Adventure Racing Mandatory Gear
Ah gear - the bane of an adventure racer's spouse's existence. You'll start off slowly with a mountain bike here, backpack and UTM plotting tool there. Then before you know it, you'll own a climbing harness and carabiners (but no rope) and a kayak paddle and PFD (but no boat). You'll have graduated from a few boxes to a closet to an entire gear room just for your adventure racing needs. Ok that may be a bit of an exaggeration for most people, but the sad truth is that adventure racing often does require way more gear than most of us would like to have to buy.
Mandatory Gear. Get used to hearing this term. In an adventure race mandatory gear really is mandatory. You will need to have it on you at all times. Some race directors will do a gear check in the middle of the race and if you happen to not have whatever piece of gear they want to see, then you might be sitting for a 15 minute penalty or potentially even disqualified.
Usually the first race is the worst. Depending on how much of an outdoor athlete you are before getting into adventure racing, you may not be set back too much, but usually there will be at least a few items you'll need to buy. Always check the web site of the race you intend to do! They should list exactly what will be required for the race well in advance. The big one is the mountain bike. Nearly every race requires it and you'll have to get one before doing a race. If you're not interested in mountain biking outside of adventure racing or you just want to try it out, then you can probably get by with a cheap used bike or a crappy Walmart bike for under 100 bucks. Then you have your biking gear - usually a helmet, spare tube, pump, maybe also a flashing light or repair kit. And of course you'll need some sort of pack to carry all this stuff around in.
Fortunately running doesn't require any specialized gear, but paddling is another story. You'll usually need at least a paddle and PFD. Thankfully most race organizations, knowing that some people would find it absurd to buy a kayak paddle when they may not even own a kayak, will offer some sort of paddle and PFD rental system usually for a nominal fee. Often in races with ropes events the race director will provide the harnesses and all necessary climbing gear as well. But sometimes they allow teams to bring their own which might enable them to travel through the ropes course faster.
Beyond that, it's highly variable depending on who is directing the race. There will usually be some miscellaneous mandatory gear like a knife or headlamp or water purification system. There will usually be some kind of first aid kit requirements. The race director may even try to micro-manage your clothing and nutrition choices and require long sleeves or a non-cotton shirt or a container that carries at least 24oz of water, for example. Personally I feel a lot of that is totally unnecessary (see below) but depending on the race you want to do, it may be something you have to put up with.
One thing that really annoys me about certain sprint adventure races is the preponderance of unnecessary mandatory gear. I don't know if some race directors are in league with their local gear shops, but they need to realize that forcing first time racers to buy hundreds of dollars worth of crap that they may not ever use again (or even use in the race!) is a huge barrier to entry. I've done a number of races where racers were forced to carry pounds worth of gear that they would almost definitely never need during the race. Waterproof jackets and long pants in 90+ degree sunny weather, iodine tablets in a 6-hour race, etc. I can't even count the number of times I've had to carry around a never-used knife or a chem light during a short daytime race.
And don't even get me started on the ridiculousness of the mandatory first aid kits. 4x4 gauze pads, medical tape, antibiotic ointment, glucose tablets, anti-nausea tablets, tincture of benzoin, nitrile gloves, etc. For God's sake it's a 6 hour summertime adventure race, not an unsupported dragon-slaying polar expedition in winter. To any race director who is reading: get with the program and pare these lists down to only what is necessary, then recommend whatever additional gear you think might be helpful, but don't make it mandatory! Races are expensive enough without all that crap. And treat the damn racers like the adults they are!
So now that you know what you need to acquire, the one key piece of advice I have is to try everything out before racing with it. Seeing a piece of gear is not the same as actually using it. A pack might not fit or rub you the wrong way after jogging in it for a few minutes, or you might not know how to use your water purification system and have to waste valuable race time trying to figure it out. And don't be that person that's removing the tags from their harness while waiting in line for the rappel. Anyway there's obviously a lot more to be said about adventure racing gear. This was just a quick primer, I will have a lot more specifics and recommendations soon on this page.