In part 1 of The Secret Set Up, I introduced the idea of how difficult it used to be to find reliable information about cyclocross, and how the mysterious cult of 'cross was hard to infiltrate. There's a lot of folklore out there, and a lot of old-school knowledge that not everyone wanted to share. If you've got a seat post in your garage that you drilled a hole in for a brake cable to pass through, or if you've ever glued tire tread to the bottom of a pair of road shoes, then you're one of those old-school insiders. For me, part of becoming a coach and wanting to promote and grow the sport meant that I've always wanted to share that information rather than hoard it.
Along those lines, wheels, tires, and tire pressure are always a topic of conversation and debate. I would imagine that only motocross racers stress about it the same way as cyclocrossers. There are so many options: skinny or fat? File or chevron? Tubeless, or tubular? There is a different tread and width that, according to legend, will work best in each condition. Unless you've got an infinite number of wheel sets (and of course, the best riders do), you can't really have a tire set up for every condition you might encounter. The first step for most people is to have a standard set up that covers most of your bases, especially since it may be your only set of wheels.
Your first decision is the type of rims and tires: clinchers, tubeless or tubulars? Most new bikes come with clinchers or tubeless, and for training, clinchers are great. There's an abundance of quality clincher tires and treads available now, and it's nice to only have to change a tube when you puncture. But clinchers are heavy, prone to flats and denting, typically have very stiff, harsh casings, and limit the amount of pressure you can run on the low end before you bottom out and pinch the tube or fold the tire in a turn. That makes them very unsuitable for racing. The one advantage to clinchers is that instead of bringing six sets of wheels to a race, you can bring a couple different types of tires and change them quickly and easily once you see the course.
These days, tubeless set ups address some of the limitations of clinchers, and they are a reasonable upgrade option. They offer a wide variety of tread patterns, also allow you some short term flexibility if you want to change tires based on a course, and allow you to run much lower pressure without risk of flatting. The sealant in a tubeless set up is also an extra shield against punctures, and if you are stuck in the woods on a training ride, you can still put a tube in it to get home. All in all, a real improvement over clinchers. With the development and improvement of tubeless rims and tires, they are absolutely the best choice for training, and a reasonable choice for racing.
For racing, tubular rims and tires are still unmatched. Tubular rims are lightweight, stronger than clinchers and tubeless by design, and don't dent as easily. Tubular tires are lighter, have much more supple, flexible casings that allow them to bend around the ground providing traction and shock absorption, they pinch flat less often, and you can run very low pressure without puncturing, again for traction, shock absorption, and lower rolling resistance. (How that works I'll explain later.) The main drawback is that they can be expensive to replace, and gluing can be a laborious process. All things considered, though, it's a worthwhile investment if you plan to have a set of wheels just for racing.
Carbon rims have come a long way in terms of durability, and are definitely suitable for 'cross. Still, they do break on impact unlike alloy wheels, and if you're on rim brakes, don't stop as well in wet weather. Alloy wheels are heavier, but less likely to fail, and sometimes repairable if you dent them, whereas carbon will instead break completely and need to be replaced. Alloy certainly makes sense for training, and carbon for racing, but again, it depends on if you value durability or performance, and what your budget is. For rim depth, value weight and durability over aerodynamics. A low profile rim in the mid 30 mm range is ideal for most riders. Rims have also gotten wider in recent years, and a 25 mm wide rim of any kind will mate with a wide 'cross tire better and provide improved performance with all tire types.
Before the UCI instituted the 33mm maximum tire width rule, elite rides not only had to worry about bringing multiple treads, but also multiple width tires to races. When I started in the late 80s, we all rode 28 mm wide Grifo/chevron-style tires pumped up to 40-60 PSI. With tires that narrow you relied on the tread for traction, not the casing, and ran higher pressure in the mud to try and cut down to the bottom and find solid ground. As wider tires became available, the style changed. Bigger volume tires meant you could run lower pressure and get traction and shock absorption. If a course was frozen or sandy, you'd want the widest tires you could fit through your frame, usually 34-35 mm. For fast, smooth courses, people continued to use 28, 30, or 32 mm tires at higher pressures. Eventually, casing materials got better, thread counts went up, tires became more supple, and a maximum of 33 mm was set. As a result, you rarely see a rider on anything narrower, and the focus has turned from tire width to tread choice, casing suppleness, and pressure variability.
With tread patterns, there are some general rules. We've come a long way from files, dots, and chevrons as our only options. These days, it's good to think in terms of terrain, and the right tread for each condition. Fast, hard pack courses that don't require much pedaling or turning traction call for a file or diamond tread with very low or no side knobs. The traction with a tire like this comes primarily from low pressure and a big contact patch, not from tall knobs that won't sink in and actually give you less traction on hard surfaces.
In muddy conditions, I decide if I'm going to go to full mud tires based on whether the mud has a bottom or not. If it's still hard packed ground but just with a slippery layer on top, I'll stick with a fast, all-conditions type tread like the traditional chevron or arrow style. If the mud has no bottom and I'm going to rely on the tread to serve as paddles giving me both pedaling and braking traction when needed, I'll got to a full mud tire.
Of course, most courses are not all of one type of terrain, and that's where an all-conditions chevron style tread is invaluable. If you only have one set of wheels to race on, this is the tread you want. It may not be the best at anything, but it always works, not matter what the course is like.
The last and most important thing to consider is tire pressure. On the road, the habit for most people is to pump their tires up as hard as they can get away with. In 'cross, the opposite is true, and you should always run your tires as low as you can get away with. (In fact, I apply that approach to the road as well, and with wider tires on the road, the trend is changing.) In 'cross, lower pressure means a larger contact area and more casing flexibility, which means better traction, better shock absorption, and lower rolling resistance. How? On a mirco level, imagine a high TPI cotton casing at 24 PSI bending around uneven ground while the rim continues to travel straight ahead, without moving up and down. In 'cross, your suspension system is made up of your body and your tires. Pumping your tires up so hard that the casing doesn't flex is like locking out the suspension on a mountain bike. Personally, at 155 lbs, my starting pressure when I warm up on a course with tubulars is 24 PSI in the front and 26 in the rear. If it's really bumpy, muddy or snowy, I'll go as low as I can without bottoming out, as low as 18 in the front and 20 in the rear. If it's exceptionally fast, if there are high force turns that bend the tire too much, or if there are rocky sections, I might go up to 26/28. If I need pressure higher than that, it means I'm racing jungle 'cross and someone's done a lousy job with their course design. If you're racing on tubeless, you can come close to these pressures, but my experience is that my equally skilled competitors who are on tubeless are usually about 5 PSI higher than I am, at least. With clinchers, add 10 PSI to that, and hope for the best.
Your limiters when deciding how low you can go should be based on when the tire bottoms out, and how far it bends when you turn. Find your comfort level with both things based on your body weight, your style, and your equipment choices. Two riders running exactly the same equipment at exactly the same body weight might run 5 PSI different simply based on good line choices and an ability to weight and unweight the bike at just the right moments to avoid smacking things and flatting or folding tires.
Many of the choices here are based on finances, commitment, skill, and even time available to spend fussing with your stuff. It's 'cross, and so what matters most is just that you're out there with whatever equipment you have, and don't stress too much about it. But when you do decide it's time to stress about it, hopefully the information here will help you make good decisions about where your resources should go.