2 Reasons Why Your Disc Brakes Don't Work

“So you just grab the rotor and push it towards the pad that’s dragging…”

“Wait,” I have to stop Jude right here. I’m sure he’s giving me a load of crap. “You grab the rotor? You’re kidding me, right?”

The one thing everyone knows is that you never, ever touch your brake rotors. Doing so can transfer oil from your fingers to the rotor and from there, inevitably, to the brake pads on your disc brake, which, in turn, leads to weak braking power. So, surely, Jude Monica is feeding me a line of bullshit. We are drinking beer. In a bar. It would stand to reason.

Jude, however, is completely straight-faced.

“I’m 100 percent serious. I mean, you wash your hands first, of course, but you can use your rotor as a tool to stop your pads from rubbing the rotor. Really.”

Here’s the thing: Jude Monica knows disc brakes.


The guy has been working on them, as Magura USA’s technical service manager for…well, since roughly the very dawn of time. The guy lives and breathes disc brakes, which is why we’re sitting at the brewery, post-ride, talking about them—or specifically, talking about the two most common complaints he hears from mountain bikers while he’s out on the road: (1) My disc brake suddenly started rubbing in the middle of a ride; And (2) My brakes don’t have air in the lines, but they aren’t stopping with the same force anymore.

The grabbing a disc brake rotor thing, however, had me flummoxed. So I had Jude demonstrate it in the video above. My Blair Witch filmmaking style notwithstanding (video production has a steep learning curve), it’s worth watching to see how Jude gets the job done. But I thought I’d add a bit of context here as well.



An Interesting Perspective on Rotor Rub

There are plenty of reasons why your rotor may begin rubbing against the brake pads. Maybe you completely taco’d the crap out of your wheel…that’s always hard to miss. Or perhaps your rotor has warped. Or maybe you never set up your disc brake caliper correctly and the rotor has been rasping away since day one.

But we’re not talking about those situations here.

I’m talking about that moment, mid–ride, when your rotor suddenly starts rubbing consistently against one of your brake pads. By consistently, I mean a constant, slight dragging against one of the brake pads. If it’s an intermittent rubbing, it probably signals a bent rotor and, well, it’s time to either true the rotor or cough up the cash for a new one.

So in this case, let’s assume your rotor has begun a constant slight rubbing against one of your brake pads. If you’re like me, you probably rectify that particular problem by loosening the caliper’s two mounting bolts and realigning the brake caliper. It works. Jude, however, suggests that this approach often misses the point. After all, it’s not as if your caliper suddenly moved during the ride—those two bolts do a good job of keeping it rooted to your frame. What has shifted, however, are the pistons’ placement inside that caliper. The pistons, Jude argues, are what you want to manipulate—not the entire caliper body—and you can use the rotor to help you do that.


If you’ve ever removed your wheel and pulled the brake lever (bad idea, of course), you know the pistons and pads will advance into that rotor-less free space within the caliper and then you’re stuck having to pry the pads apart again.

Well, when you’re riding and you're cornering, your front and (especially) rear wheel can be shimmying about, from side to side, a hell of a lot more than you might suspect. Monica argues that your rear rotor can flex as much as 5 millimeters from side to side during a hard cornering moment over rough terrain (this varies depending on axle, wheel and frame stiffness). Anyhoo, if your wheel and rotor are shimmying from side to side during these micro-moments and you hit the rear brake, one of your pistons might be able to advance farther than normal. It advances and then it stays there. You get through that corner or really rocky bit of trail and, crap, now the rotor is constantly dragging against the pads on that side of your caliper.


Again, you can solve this issue pretty easily by re-centering the caliper itself, but Jude’s approach is to simply reverse what happened on the trail. He does that by determining which side of the caliper (inboard or outboard) is getting the rotor rub. Then, after wiping his hands on something clean, he pulls the rotor into the rubbing pad with one hand, then pumps the corresponding brake lever with his other hand. Doing this helps re-center, as it were, the pistons themselves within the caliper.

This procedure is not an exact science—Monica readily admits it’s more of a finesse move, but I’ve seen him do it and, yeah, it works on every brand of disc brake. Again, the guy has worked on more brakes than most of us have ever seen, so he didn't pull this maneuver out of the ether or his backside. It sounded crazy to me, but if you think about it, it actually makes sense. Or maybe it doesn’t make sense to you, in which case, just watch the video.

If, like me, you still freak out if someone gets within a few inches of touching your brake rotor, you can always do the rotor-grab maneuver while wearing clean latex gloves.



WHEN PADS GO BAD
A loss of braking power is usually the result of one of two things: air in the system or contaminated/glazed/worn brake pads. Air in the system is pretty easy to diagnose. Straddle your bike and, with the bike stationary, pull the brake lever. If the lever simply sinks to the handlebar, you’ve got air. If, however, you can feel the pads contact the rotor in the usual spot in your lever throw, you’re probably not dealing with air bubbles in the system. Instead, it’s probably your pads.

Pads are, no big revelation here, what brings friction and stopping power to the party. If your pads are glazed over, excessively worn or contaminated by road gunk, grease, oil, something-evil-and-slippery, you might as well be squeezing a couple of ice cubes against your rotor for all the good it’ll do you.

Maybe somebody (certainly not you, right?) tried to lube your chain with an aerosol lubricant and wound up dousing not only your drivetrain, but also your brake pads. It happens. For that matter, sometimes your pads pick up contaminants while your shuttling to the trailhead and your bike is riding on a hitch rack—crap sprays up from the road, gets on your rotor and—voila—you hit the trail with a honking brake that has gone weak and grouchy.


So, remove your wheel pull the pads from you caliper and take a look at them. If they are excessively worn (there’s less than 2.5 millimeters of pad left), toss them and get some new ones. Unevenly worn in a big way? Replace them and realign your brake caliper. Still plenty of pad material, but they are slick and glazed looking or darkened by some mysterious substance? We can work with that.

I’ve always used a rough (100) grit sanding paper, but Jude Monica recommends a sheet of drywall sanding screen (readily available at hardware stores). The mesh will rough up your pad nicely (giving you back your grip and bite) and, best of all, the layers of pad material you sand off will fall away through the sanding screen—so you aren’t just rubbing contaminated brake pad powder back into the pad itself. Nice. Jude recommends rubbing the pad in a figure eight pattern, to get an even sanding. You don’t want to take off much material here at all. You’re just scraping off the crappy surface layer.


Put your freshly surfaced pads back in the caliper where they belong. Before you toss in your wheel with its contaminated rotor, clean off the with a clean rag or paper towel soaked in isopropyl alcohol. If you’re persnickety (I am) use latex gloves. You’re going through the effort, you might as well do it right. To that end, some people skip the alcohol altogether in favor of automative disc brake cleaner. That stuff is nasty, but it works great. Definitely bust out the gloves and gobbles when you're using that stuff and do your best to not huff it, as I believe the label clearly states the fumes will eat holes in your brain. I think. It's nasty, we'll just leave it there. At any rate, once the rotor is clean and has had time to dry, slap the wheel back in. You’ll want to re-bed in your brake pads to get optimal performance out of them (that’s a topic in and of itself), but either way you’re going to be a hell of a lot better off than when your pads looked as if they’d been coated in Teflon.

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