Adam W Posted December 12, 2003 Share Posted December 12, 2003 Just thinking about how much stick I want to give my standard con rods . . . I figure the main force exerted on them is rapid changes in the direction of the piston at the top and bottom of the stroke. Change in speed over time = acceleration. As the revs increase the piston goes up faster, comes down faster, so the total change in speed is greater. It also happens in less time so the acceleration is increased by a big factor as the revs go up. What happens if I double the bhp though? I will be able to go from 2000 rpm to 6500rpm a lot quicker, but the peak acceleration force on the rod will be the same as in a 50bhp engine doing 6500rpm! So, does the rate of change of acceleration impose any greater stress on the rod? Or is there some other force exerted on it? Assuming the rev limit stays the same, and that all bearings are properly lubricated etc, my theory says that the rods will be just as durable at 2000bhp as they would be at 20bhp . . . am I missing something? Quote Link to comment Share on other sites More sharing options...
Rob Posted December 12, 2003 Share Posted December 12, 2003 Crikey, there's a question. Completely off the top of my head and awaiting to be shot down... if you are increasing the power output of the engine, then more force is exerted onto the piston with each explosion pushing it down the cylinder. Therefore, the con rod acts like a column transfering the vertical load from the piston head to the crankshaft. Therefore if you increase the load there must come a point where the "column" can no longer transmit the force and it buckles under compression. Would this be "throwing a rod"? I presume that forged con rods by way of their superior construction, molecular alignment etc etc are able to take a higher load than a bog standard piston. A lighter rod will also acc/decelerate more easily. Unless you are talking about gaining more power by increasing the rpm, rather than increasing the force released by exploding the fuel. At this point we can safely say we have reached the boundary fence of my knowledge. Quote Link to comment Share on other sites More sharing options...
Adam W Posted December 12, 2003 Author Share Posted December 12, 2003 Makes sense to me! The rod isn't there just to hold the piston as it gets thrown around, all the motive power is transmitted through it, that's what I was forgetting! Quote Link to comment Share on other sites More sharing options...
Ian C Posted December 12, 2003 Share Posted December 12, 2003 Corky Bell has a bit about this. The most strain on the conrods is at TDC and BDC where direction of travel changes. This is why upping the revs wears the engine out really quickly. The increase in shove from upping the power via a turbo is mostly at 90deg crank angle, at which time the inertial loads are practically zero. Basically he says if you double the power of the engine, the inertial loads at TDC and DBC go up by about 20%. -Ian Quote Link to comment Share on other sites More sharing options...
Terminator Posted December 12, 2003 Share Posted December 12, 2003 Bugger, there goes another weight saving idea:devil: Quote Link to comment Share on other sites More sharing options...
Guest Terry S Posted December 12, 2003 Share Posted December 12, 2003 FWIW my Shot Peened balanced stock 2JZ rods were fine at 690 bhp, but I was getting a little nervous, so I went to Carrillo's, which Phil, are lighter than stock rods but stronger. The other thing to consider is the rod bolts. Quote Link to comment Share on other sites More sharing options...
Digsy Posted December 12, 2003 Share Posted December 12, 2003 Off the top of my head: IIRC the peak loading on a connecting rod is at TDC on the exhaust stroke. The load is caused by the inertia of the piston assembly being decellerated to standstill and then accelerated downwards again with no gas load behind it. This is also the load that drives the preload requirement from your connecting rod blots to prevent big end separation. Assuming that your rod is designed so that it doesnt pull itself in half after one revolution (and it would have to be a pretty poor connecting rod design to do that) then broadly speaking all "real world" failures are fatiuge failures. Fatigue is all about alternating or reversing loads. On the one hand you can calculate the tensile inertia load from the connecting rod at TDC using the piston assembly mass and max acceleration, and then you can work out the maximum compressive load. Problem is that for this you will need to know the pressure rise curve during compustion and work out where during the power stroke the maximum compressive load occurs because it may not be at TDC and therefore will not as extreme as if you just took the gas load and reacted it all straight through the connecting rod. A bit hard to explain without a diagram, but if the crank is at, say, 90deg after TDC when max pressure occurs, then the loading through the length of the rod will be less than if it was at TDC. Once you know this you can draw up the Goodman diagram for the rod design and its material and this will give you your fatigue reserve factor. Its actually not that hard to do, and I used to have an old QBASIC program that I wrote to do just this, but I lost it in a hard disk crash at work. I'll try to remember to dig some stuff out on Monday. HTH. Quote Link to comment Share on other sites More sharing options...
Adam W Posted December 12, 2003 Author Share Posted December 12, 2003 Darren, if its fatigue failure that causes most problems, then I guess the Mk3 that made 800bhp at 8000rpm on stock rods in Oz won't last too many dyno runs This is the problem, the yanks see a story like that and think "800bhp on stock rods is perfectly safe", I want my car to perform more than once without blowing up though! ie real world driving, 1000's of miles between rebuilds, not dyno queen stuff. I was wondering if I could use the tensile test rig here at work to try and overstress an old rod, I think that simulating 100 strokes a second (oo-err etc etc) might be asking too much though! It was most useful for checking out my valve springs though Quote Link to comment Share on other sites More sharing options...
Digsy Posted December 12, 2003 Share Posted December 12, 2003 Durability will always be subjective when you get into tuning. Is your rig just a tensile test rig or a full blown fatigue rig? Quote Link to comment Share on other sites More sharing options...
Thorin Posted December 12, 2003 Share Posted December 12, 2003 Usually too much alcohol. Quote Link to comment Share on other sites More sharing options...
Terminator Posted December 12, 2003 Share Posted December 12, 2003 FLMAO:D Quote Link to comment Share on other sites More sharing options...
400BHP Posted December 12, 2003 Share Posted December 12, 2003 The reciprocating mass is the main problem, you need to go for a lighter, stronger material in order to lessen the moving mass inertia. Inertial forces will obviously be greater at the same speed on a higher mass than on a lesser one, is you wanted more power, the last thing you want is more weight. Quote Link to comment Share on other sites More sharing options...
Need4Speed Posted December 13, 2003 Share Posted December 13, 2003 Lighten, balance, polish them & make sure everything is within blueprint tolerance. use the tensile test rig Not a lot of tensile loading on a rod! Quote Link to comment Share on other sites More sharing options...
Ian C Posted December 14, 2003 Share Posted December 14, 2003 Hmmm - what about at top dead centre when suddenly the rod is pulling the piston down instead of pushing it up? Isn't that tensile stress? -Ian Quote Link to comment Share on other sites More sharing options...
Digsy Posted December 14, 2003 Share Posted December 14, 2003 Originally posted by John Packham Not a lot of tensile loading on a rod! There is a whole sh*tload of tensile force on a rod, like I said, at TDC at the end of the exhaust stroke. Quote Link to comment Share on other sites More sharing options...
Need4Speed Posted December 14, 2003 Share Posted December 14, 2003 a whole sh*tload of tensile force on a rod Only due to the mass of the piston during the induction stroke! This is minor compared to the compression forces during the other three stokes. I've not come across a rod that's broken due to tension. Quote Link to comment Share on other sites More sharing options...
Digsy Posted December 14, 2003 Share Posted December 14, 2003 I agree that it may not sound logical, but the tensile inertia forces are on a par, if not larger, than the forces due to the gas loads on the firing stroke. I did some quick calcs based on the following assumptions: Stroke: 86mm Bore: 86mm Rod length: 140mm Peak cylinder pressure: 2000kPa (20 bar) at 20 deg ATDC Max RPM: 7000 Piston assembly mass: 0.4kg This gives a compressive load during the ignition cycle of 11682N at 20 deg ATDC and a tensile inertia load of 12081N at TDC. I would have expected the difference to be larger, but I had a late night last night and I've checked it through as much as I can! I've attached the spreadsheet for information. As for not coming across a rod that has broken in tension, I agree that you won't ever see a failed rod that exhibits the classic ductile fracture like you see in a tensile test. As I said in my earlier post, because the rod is subject to alternating compressive and tensile stresses, all "real world" failures will be fatigue failures after many loading cycles.rod loads.zip Quote Link to comment Share on other sites More sharing options...
Need4Speed Posted December 14, 2003 Share Posted December 14, 2003 My experience of rod failure is little end failure or piston failure leaving a rod flailing around looking for a way out of the block. Quote Link to comment Share on other sites More sharing options...
Digsy Posted December 14, 2003 Share Posted December 14, 2003 Well it has to fail somewhere. The rod section is usually at its minimum at the little end so that's as good a place as any. Another good place is on the "shoulders" at the top of the big end, where the conn rod bolt holes can act as stress raisers. Quote Link to comment Share on other sites More sharing options...
Chris Wilson Posted December 14, 2003 Share Posted December 14, 2003 Or big end bolt failure. They usually stretch a little first, this opens up the bearing clearance or reduces crush so much the shells spin, insane heat produced and massive failure quickly ensues. Quote Link to comment Share on other sites More sharing options...
Digsy Posted December 14, 2003 Share Posted December 14, 2003 I think Chris has just proved a point. You aren't going to stretch a conn rod bolt by loading the rod in compression. Quote Link to comment Share on other sites More sharing options...
Chris Wilson Posted December 14, 2003 Share Posted December 14, 2003 Originally posted by John Packham Only due to the mass of the piston during the induction stroke! This is minor compared to the compression forces during the other three stokes. I've not come across a rod that's broken due to tension. Hmm, I can't recall ever seeing a total rod failure that WASN'T caused by tension, except where bearing failures due to oil starvation or running clearance problems were the cause. Seen a few bent by excess cylinder pressures due to hydraulicing, or mega boost pressures, never come upon one that has actually BROKEN under compressive loads. Quote Link to comment Share on other sites More sharing options...
Keith C Posted December 14, 2003 Share Posted December 14, 2003 Does this mean that lighter pistons are more beneficial than stronger rods, if the objective is to increase rpms? Quote Link to comment Share on other sites More sharing options...
Chris Wilson Posted December 14, 2003 Share Posted December 14, 2003 Originally posted by Keith C Does this mean that lighter pistons are more beneficial than stronger rods, if the objective is to increase rpms? Basically, yes, hence my "obsession" with Cosworth and Omega pistons and a general dislike of US made ones. Rod length also plays a part, particularly in reducing skirt friction, hence a longer rod with the pin higher up the piston can gain power, particularly at high RPM's. It's a lot more complex a subject than it initially appears. Quote Link to comment Share on other sites More sharing options...
Rob Posted December 15, 2003 Share Posted December 15, 2003 Isn't the whole of F1 about reducing piston mass to facilitate an increase in RPM? Quote Link to comment Share on other sites More sharing options...
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