Is it safe to ride on cracked carbon rims (Zipp 404 Firecrest 2010 edition)?

Sorry to hear about your damage.

I don't think the crack is due to impact.

Maybe, maybe not, but regardless of a reason the wheel has become damaged. On the two sided likeness of damage — things are not symmetrical in practice, especially pot holes, and a crack does not have to be symmetrical.

Is this repairable and worth repairing?

It depends on whether a specific shop agrees to repair it. A wheel after such a repair essentially becomes warrantied by the repairing shop and not the wheel manufacturer. In a case the repair won't hold and you'll crash because of that, the liability will be on the shop. For this reason, many carbon repairing shops won't risk fixing such a critical part of a wheel as the braking surface. Other shops are more willing to address the problem.

I'd like a more thorough answer on what caused this crack and whether or not it is still rideable from a scientific perspective.

The wheel is certainly rideable now because you have managed to get back home on it. For how long it remains rideable before it fails, that is something nobody can tell for sure. There is likely not enough public statistics of people riding cracked carbon wheels to make any scientific predictions.

This doesn’t look like a simple scratch from grazing the side of the pothole. It almost looks like it buckled or severely deformed and now you have loose/delaminated fibers sticking out. Even if the structural integrity of the wheel were not compromised (which I doubt) you do have a crack in your braking surface. I doubt it’s possible to repair this since the braking surface has to be smooth without any layers added on top.

By the way: Great counter-example for the good old “carbon fails suddenly without warning while aluminium and steel give you plenty of early warning”.

I would not ride that wheel. Your options are:

1. Get a professional opinion from someone who specialises in carbon repair, and a quote.

2. Compare that quote to the cost of a new wheel. (side thought - check with Zipp if they have any rebates for trading up.)

3. If both choices are too expensive, you can choose to ride a replacement non-carbon wheel. That will work fine, but may look out of place. It doesn't need to be tubular, though you will have to think about your spares loadout and pack a tube/levers.

4. Accept that there are risks and choose to ride the damaged wheel anyway. You're a competent adult, and can weigh those risks.
   If you do this, be aware the bike will eat brake pads and braking performance will be lowered by some amount. Work that cost into your calculations. Also any rub will suck watts and slow you down.

You might contemplate swapping the rims on the hubs, so the front rim becomes part of the back wheel. A sudden failure of the front wheel is significantly worse than the sudden failure of the back wheel. I mean, I wouldn't want either wheel to fail personally. Also, not sure that ZIPP wheels can be parted and rebuilt like that. Not recommended.

Ride a safe wheel I'd pull a spare aluminium wheel out and ride that for now, while doing the leg work of quotes and shopping.

From a "scientific perspective" it is safe to ride until the instant when it fails.

Since wheels are pre-stressed by the spokes, it might go pop while the bike isn't even being moved, or it might go when you are riding 10 meters in front of a 30-ton truck.

There is no way "science" can tell you which option will happen. But it can make a reasonable guess about which of the above is more likely to kill you.

I don't think the crack is due to impact. It's probably due to the side grazing the pothole. There wasn't damage on the other side. If it's an impact crack, it would likely appear on both sides?

Impact, grazing. All of these common events reduce the life of carbon fiber components. You can consider yourself very lucky that the damage in this case was visible and audible. Carbon fiber components can due to unforeseen loads structurally weaken in an invisible manner, so that the only way you can see the damage is by some method seeing through the material like X-rays. Such an invisibly structurally weakened component can then later suddenly fail "just riding along".

Is this repairable and worth repairing?

In general, cracks should not be repaired, especially on carbon fiber components. A cracked metal frame is perhaps the only exception to this: it may be possible to repair a cracked frame by welding. A cracked metal rim, stem, seatpost, handlebar or any other load bearing component should NOT be repaired.

I would also say that if a wheel has very few spokes like these wheels, if they're repairable you should really consider whether to spend money on such marginal wheels or to replace them with something more durable.

Aluminum rims have a far safer failure mode: well designed rims (rims that have double eyelets and rims that are not hard anodized) bend rather than crack. Such bent rims are always repairable, although if the repair is not done perfectly, you still may have a little bit of tension imbalance in the spokes after repair.

I've read many other posts and the classic answer is "is your face worth more than 1000 dollars for a new wheel". It's of course easy to buy a new wheel, but I'd like a more thorough answer on what caused this crack and whether or not it is still rideable from a scientific perspective.

Fortunately, you need not spend 1000 dollars. I built a new front wheel for my electric road bike by spending 130.57 euros (excluding the tire, tube and brake disc). Due to the current exchange rate, it's slightly more in dollars but nowhere near 1000 dollars. I did not cut corners by reducing the spoke count: there are a total of 36 spokes in the front wheel. If you choose not to build a replacement wheel yourself but pay someone else to build it, it would probably still be below 250 euros.

Usually on wheels if you pay less, you get more (except on maybe the very cheap end). By paying less, you are more likely to have a repairable (i.e. aluminum) rim. You are also more likely to have a wheel having a full complement of spokes (36). Also, Shimano hubs are some of the best bicycle purchases you can make, and they are also some of the cheapest hubs!