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A wandering bite point can cause some serious frustration, ruining your hard earned descents. It is unsettling to say the least. Brakes need to be reliable, 100% of the time. We asked leading brake manufacturers, Hayes, SRAM and Magura, to shed some light on the matter. They pitched in on a couple other brake tech questions for us too. Time to get educated.
Why do some hydraulic disc brakes have a wandering bite point while others are reliable?
Hayes: We have studied wandering bite point and have a good understanding of what causes it. While I can’t give specifics since we consider it a trade secret I can say that in certain conditions pad clearance to the rotor is reduced while riding which results in a shorter lever stroke to the bite point. Higher ratio brake systems have a larger change in lever stroke from this compared to lower ratio systems and therefore it is more noticeable. We have developed tests to recreate this phenomena and have implemented specific countermeasures in the design of Dominion to greatly reduce the occurrence.
SRAM: Basically it comes down to material selection based on fluid choice:
- The brake fluid you choose will ultimately determine which types of rubber compounds you can use in the brake system. The rubber has to be compatible with the fluid and not shrink, swell or experience other changes in the rubber properties with exposure to the fluid. With DOT fluid based systems, EPDM rubber is the most compatible compound. Mineral oil based systems typically use an NBR compound, often referred to as Buna. EPDM offers several distinct advantages:
- EPDM has excellent resiliency when compared to other rubber compounds, which means that the seal returns to its original position very quickly after it is displaced. This is important, particularly in the caliper, because the caliper piston seals are effectively springs which pull the pistons back to their original position after the brake lever is released. Rubber with lower resiliency take more time to return, which can cause more variation in piston rollback, and therefore variation in the lever contact point. This is apparent when cycling the brake lever multiple times in quick succession, where the contact point will move further away from the bar with each pull.
- EPDM has a broader temperature operational range than NBR and most other conventional rubber compounds. With so many different types of bikes available to be ridden in an ever-widening range of conditions, this becomes very important to the overall consistency of the brake systems performance.
- On the hot side, having a higher temperature limit means that the material is able to withstand the heat generated by braking without melting, extruding, or leaking.
- On the cold side. The low temperature limit is the point at which the seal material will basically freeze. Freezing means loss of squeeze and ultimately fluid leaking. The rollback consistency and resulting contact point variation will be worse at lower temperature as a result. EPDM maintains better resiliency at lower temperatures when compared with NBR.
2. DOT fluid has a much lower kinematic viscosity compared to most mineral oils. As temperature decreases, the viscosity of any fluid will increase. This results in flow restriction during brake applications, which can exacerbate the rubber resiliency concerns mentioned above. With a DOT based system, you are getting both the best fluid and the best seal option for consistency and performance.
Magura: A “wandering bite point” is a brake operating “not as designed.” Sometimes it is the cause of multiple piston caliper’s pistons operating independently due to piston seal stickiness or caliper alignment.
Also, the wheels/chassis can experience massive violence (rock garden, high G cornering etc.) causing brake pad “kick back” where the wheel and rotor flex laterally enough to reposition the pads within the caliper’s bores while braking.
In addition, there could be a hydraulic fluid flow restriction (kinked line, improper line fitting install, poor caliper alignment) which can produce a “pump” of the lever blade caused by the inability for the brake pad (really the piston’s quadring seal) to push the fluid back through the system fast enough when the lever blade is released.
Incorrect fluid (viscosity) can also produce differing bite points and then there is the possibility of a mechanical failure and hardening of caliper or master piston seals.
Should I clean my brake rotors? If so, with what?
Hayes: Brake rotors do not require cleaning as the rotor and brake pads are designed to clear away any dirt, mud, and water from the braking surface. However, if the rotor is contaminated with by other materials like grease or oil then a thorough cleaning with isopropyl alcohol is recommended. In this case it is likely that the pads have been contaminated and need to be replaced.
SRAM: If your rotors are starting to look dirty you may clean them, but there is no reason to do so to improve performance. We recommend using isopropyl alcohol to clean rotors as it will not induce brake noise after a cleaning the way some detergents will.
Magura: Only if there is a need like contamination, otherwise leave it alone. We’ve always recommended something safe yet effective which is rubbing alcohol. You can of course use more aggressive cleaners like automotive brake clean but highly recommend removing the rotor so the excessively caustic cleaners don’t damage rubber components like brake caliper seals or the wheel’s hub seals.
What is the difference between brake pad compounds?
Hayes: Each brake compound is formulated with different targets for various performance categories like dry power, wet power, fade, speed sensitivity, pressure sensitivity, temperature sensitivity, noise, wear, and burnish. Setting the targets is a balance of giving up performance in some categories in order to gain performance in others. The constituents of the formulation are chosen and their relative volumes are adjusted to meet these performance targets. For example one compound may be formulated focusing on dry power while giving up noise and another compound may be formulated focusing on stable performance across speeds and temperatures while giving up peak power.
SRAM: Our pad offerings include the following:
- The new Power Organic configuration developed for the new G2 brake – “Lunar” Grey backing plate
- Quiet Organic pads – Black backing plate (currently used in Guide and Level brakes) – A good all-around pad, quiet, but not as powerful as the Power Organic version
- Metallic – copper backing plate – Our most powerful, hardest hitting pad compound. Long lasting and great in wet and muddy conditions. It does create a bit more noise.
Magura: Metallic and Organic pads. Metallic pads can last longer and provide a different “bite point” than the organic. This is typically a rider based preference.
Do brake and shifter cables actually stretch? Or, is it just the housing systems compressing into their fittings? Please help settle this debate.
Hayes: Cables from reputable brands will be pre-stressed which means there will be almost no stretch in usage. The length change actually comes from compression of the housing. When a cable is under a tensile load the housing experiences an equivalent compression load. This can result in housing length shortening, especially in the bends. “Compressionless” housing performs much better in this regard.
SRAM: Brake and derailleur systems can require readjustment after a new build due to housing end caps settling relative to the end of the housing once they system is tensioned. Adjustments required after the system has been in use are usually due to the housing end shifting. Over time, a derailleur housing with too tight of a bend may force some of the inner strands to protrude from the protective layer, which can slightly change cable tension.
Magura: We don’t produce nor promote non hydraulic brake actuation. That said, the cable for shifting or braking doesn’t stretch and instead compresses the housing, ferrules and fittings. Often in a poor performing cable-actuated brake or derailleur system, the housing sheath has pushed back and revealed the metal lining that acts as a compression device inside of the ferrule and results in poor shifting (or braking) performance.
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