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on January 31 2012On 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 Pedal Box install
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.
2009 Pedal Box install
Changing to a hydraulic clutch meant swapping the pedal box for a hydraulic clutch version, which is what I did. I sold the cable clutch box to a fellow Fiesta owner, and bought another Rally Design box, had it painted professionally, and bolted it in to the car.
Today (2009 onwards), the pedal box uses a 0.625" (front cylinder) and a 0.7" (rear cylinder). This gives the best possible setup in a Mk1 Fiesta in my experience.
In 2011 I removed the Tilton pressure regulator. This was plumbed in to the rear line, and was left over from when I ran the cable converted bias pedal box (above). However in 2011 it was wound fully open, and wasn't therefore doing anything, apart from leaking and making a horrible mess of the paintwork. So I removed it.
To connect the clutch slave to the master cylinder inside the car, I had Rally Design make me a custom length hose, and I bought an aluminium clutch slave adaptor from M-Sport. The IB5 box uses a push-in type adaptor on the clutch slave, so I had to look for something I could attach a braided hose on to. M-Sport make such an adaptor, so I bought one, sourced a suitable O-Ring from Brammer, and plumbed it all in.
- Parts required: M-Sport BR16020004A -4 Clutch Master Cylinder Adaptor (7/16 UNC thread) (£24.56+VAT)
- Note: The M-Sport adaptor requires a Viton 6mm O-Ring (OR6X2-VIT from Bearing Station Ltd)
- Rally Design -4 s/s flexible hose with 3/8 UNC s/s banjo for master cylinder connection
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.
