Calling all engineers and engine experts!

fryburner · Jul 27, 2007

Recently something rather unusual has been bothering me. I have been seriously worried about the crankshaft. After reading the article on wikipedia about metal fatigue and how a small microscopic crack can grow until the component fails, I thought about my crankshaft. Since the car has well over 100,000 miles on it and I have no idea how the previous owner drove it, there could be a crack in the crankshaft that I don't know about until the crack grows until suddenly as I'm driving along it snaps and I am left stranded, possibly in the way of oncoming traffic. The worst part is that this happens without warning but most of the time is happens when under load such as accelerating. And of course there is no way for me to know unless I remove the crankshaft and look at it under a microscope. And even if it were to have no cracks, that doesn't necessarily mean it will stay that way. The car is not a TDI by the way, it's a 2002 Audi A8 with the 4.2L V8. I also read that because of the compressive stress put on the shaft, it prevents cracks from forming, at the bearings at least where the compressive stress is. With 310hp and 307lb/ft, I would think there is plenty of power to break the crank. I am almost scared to drive it, especially because I go very long distances with it. I have actually lost sleep over this because I am so worried the crank will just snap when I least expect it, and when I really need power, like pulling out into traffic or merging onto the highway. If that wasn't bad enough, the frame is all aluminium and according the the wikipedia article, aluminium will fail after a time reagurdless of how little stress it is subjected to, unlike steel which has an "endurance limit" that gives it a theroetically infinite life if this stress limit is not exceeded (by the way, this limit is almost certainly exceeded by thr crankshaft). Wonderful, so now not only am I scared the engine will just break without warning, but now the frame could just fall apart too. I think my next vehicle will have pedals, oh wait that's made of metal and could snap in half without warning too!

So what can I do? If the crankshaft does have a crack in it, how much longer before it breaks? Is there anything I can do to prevent cracks and stop any cracks from getting bigger?

SuburbanTDI · Jul 27, 2007

Live with that fear and you might as well tape your thumbs down into the palm of your hand so you don't poke your eye out in case you should ever happen to fall.

If you're really losing any sleep - just sell it - and buy something with less power and "fresher" metal. But your fears are groundless and are not within reason.

(BTW: If your engine should fail you would simply coast at speed until the car were driven to the shoulder of the road by your steering and stoppped by your braking. It would be perfectly safe.)

Fourdiesel · Jul 27, 2007

fryburner - I'm a Metallurgical Engineer and I can assure you that we rarely rely on Wikipedia for advice on metal fatigue or the relative merits of aluminum or steel in engine or automobile chassis design. Let the engineers do their work. Unless you have documented and verified info on the failure rates of crankshafts in Audi V8s and that engine in particular the subject is simply not worth worrying about or discussing. As to "what if" it were to happen in the middle of a freeway, without any warning, in heavy traffic, raining hard, with the aluminum body "about to fail", and the car about to fall off a cliff... Well, what if?
By the way, If you believe what you quoted about aluminum from the Wikipedia info I would advise that you never fly and stay away from the ends of active runways at airports.

nicklockard · Jul 27, 2007

Today's materials science is so damn good that the chances of this happening for a stock car are on the order of 1 in a billion or more.

Even for cast automotive parts.

Start doing extreme modding and MAYBE you'll increase 2nd order harmonics beyond safe levels, thereby causing such failures, but...

vwmikel · Jul 27, 2007

I think it's time you stop reading Wikipedia. Don't take this the wrong way but I get the impression you're highly suggestive. Don't read anything that has to do with diseases.

The chances of your crankshaft breaking are incredibly slim. I'll be honest....I think I've heard of crankshafts breaking maybe once or twice. One time I can recall was on a 10 second drag Rabbit running a STOCK 16v bottom end that normally made 124hp. That car was making more like 500hp. So, don't lose any sleep over this, it's just not worth worrying about.

euronation · Jul 27, 2007

The failure point of the VW 4cyl block was found (the hard way) to be somewhere around 1030hp. The stock crankshaft survived.

vwmikel · Jul 27, 2007

I remember that. It was like the tractor pull engines where the block broke in half at the intermediate and blew apart.

lbhskier37 · Jul 27, 2007

I got to see a Harley V-twin on a test stand that threw a rod through the side of the case because one of the stupid test technicians didn't put enough oil in it during our endurance test. That was a pretty cool failure.

HatefulMechanic · Jul 30, 2007

fryburner said:
. I am almost scared to drive it, especially because I go very long distances with it. I have actually lost sleep over this because I am so worried the crank will just snap when I least expect it, and when I really need power, like pulling out into traffic or merging onto the highway. If that wasn't bad enough, the frame is all aluminium and according the the wikipedia article, aluminium will fail after a time reagurdless of how little stress it is subjected to, unlike steel which has an "endurance limit" that gives it a theroetically infinite life if this stress limit is not exceeded (by the way, this limit is almost certainly exceeded by thr crankshaft). Wonderful, so now not only am I scared the engine will just break without warning, but now the frame could just fall apart too. I think my next vehicle will have pedals, oh wait that's made of metal and could snap in half without warning too!

So what can I do? If the crankshaft does have a crack in it, how much longer before it breaks? Is there anything I can do to prevent cracks and stop any cracks from getting bigger?

find your nearest source of ganja. roll up, and smoke up. cause you need some form of stress reliever or something to quit worrying about that stuff!

i have seen two stock cranks fail, but they were on a 2.8 GM which is notorious for doing it.

i really dont think you should be stressing over an internet article.

jjvincent · Jul 30, 2007

Crankshaft cracks are a real problem and can put you in danger. Just imagine driving down the interstate listening to your favorite tunes. Little do you know, your crank is ready to fail. When this happens, a connecting rod breaks and it makes a big hole in your block. The hot oil sprays out and gets all over your tires. You lose control and slide into the oncoming lanes. At that point the hot oil gets on your exhaust and your car goes up in flames. You jump out just as a Kenworth T2000 smashes into your car. You roll down a large hill and get knocked out as you hit a tree at the bottom.

When you awake, you see that you are chained up in a mountain cabin. Finally, a strange man appears (played by Christopher Walken) and tells you that he saved your life. You ask about the chain and he tells you that it is for your own protection. After a few days you are up to full strength. The old man lets you roam around the cabin but you are still chained up. You soon discover that he has a daughter (played by Angelina Jolie) who is chained up and does all of the household chores. You soon realize that both of you are his personal slaves.At this point you decide to play along in hopes you'll get out.

Every day, the old man goes to town to get supplies. Every Saturday, he leaves in the evening and drives his Silver Audi A8 4.2 to play a few rounds of poker with his buddies. You start thinking of a way to get rid of this old man as you are getting tired of doing household chores (but not tired of the view you get of his daughter). The plan starts with figuring out a way to get the chain off. You take a butter knife and a file and work for about a month to make a key that works in the lock. At that point, you could just run away but this is a thick forest and you would never stand a chance against the old man.

So, every day when the old man leaves in his pickup, you go out and start his A8. You rev it up constantly when it's cold and then hose down the engine when it is up to operating temp. This is to cause those nasty crankshaft cracks to get bigger as you decide that the thermal shock to it will certainly cause the old man a problem.

After 3 years of doing this, you finally have your day. The old man gets into his A8 and as he drives down the gravel driveway, the crank breaks, causing him to hit a tree and then the car bursts into flames. He dies and you are free (along with his daughter). Both of you finally make it back to the city and decide to start a campaign for safer crankshafts and annual crankshaft magnafluxing (as part of your state inspection). The president (played by William H. Macy) signs this into law and we all live a happier life.

Fin

40X40 · Jul 30, 2007

Now I understand why some people ride the bus. I had always wondered.
You need a bus pass, and get a lawyer to sue the doctor that did the lobotomy, cause it didn't work. I am just kidding, and I hope you are too!

The cranks are known good.... except the crank behind the wheel.

Bill

whitedog · Jul 30, 2007

jjvincent said:
Crankshaft cracks are a real problem and can put you in danger.

(SNIP)

Fin

F'n Awesome.

scurvy · Jul 30, 2007

fryburner said:
Recently something rather unusual has been bothering me.
...
So what can I do?

Relax. Don't worry. Have a homebrew.

What's really bothering you is being concerned about a non-problem and not spending enough time enjoying this fantastic weather the midwest has had lately.

Dr.* Scurvy's Remedy: Turn the computer off. Get a beer. Go outside. Repeat as necessary.

* Not an actual medical doctor nor does he play one on TV. "Doctor" granted by the Church of the SubGenius only. Void where prohibited. YMMV. Quit reading this and enjoy a beer outside already.

TurbinePower · Jul 30, 2007

scurvy said:
Relax. Don't worry. Have a homebrew.

What's really bothering you is being concerned about a non-problem and not spending enough time enjoying this fantastic weather the midwest has had lately.

Dr.* Scurvy's Remedy: Turn the computer off. Get a beer. Go outside. Repeat as necessary.

* Not an actual medical doctor nor does he play one on TV. "Doctor" granted by the Church of the SubGenius only. Void where prohibited. YMMV. Quit reading this and enjoy a beer outside already.

Can it be a Coke or a Root Beer if we aren't of the drinking population?

SuburbanTDI · Jul 30, 2007

TurbinePower said:
Can it be a Coke or a Root Beer if we aren't of the drinking population?

No. This kind of stress requires the real deal.

TurbinePower · Jul 30, 2007

SuburbanTDI said:
No. This kind of stress requires the real deal.

Ah well. I guess I'll just have to substitute a nice walk in near undisturbed Boundary Waters territory to a prettier lake than any I have ever seen around my hometown.

Score one for being in Northern Minnesota all summer, in the BWCAW.

weedeater · Jul 30, 2007

I have a friend whose father was an automotive engineer. He refused to buy, drive or ride in a car that had aluminum wheels for the reason that he fervently believed the wheels would shatter when striking the first pothole they contacted.

I'm sure he would run screaming from the Volvo D5 diesel with the all aluminum block. Surely it would explode the first time it was started. And we all know how little Volvo cares about safety.

jjvincent · Jul 30, 2007

weedeater said:
I have a friend whose father was an automotive engineer. He refused to buy, drive or ride in a car that had aluminum wheels for the reason that he fervently believed the wheels would shatter when striking the first pothole they contacted.

I'm sure he would run screaming from the Volvo D5 diesel with the all aluminum block. Surely it would explode the first time it was started. And we all know how little Volvo cares about safety.

Do you want me to get into another story regarding aluminum wheels? This one stars Alec Baldwin, Nick Cage, Reese Witherspoon and James Earl Jones.

I've been traveling all day and seem to be spending my life in a few of our wonderful airports. It is after 12:00 (Eastern) so I'm entitled to drink........

scurvy · Jul 30, 2007

TurbinePower said:
Can it be a Coke or a Root Beer if we aren't of the drinking population?

I'm afeared not. If you're concerned about your crankshaft shattering, you NEED to drink the real thing!

Enjoy the lake, TurbinePower!

scurvy

Mainemike100 · Jul 30, 2007

Do any of the major auto insurance companies offer a "catastrophic crankshaft crack" rider? Think I'll call GEICO right now...

hevster1 · Jul 31, 2007

If you are really that worried about it pull it and have it magnafluxed.

Mainemike100 · Jul 31, 2007

hevster1 said:
If you are really that worried about it pull it and have it magnafluxed.

Should be done after every trip. Just in case any damage was done due to a missed shift, or even road vibration. After all, dragracers rebuild after each run.

hank miller · Aug 3, 2007

When I was first learning to drive, in a 88 dodge caravan (2.8l), the camshaft seized to the head while my sister was driving. Had about 200,000 miles on it at the time. We coasted to the side of the road no problem. After getting towed home we rebuilt the head, and got another 100,000 miles out of the original crankshaft. I'm not sure what stress you are talking about, but having one part of the engine seize while it is running has got to be harder.

I suggest you get a new hobby. Perhaps gardening would suit your temperament. (make sure you have at least 3 acres of garden, and don't use powertools for tillage - the time you spend in the garden will take away time from worrying. If you refuse to do that, taking up drinking would work, but I don't drink, and I don't recommend you do either.

whitedog · Aug 3, 2007

I was just reading a bit about a Deere engine that have over 80,000 hours on it. (Standard conversion is 45 Miles to the Hour so well over 3,000,000 miles) This was on the "original internals". I'm not sure if that means the same pistons and liners, but I'm sure it means it was the original crank and I bet that this crank has micro cracks in it.

Find something else to worry about.

fase2000TDI · Aug 6, 2007

Your car is crap. It will kill you. Sell it to me for $100

NETHED · Aug 6, 2007

fase2000TDI is trying to rip you off. I'll take it from you for free.

jjvincent · Aug 8, 2007

Don't listen to them. You can pay me $5K to take your car off of your hands and I'll even sign a contract that will state, "I will never sue you due to crankshaft failure." Trust me, this is the best deal as liability is the real cost here. When Dry Cleaners are having a rough time with liability, you sure don't want someone like me to unleash the wrath of the crankshaft chasers (they used to be called ambulance chasers) on you.

Vipervnm · Aug 8, 2007

Irrational fears make me laugh at first and then fill with pitty. What happened in your life that would cause you to focus on something without any logical basis whatsoever?

Oh, FWIW aluminum airframes are subject to stress fractures due to fatigue from in flight vibrations. However, these fractures are meticulously monitored and parts are replaced long before anything goes wrong in most cases. Steel is also subject to metal fatigue which is why bridges are monitored so heavily. Again, even in light of one bridge collapsing last week, DON'T WORRY ABOUT IT!!! It's exceedingly rare. Occupy your time with something useful.

LurkerMike · Aug 9, 2007

Vipervnm said:
Irrational fears make me laugh at first and then fill with pitty. What happened in your life that would cause you to focus on something without any logical basis whatsoever?

Oh, FWIW aluminum airframes are subject to stress fractures due to fatigue from in flight vibrations. However, these fractures are meticulously monitored and parts are replaced long before anything goes wrong in most cases.

http://en.wikipedia.org/wiki/Aloha_Flight_243

LurkerMike · Aug 9, 2007

jjvincent said:
Do you want me to get into another story regarding aluminum wheels? This one stars Alec Baldwin, Nick Cage, Reese Witherspoon and James Earl Jones.

I've been traveling all day and seem to be spending my life in a few of our wonderful airports. It is after 12:00 (Eastern) so I'm entitled to drink........

jjvincent

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