Carbon fiber spokes - Yay or Nay?

My initial reply was quite dismissive ("A fad - IMO"), but it's actually a very good question that deserves a better (and a more objective) answer. So, with my appologies to the viewer for the initial knee-jerk response, here are my thoughts on carbon fiber spokes:

I'm not really sure.

:)

OK, OK, I'll go into more details, but the truth is I haven't built any wheels with carbon spokes, and have seen very few of those in my life. That is fair to note right at the start.

Steel (material) and steel spokes (component)​

Steel as a material, and steel spokes as a bicycle wheel's component have been around for centuries (since the 90s... the 1890s :) ). They are tried and tested and, despite the current trend of people dumbing down and knowledge & experience getting lost, we can say that we know a lot about the pros, cons, limits and behaviour of steel (as a material) and steel spokes (as a bicycle wheel component).

Both aspects are important: knowing and understanding the materiral's properties, and its behaviour in a particular use case (agan, a wheel component in this context).

Carbon fiber (material) and carbon fiber spokes (component)​

Theory​

I wrote about steel, carbon fiber and other materials' properties in this article: Bicycle frame materials – explained.

The article is centred around frame building, but the parts about material properties are still relevant for this discussion. TL/DR: compared to steel, carbon fiber has about five times higher tensile strength, but less than one fifth of the steel's ductility (elongation). In other words, it is quite brittle, and doesn't elongate in a significant way.

Those are the factors that must be accounted for when using carbon as a material, and that is the main reason why carbon spokes are not 75% lighter than steel spokes.

The low elongation creates a problem when using threaded connections (nipples) as a way to keep the spokes under tension and fixed to the rim. Also, carbon doesn't fare well when trying to cut (or roll) threads onto it in order for it to connect.

So, special proprietary hub and "nipple" designs are required in order to use carbon as a spoke material.

Rims do compress to a small degree, and that is not enough to cause nipples to unscrew from steel spokes (if the wheel is properly built with enough spoke tension). However, with carbon fiber spokes that don't elongate without huge tension, standard nipples threads will tend to unscrew and loosen (if no threadlocker or other locking mechanism is used).

Another aspect to consider is the galvanic corrosion. Carbon can act as a cathode and accelerate galvanic corrosion of metals it is in contact with (it must be well insulated to prevent that). Without good insulation, it is worse in those terms compared to steel in contact with brass (as far as I know, may a reader correct me if I'm wrong on that).

To sum it up, carbon fiber is:
  • Very light compared to its strength (a definite positive).
  • Very stiff and brittle (too much of a good thing, including wheels that are too stiff, can be bad, mechanics is often nuanced - the devil is in the detail).
  • Must use some metal connection to keep it threaded, or at least some other complicated assembly to keep it tight.
  • Tends to facilitate galvanic corrosion when in contact with metals.
Those are the problems (or "challenges" as modern managers and HR folks like to call it :) ) of using carbon fiber as a spoke material.

Practice​

There are different designs of carbon fiber spokes.
  • Solid carbon fiber - made from a single solid piece of carbon fibre.
    (Mavic R-SYS wheels for example)
  • Laminated carbon fiber - made of several layers of carbon fibre cloth glued together.
    (Spinergy PBO spokes come to mind)
  • Composite carbon fiber - overlaps with the above-listed type and is a bit of a misnomer since carbon fiber itself is a composite material - but the point is in combining other materials such as Kevlar with carbon fiber, to make it less brittle, and improve impact resistance.
    (I would say that Spinergy PBO spokes fall into this category as well)
That is important to keep in mind when discussing "carbon spokes." Otherwise, we could fall into a trap similar to looking at the cheap straight Chinese spokes and saying that steel spokes are bad (while missing the high-quality swagged spokes by SAPIM and DT Swiss).

Now, I've seen a radially laced front wheel on a disc brake gravel bicycle of a strong, heavy rider. I did rebuild it properly, but the swagged steel spokes (or the hub's flanges) did not start braking before that. I've also seen many wheels with very low spoke tension last without their high-quality swagged steel spokes breaking. Finally, I've built many wheels for loaded touring, high mileage riding or racing with steel spokes.

However, I have next to no experience with carbon fiber spokes in those terms, especially the extremes. Some riders praise the Spinergy PBO spokes, to name one model & type, but those same riders also claim that their carbon spokes make the ride more comfortable and dampen road buzz (some even on MTBs). I take that with a grain of salt, since the effect of wheel itself on comfort is negligible compared to the tyre's flex (and even seatpost and handlebar flex for that matter).

This leads us to the price and availability. That is a major downside from my point of view. But that was not the question.

I would also like to point to the last photo in my latest article - something to keep in mind when reading or watching reviews videos for bicycles, bike parts and components (why usually I take most of the praises with a grain of salt):

https://bike.bikegremlin.com/21008/where-should-i-buy-bicycle-parts-and-tools/#conclusion

Relja BrevityIsNotMyVirtue Novović
 
It all depends on the conditions... If we talk about sports, yes, they have the right to exist. If we talk about normal people, I think this is not something that needs to be considered yet.
 
VlPo said:
It all depends on the conditions... If we talk about sports, yes, they have the right to exist. If we talk about normal people, I think this is not something that needs to be considered yet.
Click to expand...

Price and questionable benefits aside, my main concerns are durability and reliability.

With carbon fiber forks, stems, handlebars, frames or seatposts, sudden failure can be dangerous. But with spokes, provided that wheels have many spokes (preferably 36 :) ), failure of a single spoke shouldn't be dangerous.

Having said that, I'm not eager to buy and test such spokes - and I would be surprised if they turn out to be more durable than the good old simple J-bend swagged steel spokes.
 
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