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Crankshaft Failures

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Bill-Higdon

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Like others I thought this was history. I know that years ago the Aerovee was having some prop departing concerns and the company came up with a different crank. I have read of at least one Aerovee crankshaft break recently with the new crank. Not sure what year the new cranks came out as this video is from 2013:

Corvairs were breaking cranks but the 5th bearing seems to have cured that. The other thing that I do (even though many say it's not required) is to index the prop. But it appears in the photos of the VW prop breaks that they are separating right behind the hub. I don't see how indexing would change any of that unless somehow balance or harmonics are affected.

The other thing learned on Corvairs is that the prop, fly wheel, and ring gear should be on the same end of the crank. If you load both ends of the shaft with weights then it's gonna undergo a lot of twisting.

But I'm the dunce in the group ...
A friend had his AeroVee do the crankshaft break 2 summers ago, it broke where the lat Bob Hoover said they break. I have pictures I'll post when I find them.
 

pictsidhe

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A rash of breakages in the same aircraft type is possibly significant. The typical CX4 setup could be disagreeing with the cranks. It looks like a torsional failure to me. It would be useful if people gave full details of the whole powerplant, including prop.

Am I weird for thinking the fracture is pretty?
 

fly2kads

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Am I weird for thinking the fracture is pretty?
Not weird. One of my used bookstore finds is "Understanding How Components Fail." It has a fair number of pictures that I think are fascinating. It's sort of like fractals and other mathematically-derived art, in that way.
 

Daleandee

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A friend had his AeroVee do the crankshaft break 2 summers ago, it broke where the late Bob Hoover said they break. I have pictures I'll post when I find them.
Can't help but wonder if Sonex will at some point consider adding a 4th bearing to the case on their 2180 VW conversions as a number of builders add one from Great Plains. But doing that at this late stage might appear to be admitting a fault. While a lot of those engines don't have the crank issues, some do. I had a nose wheel version with the early VW conversion and the 1st generation crank. Never was real comfortable behind that engine for that and some other reasons.

Corvairs went through a few crank breaks a number of years ago. Seems that the well designed 5th bearing offered by SPA has corrected that, even for those of us using a GM factory crank. New cranks are also available in stock and stroker.
 

poormansairforce

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Not weird. One of my used bookstore finds is "Understanding How Components Fail." It has a fair number of pictures that I think are fascinating. It's sort of like fractals and other mathematically-derived art, in that way.
I love fractals, they're everywhere! I have a book you would love.
 

Mike Stewart

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I noticed he had a composite prop, it should have been wood. I hope when he gets his engine rebuilt he uses a wood prop or he will have the same result.
Composites have a core of wood - Prince, Catto being the only ones I know about - and are fine with the VW according to Steve. Some composites have metal rods as structure and I doubt these are acceptable.
 

Mike Stewart

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Having built several VW aero-engines, I have always stayed at 1914 cc or below using the good German VW stock 69 mm crank with the shrunk on prop hub. It takes heat and a good press to get one off. Believe the center bolt is just insurance. I have never had any problems. I still have 3 good stock 69 mm German cranks that I have been saving. I'll be using one on the 1835 cc engine that I'm building for the mud pipe buggie. Also did the Bob Hoover mods for that engine.
At one time I ask Steve at GP's about going to the force-one prop hub for the smaller VW engines and he said just a waste of money for the 1915 cc engine and below.
I have been building a 2180 aero-engine , but its going to be a flywheel drive engine.
At the time Steve passed away, there had never been a documented case of a GP crank breaking. Cranks issues were sorted out in the 70's and Steve has never used anything other than the forged, stock German crank. Since he no longer runs the company, I've no idea what's going on with GPAS.

Aerovee's have been widely known for as long as I've known anything about them as dunebuggy quality, which I would assume means cast cranks - although you'd think they'd know better by now. I don't know enough about Scat to comment. I think all one needs to know on the subject is to use the German forged crank in their engines and to use wooden props. I index my prop solely for convenience in hand propping. If there's some durability advantage in the relationship between the crank and the prop, I've forgotten what it might have been. My memory tells me there is none, but the subject is certainly worth a revisit for those curious.
 

Daleandee

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I index my prop solely for convenience in hand propping. If there's some durability advantage in the relationship between the crank and the prop, I've forgotten what it might have been. My memory tells me there is none, but the subject is certainly worth a revisit for those curious.
I indexed my prop on my Corvair. Most agree that there is no advantage and a few insist there may be. Mine is electric start so I couldn't find a reason not to do it. Engine runs extremely smooth but a well built six cylinder should ...
 

Vigilant1

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At the time Steve passed away, there had never been a documented case of a GP crank breaking. Cranks issues were sorted out in the 70's and Steve has never used anything other than the forged, stock German crank. Since he no longer runs the company, I've no idea what's going on with GPAS.
I wish that was so, but there was one failure of a GPAS supplied crankshaft with a Force One hub before Steve passed away. More here, with a link to the NTSB lab report on the crank. The crankshaft was not a stock GPAS catalog part, but GPAS sold the crankshaft to a Sonex owner who was building a "2400"cc engine. The crankshaft reportedly did not come from the usual supplier GPAS used for their "Top Bug" crankshaft.
Aerovee's have been widely known for as long as I've known anything about them as dunebuggy quality, which I would assume means cast cranks - although you'd think they'd know better by now.
What you've assumed is not correct. The Aerovees all use 82mm forged crankshafts. Absolutely no cast crankshafts in these engines.
 
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Mike Stewart

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I wish that was so, but there was one failure of a GPAS supplied crankshaft with a Force One hub before Steve passed away. More here, with a link to the NTSB lab report on the crank. The crankshaft was not a stock GPAS catalog part, but GPAS sold the crankshaft to a Sonex owner who was building a "2400"cc engine. The crankshaft reportedly did not come from the usual supplier GPAS used for their "Top Bug" crankshaft.

What you've assumed is not correct. The Aerovees all use 82mm forged crankshafts. Absolutely no cast crankshafts in these engines.
Okay thanks. How long has this been true?

Also . . . you mention "Top Bug" crankshafts. I haven't looked into this at all and there are many aspects I'm sure I'm ignorant of but I do think I distinctly remember talking to Steve about his crankshafts and he told me he used "stock, forged German crankshafts". Now . . . a lot of years have gone by since that conversation and I've caught myself mis-remembering things on other occasions so I'm in no position to seriously question your statement that Steve got crankshafts through someone named Top Bug. Your mention of it is the first time I've ever heard of Top Bug so that in itself shows how poorly informed I must be regarding VW crankshafts. At one time the subject of Steve's crankshafts was fresher in my mind and I recall discussing the subject in serious fashion with Steve (since I fly with an engine he put together himself) and to the best of my recollection what he told me was that he used "forged, German OEM crankshafts". The name Top Bug never came up in any of our discussions.

As I sit here and think about this, I actually have heard or seen the name Top Bug. It's imprinted on the top of the component that sticks up where a distributor might normally be. In the case of my engine, this component has to do with the electronic ignition.
 
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akwrencher

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My understanding is the stock stroke cranks are German, the strokers are top big. That's just from reading though, I don't have personal knowledge of this.
 

Marc W

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The Top Bug cranks are forged aftermarket cranks. Here is Great Plains crankshaft page: Crankshafts from Great Plains Aircraft! The top Bug only comes in a 82mm stroke so it would not work to make a 2400 cc engine. The crank that broke in the 2400 cc engine was probably not a Top Bug.
 

Vigilant1

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Okay thanks. How long has this been true?
The failure of the crankshaft occured in 2014. The crankshaft was reportedly sold by GPAS to the builder in 2010. That NTSB report and the lab report in linked to in my other post is worth a look. The crank didn't break at the nose/keyway, which is the "normal" break point for pulley drive installations.

My impression is that ”Top Bug" was the name Great Plains gave to the custom 82mm crankshaft they sold, I don't know who actually manufactured it.

If Steve said he used "stock, forged German crankshafts," he was probably referring to engines with a max stroke of 69mm. As far as I know (and I'm no expert), that's the longest stock stroke that was available for the Type 1 VW, so he couldn't have sold a "stock" crank with a stroke longer than that..
 
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Flyguyeddy

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There IS one more stock vw crankshaft that has a stroke of 76.5mm and it comes from the wasserboxer engine, but it also requires a custom #1 bearing
 

BBerson

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The "2400" or "1200 1/2vw" was likely a 86mm crank built up with custom weld. Scott had some fail on 1/2 VW. Still on the website.
 
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Flyguyeddy

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Pretty sure the 86 cranks from scott are aftermarket units he sources from who knows where. I do know a few of the 45hp engines had crank failures.
 

Pops

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This is what Steve put out about cranks in I believe 2010. From his web-site.

Remember many years ago, I was building a 1835 cc engine and using the stock German made crank with the skunk on prop hub with a wood prop and I ask Steve should I go to the Force One for the extra strength. He said just a waste of money, what you have will be good. It was.

 
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