(For a less instructive but more entertaining perspective on number pinning, check out Pinning Race Numbers: A Commentary.)
Placement is the key for proper pinning of your number. Race numbers need to be located so that officials can see them standing along side the race course and so that the camera (if there is one) can see it.
Luckily the ideal position works for both the camera and the officials.As you can see in these examples, numbers that are further down on the side are easier to see, and as the number goes further up onto the back of a rider, the harder it is to see. Sometimes an official or camera positioned on a raised platform can see numbers placed higher on the back but not necessarily.
A good guide to use is the middle of your jersey’s side panel—the bottom edge of your number should hit this mark. This will make sure the number is low enough to be seen from the side, but not so low that a camera positioned above couldn’t see it. Rider number 125 (top-right) in this photo is a good example of this.
Once you know the right position for the number, then it is a matter of pinning it to the jersey well. For larger numbers use 8 pins: one in each corner, and one in the middle of each side. This may seem like a lot of pins, but it will help the number lie flat and not catch the wind. Some riders try to position their numbers differently so it doesn’t catch the wind, but if they just used a few more pins, they could position their number correctly and have no problems with it becoming a sail. Learn to keep a stash of your own pins so you always have plenty at the race.
If you get smaller numbers for your shoulder, use your scapula (shoulder blade) as the guide. If the number is not centered over your scapula, it is probably too far to the center of your back, or over your shoulder too much. These smaller numbers should have 6 pins in them: one in each corner and one along each of the long sides.
The last key to knowing how to pin on numbers properly is how to use the safety pins to fasten your number to the jersey. The safety pins should be pushed through the number and jersey together, then back out through the jersey and number, as in this picture.
Many numbers come with holes punched in the corners. Ignore these! If you simply stick a pin through this hole and then through a pinch of the jersey, the number is not fastened closely to the jersey, and it will result in the number flapping in the wind, can tear the number and/or your jersey, and often results in the pins tugging at your jersey.Anyone who has pinned a number on a jersey knows that it is easier said than done. This is why you want to have good friends. Not just any ol' friend, but good, trusted friends—ideally, quick learners with pity for your plight as the person with pins plunging perilously close to your skin. It is much easier for you to put on your jersey and have your good friend pin on the number than to try to pin it yourself then put on the jersey to find that the jersey just stretched and every pin is tugging mercilessly at the fabric surrounding the number. The other benefit is that you can get your numbers pinned while continuing to warm up, if you have a trainer at the race.
Last note about your friends helping: before having someone pin on your jersey, I recommend removing any undershirt first, then put it back on once the pinning is done. If you don’t, you risk getting “pinned in” and will need help to remove pins before you can take off the jersey or (with skinsuits) even slip down your top to go to the bathroom.
Placement, number of pins, and having a friend to help—these are a few simple keys to pinning your race number properly. But what about folding, crumpling, or otherwise altering your number? Those are a no-no. (See “1K4(b). Racing numbers” in the USA Cycling Road, Track and Cyclocross Rulebook.) Some people want to fold or crumple their race number so it doesn’t act like a sail creating extra drag. If you use 8 pins to fasten your number, this should eliminate any issue you have with your number catching air. But you can always have your friend double-check that no section is poking out ready to catch air.
Hidden-in-plain-sight challenge: find a race number related error by one of the riders in the top image and leave it in the comments.
(Acknowledgements to Lowell Kellogg and his article on Numbers and Finishing on the WCA site for the image and corroboration on my experience with pinning numbers.)