Whilst I've presently got the front brakes dismantled on my MC, I've been doing some measuring and calculations.
Since the pressure applied to the brake pads is the hydraulic force in the brake pipes x the area over which the hydraulic force acts then the effective total surface of area of piston pressure is one critical measurement in assessing brake efficiency. The OEM single pistons measure 60mm diameter, therefore the area of the piston is 3.14159 (pi) x 30mm x 30mm (radius squared) = 2827 square millimeters. When the single piston presses against the inside of the brake disc, there will be an equal and opposite force exerted by the brake pad on the outside of the disc, and as such, the effective total surface area of piston pressure on the brake pads will be 2827 x 2 square millimeters = 5654 square millimeters.
I've managed to find some dimensions of piston sizes for AP Racing's 4-pot calipers sold for the MC and in a single caliper, one pair measure 38mm and the other pair measure 41mm. This means that the combined area of the 38mm pair of pistons is 3.14159 (pi) x 19mm x 19mm (radius squared) = 2268 square millimeters and the combined area of the other pair is 3.14159 (pi) x 20.5mm x 20.5mm (radius squared) = 2640 square millimeters, giving an effective total surface area of piston pressure of (2268 + 2640) = 4908 square millimeters. Therefore the AP Racing 4-pot calipers using the same hydraulic pressure in the MC's braking system are capable of producing only 87% of the pressure on the brake pads that the OEM calipers can produce: that's 13% less than the MC's OEM brakes!
The OEM brakes work on a 315mm x28mm disc and the AP Racing brakes work on a 330mm x 28mm disc. Since the braking torque is proportional to the radius of the disc, then the OEM brakes work on a lever arm equivalent to 157.5mm and the AP Racing brakes work on a lever arm equivalent to 165mm. These figures give the AP Racing brakes a mechanical advantage of 4.55% more than the MC's OEM brakes.
However, since the AP Racing calipers can only create a pressure on the brake discs which is 13% less than the OEM calipers working on 315mm discs, it would appear that, overall, the OEM brake set up should actually be a more effective braking set up than the AP Racing 4-pot kit.
The AP Racing 6-pot kit has pistons measuring, 27mm, 31.7mm and 38mm and they work on a 343mm x 32 disc. This gives the piston areas of 1145mm2, 1578mm2 and 2268mm2, respectively, which gives a total area of 4991mm2. That's still 11.7% less than the area of pressure in the MCs OEM system. However a 343 mm disc gives a lever arm equivalent to 171.5mm which gives a mechanical advantage of 8.8% more than the MCs OEM brakes.
Since the effectiveness of a car's brakes is totally reliant upon the conversion of kinetic energy to heat energy, then for brakes to be the most effective, they need to create lots of friction to generate that heat energy. On the face of it, you would expect that bigger brake pads would generate more friction/heat. However, although a larger area of contact between two surfaces would create a larger source of frictional forces, it also reduces the pressure between the two surfaces for a given force holding them together. Since pressure equals force divided by the area of contact, it works out that the increase in friction generating area is exactly offset by the reduction in pressure; the resulting frictional forces, then, are dependent only on the frictional coefficient of the materials and the FORCE holding them together. This means that the greater pressure of the MCs OEM one-pot caliper is more effective than than larger brake pads operating under less pressure with an AP Racing 4-pot or even 6-pot kit.
Therefore, from doing these calculations, it seems to me that multi-piston "big brake" kits are not necessarily the "upgrades" you might imagine them to be. I must admit that I'm rather surprised by my findings from doing these calculations. I expect that any improvement that these big brake kits might give to overall braking performance would be from the greater mass of the discs over the OEM 315mm x 28 disc, where a larger mass of disc can act as a better "heat sink".
From the information I've worked out above, I think this confirms that the basic stopping power of the OEM brakes is as good, if not better than aftermarket brake systems.
I think that improving the duty cycle of the brakes by the provision of ducting is the most effective upgrade you can make to the MC's brakes and the cheapest of all.
If you want to improve the basic stopping power of the OEM brakes then you could do this in several different ways:
1. Fit brake pads with a higher coefficient of friction.
2. Fit bigger discs.
3. Fit one-pot calipers with larger pistons than 60mm diameter.
Can anyone see any flaws in my calculations or logic on this?
Since the pressure applied to the brake pads is the hydraulic force in the brake pipes x the area over which the hydraulic force acts then the effective total surface of area of piston pressure is one critical measurement in assessing brake efficiency. The OEM single pistons measure 60mm diameter, therefore the area of the piston is 3.14159 (pi) x 30mm x 30mm (radius squared) = 2827 square millimeters. When the single piston presses against the inside of the brake disc, there will be an equal and opposite force exerted by the brake pad on the outside of the disc, and as such, the effective total surface area of piston pressure on the brake pads will be 2827 x 2 square millimeters = 5654 square millimeters.
I've managed to find some dimensions of piston sizes for AP Racing's 4-pot calipers sold for the MC and in a single caliper, one pair measure 38mm and the other pair measure 41mm. This means that the combined area of the 38mm pair of pistons is 3.14159 (pi) x 19mm x 19mm (radius squared) = 2268 square millimeters and the combined area of the other pair is 3.14159 (pi) x 20.5mm x 20.5mm (radius squared) = 2640 square millimeters, giving an effective total surface area of piston pressure of (2268 + 2640) = 4908 square millimeters. Therefore the AP Racing 4-pot calipers using the same hydraulic pressure in the MC's braking system are capable of producing only 87% of the pressure on the brake pads that the OEM calipers can produce: that's 13% less than the MC's OEM brakes!
The OEM brakes work on a 315mm x28mm disc and the AP Racing brakes work on a 330mm x 28mm disc. Since the braking torque is proportional to the radius of the disc, then the OEM brakes work on a lever arm equivalent to 157.5mm and the AP Racing brakes work on a lever arm equivalent to 165mm. These figures give the AP Racing brakes a mechanical advantage of 4.55% more than the MC's OEM brakes.
However, since the AP Racing calipers can only create a pressure on the brake discs which is 13% less than the OEM calipers working on 315mm discs, it would appear that, overall, the OEM brake set up should actually be a more effective braking set up than the AP Racing 4-pot kit.
The AP Racing 6-pot kit has pistons measuring, 27mm, 31.7mm and 38mm and they work on a 343mm x 32 disc. This gives the piston areas of 1145mm2, 1578mm2 and 2268mm2, respectively, which gives a total area of 4991mm2. That's still 11.7% less than the area of pressure in the MCs OEM system. However a 343 mm disc gives a lever arm equivalent to 171.5mm which gives a mechanical advantage of 8.8% more than the MCs OEM brakes.
Since the effectiveness of a car's brakes is totally reliant upon the conversion of kinetic energy to heat energy, then for brakes to be the most effective, they need to create lots of friction to generate that heat energy. On the face of it, you would expect that bigger brake pads would generate more friction/heat. However, although a larger area of contact between two surfaces would create a larger source of frictional forces, it also reduces the pressure between the two surfaces for a given force holding them together. Since pressure equals force divided by the area of contact, it works out that the increase in friction generating area is exactly offset by the reduction in pressure; the resulting frictional forces, then, are dependent only on the frictional coefficient of the materials and the FORCE holding them together. This means that the greater pressure of the MCs OEM one-pot caliper is more effective than than larger brake pads operating under less pressure with an AP Racing 4-pot or even 6-pot kit.
Therefore, from doing these calculations, it seems to me that multi-piston "big brake" kits are not necessarily the "upgrades" you might imagine them to be. I must admit that I'm rather surprised by my findings from doing these calculations. I expect that any improvement that these big brake kits might give to overall braking performance would be from the greater mass of the discs over the OEM 315mm x 28 disc, where a larger mass of disc can act as a better "heat sink".
From the information I've worked out above, I think this confirms that the basic stopping power of the OEM brakes is as good, if not better than aftermarket brake systems.
I think that improving the duty cycle of the brakes by the provision of ducting is the most effective upgrade you can make to the MC's brakes and the cheapest of all.
If you want to improve the basic stopping power of the OEM brakes then you could do this in several different ways:
1. Fit brake pads with a higher coefficient of friction.
2. Fit bigger discs.
3. Fit one-pot calipers with larger pistons than 60mm diameter.
Can anyone see any flaws in my calculations or logic on this?
Comment