Brakes - Page 3

Last on December 31 2015

Now for the theoretical bit

This is how you calculate the fluid pressures in a brake system. First step is to establish the formula for calculating the surface area of a piston, and it is Pi x r² where r is the radius of the piston, and Pi is 3.142

• The front master cylinder has a diameter or 0.625" (stamped on the side of the cylinder housing). Therefore the radius of 0.625" is 0.3125", which when squared is 0.0976 in², and multiplied by Pi gives a 0.307 in² (square inch) surface area.
  
  
• The rear master cylinder is 0.75" diameter, the radius is 0.375", squared is 0.1406 in², and multiplied by Pi gives 0.442 in² surface area.
  
  
• Fluid pressure is equal to the input force applied to the master cylinder, divided by the surface area of the piston.
  
  
• The pedal ratio on the Fiesta, as established earlier, is 3.571:1. So for an applied force of 200 lb on the pedal, 200 x 3.571 = 714.2 lb of force is applied to the two master cylinders via the bias assembly
  
  
• However, because the two master cylinders are joined together by a bias bar linkage, the force applied to both master cylinders, with the bias adjuster set to 50:50, is halved. This is an important point to remember. The force is divided by two because I'm using a bias adjuster which splits the load across two master cylinders. Therefore the 714.2 lb of force is now halved for each master cylinder.
  
  
• So the input force on each master cylinder is actually only 357.1 lb !!
  
  
• Therefore, the front master cylinder would produce a hydraulic pressure of 357.1 lb / 0.307 in² = 1163 psi (pounds per square inch)
  
  
• And the rear master cylinder would produce a hydraulic pressure of 357.1 lb / 0.442 in² = 808 psi
  

Which isn't particularly good news. Such a low pressure means that the calipers are going to be working at about 50% efficiency. You see, the HiSpec HS422R front brake calipers are pressure tested to 2500 psi, and are rated at 2000 psi operating pressure. The calipers also have four pistons, hence they're known as 4-pots. Each piston is 42mm in diameter, or 1.65" So that 1163 psi of pressure, for 200 lb of force on the brake pedal, now has to displace 8 pistons (2 calipers), and the surface area of each piston is 2.139 sq.in. Therefore the force on the piston in the brake caliper, is equal to the area of the piston, multiplied by the hydraulic pressure.

• So, for 1163 psi (from 200 lb of pedal pressure), the force asserted by each piston is 1163 psi x 2.139 sq.in = 2487 lbs.
  

However, in testing and in competition, the brakes don't feel that good. The pedal is wooden, and although the car slows down, it certainly doesn't want to stop in a hurry, and locking the front brakes, or indeed any of the brakes, seems impossible. An improvement is urgently required.

Aug 31^st '02: My gut feeling is that the displacement of the brake caliper pistons is insufficient. ie the pistons are moving say 3-4 mm to push the pads against the disks, with 2487 lbs of force, but if the pads are resting 1-2 mm away from the disks when the pedal is released, then the pads aren't really going to grip the disks at all. Maybe if I fit a larger master cylinder, slightly reducing the pressure and force, but increasing the displacement of the 8 pistons. That may do the trick,

Use the XR2 brake pedal

I'd been thinking about how to improve the stopping power of the Fiesta and I decided to move the linkage position on the brake pedal closer to the pedals pivot point ie from 70mm to 50mm. This would increase the leverage from 3.571:1 to 5:1, which inturn increases the fluid pressure at the front calipers (assuming the bias is set to 50:50) from 872psi for 150lb pedal pressure, to 1222psi, a whopping 40% increase in stopping power from quite a simple modification. So I removed the pedal assembly from the car, and dismantled the brake pedal, and then I decided to compare it against the spare pedal assembly hanging on the garage wall. Imagine my surprise when I discovered that the spare pedal had the linkage hole drilled at a different position to the pedal taken from the car. And where was the hole? 50mm from the pivot point. Hmm? When I built the car up in 2002, I'd used the pedal assembly from the 1300 Ghia spares that I had lying around at the time (I don't remember why). From my comparison between the two pedals, the original XR2 assembly is designed to generate a higher braking force than the one from the 1300 Ghia, and I'd not noticed the difference before, and no one had ever pointed this out to me. You learn something new every day. I've now swapped the pedals over, and the XR2 pedal is back on the car. I may have to play with the bias adjuster again to get the balance right, we'll see on the day. So my Demon Tweek for this week is: If you want to improve the braking of your Fiesta, make sure you fit the XR2 brake pedal, as it generates 40% more braking pressure for the same effort.



(Above) A photo of both Fiesta Mk1 pedals side by side. The XR2 pedal (bottom) has the linkage hole nearer the pivot point at the end of the pedal. The XR2 pedal gives a leverage ratio of 5:1 (250mm/50mm) and the 1300 Ghia Mk1 pedal gives a ratio of 3.571:1 (250mm/70mm).