Conrod weight vs Crown height
Discussion
KiaDiseasel said:
Lovely mathematical technical question. If only there were someone that way oriented on this site who could answer it 
Donfrondo said:
Hopefully. I know there are some engineers and some students on here, hopefully one of them specialises. If not a suggestion on where to ask would be well recieved.
Well now i feel like a nob. Excuse my ignorance. Thankyou KiaDiseasel said:
Hmmm. I keep promising myself I won't do this anymore but I suppose the people who need the technical information shouldn't have to pay for the idiocy of those who run the site. So Mr OP, what you should be trying to find out is how the forces on the big end bolts and also the total forces acting on the crank journal compare in your two scenarios to see if there's a clear winner. To do this you need to split the rod and piston masses into those amounts considered to be acting purely in rotation on the big end and those acting purely in sinusoidal reciprocation in the cylinder. Then the accelerations on the crank journal and piston need to be calculated and finally the forces can be found.
The correct way to find the big end vs small end masses for the conrods is to weigh them on a special jig that holds each end exactly in the centre of the big end at one end and the gudgeon pin bore at the other. Failing that, as you've been told, it's a reasonable approximation usually to assign 2/3 of the total rod mass to the big end and 1/3 to the small end.
The maximum acceleration on the piston occurs at TDC at which point you get the following situation. All of the piston mass plus 1/3 of the rod mass is considered to be reciprocating and the whole of that load acts on both the big end bolts and also the crank journal. Of the rotating mass you can visualise it as half of the conrod big end will be above the crank journal and so pulling on the big end bolts but the bottom half, the cap, will be below the crank journal and so not affecting the rod bolts but it will be acting on the crank of course.
So to summarise, the big end bolt loads are created by all the piston mass and 1/3 of the conrod mass in reciprocation plus another 1/3 of the conrod mass in rotation. The loads on the crank journal will be created by the same reciprocating mass but the whole 2/3 of the conrod mass in rotation being all of its big end. I trust that's clear enough.
Finally it just needs to run the calcs which is not desperately complicated. OK we don't actually know one of the piston weights, nor do we know the actual split of the conrod masses when properly weighed but I'll assume the piston with the steel rod is a minimum of 258.67g which then means that the reciprocating mass of both systems is the same. In practice this piston is likely to be heavier.
The longer rod will slightly reduce piston accelerations so the calculations need to take this into account. What we get at 10,000 rpm is the following.
1. Titanium rod system
Piston acceleration at TDC 45,797 m/s^2, crank accel at the big end 37,428 m/s^2.
Total forces on big end bolts - 2,300 Kg (415 Kg rotating, 1885 Kg reciprocating)
Total forces on crank journal 2,715 Kg.
2. Steel rod system
Piston acceleration at TDC 45,462 m/s^2, (crank accel as above).
Total forces on big end bolts - 2,425 Kg (553 Kg rotating, 1872 Kg reciprocating)
Total forces on crank journal 2,978 Kg.
So the titanium rod is the clear winner even if the piston for the steel rod is very light. The titanium system is a bit easier on the rod bolts, about 5%, but a whole 10% better on the crank journal and hence its bearing and tendency to fail.
In reality, it's a bit better than that (assuming you've weighed the rods with the big end bolts fitted), because the CofG of a Ti rod is significantly closer to the big end than for a steel rod, due to the density difference between the rod material and that of the big end bolts! ie the 1/3 - 2/3 rule doesn't hold true for Ti rods, and they have a great percentage of their total mass carried by the rotational component. (it's best to weigh the rod as individual parts (rod, cap, and fastenings) to understand the mass distribution.The correct way to find the big end vs small end masses for the conrods is to weigh them on a special jig that holds each end exactly in the centre of the big end at one end and the gudgeon pin bore at the other. Failing that, as you've been told, it's a reasonable approximation usually to assign 2/3 of the total rod mass to the big end and 1/3 to the small end.
The maximum acceleration on the piston occurs at TDC at which point you get the following situation. All of the piston mass plus 1/3 of the rod mass is considered to be reciprocating and the whole of that load acts on both the big end bolts and also the crank journal. Of the rotating mass you can visualise it as half of the conrod big end will be above the crank journal and so pulling on the big end bolts but the bottom half, the cap, will be below the crank journal and so not affecting the rod bolts but it will be acting on the crank of course.
So to summarise, the big end bolt loads are created by all the piston mass and 1/3 of the conrod mass in reciprocation plus another 1/3 of the conrod mass in rotation. The loads on the crank journal will be created by the same reciprocating mass but the whole 2/3 of the conrod mass in rotation being all of its big end. I trust that's clear enough.
Finally it just needs to run the calcs which is not desperately complicated. OK we don't actually know one of the piston weights, nor do we know the actual split of the conrod masses when properly weighed but I'll assume the piston with the steel rod is a minimum of 258.67g which then means that the reciprocating mass of both systems is the same. In practice this piston is likely to be heavier.
The longer rod will slightly reduce piston accelerations so the calculations need to take this into account. What we get at 10,000 rpm is the following.
1. Titanium rod system
Piston acceleration at TDC 45,797 m/s^2, crank accel at the big end 37,428 m/s^2.
Total forces on big end bolts - 2,300 Kg (415 Kg rotating, 1885 Kg reciprocating)
Total forces on crank journal 2,715 Kg.
2. Steel rod system
Piston acceleration at TDC 45,462 m/s^2, (crank accel as above).
Total forces on big end bolts - 2,425 Kg (553 Kg rotating, 1872 Kg reciprocating)
Total forces on crank journal 2,978 Kg.
So the titanium rod is the clear winner even if the piston for the steel rod is very light. The titanium system is a bit easier on the rod bolts, about 5%, but a whole 10% better on the crank journal and hence its bearing and tendency to fail.
Many, many years ago, VW were going to fit Ti rods to there polo 1.6 Gti engne which was being ticked at Cosworth where i was working at the time, and an edict came down from On High that "THIS ENGINE MUST USE Ti RODS" as the rather forthright Chief Technical Officer at the time had driven two cars, one with, one without, and found the one without to be unacceptably harsh (moving the proportion of mass carried by the rot/recip motion changes the harmonic content of engine born vibration for a 4cy inline engine of course). Unfortunately, during dev and durab, rod failures occurred, due to crack propagation and chlorine contaminants in the oil(from combustion products). Eventually, the Ti rods were dropped, after many battles in many meetings.
footnote: it turned out, much of the "difference" the CTO had felt between Ti and Steel rods was more to do with the different engine mounts fitted to one car he drove (the chassis team had stiffened the mounts to restrain the powertrain better under high loadings to make the turn-in more linear............... )
Max_Torque said:
In reality, it's a bit better than that (assuming you've weighed the rods with the big end bolts fitted), because the CofG of a Ti rod is significantly closer to the big end than for a steel rod, due to the density difference between the rod material and that of the big end bolts! ie the 1/3 - 2/3 rule doesn't hold true for Ti rods, and they have a great percentage of their total mass carried by the rotational component. (it's best to weigh the rod as individual parts (rod, cap, and fastenings) to understand the mass distribution.
Many, many years ago, VW were going to fit Ti rods to there polo 1.6 Gti engne which was being ticked at Cosworth where i was working at the time, and an edict came down from On High that "THIS ENGINE MUST USE Ti RODS" as the rather forthright Chief Technical Officer at the time had driven two cars, one with, one without, and found the one without to be unacceptably harsh (moving the proportion of mass carried by the rot/recip motion changes the harmonic content of engine born vibration for a 4cy inline engine of course). Unfortunately, during dev and durab, rod failures occurred, due to crack propagation and chlorine contaminants in the oil(from combustion products). Eventually, the Ti rods were dropped, after many battles in many meetings.
footnote: it turned out, much of the "difference" the CTO had felt between Ti and Steel rods was more to do with the different engine mounts fitted to one car he drove (the chassis team had stiffened the mounts to restrain the powertrain better under high loadings to make the turn-in more linear............... )
Ever thought of writing a book of your anecdotes and experiences similar to the 'Quality Control' one?Many, many years ago, VW were going to fit Ti rods to there polo 1.6 Gti engne which was being ticked at Cosworth where i was working at the time, and an edict came down from On High that "THIS ENGINE MUST USE Ti RODS" as the rather forthright Chief Technical Officer at the time had driven two cars, one with, one without, and found the one without to be unacceptably harsh (moving the proportion of mass carried by the rot/recip motion changes the harmonic content of engine born vibration for a 4cy inline engine of course). Unfortunately, during dev and durab, rod failures occurred, due to crack propagation and chlorine contaminants in the oil(from combustion products). Eventually, the Ti rods were dropped, after many battles in many meetings.
footnote: it turned out, much of the "difference" the CTO had felt between Ti and Steel rods was more to do with the different engine mounts fitted to one car he drove (the chassis team had stiffened the mounts to restrain the powertrain better under high loadings to make the turn-in more linear............... )
I'd buy it!
What increases the complexity and makes just looking at the numbers with doing the continuations a problem is if nothing but the rod length is changed there is a change in forces.
Bore_=_73.5_Stroke_=_68.26___Rod_Length_=_152.4_RPM_=_10000
Piston_Weight_=_295.0___Rod_Weight_=_326.0
Small_End_Rod_Weight_=_108.6666___Big_End_Rod_Weight_=_217.3333
Rod_CG_/_Distance_from_Small_End_=_101.6__GAS_PRESSURE_=_0
__________Reciprocating____Total_____Piston_Side_Piston_
Crank_Angle__Force_________Force________ForceInertia_Force_
Degree______Newtons_______Newtons______Newtons___Newtons
_.000_____18491.840_____26626.135_________.000_13513.855
Bore_=_73.5_Stroke_=_68.26___Rod_Length_=_158.75_RPM_=_10000
Piston_Weight_=_295.0___Rod_Weight_=_326.0
Small_End_Rod_Weight_=_108.6666___Big_End_Rod_Weight_=_217.3333
Rod_CG_/_Distance_from_Small_End_=_105.8333__GAS_PRESSURE_=_0
__________Reciprocating____Total_____Piston_Side_Piston_
Crank_Angle__Force_________Force________ForceInertia_Force_
Degree______Newtons_______Newtons______Newtons___Newtons_
_.000_____18356.500_____26490.794_________.000_13414.948
Stan
Bore_=_73.5_Stroke_=_68.26___Rod_Length_=_152.4_RPM_=_10000
Piston_Weight_=_295.0___Rod_Weight_=_326.0
Small_End_Rod_Weight_=_108.6666___Big_End_Rod_Weight_=_217.3333
Rod_CG_/_Distance_from_Small_End_=_101.6__GAS_PRESSURE_=_0
__________Reciprocating____Total_____Piston_Side_Piston_
Crank_Angle__Force_________Force________ForceInertia_Force_
Degree______Newtons_______Newtons______Newtons___Newtons
_.000_____18491.840_____26626.135_________.000_13513.855
=============
Bore_=_73.5_Stroke_=_68.26___Rod_Length_=_158.75_RPM_=_10000
Piston_Weight_=_295.0___Rod_Weight_=_326.0
Small_End_Rod_Weight_=_108.6666___Big_End_Rod_Weight_=_217.3333
Rod_CG_/_Distance_from_Small_End_=_105.8333__GAS_PRESSURE_=_0
__________Reciprocating____Total_____Piston_Side_Piston_
Crank_Angle__Force_________Force________ForceInertia_Force_
Degree______Newtons_______Newtons______Newtons___Newtons_
_.000_____18356.500_____26490.794_________.000_13414.948
Stan
Maybe someone can look at my calculations and see something wrong. I very rarely calculate rotation force and my answer is in Newtons. But when I convert it to kg it is double KiaDiseasel's numbers.
► (2/3)*.326*(6.826/(2*100))*(10000*2*pi/60)^2 = 8134.294
► (2/3)*.435*(6.826/(2*100))*(10000*2*pi/60)^2 = 10854.0426
Stan
► (2/3)*.326*(6.826/(2*100))*(10000*2*pi/60)^2 = 8134.294
► (2/3)*.435*(6.826/(2*100))*(10000*2*pi/60)^2 = 10854.0426
Stan
Max_Torque said:
In reality, it's a bit better than that (assuming you've weighed the rods with the big end bolts fitted), because the CofG of a Ti rod is significantly closer to the big end than for a steel rod, due to the density difference between the rod material and that of the big end bolts! ie the 1/3 - 2/3 rule doesn't hold true for Ti rods, and they have a great percentage of their total mass carried by the rotational component. (it's best to weigh the rod as individual parts (rod, cap, and fastenings) to understand the mass distribution.
Many, many years ago, VW were going to fit Ti rods to there polo 1.6 Gti engne which was being ticked at Cosworth where i was working at the time, and an edict came down from On High that "THIS ENGINE MUST USE Ti RODS" as the rather forthright Chief Technical Officer at the time had driven two cars, one with, one without, and found the one without to be unacceptably harsh (moving the proportion of mass carried by the rot/recip motion changes the harmonic content of engine born vibration for a 4cy inline engine of course). Unfortunately, during dev and durab, rod failures occurred, due to crack propagation and chlorine contaminants in the oil(from combustion products). Eventually, the Ti rods were dropped, after many battles in many meetings.
footnote: it turned out, much of the "difference" the CTO had felt between Ti and Steel rods was more to do with the different engine mounts fitted to one car he drove (the chassis team had stiffened the mounts to restrain the powertrain better under high loadings to make the turn-in more linear............... )
I was looking at Titanium rods for my road bike engine a while back. Many, many years ago, VW were going to fit Ti rods to there polo 1.6 Gti engne which was being ticked at Cosworth where i was working at the time, and an edict came down from On High that "THIS ENGINE MUST USE Ti RODS" as the rather forthright Chief Technical Officer at the time had driven two cars, one with, one without, and found the one without to be unacceptably harsh (moving the proportion of mass carried by the rot/recip motion changes the harmonic content of engine born vibration for a 4cy inline engine of course). Unfortunately, during dev and durab, rod failures occurred, due to crack propagation and chlorine contaminants in the oil(from combustion products). Eventually, the Ti rods were dropped, after many battles in many meetings.
footnote: it turned out, much of the "difference" the CTO had felt between Ti and Steel rods was more to do with the different engine mounts fitted to one car he drove (the chassis team had stiffened the mounts to restrain the powertrain better under high loadings to make the turn-in more linear............... )
I was advised by the rod manufacturer that the only real benefit to using Ti instead of steel was to their bank balance...
Stan Weiss said:
Maybe someone can look at my calculations and see something wrong. I very rarely calculate rotation force and my answer is in Newtons. But when I convert it to kg it is double KiaDiseasel's numbers.
? (2/3)*.326*(6.826/(2*100))*(10000*2*pi/60)^2 = 8134.294
? (2/3)*.435*(6.826/(2*100))*(10000*2*pi/60)^2 = 10854.0426
Stan
You're taking 2/3 of the rod weight i.e. the force on the whole journal not just on the big end bolts. I'm using 1/3 to start with and doubling it to get the force on the journal.? (2/3)*.326*(6.826/(2*100))*(10000*2*pi/60)^2 = 8134.294
? (2/3)*.435*(6.826/(2*100))*(10000*2*pi/60)^2 = 10854.0426
Stan
KiaDiseasel said:
You're taking 2/3 of the rod weight i.e. the force on the whole journal not just on the big end bolts. I'm using 1/3 to start with and doubling it to get the force on the journal.
Thanks, yes I am calculating the rotation force. I misunderstood your post and believed that that is what your were calculating also.Stan
nsa said:
Hainey said:
Ever thought of writing a book of your anecdotes and experiences similar to the 'Quality Control' one?
I'd buy it!
Me too. I'd buy it!
What do you mean "Quality Control one"?
As mentioned above, it's a 16v sprint engine. Works out at about 1155cc, with the bmw 16v conversion. Brett Simms doing the head, ported with bigger valves, tougher springs, 300 degree cams, 40mm throttle bodies and 12.5:1 CR. MED have done the short block, with their arrow 68mm crank, titanium rods, 73.5mm forged omegas, balanced up with their lightest flywheel and clutch assembly. Gearbox is the only part undecided so far, I'm probably going to put a clubman-spec SCCR 'box on it off my road car to dyno it, see how it makes power before i decided which gearset and final drive to put with it in the car.
Was at the engine builders yesterday, the short height piston is only 10g lighter than the full height. 297g vs 288g with pin. Is there anything else to consider with a short crown height piston? Like less friction because less skirt? or tipping effect because of the center of gravity in relation to the pin?
Donfrondo said:
Was at the engine builders yesterday, the short height piston is only 10g lighter than the full height. 297g vs 288g with pin. Is there anything else to consider with a short crown height piston? Like less friction because less skirt? or tipping effect because of the center of gravity in relation to the pin?
The mathematics of how minimum possible piston weight varies with crown height is relatively simple which is why I said above that the steel rod piston would be heavier than the 258.67g needed to equate the reciprocating masses. Hence the titanium rod system is an even better choice than the best case piston weight scenario I used for argument's sake. I wouldn't overthink anything else. The differences in friction or side forces are going to be too small to fret about and any conceiveable differences in power too small to even measure.As a further aside, you probably won't know much about the magnitude of rod bolt and crank journal forces in general engines. To put it into perspective the 2,300 kg bolt force I calculated for your engine is only about the same as a bog standard Ford Pinto engine at 6,500 rpm and even its stock chocolate rod bolts will stand 8,000 rpm for limited periods. Your components are so light that even at 10,000 rpm nothing is ever going to break. It would barely be taxing the bolts at 12,000 rpm. So don't waste money on any super duper bolts. No doubt the rods come with ARP 2000 ones anyway and they'll be more than enough.
Donfrondo said:
Was at the engine builders yesterday, the short height piston is only 10g lighter than the full height. 297g vs 288g with pin.
It's actually not that difficult to write a simple spreadsheet or other program that will calculate piston weights for you very accurately with a few data input points - wall thicknesses, crown thickness, material specific gravity etc. When I was designing and making pistons many years ago I did this and found pretty good agreement between the calculations and the final product. Obviously CAD can get you spot on but we didn't have that at the time.In your case all that really needs to be done to change the compression height is change the length of the pin bosses. The rest can stay the same. So assuming an 18mm pin on a small piston like this we get a boss about 25mm wide ( 3 or 4mm on either side of the pin bore). Boss thickness is probably going to be about 12 to 15 mm along the pin axis (say 13.5 mm). Changing the compression height by 0.25" (6.35 mm) we get a total boss volume of 2.5 x 1.35 x 0.635 x 2 = 4.29 cc
At a material sg of 2.8 that's 12 grams. So what you found basically. It was very unlikely to have been much more.
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