Why ventilate a hub motor? Well, as covered in the temp sensor page it will you allow to push more power through your setup without the motor overheating and destroying itself. Perhaps more importantly though, a cool motor runs much more efficiently. This means a perceivable increase in power and range.
You'll need a power drill, and titanium drill bits. Regular bits work, but not as well. I had to use a standard 3/8th bit for the large holes. A hammer and punch, a vise, a measuring tool, a sharpie, and vacuum also help!
Some theory on how to do this correctly first. Just drilling hubs all willy nilly in the motor does work; it allows heat to easily escape. But there is a much better way. After all, a hub motor is spinning quite fast. And what else spins fast? A fan! And in this case we're going to turn our hub motor into a centrifugal fan.
So the principle is simple: centrifugal force forces air to the perimeter of the hub motor creating a high pressure zone there and a low pressure zone near the center. All you need to do is drill holes near the center for intake and as close to the perimeter as possible for exhaust. This actively pumps air through; cooling your motor.
In this picture you can see that the drill holes are as close to the center and the perimeter as possible. Remember, you want the air to pass over the hot windings of the motor.
Another thing that helps here is to correctly angle the holes. On the intake you want the holes to make it easier for air to flow in. For the exhaust the opposite is true. Think of it as a cheese grater if it helps. The direction the covers are spinning is very relevant here. This is why I suggest marking the direction each motor cover spins with a sharpie or something. The disc side turns counter clock wise, and the drive side turns clock wise.
I guess we should talk about what drill bits to use now. Titanium coated drill bits! I just used the biggest ones I had. You can go bigger with circle cutter if you have the time and money, although I wonder about the strutural integrity of massive holes since the covers are load bearing. Anyways, since there are less intake holes, I used a bigger bit for those. I actually made a spread sheet to make it easy to match up drill bit sizes and hole numbers so that the area of the intake holes matches the area of the exhaust holes.
I used a 3/8th for intake and 5/16th for exhaust.
Once all the holes were done, I came back and angled them. This was a bit trickier and required some playing around to get it right.
You want the holes to be angled so that air flows as easily as possible. On the intake they need to cut into the air relative to the direction it is spinning. On the exhaust it's the opposite. Use the picture above if you need help figuring this out. Don't mess this up because it's hard to fix it!
To find the correct angle for the holes imagine a line going from the center of the axle to the center of the hole. The drill bit should be angled 90 degrees from this in the correct direction relative to which way they are spinning. Then just hold the cover down, put the drill bit in, and pull down on it.
Alright that's it! One final thing though:
Vacuum out all the metal shavings! You don't want them in the bearings and if they get in the hub you won't ever get all of them off the magnets!
My only issue with the example covers are that I think you went too far with the intake holes. The center has less aluminum supporting the axle to begin with and the holes are closer together than 1 diameter, so you're left with less than half the strength. You're essentially supporting the bike via 18 (9 per side) cast aluminum spokes that are relatively thin. To get the same intake cross sectional area you could do to rings of holes totaling the same number with the second set in line, but at slightly greater radius. At the perimeter you could go to twice as many holes of the same size, because you're still left with plenty of material overall. - john in CR
Basically, I could double the ventilation area AND strengthen the covers by drilling the intake holes a bit farther out and doubling their numbers. Also I could of doubled the exhaust holes. I think i'll be fine, but it's a thought.