First of all let's answer the question "what makes ribs juicy?". Simple answer, "Water". No, juiciness is not all from marinades. Marinades will help, but that's not the total solution. Marinades are used mostly to tenderize and flavor meats.
With beef ribs most folks (like me) buy them from the local grocer and there's not a lot of meat on them unless you buy short ribs. Since most frequently that is the case, we'll concentrate on those less meaty ones.
Pork spare ribs are the common purchase and these will most likely need tenderizing. Baby back ribs are easier to tenderize and the following method can be used but steaming time will be a bit shorter.
Let's get to the "Meat" of the subject. Beef and pork can be flavored in various ways, and this article will not address that point. Juiciness is what we're going after here. And, you should understand that when ribs are cooked perfectly the rib tissue that keeps the meat on the bone should still be connected but not holding tight, and, not falling easily away. The end result should be a rib that is cut easily, stays on the bone for handling and pulls away easily with the teeth.
Introduction of moisture, that's the objective, and we have a few choices.
You could boil the ribs before putting them on the pit but this releases precious natural meat juices into the water that can't be recovered. So boiling is an option but not the recommended option.
Steaming the ribs is the better option, and, steaming at the end of cooking is when you want to do this... now let's get to it. Keep in mind that if you're doing beef and pork ribs one may be done before the other.
The simple 10 step process:
Get a pan large enough to hold the ribs and some foil to cover the pan.
1. Marinade and season the ribs as you normally would.
2. Brown the ribs well on both sides over the fire.
3. Move the ribs off to the side.
4. After about 20 minutes add sauce and cook onto the meat well (about 15 minutes).
5. Sauce the meat once more.
6. Put the ribs in the pan and add just enough water to cover the bottom of the pan.
7. Cover the pan with the foil lightly crimping to hold in the steam.
8. Steam over a low fire (you can do this in the oven at about 275ºF also).
9. Start checking the doneness after about 20 minutes. Be sure there is always water in the pan.
10. When done (test with a fork), remove from the heat, leave covered, and, let sit for about 10 minutes before slicing.
That's it.
The end result is a nicely browned rib, full of flavor and
really juicy. You may notice a nice sauce at the bottom of the pan. You
can brush the ribs with it after they're cut. If the pork ribs are done before the beef ribs (or vice-versa) remove them from the steam pan and lightly wrap them in foil, then continue to steam the others..
If you want smoke flavor you'll have to do that at the beginning. Once the ribs are covered in the pan the smoke won't get to the meat.
Click here for more info on barbecue in general.
Gook luck...