Tech Forum : Brake caliper calculation

Horstl	Posted Tuesday, Feb 20th 3:03pm [Edit] [Quote] [IMS] [View car]
Member Post nr. 25 Austria 323i / 2.7, 323i, 315, M3	Folks, I would need your ideas in some mechanical / physical engineering. I'm in progress of designing a low cost big brake solution for my E21: 284mm vented disk with 1-piston (57mm) caliper. Here are my concerns: The E21 originally has a kind of 2 piston (48mm) caliper (don't know the exact wording for this type). If my memory serves me right the calculation of force is surface x pressure So in my case on assumption of a gives pressure I would get less force applied to the brake pads when I move over to the single piston caliper: E21: 2x48mm E28: 1x57mm I also asked this on the german board, but they try to tell me that the e21 2 piston caliper design only applies the force equal to one 48mm piston to the pad and basically the 57mm is better than 2 48mm pistons... what's going wrong here? rgds Horst

ichiwan2	Posted Tuesday, Feb 20th 3:59pm [Edit] [Quote] [IMS] [View car]
Member Post nr. 268 Kuala Lumpur, Malaysia BMW320/6(e21)	Dunno if this helps, my thots (for the record, I am not a mechanic/engineer). The basic idea of improving brake performance is to stop better. You could do three things, 1. For a given amount of force applied, increase the surface area of the friction material, i.e., brake pad and disc combination. 2. For a given amount of pad/disc combination, you increase the amount of pressure on the pad. 3. You could do both. The application of force using the brake fluid as a medium involves fluid dynamics. I am no expert in this case either. My guess is more fluid pressure/volume will be needed to drive two 48mm pistons than it is to drive one 57mm piston. However, the single piston caliper needs to do more work to overcome some of the friction between the floating calipers. I have done it both ways, i.e., Girling 4pot calipers and bigger servo (from e28/e30). My next project is to fit a mechanical anti-lock brake gadget to reduce the likelihood of locking up in emergency conditions. Hope this helps. If I am totally wrong, please correct.

Madhatter	Posted Tuesday, Feb 20th 5:06pm [Edit] [Quote] [IMS] [View car]
Member Post nr. 119 Australia - QLD E21, E24, E30	This really isnt a basic topic like you might think, there are so many things which have an impact, it can really do your head in. Talking cylinders, it actually depends on the size of the master cylinder too. increasing caliper piston total area requires more fluid transfer, and travel from the piston in the master cylinder in order to provide the same amount of force at the wheel cylinders. A larger master cylinder piston means more fluid is displaced, at less travel while generating the same amount of force. However, the larger piston requires more force to move, so the pedal actually becomes firmer requiring more effort to be applied. So if you increase the area of the pistons in your brake caliper (doesnt really matter a whole lot how many you have), you quite often need to increase the size of the master cylinder piston in order to decrease the increase in pedal travel. So to make it simple; A larger master cylinder displaces more fluid, with less pedal travel, but requires more force to operate. A smaller master cylinder displaces less fluid, requires longer travel, but requires less force to operate. Now as far as increasing braking force, it depends on the below. friction coefficients, ie between pad material and rotor. Largely a factor of pad compound. radial height of the rotor, or basically how large the rotor is, and what is the actual area that can be used. clamping force the caliper can physically apply, largely dependant on piston sizes and the increase in line pressure (see below) brake line pressure, simply based on master cylinder sizing. Pad material and friction is a given, so there is no need to really explain it here. Rotor area is pretty simple, the larger the rotor the more leverage it has (assuming both have a caliper that uses the outside edge of the rotor), this increases the potential force it can generate. However, note that actual area that can be utilised is more important than actual radius. It isnt good enough to have a huge rotor but only having the caliper acting on a small portion of the height. clamping force is bascially how well it can grip the rotor. It's pretty much determined by line pressure and the size of the caliper pistons in use. It actually works in reverse to the master cylinder here, the larger the pistons (and it is all basically surface area), the more clamping force the caliper can generate. Again, take note that larger pistons require more fluid and more force to operate. You can look at it by looking at load displacement. A larger surface area is more effective at displacing a load than a smaller area, so to transmit the same level of force, you need to increase the amount of force (in this case fluid pushing against it) acting against it. Line pressure is dictated by master cylinder size, the amount of pedal travel, and your piston sizes. A larger master cylinder piston actually generates less pressure and less travel results. How does it all work? well braking ability is basically calculated by taking the effective radius of the rotor, multiplied by the clamping force of the caliper, multipled by the friction coefficient, then divided by a known factor. Then that is before you even start talking about fluid volumes it's way to technical for the average person to get involved with as there are many usefull calculators out there which will work out the above factors given piston sizes and line pressures. What does it all mean? bugger all if you dont know what you want and what you are doing The proper way is to either calculate things out and do it on paper, then put it into practice, or simply put it in to practice and monitor line pressures to make sure the above still performs correctly. If you add more pistons and travel increases, you should look at increasing the size of the master cylinder to compensate for the travel if needed. If your travel is decreased, the pedal becomes hard and requires more force to operate, you need to look at decreasing the size of the master cylinder in order to increase line pressures and reduce the force required. A way around the above is to increase either the rod ratio of the pedal or upgrading the booster to increase the amount of mechanical force (ie your foot) you can apply to the master cylinder. However, you can have issues with regulating pedal pressure when this happens, so if you find your brakes are pretty touchy, this is a byproduct of it and you should reduce the force applied. So in reply to your question, no it isnt that simple. Multiple pistons have the potential to generate more clamping force, but without calculating the difference in pressures (and what you could do about it), you cant just simply say which is going to provide more force. Pressure issues aside, more pistons are always going to provide more clamping force potential, so force applied and pressures being equal, 2 is going to be better than one, and fixed is better than sliding. Non sliding calipers apply the force evenly to both sides of the caliper, compared to sliding setups which work on the basis that any action requires an opposite reaction, so the piston is doing all the work on one side wedging the other pad against the cradle and squeezing both sides on to the disc. What actually happens when changing to a sliding setup? you require more pressure to achieve the same clamping force, plus an increase in fluid volume because of the larger piston. So you will probably find the master cylinder is actually a little large for the single piston, so the pedal will be firm, but you may find braking ability has been reduced and you need to increase the travel in order for it to return (see the above for ways to counter act it). Edit: If you want some basic formulas; Pressure = Force divided by Area (of the master cylinder, which is Pi R, squared) Force = Pressure multiplied by area The one factor you really need to know is force values, in this case it is the actual force applied to the master cylinder. This is done by taking the force applied to the pedal which is then multiplied by the pedal ratio. You need to know the ratio or measure the pedal distances, pad to pivot, pivot point to master cylinder connecting rod.