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Brakes
On a road car you can pretty much take them for granted. They're engineered by departments that spend millions, on finding the right master cylinder, slave, and servo combination. And many more thousands of hours are spent on testing durability, stopping force etc.
On a competition car, fitting uprated brakes can be, and sometimes is, a bit of a lottery. For one thing, they must work harder than those fitted to a road car. Braking from speeds of over 120 MPH, regularly down to below 20, they undergo tremendous stresses as heat builds up over very short distances and times. Guided by magazine articles, pages on the internet, and a very informative book (Brake Handbook by Fred Puhn), I've completely thrown away the standard brakes on the XR2, and fitted uprated competition brakes all round.
This is a technical account of how I've modified the brakes on my Fiesta.
The mechanics of the hydraulic system
2006 update
Previously, I ran a bias assembly in place of the brake servo. In 2006 I ditched it all, for a modified Rally Design pedal box. The pedal feels amazing with the new pedal box. It eliminates all the linkages and transfer bars on the original XR2 system, which means more pedal effort is turned in to producing the high pressures required by modern braking systems. And the new system weighs in at just 3.9KG all in, so its 4KG lighter than the previous setup.
The Rally Design Mk2 Escort bias pedalbox does need some modifications for it to fit in the Fiesta.
First of all, there are two versions of the pedal box available from Rally Design. One for a cable operated clutch and another for a hydraulic clutch. I chose the cable operated clutch type.
I then combined the Fiesta clutch pedal with the Escort clutch pedal, as I wanted to retain the ratchet mechanism which automatically adjusts the clutch cable tension. The shaft in the pedal box, is a smaller diameter than the shaft on the Fiesta pedal assembly, therefore a bush needs machining to allow the Fiesta clutch pedal to sit on the shaft. To save weight, I used an aluminium bush which is loctited to the steel shaft, so the clutch pedal/ratchet mechanism rotates on the aluminium bush.
The next mod was to weld an extra bracket to the top of the pedal box so that it bolted to the existing support bracket, which sits above the pedal box. So in all, the box is held against the bulkhead with four M6 bolts, and is supported from above by an additional nut and bolt. This seems to have given the pedal box enough support, and it certainly doesn't flex or move when pressure is applied to the brake pedal.
Once modified, I then had all the parts shot blasted, and powder coated for a more professional finish. In testing (April 2006) the brakes are fantastic. There's a real sense of urgency with the stopping power now. However, the rear wheels lock up when the pedal is pressed very hard, so a Willwood pressure regulater is required in the line to the rear brakes, to reduce the pressure further. This allows the front:rear bias to run at 50:50 which gives a progressive feel to the pedal, and excellent stopping power. Note: I have used the 0.625" cylinder for the front brakes, and the 0.7" cylinder for the rear. There is a 0.75" cylinder available, which would further reduce the pressure to the rear brakes.
I also fitted a Willwood brake pressure regulator in line with the rear brakes. This allows the pressure to the rears to be restricted, as under heavy braking it is all too easy to lock the rears.
Prior to 2006...
Operating through a standard XR2 brake pedal mechanism, the right hand drive Fiesta is immediately at a disadvantage to the LHD models. The Ford Fiesta has the brake servo mechanism fitted on the left hand side of the car, and therefore on the RHD car, the force applied to the brake pedal must be transferred across the car's bulkhead, in to the master cylinders. A 3/4" diameter steel tube (transfer bar) connects the brake pedal linkage to the servo, as can be seen by the photo. The disadvantage for the RHD car comes in the form of flexing of the bulkhead when the brakes are pressed. I've reinforced (braced) the bulkhead on my Fiesta, to prevent the bulkhead from moving too much. Its a favourite for scrutineers though. They'll always make a comment regarding the flexing of the bulkhead, despite whatever steps you've taken to strengthen things.
The transfer bar features 60 mm levers on each end, giving a mechanical ratio of 1:1. The input force from the brake pedal is transmitted along the bar, and in to the master cylinders.
The actual brake pedal lever (inside the car) is 250mm in length. Pivoting at the top of the lever, a linkage 70mm down, connects the lever to the transfer bar. This gives a pedal ratio of 250 / 70 = 3.571:1.
So we've so far established how the force applied to the brake pedal, is multiplied by a factor of 3.571:1 and then transferred across the bulkhead via the transfer bar, and in to the brake master cylinders.
The Servo (or the Power Assist Vacuum Booster)
On the stock road-going version of the Fiesta, a brake servo (or booster) is used to translate the force applied to the brake pedal, in to a much higher mechanical force. In simple terms, it takes the force applied to the brake pedal, and amplifies the force, which is then fed in to the brake master cylinder.
Brake servo's are not common on race cars, and indeed are often fitted to compensate for an error in the design of the braking system. They take a vacuum feed from the inlet manifold, which is quite tricky to plumb in when you have a manifold like the one I'm using, with four seperate isolated runners, individually feeding each inlet port. I'm not using a Servo, instead I've chosen to use a bias adjuster assembly as detailed below
Front brakes
I opted for HiSpec Engineering billet 4-pot front calipers. These allow the use of 285mm ventilated disks, and fit on to the existing Fiesta hubs by way of an adaptor. Overbraided front hoses were made to my design by Earls, and these connect the calipers via M10 fittings, to the cupro-nickel hydraulic pipe.
I'm using Ferodo DS2500 brake pads in the front and rear brakes.
Rear brakes
I bought the Fiesta Centre rear disk brake conversion kit. It comprises of a machined aluminium plate which the caliper bolts on to, a modified rear brake drum which is machined to allow a disk to be fitted, a pair of Fiesta Mk3 solid front brake disks, and an assortment of fixings. You will also need to buy a pair of Ford Granada (Mk3) Scorpio rear brake calipers. I bought my calipers from Halfords. The calipers have identical part numbers to those used on the Sierra Cosworth.
I opted to modify the mounting brackets supplied by the Fiesta Centre. My rose jointed adjustable trailing arms are manufactured by TAS, and interfere with the caliper if you mount it in the 8 O'Clock position. I drilled one new hole in the mounting plate, which allowed it to be rotated to mount the caliper at the 12 O'Clock position.I also had to have the drums turned down a fraction on the lathe, as they hadn't been machined properly, and when bolted in place, locked against the mounting plate. The Fiesta Center were made aware of the problem, but at the time had used a different manufacturer of brake drums, so there were some difference in tolerances.
Master Cylinders (pre-2006)
I'm using 0.625" front master cylinder, and a 0.75" rear. The input force applied to the master cylinders is halved, because of the brake bias assembly. So applying 714 lbs of force on the brake pedal, results in 307 lbs of force on each master cylinder, assuming that the bias bar is set at 50:50.
Using a hand brake with Sierra calipers
You will need to use an Escort RS 2000 4x4 handbrake cable, available from any motor factors for about £20. I had to fabricate a bracket which bolts under the floor of the car, and holds the cable in place. The bracket is made from 5mm steel, and was powder coated before I bolted it to the car.
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 31st '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 right 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).
Brake systems - links to other sites
Howstuffworks.com - www.howstuffworks.com/brake3.htm
Hydraulic brakes - www.tpub.com/fluid/ch1j.htm
A short course on brakes - www.familycar.com/brakes.htm
Basics of Hydraulic brake systems - www.babcox.com/editorial/cm/cm20119.htm
HotRodHeaven.com - www.hotrodheaven.com/tech/brakes/brakes1.htm
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