DIY hobie crank arms (again)

I've posted several threads on this topic before, but I didn't put much detail in any of them. Every time I make a new pair, I always seem to have to relearn something, so I thought I'd make a new thread with all of the details, This may be more for my own benefit than anyone else...

I start with 3/4" square 6061 aluminum bars, which are cut to 13" in length. I've got a piece of aluminum channel with the precise spacing carefully marked.

After using a punch to make a small indentation, I carefully drill, making sure to get things exactly centered.

Next, I drill a series of holes of increasing diameter, each about 1/2 way thru. This makes it easier to keep the holes centered.

The hole where the adjustment pin goes (shown in the picture below) is 5/16".

The hole at the bottom is where the bolt attaches the crank arm to the drum. The bolt is 3/8", but a 3/8" hole is too tight of a fit, so I use a size "W" drill bit. If I recall correctly, it is 0.011" bigger then 3/8", and that's just enough to make for a perfect fit.

For these, I plan to use regular 9/16 bicycle pedals. That requires a 33/64" hole. This is the most difficult of the holes to drill. It's also  super-important to get this one perfectly centered, as there is not a lot of extra metal to the front and back.

Finally, I cut threads in the 33/64" hole (RH thread for the right pedal and LH thread for the left pedal). It's super-de-duper-important to make sure the tap goes straight into the hole, as even being slightly off-angle will be a problem. Also, once the tap is solidly in, I only do about 1/4 turn of cutting before backing it out, and I back it out 3/4 turn.

It doesn't take a great deal of strength to cut the threads, but this grade of aluminum (6061) is very "gummy", and it does like to clog up the tap (interestingly, I don't find this to be an issue when drilling it, just when cutting threads). If you don't back it out far enough with each new cut, it can get jammed up in a hurry.

When cutting the threads, I have not noticed much difference between using oil, and dry cutting. But, it does seem to be slight easier to get it started with some lube, so it is probably worth it.

This time, I made 2 pairs, and I finished them by sanding, using a random orbit sander, and 120 grit paper. It seems to give a nice faux sandblasted look.

Just few other random things...

If you use solid aluminum crank arms, I would suggest replacing the stainless bolt that connects the crank arm to the drum with a nylon bolt. Stainless in aluminum will result in galvanic corrosion when exposed to saltwater, and the stainless bolt can become very difficult to remove. You won't have that problem with nylon, and there is no issue wrt the strength.

The same aluminum/stainless corrosion issue can/will happen to the adjustment pin, and it could get so bad that you won't be able to adjust the setting by hand. So, you want to grease it, and I'd recommend that you do so at least once each season. I've thought about replacing the stainless pin with something else, but I'm not sure that aluminum would be strong enough, and I can't think of anything else to try.

The 6061 aluminum that I used in this thread is probably indestructible for this application. It is also fairly cheap, and has great corrosion resistance. But, if you want to use something even stronger, I've made crank arms out of 7075 aluminum, and the process is the same. In fact, cutting threads in 7075 is actually easier than 6061, as it does not have that "gummy" characteristic of 6061. The corrosion-resistance of 7075 is not as good as 6061, but if you wash your Mirage drive after its in the salt, I don't think you'll have any problems with it. One down side is that 7075 is about 3x more expensive.

I really like the C-clamp to the cap rail for a hold down while cutting the threads. Simple. Effective. Years ago I purchased from Hobie a solid pair of bike threaded drive arms but have also made a pair for a second drive. I did buy off Amazon the 9/16  tap/threaders for 20 bucks. Right and left handed threads. Worked fine. The big advantage in solid threaded drive arms is the strength and the option of bike pedals. I use SPD mountain bike pedals and clip in with mountain bike sandals with cleats. Great way to spread the work among the leg muscles by being able to pull back and push forward. Hobie plastic pedals I never liked.

These things are sweet. Definitely gonna hold up to long time use. I installed the 3/8 nylon bolts that NowhereMan recommended, cut the extra thread on the inside of the drum and now I need to get the actual pedals attached. Thanks NowhereMan!

Those are pretty sweet.  Have you considered making them a little shorter or longer?  Could be a good way to help adjust travel for longer/shorter legs, or to really refine how much leverage and fin stroke per/inch of pedal travel.

Those are pretty sweet.  Have you considered making them a little shorter or longer?  Could be a good way to help adjust travel for longer/shorter legs, or to really refine how much leverage and fin stroke per/inch of pedal travel.

It's funny you mention that, as I've made a bunch of longer ones, up to 2" (maybe even 2.5") longer. There is probably an old thread here about that topic.

Why longer? Years ago, I owned a couple of recumbent bikes, and the consensus then seemed to be that for maximum power on a recumbent, you wanted the seat laid way back and the pedals up higher than you'd expect. So, my thought was that longer pedals would get my feet up higher and might result in more power/less effort. I found that about 1 inch longer seemed best, but on my AI, longer is more trouble, as the ropes are routed near the pedals. Recently, I raised the seat on my (pre-2015, pre-vantage seat) AI, and now I actually find that standard length (13 inches) is better than longer. I never considered shorter crank arms, as my heels would drag on the kayak if the cranks were any shorter.

Anyways, my conclusion is that there is probably no universal "best" length, or even a simple formula that will tell a person what is best for them. The only way to find out would be to experiment.

Btw, if anyone wants to experiment with shorter cranks, LMK as I have a few pairs around that I'm not using, and shortening them is easy.

Those are pretty sweet.  Have you considered making them a little shorter or longer?  Could be a good way to help adjust travel for longer/shorter legs, or to really refine how much leverage and fin stroke per/inch of pedal travel.

It's funny you mention that, as I've made a bunch of longer ones, up to 2" (maybe even 2.5") longer. There is probably an old thread here about that topic.

Why longer? Years ago, I owned a couple of recumbent bikes, and the consensus then seemed to be that for maximum power on a recumbent, you wanted the seat laid way back and the pedals up higher than you'd expect. So, my thought was that longer pedals would get my feet up higher and might result in more power/less effort. I found that about 1 inch longer seemed best, but on my AI, longer is more trouble, as the ropes are routed near the pedals. Recently, I raised the seat on my (pre-2015, pre-vantage seat) AI, and now I actually find that standard length (13 inches) is better than longer. I never considered shorter crank arms, as my heels would drag on the kayak if the cranks were any shorter.

Anyways, my conclusion is that there is probably no universal "best" length, or even a simple formula that will tell a person what is best for them. The only way to find out would be to experiment.

Btw, if anyone wants to experiment with shorter cranks, LMK as I have a few pairs around that I'm not using, and shortening them is easy.

That's kinda funny - I was thinking a shorter arm would be better for me.  I have relatively short legs for my height, and often wished the stroke length from front to back was a little shorter. I might have to fab up a couple arms for my drive.

I use SPD mountain bike pedals and clip in with mountain bike sandals with cleats.

This is a fantastic idea - I even have a spare set of pedals, haha.

Any issues with saltwater vs pedals?

This time, I made 2 pairs, and I finished them by sanding, using a random orbit sander, and 120 grit paper. It seems to give a nice faux sandblasted look.

Wow! Handyman!

 I use SPD mountain bike pedals and clip in with mountain bike sandals with cleats.
[/quote]

This is a fantastic idea - I even have a spare set of pedals, haha.

Any issues with saltwater vs pedals?
[/quote]No. I rinse afterward and use grease on the pedals. Same for cleats.