Questions about cracks in my rim

You are getting some cracks around the spoke holes. If the rim had been built with much too high spoke tension, that would be one possible cause. I'm not sure how likely that is; the manufacturer would presumably have done at least a cursory check of the tension to see if it was within their limits. It's more likely poor rim material, e.g. they chose an alloy that's too brittle, or maybe impurities in the batch of aluminum they got, or maybe some other manufacturing flaw. If you keep riding the rim, eventually a spoke will pull out or the rim will fracture around a spoke, which will send it out of true.

You definitely can't reuse the spokes. They might not be the correct length for another rim. In any case, spokes also have a fatigue life, and it would be better to replace them.

The state of the hub is an open question. A builder will generally inspect the hub flanges where the spokes go into the hub. If they're cracked, the hub is toast. They will crack eventually, but hubs can be reused, potentially several times, unless the flanges were built too weak or there were material impurities. Aside from that, we don't really talk about the life cycle of a hub - the bearings would be another question, but those can be removed and replaced. You have a lower-end OEM wheelset, and the bearings are likely to need replacement. Last, it's possible that the axle has taken damage over the hub's life, e.g. it could be bent. This isn't that likely unless the axle were made too light - which happened to me, but it was a lightweight aluminum axle, and most axles are steel.

This does raise another question: is it economical to reuse the hub, considering the cost of repairs? Rim brake wheels are going out of style, as most new bikes are disc brake. It's possible that you could get a second hand set of rim brake wheels on eBay or similar channels. However, the current pandemic has caused a shortage of bike parts, and prices on the used markets may be higher than normal. Correspondingly, it's possible your bike store might not be able to get usable replacement parts.

I don't believe that this type of failure is typical on even a lower end OEM wheel, but it is too late to seek any warranty recourse. In any case, if the brake tracks are visibly concave, the rims may have been approaching end of life anyway.

You've done reasonably well out of those rims.

I got Ace 17s on my 2010 hybrid. The rear failed in a similar way, though closer to a spoke hole after approx 10-12,000 km, but I'm heavier than you and had a rear child seat on there for some of that time. The front is still going strong after another few thousand km (I'm missing my earlier records).

Failure is likely to be largely fatigue, though at some point an impact may have been the last straw. In my case I noticed it when truing the wheel and it suddenly got worse for an adjustment that should have made it better. Checking for trueness should be enough, given that a single failure is quite unlikely to cause the wheel to collapse. With rim brakes that's easy. If the wheel is out of true, it's worth inspecting more closely.

Unless your hub is particularly good, and either almost new or extremely well cared for, there's probably no point having a wheel rebuilt on it.

When I last looked, a pre-built (rim-brake) wheel was cheaper than getting one made on an existing hub. I now build my own wheels and the components come to a similar price to a pre-built wheel of comparable specification. I've always wanted something a bit odd, or have been reusing something, so it saves me money given that my time is free and I quite like it. You may have to shop around, but plenty of rim brake bikes are still sold, along with components for them.

If you're on a really tight budget, and want to build your own wheel, then buying a rim with the same ERD (effective diameter), and same number of spoke holes would allow you to reuse the hub and spokes. There's a slightly increased risk of spoke failure, both from reuse and from wheelbuilding being a bit tricky at first

I have a set of AlexRims ACE 19 rims that came on a 2011 Specialized Tricross

This rim is a design fault.

Good rims have double eyelets (also called sockets) to distribute spoke tension across both rim walls of a double wall rim.

This rim has only single eyelets. They cannot distribute the tension. Thus, the inner wall (the one near to the hub) cracks.

Double eyelet rims are generally incompatible with tubeless tires, so we can further conclude that good tires (those that can be mounted on good rims) are traditional tube-type clincher tires.

They have about 18,000km on them, mostly flat terrain, and a mix of pavement (90%) and gravel/ rail beds (10%).

That's about what single eyelet rims last. Double eyelet rims would last several times that, or until the rim brakes wear out the rim sidewalls, which can happen quite fast in wet weather but in dry weather takes practically forever.

Is this a reasonable state for this type of rim to be in given the age/ mileage/ terrain being ridden? FYI, I weigh around 170 lbs.

Yes. Single eyelet rims do just that. Your weight doesn't matter unless you need to crank up spoke tension to prevent loosening spokes, in which case too high spoke tension can accelerate rim cracking.

How does damage like this generally happen? Would each of these cracked spoke entry points have been a single transient event like hitting the edge of a pothole, or is this a slow process from mechanical fatigue?

It's fatigue from loads that are concentrated due to not having double eyelets.

If I hadn't caught this issue and kept riding these, what's the likely progression of the failure? Spokes pop out of the wheel one at a time, flagging the issue to me via wheel out-of-trueness? Or sudden catastrophic disintregration of the rim some day?

I wouldn't rely on the future being noncatastrophic. The rim has already failed. Whether or not it hurts you is anyone's guess.

How often is it recommended to clean off your rims and inspect them up close to catch this type of damage (preferably before it progresses this far)? Ideally I would have caught the first two examples while they were still in the state of the last example, if they went through a progression like that.

Buy double eyelet rims. Then you don't need to do any inspection.

Is it safe to reuse the hub and spokes, or should I assume they're near end of life too?

If you have not used radial spoke lacing, the hubs are most likely fine. Spoke is just tensioned wire and won't fail unless it has already failed. As a matter of fact, a spoke that has withstood the test of the time is more durable because it probably has already been stress relieved and has demonstrated having no material faults. Steel in spokes has different fatigue characteristics than aluminum on rims.

I had the same thing happen on two bikes, both from the same manufacturer but 4 model years apart. The rim in my case was a Schürmann YAK 19 (without eyelets). Both times I started noticing after some 2 years of daily use on the road that the rear wheel was out of true, and upon closer inspection noticed the cracks.

What caused the rim to fail?

Given that I had the same thing happen on two bikes with the same rim model, I would attribute the failure to poor quality of the rims. I did not notice any damage to the spokes or the hub. Both times the rear wheel was affected – after all, the weight of the rider primarily rests on the rear wheel.

Can I reuse the hub and spokes?

As long as the spokes and hub still appear to be in good condition, you should be fine reusing both. However, if your new rim has a different ERD from the old one, your spokes may not be of the correct length (unless the difference is minimal) and you will need new ones. If you get the same model of rim again, spoke length is not an issue, but the new rim might crack just like the old one.

On both bikes, I just replaced the rim with the same model and kept the hub and spokes. The first bike got stolen a few months after I replaced the rim, so I cannot tell for sure what would have happened to the wheel. For the second one I got the same model of rims but with eyelets (ERD differs only minimally, so reusing the spokes was OK). Eventually the spokes on the cassette side started failing, but I attribute this to an accident in which my rear derailleur got warped, causing the chain to repeatedly fall between the cassette and the spokes, damaging the spokes until one failed.

Most rims are made out of an aluminium alloy, which is a lot more brittle than the stainless steel spokes are made of – in other words, I would expect the spokes to outlast the rim. Also, spokes are unlikely to fail all at once – more likely, they will fail one at a time, and when the first one does, you should assess the damage and replace what is necessary.

That being said, if you are experiencing other issues with your rear wheel (such as the hub going bad or the cassette being worn out and not coming off no matter how hard you try), replacing the entire wheel may be the cheaper and easier option.