By which, I mean not your lactate threshold, but the point at which something is meaningful, successful, or worthwhile. Grab a cup, this one's long.
In deference to GeWilli (the concept of which just made me throw up in my mouth just a teeny little bit), here is the "TLDR" version, NOT the abstract: Every piece of equipment has a level at which its benefits matter, or its shortcomings become a barrier to use. How you interact with those benefits and shortcomings, and how the affect your riding, is the heart of the matter.
The question that inspired this topic is one you see quite often on the forums - "is (component - usually 'rim/wheel x') even any aero at all?" It's a challengingly, and consistently, worded question, for what does it mean? Is saving one second per kilometer at 30mph "aero at all"? Because that's what a Rail 52 or 404 get you versus a Velocity A23. Let me clarify that this is not a knock on the A23, it was just the benchmark in the test we did that allows me to make the preceding claim. Is 1/3 of that significant? A lot of "semi-aero" (the only phrase that rivals "gravel grinder" for my enduring enmity) wheels will be in about that range versus a mid-depth carbon wheel set like an Enve 3.4, Rail 34, or 303.
Unlike what most people claim, I actually get worse at parsing out aero differences in wheels just from riding them. Riding at the speeds it takes to really know what's up hurts, and hurting is hurting. There are WAY too many variables on any given day for me to give any credence to my thoughts of "yeah these are faster than those," so I don't even allow myself the luxury of those thoughts. The only way I can reliably tease out those answers is to go on one of a few group rides that I know well, ride it in the way that I do, and see how much time I spend coasting. The more coasting, the "more aero" the wheel - assuming I do an okay job of reducing the effects of other variables.
Maybe having "more aero" equipment means getting you to the finish line fresher and more capable of fighting out the finish, maybe it means a better TT, maybe it means hanging on to the A group through one extra selection, maybe it means finishing the Sunday morning worlds without becoming so smashed that you can't mow the lawn and you can actually stay awake during the kid's soccer game. Whatever it is, it is.
Brake heat resistance is another threshold type thing. We recently surveyed some rims just to see what was out there. We have a favorite hill in Vermont, one where one of our first carbon clinchers died an untimely heat-related death some years ago. East Mountain Road in Killington is a certified 5 star wheel killer. The last 3 miles or so of this thing are crazy mofo steep, twisty, and undulating. Hit the entrance to the section at 20 and then, using only your rear brake, hold your speed at 15mph constant. The KOM time of this descent is 3:22. Done correctly per the test, you're looking at about 10:40. Yup. It's an absolutely idiotic thing to do on a bike, no one should ever ride like this, carbon wheels or no but especially not on carbon wheels. This is basically wanton abuse of a wheel. Far from claiming that Rails are perfect, they aren't and nothing is, but they pass this test. The rims we tried recently did not. Does that mean they're bad rims? Not a rehotorical question.
Tubulars will, for the indefinite future, remain the gold standard for cross. Tubeless fans can say "well look at mountain bikes, they use tubeless at the highest level." To that I would say "look at Julien Absalon and Nino Schurter's bikes, what do you see for tires - tubulars, that's what." If you're a pro, exceptionally good, or even a platinum-elite level stayer at Holiday Inn Express, tubulars are the cat's ass (a phrase Mike called me out for using recently, except that it's legit). If you've got a budget and limited mind- and trunk-space for toting around 9 different sets of wheels, then tubeless promises great things in terms of economy and convenience. That doesn't count unless they work well enough - but again, what does that mean? As well as we can test it, it means "they stay on without any burping up to the point where a tubular would reliably have rolled or otherwise failed." As recently as last year, this wasn't the case, they couldn't reliably do it. Tubeless cx may never offer the sublime suppleness and feel of skin to skin contact that the best tubulars do, and a lot of people who are good enough to know the difference (or make like they do) and able to tolerate the price of entry to really good tubulars will stick forever with tubulars. And that's totally fine. As much as we're out to convince anyone that tubeless cx is a viable option, it's not the front row of the elite race that we're targeting for that (and we make awesome tubulars, too). When I lived in DC I'd hear this phrase "don't let perfect be the enemy of the very good," and you could be sure that the speaker was someone you'd probably want to strangle, but in this case it's valid. Good tubeless cx does what it does very very well, but it isn't an FMB tubular. Is that a crime?