The clutch or brake pedal of a car is an example of either a first or a second class mechanical lever depending upon the design of the pedal.
The pedal ratio or the mechanical advantage is the ratio of the output force (Fo) to the input force (Fi) or the ratio of input lever length (X) and output lever length (Y):
Pedal ratio or mechanical advantage= Fo / Fi = X/Y…………………………….eq.1
What is the importance of the ratio for automotive braking system design calculation?
We can conclude, from the eq.1 above, that increase in the ratio means:
Typically, the pedal ratio lies between 4:1 to 6:1. Power brake requires lower ratio and manual hydraulic system require higher ratio.
What is a variable ratio pedal?
As the name suggests, in case of a variable ratio, the brake (or clutch) pedal ratio changes as the driver presses the brake padel. How? See the below figure of a VR pedal:
The output pivot is no more a fixed one, rather it is at the end of the link-2, the other end of the link-2 follows the slot. As a result, with the depression of the pedal, the output pivot point comes closer to the main pivot and thus the length of the output arm (Y) reduces and causes the overall leverage ratio to increase.
What it means to the driver is: at the end of the pedal stroke the driver needs to press the pedal less harder which in-turns will increase the drivers operational comfort because towards the end of the pedal stroke, normally, the braking system (or clutch system) demands more force.