Using these measurements, in addition to the ERD (Effective Rim Diameter) of the rim you intend to use, you can calculate, using one of several online spoke calculators, what length spoke you'll need.

We DO NOT recommend radial laced wheels. There is an excellent chance that you will stretch the hub bores and loosen the bearings if you do. The weight savings is minimal, the aerodynamic advantage is negligible on a BMX wheel, and it's weaker than crossed pattern lacing, so other than the fact that it looks kinda cool, there's no advantage to radial lacing.

Keep in mind, the spokes in a 20" wheel are much shorter than the spokes in a mountain bike or road wheel, so you don't need to be as precise as you do with a larger wheel. On the vast majority of the BMX wheels that we build here, we use the same length spokes front and rear, even on the Mini Cassette hub with it's differently sized flanges. When you're calculating the length of a set of BMX wheels, you can probably take the average of the three lengths your calculations give you and just buy one length of spoke. This is assuming that the three lengths are within 2-3mm of each other. If the difference is more than that, you should order the different lengths in order to get the strongest results.

The wheels we build in-house are built with Sun Rims. The majority are built 4x, but we can build 3x as well. Here are some of the most common wheels we build here.

lacing a wheel

Lacing a wheel is one of the hardest, but most satisfying things to do, as far as working on your bike goes, at least. It's not impossible, but it's not simple either, and while lots of people have done it, there's a lot of little steps to take so that it's done right. For instance, I see lots of kids who build their own wheels who don't get the tube valve hole situated in the right position, so it's always a bitch to put air in the tire. But with a little planning a a decent amount of attention to detail, you can get your wheels set up properly so you'll never have that problem.

Start off with a new rim and spokes, if you can afford to, because used spokes are stretched and bent and possibly chewed up by that rough concrete ledge you were trying feebles on the other night, and used rims have hops, bends, and bent braking surfaces that you'll have to fight to make as straight as you can, and they'll never be as perfect as a new wheel, obviously.

Use Loc-Tite (blue non permanent) or Wheelsmith spoke prep on the threads of the spokes before you start building. It will help to lubricate the spokes during the build and help to keep them tight down the road.
Start the wheel build by making sure the lable on the rim is right side up so that you can read it from the right side of the bicycle.

We're going to divide the wheel into four different sections of spokes- Section One spokes will be heads down on the right side of the bike, section two spokes will be heads up on the right side of the bike. Section three spokes will be heads up on the left side of the wheel and section four spokes will be heads down. Heads up spokes are placed into the hub flange from the near side, or dropped into the holes. Heads down spokes come in from the back side. For now, we're going to concern ourselves with the first two sections. Start out on the right side of the hub and wheel and place a spoke head down on the hub and into the spoke hole to the left of the valve hole on the rim. Thread a nipple onto the spoke 2-4 turns. Skip a hole on the hub, and place another spoke head down, and skip 3 holes on the rim, placing it into the 4th hole, and thread on the nipple. Repeat this process twice more, so that you have 4 head down spokes. These, and the remainder of the head down spokes on this flange, are all section one. Since we're building a 4 cross wheel, now it's time for the crossing. Drop a heads up spoke in the hole to the right of the 4th spoke, and cross it under the first three spokes, and then over the fourth. It's important that you go over the 4th spoke and not under it. Skip one hole on the rim, and thread on a nipple. It should look like this:
Next, place the rest of the head up spokes on this flange, and follow the same pattern (a spoke, skip three holes on the rim, and the next spoke.) Make sure you cross over the remaining 3 spokes from section one.
Here's a close up of what the hub should look like at this point:
Now we get to the exciting and scary part where we start putting spokes on the opposite flange. Exciting because we're getting that much closer to being done, and scary because this is where you can screw it up. Luckily, I've come up with a nearly fool-proof method of locating where the spoke is supposed to go. You first want to flip the wheel over so that the side with all the spokes in it is on the bottom. Locate the spoke on the opposite flange that is one spoke hole away from the valve hole. Take a spoke and drop it down (head up spoke) through the spoke hole that is one hole in front of the spoke hole on the opposite flange. You can tell if you have the correct whole because if you hold the spoke so it's parallel with the axle, as I've done in the picture, it should fall just in front of the spoke on the opposite flange that is one hole on the rim from the valve hole. If this confuses you, take a look at the enlarged picture.
You'll have to weave the spoke through the "V" of the spokes on the opposite side of the wheel, and then bend it back to get it into the proper position next to the valve hole of the rim. Be careful not to scratch the rim during this part of the proceedure. Next, you'll fill in the rest of the head up spokes in this section. You'll have to weave them through the "V" on the other three crosses of the first side, but the remainder should be fairly easy. Just remember to count three holes on the rim, and place the spoke into the fourth.
And finally, the completed section.
Now we'll hit the last 5 spokes of section one. You'll drop them down from the top, so when you position them, you won't have to worry about twining them through the other spokes. Follow the same pattern as the original 4 spokes you placed.
Once it's completed, you'll have this:
Now that you've got the first side (sections 1 and 2) completed, drop the 4th section spokes down through the remaining spoke holes. You can tell you're correct because the rim lable is not upside down. Another clue is that you have a group of three spokes, an empty spoke hole, and then three more spokes all the way around the wheel. Now, insert head down spokes into the section 4 spoke holes on the left flange.
Take one of the spokes, and opposite to the direction of the spokes that are already laced on this flange (3rd section), cross over 3 spokes, go under the 4th, and attach it to a nipple in the spoke hole that the spoke should be pointing at. Repeat for the rest of the spokes in this section, and you've laced the wheel. Once all the 4th section of spokes are attached, your wheel is completely laced.

Next, tighten all the spokes down until there is 2 or 3 threads still showing on each spoke all the way around the rim.

Now all you have to do is true it, which is essentially a trial and error process, making sure that you keep more or less even tension on every spoke. In addition to proper dish, which is centering the rim between the locknuts/conespacers of the hub, you'll also have to concern yourself with the roundness, or the up and down movement, as well as the side to side movement. A wheel truing stand is best for this, but you can use your brakes on your bike in a pinch. Work on the up and down, and then the side to side and the dish, and then go back to the up and down and then back to the side to side. You should be turning the spoke wrench a quarter turn to a half a turn at a time. The more time you spend here now, the less time you'll spend in the future fixing your wheels, so take the time to do it right. Finally, you'll have a straight wheel with little to no hops, or you might even have it perfect. Proper spoke tension is not so tight that the spokes will not flex at all, but they shouldn't move as much on a BMX as they might on a larger wheeled bike, where you want a bit of flex built into the wheels. When you squeeze two crossing spokes together, they should move a millimeter or two.

Now that you've gotten your wheel to this point, you should make sure the axle nuts are tight, and place the wheel on flat on the ground. Hold onto each side of the wheel and kneel on it, applying most of your weight to the wheel, and then rotating it 90 degrees, and kneel on it again, until you've gone 360 degrees. Then flip the wheel over and do it repeat the kneeling process on the other side, and then put it back in the true stand and make it perfect again. Doing this makes sure all the spokes are seated properly and helps to even out the tension. It all adds up to a stronger wheel down the road. If you're particularly anal, you can repeat this entire process, but it's not strictly needed more than once.

Then, put a proper rim strip on the rim, as opposed to a bunch of old stickers, mount up your tire and tube, and that's it!