Brakes - Page 1Last on March 28 2017
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 ditched the standard brakes on the XR2, and fitted uprated competition brakes front and rear.
This is a technical account of how I modified the brakes on my Fiesta.
The mechanics of the hydraulic system
2006 Pedal Box installWhen I first built the Fiesta, I ran a bias adjustable brake assembly in place of the brake servo, which sat in the engine bay complete with master cylinders. But I never really got this setup to work, and at the back of my mind, I always thought that fitting a pedal box would be a better solution.
So in 2006 I decided to switch to a modified Rally Design Mk2 Ford Escort pedal box. It fitted quite easily, in place of the standard pedal box. And immediately the pedal felt better, having eliminated all of the linkages and transfer bars from the original Fiesta Mk1/Mk2 system. This also meant that more pedal effort was turned in to producing the high brake fluid pressures required by the modern 4 pot brake calipers on the front of the car. And the replacement pedal box weighed in at just 3.9 KG, so it was 4.0 KG lighter than the original Ford brake setup (including the transfer bar and other linkages).
The Rally Design Mk2 Escort bias pedal box did need some modifications for it to fit in the Fiesta. First of all, there are several versions of the pedal box available from Rally Design. One box exists for a cable operated clutch and another for a hydraulic clutch. So for the first pedal box I chose the cable operated clutch type, as I was running a cable clutch at the time. I had to combine the Mk1 Fiesta clutch pedal with the supplied Escort clutch pedal, as I wanted to retain the cable ratchet mechanism from the Fiesta which automatically adjusted the clutch cable tension. But the clutch pedal shaft in the Escort pedal box, was a smaller diameter than the shaft from the Fiesta pedal assembly, therefore a bush needed machining to allow the Fiesta clutch pedal to sit on the shaft. To save weight, I used an aluminium bush which was loctited to the steel shaft, so the clutch pedal/ratchet mechanism rotated on the aluminium bush. If the aluminium bush were to rotate on the steel shaft, it would have worn away very quickly, as aluminium is very soft compared to steel.
The next mod was to weld a bracket to the top of the pedal box so that it bolted to the existing Fiesta support bracket, which sits above the pedal box. So in all, the box was held against the bulkhead with four M6 bolts, and was supported from above by an additional nut and bolt. This seemed to give the pedal box enough support, and it certainly never flexed or moved when pressure is applied to the brake pedal. I also had to run a small slab of aluminium between the pedal box and the front bulkhead, to allow the pedal box to clear the steel reinforcment ridge that exists on the bulkhead.
Once modified, I then had all the parts shot blasted, and powder coated for a more professional finish. In testing (April 2006) the brakes were fantastic. There was a real sense of urgency with the stopping power which I never achieved with the original setup. However, the rear wheels locked up when the pedal was pressed very hard, so a Willwood pressure regulater was equired in the line to the rear brakes, to reduce the pressure further. This allowed the front:rear bias to run at 50:50 which gave 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.
A Willwood brake pressure regulator sat in line with the rear brakes. This allowed the pressure to the rear brakes to be restricted, as under heavy braking it was all too easy to lock the rears.
2009 Pedal Box installChanging to a hydraulic clutch meant swapping the Escort 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. Again I modified it to add a bracket to the top of the pedal box, and bolted it to the bulkhead.
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, although the rear end does still have a tendancy to lockup. In 2011 I removed the Tilton pressure regulator. It leaked brake fluid from the 1/8 NPT jounts, and made a horrible mess of the paintwork on the floor of the car. This has reintroduced the tendency to lock the rear brakes under heavy braking which I've combatted by winding more bias to the front brakes.
Connecting the hydraulic clutch to the IB5/IB5+ slave cylinderTo 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, 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.