(11/24/2013) So, how do you know when you need to change your brake pads? Go to your maintenance manual and look for the specification. In my HD Service Manual (Part No. 99483-06), Chapter 2: Chasis, the Motor Company tells us that if the pad thickness is less than 0.04 inches (1.02 mm), it's time to replace them.*
In the photo at right, you can see one of my old front pads. At the thinnest point on any of the four, the pad thickness measured 1/32" or 0.01325 inches (0.8 mm). Replacement was clearly overdue.
By the way, these old pads were HD Original Equipment Manufacturer (OEM) and lasted me about 45,000 miles. I can live with that. They were my second set. The original pads lasted until about 45K miles and now I'm at 90K.
Measuring pad thickness
Check out this helpful video from J&P Cycles:
http://www.youtube.com/watch?v=BEKaI_gKJ8Q
In this video, the mechanic (or "tech") shows you how to measure the front pads' thickness while they're still on the bike. By the way, look closely: the pads he's removing look pretty good to me. Guess that's OK when you're point is to make an instructional video and someone else is paying. :-)
Brake pads come with grooves etched in them perpendicular to the motion of the rotor. If you've worn the pads down so the grooves are gone, it's time to change them. As you can see in the photo above--no sign of any grooves. They're long gone. Unfortunately, you really can't see the grooves--or lack thereof--until you've got the darned things off.
Bolt head oddity
The first thing you should know before you begin to remove your old brake pads is the odd mix of tools you will need: for the calipers, you need a 10mm, 12-point socket or box wrench. For the pad pins, you will need a 1/4" 12-point socket or box wrench. Yes, one is metric and the other English. And I'd swear that those bolt heads are Torx, but 12-point sockets do the trick. I tried Torx sockets and they don't work, and you sure don't want to strip these bolt heads!
If you have some painter's tape handy, use that to cover your fender so you avoid damaging it when you slide the caliper off the rotor.
You do not, apparently, have to remove your front wheel to change pads. This is my second time replacing front brake pads and I've removed the wheel both times. It takes longer, but I find it diffcult to get the calipers past my fender otherwise. (Maybe next time I'll try doing the job without taking the wheel off and use some patience instead.) I also remove the rear wheel when I change pads, which apparently is unnecessary as well. I guess I tend to do things the hard way.
I'm not going to go into the details of the process. The maintenance manual and YouTube videos do a great job of that. However, I will say that you should go to several sources for instructions before you begin. Even if you've changed pads before, as I have. It's a task you do so infrequently, it's easy to forget something important; and you might learn something new.
For example, in one of the videos I watched, I learned that there's something called "pad lube" that you apply to the backs of the new pads when you install them. Unfortunately, I learned about pad lube after I installed me new pads. Doh! I'm going to replace my rear pads in March and when I do I will pull these front pads off and apply some of that lubricant.
I added some brake fluid and took her out for a ride yesterday. The new brakes feel the best they've ever been. Unfortunately, I hear a faint sqeaking noise when I pull on the front brake lever. Let's see if it lasts for very long. If they continue to squeak, you'll be hearing from me in this post with a follow-up bitching about my new EBC organic pads. Let's hope that doesn't happen.
Keep the shiny side up and pray for a short, mild winter!
-The Long Rider
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*Important note: I heard a guy in one of the YouTube videos say that when the pads reach "sixteen thousandths of an inch" it's time to change them. That would be 0.0016 inches. I am confident that he's very wrong. That's paper thin; another tap of the brakes and you'll be eating rotor.