Rock solid Hobie drive arm upgrade group buy ... UPDATED WITH ORDER INFO!!

I've looked at doing this too .... the material cost is ~$25 if you are just making 1 set .... lessens greatly as you do more sets.  All that is really needed is a drill press.  Nothing with tight tolerance.  Anodize is mainly cosmetic in this case.  If you want to get "fancy", you can drill the pilot hole oversize & press in a set of bronze bushings at the pivot point to increase life.

I had a shop lined up about 2 years ago to do it in quan. but pulled the plug due to loss of interest on my part.

Is that $25 for a 6061-T6 aluminum?

count me in for 2 sets.

I've looked at doing this too .... the material cost is ~$25 if you are just making 1 set .... lessens greatly as you do more sets.  All that is really needed is a drill press.  Nothing with tight tolerance.  Anodize is mainly cosmetic in this case.  If you want to get "fancy", you can drill the pilot hole oversize & press in a set of bronze bushings at the pivot point to increase life.

I had a shop lined up about 2 years ago to do it in quan. but pulled the plug due to loss of interest on my part.

Cool; i guess this is gonna be my next project before Albion! I got a drill press, taps and a 2-ton press... just need to hit up Naylors and pick me up some metal....

Make me a pair out of of ss 303 dude.

Make me a pair out of of ss 303 dude.

I'll find out how much it is and let you know... I just hate cutting into S/S.... slow and painful..... but youre my pare, so I'll try!!!

303ss is crap!!! For salt you want 316

It is mind boggling that you are selling drive arms that "WONT BREAK" without doing any engineering stress tests. You are using the exact same dimensions as existing Hobie arms, just using solid aluminum instead of aluminum tubing. The problem here is not aluminum's strength, it ability to withstand an applied load without failure. You would be hard pressed to break a pair of new Hobie v2 arms by doing a leg press. And the breaks posted on NCKA did not happen that way. They occurred with repeated application of stress much smaller than the ultimate strength of aluminum. This would suggest the problem is more likely fatigue strength. Take a look at the graph below.



Steel alloys have a fatigue limit, below which no number of cycles will cause failure. Whereas aluminum alloys have no such limit, and will fail with even small stress at high duty cycles. So changing from hollow aluminum tubing to solid aluminum will not solve the fatigue problem. For better fatigue strength, stainless steel is the way to go.

Is that $25 for a 6061-T6 aluminum?

The material cost for 6061-T6 is only about $10.

It is true that you can make crank arms yourself with a drill press---I've made a few myself. But, if you're paying a shop to do it, they'll work to very tight tolerances, they have to pay (skilled) labor costs, and they use expensive equipment that wears out. I'll be surprised if any such place will do a few dozen crank arms for a price that people will consider reasonable. Of course, I hope to be proved wrong...

It is mind boggling that you are selling drive arms that "WONT BREAK" without doing any engineering stress tests. You are using the exact same dimensions as existing Hobie arms, just using solid aluminum instead of aluminum tubing. The problem here is not aluminum's strength, it ability to withstand an applied load without failure. You would be hard pressed to break a pair of new Hobie v2 arms by doing a leg press. And the breaks posted on NCKA did not happen that way. They occurred with repeated application of stress much smaller than the ultimate strength of aluminum. This would suggest the problem is more likely fatigue strength. Take a look at the graph below.



Steel alloys have a fatigue limit, below which no number of cycles will cause failure. Whereas aluminum alloys have no such limit, and will fail with even small stress at high duty cycles. So changing from hollow aluminum tubing to solid aluminum will not solve the fatigue problem. For better fatigue strength, stainless steel is the way to go.

Is there a need to go to SS when there have been no reports of breakage of the original Hobie solid aluminum arms? They have been in use for a long time now on earlier Hobies. Actual water testing over time indicates that solid aluminum arms are all one needs for years of pedaling. The rest of the drive parts like drums and cables will break before the arms do.

303ss is crap!!! For salt you want 316

You are correct.  Found this.

316L stainless steel is an extra low carbon grade of 316 stainless steel; generally used in stainless steel watches and marine applications.

Make me a pair out of of ss 303 dude.

I'll find out how much it is and let you know... I just hate cutting into S/S.... slow and painful..... but youre my pare, so I'll try!!!

Thanks pare ko!  I'll buy the ss316L for both of us.  Just let me know the size(LxWxH) and type(solid square bar, square tube) you want to use.  I think mine is getting close to that fatigue limit of aluminum.  So I better have a backup.

A Loose Screws Group buy? Lots of Hobies in the group.

But if you wanna go all out .. Use Titanium . Almost as light as aluminum and almost as strong as stainless steel .. And also has exceptional resistance to sea water ..

But if you wanna go all out .. Use Titanium . Almost as light as aluminum and almost as strong as stainless steel .. And also has exceptional resistance to sea water ..

How about solid carbon fiber arms like really expensive bike cranks. Light, never rust, no fatigue. Maybe $500.00 a pair but what the hell. Cheap mental insurance. As to aluminum v SS. Ask yourself if you would feel safer in a commercial jet if the craft were build of SS rather than aluminum alloys. Countless hours, millions of miles do those flexing wings stay attached.