Hi all

My brake set up is very similar (if not the same) as a Westfield - 4 pot front calipers (Caterham - 1" I think) and Sierra rear calipers.

I have been messing around with master cyls. for a while now since removing the servo. I tried 0.75 running a single circuit and that was rock solid so i went down to a 0.7 which is OK but a little wooden. pedal ratio is around 41/2 : 1

I want to end up with dual circuits if I can but dont want to make up a pedal box so am looking at tandem master cylinders.

The thing I dont understand is the Westy tandem master is 0.812 which in my set up would be rock solid.

Does the fact that the master is running in effect separate chambers mean that each circuit needs more fluid to get it to work. I am missing a link in the logic here somewhere.

Help greatly appreciated

Thanks
Gavin

No, you're right; A tandem cylinder would feel the same as a single of the same diameter. The larger esty one would feel harder. Are you used to a non-servo brake? They always feel hard compared to a modern car.

However a single system is illegal AND dangerous! You need either a twin cylinder (+bias bar) system or a tandem one.

With twin cylinders 0.625" front and 0,7 rear cylinders are common. For a tandem master, swapping to the cylinder from either an early non-servo Fiesta or Fiat cylinder is often used to give better pedal feel (sorry, don't know what diameter they are).

I had to re-read your problem and re-jig the answer I was going to give, I think it boils down to 3 things
(1) I think you need to consider the what pad friction material you are using --- it sounds as if it is much too hard
(2) Change to a Fiat 124 master cylinder which is 3/4" and very cheap from Brakes Int.
(3) With non-servo disc brakes you control the stopping with pedal pressure rather than pedal travel as you do in a modern tintop, if you have never used a non-servo system before it takes a bit a of mental adjustment.


I would also check the bore of the front callper pistons as 1" bore 4 pot gives a half effective hydraulic area of a Sierra or Cortina/Escort calliper. On a 4 pot calliper each piston needs to be 1.42" to give the equivalent effective hydraulic area to the standard Cortina or Sierra calliper

British Trident

Interesting point about the set up - thanks

I reckon that the Sierra caliper is around 1 1/2" so I divided it by two as there is only one cyl to represent a conventional caliper. I equated this to 4 x 1" front and 2 x 3/4" at the rear.

Is there a better comparator?

Thanks too to adithorp. I'll look into the masters you suggest.

I wonder if I need a little more control on the pressure reaching the rear?

Thanks
Gavin

You don't divide by two ---- A combination of hydraulic principals and Newton's laws prove the reaction force is provided by the hydraulic area rear face of the Sierra calliper casting

Sierra callipers are either 54mm or 60mm bore Taking the smaller size each calliper has an effective hydraulic area of 2300 mm^2

In 4 pot calliper only the area of the pistons on one side of the calliper count so each piston needs to have an area of 1150 mm^2 which if you work it out requires 38mm dia for each piston.

Hi all

Done some re checking and realise that the background I provided is wrong - oops. Sorry.

The Sierra rear caliper (slider type) is 48mm diameter and the front 4 pots are 38mm diameter.

I fitted a restrictor to the rear circuit to stop the back end from going live because of too much rear braking and this has helped a lot - but with the 0.7 master (still single for the moment until I sort this out) the pedal travel is fine but even with the hardest push I can muster, I cant get the fronts to lock up and I think that with such a push, I should be able to.

If I went down to a 0.625 master, I know that the pedal travel will increase but would this have the effect of requiring more or less of a push?

Thanks
Gavin

What pad material have you got ?

Ferodo 2500