Conrod weight vs Crown height
Discussion
Im looking for someone really clever to give some detailed advice on pistons and conrods of a race motor.
Ive two options, i can use a 6" titanium conrod weighing 326g with a std crown height (~1.5inch iirc) forged piston weighing 295g. Or i use a steel 6.25" rod (435g) with a low crown height (1.25") piston which is a little lighter too.
The engine has a 68.26mm stroke, 73.5mm bore, 16valves, and is expected to rev to 10000+rpm
The engine builder is suggesting option 1 as I already have the ti rods, but the rpms vs the crown height is bothering me. Does anyone know if there's cause for concern
Ive two options, i can use a 6" titanium conrod weighing 326g with a std crown height (~1.5inch iirc) forged piston weighing 295g. Or i use a steel 6.25" rod (435g) with a low crown height (1.25") piston which is a little lighter too.
The engine has a 68.26mm stroke, 73.5mm bore, 16valves, and is expected to rev to 10000+rpm
The engine builder is suggesting option 1 as I already have the ti rods, but the rpms vs the crown height is bothering me. Does anyone know if there's cause for concern
The answer, is, unfortunately "Do the Math" as our american cousins are want to say!
It also rather depends whats sort of "race motor" this. ie a clubman engine, that has to do 3 seasons without being looked at, and where a major failure (like a Ti rod letting go....) will stop the project stone dead, or a proper works type job, budget unlimited, win at all costs, type affair?
IME, Ti rods bring advantages, but with penalties (such as the unknown fatigue life / complex crack propagation / chemical poisoning issues). In most cases, a properly designed steel rod can deliver 90% of the performance with none of those penalties.
Have you calculated your bearing loadings at max rpm at zero load? Can you support the heavier rod without exceeding the surface pressure limits of the big end bearing?
Max Rod angularity isn't too different in either case (0.220rad vs 0.212rad) so the short rod increases piston thrust loading by just ~4%
Piston height is another thing entirely, at 10krpm, your piston motion is likely to be a complex orbit, and getting the pin offset, and bore clearance correct across the complete load/speed range is difficult without either complex simulation or quite a bit of trial and error! Low crown heights obviously drop the pistons CofG towards the crank, and allow you to use a longer rod, but they can push skirt loading up, and often this is a weak area for most pistons.
So, the simple answer is "there is no simple answer", sorry!
It also rather depends whats sort of "race motor" this. ie a clubman engine, that has to do 3 seasons without being looked at, and where a major failure (like a Ti rod letting go....) will stop the project stone dead, or a proper works type job, budget unlimited, win at all costs, type affair?
IME, Ti rods bring advantages, but with penalties (such as the unknown fatigue life / complex crack propagation / chemical poisoning issues). In most cases, a properly designed steel rod can deliver 90% of the performance with none of those penalties.
Have you calculated your bearing loadings at max rpm at zero load? Can you support the heavier rod without exceeding the surface pressure limits of the big end bearing?
Max Rod angularity isn't too different in either case (0.220rad vs 0.212rad) so the short rod increases piston thrust loading by just ~4%
Piston height is another thing entirely, at 10krpm, your piston motion is likely to be a complex orbit, and getting the pin offset, and bore clearance correct across the complete load/speed range is difficult without either complex simulation or quite a bit of trial and error! Low crown heights obviously drop the pistons CofG towards the crank, and allow you to use a longer rod, but they can push skirt loading up, and often this is a weak area for most pistons.
So, the simple answer is "there is no simple answer", sorry!
Donfrondo said:
All the components are off the shelf, so it's just a case of mixing and matching to create the combo that's going to work best. The motor can be pulled apart as often as I like, I anticipate I'll have it apart over winter once a year to check it over.
Out of interest, what engine is it? Piston manufacturers will sell 'performance' pins if you go down that route. An A-series with a sprint spec 16v conversion. The omega forged pistons already use a smaller than std pin which mate to the arrow rods which have a little end only suited to the smaller pin. If i didn't already have the ti rods i probably wouldn't be asking the questions, i would have just gone with the longer rods. If the suggestion is there's nothing in it between the two options then I'll use what I've got. Its more a case of if I'm loosing out by using the combo?
Donfrondo said:
An A-series with a sprint spec 16v conversion. The omega forged pistons already use a smaller than std pin which mate to the arrow rods which have a little end only suited to the smaller pin. If i didn't already have the ti rods i probably wouldn't be asking the questions, i would have just gone with the longer rods. If the suggestion is there's nothing in it between the two options then I'll use what I've got. Its more a case of if I'm loosing out by using the combo?
Surely if this is a tried and tested package....the seller of these kits can advise ?Is that using the "5...sort of lol" main bearing crank and straight rods, or a normal type crank and rods ?
The new cranks look class.
Yea MED. Steves been great and his advice was to go with the ti rods because that's what I have, he said he builds loads of race engines with the full height pistons and never has any issues, I've no reason to doubt him. Im using his 68mm short stroke crank which I'm starting to think would suit the shorter height piston. But then the complication is the fact the rod is over 100g lighter, a weight I'd never save with a shorter height piston.
Donfrondo said:
...the complication is the fact the rod is over 100g lighter, a weight I'd never save with a shorter height piston.
But classical slider-crank analysis would only assign a third of that weight saving to reciprocating mass, so if your piston (+ pin and hardware) is more than 33g lightee you're probably ahead with the steel rodEdited by AER on Saturday 1st October 23:07
Max_Torque said:
The answer, is, unfortunately "Do the Math" as our american cousins are want to say!
It also rather depends whats sort of "race motor" this. ie a clubman engine, that has to do 3 seasons without being looked at, and where a major failure (like a Ti rod letting go....) will stop the project stone dead, or a proper works type job, budget unlimited, win at all costs, type affair?
IME, Ti rods bring advantages, but with penalties (such as the unknown fatigue life / complex crack propagation / chemical poisoning issues). In most cases, a properly designed steel rod can deliver 90% of the performance with none of those penalties.
Have you calculated your bearing loadings at max rpm at zero load? Can you support the heavier rod without exceeding the surface pressure limits of the big end bearing?
Max Rod angularity isn't too different in either case (0.220rad vs 0.212rad) so the short rod increases piston thrust loading by just ~4%
Piston height is another thing entirely, at 10krpm, your piston motion is likely to be a complex orbit, and getting the pin offset, and bore clearance correct across the complete load/speed range is difficult without either complex simulation or quite a bit of trial and error! Low crown heights obviously drop the pistons CofG towards the crank, and allow you to use a longer rod, but they can push skirt loading up, and often this is a weak area for most pistons.
So, the simple answer is "there is no simple answer", sorry!
I love reading your posts, I always learn something It also rather depends whats sort of "race motor" this. ie a clubman engine, that has to do 3 seasons without being looked at, and where a major failure (like a Ti rod letting go....) will stop the project stone dead, or a proper works type job, budget unlimited, win at all costs, type affair?
IME, Ti rods bring advantages, but with penalties (such as the unknown fatigue life / complex crack propagation / chemical poisoning issues). In most cases, a properly designed steel rod can deliver 90% of the performance with none of those penalties.
Have you calculated your bearing loadings at max rpm at zero load? Can you support the heavier rod without exceeding the surface pressure limits of the big end bearing?
Max Rod angularity isn't too different in either case (0.220rad vs 0.212rad) so the short rod increases piston thrust loading by just ~4%
Piston height is another thing entirely, at 10krpm, your piston motion is likely to be a complex orbit, and getting the pin offset, and bore clearance correct across the complete load/speed range is difficult without either complex simulation or quite a bit of trial and error! Low crown heights obviously drop the pistons CofG towards the crank, and allow you to use a longer rod, but they can push skirt loading up, and often this is a weak area for most pistons.
So, the simple answer is "there is no simple answer", sorry!
Unless you have the small and big end rod weights the norm is to use 1/3 the rod weight when calculating the reciprocating forces.
► (435 / 3) - (326 / 3) = 36.3333 is the piston difference greater or less than this?
Bore = 2.893701 Stroke = 2.687402 Rod Length = 6.0 RPM = 6500
Piston Weight = 400.0 Rod Weight = 326.0
Small End Rod Weight = 108.6666 Big End Rod Weight = 217.3333
Rod CG / Distance from Small End = 4.0 GAS PRESSURE = 0
ATDC - 2213.252
Bore = 2.893701 Stroke = 2.687402 Rod Length = 6.25 RPM = 6500
Piston Weight = 363.6 Rod Weight = 435.0
Small End Rod Weight = 145.0 Big End Rod Weight = 290.0
Rod CG / Distance from Small End = 4.166667 GAS PRESSURE = 0
ATDC - 2196.765
Rotation forces is another store.
Stan
► (435 / 3) - (326 / 3) = 36.3333 is the piston difference greater or less than this?
Bore = 2.893701 Stroke = 2.687402 Rod Length = 6.0 RPM = 6500
Piston Weight = 400.0 Rod Weight = 326.0
Small End Rod Weight = 108.6666 Big End Rod Weight = 217.3333
Rod CG / Distance from Small End = 4.0 GAS PRESSURE = 0
ATDC - 2213.252
Bore = 2.893701 Stroke = 2.687402 Rod Length = 6.25 RPM = 6500
Piston Weight = 363.6 Rod Weight = 435.0
Small End Rod Weight = 145.0 Big End Rod Weight = 290.0
Rod CG / Distance from Small End = 4.166667 GAS PRESSURE = 0
ATDC - 2196.765
Rotation forces is another store.
Stan
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.
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.
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