Piston area: floating vs fixed

[vette427sbc]

Not corvette related (unfortunately!) But I am trying to adapt a bigger caliper to my daily driver, and Im curious about differences in piston area.
I need to retain the stock master cylinder, and I want to maintain the designed F/R bias for the ABS/traction control.

Stock caliper is a floating 2 piston design with 43.9mm pistons. Total piston area comes out to 3027.25mm^2

The caliper I am hoping to use is a fixed 8 piston with (4) 31.9mm and (4) 27.9mm pistons. Total piston area comes out to 5642.32mm^2

Am I right in thinking that I will have a similar pedal feel since the pistons in the fixed caliper will only have to travel ~half as far to achieve similar clamping force?

Picture because everyone loves cool hardware
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[phantomjock]

Chris - I won't offer a guess -- sorry. But I will offer a "LINK."

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It it to Jake Latham's Dynamic Weight Transfer Calculator, Torque Estimator, and Brake-Thingie. It has been down for a number of years - but I fired up the WayBack Machine.


I've a number of simpler spreadsheets - but this one is readily available. Its a Java Script and could be "ripped" and saved (Jake said that's OK too.)
I've only input a few values and it all seems to calculate.

Something to do on a wet cold day, anyway.

Cheers - Jim

Oh Yeah - Radial mounts too -- COOL.

[BBShark]

I want to say of course they are the same. Work=Force x Distance. But then there is that voice in the back of my head that makes me question, in this particular example, whether that's right.

So working backwards from the brake pad, whats the unit area of the pad? Both styles of calipers are the ~same. That means the force for the floating caliper is the same as a fixed. My money, fixed caliper needs 2X the area of the floater.





I think

[vette427sbc]

Originally Posted by phantomjock

Chris - I won't offer a guess -- sorry. But I will offer a "LINK."

[Only registered and activated users can see links. Click Here To Register...]

It it to Jake Latham's Dynamic Weight Transfer Calculator, Torque Estimator, and Brake-Thingie. It has been down for a number of years - but I fired up the WayBack Machine.


I've a number of simpler spreadsheets - but this one is readily available. Its a Java Script and could be "ripped" and saved (Jake said that's OK too.)
I've only input a few values and it all seems to calculate.

Something to do on a wet cold day, anyway.

Cheers - Jim

Oh Yeah - Radial mounts too -- COOL.

Jim- Thats a great link... Ill definitely be playing with that tonight, Thanks!
Radial mount 8 pots, supercharged 5.5L, AWD, 6600lbs Its a fun vehicle, just needs the brakes to match

Originally Posted by BBShark

I want to say of course they are the same. Work=Force x Distance. But then there is that voice in the back of my head that makes me question, in this particular example, whether that's right.

So working backwards from the brake pad, whats the unit area of the pad? Both styles of calipers are the ~same. That means the force for the floating caliper is the same as a fixed. My money, fixed caliper needs 2X the area of the floater.





I think

Im still attempting to wrap my head around this as well... The one thing that Im not sure how to "input" into my head is that technically the pistons are traveling half that of a floating caliper. The piston on a floating caliper must take up the pad gap on the front and backside of the rotor, while the piston on a fixed caliper only takes up the pad gap on that side.

Someone on another forum posted this link regarding something else, but theres some mention of piston area and clamping force in here. Its a good read.
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As far as pad/swept area, rotor size and pad coef, Im leaving those calcs out until I can figure out the piston area.
My napkin math says that if you only take half of the fixed caliper area, the 8 pot calipers will be 7% undersized, and deliver a 14% stiffer pedal, all other things equal. I am upsizing the rotors 10% (315mm to 350mm) so I dont know where that number would come into play. It looks like if I start messing with Jims link I can get some kind of numbers to crunch. Unfortunately, Im working off of guesses for master size, booster efficiency and pedal ratio.

[rtj]

I did a search as this is interesting, some discussion here:



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[69427]

Originally Posted by BBShark

I want to say of course they are the same. Work=Force x Distance. But then there is that voice in the back of my head that makes me question, in this particular example, whether that's right.

So working backwards from the brake pad, whats the unit area of the pad? Both styles of calipers are the ~same. That means the force for the floating caliper is the same as a fixed. My money, fixed caliper needs 2X the area of the floater.





I think

I agree.

In a fixed caliper, only one end of the caliper "cylinder" is doing any pressurized work (the piston), while on a floating caliper both the piston and the cylinder opposite wall are doing pressurized movable work.

[phantomjock]

Chris -

Here is a formula for CLAMP LOAD that provides some insight:

The clamping load is assumed to act on all friction surfaces equally. For dry disc brakes it doesn’t matter whether the brake is of the sliding type or opposed piston. Newton’s Third Law state every force has an equal and opposite reaction and a reaction force from a sliding caliper is the same as an opposed piston one.

emphasis mine...

For more details and other calculations the page is:[Only registered and activated users can see links. Click Here To Register...]

Cheers - Jim

[vette427sbc]

It looks as if we’re correct in the doubling of the area... I used Jim’s link to the calculator (which is a fantastic tool btw... if you’re saying the owner of that website gave the ok to rip the JavaScript or whatever it is, that should be saved in the downloads section) to come up with a few comparisons...
I do not have the brake pedal ratio, master cylinder size, or brake booster assist.
I’m going to input a 6:1 pedal ratio because that’s a common manual brake ratio and I am familiar with leg effort numbers here. This will also be so I can leave the booster assist numbers out of the equation. There is a 27 stamped on the master, which I am going to assume is the bore size in mm. It falls inline with other power brake master cylinder bore sizes. Master bore size, pedal ratio, brake pad coefficient of friction, and brake pad height are constants in these calcs, so we can get a fair assessment of what will change with just changing the caliper and rotor.

50lb input on the brake pedal, with stock caliper and rotor (315mm): 917 lb/ft torque
50lb input on the brake pedal with the 8 piston and stock size rotor: 853 lb/ft torque
50lb input on the brake pedal with the 8 piston and 350mm rotor: 964 lb/ft torque
47.5lb input on the pedal with 8 piston and 350mm rotor: 916 lb/ft torque

So forget the torque numbers as a rating, but use them as a comparison. On paper, a mildly stiffer brake pedal would net slightly better braking tq with only a slight change in bias front to rear.

[phantomjock]

Just put a Zip version in Downloads. Once it proves out it should be loaded. Lets you bypass the Wayback Machine. Should just unzip, the when you click on it - should open in your browser. And start calculating...

Here's hoping! just like this one -- when I have some time I may add a header to it. We'll see.

Cheers - Jim

(just checked this one and it goes!) Newest version is loaded in next post...

[BBShark]

Thanks for the heads up. Just approved it.

[phantomjock]

Thanks - I may have pulled the trigger too soon. Just cleaned it up a bit. I"ll reload here and in downloads too. Sorry for the extra work.

They both work - the header on this one is cleaner and gives Jake credit.