single piston vs. 4 piston calipers

[rbohm]

reenmachine, the early kelsey-hayes calipers did have one problem though, and that was the caliper twisted and flexed during braking, causing inconsistant performance. i believe that this problem was corrected in later years though, but it still suffers from that problem to an extent. that said though, the advantages of the KH four piston calipers far outweigh its disadvantages, and are superior, imo, to a single piston caliper. the later two piston PBR design calipers are an excellent design though, and work very well in most applications.

 

[reenmachine]

reenmachine, the early kelsey-hayes calipers did have one problem though, and that was the caliper twisted and flexed during braking, causing inconsistant performance. i believe that this problem was corrected in later years though, but it still suffers from that problem to an extent.

That's also a great point to add to the archive here -- caliper rigidity. Any force wasted flexing a caliper isn't being used to slow you down.

 

[Rusty67]

That's also a great point to add to the archive here -- caliper rigidity. Any force wasted flexing a caliper isn't being used to slow you down.

Yes, and thats not only true at the caliper but anywhere in the system. Anywhere there is flex instead of fluid movement you lose braking force.

 

[Hack]

The surface area of the pads has more to do with "stopping force" than piston area does, not that piston size doesn't also affect stopping force of course. Larger pistons require less pedal effort to do the same "work" but as Hack said they require more fluid to do the same work. I put a MC on mine that holds 25 cubic inches of fluid!

I'm going to have to disagree somewhat with what you are saying here.

I think you and I mean the same thing, but a larger piston means you push with the same force as before, but your car stops faster. It's a similar effect to stepping harder on the brakes.

Yes, I believe that pad area will matter, but I would expect that doubling the force that the pads apply to the rotor would change the amount of braking much more than doubling the size of the pads. I would think that the coefficient of friction is related directly to normal force, not pad size. Doubling the pad size actually cuts the pressure applied by the pad in half. Same total force, pressure decreases.

 

[Edbert]

We do agree it is mostly semantics, some bad choices of verbage on my part up there too.. Total friction is what causes the braking not just force. One pound of pressure on one square inch of surface does not cause as much friction as one pound of pressure on two square inches of surface. Similarly one pound of force on one square inch of surface does not cause as much friction as two pounds of force on one square inch of surface.

So if the pads are the same size the larger piston displacement will be better. If piston size is the same then the larger pads will be better. The amount of foot pressure has more to do with bore size than either pad or piston size.

Then you introduce power boosters, line size, type of flex hose and it all gets mixed up

 

[iskwezm]

so is it worth putting money into my 4 piston KH calipers???

 

[Jester67]

IMHO yes it will be cheaper and they will do a fine job for all but the most demanding multi-high speed stops

 

[rbohm]

not cheaper than single piston calipers from say a 68-73 mustang, virginia classic mustang has them for $299 a pair new, and autozone, and others, have them rebuilt for about the same money. however i feel that the four piston calipers offer better, and more consistant, braking. so yes they are worth putting your money in them. i am going to upgrade the drums on both my falcon and my mustang to four piston calipers.

 

[iskwezm]

so i guess ill just rebuild the drivers for now, i need the money for other areas still

 

[Hack]

snip

So if the pads are the same size the larger piston displacement will be better. If piston size is the same then the larger pads will be better. The amount of foot pressure has more to do with bore size than either pad or piston size.

snip

(force at the brake pad)/(force at your foot) = (area of piston)/(area of M/C bore)

I'm pretty sure the relationship will look like that. A change to either of the two areas really has an equal effect.

The M/C bore can't really change too much, given that you want the pedal to travel a certain distance (so the pushrod movement is defined), you want to apply a reasonable amount of force to the pedal, and the M/C has to be capable of delivering the proper amount of brake fluid to move the caliper pistons in front and whatever you have at the rear wheels.

If it were me, I would pick the brake calipers I want and then that defines the M/C that will work. I don't understand how you're going to change the pad size. Typically the pad is just sized to fit into the caliper. I wouldn't even think about pad size, just pick the caliper piston size you want and go from there.

 

[Rusty67]

Also, don't forget the K-H brakes use an 11 inch rotor and I believe the 68-73 is a 10 inch ?

 

[BlackenedSVT]

Why not some Brembo 14" rotors with 6 piston calipers =)

 

[Hack]

Also, don't forget the K-H brakes use an 11 inch rotor and I believe the 68-73 is a 10 inch ?

That makes a big difference.

 

[Rusty67]

Why not some Brembo 14" rotors with 6 piston calipers =)

As long as you're buying I'll take the rear kit as well ;-P

 

[degins]

Also, don't forget the K-H brakes use an 11 inch rotor and I believe the 68-73 is a 10 inch ?

The 68-73 rotors are 11.25", Granada are 11.06".

As I said before, all other things being equal, the cross sectional area of the caliper piston(s) is the determining factor in applied force. As Reen said so well, it doesn't matter if there is a single or 2 opposed pistons. This assumes that there is only a small inefficiency in the float or sliding mechanism, and that the float or slide of the caliper works per design.