A question about connecting rods
A question about connecting rods
(OP)
Ran across a discussion about the merits of I beam vs H beam con rods. It was a "which is best" question.
I would lean towards the I beam for the proper application of beam loading. I would like to see what you folks have say about this.
I would lean towards the I beam for the proper application of beam loading. I would like to see what you folks have say about this.





RE: A question about connecting rods
RE: A question about connecting rods
BTW, I have a real world experience of rods being "squished" (shortened) very slightly, under the stress of knock, without any visible bending. I did a compression stress analysis that indicated this was indeed possible with cylinder pressures exceeding 2000 psi and the connecting rods in question. The plastic strain was approximately 0.020" in the worst rod, IIRC.
RE: A question about connecting rods
I am of the unproven belief that the long thin ribs of the H beam are more prone to deflection under compression of the rod than is the centre of the I which is restrained by the shorter thicker ribs typical of I beams
Regards
Pat
See FAQ731-376: Eng-Tips.com Forum Policies for tips on use of eng-tips by professional engineers &
http://eng-tips.com/market.cfm
for site rules
RE: A question about connecting rods
RE: A question about connecting rods
I beams as in say a large gantry crane have the load situated in such a way that the flanges on the top and bottom are in compression and tension. So how accurate is FEA? Does it take into consideration the common equations for I beam vs H beam design? What is FEA based on? Old strain gauge measurements? Brittle Lacquer cracks?
I'm curious.
RE: A question about connecting rods
RE: A question about connecting rods
RE: A question about connecting rods
Regards
Pat
See FAQ731-376: Eng-Tips.com Forum Policies for tips on use of eng-tips by professional engineers &
http://eng-tips.com/market.cfm
for site rules
RE: A question about connecting rods
Anyhow, his recommended (not mandatory) orientation is H-beam, like a Carillo rod. One of the explanations offered is heavy unsymmetrical loading can cause greater bending moments than inertial "whip".
The design details other than beam profile are more more important for durability, I'd say.
Most of the cracks in connecting rods I saw originated from the terrible profile ( a notch, really) to accept the rod bolt head in many stock rods. H beam rods almost invariably thread the rod and use a bolt coming up thru the cap. The resulting detail where the beam joins the big end is much more nicely shaped.
RE: A question about connecting rods
Larry
Larry Coyle
Managing Partner
Cylinder Head Engineering, LLC
CNC Porting
De Soto, KS 66018
RE: A question about connecting rods
I have seen I beam rods that have cap screws from the bottom as well. It certainly does improve the strength at the shoulder which is a common weak spot on rods with through bolts.
To compare a Carrillo with an OEM I beam is not exactly a real comparison. To compare rods of equal quality and weight to see which is strongest is more relevant. Strength for bulk might also be an issue as is fatigue strength vs weight as it affects the material choice. The balance of these properties obviously need to be tied to the individual application.
Regards
Pat
See FAQ731-376: Eng-Tips.com Forum Policies for tips on use of eng-tips by professional engineers &
http://eng-tips.com/market.cfm
for site rules
RE: A question about connecting rods
Does anyone have any photos of F1 con rods?
RE: A question about connecting rods
To my unpractised eye they look almost identical to a road car's.
"there has been no honest study other than FEA, on connecting rod bending forces."
I don't know what to make of that sentence. I've seen straingauged conrods going into engines, I rather imagine that the engineer involved discovered what he needed.
Cheers
Greg Locock
SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: A question about connecting rods
Cheers
Greg Locock
SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: A question about connecting rods
RE: A question about connecting rods
You mention comparing a Carillo vs OEM I beam, how many OEM H beam rods are you aware of? The original question was comparing H beam to I beam. Someone mentioned Carillo and I do have direct knowledge of this product and others.
You statement regarding I beam rods using cap screws is the best solution when you can use it as limited by individual application rules.
Larry
Larry Coyle
Managing Partner
Cylinder Head Engineering, LLC
CNC Porting
De Soto, KS 66018
RE: A question about connecting rods
I personaly only know of one application for a H beam rod in a old airplane engine, and it was chosen I think for the clearance space it provided for the 2 close articulated rods, and not so much because it was the strongest ie best as far as strength.
If you compair an H beam vs I beam con rod, looking at them straight on, that is in line with crankshaft centerline axis. H beam rods are always wider, so before jumping on the H beam band waggon, a fair comparison would be to have an I beam of the same width and flange cross section thickness. The I beam always wins, must be why all the OEMs' In large industrial engines win.
RE: A question about connecting rods
Regards
Pat
See FAQ731-376: Eng-Tips.com Forum Policies for tips on use of eng-tips by professional engineers &
http://eng-tips.com/market.cfm
for site rules
RE: A question about connecting rods
I had someone at the door when I posted that last one. And looks like I really goofed up.
"Large industrial engines use them"
RE: A question about connecting rods
Regards
Pat
See FAQ731-376: Eng-Tips.com Forum Policies for tips on use of eng-tips by professional engineers &
http://eng-tips.com/market.cfm
for site rules
RE: A question about connecting rods
The cost of a forged rod (I-beam) will make it more attractive than a fully-machined rod (H-beam).
(of course) none of the strain gaged rods were h-beam. nobody uses them in production.
RE: A question about connecting rods
In a real world I looked at a lot of broken rods and after a discussion with a con rod manufacturer, the conclusion is that the rod should be a tapered tube. That reminds me of Myer Drake's welded tubular rods of the Offy days.
The Carrillo rode are both forged and fully machined that cures one of the weaknesses in forged metal.
That being inclusions at the surface being driven into the part.
Although I believe VAR cures part of that, The fully machined component has a distinct advantage.
I guess my point is besides the 'I' factor of the cross section there are apples and oranges in the way it is manufactured.
Here is an starting point http
I don't know anything but the people that do.
RE: A question about connecting rods
Cheers
Greg Locock
SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: A question about connecting rods
I would like to see some real data compairing them, not FEA, but compression and tension bench tests, bending loads etc. Cost is not an issue on some engines, especially huge industrial engines, and top of the line engines like the VW W-16, if there is a superior rod design the manufactures would latch on to it. There are outfits like GMPP that puts H beam rods in some engines, so its not a cost issue I'm looking for its a what rod is the true best? I lean towards the I beam, it is the much stronger design, for the loads it is asked to take. And especially at high rpms. I've seen some discussions saying that not all F1 engines use I beam, but the ones I have pictures of are I beam type, and that makes alot of sense for the rpms that are encountered.
RE: A question about connecting rods
Cheers
Greg Locock
SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: A question about connecting rods
Of both types of rods. During engine operation the rod experiences cyclic bending. Simple proof of how a rod can bend is the typical hydraulic lock bend.
H beams would have to carry tensil and compression loads in those very narrow edges of the H flanges. And if the FEA is really up to snuff, it should show them as very highly stressed members.
RE: A question about connecting rods
I for one am not in the least surprised that F1 uses FEA to design stressed components.
Regards
Pat
See FAQ731-376: Eng-Tips.com Forum Policies for tips on use of eng-tips by professional engineers &
http://eng-tips.com/market.cfm
for site rules
RE: A question about connecting rods
High performance rods (steel or Ti) are typically hot forged blanks, that are subsequently 100% machined on all surfaces to remove the metallurgically unsound outer surface layer that is contaminated during the hot forging process. Machining an "H" beam configuration rod can be done with larger cutters than an "I" beam rod. Thus it is quicker and less expensive.
The small difference in strength, weight and stiffness between an "H" beam rod and an "I" beam rod is generally not an issue, except in ultra-critical applications like F1. Where cost is no object.
Regards,
Terry
RE: A question about connecting rods
I beam is the standard connecting rod style used by most all OEM's. So are we all talking the same I vs H beam here?
RE: A question about connecting rods
RE: A question about connecting rods
I have been to Fred Carrillo's shop and the cutter that cuts the sides is about 4" in diameter and although they are forged blanks they still are in the rough shape of the finished product.
I don't know anything but the people that do.
RE: A question about connecting rods
From a purely structural standpoint, the I beam rod is slightly preferable to the H beam rod. Most production automotive gas engine rods are now made from powdered metal (PM) blanks, due to cost. PM blanks give adequate strength, have very little scrap, and can be produced very close to the net finished shape. A PM rod blank can just as easily be produced in either an H beam or an I beam configuration, but they are universally I beams.
As thundair noted, old man Carrillo made the H beam racing rod ubiquitous. But as I noted in my previous post, Carrillo does this for manufacturing cost reasons. Carrillo makes dozens of different rods, but they all come from only 3 or 4 forging sizes. Which is one of the reasons Carrillo must 100% machine his rods. Carrillo is a smart business man and a good machinist, so he quickly figured out that removing all of that extra metal from the forged blanks was accomplished more quickly and cost-effectively with large cutters. Can you imagine how long it would take to machine the I beam web pockets in a typical alloy steel rod with a little 1/2 inch diameter ball end mill?
Here's a current titanium F1 rod:
Regards,
Terry
RE: A question about connecting rods
That is a completely different look to the 2000 Ferrari ones.
Cheers
Greg Locock
SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: A question about connecting rods
RE: A question about connecting rods
If bending was the big issue, an I beam would be far superior to an H beam, since the I beam puts the material away from the neutral bending axis where is does the most good. But a rod beam is more a compression/tension member, due to the fact that the load transfer joints at each end of the rod have a rotational DOF in the bending plane. So the only bending that can be produced in the rod beam is from friction moments at the joints and inertias due to the rod mass whipping about. Of course, one must also account for compressive buckling if the rod beam length is long and the cross-section limited.
One tangential benefit of the H beam is that I believe it gives a more uniformly stiff back-up structure for the upper rod bearing shell.
Look closely at this photo and you'll see a big old honkin' H beam rod:
Regards,
Terry
RE: A question about connecting rods
I think there were a couple of I beam master rods used on radials, but in general, no one sorted out how to do the transition required for the slave rods ( which were invariably I beams). I believe Liston, Taylor and Ricardo discuss this engineering dilemma in some detail. It is funny you picked a radial as an example, as the master rod I think does see significant bending stresses due to its role in restraining the motion of the slave rods and their lower link not being in line with the center of the crank.
Charles
RE: A question about connecting rods
Wright used H beam master rods. Pratt & Whitney used I beam master rods.
The big Pratt radials were far better engines. But I don't think it had to do with the rod beams.
Pratt R2800 master rod:
Regards,
Terry
RE: A question about connecting rods
Regards
Pat
See FAQ731-376: Eng-Tips.com Forum Policies for tips on use of eng-tips by professional engineers &
http://eng-tips.com/market.cfm
for site rules
RE: A question about connecting rods
Pat well then lets see some hard data compairing these 2 designs. Yes H will work. Yes H will buckle under pressure.
Bending loads???? Have you ever seen a hydraulic locked rod? High pressures in a cylinder will cause bending. And so true about the radial engine master rod feeling some bending loads.
Take note of the material surrounding the wrist pin hole.
H vs I beam.
RE: A question about connecting rods
Most comparisons of H vs I are a racing H vs a production I so not a fair comparison.
I thought the radial engine, thread going in circles pun was an obvious joke.
I have seen a bent rod from hydraulic lock. It was a production I beam and was very bent. I have no doubt a H beam would have been equally bent so that does not really prove anything.
Re the production radials, one uses H beam for the master only and one uses I beam. Also totally non conclusive.
This really is going nowhere unless someone comes up with hard back to back data, and even then, it might depend more on the quality of the individual pieces rather than the basic I vs H design. This has all been repeated several times in this thread alone so I really have finished with it.
Regards
Pat
See FAQ731-376: Eng-Tips.com Forum Policies for tips on use of eng-tips by professional engineers &
http://eng-tips.com/market.cfm
for site rules
RE: A question about connecting rods
The usage of the two designs in the old radial engines is conclusive as we disscused why, the H was chosen.
It simply needed the cut outs to accomidate the articulated rods. It is the most effcient way to do the job and quickly, but note the articulated rods are of I beam design. The engineers most likely figured the cut outs for like the PW rod would weeken that area, and decided the H beam would sufice and be a one shot wheel cutter machining job. It was a cost cutting thing not a this is the best structure for the application thing.
Also as a side note they chose forged steel for the crankcase vs the forged aluminum that the Pratt & Whitney used. Both engine designs are exemplary, and have proven very reliable and longlasting.
RE: A question about connecting rods
It depends on the application. :)
RE: A question about connecting rods
It depends on the application. :)
Niether one.
And this post has nothing to do with the topic.
The application has nothing to do with a design that is stronger than the other.