Mirage drive repair

Wow, I've been an engineer for 20 years and I'm very impressed with the smart thinking that went into this repair. FG, maybe you missed your true calling in life? ;-)

That is some real ingenuity!

Now THAT is impressive. Great job FG.

Very, very cool.  Time to get a spare pedal.

Awesome, great work!

Damn Bill, that's some fine work!
You are a true friend indeed.

-Spot-

The pedal crank failure caused me to examine my own pedals and find replacement cranks.  The bare pedal bar is a right/left product though it seems to me the crank is symmetrical. I have made an inquiry.

Also, I pulled the cap plugs on the actual pedals to see how the pedals are attached. They are attached by stainless washer and a nylock nut.  One side was filled with salt water and slightly corroded.  The other side was dry and shiny.

I rinsed and dried the wet side, then filled both sides with Marine Wheel bearing grease, and replaced the cap plugs.  The plugs pry out with a flat screwdriver and have a ridge meant to be a seal.  You might want to check yours.

Nice work Bill! I can't afford a spare drive but I can definitely afford those parts to carry along on trips. Hmmm

Dino Abulencia
HOW-NWest

Well, here we go again!  Less than 2 months since my last broken pedal arm...



Just to be clear, I am always one praise the brilliance of the mirage drive.  It's an incredible machine.  However, I think it's time for hobie to recognize this issue and improve on an already-great product.

I understand these pedal arms are hollow to reduce weight.  But it creates a very weak point where the pin goes thru.  A solid aluminum pedal arm would not be much heavier, and a worthy trade-off to NOT have this happen. 

Murphy's law dictates failures like this happen at the worst possible times...i.e. right at the beginning of a fishing weekend, or when you're at the furthest point from the launch and you have to paddle back against the wind and current.  Not to mention a ruined fishing day.

It's almost gotten to the point where I'm thinking of bringing a spare drive out on the water with me.  Not even kidding.  You HAVE to  be able to rely on these machines, and you just can't.  And these boats I have (outback and PA) are NOT made for paddling.

So for me, that's 2 broken pedal arms (and I've seen it happen to somebody I was fishing with), and 3 broken masts.  And only one of those times was I lucky enough for it to happen near the launch.

If any of you guys have the ear of Hobie HQ, send it up the ladder!  I'd appreciate it! 

I understand these pedal arms are hollow to reduce weight.  But it creates a very weak point where the pin goes thru.  A solid aluminum pedal arm would not be much heavier, and a worthy trade-off to NOT have this happen. 

As a mechanical engineer, and a stress analyst by specialty, making it solid would not help as much as you think.  If you look at the stress, and load carrying, through the cross-section, the portion that is currently hollow doesn't do that much.  What they need is a consulting engineer (ahem, shameless plug warning) to do some loads analysis followed by stress analysis to determine the best way to optimize that structure for weight and strength.

Will Work for Adventure

I can't remember if you are coming to ORC or not, but if you are do you need a spare drive for the weekend?

I can't remember if you are coming to ORC or not, but if you are do you need a spare drive for the weekend?

Unfortunately I am in FL so I won't be able to make ORC.  But thanks for the offer!

I understand these pedal arms are hollow to reduce weight.  But it creates a very weak point where the pin goes thru.  A solid aluminum pedal arm would not be much heavier, and a worthy trade-off to NOT have this happen. 

As a mechanical engineer, and a stress analyst by specialty, making it solid would not help as much as you think.  If you look at the stress, and load carrying, through the cross-section, the portion that is currently hollow doesn't do that much.

Will Work for Adventure

Wow you really think that a solid 3/4" bar would have a similar failure point after a similar amount of time to the hollow bar? What about different alloys? I am going to try and make some from either 2024 or 7074.
Thoughts?

Sent from my Motorola Flip phone.

I understand these pedal arms are hollow to reduce weight.  But it creates a very weak point where the pin goes thru.  A solid aluminum pedal arm would not be much heavier, and a worthy trade-off to NOT have this happen. 

As a mechanical engineer, and a stress analyst by specialty, making it solid would not help as much as you think.  If you look at the stress, and load carrying, through the cross-section, the portion that is currently hollow doesn't do that much.

Will Work for Adventure

Wow you really think that a solid 3/4" bar would have a similar failure point after a similar amount of time to the hollow bar? What about different alloys? I am going to try and make some from either 2024 or 7074.
Thoughts?

Sent from my Motorola Flip phone.

I didn't say it wouldn't be better.  Just that the improvement isn't as big as you'd think.

For the same weight addition as making the bar solid you'd get far better results from adding that material to the outside.

I don't know what alloy Hobie's supplier is using for the arms, but for the alloys you mention 7075 would be better in terms of fatigue life, depending on temper anyway.  At least I assume you meant 7075, rather than 7074.

But 2024 has a higher fracture toughness (resistance to complete failure after a crack has started).

I can't recall which is better from a corrosion standpoint.

Anyway, for 7075-T6 300MPa peak stress results in about a 1-million cycle life.    For 2024-T3 1-million cycles corresponds to about 250MPa.  There's a LOT of scatter in fatigue life data though, so use it with caution.

Again though, it depends on the temper of the material, as well as the smoothness of any machining you do.  Cut the holes with a nice sharp bit, at the appropriate speed and you'll get longer life than if the hole is cut with a crappy bit, at the wrong speed (partly because it makes a rougher hole, partly because the heat generated can change the local temper).  Also make sure the hole is drilled with a good drill press with little washout so you can maintain concentricity.  Make the hole as close in size to the diameter of the bolt as you can while still making assembly possible (i.e. minimal clearances).  Use a shoulder bolt if you can.  All those things will help with reducing stress concentrations that can drive up the stress in local areas and cause cracks to initiate.