Brake theory question for you all

[Skip Fix]

OK if we look at caliper piston size for systems for a fixed we only use 1/2 the pistons vs a floating caliper we use all(to me seems both sides of a fixed squeeze, as much as a floating does.) why are most of the upgrades a fixed caliper like later Z06s, Porsche,CTSV,new Camaros/Mustangs, etc etc?

[RobNoLimit]

It's kind of difficult to explain with all of the hydrolic equations, but the total clamping force can simply be based on one side of the rotor centerline. Even though a 4 or 6 piston caliper has pistons on both sides of the rotor, you only get to count the piston area on one side.

[Skip Fix]

So a PBR sliding caliper with two 43mm pistons would then be an upgrade for a fixed CTS-V Brembo with a 36mm and 40mm pistons since we only count 2?

[SLO_Z28]

why are most of the upgrades a fixed caliper like later Z06s, Porsche,CTSV,new Camaros/Mustangs, etc etc?

Style, not function.


So a PBR sliding caliper with two 43mm pistons would then be an upgrade for a fixed CTS-V Brembo with a 36mm and 40mm pistons since we only count 2?

There are several things to look at, first and most important is the pads being used, the material and size matters a lot. Second is the caliper strength and contrary to popular belief monoblock calipers aren't necessarily strong and lose rigidity the hotter they get. Third is the weight, as this is all sprung weight, and the rotor is reciprocal mass. Remember that mass isn't a measurement of weight(as that id dependent on gravity) but is a measurement of inertia. The more mass you have moving farther on an axis the force generated can become rather large, and without getting into a lot of math the larger rotors not only reduce your horsepower on drive axles, they slow turn in, make the tires less responsive, and reduce the tires overall grip. The point of all this is you want the absolute lightest rotor you can use, with an appropriate pad in both compound and surface area. The master cylinder will be determined by the piston surface area of the calipers, the pedal ratio, and the drivers desired feel.

[jpgolf14]

Fixed calipers have a few advantages over sliding calipers. First, in a sliding caliper, fluid pressure is used to slide the outside of the caliper. I have heard up to 100psi, but a quick hand calc makes me think that is a bit high. Either way your first few psi are used to move the caliper instead of squeezing the rotor, this will also slightly increase your pedal travel. Fixed calipers may be considered to be slightly more reliable as the slider pins can become stuck, bent, etc.... The only moving parts are the pistons in a fixed caliper. Finally fixed calipers tend to be easier to change pads, as you typically remove the bridge and the pads come out, no need to remove the caliper like most sliding caliper. This may sound minor, but it sure makes changing pads at the track much easier.

The disadvantage of fixed calipers is they are more expensive and require greater wheel clearance.

At the end of the day, there is nothing wrong with a quality sliding caliper. Probably the biggest benefit of all is the fixed calipers look cooler.

Now the next question was regarding piston area. You seem to associate larger piston area with better. That is a common misconception. It really depends on the car and the driver. Larger pistons will create a larger force on the rotor (for the same fluid pressure), but at the expense of a longer, softer pedal. Personally I like a short firm brake pedal. The real beauty of the make of the kits you mention is the larger diameter rotor. The larger rotor provides "free" brake efficiency. You get a higher brake torque without changing the length or feel of the pedal. The downside of a larger rotor is it also typically has a larger moment of inertia which as SLO_Z28 has pointed out, has its own set of disadvantages. Like anything on a car, it is a compromise.

Now take a look at the chart below. This is a comparison of brake kits commonly installed on 3rd Gen (82-92) F bodies. If you look at the last two columns, you can see I have included the change in brake pedal effort and travel of each system compared to stock for the same deceleration rate. Now, GM made a special version of those cars called 1LE for factory stock racing. The major upgrade to those cars was a change from a 1 piston 10.5" brake system to a 2 piston 12" brake system. If you look at the last two columns you will find that the 1LE brakes are actually less efficient than stock!!! Yet most people consider them to be a significant upgrade to stock. Remember, its all about driver feel. Again, I prefer a shorter stiffer pedal.

https://static1.pt-content.com/images/pt/2012/07/7548242854_1ccb423c17_h-1.jpg

[ace_xp2]

The design issue with most sliding calipers is that normally the bridge section is a "C" directly over the rotor. As opposed to modern fixed designs which employ more of an "O" shape so as to reduce the bending forces acting on its bridges. Sufficient bridge rigidity for a sliding caliper typically comes through an increase in bridge height which means less rotor in a given rim.

[Skip Fix]

I agree you need a balanced system but keeping the PSI the same larger pistons with the same pad size and coeff and rotor can allow more torque to be applied. WIlwood used to have charts for all their caliper and the piston sizes with pressure applied-bigger pistons more available pressure at the pad.

Larger rotors are heavier but do give more lever arm length for the caliper to work on stopping the inertia of the wheel as well as more swept area for cooling-why the 1LE system was probably used for the cars GM set up for track use.

Also figure the stock second gen floating front caliper was a 2 15/16" cast iron-so an aluminum 2 piston small piston sq in of a Baer kit really an upgrade as they would advertise?

My biggest question was why the fixed are "supposed" to be so much an upgrade over a floating design when you really can't use all the pistons in the equation. GM orignally used a 4 piston fixed caliper on the 67 Chevelles/GTO and early Camaros yet dumped it for a floating caliper and the same vehicles the next year and up. Maybe the advantage is looks and advertising like the original Baer kits.

Nice chart thanks for it.

[jpgolf14]

I agree you need a balanced system but keeping the PSI the same larger pistons with the same pad size and coeff and rotor can allow more torque to be applied. WIlwood used to have charts for all their caliper and the piston sizes with pressure applied-bigger pistons more available pressure at the pad.

Larger rotors are heavier but do give more lever arm length for the caliper to work on stopping the inertia of the wheel as well as more swept area for cooling-why the 1LE system was probably used for the cars GM set up for track use.

Also figure the stock second gen floating front caliper was a 2 15/16" cast iron-so an aluminum 2 piston small piston sq in of a Baer kit really an upgrade as they would advertise?

My biggest question was why the fixed are "supposed" to be so much an upgrade over a floating design when you really can't use all the pistons in the equation. GM orignally used a 4 piston fixed caliper on the 67 Chevelles/GTO and early Camaros yet dumped it for a floating caliper and the same vehicles the next year and up. Maybe the advantage is looks and advertising like the original Baer kits.

Nice chart thanks for it.

Not being able to use all the pistons in the equation has nothing to do with the performance of the braking system. In fact if by some weird science you did, those calipers would be awful. We already discussed why they are better.

Fixed calipers have a few advantages over sliding calipers. First, in a sliding caliper, fluid pressure is used to slide the outside of the caliper. I have heard up to 100psi, but a quick hand calc makes me think that is a bit high. Either way your first few psi are used to move the caliper instead of squeezing the rotor, this will also slightly increase your pedal travel. Fixed calipers may be considered to be slightly more reliable as the slider pins can become stuck, bent, etc.... The only moving parts are the pistons in a fixed caliper. Finally fixed calipers tend to be easier to change pads, as you typically remove the bridge and the pads come out, no need to remove the caliper like most sliding caliper. This may sound minor, but it sure makes changing pads at the track much easier.

What more are you looking for?

[Skip Fix]

I understood all that-no problems and understand the advantages and disadvantages of both styles. I don't understand why they are always pushed as an "upgrade". GM obviosuly didn't think they were going backwards on the A and F bodies going to a floating caliper from fixed-or going from a fixed caliper on C2/3 Vettes to C4-6 floating calipers, yet look at all the "upgrade" kits to add fixed caliper Wilwood,Aerospace,CTS-V, Camaro calipers on cars with a decent floating caliper system already. To me an upgrade should perform better shorted stopping distances, better pad size or clamping force, bigger rotors to have better lever arm and heat dissipation.

Yes I like changing pads easily on a fixed, and no I have never seen a bent caliper pin/bolt on a floating, never seen a bent pad on a big single piston floating vs a twin piston floating. The only slightly stuck pin/bolt wee on off road driven trucks with little maintenance to clean off the sludge.

Still don't understand why on a fixed if you have pressure in pistons on both sides on a fixed. Rotors can't move so how are the pistons on both sides not exerting clamping in the equation with pressure applied? I'd bet it you took an old Wilwood Dynalite with the tube connecting each side of pistons and just used one side at a time each side individually you would still get decent clamping.

Wilwood and Howe have had lighter aluminum GM style calipers for a long time used in the circle track for lightness, but most guys there will admit a cast iron factory caliper has way less flex. Heck multi pistons in a fixed caliper have more to leak or get stuck(how SSBC started doing C2-C3 SS sleeves in calipers)

[jpgolf14]

I think you are getting to caught up in fixed vs sliding. Instead you should be looking at the mechanics of the system.

I don't understand why they are always pushed as an "upgrade".

How do you sell brakes kits if you don't sell things as an upgrade? The concept of fixed calipers is an upgrade over the concept of a floating caliper. But like everything in life, the details are what make a setup good or not.

GM obviously didn't think they were going backwards on the A and F bodies going to a floating caliper from fixed-or going from a fixed caliper on C2/3 Vettes to C4-6 floating calipers

I don't pretend to have any knowledge of those specific setups. However GM is in the business to make money. Fixed calipers are more expensive than sliding. I think that is your explanation right there.

yet look at all the "upgrade" kits to add fixed caliper Wilwood,Aerospace,CTS-V, Camaro calipers on cars with a decent floating caliper system already.

The price of machining has come way down in the last 10+ years. It is now much cheaper to machine calipers or anything else than it used to be. If you can get all the benefits of a fixed caliper with not very much extra cost, why not do it? On top of that, the OEM's have lots of resources for designing and testing calipers. The aftermarket caliper makers probably do not. At the end of the day, a fixed caliper is much easier to design than a sliding caliper. Why make it more complicated? Fixed calipers are amazingly simple. A mechanical pencil is a more complicated device.


To me an upgrade should perform better shorted stopping distances, better pad size or clamping force, bigger rotors to have better lever arm and heat dissipation.

Then you should start looking at all the factors that really matter. Pick a kit based on your requirements. I think you will find that all the good options, with perhaps the exception of the C5/C6 setups, will have fixed calipers. There are no good sliding calipers out there designed for a 14"+ rotor. If there were, you would probably see some aftermarket kits out there.


Still don't understand why on a fixed if you have pressure in pistons on both sides on a fixed. Rotors can't move so how are the pistons on both sides not exerting clamping in the equation with pressure applied?

It is Newton's third law. Every action must have an equal and opposite reaction. For a sliding caliper, the pistons on the inside of the rotor push on rotor which causes the outside slider mechanism to move. The outside pad comes in contact with the rotor and reacts the force produced by the pistons. The force is equal to the fluid pressure * piston area. The reacted force is equal to the piston force. The sum of the forces on the rotor in the piston direction is zero.

Now take a fixed caliper. As fluid pressure rises, all the pistons on both sides of the rotor move equally. As they contact the rotor, they are pushing on the rotor from both sides equally. The force from the outside pistons is equal to the inside pistons. No different than the sliding caliper. The sum of the forces on the rotor in the piston direction is zero.

The pedal travel is also the same b/c the fluid requirements are the same. Lets say the pads ride 0.005" off the rotor face. In a sliding caliper, the pistons move the first 0.005" to close the gap between the inside pad and the rotor, but the 0.005" gap still exists between the outside pad and rotor. So the pistons need to extend another 0.005" to force the slider mechanism move 0.005" to close the gap between the outside pad and rotor.

In a fixed caliper, the pistons on each side only need to move 0.005" to close the gap on each side. So you have twice as many pistons, but they are only extending half the distance. So the fluid requirements are the same.


I'd bet it you took an old Wilwood Dynalite with the tube connecting each side of pistons and just used one side at a time each side individually you would still get decent clamping.

If you make enough assumptions about the rigidity of the components, sure, you would still have full brake force. But that load has to be reacted somewhere. You are talking about ~5000lbs of load. How do you think the rotor would like being pushed by 5000lbs. The rotor hats would likely bend. What about the caliper body and lugs? What about your brake bracket? What about your spindle, struts, outer wheel bearing, wheel, spindle snout. The whole point of having pads and reacted loads is to alleviate the load from somewhere else. Without the reaction, you put a whole bunch of stuff in bending that was never designed for it.


Wilwood and Howe have had lighter aluminum GM style calipers for a long time used in the circle track for lightness, but most guys there will admit a cast iron factory caliper has way less flex. Heck multi pistons in a fixed caliper have more to leak or get stuck(how SSBC started doing C2-C3 SS sleeves in calipers)

Caliper flex affects a few things including maximum brake force and pedal feel. If you have locked up the tires, you have exceeded the required brake force. Brake systems produce way more brake torque than required. So losing some to caliper flex is no big deal. Now the real down side of caliper flex is it makes your brake pedal longer and harder. Not good.

I really think you need to get over the assumption that more piston area is always good. It is only good if you want a longer softer pedal. Most GM cars already have a long pedal, the last thing I want is to make it even longer.

[Skip Fix]

I understand the unidirectional force vectors of a fixed from one side flexing the rotor,bracketr etc etc. But you stil are having pressure applied to the rotor surface based on line PSI and piston sq in. Now add another set on the other side to offset the unidirectional forces/flex that also are applying force in a similar manner but do not get figured into the equation.

If you want a hard short pedal sit in the Camaro with a 1.0325 MC, 6:1 pedal, 2.5" rear metric GM calipers and 1.75 Dynalites on the front!

I think larger pistons with an adequate larger MC IF they also have a larger pad can be a good thing-other wise why do I need those 6 piston calipers:)

[jpgolf14]

I understand the unidirectional force vectors of a fixed from one side flexing the rotor,bracketr etc etc. But you stil are having pressure applied to the rotor surface based on line PSI and piston sq in. Now add another set on the other side to offset the unidirectional forces/flex that also are applying force in a similar manner but do not get figured into the equation.

Because without the reactive force there is nothing to support the rotor on the opposite side. Imagine you are 200lbs and standing on a concrete pad. When you stand on the pad, you don't sink in right? That is because the pad is supporting your weight. It is pushing up on the bottom of your feet the same 200lbs that you are pushing down. Does that mean that there is really 400lbs of force between you and the pad? NO. If you don't buy that, you need to take it up with Newton.

If you want a hard short pedal sit in the Camaro with a 1.0325 MC, 6:1 pedal, 2.5" rear metric GM calipers and 1.75 Dynalites on the front!

I think larger pistons with an adequate larger MC IF they also have a larger pad can be a good thing-other wise why do I need those 6 piston calipers

You keep forgetting, there is nothing magic about 6 piston calipers. Its the piston area that matters. For example, my 6 piston CTS-V calipers are giant, 13" long. But they actually have a smaller piston area than my OEM 1 piston caliper. Did you bother to look at the chart I posted above? You will find that the OEM 1 piston caliper has a greater piston area than every single other caliper on the list.

Also, a larger MC will produce even less pressure. If you want to increase the fluid pressure, you need to decrease the master cylinder size. As long as the ration between MC area and caliper piston area remain the same, the force at the pad will remain the same. 1 sq in MC teamed with 4 sq in calipers will be the same as a 2 sq in MC teamed with an 8 sq in caliper.

Basically stopping the car goes like this:

Lets assume a maximum effort stop where you are pressing on the brake pedal with 100lbs of force. Using a 6:1 pedal ratio, that force is up to 600lbs at the pushrod for the vacuum booster. IIRC vacuum boosters give about 3:1 assist. Now you have 600*3 = 1800lbs of force on the master cylinder push rod. Assuming the MC has a 1.25" bore, that is 1.227 sq in. Fluid pressure is measured in PSI, pounds per square inch. 1800/1.227 = 1467psi. Pressure is constant in a closed system, so 1467psi at the mc is 1467psi at the caliper. Now assume we are using 4 piston calipers with 1.75" pistons. Caliper piston area is 2*(1.75/2)^2*3.14 = 4.8 sq in. Now the fluid pressure is evenly distributed on the pistons. The total force on the pistons is 4.8sq in*1467psi = 7041lbs. That force is transmitted through the pad and is applied to the rotor. But there are losses in that transfer due to sliding. Lets assume the coefficient of friction between the pad and rotor is 0.4, a common value for street pads at low temperature. So the force on the rotor is 7041*0.4 = 2817lbs. Now that is acting on a moment arm from the center of the hub to basically the center of the pad. Lets assume a 14" rotor which would net a moment arm of about 6". So the moment at the center of the hub would be 2817 * 6 = 16900 in lb or 1400 ft lbs. That 16900 in lbs acts on the tread of the tire. Lets assume a 26" tire, so the moment arm is 13". The force on the tread would be 16900 / 13 = 1300lbs. So what who cares. Lets assume this is a heavy pro touring car with grippy tires. On a max effort deceleration a lot of weight transfers forward. So lets say during the deceleration the weight on the tire is 1250lbs. If we assume the grippy tires have a coefficient of friction of 1.0 between the tire and road, that means we can tolerate 1250*1.0 = 1250lbs of tread force. So based on all these calculations, we are using 1300/1250 = 104% of the available grip. In other words the tire will lock up.

[ace_xp2]

Generally the six piston gain is reduction in pad taper as the clamping load may be more even. Some companies (most?) even use the same pad between the two.

[jpgolf14]

Generally the six piston gain is reduction in pad taper as the clamping load may be more even. Some companies (most?) even use the same pad between the two.

That is true. Although a small detail with respect to the basics we are working on here.

[Skip Fix]

"You keep forgetting, there is nothing magic about 6 piston calipers. Its the piston area that matters. For example, my 6 piston CTS-V calipers are giant, 13" long. But they actually have a smaller piston area than my OEM 1 piston caliper. Did you bother to look at the chart I posted above? You will find that the OEM 1 piston caliper has a greater piston area than every single other caliper on the list."

I understand that- but the folks pushing "the magic" and those buying the "upgrade" do not is my main point here,especially if the bigger piston was designed as a system. Baer when they first started advertized their 2 piston PBR as an upgrade to the single piston GM setup-smaller piston diameter and smaller pad. Wilwood's 10.75 dynalite for 30 years has been pushed as an upgrade top the Camaro single piston factory setup-other than a lighter hub/rotor it has a WAY smaller pad, slightly smaller rotor and smaller piston size.

And I understand the argument for better more even pad clamping with more pistons. More theoretical than real just liek the bent single piston pads?

"Because without the reactive force there is nothing to support the rotor on the opposite side. Imagine you are 200lbs and standing on a concrete pad. When you stand on the pad, you don't sink in right? That is because the pad is supporting your weight. It is pushing up on the bottom of your feet the same 200lbs that you are pushing down. Does that mean that there is really 400lbs of force between you and the pad? NO. If you don't buy that, you need to take it up with Newton" But my weight does compress the pad into the soil somewhat causing pressure on the underneath side(a delta psi on the soil different than if I am not on it-compare this to the flex in brackets) if I do have another 200 lbs of pressure on the bottom side(like another set of pistons) the soil gets not compression(flex in the brackets).


"Also, a larger MC will produce even less pressure. If you want to increase the fluid pressure, you need to decrease the master cylinder size. As long as the ration between MC area and caliper piston area remain the same, the force at the pad will remain the same. 1 sq in MC teamed with 4 sq in calipers will be the same as a 2 sq in MC teamed with an 8 sq in caliper." Why this was added "I think larger pistons with an adequate larger MC IF they also have a larger pad can be a good thing".


I have some concept of hydraulic fluid dynamics,pressures, piston diameter generating pressure(has been a few years since college physics) just still can't quite understand why we drop half the fixed calipers pistons out of the equation since they are adding some pressure/force to the pad/rotor action. I would think with less caliper flex than the back non piston side of a floating caliper of reasonable weight.

The Camaro project is set up as a drag car also using some extra factory rear discs I have so not the ideal balance but with larger rear tires should be adequate, probably lock the skinny fronts up even without the power assits added in, and is a manual brake setup. I think the power assist have less than 6:1 ratios since mine was moved up for the manual setup.

To add second gen specs to your chart the 79-81 TA WS6 rear caliper had a 2.5" caliper piston and a 154 pad like the metric fronts GM used, 11.15"diam./min .90" thick rotor if you want to add that to your chart to round it out-again nice chart. Also most second gens front caliper was 2 15/16" and if you swap to the larger D61 Caprice B body pads you get even more pad size than the D52 .

[jpgolf14]

"You keep forgetting, there is nothing magic about 6 piston calipers. Its the piston area that matters. For example, my 6 piston CTS-V calipers are giant, 13" long. But they actually have a smaller piston area than my OEM 1 piston caliper. Did you bother to look at the chart I posted above? You will find that the OEM 1 piston caliper has a greater piston area than every single other caliper on the list."

I understand that- but the folks pushing "the magic" and those buying the "upgrade" do not is my main point here,especially if the bigger piston was designed as a system. Baer when they first started advertized their 2 piston PBR as an upgrade to the single piston GM setup-smaller piston diameter and smaller pad. Wilwood's 10.75 dynalite for 30 years has been pushed as an upgrade top the Camaro single piston factory setup-other than a lighter hub/rotor it has a WAY smaller pad, slightly smaller rotor and smaller piston size.

Don't know what to tell you. The Dynalite has no business on the front of any sports car over 3000lbs IMO. Very small caliper and pad. Just so you know the pad volume is not a major contributor to the performance of the brake system. It mainly determines how long the pad will last. And of course a larger pad can absorb a little more heat, but that is not the pad's job.

Regarding the Baer's. Remember small pistons is not a bad thing. Many people prefer the shorter firmer pedal, myself included. I would much prefer the feel of that setup to the OEM.

And I understand the argument for better more even pad clamping with more pistons. More theoretical than real just liek the bent single piston pads?

If you are pushing the car to the limits it may make a difference. To be honest though, good caliper design plays a larger part in pad wear than the fact there are 6 pistons. In reality a well designed 4 piston will be just as good.

"Because without the reactive force there is nothing to support the rotor on the opposite side. Imagine you are 200lbs and standing on a concrete pad. When you stand on the pad, you don't sink in right? That is because the pad is supporting your weight. It is pushing up on the bottom of your feet the same 200lbs that you are pushing down. Does that mean that there is really 400lbs of force between you and the pad? NO. If you don't buy that, you need to take it up with Newton" But my weight does compress the pad into the soil somewhat causing pressure on the underneath side(a delta psi on the soil different than if I am not on it-compare this to the flex in brackets) if I do have another 200 lbs of pressure on the bottom side(like another set of pistons) the soil gets not compression(flex in the brackets).

You can only consider one body at a time. Maybe drawing a free body diagram would help, but I don't have the energy to discuss this further.

The Camaro project is set up as a drag car also using some extra factory rear discs I have so not the ideal balance but with larger rear tires should be adequate, probably lock the skinny fronts up even without the power assits added in, and is a manual brake setup. I think the power assist have less than 6:1 ratios since mine was moved up for the manual setup.

If its a drag setup, run whatever brakes you want. They will stop the car one time. Why are we having this discussion for a drag car?


To add second gen specs to your chart the 79-81 TA WS6 rear caliper had a 2.5" caliper piston and a 154 pad like the metric fronts GM used, 11.15"diam./min .90" thick rotor if you want to add that to your chart to round it out-again nice chart. Also most second gens front caliper was 2 15/16" and if you swap to the larger D61 Caprice B body pads you get even more pad size than the D52 .

Will do.

[Skip Fix]

My drag car is not the center of this discussion I started-fixed vs floating and piston diameter was the main topic. It was just an example of a OEM large piston caliper that does have a short firm pedal.

Back the the concrete pad + 200lbs person example it does take some additional force on the underside to zero back the original downforce of just the concrete pad(using that as a baseline) compared to adding the 200lber on top.

But would not also a larger pad have more surface area to apply a coef of friction to? I thought that was a factor in a brake system, otherwise why have larger pads on a 3/4 ton truck vs a 1/2 ton, Police car(D61) vs regular Caprice(D52). Alot hard to push a 1x2" eraser sideways than a small pencil top eraser due to surface area that coef of friction is applied to.

[jpgolf14]

My drag car is not the center of this discussion I started-fixed vs floating and piston diameter was the main topic. It was just an example of a OEM large piston caliper that does have a short firm pedal.

Cool, no problem.

Back the the concrete pad + 200lbs person example it does take some additional force on the underside to zero back the original downforce of just the concrete pad(using that as a baseline) compared to adding the 200lber on top.

I'm not sure I'm following you. Are you referring to the fact that the concrete pad has weight? Lets assume it weights 100lbs. That means the ground is pushing up on the concrete pad with 200+100=300 pounds of force.

But would not also a larger pad have more surface area to apply a coef of friction to? I thought that was a factor in a brake system, otherwise why have larger pads on a 3/4 ton truck vs a 1/2 ton, Police car(D61) vs regular Caprice(D52). Alot hard to push a 1x2" eraser sideways than a small pencil top eraser due to surface area that coef of friction is applied to.

Yes a larger pad has more surface area to apply the friction to. But back to basics. The friction force, Ff, equals the normal force, Fn, times the coefficient of friction, mu. Ff = Fn * mu. The normal force is the force pushing down on the pad, so its the force produced by pistons. Remember the force is acted on the whole area. So while the pad maybe larger the normal force at any one point must be lower since the total force is spread across a larger area.

If we have a total piston force (normal force) of 10000lbs and a pad area of 2 sq in. That means at the pad face the pressure, psi, is 10000/2 = 5000psi. Now if we have the same 10000lbs of normal force, b/c we haven't changed the caliper, but this time the pad is only 1 sq in. The pressure at the pad face is now 10000/1 = 10000psi. It is an inverse linear relationship between pad area and pressure. At the end of the day its still the same total force pushing down though.

Now this applies very well for stiff materials. Brake pads and rotors are very stiff, they don't deflect much even under 10000lb loads. Rubber (eraser, tires, ect...) on the other hand has very low stiffness. The coefficient of friction actually decreases with increasing normal force. I suspect this is because the rubber deflects. So now the normal force is not perpendicular with the tread. With such a soft material I bet the tread can actually reach a buckling situation on hard cornering, depending on the tread depth and tread block size. Once the tread is buckled, there is effectively zero shear stiffness and zero load carrying ability. Fortunately there are lots of other treads near by that share the load, so the treads don't fully buckle, but I bet that is why the coefficient of friction changes with normal force.