Enter Myth cranks. Design dating from 2006. Weight: sub-500g including BB cups & proprietary chainrings (89g/set, blanks made by Praxis Works in '07 and further CNC'd, Ceradure coating). Patented clamping mechanism. Very very stiff, but we're still in the testing phase. If there is ever a final version, it'll differ and there's no specific release goal yet.Estelja wrote:How about something like the Sweet Wings connection with a much larger overlap and larger splines?
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.
Have you looked at Praxis' M35? http://praxiscycles.com/pages/m35 ? It mentions the possibility for carbon spindles.
What it does is move the bearings outside of the BB shell to make room for the spindle. But the bearings will leave little room for crank arms. Plus the cup/BB interface will give rise to additional flex. Outboard bearings from the very beginning was an attempt to accommodate the old standard, like patch work. Now that I am making both the frame and the crank, I can indulge myself a bit more.
The million dollar question is: WHAT IS THE IDEAL DIAMETER OF A CARBON SPINDLE?
Look says 50mm, but theirs has a concave in the middle, so it will not act as a true 50mm.
Q factor: definitely on the lower side, consistent with the big 3 or maybe 1-2mm lower. 144mm-ish.
How does that sound?
Spider design: If I don't hear any major objection, I will go with the Campy/New Red 5th hidden arm approach.
BCD: guess I will not go too crazy on this one and stick to the traditional rather than EE's approach.
One of the key points I will address is the NDS crank arm stiffness. Most tests show NDS stiffness numbers much worse than DS. Need to fix that.
What about a derlin shell that could go around the spindle non-drive side. The spindle could be tapered, and the shell would 1. act as a shim between shell and bearing. 2. be threaded to allow for preload adjustment (a la many of truvativ's pf30 specific cranks 3. be machined at the end to allow for the non-drive crank connection 4. the connection wouldn't be tapered, but act similar to the hollowtech 2 cranks in the sense of there being pinch bolts and a way to insure the arm is completely engaged with the interface.
BSA frankly was most appropriate for steel. We have gone to 30mm spindle after several decades, which apparently is fairly suitable for alu. But I believe Carbon requires a larger spindle to make sense. THM going back to alu spindle on the M3 is evidence.
Fortunately I am putting together bike frames myself so I am going for better integration this time.
Design wise, while I dont have the answer yet, I am fairly confident I will find it. THe biggest problem is probably with carbon layup, which my engineers are less familiar with on the crank. FSA cranks are known to be soft but THM known to be fairly stiff. Must be the layup technology. Need to do more work on that front...
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