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by Caley Fretz
March 21, 2018
Your new bike shines in the same untouched way a new car smells, glistening from every curve and component. Its disc rotors catch morning light as you kit up for the maiden voyage. You pedal down the block, euphoric in its novelty, and for the first time in your cycling life, grab a fistful of disc brake, just for the hell of it. Just to feel the power.
But there is no power.
Don’t worry, there’s nothing wrong with the brakes, or with your hands. That lack of power with new pads, new rotors, or both, is completely normal. You just need to bed them in.
A bed-in process is required in any metallic brake system, including the brakes on a car, and your disc-brake gravel or road bike is no exception. It’s a simple procedure, less than 10 minutes long, and ensures that the first time you really need those discs, they’re working at full strength.
It’s not just about those first few pulls of the lever, though. Fail to bed-in your disc brakes and you could suffer vibration, noise, poor modulation, and low power. Nobody wants that.
To put it in terms familiar to rim-brake aficionados, hopping on your bike without bedding-in new disc rotors and pads is like rolling down a mountain pass with rim brake pads that aren’t properly adjusted — toed out, for example. They’ll work, but not as well as they should.
Since more riders are currently jumping on their first disc-brake bike than at any time since discs swept mountain biking in the early 2000’s, let’s talk about how to bed-in your disc brakes properly.
Disc brake bed-in is the process of preparing new disc rotors or pads for heavy use by transferring material from brake pads to the rotor through a careful heating/cooling cycle, resulting in improved brake power and control.
This transfer of material from pad to rotor increases the friction in two ways. First, it gives the pad something other than bare steel to grab on to. Second, the bedding-in process sculpts the pad and rotor so that they mirror each other, maximizing the contact area between the two surfaces. This is why the bed-in procedure needs to be performed any time either the pads or rotor are replaced.
A set of disc brakes that hasn’t been bedded-in properly is easy to spot, and even easier to hear. Loud squealing noises are often caused by poor or non-existent bedding. Without a proper bed-in process, pad material accumulates unevenly on the rotor, which manifests as high-frequency vibration. High-frequency vibration results in noise, particularly when things get wet.
Basically, if your disc brakes aren’t working right, the bedding-in process is a likely culprit.
Every disc brake and pad manufacturer has its own bed-in procedure, but all are slight variations on a similar set of steps. The key is to heat and cool the brakes in a repeatable and consistent manner, without accidentally scoring the pads or rotor by braking too hard, too soon. Again, the goal is a consistent transfer layer. Here’s what we have found works:
SRAM says to perform 20 slowdowns from a medium speed, and then another 10 from a higher speed. Shimano has a slightly different take, suggesting 10 stops with each brake. Pad manufacturer SwisssStop recommends dragging each brake individually for 20-30 seconds down a gradual slope two or three times, then doing the same down a steeper slope, then swapping the front and rear brake pads.
The number of times you repeat the process isn’t as important as properly executing the elements of the process. Gain sufficient speed, slow consistently but do not come to a stop, and repeat.
Since the bedding-in process is dependent on heat, you’ll need more speed (and possibly more runs) if you’re running larger rotors. This is mostly a concern with mountain bikes, where 180mm and 200mm-diameter rotors are much more common, but also if it’s very cold outside.
Heating the rear brake is more difficult than heating the front brake. Avoid the temptation to pedal while dragging the rear brake. Pedaling makes it difficult to maintain an even brake pressure. Instead, do a couple of repeats using only the rear brake, and remain seated through the whole process to increase rear tire traction.
Most importantly, never come to a complete stop in the middle of the bedding-in process. The transfer layer applied to the rotor needs to be consistent, and coming to a stop may leave extra pad material in one spot on the rotor. This can affect modulation and cause noise later on.
Perhaps your discs work just fine without a formal bed-in process. That’s not too surprising, actually. Most rides feature multiple slow-downs, few complete stops, and put plenty of heat into your brakes; quite a lot like a bedding-in process, in fact. But the first few stops on new pads or rotors are always highly compromised since new rotors or new pads simply do not have the power of a brake system that has been properly bedded-in. And the chances of accidentally doing something that causes noise, power loss, or poor modulation down the road are high. When it comes to brakes, better safe than sorry.
We called up Nate Newton, SRAM’s Road Marketing and Technical Rep, to find out if there is anything riders can do if they didn’t bed in their brakes and are having problems with power, modulation, or noise. His answer: It depends on the symptoms.
Decreased power is often caused by glazed pads, which can be caused by braking too hard (generating too much heat) before a proper bed-in. Pad contamination is another likely culprit — it takes just a tiny bit of oil or grease on the pads to decrease power dramatically. That’s why you should never touch disc pads or with your oily fingers. Both are issues that may be solved with some sandpaper, if you’re lucky.
“If you glazed your pads, for example, you can try sanding them down and then do a bed-in,” Newton said. “Or if you just contaminated the pads and rotor, cleaning both with rubbing alcohol and sanding the pads down might work.”
Vibration and noise are a different story. “If you are getting a vibration or excessive noise, it’s likely coming from a rotor surface that has an uneven pad transfer layer burned in,” Newton said. That uneven layer grabs at the pads intermittently, causing the vibration.
In this case, cleaning the rotor with rubbing alcohol isn’t going to make a difference. The vibration has literally been baked into your rotor. New rotors, and likely new pads, are in order.