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Carbon fiber rods


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#1 MatsNorway

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Posted 06 November 2018 - 21:01

http://www.speedhunt...carbon-conrods/

 

Thoughts?



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#2 Greg Locock

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Posted 06 November 2018 - 22:20

It is a great shame that they merely took a metal one and copied it in CF (looking at the big end in particular). That doesn't really seem to play to CFs 'strengths'. 



#3 Fat Boy

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Posted 07 November 2018 - 00:34

It is a great shame that they merely took a metal one and copied it in CF (looking at the big end in particular). That doesn't really seem to play to CFs 'strengths'. 

 

It'd be hard enough to get an engine builder to try these in the first place. Getting one to put them in an engine if they looked different from a steel one would be damned near impossible!



#4 Greg Locock

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Posted 07 November 2018 - 06:25

We used to flog carbon fibre propshafts. They were aluminium propshafts wrapped with a layer of CF. The whirl speed was a bit higher, but that was a classic example where the gain was tiny because there was no real redesign.

 

In the case of the conrod, how does the female thread work? I bet there's a bloody great lump of metal in there.



#5 Fat Boy

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Posted 07 November 2018 - 20:31

That's likely. If you're really building the connecting rod right, you'll probably have a crank that has to be split. That's it's own issue.

 

I've run a car with the ~100mm diameter carbon front anti-roll bar. It would have been _heavy_ in steel, but in carbon it was a good weight saver.



#6 Greg Locock

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Posted 08 November 2018 - 00:47

Black paint on a  drainpipe and you're good to go. More war stories on CF - a helicopter company used to insist that the hardpoints (aluminium bits) were pop riveted into the CF layup, carefully cutting all those expensive fibres. I worked with a guy who used to build CF motorbike frames for races, Jim Cook. He spent most of his time figuring out how to transfer loads from the hardpoints into the CF layup. The actual CF side of it was comparatively straightforward. It's actually an impedance mismatch problem, if you have an electrical brain. Basically you have to encourage the stiff fibre and the somewhat stretchy metal to move by the same amount at the point of contact, for the same load, with the aid of some even more stretchy glue. Or if you switch to steel, stiff metal to stretchy fibre.



#7 Bikr7549

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Posted 08 November 2018 - 04:32

Point loads are difficult for composites to handle well, a plate/fitting bonded to the outer surface would be much better than point loads from rivets, especially if the stiffness of both were designed to be the same. It can be amazing how much bond area there can be from a fairly simple joint, and a good bonded joint made if proper materials are used. A composite engineer acquaintance once mentioned that composite engineers don't ride composite bikes, tho I suspect this was due to detecting damage and ply failures rather than design of the frame structurally.

 

As to the con rod joint the metal insert could be made in a few ways, the most likely to me looks like having a bushing with a large area on the fastener mating surface to address compressive loads, then either continue at that diameter all the way thru to the mating surface of the rod halves, or have it neck down to a reduced diameter at some point under the fastener interface. Do the two metal inserts meet at the joint or is there a gap between them is a big question. Spreading the fastener load over enough surface of the composite would be tricky. I agree that a jointless rod on a built up crank is a possibly better way to go, from the perspective of a composite rod. I am guessing that there is a nut for the female side of the thread, or the insert on the other side is internally threaded.


Edited by Bikr7549, 08 November 2018 - 04:34.