I've been browsing 700c carbon rims on ebay, and noticed most are listed with the 33/50/60/88/etc mm in the title. What does this represent? It doesn't appear to be spoke/nipple holes.
Additionally, what's the general life span of a carbon rim?
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That dimension refers to the depth of the rim. See this article on some particular carbon rims, that includes a cross-sectional diagram. Deeper wheels tend to be more aerodynamic, though they are heavier, and handle worse in crosswinds. Thus, different rim depths are more suitable for different types of races. 88mm rims are almost exclusively used in time trials and triathlons, while shallow rims will be more popular for races with lots of climbing. For riders without the luxury of multiple wheelsets, 38-60mm seems to be the standard depth.
The life span of carbon rims (or any rim, for that matter) depends greatly on the type of riding done. Typically, a wheel will need to be replaced because the braking surface has worn down. This means that the wheel's longevity is dependent on the type of riding done. Long descents with lots of braking will accelerate the wear. Riding in the rain will accelerate the wear. Keeping the brake pads and rims clean will increase the longevity. Also, some carbon wheels have carbon braking surfaces, which are more sensitive, while others have aluminum braking surfaces, which are more durable.
So I am hesitant to give an exact number, but I'd say a reasonably-well maintained wheelset should probably last 10,000-20,000 miles.
These 38/50/60/88 numbers are the 'depth' of the wheel's aerodynamic rim, in millimeters. Let's take 38 for example. This means that from where the tire meets the wheel, the rim extends an additional 38 millimeters towards the hub. When you see 88, that means the rim extends 88 millimeters from the tire.
Why does that matter? One of the biggest benefits of carbon wheels is the aerodynamics. They play a major role in how fast, and how efficiently a cyclist can ride. Think back to when you were on a ride with your buddies, and drafted behind another rider. It's much easier to keep up with them, because of the better aerodynamics when you are drafting.
Your spinning wheels are the biggest source of slow aerodynamic turbulence. To improve this, we build wheels with aerodynamic advantage as a top priority. The way we do this is to increase the 'depth' of the rim. The deeper the rim, the more aerodynamic the wheel will be and the more efficiently and faster you'll be able to cut through the wind. An 88mm deep rim is more aerodynamic than a 38mm rim, and will be faster in situations where aerodynamics are very important, like long fast solo riding and triathlons.
Why would anyone choose 38mm rims, if 88mm rims are more aerodynamic? The more a wheel becomes aerodynamic for going forward, the more it can be negatively affected by cross-winds, or winds coming from the side. Let's imagine you are riding with an 88mm deep front and rear rim. You will be able to go very fast because of the aerodynamics of deep wheels, but if a gust of wind comes from the side, that wind will push sideways against the large rims and push the whole bike sideways. This isn't such a big deal if you're riding alone, such as in a triathlon, but if you are riding in a group of cyclists, this sideways push could cause an impact with other riders, or at the very least it would require extra concentration from you.
What's the solution? A wheel with a 'shallower' rim depth, like a 38mm, will be less affected by crosswinds, but will also have less aerodynamic advantage.
If that wasn't enough, you must also consider weight. A 38mm deep wheel will be lighter than an 88mm deep one, because less carbon is used. A lighter wheel will accelerate faster and will be much easier to ride up hills. This means that if you are sprinting and hill climbing a lot, a shallower rim, like a 38 will be best.
Why do I see some riders with different front and rear wheels? Due to the fact that the front wheel can turn for steering, its more susceptible to cross-wind interference. The rear wheel is fixed in a straight line with the bike, so if a cross-wind hits it, the impact on the bike won't be as much. This means that you can use a deeper rim on your rear wheel without impacting handling as much as that same rim would affect the front wheel. Wheelsets such as the 38-50 to take advantage of this fact. This wheelset uses a 38mm front rim, and a 50mm rear rim. This way you can get more aerodynamic advantage without sacrificing as much stability in windy conditions.
How do I choose? This is the most difficult choice for riders looking to purchase new carbon bicycle wheels. You must think about the situations you ride in. If you ride mostly hills you will want to choose a 38mm wheelset, if you are an all around rider choose a 50mm wheelset, if you are an all around rider looking for more aerodynamic advantage choose a 60mm wheelset, and if you are a solo rider, or a triathlete choose an 88mm wheelset.