FRONT BRAKES AND MASTER CYLINDERS
Triumph derived suspension - two types Vitesse/GT6 and Spitfire/Herald.
The Spitfire Herald on the whole uses a 9.0 inch disc and the Girling type 12 calliper, the GT6 Vitesse uses are larger 9.6 inch disc and the bigger type 16 Girling calliper
Dutton/Escort strut derived suspension
Escort MK1‘s use a 8.6 inch disc with the Girling type T12/16 and the MK2 a larger 9.6 inch disc and M16 Girling calliper.Early mark 1's had small discs to get under 12 inch wheels later upgraded with larger discs and calipers on the faster Mk1's and all the Mk2's.
Later 9.6 inch discs cannot be fitted to the earlier 8.6 hubs as the whole hub assy. is smaller – for the bigger Escort brakes changes to the stub axles and steering rack need to be carried out to upgrade to MK2 brakes – This does has the advantage that if desired you can fit a spacer kit to the M16 calliper and with a little work the Capri 2.8i vented disc can be fitted.
You can upgrade without changing the steering rack if you use the Capri strut as the steering arms are the same as for the MK1 Escort even on the late Capri’s as I am sure you are aware by now there is an added bonus in that the Capri strut has the stub axle positioned an inch further up the strut compared to the Escort effectively lowering the suspension 1 inch with no change in the suspension setting.
There is a simple advantage with the bigger brakes – the M16 has a bigger piston than the T12 so more force is available on the pad – the bigger area of the disc and pad reduces the local heating and aids the dissipation of the heat generated in the braking process making the chance of the brakes overheating much less likely – Vented discs reduce the chance even further.
For a car as light as the Phaeton with such an aerodynamically restricted top speed the 8.6/9.0 inch discs of the Spitfire/Escort Mk1 are more than capable of dealing with enthusiastic driving without any problem – If you are thinking of a more sporting application or a lot of power then the Vitess/GT6 – Capri/Escort MK2 are a wise modification.
Discs callipers and pads are only any good if sufficient pressure is present to operate then correctly – there is nothing wrong with lots of pressure being generated by the braking system but there is with not enough.
Not enough pressure in the braking system is a really bad thing and leads to poor braking performance in longer stopping distances, overheating due to prolong friction period and brake fading due to the heat build up. It also does nothing to the confidence of the driver.
Most Phaetons have the donor master cylinder for the brakes with the standard if slightly modified brake pedal assembly – on the whole these are good and more than adequate for the application, modifications from this can create some problems.
As an example of a bad braking system I will use my S1 V8 as an example – Overall the spec looked good and on sitting in the car and pressing the pedal it had a good “solid” feel. Upon driving the car up the road it became frighteningly clear that the feel was firm wooded and dead and really not very effective at all – in fact the same amount of braking effort would be available from a “Flinstone-esque’ hole in the floor and a couple of clogs on your feet!!!
The fried offside Girling calliper from My S1, the rubber dust caps had melted, the pads had degraded and fallen to bits and the piston was well and truely seized in but not beyond repair.
The Technical bit
A bit of work and reading later and I have found that the accepted minimum formula for effective brakes is that the pressure created in the master cylinder by a 100lb force on the brake pedal should be at least 1000PSI.
This pressure is created by the mechanical function of the pedal and its leverage and the action of this force on the cross sectional area of the master cylinder piston.
Both the Escort and the Spitfire pedal have a mechanical leverage of about 4.0 - 4.5:1 meaning that the 100lb force is multiplied to a 400-450lb force onto the master cylinder via the connecting rod. If you divide this by the cross sectional area of the master cylinder you have the pressure in Pounds per Square Inch PSI.
Now the S1 had a leverage of 3:1 the pedal was 9 inches from pivot to pedal and 3 inches from pivot to connecting rod pin. 9/3 = 3:1 ratio giving 300lb force on the M/C piston for a 100lb push. – The master cylinder area was .875 inch and using good old Phi r² for the area of a circle (bore) we have an area of 0.601 sq inches 300/.601 = 500PSI – Oh dear this is not 1200PSI and goes an long way in agreeing that this will not give adequate brakes.
To also confirm what I had found out on inspecting the brakes when I bought the car I found that the offside calliper pads were like blocks of charcoal and just as crumbly tying in with a comment that the owner had set fire to that calliper at one point whilst driving enthusiastically – basically it would have taken very long applications of the pedal to sloe the car from the speeds it was capable of – this would have been very similar to trying to slow the car using a handbrake/ driving with the handbrake on which makes things very, very, very hot indeed. (photo above)
Was this had shown was that on all fronts the brakes were designed very badly and as I intend to compete with the car something very drastic need to be done.
Step one was to move the pivot point of the calliper to improve the mechanical advantage – this was done by deciding that I wanted a 5:1 pedal ratio and moving the pivot accordingly to 9/5 = 1.8 inches from the pivot point instant improvement from 300 to 500 pounds on the piston – the next was to find a master cylinder with a smaller bore – Early Cortinas/Spitfires/Escorts used a 0.625” cylinder and later Capri’s and Escorts used 0.700” and 0.750” as the brake callipers got bigger – As I plan to use bigger brakes in the future and I had a good leverage I went for a 0.750” and found one on e-bay – NOS( new old stock) for £11 using this I have a calculated cylinder pressure of 1131PSI – which is pretty good and a definite improvement over the 500PSI available previously.
The reason for the bigger master cylinder is that bigger brakes with bigger callipers with bigger pistons need more fluid volume to move the piston.
As an example if I had used a 0.625” M/C the pressure would have been over 1600PSI – nice pressure but the pedal travel would be longer. I have written a small program using MS excel that does all the calcs HERE. for you should you wish to know what’s going on in your system.
Callipers are a simple device, the pressure in the system acts on a piston that in turn presses a friction material pad directly onto the spinning brake disc – the force is a function of the piston diameter/area and accordingly the pad area and the diameter of the disc. The diameter of the disc increases the swept area of the pad and also the action of the leverage to the wheel diameter.
If the calliper piston is 2.0sq” then a pressure of 1200PSI will give a force of 2,400lbs onto the pad – yep that’s a ton of force on each of the pistons for a 100lb press on the pedal – in anger you should be able to exert up to 175+lbs which equates to over 10,000lbs of force with a 5:1 and a 0.625 cylinder on an M16 calliper piston !! which is why brakes lock-up.
In my cars I have decided that following is a good way to go.
S1 V8 with modified Capri 2.8i discs on triumph hubs with M16 Callipers – M/C of 0.750 with modified Triumph pedal assy’ to give 5:1 pedal ratio.
S4 with Cortina uprights and Capri 2.8i vents with M16 callipers – a M/C of 0.700 inch on standard Ford pedal ratio of 4.6:1.
S3 with Escort Mk2 solid 9.6 inch discs and M16 callipers – M/C 0.700 inch on standard Ford Pedal Ratio of 4.6:1
Other important factors to check for when inspecting the system:
That the pedal can move to the full travel of the master cylinder and that it does not hit a bulkhead before the full travel is achieved.
That the push rod action is centred and parallel to the travel / location of the master cylinder.
That the brake lines and hoses are sound and in good condition without kinks or chafes.
The fluid is in good condition – its not expensive so change it at least every 2 years with a good DOT4 grade fluid.
That the callipers are in good order and not seized or have damaged dust covers. A sticky piston will cause overheating of the brakes.
That the discs are smooth and not warped – a warped disc will dramatically increase the pedal travel and cause vibration.
Make sure that complete system is free from leaks.
That all pads have more than the minimum recommended pad material thickness.
This is not a comprehensive list and if you do not feel confident in your ability as a mechanic to check or undertake these tasks get them carried out by someone who is.