msauve

Such a relationship only applies when compressing a gas. Boyle's law does not not apply to fluids. Once the "slack" in the system is taken up, very little movement is required for significant increases in pressure.

You have not given information on the working area of your caliper pistons vs. stock. If it's greater, they take less pressure, BUT MORE FLUID, to operate, and the amount of fluid determines pedal movement more than the amount of pressure.

ryoung

The behavior of a real gas deviates from Boyle's ideal gas law.

Many brake fluid manufacturers brag about the low (not zero) compressibility of their products. DOT 5 silicone brake fluids should be avoided because it's twice (or more) as compressible as DOT 3 or 4 glycol ether brake fluids. Quoting from Stoptech's <a href="http://www.stoptech.com/whitepapers/brakefluid1a.htm">Brake Fluid 1A</a>,

<i>"Now, all fluids have a certain amount of compressibility to start with, but adding even the smallest amount of air into the solution can dramatically increase the amount of elasticity in the system. In the case of silicone-based fluids, air is quite happy to take up residence between the brake fluid molecules, and as a result the fluid compressibility goes down. This is felt at your foot like stepping on a big spring. As you can imagine, more air = more spring."</i>

Last but not least, <a href="http://www.prosystembrakes.com/brembobrakefluid.htm">Brembo</a> shows the effect of a 10% reduction in brake fluid compressibility on hydraulic pressure and pedal travel. Like rubber brake lines or a flexible firewall, the compressibility of brake fluid just makes it another source of compliance in the system.

<img src="http://www.prosystembrakes.com/images/brakefluidchart.jpg">

msauve

But it's 99% of the way to the incompressable ideal, vs. a gas, which is what you were basing your argument on. Brake fluid compressibility does not account for 1.5" of brake pedal travel when increasing pressure from 1050 PSI to 1500 PSI (which your argument explicitly claimed).

i.e. you calculated that 1050/1500 = 0.7 and then applied Boyle's law to claim that pedal travel would be reduced to 5" x 0.7 = 3.5", or 1.5" less.

You were clearly applying Boyle's law as if brake fluid behaved like a perfect gas. You were wrong, it is VERY far from it. That real fluids are somewhat compressable doesn't mean they behave like gases.

The chart is of course worthless without more detail, such as the volume of the compressed fluid, which will have a linear relationship with the difference.

ryoung

Since the chart is from an article on Brembo's brake fluid, I believe it's reasonable to assume that this represents data from a test rig that resembles a braking system.

The Brembo curve isn't perfectly linear, but it's close enough to say that hydraulic pressure is proportional to pedal travel. Note that a 1/3 reduction in hydraulic pressure from 100 bar results in about a 25% reduction in pedal travel. My S8 started with 5" of pedal travel; for a 1/3 or 500 psi reduction in hydraulic pressure, the reduction in pedal travel would be more than an inch.

OK ... No one's under my dashboard measuring brake pedal travel during 1g stops, but when I switch to a high friction pad, I definitely feel more than an inch of pedal travel reduction (or at least I did before The Block).

<img src="http://www.prosystembrakes.com/images/brakefluidchart.jpg">

akula-ssh

Reason I ask: I don't see stepping on the pedal alone could generate 1kpsi. Now if pedal travel is proportional to the amount of force whatever pump generates to create the pressure, we need something that's not an ideal gas, and the relationship between hydraulic pressure and pedal travel would be linear, or very close to.

ryoung

See <a href="http://auto.howstuffworks.com/brake.htm">How Stuff Works</a> for the basics. The pressure from your foot acts on a lever arm with about a 5:1 mechanical advantage, which pushes on a hydraulic piston in the master cylinder, with a big assist from a vacuum booster. More than 1000 psi at the brake caliper during a maximum effort stop is not at all unusual.

akula-ssh

Or is it "always on" with the same amount of assist all the time?

msauve

they're measuring brake fluid, so in fact it's likely that they've made every attempt to eliminate all other variables, such as caliper flex, brake line expansion, pad compression, etc. It's not reasonable to expect their results to accurately reflect a real braking system.

Furthermore, the simplistic notion that there's a definite benefit from reducing pedal travel is unproven. The goal is not to get to maximum deceleration in a hurry, but to be able to get there repeatably and without overshoot (i.e. without locking the brakes). Reduce the range of pedal motion and your're also reducing the ability to modulate braking, which can work against you.

You still haven't quantified the piston area in your brakes vs. stock brakes. That makes a difference in pedal travel.

Jkay

PWNED! Don't mess with RY. You're just flailing helplessly now.

msauve

If you knew anything about WTF was being discussed, you wouldn't be flouting your ignorace so openly.