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PostPosted: Fri Jul 12, 2013 10:32 pm 
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Location: Sarasota,Key West FL
I went ahead and did an FEA stress analysis on the mirage pedal, and you guys are actually onto something.

Here is the stress plot:
Image

What this is telling us is with 100 lbs force on the pedal, the max stress is about 44,000psi around the area where the pedals seem to always break. This is all well and fine but the yield strength of the aluminum is only 31,000 psi. I did my analysis using aircraft grade aluminum (the really good stuff), so actual results will probably be less ( I don't know the actual grade they are using).
This doesn't mean that it will break at 100 lbs force, what it means is each time you hit 100 lbs force, the metal gets a little weaker and yields just a little (metal fatigue), and eventually just breaks and falls off. You can do this at home folks, take a piece of 1/8 aluminum and bend it back and forth 10 or 20 times, it keeps getting easier, then all of a sudden after so many flex's it just breaks, that's just the way aluminum is, sorry to burst everyone bubble about the strength of aluminum....
What this means is all of our mirage drives are ticking time bombs. I know in my case I'm going to take mine apart and drop in a couple 1/8" x 1/2"x2" pieces of aluminum, I will pack some paper into one end, grease up the pin (the pin used for setting the mirage pedals), then pour in epoxy over the 2 pieces of aluminum (one on each side of that mirage setting pin), basically filling the inside around the problem area. This should add enough strength to the problem area to eliminate the fatigue problem. I imagine just a solid slug of epoxy (without the aluminum strips) will probably also work, we only need to distribute the load to a wider area. Basically anything in there to strengthen the weak area will work.

Hope this helps
Bob


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PostPosted: Sat Jul 13, 2013 5:56 am 
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Location: Amelia Island, FL
Bob,

WOW, impressed with your analysis! Not the typical biased BS responses. Not sure I like your findings but......

Thanks


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PostPosted: Sat Jul 13, 2013 6:16 am 
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Location: Columbia, MD
Great analysis Bob!

Please correct me if I'm wrong...

The model looks like it's been run in one axis? If you take torsion into account the situation only gets worse. The peddalers feet are applying force a couple inches off the strut centerline, so there's always a twist applied. Only gets worse when the boat is canted while turning etc. That twist is (mostly) resisted at the pin where the breaks occur.

So you get a double whammy of axial stresses AND torsional stresses concentrated at the pin point.


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PostPosted: Sat Jul 13, 2013 11:15 am 
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Location: Escondido
bram wrote:
On only the resistant of the fin against the water it is impossible to brake the shaft of the pedal. It can happen but only when there is a construction failure. Puth the pedals next to eighother and then you can give more power as the drive can handle.
Bram, great description! I think you hit the nail on the head.

fusioneng wrote:
I went ahead and did an FEA stress analysis on the mirage pedal....This doesn't mean that it will break at 100 lbs force, what it means is each time you hit 100 lbs force, the metal gets a little weaker and yields just a little (metal fatigue), and eventually just breaks and falls off
Bob, excellent analysis as always!

As I recall, Hobie identified "fisherman's feet" as a major cause of drivewell cracks a few years ago. Unfortunately the plastic does the same thing the aluminum does -- flexes and eventually cracks. So do the masts. Hobie has since strengthened the drivewell to minimize stress cracks. So here's the question -- if you strengthen the crank arms, does that increase the chance of cracking the boat? Which is less expensive to replace? IMO there should ideally be a designated sacrificial part barring wear and tear and defective parts. 8)


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PostPosted: Sat Jul 13, 2013 2:56 pm 
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Location: Auckland NZ
Roadrunner, does that mean that my boat will sooner or later but inevitably crack in the area of the drivewell due to the stresses of pedalling ?!?


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PostPosted: Sat Jul 13, 2013 4:17 pm 
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Respectfully, why should there be a "sacrificial part?" Design the system to perform correctly. We're talking about a pair of metal tubes & a plastic socket.

If you had to pay a little more for no hull cracks & no drive failures, wouldn't you? I don't think either of those problems are so complex that they can't be solved relatively easily. The Mirage Drive is Hobie's premier product & the reason we all bought their boats. It deserves to be as bulletproof as possible.

The hull and drive are mission critical parts. I don't want the "sacrifice" to be me because it happens at the worst time in an unpredictable way.

Just my opinion.


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PostPosted: Sat Jul 13, 2013 4:33 pm 
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Location: Florida
Well, I think that Hobie is smart to keep offering items like the stainless steel sprockets and the solid crank arms.

In most cases nobody needs to retrofit their mirage drive, but it is nice that you can. Viva la choice!

Personally, I have modified all my drives to have solid crank arms. If your pedal wears out on the hollow crank arms you must buy a new crank arm/pedal assembly anyway so get a solid arm and some comfy bike pedals and you are good to go.

If you use turbo fins and are a heavy user and pedal in salt water (and not wash drive completely) and just want that extra peace of mind a set of solid crank arms might be worth the small investment.

I don't think the hollow crank arms are defective, but I am in favor of making all my gear as bulletproof as possible.


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PostPosted: Sat Jul 13, 2013 5:43 pm 
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stobbo wrote:
Roadrunner, does that mean that my boat will sooner or later but inevitably crack in the area of the drivewell due to the stresses of pedalling ?!?
No, of course not. You could blow a scupper tube or have it mauled by a shark!
Image
:mrgreen:

Seriously, not too many things can be used daily and last forever. Look at automobiles for example -- sooner or later they wear out and get recycled. You've probably observed plastics becoming brittle with age and sun exposure. I watched a guy put his foot through the footwell of an Ocean Kayak once -- the plastic just shattered. The nose of the drivewell is the most stressed part of the Hobie Mirages and plastic cannot be flexed indefinitely. You better order your new Adventure before it's too late! :lol:


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PostPosted: Sat Sep 07, 2013 11:45 am 
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Just resurrecting this post to say that I have just had another pedal breakage. In exactly the same place as the photograph and consistent with the engineering stress analysis provided by other posters above. This breakage has occurred only a few months after replacing the pedal and I have no doubt that it is one of the new ones. If I experience another breakage in a similar timeframe I am going to be looking to get my hands on some of the older pedal shafts as this new one has lasted very little time compared with the pedals already on my drives.


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PostPosted: Mon Feb 17, 2014 3:45 pm 
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Location: Jaco, Costa Rica
stobbo wrote:
Just resurrecting this post to say that I have just had another pedal breakage. In exactly the same place as the photograph and consistent with the engineering stress analysis provided by other posters above. This breakage has occurred only a few months after replacing the pedal and I have no doubt that it is one of the new ones. If I experience another breakage in a similar timeframe I am going to be looking to get my hands on some of the older pedal shafts as this new one has lasted very little time compared with the pedals already on my drives.



X2 but in 2 months of usage.

_________________
Mark
Adventure Island- 2014
Revolution 13- 2013


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PostPosted: Tue Feb 18, 2014 12:31 am 
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Does anybody know what the weight difference between a set of the stock pedal shafts & the upgrade solid shafts is? I'm thinking about upgrading if the weight difference isn't too much. FWIW, I haven't had a problem on any of my 3 drives as of yet so this would be a proactive upgrade. Thanks, jim


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PostPosted: Tue Feb 18, 2014 1:29 am 
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Location: Canberra, Australian Capital Territory
Thanks to all posters, especially The Tall Man, Bram and FusionEng. A cautionary tale for all of us with Mirage drives.

I notice when I sail my Sport that I put a lot of force (and probably some twist) on the two pedals, particularly when going to wind (pedals together, to get maximum 'centreboard' effect, and a bit of hiking out). Always wondered if this was damaging in any way. Sounds like it could be.


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PostPosted: Tue Feb 18, 2014 5:46 am 
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Location: High Point, NC
All materials and structures have a built in fatigue life. Every time you flex something (in the elastic range), or generate a "cycle," you damage the item by some amount. The deeper the cycle the more damage is done.

Materials and structures are selected and designed so that they are able to perform their tasks for the required amount of time before they fail. This could be millions of cycles in some cases, hundreds or only dozens in others.

I have no idea if the few Mirage Drives that fail have simply reached their normal fatigue life, or if something about those particular drives weren't up to spec and therefore failed long before they should have.


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PostPosted: Wed Feb 19, 2014 12:19 pm 
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I am 6 foot 2 inches tall and own a 2011 Outback. I fish in saltwater almost every day for 7 months a year,as I am retired. I have the original mirage drive with no shaft breakage. I meticulously rinse the mirage drive in fresh water after each use and apply an anti-corrosion spray every day as well. I am not accusing anyone of poor maintenance because that is for each individual to ascertain for themselves. One thing I am curious about is looking for common denominators here. How many of these breaks occurred when using turbo fins for example? I have never made that conversion to larger fins for several reasons, more hull cracks have occurred with them and it reduces my ability to fish the flats with speed. In addition was there any corrosion going on a the site of the fracture especially where any dissimilar metals meet each other?


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PostPosted: Wed Feb 19, 2014 3:49 pm 
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Location: Missoula, Montana
The fact that many Mirage Drive pedal shafts have been breaking indicates to me that the shafts are badly designed. The Mirage Drive should be designed to withstand many years of heavy use without any failures at all. For example, bicycle pedals and cranksets essentially never break. Owners of Hobie kayaks should not have to worry that their Mirage Drive's pedal shafts will break if they pedal too hard. I would not buy a kayak paddle which was known to break when paddling hard. The number of Mirage Drive pedal shaft failures which have been reported over many years worries me. I don't like having to worry that a pedal shaft will break when I am miles from shore. Sure, I can paddle the kayak back to shore. But I shouldn't have to worry that the pedal shafts on my Mirage Drive may fail at any time. Hobie has known about this problem for many years. It should improve the design of the pedal system on Mirage Drives, and it should replace older pedal shafts which break without charge, even if the warranty period has passed.


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