How does this work? Twin piston dble circuit brake caliper

[OldDuffer]

How does this work?

I'm messing about with brakes again, I'm looking at a Talbot Express camper van caliper.

Only I'm confused. It has 1 x 38mm piston + 1 x 48mm piston, each fed via separate circuit. How does that work then? It seems the 38mm is to the front of the caliper. The leading edge.
The separate circuit I get, safety et al, but why differing diameters? i've asked before but never really got a clear-cut answer. I just like to know this stuff.

My theory: The self-servo effect more prevalent in drum-shoes must also apply to a lesser extent on a pad. Minded to this, I'd be thinking the rear of the pad needs the most force, which it gets here? Else we're going to see uneven pad-wear. But that doesn't reallya dd-up. Most calipers have equal pistons and manage well enough?

The pad is symetrical. Left caliper shown. Anyone with a proper explanation?

Edited by OldDuffer on Monday 9th September 14:13

Edited by OldDuffer on Monday 9th September 14:17

[Fastdruid]

Due to the frictional force on the pad it will try and "rock", with even sized pistons this results in more wear to the leading edge of the pad.

To make the pad more effective and to even out the wear (and braking) the leading piston is of a smaller diameter. This ensures braking force is even across the entire pad.

Don't know why they are entirely different circuits however!

[Krikkit]

Fastdruid said:

Don't know why they are entirely different circuits however!

A safety feature - I'd guess there's a pot working on all the brakes in case of circuit failure, much more controllable than having the crossed-over braking that you usually get.

[OldDuffer]

FastD, yup that was my theory. And two circuits for safety. Only if uneven pad-wear is the issue, I've not seen it before. And most calipers have equal pistons - so what's the difference here?

Edited by OldDuffer on Monday 9th September 14:38

[Fastdruid]

It's very common to have different sized pistons but of course it's cheaper to have them the same size, especially if you don't particularly care about performance.

[OldDuffer]

OK, makes a bit more sense, but even so, this is a Talbot Express van from 1993. It's not exactly top-line is it? Here's a sister callper using the same casting from another Iveco/Fiat vehicle (Santana PS10 Massiff) and it's running 48mm x 2.

[The Wookie]

OldDuffer said:

OK, makes a bit more sense, but even so, this is a Talbot Express van from 1993. It's not exactly top-line is it? Here's a sister callper using the same casting from another Iveco/Fiat vehicle (Santana PS10 Massiff) and it's running 48mm x 2.

May have been marginal in one area of performance and the performance benefit of staggered pistons was the cheapest tweak to get it over the line vs going up in disc size or changing caliper casting.

Many ways of skinning a cat with brakes and being a precision engineered part of the car getting the most cost effective solution for the performance requirements can be a balancing act.

[OldDuffer]

Thank you Wookie. Even if you're wrong, and I suspect you're not far off, that's starting to sound more rational. The thing was borderline in a design parameter, this was the cheapest tweak. They had this casting in the parts bin, so they spent a week messing with various piston-sizes until...

Edited by OldDuffer on Monday 9th September 19:54