In Depth Brake Question

[bbejj123]

So im looking for someone who understands fluid dynamics, so after talking to a few people ive gotten the understanding that Bigger bore of a brake master cylinder doesnt always mean better braking.

Now can someone explain to me why instead of having a stock 3/4 bore bmc and having a 7/8 bore bmc will create less pedal travel and stiffer pedal feel but have less pressure to the pads on the calipers.

I understand why there is less pedal travel and its stiffer but i dont understand why there is less pressure at the pads.


thanks, Brett

[okayfine]

I understand why there is less pedal travel and its stiffer but i dont understand why there is less pressure at the pads.

'Cause there's less pressure in the system required to move the same amount of fluid (amount necessary to move the pads into contact with the rotor), due to the increase bore size. Pound for pound, the smaller bore will exert a higher pressure. To equal the pressure at the caliper you will have to push harder on a larger-bore BMC, hence the "stiffer" pedal.

But I'll get in line with you to await the fluid dynamics experts.

[thisismatt]

Yeah...I don't really see how it's much of an issue of fluid dynamics...or it's at least a very simple issue. You're exerting the same force as normal, but over a greater area with the larger bmc, so your pressure is less. You could change your pedal setup to leverage more force on the bmc piston.

[JohnR]

The pressure in the system is a result of both the area of the piston in the bmc and the area of the piston in the caliper and the wheel cylinders or rear calipers. So you are moving more fluid which may result in faster application and a firmer feel because you may physically have to push harder, in order to achieve the same stopping distance. In numbers if your bmc is 1 sq in. and you push with 1 pound force you have 1 psi, if the caliper piston is 2 sq in. you will apply 2psi to the brake pads.

[datzenmike]

Like an equation... effort (energy) in, equals effort (energy)out. A bottle jack can lift a 1 1/2 ton car 6" with about twenty pumps of 40 lbs. You could lift it that far with one pump but it would require a 3,000 lb down push on the lever.

[5teN]

Hey Brett,

If you're only changing M/C size and you want to decelerate at the same rate, you would need the same brake line pressure.

P=F/A

If you increase A, by increasing piston DIA, you need to increase Force (at the pedal) to keep the same pressure, P.

To make the pads contact the rotor, you need to displace the same volume of fluid regardless of M/C diameter. So, the reason you don't move the pedal as far is simply because with the bigger diameter piston, you displace more volume of fluid as the piston moves through the bore.

volume change = A*x

[bbejj123]

thanks guys, as soon as someone brought it up as being a formula it made perfect sense haha

[icehouse]

I think of the brake master bore and the caliper/wheel cylinder bore like ten speed gears. A big gear in the front (master) and a small gear in the back (caliper) equals a bitch to pedal. A small gear in the front and a big gear in the back equals easy thus why we never took our ten speed out of first gear.

[heirfaus]

...thus why we never took our ten speed out of first gear.

You must have been riding low, because you were definitely riding slow