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I've got a set of Zipp Firecrest 404 Tubulars (2010). I went over a pothole recently but did not crash. It was a pretty big pothole. After going over the pothole, I heard a rubbing sound from the front wheel when braking. Continued the ride home (20Km) and inspected it when I got home. I found a crack on the side of the rim. The tubular tires look fine. At the point of the crack, there's no visible damage to the tubular tires or the other side of the rim. The tubular tires also still hold air perfectly. I've attached some pictures below. A couple of questions:

  1. 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?
  2. Is this repairable and worth repairing?
  3. 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.

Image 1 Image 2 Image 3

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    I don't think anyone is going to certify your ten year old cracked wheel as safe via the internet.
    – thelawnet
    Sep 21, 2020 at 6:32
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    I'm not looking for certification that it's safe. I'm looking for answers (or opinions) from people with more experience and knowledge than me.
    – jkschin
    Sep 21, 2020 at 7:31
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    Note that just a nick in carbon fiber means a lot more than a nick in steel, let alone a crack because carbon does not yield. When it goes, it goes very suddenly. Model airplanes with carbon spars have been lost this way and it is why some stick with metal. The brittle nature of carbon means that as soon as it crosses the line everything will collapse like a chain of dominoes. There will be no warning. The fact the wheel held up after a pothole is about as good a warning as you can expect and indicates it was overdesigned to account for the pothole scenario. It won't do it a second time.
    – DKNguyen
    Sep 21, 2020 at 13:40
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    Simply stop using that rim. You're not a cat.
    – Carel
    Sep 21, 2020 at 15:33
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    @jkschin Yes, tubulars are not pinched so easily (but if you try hard enough, you can pinch even a tyre).
    – Vladimir F Героям слава
    Sep 21, 2020 at 16:58

5 Answers 5

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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.

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  • A warranty isn't much value to you when you are dead, whoever provided it
    – alephzero
    Sep 21, 2020 at 14:49
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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”.

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    It looks to me like impact from the edge has caused some delamination. It must have buckled to do that. I've destroyed a badly cracked (useless) CF badminton racquet in the past. I had to beat it against the ground repeatedly with all my strength (the original damage was caused by leverage) - but the failure modes can be directional.
    – Chris H
    Sep 21, 2020 at 9:58
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    The fact that the wheel held together for the rest of the trip is only down to the tire being a tubular one: The rim plays no role in handling the forces resulting from the tire pressure. If the tire had been a normal beaded tire, the same damage would very likely have meant instant death of the rim and tire (with the rim's flange giving in to the tire pressure, and the tube blowing out of the hole in the rim).
    – cmaster - reinstate monica
    Sep 21, 2020 at 15:04
  • In the peloton, I was the only one with tubulars. All other clinchers had a ruptured inner tube.
    – jkschin
    Sep 21, 2020 at 16:42
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    @ChrisH Carbon fiber components can withstand a lot of beating in the right direction. But it is dead easy to brake a carbon fiber skiing poles into two, for example. I've run through several.
    – Vladimir F Героям слава
    Sep 21, 2020 at 21:31
  • @JKschin Man, that pothole really seems to have ruined your day. You guys have my sympathy.
    – cmaster - reinstate monica
    Sep 22, 2020 at 8:31
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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.

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    I believe I have heard one major carbon repair shop, Ruckus Composites, say they don’t usually repair carbon rims. This was on the Marginal Gains podcast. I can’t put a time stamp on the comment. They’d be able to advise if the rim is safe, though. That said, we can see clear delamination, I.e. the layers of carbon are coming apart, as opposed to just a chip in the top layer of resin.
    – Weiwen Ng
    Sep 21, 2020 at 10:23
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    You can't check a carbon fiber structure just by looking at it. At the minimum you would need to do an ultrasound scan, and more likely you need to X-ray it. For something as cheap and safety-critical as a wheel, it's not worth the cost of checking it. Of course for a carbon fiber part of a jet engine with a price tag of $100,000, it probably is worth the cost of checking it!
    – alephzero
    Sep 21, 2020 at 14:56
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    The Ruckus Composites website says the will inspect wheels and forks (using ultrasound) but not repair them, because "in most cases, the repair is more expensive to fix than replace." ruckuscomp.com/repair (at the bottom of the FAQs)
    – alephzero
    Sep 21, 2020 at 15:03
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    @alephzero thanks for the corroboration. now that you mention it, I do remember Shawn Smalls (owner of Ruckus) saying as much on the podcast.
    – Weiwen Ng
    Sep 21, 2020 at 16:47
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    Worth asking Zipp about a crash replacement. They have an official published policy from the 2021 model year that they will do 50% off retail on a new wheel. But I have heard reports of them offering discounts on replacements before this.
    – Ivan McA
    Sep 22, 2020 at 5:48
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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.

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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!

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    The contention that carbon should not be repaired is wrong. It can be repaired, and if competently repaired it will be as strong or better than the original tube. Practically speaking, it may be uneconomical to repair rims, but frame tubes have been repaired. There are a number of reputable shops, e.g. Rucuks Composites, which had podcast interviews on Cyclingtips and Marginal Gains.
    – Weiwen Ng
    Sep 21, 2020 at 17:28
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    Also, strictly speaking, if an aluminum rim has bent, it should be replaced. If you bend it back, you do risk a later failure when you aren’t expecting it. This is true with RD hangars as well as rims. It’s an issue related to fatigue life. Also, in this failure mode (hit pothole), would the rim walls be hard to bend back in practice?
    – Weiwen Ng
    Sep 21, 2020 at 17:31

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