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Welcome back to the Bikerumor Ask A Stupid Question series. This week, we’re back on the brakes, with Ricard Bages from Galfer fielding your tech questions on how rotor size relates to modulation, the brake bleeding process, and whether or not the oft-forgotten hose can offer some marginal gains in braking performance.
Regardless of rotor size, once the rotor is locked by the caliper, the wheel will skid. Also, as was mentioned in one response (with reference to a previous installment of the Ask A Stupid Question series), as rotor size increases, modulation decreases. Might it make sense that a smaller rotor might make more sense to maintain control and sometimes might actually stop faster because it allows more modulation and requires more force to lock up? Of course, bike handling skill is a big variable in a skid, so the question is kind of academic.
Galfer: Rotor size increases the braking torque proportionally. For example, jumping from a 160mm to a 223mm rotor, braking torque is around 40% increased. On the other hand, cooling power is increased too. The ratio between force on the lever and braking torque changes when rotor size is increased, but the human brain is easily capable of fitting the new ratio according to the floor grip.
To get a good predictable braking torque, it is very important to choose the pads correctly. Some friction materials change drastically the friction power depending on the surface pressure. Others give an aggressive grip from the very beginning without modulation. Galfer pads are known to offer good modulation and keep the performance along the duration of the pad’s life.
Why don’t hydraulic disc brakes bleed similarly to how automotive brakes are bled? E.g., empty the reservoir, fill it with fresh fluid, then pull the lever (or brake pedal in a car) crack a bleed screw, have the fluid come through the caliper, tighten the bleed screw, then repeat until air free/clean fluid comes through. A lot easier and only requires two very cheap tools to do.
Galfer: The bicycle bleeding system is very similar to the automotive/motorcycle on many brands but thought to be cleaner. Note that oil is spilled when the bladder is assembled on a motorcycle. For a Japanese brand, the procedure is the same as you describe, but it uses an additional reservoir to make the process cleaner. Others uses syringes to remove the old oil and bubbles quickly.
What’s the relevance of the hose diameter and material when it comes to braking performance? Are there marginal gains to be had by upgrading this component?
Galfer: It effects on the stiffness under pressure and temperature. On longer hoses (rear brake) stiffness is much more important because the volume inside the hose is much increased. Keep in mind that feeling is not the same on right and left lever. Using a stiffer hose on the rear brake, could help to get the same consistency on both brakes.
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