Rock Solid hobie pedal arm upgrade group buy .... UPDATED WITH ORDER INFO!!!

I always wondered why they were not aluminum.  Bicycle crank arms are and they take forces much greater than these. 

The Hobie arms are aluminum, just hollow bar. I also believe they are fairly low grade aluminum. Bike cranks are solid or highly engineered hollow cranks.

I always wondered why they were not aluminum.  Bicycle crank arms are and they take forces much greater than these. 

The Hobie arms are aluminum, just hollow bar. I also believe they are fairly low grade aluminum. Bike cranks are solid or highly engineered hollow cranks.

Yeah, there's aluminum, and there's aluminum.  The strength and fatigue resistance of different alloys (and the quality of a given alloy based on the supplier) varies greatly. 

Also, a big difference between a cheap hollow bar extrusion and the (I assume) expensivly forged shapes of bike cranks doesn't really compare.  Even if they did, look at the cross-section of your bike's crank, and then at the cross-section of a Hobie pedal arm.  The bike crank will be a lot beefier.  The bike manufacturer would have gone through some pretty sophisticated stress analysis before it was produced.  Hobie probably went to Home Depot and bought some cheap aluminum tube stock, tested it in a prototype for a month, and called it good.

FYI, response from one of the guys at Hobie about this issue:

We have experienced some issues with this item - we have the arms extruded for us by an outside vendor. They've had some hardness issue - we now do 100% hardness test in-house before the cranks get assembled - and the issue hopefully is behind us. We are also making an upcoming change to this part and increasing the size of the section - so in the future (likely new model year) the crank arms are going to get a slight change - which we make a change to the adjuster handle as well.

In for a pair.

I have a question for the engineers of nwka.  Is it possible the drive arms are designed to be a weak point, absorbing damage that would otherwise damage the drive well?

I have wondered the same thing. I've also wondered this: Since the maximum amount of force that can be applied to the pedal is whatever the water resistance is at the fin plus whatever resistance is on the other pedal, are the arms breaking more easily than expected because of the natural tendency to not lift your non-drive leg? I.e. you're pushing the fin, plus you're pushing your other leg up, and possibly you're even exerting force without thinking about it with the non-drive leg.

I think this is a cool product, I'm not down on it at all, but I'm curious why the arms are breaking in the first place. It seems to me that if I take one of my feet off the pedals altogether and push as hard as I can with just one leg that I would never break the arm. But perhaps that is not true in some conditions. I'm not playing down the breakage others have experienced at all, but I'm curious if it is relatively easy to avoid.

I'd have to be able to look at the fractured face to be able to tell, but I think these are fatigue failures, not single overload failures.  If it is fatigue then beefing up the arms will only help overall drive durability.  If it's single overload failures then yes, beefing them up could cause a more expensive failure elsewhere in the drive, though that is far from certain.

I've examined my fracture faces pretty closely...one appears to be a clean break (the one I suffered when my drive came out of the water climbing a big wave face) and the other looks like it started as a crack at the inboard aft corner and propagated into a failure...I still have them on my workbench for some reason.

Beyond the cost, imagine if you will..... "carbon fiber".   

Carbon fiber is cool and all, but the problem with it is A) it really doesn't having holes in it (esp woven and unidirectional fiber layups), and B) even if you get the holes right (which requires winding fibers around the circumference) if it fails it can do so without any warning.  The aluminum arms that failed probably had cracks that an inspection looking for such would have turned up perhaps a dozen trips or more before final failure.  Composites, especially carbon, are notorious for not having any visible fatigue damage prior to catastrophic failure.

You have to remember that many manufacturing companies design things to last so many cycles before fatigue sets in and a part fails.  Some of this is intentional to sell parts, some to protect other (more expensive) parts from breaking, and some to save on manufacturing costs.  Not sure which reason hobie uses, but beware that you may break something else.

All that being said... I'm gonna try them out

From what I've seen as an engineer it's usually a cost cutting move that has little thought as to consequences attached to it.  Planned obselence is usually more work than people want to put into it.

From what I've seen as an engineer it's usually a cost cutting move that has little thought as to consequences attached to it.  Planned obselence is usually more work than people want to put into it.

.  That's usually more related to the auto industry

I've examined my fracture faces pretty closely...one appears to be a clean break (the one I suffered when my drive came out of the water climbing a big wave face) and the other looks like it started as a crack at the inboard aft corner and propagated into a failure...I still have them on my workbench for some reason.

Thanks for the feedback. Also the quote from Hobie via Rawkfish is pretty interesting.

... the quote from Hobie via Rawkfish is pretty interesting.

Very...especially considering that both my failures were isolated to my 2013 V2 drive while my 2006 V1 drive is going strong.

I've noticed the same thing.  My older drive looks fine, but the newer drive was the one that failed.  The arm on the newer drive that didn't fail looked like it was going to so I replaced it too.

... the quote from Hobie via Rawkfish is pretty interesting.

Very...especially considering that both my failures were isolated to my 2013 V2 drive while my 2006 V1 drive is going strong.

has me scratching my metallurgy brain and wondering if anyone has collected/posted any data on what year V2 drives are showing the most failures---
2010?
2011?
2012?
2013?...

that would be interesting data, might help differentiate between an temporary alloy/processing/heat treatment issue (mostly effecting V2 drives over a limited period of time) or a design issue (fairly uniform for all V2 years)...cheers, roger

I have two 2011's and both have held up fine.