[v12-engine] Con Rod Design

Does anyone know why the ‘‘H’’ beam conrod’s are reputedly superior
to the conventional ‘‘I’’ beam design.
The ‘‘I’’ beam would appear to make better use of the steel by
putting more of it further from the centreline to resist bending
yet I am told the ‘‘H’’ shape is better.
Any ideas?–
MattFurness
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MattFurness wrote:

Does anyone know why the ‘‘H’’ beam conrod’s are reputedly superior to
the conventional ‘‘I’’ beam design. The ‘‘I’’ beam would appear to
make better use of the steel by putting more of it further from the
centreline to resist bending yet I am told the ‘‘H’’ shape is better.

I can give you two reasons why the H shape is considered better:

  1. Carello rods are H shaped, and last I heard the company
    guarantees they won’t break. Period.

  2. Rods see a lot of stress – tension, compression, and bending.
    The bending is all in the direction that you would expect the I shape
    to be most beneficial. However, the bending stresses are probably
    relatively small compared to the compression and tension stresses,
    since the bending stress is only a result of the bottom end of the
    rod moving side to side as the crank turns. And this is the
    direction in which a rod can be made quite broad to handle this
    stress.

Tension, of course, scarcely cares about the shape of the rod, it
could be shaped as a length of flexible cable.

Which leaves compression. Failures in compression in rigid
structures often involve a shear fracture at a 45 degree angle to the
direction of the compression loading – but since the tension and
compression loads are pretty similar (both caused by piston stopping
and reversing direction), frankly I’d expect a failure in tension
before you reached that point.

Items in compression can fail sooner, though, if they “buckle”.
Buckling is what happens when the item in compression bends in the
middle so the center portion moves out of line with the compression
load.

Now, it just so happens that the H shape would be the optimal
configuration to resist such buckling. Since the conrod is narrower
in the direction parallel to the centerline of the crank than it is
in the plane of conrod movement, one would expect it to buckle in
compression in this direction. The H cross section conrod puts the
most steel along the sides in that configuration and thereby resists
this buckling the best.

Is buckling in compression really a serious issue with conrods?
Really, I have absolutely no idea. But it would explain the
preference for the H shaped rod.

BTW, I really don’t think either the H shaped rod nor the I shaped
rod is the optimum shape. I believe the optimum shape would be a
rectangular box cross section – and being a conrod, it’d be a bigger
rectangular box cross section at the crankpin end than at the piston
pin end. A tapering hollow trapezoidal box conrod. Thing is,
though, is I have no idea how you’d make such a thing. And if you
could make it, I dunno how you’d keep the chamber inside from filling
up with oil, which would probably cause all sorts of problems.

– Kirbert

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In reply to a message from Kirbert sent Tue 15 Apr 2008:

There were one or two people who used fabricated titanium conrods
in British singles in the old days. Those guys used to take the
conrod from one engine to the other when they replaced motors.
Titanium is a lot cheaper since the Iron Curtain fell I believe.–
The original message included these comments:

BTW, I really don’t think either the H shaped rod nor the I shaped
rod is the optimum shape. I believe the optimum shape would be a
rectangular box cross section – and being a conrod, it’d be a bigger
rectangular box cross section at the crankpin end than at the piston
pin end. A tapering hollow trapezoidal box conrod. Thing is,


66 2+2, 78 RAM D-type replica
Cambridge, United Kingdom
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In reply to a message from Kirbert sent Tue 15 Apr 2008:

Yes, but the rod is also well restrained from deflecting in this
direction by the big end bearings and the small end bearings so the
line of action would be pretty well down the centreline making this
axis the least likely for buckling failure.Also…I’m not sure the
shape is optimised to resist buckling…it is broader down the big
end…(simple and effective nomenclature…)…presumably for a
reason…
Another thought…maybe it is a fatigue consideration where the
stresses are better spread into the bearing support
arrangement…or does it reduce the moment of inertia in the
direction of oscillation and thus reduce these loadings???
Hmmmm…–
The original message included these comments:

Now, it just so happens that the H shape would be the optimal
configuration to resist such buckling. Since the conrod is narrower
in the direction parallel to the centerline of the crank than it is
in the plane of conrod movement, one would expect it to buckle in
compression in this direction. The H cross section conrod puts the


MattFurness
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MattFurness wrote:

Yes, but the rod is also well restrained from deflecting in this
direction by the big end bearings and the small end bearings

Ooooh, gotta disagree there. At the loads we’re talking about –
tens of thousands of G’s – the soft bearings in the ends of the
conrod are not going to provide any significant hindrance to conrod
buckling. If they’re working right at all, the bearings are skimming
on an oil film, and the oil film will provide NO hindrance to conrod
buckling.

Also…I’m not sure the
shape is optimised to resist buckling…it is broader down the big
end…(simple and effective nomenclature…)…presumably for a
reason…

Well, the big end is where all the sideways G’s are. The little end
is reciprocating, there are no sideways G’s at all, so no need for
breadth to provide bending strength.

The big end is also where the big diameter journal is, and the loads
transmitted by the conrod must be distributed around that journal.

… Another thought…maybe it is a fatigue consideration
where the stresses are better spread into the bearing support
arrangement…

Fatigue is certainly a consideration with conrods, but I don’t know
how the H configuration would help. In fact, I’d almost expect it
would make it worse, what with all those edges of the H.

…or does it reduce the moment of inertia in the
direction of oscillation and thus reduce these loadings???

Are you asking if the H design makes the conrod lighter? Or if it
makes the conrod more flexible and therefore less likely to suffer
stress spikes?

– Kirbert

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In reply to a message from Kirbert sent Wed 16 Apr 2008:

Kirby

The oil film thickness maybe a few thousands of an inch above the
soft metal underlay but this thin film of oil is actually designed
to withstand the bearing loads. You would think that if buckling in
this plane was an issue you would see wear on the edge of the
bearings where the oil film had broken down. I have not seen this
in the engines I have had to bits…
Also…the clearance of the piston in the bore and the length of
the piston skirt by the gudgeon pin probably prevents the
application of any moment (along the plane of the crankshaft) into
the rod via the small end bearing. If this is the case then the
piston is pretty much applying direct thrust down the rod
centreline…mind you…the bearing ABSOLUTELY removes the
ability to apply a moment in the other direction…so maybe the sum
of the clearances in the other direction can add a significant
moment to the rod…
I was wondering if the H design can have better radii down onto the
big end journal and thus improve the fatigue situation at the
change of section.
The design may also have less inertia in the ocsillating direction
and reduce these loads.
I don’t think it is an issue of flexibility to reduce stress spikes
because I imagine you want very stiff reciprocating components to
assure predictable motion at high loads and speeds.
Normally with this sort of stuff the original designers get it
pretty right which is why the standard rods all look pretty similar.
Then along comes the ‘‘H’’ rod and it is supposed to be better.
It may well be an improvement in some second or third order
consideration which the original Engineers weren’t worried about.
I reckon that buckling would be one of the major considerations.
Maybe rotational inertia, maybe windage losses??? maybe something
else???–
The original message included these comments:

tens of thousands of G’s – the soft bearings in the ends of the
conrod are not going to provide any significant hindrance to conrod
buckling. If they’re working right at all, the bearings are skimming
on an oil film, and the oil film will provide NO hindrance to conrod
buckling.


MattFurness
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MattFurness wrote:

You would think that if buckling in this
plane was an issue you would see wear on the edge of the bearings
where the oil film had broken down.

To be clear: buckling would happen exactly once. And you’d be lucky
to find enough pieces of the bearing afterward to do such an
analysis. Buckling is not something that can occur on an ongoing
basis.

Also…the clearance of the piston in
the bore and the length of the piston skirt by the gudgeon pin
probably prevents the application of any moment (along the plane of
the crankshaft) into the rod via the small end bearing. If this is the
case then the piston is pretty much applying direct thrust down the
rod centreline…

All true, and all unrelated to buckling failure. A buckling failure
presumes pure compression load, no moment; the sideways buckling is a
result of the pure compression.

Let’s see if I can clarify buckling theory a bit. Presume the center
of a column under compression loading is deflected a small distance
to one side. The compression loading then works to increase this
deflection. There is also a “restoring force” generated within the
column itself, as a result of its stiffness, that tends to pull it
back straight. As long as the restoring force exceeds the
compression’s tendency to bend the column further, the column won’t
buckle.

The amount of deflection is the issue. If the center of the column
is deflected a lot, it doesn’t take much compression to overpower the
restoring force and buckle the column. The higher the compressive
load, the less you can deflect the center of the column before you
get into trouble.

There’s a limit, though; once a particular level of compression is
reached, any deflection at all will cause buckling – which means, in
essence, that no deflection is required, it will generate its own
deflection. The column will buckle under pure compression with no
bending moments or sideways deflections input. This is the failure
mode I’m talking about with conrods subjected to tens of thousands of
G’s.

Buckling is a catastrophic failure. Once buckling commences, it very
quickly accelerates to complete folding of the column because the
deflection reduces the load the column can carry. The very first
time a conrod tries to buckle, you will have a bent or broken conrod.

I was wondering if the H design can have better radii
down onto the big end journal and thus improve the fatigue situation
at the change of section.

Possible, I suppose, but I’m not sure what’s wrong with the radii on
the I configuration.

Normally with this sort of stuff the
original designers get it pretty right which is why the standard rods
all look pretty similar. Then along comes the ‘‘H’’ rod and it is
supposed to be better. It may well be an improvement in some second or
third order consideration which the original Engineers weren’t worried
about.

There’s also a design criteria difference. The engineers designing
the production conrods are looking for long fatigue life at moderate
loads and speeds coupled with ease of manufacture. The aftermarket
rods, though, are typically designed with competition in mind, so the
emphasis is on high load and high speed with less concern for ease of
manufacture.

It wouldn’t surprise me at all if the I conrod is significantly
easier to mass-produce than the H rod. Of course, it may just be
that the big manufacturers are geared up to make I rods and don’t
want to retool to make H rods.

Maybe rotational inertia…

Offhand, I’d suspect that H rods have larger rotational inertia. The
I has one broad rib and two short ones, while the H has two broad
ribs and one short one. Of course, presuming all these ribs are
roughly equivalent in thickness might be a mistake.

maybe windage losses???

Might the H form little scoops to catch air?

maybe something else???

Of course. We’re just guessin’ here.

– Kirbert

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In reply to a message from Kirbert sent Thu 17 Apr 2008:

Kirby.
Simple Euler Buckling theory assumes direct thrust and results in
amazing column capacities. But… the real world often results in
applied moments to the column and these relatively small moments
can have dramatic results in the column carrying capacity.
That is what I was trying to imagine as a source of failure.
A small amount of eccentric loading will dramatically reduce the
euler critical buckling load. I agree that buckling is catastrophic
but if there is a tendency to apply slight eccentricity to the
conrod then it would possibly show as wear patterns in the
bearings.
It may well be that the I beam is easier to cast. You can imagine
the rod lying flat on the bench with the split line halfway through
the thickness of the rod. Voila. The top mold is removed and there
is your classic I beam.
The ‘‘H’’ on the other hand has a part line which is at right angles
to the cores for the bearing surfaces. More difficult and expensive
to mass produce these.

So…maybe the ‘‘H’’ rod is marginally better than the ‘‘I’’ rod if it
is carefully designed with better attention to detail and less
emphasis on production costs.
Actually…it just occured to me…here’s the acid test…what do
they use in Formula One engines???–
The original message included these comments:

All true, and all unrelated to buckling failure. A buckling failure
presumes pure compression load, no moment; the sideways buckling is a
result of the pure compression.


MattFurness
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In reply to a message from MattFurness sent Thu 17 Apr 2008:

To Quote Allan Lockheed’s Engine Expert ‘‘Critical g Load,
Ring Flutter, Critical RPM, and Con Rods
The weakest components in most, though not all,
high-performance engines are the connecting
rods and the parts attached to them. (Some other sources of
failure are inadequate oiling systems,
detonation, and valve train breakage. However, the program
does not deal with these items at
this time.) That is, the likelihood of your engine breaking
at high RPM because of a weak part is
usually directly related to the reciprocating loads on the
connecting rods. In a naturally aspirated
engine, the biggest of these loads occur as the piston stops
and reverses direction at TDC overlap.
Our experience indicates that well built engines, with
balanced and polished high-performance
steel rods, upgraded rod bolts (7/16 inch aircraft steel
capscrews for example), forged high
performance pistons, high performance cranks (high nodular
iron or steel), and 4- bolt cylinder
blocks fail only when the peak acceleration across TDC
approaches 4,350 g’s.’’

Another tid-bit of trivia is the difference between street
car and racing bearings. Besides being slightly different
material the biggest difference is the racing bearing is
thinner at the ends of the bearings. Where the rod splits.
Why? because they (clevite) have measured over .010’’
deflection of the big end of the rod in high rpm racing
engines, as normal operation. Mainly due to the ‘‘g’’ load of
the at TDC.
Just thought you might like to know.
Regards,
John–
John Aller www.audietech.com
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In reply to a message from MattFurness sent Thu 17 Apr 2008:

''It may well be that the I beam is easier to cast. You can imagine
the rod lying flat on the bench with the split line halfway
through the thickness of the rod. Voila. The top mold is removed
and there is your classic I beam.

The ‘‘H’’ on the other hand has a part line which is at right
angles to the cores for the bearing surfaces. More difficult and
expensive to mass produce these.’’

Rods are forged, not cast, but if you subsitute die for mould you’d
still be right about ease of manufacture (but see below).

Forging also results in greater strength because the ‘grain’ of the
metal can be arranged to run along the stress lines rather than
across, or in no particular direction like casting.

The H beam rods will be machined from forged blanks I’m sure, just
like OEM I rods. But instead of a rough-finished forging that drops
out of the dies easily, and is cheap to make because it needs only
to be machined at each end, H-beam forgings are machined all over.
So the final H shape has probably got nothing to do with how easy
it is to forge because it is that shape purely as a result of
machining the original blank on every surface. This gives better QC
for a start, but ism ore expensive, obviously.

Some H rods - especially exotic metal ones - may be machined from
billet.

I think that sound rods don’t generally fail from compression -
though there is probably some top-fuel dragster somewhere that runs
eye-watering boost to prove me wrong. I think they generally either
fail from tension (classic scenario is changing down a for a
corner) or by bending that is secondary to some other failure.

Eg: Rods that snap near the top can be broken from sideways bending
such as if the gudgeon pin twists in a broken piston boss,
especially with floating pin designs if a circlip is left out and
the pin drifts over to one side.

The higher the rod strength, the more gentle the progression from
central beam section to the eyes. In side view of an H section rod
like a Carillo, the small end is just a rounded top to the dead
straight sides and there is no ‘swelling’ where the piston pin
sits. They just machine a slot in the sides of the beam that leaves
the necessary meat at each end. The big end also has a nice gentle
curved web each side, precisely machined and with excessive overall
mass avoided because you put the metal where it’s needed most and
can machine away every other milligramme, unliek a ofrged rod where
you end up leaving a little bit extra because it’s a forging?

‘‘Actually…it just occured to me…here’s the acid test…what
do they use in Formula One engines???’’

I have no data, and you can design an I beam to be very strong -
especially on ultra-short stroke engines - but I’d expect all
modern F1 engines to use H beams. This item makes even a ‘cooking’
Carillo look like something from a steam engine…

http://tinyurl.com/6l5fju

Pete–
66 2+2, 78 RAM D-type replica
Cambridge, United Kingdom
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doctorpipe wrote:

Another tid-bit of trivia is the difference between street
car and racing bearings. Besides being slightly different
material the biggest difference is the racing bearing is
thinner at the ends of the bearings. Where the rod splits.
Why? because they (clevite) have measured over .010’’
deflection of the big end of the rod in high rpm racing
engines, as normal operation. Mainly due to the ‘‘g’’ load of
the at TDC.

Cool! So the piston is pulling so hard on the conrod that the big
end ovalizes, squeezing the sides in closer to the journal. And they
open up the bearings so that this doesn’t close the clearance up too
much.

– Kirbert

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Guys,

I think I should add that the G’s, while a contributing factor, are not the
critical factor and can be misleading.

In the case of the rod, the peak force is occuring at TDC and is basically
taken by the big end cap and bolts (hence all the fuss about your bolts) and
are proportional to the G’s (which is RPM and stroke related) AND the
dynamic mass, which is predominately the piston, pin and rings (plus a bit
of the rod).

The bolts must clamp with a force greater than the peak loading, so that
things stay together. So there are issues about how tight the suckers need
to be.

To reduce the stresses involved, the focus is generally on reducing the mass
of your piston (and pin) if at all possible.

Of course, most just reduce RPM, but this is not practical if you are aiming
for top HP.

If anyone has data they want to throw me, I can calculate the kinds of
forces occuring. The important number is the big end force in Newtons (sorry
I’m metric).

I include here some data I posted on a previous thread:

Jag 6.7 litre (410cu?) V12
Forged Pistons (95mm) at 486gm ea (inc rings).
Rods Chev Carillo at 686 gm.
Pin is 139gm (inc clips).
Big end bolts ARP at 74 gm.
Stroke is 79mm.

Big end force is 34,600 N at 8500 rpm. (Max piston G’s 4,000)

NASCAR V8 running 400gm pistons, 72gm pin, 525 gm rods @ 10,000 rpm has
37,000N big end loading. (5800 G’s)

So I figure with decent rods and pistons in the sort of config we are
talking, 8500 is not out of the question and is still 6.5% less than “state
of the art”.

You might notice that even with what is “state of the art aftermarket” with
our Jag engine, there are large areas of impovement that could be made in
reducing piston, pin and rod mass, if price (and durability) weren’t an
object.

Titanium rods would drop your rod weight by 1/3rd. But this impacts dynamic
mass only by 1/9th. So for the cost, it would drop my peak forces from
34,600N to 31,600N or allow an increase in RPM from 8500 to 8900. So I
figure I can live with what I have got. (I could achieve the same result by
removing 46g from the piston and 39g from the pin, both of which should be
relatively simple.)

I also note, that pin weights are now down to 50g and both piston and pin
and rod weights are limited by regulation to try to restrain costs. Some
rods now have no little end bearing to reduce weight and run direct on a DLC
(Diamond Like Coating) coated pin.

Regards
Mark Eaton

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PeterCrespin wrote:

I have no data, and you can design an I beam to be very strong -
especially on ultra-short stroke engines - but I’d expect all modern
F1 engines to use H beams. This item makes even a ‘cooking’ Carillo
look like something from a steam engine…

http://tinyurl.com/6l5fju

I dunno what a “cooking” Carillo is, but that’s an interesting
picture – and from 1993 no less.

Seeing this does make it clear that the H design results in a cleaner
meeting between the webbing of the H and the threaded boss that the
big end cap bolts thread into. That alone may be enough reason for
its superiority.

There are a couple of things I don’t get about the parts in that
picture. I don’t understand the notched appearance on the sides of
the big end. I also don’t understand why the conrod appears to be
stepped a bit narrower in the H area than it is at the big end; I
would expect you’d want to keep it full width over its entire length.

I’m betting that engine’s really oversquare. I can’t recall the last
time I’ve seen a piston that was that much larger than the big end on
the conrod.

– Kirbert

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In reply to a message from Kirbert sent Fri 18 Apr 2008:

Well to me a Carillo is ‘the bees knees’ as we say over here, and
far from ‘cooking’ (term we use for bog standard old engines) UNTIL
you stand it next to the Ferrari part. At that point the average
Carillo looks somehow old tech…

I’d guess the grooves in the side of the big end are for lube
escape to avoid friction between the cheeks and the crank or
between adjacent rods at the rpm these things run at.

The waisted rod above the big end is probably just a case of only
having as much metal as you need and no more. If the deep webs take
care of much of the load in the rod beam, they probably don’t have
to have them full thickness as well.

Not sure about this engine but around that time Ferraris were V12
with something like 85mm bore versus 59 mm stroke - ultra
oversquare. That rod/piston looks like it is from that type of
engine. I think when you get such dimensions you end up with
pistons rocking in the bore due to practically no skirt, but they
obviously sussed it all out.

Pete–
The original message included these comments:

picture. I don’t understand the notched appearance on the sides of
the big end. I also don’t understand why the conrod appears to be
stepped a bit narrower in the H area than it is at the big end; I
would expect you’d want to keep it full width over its entire length.
I’m betting that engine’s really oversquare. I can’t recall the last


66 2+2, 78 RAM D-type replica
Cambridge, United Kingdom
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In reply to a message from Mark Eaton sent Thu 17 Apr 2008:

This is why I suspect there are effects or forces other than simple
compressive loads at work on a conrod. 34600N is not a large force.
Actually I would be happy supporting this load with a piece of
1/2inch medium tube 200mm long which has a critical buckling load
of 10 time this value!!!
The fatigue cycles will be through the roof and there will be
induced moments from the oscillating movement and so on and so
forth.
The fact is that the ‘‘I’’ conrod is fundamentally better at
resisting moments in a completely different plane than the ‘‘H’’ rod.
The ability to resist pure compression is dependant on the moment
of inertia of the section and both can achieve a suitable value to
do the job.
It may be that the ‘‘H’’ rods get more attention to detail to better
resist fatigue and reduce localised stresses and are designed with
scant regard to production costs.
I wonder what the drag racing fraternity use in their amazing
engines.
These things are right on the leading edge of the load envelope and
you can bet a lot of dollars that what they use is as strong as you
can get. They only have to do a few thousand cycles and their work
is done.
If they run with the ‘‘H’’ rod and the F1 guys use it then it’s gotta
be fundamentally better.
Be interesting to know exactly why?–
The original message included these comments:

Big end force is 34,600 N at 8500 rpm. (Max piston G’s 4,000)


MattFurness
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MattFurness wrote:

This is why I suspect there are effects or forces other than simple
compressive loads at work on a conrod. 34600N is not a large force.

I’m not used to working in Newtons. That’s about four tons, isn’t
it? Standing on your conrod? Yeah, perhaps not that big a deal.

Actually I would be happy supporting this load with a piece of 1/2inch
medium tube 200mm long which has a critical buckling load of 10 time
this value!!!

Well, I suppose I could point out that a circular cross-section is
considerably better at resisting buckling than the shape of a typical
conrod! Point taken, though.

The fact is that the ‘‘I’’ conrod is fundamentally better at
resisting moments in a completely different plane than the ‘‘H’’ rod.

Yes, and at first blush it looks like the I rod has the strength in
the direction needed!

It may be that the ‘‘H’’ rods get more attention to detail to
better resist fatigue and reduce localised stresses and are designed
with scant regard to production costs.

There’s no doubt that the H rods – either Ferrari or Carillo – are
made with more care than the typical mass-production I rod. But we
need to separate the quality of workmanship from the basic
configuration. Ferrari and Carillo could just as easily be putting
the same care and workmanship into I rods – but they’re not, they’re
making H rods. What is behind that decision?

If they run with the ‘‘H’’
rod and the F1 guys use it then it’s gotta be fundamentally better. Be
interesting to know exactly why?

OK, so you’re not buying the buckling explanation. Lemme try another
one: Maybe there’s a resonance issue. When you get an engine
screaming, there are all sorts of resonances involved, and they have
all sorts of detrimental effects. For example, resonances in the
block can dynamically misalign the main bearing journals, so when you
take the engine apart everything looks fine except the bearings are
wiped out – and you wrongly blame the bearings because you can’t
find any cause, the bearing journals are perfectly aligned when
measured on the bench.

It’s apparent that an H rod would be stiffer in the direction of the
depth of the rod while the I rod would be stiffer in breadth. Due to
the rod being broader than it is deep, going with the H cross section
would tend to bring the stiffnesses in the two planes closer to one
another. Given the direction that the centrifugal loads are applied,
you’d expect to want more stiffness in the breadth and therefore the
I conrod would be better, but perhaps the centrifugal loads are not
that overwhelming and resonances are actually the more serious
concern, and somehow stiffness in the depth direction would help
address them.

– Kirbert

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In reply to a message from Kirbert sent Fri 18 Apr 2008:

I don’t follow this line about an I con rod being stiffer in some
direction than an H con rod. H con rods are deeper in the direction
of rotation and wider in the line of the crank. Gonig from meory
but that’s how I recall my Carillos. At a guess I’d say my Carillos
were stronger in both directions than the Triumph rods they
replaced, although of course those rods were forged alloy with a
steel cap, so they were chunkier all round than a steel rod would
have been. but I’d see them as stronger than a typical I beam rod
also.–
The original message included these comments:

The fact is that the ‘‘I’’ conrod is fundamentally better at
resisting moments in a completely different plane than the ‘‘H’’ rod.
Yes, and at first blush it looks like the I rod has the strength in
the direction needed!


66 2+2, 78 RAM D-type replica
Cambridge, United Kingdom
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In reply to a message from Kirbert sent Fri 18 Apr 2008:

Hmmmm…also…maybe torsional resistance is improved with the ‘‘H’’
correctly desinged. I’m off on holiday for a few days…I might
ponder this while I’m relaxing with a beer.
Or not.–
The original message included these comments:

one: Maybe there’s a resonance issue. When you get an engine


MattFurness
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PeterCrespin wrote:

I don’t follow this line about an I con rod being stiffer in some
direction than an H con rod. H con rods are deeper in the direction of
rotation and wider in the line of the crank.

I’m speaking, of course, of two conrods that have the same exterior
dimensions – same width, same depth. The exterior dimensions of a
conrod are usually limited by other factors; for example, the width
in the line of the crank can’t be more than the clearance between the
counterweights cast into the crank on either side of that journal.
The breadth in the direction of rotation is usually limited by having
to clear the bottom edge of the cylinder at the 11:00 and 1:00
position.

If you need stronger rods and the rods you have aren’t already out to
these limits, the best way to get stronger rods is to go out to those
limits. You can make the actual webs of the conrod very thin if
they’re waaay out far from the centerline.

The question at hand is, presuming you’re already at those maximum
dimensions, do you configure the rod as an H or an I? An I will be
stiffer in the direction of rotation. An H will be stiffer in the
line of the crank.

AFAIK, we’re still at a loss here as to why the state of the art
seems to be the H configuration.

– Kirbert

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In reply to a message from Kirbert sent Sat 19 Apr 2008:

‘‘AFAIK, we’re still at a loss here as to why the state of the art
seems to be the H configuration.’’

Indeed - that’s because we’re all guessing.

I don’t see why an I is stronger in the direction of rotation
though, Two webs may be better than one, even if the single I web
is fatter. In fact an H might be the best way of getting stiffness
in that plane, without getting to unfeasible mass. I don’t think
stiffness in the line of the crank is a major criterion since there
is really not a major force in that orientation if all is well.

But I’m still only guessing.

Pete–
The original message included these comments:

The question at hand is, presuming you’re already at those maximum
dimensions, do you configure the rod as an H or an I? An I will be
stiffer in the direction of rotation. An H will be stiffer in the
line of the crank.


66 2+2, 78 RAM D-type replica
Cambridge, United Kingdom
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
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