Hydraulic theory question

[jimmystratos]

I'm struggling with the concept of the equivalent set up with tandem and dual M/Cs
A car has a tandem mastercylinder of 23mm diameter.
If you change to dual (parallel) mastercylinders of 23mm diameter, and change nothing else, what changes?
Am I right that:-
For equivalent braking, the pedal travel will be the same, but the pedal effort will be double, as you're pushing twice the area?

[Mave]

jimmystratos said:

I'm struggling with the concept of the equivalent set up with tandem and dual M/Cs
A car has a tandem mastercylinder of 23mm diameter.
If you change to dual (parallel) mastercylinders of 23mm diameter, and change nothing else, what changes?
Am I right that:-
For equivalent braking, the pedal travel will be the same, but the pedal effort will be double, as you're pushing twice the area?

? Isn't parallel essentially the same as dual from a hydraulic viewpoint, just arranged differently? For an inch of pedal travel, you displace the same amount of fluid?

[GreenV8S]

I think the normal way a tandem master cylinder works is that there is a floating piston that separates the primary and secondary circuit, so they both see equal pressure. In terms of hydraulic advantage it's just like a single piston with a 'T' piece feeding both circuits. In other words the total displacement is simply the cross section area times the travel.

If you replace this with two separate master cylinders mechanically connected in parallel the displacement is the combined cross section area times the travel, i.e. you have just doubled the displacement, so you'll need twice the pedal force and you'll get half the pedal travel for the same braking effect (other things being equal).

[jimmystratos]

GreenV8S said:

In other words the total displacement is simply the cross section area times the travel.

But since the tandem system has two compartments moving simultaneously, the displacement is twice this, is it not?

[Fume Troll]

http://www.autoshop101.com/forms/brake02.pdf

Cheers,

FT.

[GreenV8S]

jimmystratos said:

GreenV8S said:

In other words the total displacement is simply the cross section area times the travel.

But since the tandem system has two compartments moving simultaneously, the displacement is twice this, is it not?

No, I don't think so. Since the secondary (floating) piston occupies a constant volumne, the total amount of fluid displaced from the m/c is purely the area of the primary piston times its travel. In terms of hydraulic force/pressure/displacement/leverage etc, it behaves exactly the same as a single piston connected to both circuits via a T piece. The only significant difference is that the secondary piston limits the amount of fluid that can be displaced into each circuit, and once it has hit its end stop the affected circuit is effectively isolated, allowing full pressure to be applied to one circuit even if the other one has a leak.

[jimmystratos]

Yes, I understand now.
If you replace the tandem with two separate master cylinders in parallel the displacement is the combined cross section area times the travel, i.e. to displace the same volume of fluid requires half the travel. However, since the pedal force is now applied to twice the area, you have to press twice as hard on it to achieve the same psi in the hydraulic system.

So now I think the pedal travel will half, and the pedal pressure double, to achieve the same effect at the caliper.

[Mr2Mike]

Right, but since each master cylinder now only feeds two calipers/slave cylinders, the area of the MC pistons can be halved compared to a tandem cylinder which will then give simmilar pedal travel and pedal pressure.