I mentioned to my son that Connecting rods stretch under extreme conditions and of course he asked why. I performed due diligence (Google and HAMB search) to no avail. I know that heat and high RPM factor in but it seems there would be more of a compressive rather than a tensile force. Does it have anything to do with the angle of the rod in relation to the piston at the moment it is "******ed" downward by the crank? Try to keep it as simple as possible. I'd like to be able to explain it to the boy (as well as understand it myself.)
Connecting rods are exposed to both compression and tension - compressive force as it travels to top dead center and tensile force as it travels down the bore. I believe they stretch because the rods are weaker in tension than they are in compression. I'm sure there will be better responses, but that's the general idea.
What part of the rod is stretching? Maybe the big end, as it is pulling the piston down at very high RPM?
Slack Just for giggles do a google search on aluminum racing connecting rods. They are probably discussed more than their steel counterparts. Especially take note as to why they are not suitable for street use.
Other way round . Rods are under tension at TDC . The B/E caps will oval slightly at TDC at high RPM.
You are right. What I meant to say is that the connecting rod is in tension the moment the piston begins going down the bore.
During the downward intake stroke every other crankshaft revolution of 4 cycle N/A engine. If it's running right there should be pressure above the piston putting the rod in compression during the downward power stroke. The rod is under constant compression in 2 cycle engines. Ed
The bolt stretches before the rod. Realize that it is moving: Kinetic Energy; .5m*** X velocity squared. When the piston gets ******ed down at 2000rpm it is 4 times greater in weight as it is at 1000rpm, when you ****** a piston down (intake stroke) at 8000 rpm that piston is serious heavy.
Simple explanation, the Piston is going about 40-50 mph, getting exploded on, and instantly reversing direction, continuously. The rod has to take that force, continuously, so it deforms over time Steel, vs aluminum is a materials discussion, fatigue life etc.
I think Jager aluminum rods are OK to use on the street. But obtaining them might be a problem? As well as their cost.. pdq67
oj is on to it...the piston lags behind what the crank 'wants' to do. The crank is trying to pull the rod apart and sometimes succeeds. The rod cap will deform first and the reason every machinist will check the big end and resize the hole. If a rod bolt is weak it will certainly fail in tension. The con rod beam rarely fails in tension...simply too much cross sectional area...except for something like a stock rod under a blower or similar. examples: .
This is a good discussion asking just-what-is-happening? I'd have to agree w/73RR, 'cept for the cycle that commences just prior to TDC, when the piston transfers thrust to the piston-pin...
Inertia. it is just simple physics. Tie a large nut to a long rubber band and swing it around your head for a simple experiment to show him why or how it happens.
Con rod stretch is critical in understanding when it comes to building high performance engines. The builders of F1, INDY, and NASCAR motors actually calculate and measure how much the rod stretches in order to determine proper clearances. You don't want the rod stretching enough so that the piston starts tapping the valves or cylinder head in your cup motor. The chart below shows a comparsion between a 10,000 rpm cup motor and a 20,000 rpm F1 motor. You can see the F1 motor actually has a slower piston velocity even though it is spinning at twice the rpms, but due to the higher rpm, it experiences higher accelerations, despite the short stroke and large rod ratio. At the end of the day, even a lightweight F1 piston that may only weigh 15 oz, puts a very large tensile load on the conrod when it is asked to decelerate at the top of the stroke and experiences 7000 g's. That equates to some 9000+ lbs of tensile load. More Reading http://www.pureluckdesign.com/ferrari/cdcolumn/ http://www.epi-eng.com/piston_engine_technology/comparison_of_cup_to_f1.htm
Jim On the big end you will get bolt stretch before rod stretch, it is the only part of the rod that is separated. At least on most of the motors that we fool with. The rod bolts will be your weak link. it has always amazed me that a rod will separate more often than it will loose a cap or break a rod bolt. It seems that most of the stress on a connecting rod is in the beam and not on the ends. This is of course not scientific only observation on my part.
Alright george, maybe I didn't word that properly (or correctly at all). I also should have included that the higher RPM does have an effect the accelerations at this point. It is true that at the same given rpm, a smaller rod ratio will have a higher peak acceleration. It is corrected. Damn brain got ahead of my fingers again.
Ideally you want the piston to touch the head at speed. I don't want to get into quench and such but it is very beneficial to run the minimum clearance. Typically my hemi motors needed about .055 piston to head to avoid mashing the bearings (top) to smithereens. Early on, when we were trying to understand what was going on, I would machine a flat on the end of the rod bolt and measure the total length after every run. Obviously, the steel rod engines could be measured while bolted up.
The early Ford 427s actually had nubs on top of the pistons that closed the valves @ peak RPM. A properly clearance and ***embled engine the only valve that should have a problem with piston interference would be the exhaust. The intake should be chasing the piston down the hole. Gene should remember this if no one else does, he has been around a while. Way back when, some of the cam grinders would grind a dual profile cam for blower motors that held the exhaust open longer and at a greater lift than the intake. The piston actually chased the exhaust valve as it was seating. if you overreved you would take the piston (and other parts) out because of the piston smacking the exhaust valve.
Another thought occurred while reading all these great replies. Y'all know waaaay more about this than I do, so if this is a stupid question, just ignore it. Is there ever a point, when the piston is being ******ed down, that both valves are closed and there is a microsecond of vacuum to overcome? Thanks to everyone for the input, I'll go back to lurking for a month or so. Merry Christmas.
Pulled down, as in the intake cycle. Overlap degrees, would mean both valves are open. Hopefully the other event, the power stroke cycle would be providing lots of pressure. We need those flame-front science guys here!
I've played "Russian Roulette" with my high rpm rod stretch calculations before. On one of my heavily modified dirt bike engines I thought I could push the the quench clearances that I knew of from bigger motor experience since things or smaller and lighter. Even with the closed big end rod and tight fresh everything when pushing the piston out of the deck into the head gasket's territory and avoiding over rev fascinating stuff happens. Let's just say sometimes the piston is going to want to clean the carbon for you. Don't forget that piston rock can make some rattle up there too.
Not on a otherwise healthy engine. if there is compression of just air, there will also be expansion so no vacuum will be created. if the spark is cancelled there will be no ignition, but the compressed cylinder pressure will need to return to atmospheric. I like that. A paddle ball would also be another good example.
Has anyone considered the effect exhaust scavenging has? Good scavenging creates a vacuum. This is applied to the combustion chamber during overlap. Part of the combustion chamber is the piston face area. That vacuum pressure reacting on the piston area creates a total force that is essentially equivalent to extra m***. Add in the extra rpm and the exponentially greater g-forces and you have a recipe for rod failure under tension.
Scott - A good vacuum from the exhaust only scavenging is pretty minimal. You can see it easily from the original style crankcase scavenging from the exhaust before the external pumps came to be. The exhaust as I recall is only able to pull 8 or 9 inches, even on a Pro Stock engine, 4 to 7 on an "everyday" car. This is why the mechanical vacuum pump came into existence. To your question, no, even 30(ish)in. like the mechanical pumps can pull, it's a gas, a compressible, stretchable gas. So there is little to no effect on pulling on the pistons, even at today's higher vacuums. Mike
A 4' stroke engine @ 6000 rpm has a piston moving at 66 ft per second changing direction in 100 th of a second.
The 1959 Pontiac 389 engines had rods that where bad to stretch. It was common to hear someone state that they had rubber rods. a redesign in 1960 made then better.
Somewhere, probably in Circle Track when it was a super tech magazine, I saw a chart of piston loads studied by GM on 301-302 small blocks...development to get some numbers for racing programs. I think that was an excellent engine choice for HAMB thinking, a high performance engine planned for hard use but not exotic or very expensive design. No unobtanium connecting rods, no crazy loads or detonation brought on by nitro or insane RPM. Biggest load was stretch at the top of the exhaust stroke, piston going up with no cushion of compression. as I recall, I think it was close to twice the other direction top load on the power stroke. Yunick at about the same time ****yzed rod failure: Stretch ovals the big end, distorted bearing at parting line s****es all the oil off the journal, big end decides it wants to rotate instead of reciprocating. Bang.
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