# Thinking about a p85 build.

Just a fun math exercise to show what I mean.

80lb bike traveling at 20mph has 1,067ft-lbs of kinetic energy, or 1,446.658 Joules.

80lb bike traeling at 40mph has 4,284ft-lbs of kinetic energy, or 5,808.324 Joules.

I had 203 front and rear with the hydraulic pull calipers (same ones Damien uses) an it had more than enough stopping force. Enough so that I had to set my rear brake slightly "looser" than the front for my dual pull brake lever in order to get more front bias.

As I said, 160 can be sufficient for a standard bicyle, but 160 and 140 brake setups were originally meant for standard pedal bikes. The prolification of larger brakes came from racing bikes and e-bikes. Bikes that are heavier and/or much faster than a regular mountain bike with a slightly fit person in the seat. I don't play too many "Good enough" games when it comes to safety stuff, and brakes are a safety item. I like being able to slow down in ways other than leaping from the bike or running into stuff.

Remember, energy is the square of velocity times mass. Going twice as fast doesn't mean double the energy, it means 4 times the energy that then has to be converted into heat via friction. Your 80lb gas bike going 40mph takes roughly 4 times the brake force to stop at the same distance as an 80lb bike at 20mph.
Yeah, I have the same calipers as Damien. If I can get the rotor to run ok I'll see if we'll they work on these hills, if not I'll upgrade it. Thanks for the information!

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