Posted by Paul (in NY) on Thursday, January 05, 2006 at 8:31PM :
In Reply to: Re: Will posted by Kaegi on Thursday, January 05, 2006 at 7:31PM :
As an illustration, think of it in this manner. In real life, most Master Cylinders are never pushed to full travel. Brakes are applied (even in a panic stop) long before the pedal is full on the floor with MC piston fully pushed.
This area where the piston never is pushed into can get
corroision and some possible rust. Pushing the piston past its normal travel can cut the seals on the piston and allow it to blow by. But thats another issue to be thought of when bleeding your brakes.
So lets consider that no matter what size bore we have on the test Master Cylinder the stroke is always 2 inches (just to grab a number).
So lets put a MC on a PW with disk brakes with a bore of .500". We can push the MC piston to full stroke of 2 inches and nothing will happen in the calipers. We have not moved enough fluid. Now, lets increase the bore to .750" and repeat the test. We may feel/see a slight amount of caliper action, but not nearly enough for stopping. Now increase the bore to 1.250", push it and we have good caliper action (good brakes), but the piston is bottomed at the end of the MC.
We now increase the bore to 1.3125" and repeat the test.
Now with the same piston travel we are pushing a lot more fluid and find good caliper action and brakes at only 1.0" of piston movement. For the final test in the illustration, increase the bore size to 2.00" and repeat the test. You may find that the slightest push on the pedal, sends you thru the window...so to speak. We have moved a huge amount of fluid to the caliper with only a very short stroke required, due to the large bore.
So it all depends on available pedal movement which results in piston stroke moving oil. The larger the piston (within reason) the shorter the stroke required for good caliper action and thus higher pedal. We MUST have a good reserve pedal ( high pedal). Because in an emergency stop, the pedal is pushed further than normal braking. If your already next to the floor on a normal hard stop, you will be ON the floor in an emergency stop and you still need more pedal movement for that emergency stop which is not available.
As a last thought, manufactures building cars/trucks with 4 wheel disk brakes run some type of proportioning or balance control for the brakes. Its a Matter of Physics that as you stop, weight is transfered forward, removing weight from the rear wheels. We know that a tire skidding is not effective in brakeing. We can not control the weight transfer forward, but we can contol the amount of brake applied to the rear wheels. This is done with a proportion valve. We limit the fluid to the rear wheels in proportion so that in a hard stop, the rear wheels dont lock up before the fronts do. In an ideal situation, under very hard brakeing all 4 wheels will be right at the point of still rotating, but not skidding. Proportion valves are adjustable and must be set up for each vehicle they are installed on. No two vehicles are the same. Instructions come with the valve for proper set up. I am running Summit Racing Proportion valves on the Bumblebee and Sweetheart. The Bandit will also get one later in build.
And vehicles with front disks and rear drums usually dont need proportion valves since the rear drums are much less effective than the front disks, lock up of the rear drums before the front disks is impossible.
Paul
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