H beam vs. I beam rods
- Thread starter Alky V6
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I turn wrenches for a top, race engine builder on some weekends. It's mostly BBC stuff. From what I've seen from his and other "modern" engines that I've torn down. I beam rods are used in the lower HP applications for cost savings. H beams are used for the upper HP stuff. I'm not saying that I know why. I'm just saying that's what I'm seeing. These are high HP drag engines that go in boats and cars. Also, The "weakest link" that I've seen on most high rpm engine failures is not the rods. It's from the valvetrain not being able to handle the RPMs and fails. Thus taking out the bottom end. Guess what rod many of the top Pro stock teams are running in their engines right now? Many are switching back from alum to steel H beams.
Fun thread.
Mike Barnard
Is this blown, turbocharged stuff? Big cylinder pressure stuff? Or big cubed N/A stuff?I turn wrenches for a top, race engine builder on some weekends. It's mostly BBC stuff. From what I've seen from his and other "modern" engines that I've torn down. I beam rods are used in the lower HP applications for cost savings. H beams are used for the upper HP stuff. I'm not saying that I know why. I'm just saying that's what I'm seeing. These are high HP drag engines that go in boats and cars. Also, The "weakest link" that I've seen on most high rpm engine failures is not the rods. It's from the valvetrain not being able to handle the RPMs and fails. Thus taking out the bottom end. Guess what rod many of the top Pro stock teams are running in their engines right now? Many are switching back from alum to steel H beams.
Fun thread.
Mike Barnard
I'm gathering by what I've discovered so far that a H beam rod is preferred from a rpm point of view. Maybe with the mass concentrated central to the beam, there is less inertial weight being thrown around. What does this mean for the strength of the rod? That is the question on my mind. A H beam might be an advantage from the viewpoint that I shared above, but is there a compromise with the strength of the arrangement?
I'm sure that the Pro Stock class is less interested with durability if it means even a couple hundredths advantage on the timeslip. That is a tight class that will spare no expense to find a timeslip advantage. If they feel they're maxed out on the heads, they will turn to rotating mass. I'm sure we all know that reducing rotating mass or inertial forces does not necessarily go hand in hand with durability.
I wonder what con rod design was engineered into the Bugatti Veyron?
http://www.youtube.com/watch?v=btwKj2I0_wE
http://www.youtube.com/watch?v=btwKj2I0_wE
Looking at the con rod strictly from a structural support beam point of view, does centrally locating the mass of the beam give you a stronger beam? I'm considering mainly bending and twisting forces.
If we take the concept of centrally locating the mass of a beam to an extreme, we end up with a round rod. How well does a round rod cope with bending and twisting forces?
Going back to the explanation that Blazer406 gave, a support beam that has to deal with bending forces in one axis is much, much stronger if oriented one way over another.
In which axis does an automotive con rod see the most bending force?
If we take the concept of centrally locating the mass of a beam to an extreme, we end up with a round rod. How well does a round rod cope with bending and twisting forces?
Going back to the explanation that Blazer406 gave, a support beam that has to deal with bending forces in one axis is much, much stronger if oriented one way over another.
In which axis does an automotive con rod see the most bending force?
In my example I used a S4 x 7.7 (I beam). Its shape yields the properties I put in the post. If you are designing a rod from scratch..... you can play with the thickness' of the web and flanges....as well as the width...... to give you the strengths you need in the strong axis and weak axis. Another term thrown around by engineers is "column buckling". This has to do with the deformation of a column (i.e. connecting rod) when it is under a compressive load. It would not be straight forward to analyze this on a conecting rod.... because it's cross sectional area..... and actual shape changes as you go from the little end to the big end.
With all that said...... I wish I had time to try and analyze a particular rod design (like the K1 H-beam) and compare it to a popular i-beam design just to see what the analysis yields..... but alas.... I can't right now.
I PM'd Buddy Ingersoll and mentioned this thread hoping he would possibly come chime in.
What I am visualizing in my head is the occasional occurrence of a piston pin sticking or dragging due to high or extraordinary loads causing a paricular bending force on the rod, and the certain axis of that bending force. IMO the I beam is more able to cope with that particular situation, if we call on the information that Blazer406 supplied.
Bent wrist pins give rise to the possibility of a bending wrist pin under high load causing a drag on the small end bearing surface. Any sort of drag on that bearing surface would impart a certain bending action mainly on one axis to the beam of the con rod.
Bent wrist pins give rise to the possibility of a bending wrist pin under high load causing a drag on the small end bearing surface. Any sort of drag on that bearing surface would impart a certain bending action mainly on one axis to the beam of the con rod.
The greatest tensile load on the rods is at tdc on the exhaust stroke, so that's when rod bolts almost always break, so never rev an engine with no load. The two most popular times for bolts to break are coming out of the waterbox and as you lift crossing the finish line. On the compression/power strokes those forces act to lessen the tensile load so the bolts are usually ok and you just have to worry about getting the cylinder pressure up enough to start to bend the rod, or getting the rpms too high in some way, or hydrolocking a piston after blowing a head gasket . Pound for pound the shape that resists bending and twisting the best is a hollow tube, since diameter trumps everything. If you do some reading on moment of inertia as Brian posted you will see that it goes as basically the diameter if a tube or height of web if an I beam or piece of angle, cubed, and the stiffness is directly proportional to the moment of inertia. I beam rods are much cheaper to make than H beams. This comes up every now and then over at Speed Talk: Interviews - Racing Books - Racing Forum, where several top, top engine builders post, and the best consensus I can report from there is that "h beams may be better, but maybe not", so you won't get a simple answer with a simple geometric picture to explain it that covers the real world on this one. Each resists the greatest force in a direction perpendicular to the other but engines find a way to break either one, and top rod makers make both. Polishing and shot peening do the same as rolling the fillets on the cranks - they help increase the fatigue life but don't increase the strength.
Blazer, there have been some fea stress analyses posted over at speedtalk if you have some time to search.
. Pound for pound the shape that resists bending and twisting the best is a hollow tube, since diameter trumps everything. If you do some reading on moment of inertia as Brian posted you will see that it goes as basically the diameter if a tube or height of web if an I beam or piece of angle, cubed, and the stiffness is directly proportional to the moment of inertia. I beam rods are much cheaper to make than H beams. This comes up every now and then over at Speed Talk: Interviews - Racing Books - Racing Forum, where several top, top engine builders post, and the best consensus I can report from there is that "h beams may be better, but maybe not", so you won't get a simple answer with a simple geometric picture to explain it that covers the real world on this one. Each resists the greatest force in a direction perpendicular to the other but engines find a way to break either one, and top rod makers make both. Polishing and shot peening do the same as rolling the fillets on the cranks - they help increase the fatigue life but don't increase the strength.Blazer, there have been some fea stress analyses posted over at speedtalk if you have some time to search.
I sure wish we could get a high end endurance engine designer to chime in. I'm sure there has been computer analysis done on the different rod designs. I just find it so coincidental that an I beam shank is so widely picked for the vast majority of high powered endurance engines. Maybe an I beam is cheaper to manufacture, but does being cheaper to make necessarily mean that it is the inferior design?
Looking at the Manley catalog, it seems to me that with each mention of a H beam rod choice comes a recommended max hp rating to go with that rod. That is not necessarily the same with the I beam rods.
It seems more common to see an I beam rod made more massive to handle high output situations than it is to see a H beam rod with added material to the flanges to handle extraordinary power levels.
It seems more common to see an I beam rod made more massive to handle high output situations than it is to see a H beam rod with added material to the flanges to handle extraordinary power levels.
Don,
I found this post on a Corvette site. If you do a Google search there is a lot of discussion on this topic. Don, if you read down the posts, there is a comment about hitting nitrous at low rpms and the amount of torque that is created.
Jeff
Finally, an analysis of I-Beam rods vs H-Beam rods - Corvette Forum
I found this post on a Corvette site. If you do a Google search there is a lot of discussion on this topic. Don, if you read down the posts, there is a comment about hitting nitrous at low rpms and the amount of torque that is created.
Jeff
Finally, an analysis of I-Beam rods vs H-Beam rods - Corvette Forum
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Most of the stuff that I wrench on is 500+ CID N/A BBC engines. Most make over 1000 HP and are run to 8,000-10,000 RPMs. On the Pro Stock thing, many have switched from the lighter alum rod, back to a steel rod because it doesn't expand as much with heat. This allows tighter clearances for oiling and piston to head squish. Some feel that this gain is worth the penalty for a heavier steel rod.
Mike Barnard
I caught that link before I started this thread. It is an interesting read.
I figured the expansion rate differences had something to do with it. Thanks for the input.
A very interesting link.
Comparing NASCAR Cup engines to Formula One engines, by EPI Inc.
A quote from the article; At max compressive load, the stresses and buckling margins in both rods are acceptable (although “I” beam rods have considerably more buckling margin at the same loads than “H” beam rods of similar cross section),
Comparing NASCAR Cup engines to Formula One engines, by EPI Inc.
A quote from the article; At max compressive load, the stresses and buckling margins in both rods are acceptable (although “I” beam rods have considerably more buckling margin at the same loads than “H” beam rods of similar cross section),
copied from Carrilo's website...
Straight H-Beam
The stoutest beam configuration available. It is capable of handling the most abusive forces in an engine.
My guess that they are touting it as the strongest they make........ which apparently..... some people here have bent..... or broke....... so apparently they don't make a rod strong enough for a max effort turbo V6.....and that is why most with the experience have moved on to Olivers..... or something else...
