Cartridge hubs vs loose-ball hubs

Most hubs these days claim to be "sealed". This basically means that if you look the hub from the axle end you can't see the balls of the ball bearings, because there is, at least, a "dust shield" that leaves only a very narrow opening between rotating hub and fixed axle.

There are also "cartridge" hubs, which were the original sealed hubs popularized by Phil Woods, back when standard bearings had no dust shields and were pretty crappy. These use a (theoretically) replaceable cartridge on each side of the hub, where the cartridge is an industrial bearing cartridge similar to what's used in electric motors (and similar to what's used in bottom-bracket cartridges). However, replacing the cartridges requires special tools, and likely many bike shops would just give you a dumb look if you talked to them about replacing the cartridges.

Both of these schemes are a significant improvement over the original open bearing style, since, not only do they keep most moisture out, they also keep out dust and dirt. For the average rider, though, the cartridge scheme is probably not worthwhile, since it is not possible to simply rebuild the bearings.

The tools are relatively expensive compared to cone wrenches, but all cartridge bearings can be replaced if needed.

http://www.enduroforkseals.com/id176.html

You need both a puller and a press of some kind.

Cartridge bearings last a very long time with no maintenance at all.

Since installing fancy ceramic bearings is part of high end road cycling these days I would think most competent shops would have the tools and knowledge to replace a cartridge bearing. But if you don't ride in the rain or power wash your bike, it's unlikely you'll ever need to replace them during the life time of a single rim.

For road bike wheels, I don't have preference either way. It really depends whether you have the tools, skills and time to keep a traditional cone and ball hub working.

Both kinds of bearings are equally good for wheel hubs. Because of ease of replacing traditional loose balls bearing with standard tools, I would always go with those but recently I've got a set of new wheels with cartridge bearings only because I liked the wheels (and the price) and because I'm curious about how they will hold. Considering the length of time both kinds of bearings will last, the cost of hiring a professional to rebuilt them (if needed) is irrelevant. I like restoring vintage bikes and never had to discard a wheel because of the problem with loose balls bearings.

Many trailers for Vehicles / ATVS use an "open bearing" design. This enables a person to do maintenance on them without completely replacing them out. This is cost effective at this size / scale but when it comes to a bicycle... I would highly suggest you stick with sealed bearings.

I'm expanding on an answer I made to a related question that was recently asked and closed for being a duplicate.

Earlier in cycling's history, most hubs were cup and cone. This blog post by Wheel Fanatyk thinks that Phil Wood, Maxicar, and possibly Chris King were among the pioneers of cartridge bearings in bicycle hubs (Phil) and headsets (CK), possibly starting around the 1970s. Cartridge bearings have become standard in industrial applications. They are widely available in standard sizes, many of which are applicable to bicycles. Among performance bicycles, the vast majority of wheels use cartridge bearings, barring wheelsets by Shimano and some Campagnolo/Fulcrum models (Fulcrum's parent is Campagnolo). To be honest, I'm not as familiar with lower price point wheels, and cup and cone hubs could be cheaper at that price point. Nonetheless, I suspect that most of the wheels the LBS in this question had access to used cartridge bearings. The question I linked to asked why cartridge bearing hubs weren't more common, and the answer is that they are actually very common.

Every bearing arrangement has some form of seal or shield against contamination

Both questions have the misconception that only cartridge bearings are sealed. In fact, cup and cone hubs can have seals to protect against dirt and water intrusion. The picture below is taken from a Campagnolo service manual. For the record, it's from this technical manual, revised in 2017, and it covers wheels like the Eurus, Shamals, and Neutron Ultras, which I think are an older generation of wheels (covering the mid to late 2000s until early or mid 2010s at least).

The white ring is a removable rubber seal. I haven't held a Shimano hub in some time, but this 2013 blog post describes them as having both contact and labyrinth seals in different spots. Also, you can take the seals out of your cartridge bearings. You should not do this, unless perhaps you have a shot at the hour record (indoor tracks are very clean) or perhaps the bike is being exclusively used for competitive Zwift racing. The point is merely to demonstrate that seals are not an essential property of either bearing style.

As already demonstrated in other answers, bearing cartridges can be removed with bearing pullers. Here, the standardized dimensions and wide aftermarket availability are one potential advantage. If you ruined a hub's cups or cones, you most likely need to approach the original manufacturer for replacements. After some time, stocking these items for older hubs may become a chore and they may decide not to support their equipment in this way. In contrast, in cartridges, the inner and outer races (equivalent to the cup and cone) are an integral part of the unit. You just pull the whole thing out. The bearing number, e.g. 6806 or 6903, will correspond to a standardized inner diameter, outer diameter, and bearing width. If you get a bearing of the same number, it will fit (there may be some cases where the manufacturer specifies a slightly non-standard number, e.g. I believe Campagnolo Ultra Torque bearings are close but not identical to a standard bearing number). If you don't see the number but you measure the 3 dimensions listed earlier, you can find the correct bearing designation.

Why have most hub manufacturers gone cartridge?

Why have many manufacturers ditched cup and cone bearings? There's a classic make or buy decision at play here. Depending on your requirements and your capabilities, you might be able to make everything yourself, or you might be better off buying some parts from someone else. Given the wide availability of cartridge bearings of different sizes and quality, I think most cycling companies have chosen to go and buy their bearings from someone else, e.g. Enduro or SKF bearings. This could allow companies to focus on other aspects of their product, be that hubs, bottom brackets, or something else with bearings.

There are some exceptions. Chris King, an eponymously named small boutique manufacturer, uses cartridges but they make their own bearings in-house. I recall a podcast where King described how he scavenged a large bin of barely used medical bearings, cleaned them out, and put them in headsets. Shimano is a very large manufacturer of many types of cycling equipment. As I mentioned, they still use cup and cone bearings in their hubs. They certainly have the scale to keep producing high-quality cup and cone bearings, and I wonder if their institutional conservatism (or at least, that's how I perceive them) may have pushed them towards deciding to make, rather than buy. Campagnolo seems to have recently chosen to buy at least some of their wheel bearings, despite long prior experience with making very good cup and cone bearings. Campy is a much smaller operation than Shimano. This is pure speculation, but their smaller scale might have pushed them in the opposite direction from Shimano. I believe their higher end wheels may still have cup and cone bearings even today.

Hub service for cartridge bearings

I don't necessarily agree with @Fred that cartridge bearings necessarily last a long time with no maintenance. They can last a long time without maintenance. It does depend on, among other things, the quality of the bearings - it's not just the roundness of the balls (i.e. the ABEC grade), it's the quality of the steel of the balls and races, it's whether the specified bearings can adequately bear the loads they will encounter (e.g. 688 bearings are very small and are used in some weight weenie front hubs, one of which I had, but they don't last long at all), it's about how widely spaced the bearings are, and many other elements. James Huang Cyclingtips recently had an extensive podcast interview with someone from Enduro, which makes bearings. People deeply interested in this topic might be interested, but it is long and nerdy.

I agree with Fred that many good bike shops should now have the tools and experience to remove cartridge bearings from hubs. Consider that they do this all the time with press fit bottom brackets.

A quick word on cartridge bearing maintenance. No seal is impervious. Eventually, contamination can build up in the bearing. It is possible to remove the seals, clean out the bearing, and add new grease. I recall some posters here have said that this is not preferred, because you can damage the seal. So, the appropriateness of regreasing a cartridge bearing may be a matter of opinion. You may find it difficult to obtain a replacement seal. If you do this, I would suspect that it can restore a bearing that's a bit rough, but you need to be careful removing the seals. In some cases, the manufacturer specifically requires regular service. Chris King does, and they design their seals to be easily removeable. Kogel, an aftermarket supplier of ceramic bearings in bottom brackets and derailleur pulleys, also requires annual service. They sell refresh kits with new seals and grease. They communicated to me that they are including the seals to be sure, but that it was possible to remove the seals without damaging them.

As to bearing life, I have had bearings (and the hubs, naturally) that outlasted the rim. I have had a couple of sets where the rims outlasted the hub bearings. The stories would be too much a digression, but they are actually design flaws rather than issues with the bearings per se. Assuming you get a quality hub, I would be comfortable with either bearing design in isolation. You do need to monitor cartridge bearings and replace them when they get rough; for home mechanics, bearing pullers and presses tend to be a fairly expensive investment, so be aware of this.

According to Shimano on bicycle hubs:

Angular contact bearings of the cup & cone type offer greater strength than sealed cartridge industrial bearings due their ability to displace lateral and vertical loads more effectively for super smooth rotation and longer durability. Angular contact bearings also allow easier maintenance, adjustability and serviceability.

Cup & Cone Bearing