All very confusing, but it seems line pressure for a given pedal pressure, is inversely proportional to the surface area of the master cylinder piston. Bigger piston equals less pressure, but less pedal travel.
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It’s all about pressure, Kevin - not pedal/piston travel…
As pedal is released the pistons retracts by seal elasticity - basically within runout spec; 0,002". At the initial pedal application the pistons moves into contact with the discs - requiring fluid volume; ‘total piston area X 0,002’ delivered by the master cylinder. Pedal moves accordingly to push this volume to the calipers from the ms - with excessive disc runout the pedal travel increases. Theoretically, there shouldn’t be any additional pedal movement as pedal pressure is increased - but in practice there is some ‘give’ in the system…
With pedal force of 10 lbs applied to the ms, with has a square are of roughly 0,6 square inches; system pressure is then about 17 psi which is multiplied by piston area top give the force applied to each brake pad. Note that the force on the actual pedal is modified by the lever action of the linkages to the ms. The booster acts directly on the ms, and maximum booster force is in the vicinity of 500+ pounds, giving a ms pressure of some 1000 psi - in addition to actual pedal pressure. All multiplied by piston area - all of which explains why braking the car without booster assist is an effort…
As an aside; braking effect is limited by the road-to-rubber friction of the tyres - exceeding that and the wheels slip. Which is why fore-and-aft balance is important; under heavy braking some weight shifts forward, giving more front grip and allowing more braking on the front wheels before lock-up. However, under more normal braking; an even distribution of force is more desirable, both axles bearing a fair share of the work. And with the car fully laden it is important that the rear axle is not ‘underbraked’ - it will impair overall braking…
Frank
xj6 85 Sov Europe (UK/NZ)
Frank
xj6 85 Sov Europe (UK/NZ)
