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dodge8564 (Automotive) (OP)
13 Aug 03 2:23
Can someone please explain how load is transmitted through the drivetrain to the connecting rods?
I am in a discussion in another forum about how an engine needs stronger connecting rods for towing, even if the engine never reaches above 3500 rpm.  
Another person is arguing that load is not transmitted through the drivetrain to the engine itself, and the only reason to have stronger rods and bearings is for high rpm's.
patprimmer (Publican)
13 Aug 03 3:55
I can't see how anyone can fail to realise that the force is generated by expanding gas in the combustion chamber, then transmitted via the pistons, rods, crank, clutch, gearbox, tailshaft, final drive, axles, wheels, then tyres.

Slightly different for front wheel and automatic transmission drives of course.

However, full power is full power, so the stress on the rods at WOT is the same, towing or not, the only difference is that when towing, more power is required to maintain the same speed when compared to normal.


dodge8564 (Automotive) (OP)
13 Aug 03 4:04
Correct, but what I mean is that towing a load causes more pressure to be exerted on the moveable parts, because of increased resistance to forward motion.  Basically I want validation ;D for (load)=(stress)
Helpful Member!  andyv8 (Automotive)
13 Aug 03 8:08
The load on the con rods will be highest at peak cylinder pressures or maximum rpm. The peak stresses in a conrod  are defined by rpm/fueling/ignition and will peak at the engines peak torque figure or max rpm. If more load is put onto the engine (ie by towing) that the engine has available torque, then it will start to slow the engine down and eventually stall it. Adding more load will not increase the peak cylinder pressures and will therefore not increase stress in the con rods.
Once upon a time, BMW tried to put their V12 (car engine) into the Range Rover, but the torsional shocks transmitted through the drivetrain when driving off-road resulted in quite a few twisted cranks...... This wouln't be a problem for the con-rods though as they will only ever see the same pressures as  seen during full-load testing on the dyno.
patdaly (Mechanical)
13 Aug 03 8:22
At least from my perspective, the power transmitted is more or less compressive, hence virtually any stock rod made will handle stock type RPMs, HD useage or not. At least in my experience, the only time things come unhinged is when the RPMs climb, and the Tensile stresses exceed the stock rod ( or Rod bolts ) capabilities.
mikikanazawa (Automotive)
13 Aug 03 20:33
If the engine is stock, peak power is peak power so towing would not normally exert any more compressive force on the conrods than would accelerating at wide-open throttle.  Towing would just increase the amount of time peak compressive forces are on the rod.

Unless, of course, the engine is capable of making torque in an RPM range where the oil pump is not providing sufficient pressure in the main and rod bearings.  This could possibly cause a shock load on the rod that could buckle it.
dodge8564 (Automotive) (OP)
13 Aug 03 21:27
Hmmm, okay, I was wrong.  Darn, I hate being wrong....  Thanks for the thoughts guys, I appreciate at least knowing I was wrong.
JayMaechtlen (Industrial)
13 Aug 03 21:58
what about fatigue life?
in a towing app, you can be running near peak torque for much longer periods of time. (or ranging between peak torque and peak power!)
That means a lot more load cycles. Are con rods typically stressed to a level where that would be a factor- or are they in the "infinite life" part of the curve?

Jay Maechtlen

mikikanazawa (Automotive)
13 Aug 03 22:27
Actually I think what you mean is more load per cycle, rather than more load cycles, right?  Ok unless you've selected a lower gear for towing a load while keeping vehicle speed the same.

Even at that, a compressive force on a conrod does not fatigue it.  The tensile force (when the piston is yanked away from TDC) is the primary cause of fatigue.  A properly designed and implemented conrod will not experience fatigue to speak of if the reciprocating mass and RPM are kept in check.  Ever wonder why a conrod bolt doesn't break even though it can be cycled millions of times?  The clamping force is greater than the tensile load.
JayMaechtlen (Industrial)
14 Aug 03 1:13
No, I mean number of cycles.
I know that the bolts, if properly sized and preloaded, see only a fluctuation in tensile stress.
The rod beam sees a full reversal, from full compression to full tension, repeatedly.
(actually, under load, will only see tension every two revolutions, on the inlet. It sees reversals at the transition from exhaust to intake, then from intake to compression, right?)

Jay Maechtlen

patprimmer (Publican)
14 Aug 03 1:19
Towing does tend to increase rpm by two scenerios

1) Going up hill, it is often necessary to select a lower gear to just maintain road speed.

2) Going down hill it is prudent to use a lower gear to take advantage of engine breaking and preserve the brakes in case they are needed. The rule I was taught was always decend a hill in the same gear you would need for the acent.

This might not apply to large modern cars with light trailers, but in my experience, it starts to come into play as the towed weight gets to within about 3/4 of the tow car weight, and is becomeing critical at about even weight.

Running down hill on a trailing throttle in a lower gear will tend to try to stretch the rod bolts, and the gudgeon pin area of the rod, but most modern cars have huge safety factors in this department, and rods, rod bolts and guedgons last well beyond the life of the rings and bearings and valvetrain, and these mostly outlast the body, trim and suspension.

Back to the original question. It has already been answered to varying degrees at least 3 times, but here goes again. A trailer has no direct effect on the load on the engine components, as full cylinder pressure and maximum rpm produce certain forces within the engine, no matter what. A trailer indirectly effects the engine in that the vehicle requires more power to maintain speed, so on average, the throttle will be opened more and lower gears will be used more often as it is necessary for the engine to produce the extra power to cary the extra load.


mikikanazawa (Automotive)
14 Aug 03 1:36
Actually, properly "stretched" rod bolts see zero fluctuation in tensile load.

If they are providing 10,000 lbs of clamping force, and the highest tensile load is 9,999 lbs then the bolts receive no more "stress" than they are already preloaded to.
CoryPad (Materials)
14 Aug 03 9:17

You are incorrect.  Even properly stretched fasteners will experience pretension variation.  Fasteners act as parallel springs, so the amount of the joint force that is added to the pretension is a function of geometry and material properties (load introduction point, screw compliance, and joint compliance).  You can learn more at:



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strokersix (Mechanical)
14 Aug 03 12:56
Rod bolts also experience bending loads, primarily due to deflection of the rod big end under tensile stress at TDC.  I understand that this is one reason why bearing inserts are tapered in thickness toward the rod cap parting line, preventing metal-to-metal contact.  How large of an effect this is relative to linear clamp loads I don't know.  I'm sure others can answer this.
mikikanazawa (Automotive)
14 Aug 03 13:07
That's true, but the bending forces are "negligible" when discussing a properly designed rod and bolt system.  Or so I've been told.  Personally I haven't had a rod failure yet, but then I've only built around 30 engines or so.  (Four bangers, displacing from 1.6L to 2.2L and all generating at least 150 HP per liter.)
xr7755 (Mechanical)
31 Aug 03 4:59
You mention towing. The important thing happening in towing that isn't happening in a lighter vehicle, especially in mountainous areas. Is constant load. The average auto engine is not designed to put out continuous advertized HP.
Most likely they would melt down from the heat generated. Kinda like the dragster engines. Most have no cooling systems because they only put out the HP for a very short time. As far as the connecting rods needing to be any stronger. As others have said there is no reason for it. The peak pressure load is a compressive load and the con rod is acting as a column when it is applied. I'd say you would blow holes in pistons before you would over stress a stock con rod in that situation.
bettonracing (Automotive)
8 Nov 03 9:42
I think u guys did a good job collectively of analyzing the topic but alot of the 'basics' were overlooked.

I agree (to a certain extent) that there is more loading on the connecting rod when towing. The fact that the weight of the vehicle tends to resist the engine rotation thru the drivetrain (can be simulated by using some sort of brake to hold the flywheel from turning) and that the same pressure (let's assume peak cyl pressure) is being applied from the piston, there is more compressive load on the connecting rods.

However as mentioned before, the max pressure is at *max*  when there is peak cylinder pressure. (actually max should be when the resistance overcomes the peak cyl pressure and the engine is slowing down @ wot.) but since conrods are usually built to handle peak cyl press + some extra, the weakest link would turn up to be the piston (and in few cases the crankshaft), also prev stated.

Conclusion: Towing doesn't justify a new set of rods, but they are put under additional strain (esp obvious when towing is compared to not towing).  

my $0.02....make that $0.01... :D
ShaunT (Automotive)
8 Nov 03 10:01
I think 2 real factors that have been missed here are Detonation and Fuel grade. These go hand in hand and when wrong will kill anything quickly. Especially something that you are leaning on pretty hard.


bettonracing (Automotive)
8 Nov 03 10:08
Those two factors more affect the piston than the con rod (even tho they do indirectly affect the rod). But nobody worries about their rods breaking from detonation (and fuel grade). They worry about burning holes in the pistons.

*see original message*
unterhausen (Automotive)
9 Nov 03 11:03
I don't really see the answer to original question.  The answer is that if all that is varied is load, a higer load will cause higher stresses to develop in each part that transmits force from the piston to the wheels.  Thus the proper answer to the origninal question is that both people in the argument were wrong.

Consider, if you will, an ideal engine with no possible failure modes and no friction and no mass.  The force on the connecting rods is then the force developed by the piston in opposing a load.  If there is no load, the engine quickly goes to infinite rpm and leaves it's earthly bounds.  On average, the torque required by towing will be more than the torque required by not towing, and thus the average force on the connecting rods will be higher.  The reason that people don't worry about this on real engines is that connecting rods are stronger than they need to be to counteract buckling.
xr7755 (Mechanical)
10 Nov 03 14:21
"Can someone please explain how load is transmitted through the drivetrain to the connecting rods? "
Yeah guess we did miss the question.
The load starts at the tire contact radius, and is transmitted through the axle, to the differential gear set, to a drive line or propeller shaft if it is hotchkiss drive, then to the transmission then to the crankshaft and of course then to the rods.
All gear sets in between are torque mulitpliers. And in the end, in theory, you see close to the same HP at the rear wheels that the engine puts out. Actually though it isn't because of losses.
Rob45 (Automotive)
21 Nov 03 17:04
The greatest load on a connecting rod is inertial load, occurs at top dead center on the exhaust stroke,  and produces a tensile stress, not a compressive stress.
This is true regardless of engine load,  and depends ONLY on the RPM for a given engine.
ivymike (Mechanical)
21 Nov 03 18:06
The greatest load on a connecting rod is inertial load, occurs at top dead center on the exhaust stroke,  and produces a tensile stress, not a compressive stress

Is it fair to say, then, that you believe there is no such thing as an engine where the firing load exceeds the inertia load on the conrod?

patprimmer (Publican)
21 Nov 03 20:08
I think that is approximately true for normally aspirated engines, but once you get into high boost and/or methanol and nitromethane fuels, the compressive loads can get very high, but this has nothing to do with the original question.

The highest load on the bolts is at maximum RPM with no load. ie a closed throttle as the vacuume in the inlet manifold helps to stretch the bolts by reducung the pressure on the piston that might otherwise help it to change direction at TDC. I think the highest stress is at the highest piston acceleration rate on its way down the induction stroke. That position changes a bit depending on rod to stroke ratio, and I don't see a need to calculate it accurately, but it is definitely well after TDC.

At TDC, the piston is instatainiously inertia free, as it is instantainiously stationary.


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ivymike (Mechanical)
21 Nov 03 21:29
That position changes a bit depending on rod to stroke ratio, and I don't see a need to calculate it accurately, it is definitely well after TDC.

Hey Pat,  it might be worth double-checking your logic on that one.  The piston is instantaneously motionless at TDC (excluding secondary motion), but it is certainly not without acceleration.  

patprimmer (Publican)
21 Nov 03 23:24

A moment of brain fade, it must be subject to acceleration to stop and start again.

It's to long since I studied simple harmonic motion and I must admit I forget how to do the sums.

I know from experience that rod bolts most often break as you lift of the throttle at maximum RPM.

The lift of adds manifold vacuume to the forces on the piston, so the maximum force must be after the exhaust valve closes, unless the pulse tuneing and gass inertia pulls a stronger vacuume in the exhaust than the closed throttle does in the inlet. I can't imagine this ever being so, so maximum suction on the piston must be after the exhaust valve closes, and the resultant force from vacuume over the piston and inertia must therefore be greatest after the exhaust valve closes, otherwise you would not get more failures on a closed throttle


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xr7755 (Mechanical)
22 Nov 03 12:24
Pat, just curious. On the bolt breakage. What engine and what rpms? How many bolts broke?
crashbox455 (Automotive)
22 Nov 03 19:16
i don't understand one thing- if an unloaded vehicle under WOT reaches the same cylinder pressure as a loaded one under part throttle, why does the loaded one detonate?

resisting the motion of the piston will increase cylinder pressure, perhaps beyond the "shock" tolerance of the rod?  i'm not a structural engineer, so maybe someone who is can explain why i'm wrong.  but it seems to me that putting the vehicle under load will spread the peak pressure out over an extra degree or two of crank motion, possibly causing premature rod failure.

anyone care to counterpoint?
patprimmer (Publican)
23 Nov 03 21:00
A long time ago, (197?) once only, the rest are only anecdotal

Roadracing 350 Chev LT1

7500 rpm stock (pink spot) rods, 3/8" ARP bolts, endurance races.

The engine peaked out at 7000 rpm, but the driver had a habit of changing down comeing into a downhill corner.

Only one bolt broke that I saw, but a number of comments were made by others who saw it, that it always happens as you back of and it goes lean. I knew the goes lean was wrong and has nothing to do with it, but the guys who commented were much more experienced than I, although not nearly as scientific.

I always attributed it to the extra vacuume as the throttle closed at very high rpm.

Since then I always used aftermarket rods and 7/16" bolts. No more failurs.


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patprimmer (Publican)
23 Nov 03 21:23
The engine at WOT is always under load, even if it is it's own inertia. Low external load will see a very rapid increase in rpm as the cylinder pressure overcomes the inertia load. If there was no load, rpm increase would be instantainous instead of very rapid.

The initial question was not about no load vs loaded to stall, it was about the variables from the load of an unladen car to a heavily laden car, say an increase of 1400Kg to 3000Kg as an example.

I have never ever seen a standard rod in a standard engine built since the 1960's fail in compression, unless hydraulic locked or seriously flawed in the first place (actually I have never seen that either, but consider it possible).

I have seen engines that have been hydraulic locked. They  pull every head studor blew the porcelain out of spark plugs, but still not bend rods.

I have seen rod bolts break, once in an engine I was involved in (see above), and in other engines, in which I was not involved.

I have never seen an engine failure that could be directly and solely attributed to a rod or rod bolt failure in a standard engine in an everyday drive car. Any failed rods in these cases had other damage such as spun bearings, broken gudgeons etc


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GregLocock (Automotive)
23 Nov 03 21:35
My partner in crime once assembled a diesel engine from two donors. He had not realised that the block height and the conrod length were matched, and used the block from one and and the rods from the other.

On reassembly the engine ran OK, but very quickly blew the head off, by stripping the head bolts.

We bodged it back together only for the same thing to happen again.

This time he looked around carefully and noticed that the serial number stamped into the piston's crown was clearly visible on the head (or vice versa). The piston had been hitting the head hard enough to break the head bolts.

The con rods were fine.


Greg Locock

ivymike (Mechanical)
24 Nov 03 1:18
couple of questions -

* Is it necessarily the case that impacts between the piston and the head cause the conrod to be loaded in compression (which is what I think you were implying above)?

* Are you guys (pat and greg) trying to tell us that any conrod that is capable of withstanding the tensile loading in a (naturally aspirated, if you must) engine can withstand the compressive loading that it experiences in the same engine?  (presumably the reason you would assert such a thing is to counter the previous statement by unterhausen?)

patprimmer (Publican)
24 Nov 03 4:31
If a piston is hitting the head hard enough to pull the studs, I think it is safe to presume the reaction is transmitted back through the piston through the pin to the rod.

I know that whenever I walk into a closed door, my nose certainly compresses, even if I do not hit it hard enough to pull the hinges off.

I certainly am NOT saying that because the tensile is adequate, this means the compressive is also. This would imply that strong rope could work as a con rod, as it has good tensile, shame about the compressive though.

I am saying that in all engines that I have seen, used or have relevant information on, which is quite a few, the conrods are built with such a safety factor in both tensile and compression, that they never ever fail, unless something else happens to break them.

For tensile, they simply need enough cross sectional area of a suitable strength steel. Design (appart from notches) does not matter.

For compressive, they need a beam with depth of section in height and width (if you presume that length is pin to pin direction) so as to resist bending.

I see no real reason to counter much of unterhausen's post. despite him saying the question was not answered, he gives much the same answer that is repeated over and over again, with slightly different symantics.

The original question is well and truely answered, and this thread is starting to get really hyperthetical and boreing, so I don't think I will have anymore to say on the matter


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Rob45 (Automotive)
24 Nov 03 18:13
To InHiding, who wrote:
"Is it fair to say, then, that you believe there is no such thing as an engine where the firing load exceeds the inertia load on the conrod?"

The short answer is, 'Yes.'
Note all the other stories above about rods failing when the throttle is closed and the cylinder pressure drops dramatically.

And 'beliefs' have nothing to do with it;  I've done the math, and instrumented and tested the engines.
Tmoose (Mechanical)
24 Nov 03 19:18
I've heard several stories about rods failing at closed throttle.

There are a few pieces of those stories I have never managed to fit together well enough for my satisfaction.
1 - fatigue failures (cracks) can take hundreds or thousands of heavy stress cycles to progress from initiation to the big bang grand finale.  With the aid of the magnaflux process I have seen dozens, if not hundreds of rods and crankshafts with cracks in the expected high stress areas. Those cracks have varied from so tiny as to be removed by a little polishing, to so deep and wide that it was only half a joke to say that the owner had turned off the ignition on the engine revolution just before the part broke in half.
2 - I've seen a few busted rods and crankshafts and valve springs.  The crack Always (even the battered parts) showed the text books signs of crack progression. Really, the only question might be how many cycles it took for crack to hurry thru the last part of it's final journey.
3 - I figure The tensile force on a 4-stroke connecting rod at the exhaust TDC is always un-opposed by significant cylinder pressure.  

So, I have long felt that heavy reversing stress cycles accumulate quickly at full throttle max rpm. They may accumulate at most twice as fast at closed throttle max rpm.
So, in my mind, 2 minutes of max rpm full throttle is much like 4 minutes of max rpm closed throttle.  Since the forces vary as speed^2, an extra 500 rpm (on a bad down shift) is probably a much greater danger than simply 'backing off."

Well prepared parts (magnafluxed, well radiused and shot-peened) have a nice fat bank account of useful stress cycles to spend before failure.  A this point it would surprise me if the "fatigue checks" only get cashed when the throttle is closed.  If I am missing the reason part throttle stresses are SO HIGH, then it would seem likely that several rods would often break at once, and maybe all the rods would break at the same time once in a while.
Skeebo (Automotive)
24 Nov 03 22:15
Pat finaly some one said it.The highest cyl press. is when the throttle is snapped shut after WOT.This causes a Hydrolock affect in the cyl.You can prove this by watching the KV scale on any secondary ignition scope.
This is why you cant run alluminum rods on the street they will fly apart on far as having to spend big bucks on a set of rods just debur all edges and sides verticaly and pollish the rod will eliminate any stess fractures from starting.we ran stock rodsfor ten years in the national tractor pullers asso.never broke a rod.we ran twin 396 chevy's approx 1200 hp each.the only break downs where usualy caused by valve failure.
Todd Sternberg
GregLocock (Automotive)
24 Nov 03 22:54
I find it hard to believe that. Where does this pressure come from?

Look at the load on a piston as it approaches TDC.

Acceleration at TDC is roughly r*w^2, or at 6000 rpm with 100mm stroke, that is 0.05*(6000/60*2*pi)^2

So that is a force of about 11 kN on a 550 g piston. In the absence of any gas forces that is all supplied, in tension, by the rod.

If it is compressing gas then there is downward pressure of CR*piston area*atmospheric pressure, or say 9.5*pi*.046^2*101325= 6kN, on the piston. Therefore the load on the connecting rod has HALVED at full throttle compared with the unloaded case at the same speed.

So when the old guys talk about the intake charge cushioning the piston they were right.



Greg Locock

ivymike (Mechanical)
24 Nov 03 23:44
I've done the math, and instrumented and tested the engines.

That's a bold statement - to prove the assertion that no such engine exists, you'd have to test every engine in existence, wouldn't you?  Blanket statements are dangerous; I didn't figure I'd have any "takers" on that one.

See if these figures work for you:

peak cylinder pressure 170bar
rated speed 1800rpm
cylinder bore 155mm
stroke 165mm
rod length 310mm
piston and pin assy mass 7.21kg

conrod load due to pcp: ~320kN
conrod load due to inertia at tdc exh: ~27kN

I know which one I'd be more worried about.  There may be at least one more such design out there.  

My point is simply that one must check both buckling and tensile loads when doing classical checks on a connecting rod.  Sometimes one is big, sometimes the other.  It's often easy enough to guess which will be the big one, but if you've done the calculations once, you can run through them all in about 5 minutes with a hand calculator, so why not do ALL the checks instead of just waving hands and pointing thumbs?
ivymike (Mechanical)
24 Nov 03 23:49
...for accn at TDC, I prefer 0.5*(stroke)*(w^2)*(1 + 0.5*stroke/rod_length).  With common ratios of rod/stroke, the number is about 25% higher than the one you'll get with the abbreviated formula above.
framorgal (Automotive)
4 Dec 03 16:57
I think that many of the participants on this thread are confused whith the concep of "towing". Really is the same if the engine is used for towing or if is instaled into a test bench performing a Power curve. If the engine runs stationary at a given speed -WOT-, this means that there is an equilibrium between the power developed for the engine and the power consumed by...wheels, propeller, dynamometer... is the same!, if the engine doesn?t runs stationary and decelerates, this means that is needed more power from the engine, and if is possible the system (engine-consumer) stabilize at a different engine speed, if not the engine stalls.
About the rod mechanical loads, you must to consider a complete engine cicle -720?- and apply to the rods THE SUM of all the forces calculated each degree (for example):
 Piston inertia, Gas forces, Rod inertia, bearings resistence...
In the worst situations:
 Max. engine speed (with and without load), max. torque speed (full load), sudden deceleration at max engine speed, sudden acceleration whitout load
All that forces produces traction, compresion and bedding in the rod, and after you must consider that the real system is not static (crankshaft torsional vibrations among others) and the max. peak pressure can be higher (Knock), for this reason you must to apply a safety factor if you want to run sure.
In sport you usually play with this safety factor (taking mass fron the rod) when you need an high rev. engine, or when you want sudden acceleration, but if you use the engine in almost stationary speed (like towing) the rod mass doesn?t represent any problem, dont spend your money in lightweight rods.

Sorry for my English, i'm learning
ElwoodGreen (Mechanical)
4 Dec 03 20:48
And here is my 2 cents worth:  In summary, it depends on the additional load created by extended towing and the original design constraints.  The rods will experience cycles of compressive and tensile forces as mentioned in the better posts above.  ANY rod will break if the combination of load and number of cycles exceeds the fatigue failure threshold of the material.  A crack will propagate across the material and it will break.  A stronger material, or the same material that has been better prepared, will last longer.  What are the goals?  Do you care if the engine only lasts 50,000 miles because it was operated closer to maximum load most of the time?  

My old F-100 has over 300K miles on stock rods (FE series 360 cid), but I only tow a few times a year, and I don't often exceed 3500 rpm.  The engine design seems to have perfectly matched my driving habits, but I plan to build a bit more power and torque in the next one.  I have a set of stock rods that will be polished up a bit, but I didn't feel the need to spend big bucks on heavier duty pieces.  I definitely don't need race pieces.

I have seen 2 such instances of rod breakage on very large industrial gas engines operating at only 900 rpm.  These are fully machined rods, by the way.  The main difference between these and most automotive rods is in the cap stud bolting arrangement.  The big end is cut at an angle, and the studs are threaded into the rod directly.  This facilitates removal.  Otherwise, the big end won't fit through the bore.  The cracks started at the innermost end of the threads for the upper stud and spread from there.  The manufacturer, after 2 rounds of rod change-outs on 3 engines and 2 new shortblock assemblies, says they fixed the manufacturing problem.  We are now approaching the operating hour range when the first ones broke, and I am getting very nervous.  We reduced average loads by about 5%.  Will this be enough to avoid additional fatigue failures?  Only time will tell.

Happy holidays.
tony90 (Industrial)
23 Nov 04 22:26
why is a connecting rod shaft tapered?

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