Preamble:
This is a work in progress, doen quickly to annot Matt as he is hard of learning so any errors please let me know and Ill update it.
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Here is just a quick how to set up an Escort bias pedal box and a few explanations of why it’s done this way.
First thing you should do is have a good look at the pedal box, familiarise yourself with the parts and how they all fit, you should notice that the pedal top is not central to the two master cylinder holes; the front master cylinder should be fitted to the hole that’s nearest the pedal and there is a good reason for this; as it already has some built in bias. In an Escort we only want about 20% of the braking force at the rear and in a stripped out race car maybe even less.
Most common brake setups with disks all around use a 0.625 inch bore master cylinder on the front and a 0.7 or 0.75 on the rear (but this does depend on the actual size of the piston in the calliper or wheel cylinder too big and your brakes will be hard but with little effect, to small and you will have excessive pedal stroke but lots of power, most suppliers can advise on sizes for common callipers I will list some below soon) on an Escort Bias Box the master cylinder exerting the most pressure (smallest) will always go to the front, it’s a common misconception that the larger the master cylinder the more breaking power is achieved, it is in fact the complete opposite. Breaking power is achieved by 2 multiplications of force the first is mechanical and the second hydraulic:
The pedal is the mechanical multiplier; the master cylinder rod is much nearer the pivot point that the pedal rubber where you apply the force, if we say the ratio of the pivots is 5:1 for every 1 lb of pressure that we apply to the pedal 5 lb of pressure is applied to the master cylinder pistons.
The Bias bar is simply a mechanical divider this divides the mechanically multiplied pressure across the two master cylinders.
The differential size of the Master Cylinder Piston Area and the Calliper Piston Area is the second multiplier take the area of the 0.625 master cylinder (0.307) and the area of say a Cosworth 2wd Front Calliper (3.87) the ratio is 12.6:1 so now for every 1 lb of force we exerted on the pedal we have 63 lb of force being exerted on each brake disk (1 x 5(mechanical) x 12.6(hydraulic)). Out of interest the pressure applied to the brake disk in the same scenario using a 0.75 piston (Area = 0.44) is 44 lb that’s only 70% of the braking force !!!!!!!
You can also see the amazing forces involved too forget the bias bar for a second and say that 20 lb of the pressure you apply gets to the front master cylinder and you have 1270 lb at the disk / drums.
One other issue you may find is even after all the bias settings your still getting too much rear brake then there is a simple answer, fit an inline proportioning valve and adjust as required.
Pad Area has no measureable effect on the Pressure applied to the disk, however the pad area is important; the friction material works best between certain temperatures, too hot or too cold and they won’t perform, the smaller the pad area the hotter it will get the larger the cooler so it’s all a compromise.
Back to assembly, before bolting everything up we need to modify the front cylinder, simply chop 8mm off the end of the rod otherwise with the bias would all the way forward the rod will hit the bias bar and bind up. Your ready to assemble now grease up the balance bar and screw on the clevis pins these need to be the same distance apart on the bias bar as the distance between the centre of the two cylinders, think about it as this is quite important. Fit the cylinders remember the front one goes nearest the centre of the pedal top!
Most Common cylinders have -4 (dash 4) Fittings for the input and -3 for the outlet. Make sure there are no uphill sections in the inlet hoses where air can hide.
*Note: the area of callipers can be calculates as follows:
Single Piston on a slider (Std Sierra front and rear) = Area of One Piston
Twin Opposed Pistons (M16 Escort Std Fitment) = Area of One Piston
4 Pots (most aftermarket and Cosworth 2wd) = Area of 2 Pistons
(Cosworth 2wd Pistons = 40mm Front and Rear)
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