Well, the bug has bitten. Now I can't stop.

Been thinking about a new crank design for a year now. See: viewtopic.php?f=3&t=96497&p=819164&hilit=pushed+cranks+space#p819164" onclick="window.open(this.href);return false;

Target:

Carbon spindle,
Some sort of integration
Compact,
4 arm (Campy 4 arm, not Zipp 4 arm),
(reasonably) low Q factor,
Light weight,
Stiffness.

Apparently LOOK has a patent on one-piece carbon cranks (hasn't BMX had the design for ever?), so full integration is out.

I tried to explore a thin bearing that will fit a PF30 shell to allow a bigger spindle than 30mm. That didn't go very far. I might have to explore a larger frame BB shell. Fortunately, commercial feasibility isn't a factor, so anything goes, for brainstorming purposes.

That aside, I need to figure out the best JOINT mechanism for this crank, on the drive side -- the inherently bulky joint will overlap with the bulk of the spider. At least we can have a sleek piece on the non-drive side.

Hirth is out as it's more appropriate for Steel. Too many carbon/metal interfaces will defeat the purpose. Probably no spline either.

As stupid as it sounds, i am actually thinking about something along the lines of a square tapered structure but in carbon. Alternatively the spindle could be round with several smaller fastening bolts from different angles (4-5).

Any input welcome.

Sorry for the sh*tty drawing.

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but seriously, the first thing that came to mind, when you mentioned possibly square taper, and reducing/eliminating metal inserts, was the PMP MicroII:

the bolt expands the four sides of the taper. i was thinking of coarse threads made with structural foam epoxy, like on the inside of thm and easton forks.
a better pic of the PMP system is in their catalog, page8
http://www.pmpbike.net/image/CatalogoCatalogo_2011PMP_Eng_1.pdf

I would look at doing spindles co molded onto each crank arm, both full width of the shell, one nesting inside the other. At both ends, each tube would nest into an ISIS type spline receiver on the crankarm, one being an "inie" , the other an "outie".

A little complex to mold, but it would require metal only for thread inserts to keep the arrangement together, not to resist bending moments or bearing loads.

How about something like the Sweet Wings connection with a much larger overlap and larger splines? With them being made out of carbon you could sleeve the NDS into the DS side almost full length without much weight penalty.

Interesting thought Spud. Let's hope Shimano or Sram doesn't try to patent it this week...

Quick thoughts: The fewer teeth, the better, 2-4 should be plenty to give the greatest amount of support to each one. Also, I would think about leaving a lenticular void between the two spindles with a taper to them (the spindles) at each end... Hope that makes sense... Here's a really quick sketch of what I'm thinking...

Now there's prior art...

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"Organization is for the simple-minded, the Genius controls the chaos." - Jens

Lance,

Exactly. Though I would build it with a very slight interference fit between to two spindles, so that they reinforce each other in the middle of the span, where bending deflection would be greatest.

Have you checked the nature of the patent? It seems like it would either have to be a design patent or a utility patent covering their process. If not, I would expect that with a bit of legal assistance you would be able to use the "obviousness" provision that would nullify the patent if they are claiming a utility patent on the concept of a bmx-style crankset made of carbon...

Though honestly, I like a joined crankset better for some reason. Probably has to do with the ratio of the size of the BB shell necessary to the size of the crankset...

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"Organization is for the simple-minded, the Genius controls the chaos." - Jens

An oval axle end should be the best solution with a carbon spindle. It would be easier to manufacture too

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Lance -- I did send them an email asking how I can avoid stepping on their toes, and they promised someone will get back to me.

Eric -- Oval is easy to make. I am just a bit worried about the fit which will be a bit hard to ensure due to the nature of carbon and the manufacturing process (soft cloth soaked in resin and backed in a mould, v. say CNC'ing aluminum). Especially since you need to figure out a easy way to take it out. What do you think?

Spud -- what's the thinking behind having a sleeve and essentially 2 fastening ends? I assume to allow most fastening area within a given small shaft diameter?

I think we can all safely say the biggest challenge w carbon is the lack of surface hardness (don't know the techinical term) and difficulty to achieve high precision. Therefore whatever joint we use, it will have to get around these two weaknesses.

I am playing with 3-4 ideas right now, including the sleeve concept although not full length. SPlines face the challenge of precision on a glue jobs. I thought about doing a "monster hirth" design with like 4-6 teeth on pure carbon. Not too sure about it though.

Here is my thinking:

1. if a conventional spindle size (30mm or whatever that can be squeezed into 46ID BB), I will put the fastening mechanism OUTSIDE the BB shaft, because I believe there is no structure as solid as a simple round spindle that goes through the BB shaft, especially since spindle diameter is the main limiting factor.

2. If a larger BB shaft like 65mm, then there is plenty of room to get the fastening mechanism into the shaft. But even then, nothing beats a simple round spindle. On a 53-39 crank, I will have a pretty large spider which I can figure out a way to fasten to the spindle...

Need to think a bit more.

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Elviento,

my thinking is as follows. If you don't use two long sleeves, much of the bending load will be carried at the spindle crank interface, necessitating a robust connection, which likely means weight. Or you will put a big bending moment in the center of the spindle, which will also require a robust fastening mechanism, most likely metal. By using full length sleeves, you eliminate bending concerns at the interface, which allows you to stay in carbon. It also removes bending stresses at a mechanical fastener in the center.

Regarding high precision molding and machining, a simple solution would be to cut half rounds into the edge of each mating circle, and use slugs as key ways. Then you only have to use an indexing fixture to mill the rounds into the mating circles, so it is easy and cost effective. Not quite as nice as a fully molded high precision part, but the tooling would be easy.

I don't know if you have fully understood what I was talking about



You don't need a typical crankarm/axle interface with this system that would be much more difficult to do in carbon

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At the end you have two half thickness spindles (one for the DS and other for the NDS) fastened in the opposite crankarm?

If that's your idea, I think you will have a hard time designing a fastening system for a crankarm that already has its own supportive axle

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AFAIK the patent is more to do with the way the it installs rather than what it's made from or being one piece. With the concave section it 'rolls' in.


The 'Ashabulta' steel one piece crank found on many cheapy bikes could be easily adapted to carbon but you'd simply need to make a huge BB shell to get away with a decent dia. spindle.

Spud, so you are saying this takes the load off the interface but rather puts it on the extended sleeve into the other half. That's definitely worth more exploring. I just want to throw a little extra complication into the mix. The outside sleeve will be supporting a bearing or two which rests against the frame. A huge amount of the load will also be applied to the bearings and back to the outer sleeve.

Therefore, the outer sleeve will need to be pretty robust anyway. Is there then any way to capitalize on that roubustness to also help support the spindle-cankarm interface?

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I would like to remind you all that I have no idea what you are talking about. I just hope it all turns out nice and your cranks weighs 250grms and makes you go 20mph faster uphills .