Disc brake breakdown: The experts answer our questions
By Josh Patterson and Karl Rosengarth
There’s a lot of information flying around out there about disc brakes, some more accurate than others. Here’s a cribsheet of what you need to know from brake experts at SRAM, Magura and Finish Line.
What does breaking, or bedding in, new disc brakes really do?
All new disc brakes—mechanical or hydraulic—have a break-in period where performance will be significantly lower. During this break-in period two things need to happen to achieve full stopping power: the surface of the brake pads needs to be roughened, and material from the pads needs to be embedded in the rotors. Both these things can be accomplished by going for a quick spin around your neighborhood and gently applying the brakes without allowing yourself to come to a complete stop.
Mineral versus DOT fluid
DOT stands for Department of Transportation; this brake fluid is tested and regulated. It must meet or exceed specific requirements for boiling point, consistent viscosity and compressibility under a wide range of conditions. DOT fluids are also readily available at most gas stations and all automotive stores.
When it comes to mineral oil—which is NOT the same stuff you can buy at the pharmacy—there are no such standards. So manufacturers produce or purchase fluids to meet standards they set for their products. DOT fluids tend to have a higher boiling point, but will absorb moisture over time, leading to a decrease in performance. Finish Line manufactures lubricants and brake fluids for the bicycle industry. As a company that produces both mineral and DOT brake fluids what’s their take? David Clopton, of Finish Line, feels many riders get to too hung up on the pros and cons of one fluid over another. “There are much more important factors to worry about. I’d recommend picking the brake system you like the best,” says Clopton. One is not necessarily “safer” than the other either. Both should be handled with care—wear nitrile gloves and eye protection when bleeding brakes using either fluid.
Which brake uses what?
DOT 4 or 5.1
- Avid / SRAM
Not all DOT Fluids are the same
DOT 3, DOT 4, DOT 5.1 Fluids are glycol-based fluids. DOT 5 is a whole ‘nother animal. DOT 5 is a silicone-based fluid. If you find yourself wondering if you can replace the commonly used DOT 4 or 5.1 in your disc brakes with DOT 5 fluid, STOP! “A black jelly-like substance will form where to two fluids mix. At the very least you would need to changes hoses to salvage your brakes,” says SRAM brake engineer Paul Kantor.
You can, however, bleed your DOT 5.1 brakes with DOT 4 fluid and vice versa. The brakes will feel the same; the only change will be the boiling point of the fluid. “Dot 5.1 manages heat better,” says Kantor. While most DOT 5.1 fluids (never to be confused with DOT 5) do have a slightly higher boiling point, DOT 4 is more readily available.
What would happen if I bleed a DOT system with mineral oil…?
There are no stupid questions…just stupid people who should have asked questions. Bleeding a DOT fluid brake system with mineral oil is like overdosing on Viagra—in both cases the result is unwanted swelling. In the case of your brakes, the seals and O-rings will double or triple in size—prepare for an expensive rebuild!
Metallic versus organic pads— what ’s the difference?
“Organic” is something of a misnomer. It’s not like they’re going to sprout leaves when wet. They have less metallic content, and more fiberglass and resin compounds, than their metallic counterparts.
Organic pads run quieter, particularly in wet weather. They have a smoother feel when engaged and have a more consistent feel under a wide range of conditions, but wear quicker and have less overall stopping power. Magura Product Manager Stefan Pahl notes that organic pads transfer less heat to the piston/caliper. “This helps keep the brake fluid cooler, “ says Pahl. Less change in the temperature of the brake fluids means there will be less change in power as the brakes heat up. For the weight weenies among us, another benefit of organic pads is that organic material can be bonded to aluminum, as opposed to steel backplates, for weight savings, this saves about 10 grams per wheel.
Metallic pads generally last longer and have more overall stopping power. They also tend to produce more noise than organic pads. For those who frequently ride in gritty or sandy conditions, metallic pads will significantly outlast their organic counterparts.
My brakes feel fine, why should I bother servicing them?
According Kantor, brakes should be bled once or twice a year. “There is a certain permeability in all brake systems. Outside moisture will creep in. DOT fluids have a wet and a dry boiling point. The wet point is quite a bit lower than dry boiling point,” says Kantor. Finish Line’s David Clopton adds, “The dry numbers are so off the chart who cares? It’s the wet numbers that matter, they approach the temps where you could actually notice brake fade.”
Even the best hydraulic brakes will get contaminated with air and water over time. DOT fluids will absorb moisture from the surrounding climate. The absorption of water into DOT fluid significantly lowers the boiling point of the fluid, leading to decreased braking performance and fade during hard braking and sustained descents.
Mineral oil does not absorb moisture from the air in the same way DOT fluids do, though air can still contaminate brake systems over time, which can result in a similar decrease in performance.
Three easy things you can do at home—in increasing order of mechanical aptitude:
1. Keep contaminates off your brakes. When lubing your chain, drip bottles are a better option than slathering your drivetrain with a spray bottle. When washing your bike try not to wash grit, grime and grease from your frame and tires onto rotors or into the caliper.
2. Clean rotors and pads with isopropyl alcohol. There are a lot of products on the market that claim to eliminate brake squeal, but regular cleaning of the rotors and pads in your best bet. “Some people get hung up on disc brake cleaners, we find these tend to bring on more squeal issues,” says Kantor. If you have frequent squeal or pulsing issues gently sanding the pads and rotors with fine grit sandpaper can also silence them. Pahl recommends also making sure everything on the bike is appropriately tightened: Check the bolts, pivot points, hubs and spokes.
3. Pull a vacuum on the master cylinder. Short of a full bleed, this is the best way to keep your brakes performing at their best. It is not uncommon for new brakes to have some air in the line as well. Pulling a vacuum on the master cylinder is a quick way to remove trapped air bubbles.
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