is used were engine expected not have any
chance of damaging-detonation , its stiffer in that plane
and would increase damage
H-Beam = NASCAR , etc
I-Beam = more flex in that plane, more forgiving , so used more in Nitrous classes where exact tuneups are harded
to get quickly, and more detonation chances
the H-Beam is better when things are perfect or close to
Larry Meaux (maxracesoftware@yahoo.com)
Meaux Racing Heads - MaxRace Software
ET_Analyst for DragRacers
Support Israel - Genesis 12:3
RE: Connecting rod shape
The "H" style was made popular by the excellent quality of the rods made by Carillo from about 30 or 40 years ago, and the fashion continues, probably as it is "obviously different" to stock rods.
Technically they are equal strength to "I" rods under tension, which is were virtually all rod failures occur in normally aspirated engines.
Under high compressive loads, like in high boost forced induction systems, "H" beams have a slight dissadvantage as the long thin ribs can flex and allow the rod to bend.
Regards
pat
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
RE: Connecting rod shape
RE: Connecting rod shape
The result was that for the same weight, the I shape was around 20% stiffer, and the weigth distribution more closer to the ideal.
I'm agree with Pat arguments.
Paco
RE: Connecting rod shape
"The theoretical loading on the connecting rod is easily calculated ..... the actual stress on the rod column may be far greater than such a calculated value on account of unsymmetrical loading, vibratory stresses (including the piston in rotation), and stress concentrations. Heavy unsymmetrical loading comes from the piston axis being out of line with the rod axis, more or less common to all engines. It has been found that such loading may cause greater bending moments in the rod than "whip", and it is one argument for orienting the "H" section of the rod 90 degrees from the usual direction. This design also simplifies forging and machining in many cases".
RE: Connecting rod shape
RE: Connecting rod shape
RE: Connecting rod shape
So all arguements aside. How many failures have you guys/gals, seen in both styles? As someone mentioned Carillo rods were (are) the best. And they have always been
the 90deg I or "H". I have no experience with H but I have seen lots of "I" rods bent and in pieces.
RE: Connecting rod shape
Crower, Manley, Argo, Precision Automation and Robotics, and even Eagle make some excellent rods now.
The F1 rod is an "I" beam, and I expect Ferrari could afford Carillos if they thought they were better suited to their engines.
The Ferrari rod probably takes nearly twice as long to machine as compared to a Carrilo.
Regards
pat
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
RE: Connecting rod shape
RE: Connecting rod shape
RE: Connecting rod shape
A decade or so magnufluxing passenger car, motorcycle, and race car rods suggests crappily designed or poorly executed transitions between I/H-beam and big-end or wrist pin end have brought WAY more rods to their demise than the orientation of the beam section.
I think Taylor also points out the (Carillo) H beam lends itself nicely to the threaded rod instead of bolt-n-nut design, which tends to improve the beam-to-big-end transition a lot simply by removing the whopping notch for the bolt head.
RE: Connecting rod shape
Rod
PS: The only rod I have ever seen 'bent' was from an early Chrysler Hemi drag engine. I think it lost fire and hydraulicked a cylinder. You will have to ask Pat on that one. V8's are a little out of my area.
RE: Connecting rod shape
The only noticably bent rod I ever saw was an OEM unit out of a 308 CI Holden V8. Similar story. It had a poorly installed water injection system, which siphoned water into a cylinder while parked on a slope. It hydraulic locked and bent a rod.
My comments re "H" vs "I" for high boost applications is precautionary, as to me the chance of the long thin ribs on the "H" are somewhat more susceptible to flex, and allow the beam to bend.
Regards
pat
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
RE: Connecting rod shape
RE: Connecting rod shape
Yes I hold the opposite view.
Even though the ribs on the "H" style are somewhat deeper, which should increase stiffness in the plane of the gudgeon, the ribs are so long and thin, that they are inclind to flex at the outside edge, alowing the beam to bend.
An 'I" beam puts a lot more metal in the area of the outer fibre when the rod is being bent in the plane of the gudgeon.
Much of this is based on intuition and anecdotal evidence.
I would really like to see some finite element analysis of a few structures. I might be converted.
Regards
pat
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
RE: Connecting rod shape
RE: Connecting rod shape
RE: Connecting rod shape
Regards
pat
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
RE: Connecting rod shape
RE: Connecting rod shape
Theory for this is that the pistons on the exhaust stroke are flung to the end of their stroke unrestrained and then jerked back putting large strain on the rod .Solution stronger rods that are lighter and lighter mass at the end of the rod (pistons)
2: compression failures almost always due to detonation or Pre ignition caused by too much compression and too much spark lead. Or just plain too much fuel load IE nitro or turbo boost too high.
RE: Connecting rod shape
RE: Connecting rod shape
-----------------------------------
on a normally-aspirated engine ,
if the headers are working correctly,
you should/want a negative pressure wave to
be centered on the overlap period
so the rod should be in more tension
during overlap period
in normally-aspirated engines with headers working
Larry Meaux (maxracesoftware@yahoo.com)
Meaux Racing Heads - MaxRace Software
ET_Analyst for DragRacers
Support Israel - Genesis 12:3
RE: Connecting rod shape
RE: Connecting rod shape
The only two orientational differences I have seen between Aus and USA are:-
1) Here the sun travels from right to left, whereas in the USA it travels from left to right. Singapore is quite confusing in this regard though.
2) You guys drive on the wrong side of the road, as you are probably very well aware, being of British origin.
Oh yes, I almost forgot, we also have the Gay Mardigra, but then again, you guys have San Francisco.
I guess turbos are a bit easier on rods re tensile as there is no effective exhaust scavanging, and there is always some residual back pressure in the exhaust port, as well as boost pressure in the inlet, but TDC overlap is still probably the highest tensile load.
Regards
pat
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
RE: Connecting rod shape
RE: Connecting rod shape
I just want to add about the effect of buckling due to the combustion pressure when comparing between I and H conrod.
If we simulate both the I and H for buckling, we will notice that the H conrod is stronger due to more cross section area at the critical points.
Overall, it is still difficult to predict which one is better. For turbocharged application, we have to consider a lot about the buckling effect. On the other hand, for NA application like the F1 where it revs up to 19000rpm, definitely centrifugal effect and repeated compression and elongation effect will have to be considered. Not to mention about critical fastener attaching both the big and small ends.
In addition to that, the effect of bore and stroke will also have to be considered. The Honda F1 turbocharged engine has longer conrod if compared to the new F1 conrod. This will yield different result entirely.
AO
RE: Connecting rod shape
RE: Connecting rod shape
why do majority of Hi-Perf rods use bolts rather than studs ?
especially in aluminum rods where you would think they would be beneficial in protecting aluminum threads
by using studs rather than bolts .
Answer=> ? .... the stud has a too much of a
"lever-effect" and transmits distortion to the journal hole
(any motion of the stud distorts the journal hole ?)
any other opinions / insight ??
Larry Meaux (maxracesoftware@yahoo.com)
Meaux Racing Heads - MaxRace Software
ET_Analyst for DragRacers
Support Israel - Genesis 12:3
RE: Connecting rod shape
Most big end bolts are through bolts with nuts on the end.
Why wouldn't Al rods be counter bored and use the same? Much stronger than threading the Al. It would be a bit difficult to aquire any stretch threading into Al, especially the minimal amount on the big end.
RE: Connecting rod shape
RE: Connecting rod shape
One solution is to make the bolts longer so the head is further from the bore of the big end, but this makes the rod and the bolt heavier, and increases the room taken for the crank to rotate at a given stroke. Many high performance engines are pushing the limits re RPM and capacity, so extra weight and bulk are a significant dissadvantage.
Most good high performance rods use a pair of tube dowels to locate the cap on the rod, and to reduce flex at the parting line, thereby reducing fatigue due to bending of the bolt, stud, cap screw.
The above applies to both steel and aluminium rods to varying degrees.
Extra considerations re aluminium are:-
The extra bulk is even more critical as aluminium rods are already bulky
The notch is more critical, as aluminium (to the best of my knowledge) is more notch sensitive than steel.
The longer the bolt, the greater the dimensional change difference, due different co-efficients of expansion between aluminium and steel.
I would think the ideal design would be a stud, mounted in the rod as described by EnglishMuffin, with tube dowels, and possibly a very small pilot hole right through to avoid trapping air under the stud, as this might expel and blow out the stud retaining adhesive (locktite).
This would allow significant strength to be retained in an area where (from anecdotal evidence), rods are weakest , while reducing the weight and bulk of aluminium rods and the bulk and to a lesser extent, the weight of steel rods.
The reduced bulk would be in the area that often interfers with the cam lobes in a typical American V8, and obviously the cam lobe is an area where interference is difficult to measure and correct for.
The linking portion on the "H" beams sounds like a good idea to me in high compressive load situations.
The cross sectional area of the "H" beam is much lower near the outer fibre which is resisting bending.
Regards
pat
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
RE: Connecting rod shape
RE: Connecting rod shape
RE: Connecting rod shape
RE: Connecting rod shape
[...] appear to me that it would probably have something to do with the fact that studs are usually designed to be tightened until their threads jam against the end of the mating female thread. If this is not done, I can't see..."
Hmmm, I wonder about your installation procedure. I suggest you look at the ARP site (http://www.arp-bolts.com/index.html), go to Product, then the Main or Head section, then the How To Install section. They say to finger tighten only.
Some time ago in Precision Engine (formerly Precision Machining) magazine they said to back off the stud 1/4 to 1/2 turn.
RE: Connecting rod shape
RE: Connecting rod shape
I guess it is much better to use bolt engaged straight to the conrod upper end. This way the bolt would have longer engaged thread if compared to the nut engaged to the stud bolt.
RE: Connecting rod shape
RE: Connecting rod shape
RE: Connecting rod shape
For high revving engine, we want the crankcase and the oil scrapper to be as close as possible to the conrod locus. This will ensure compact design as well as low friction loss from the splashing oil.
Therefore, with the nut instead of bolt head, I wont be able to keep the oil scrapper very close to the conrod locus. The closer the oil scrapper is to the conrod locus, the more horsepower I would get out of the engine.
With the stud bolt, I need to use a special nut to enable it to be closer to the oil scrapper. With this combination, the weak point is shifted away from the main conrod structure.
As for the FEA, yes we did. We did it over titanium bolts (not the stud bolt though). The factor of safety is very close to 1.0. It's just enough for one Sunday afternoon race. To join the 18000rpm NA engine club, the conrod, piston and crankshaft need to be extremely lighweight, stud bolt for the conrod does not belong there.
AO
RE: Connecting rod shape
With my very limited engineering backround, I have just completed my first semester of college, I have to disagree. I would think that the majority of the forces put on a connecting rod are bending forces as a result of the piston pushing downwards on the rod and the rod being at an angle due to rotation of the crankshaft. I would think that you would want to concentrate a majority of the metal towards the outside of the rod hence the use of I-beam rods. A tubular rod would have very little metal towards the outside with most of its mass concentrated in the center. This means the tubular rod would be more prone to bending, especially with small rod/stroke ratios. Tubular connecting rods are used, though. Our ship's service diesel generator, about 450kw@910rpm for an idea of size, has solid round rods. An ideal application for tublar rods would be in slow speed 2 stroke diesels as they have an articulated rod. The piston rod moves only vertically and has no side loads on the rod. For the most part they are tubular. The actual connecting rod which joins the piston rod and the crankshaft is of an I-beam style.
Here is a link to the best example of an articulated rod I can find. http://www.haskey.com/johnh/railfair99/16_porter.jpg
RE: Connecting rod shape
Interesting. We did several classical calculation comparison between I,H and cross shape conrod. The H appeared to be strongest. The engineer in charge did inform me about the hollow rectangular shape conrod. This will yield better result. My respond to his suggestion was, how can we manufacture the hollow conrod.
If you can manufacture for me a tubular conrod while at same time having the small and big ends as strong as the conventional conrod, you should get your process patented.
As for my choice of conrod for high revving engine, my first choice will still be H type conrod. With the I shape conrod, the thick outer metal is in neutral position to resist bending movement. As for the H type conrod, I have two thick structure opposing the bending movement.
Anyone can disagree, but as a designer I am free to choose the design. The moment of truth comes during the WOT durability test.
AO
RE: Connecting rod shape
RE: Connecting rod shape
RE: Connecting rod shape
I dont think it is an overkill unless you overdesign the thickness of the H conrod for the required application. Many racing bikes and cars that rev at very high rpm (some at double the speed of NASCAR max rpm) use H type and there is no clear rule for forced induction or NA engines.
It is the preference of the engineers, some use I type and get better result and some use H type and get better result. No matter which one we choose, it is the details of the thickness, radius, length, tolerance, manufacturing process, development history, FEA calculations, R&D, etc. that will determine the final result. In racing, the final result is the one that matters.
AO
RE: Connecting rod shape
We used 2 ΓΈ3.0mm by 6.0mm dowel pins (parallel to bolt centerline) for locating the cap relative to the rod instead of hollow dowels, concentric to the rod bolts.
Prior to this, we were building our own rods and ours looked like they were chopped out of a block of wood (they were designed and built by passenger car engineers who had never experienced what race engineering is about).