Squishy/Firm Brakes? - Wrist Twisters
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post #1 of 7 Old 11-15-2017, 10:32 AM Thread Starter
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Squishy/Firm Brakes?

Over the years of owning the 919, I've had different brake feels during different situations. I don't know what causes them.

When I did a track day at limerock park which is very hard on the brakes, I noticed my lever slowly required more and more pull to achieve the same amount of brake. The total amount of force required was the same, but the "bite" was less, assuming that means how much travel is needed from the initial contact of the pads to the rotors, to full stopping power. It doesn't return to the way it was before after cooling down. It takes maybe days before it's back to the way it was. This happens every time I really go hard on the brakes during a very spirited ride, like on the tail. And then some days, typically on colder and very humid days when the grass starts getting dew, my brake feels like a rock. Again, the same amount of pressure, only much less travel. I'm just curious to know what the science is behind this, as everywhere I look, people are talking about brake fade or hydraulic issues.

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post #2 of 7 Old 11-15-2017, 12:09 PM
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TLDR: sounds like the perfect example of stock rubber brake lines.

When it gets hot the rubber lines are more susceptible to expand under pressure ie when applying the brakes -> squishy feel.
When it's cold the opposite happens (rubber tightens up with lower temperature) -> firm feel.

You should upgrade to stainless steel brake lines and you'll get the firm brake feel in all riding conditions which in turns will make braking feel the same at all times.
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post #3 of 7 Old 11-15-2017, 12:57 PM Thread Starter
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This is what I had suspected. Thanks for confirming it, as I couldn't find sufficient evidence of it anywhere else. Again, thanks!

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post #4 of 7 Old 11-15-2017, 02:46 PM
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Brake Fluid is Hygroscopic. As it ages it absorbs moisture from the air. That moisture in the brake fluid when heated during aggressive brake use through the backing plates of the brake pads and into the caliper pistons etc causes the brake fluid to vaporize the water into gas which is now compressible making more lever travel required to get the same amount of braking force at the lever.

Quite simply liquids are not compressible, gasses are.

OEM Style rubber brake lines can exacerbate the condition because they also bulge and flex more than braided steel lines do, but the root cause is too much heat and/or old brake fluid.

Many times when it comes to this condition amongst riders at trackdays it is generally caused by one of two things:

1. The use of brake pads that are not up to the task of trackday or race pace. EBC HH or Galfer pads etc

2. Overuse of the brakes (i.e. using the brakes way more than actually required).

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post #5 of 7 Old 11-15-2017, 03:40 PM Thread Starter
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Why does it get stiff on a cold damp day? It's most noticeable when it's like 80 during the day at 55 at night

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post #6 of 7 Old 11-15-2017, 05:09 PM
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You are not riding as hard on a cold cool day

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post #7 of 7 Old 11-16-2017, 10:30 AM
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This post has motivated me to clean and service my brakes. Starting with the rear I will order some brake grease and brake fluid. I'll remove calipers and clean seals, pistons and such. Got a castrol fuel agent nearby so I'll find out what they have or I'll go to my honda dealer for stuff. I might consider a rear brake stainless braided line since my rear brake is more squishy than front.

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