After twelve years of owning my probe, I have decided to revamp it. One of the many things to go on my car are the stock brakes (pads and rotors are quite worn). With many posts of big brake upgrades, and unsafe upgrades, and many incomplete conclusions, I felt I should put a little engineering light on the subject. There will be many simplifications made here to keep it understandable, but in the end I hope this post will allow you to choose a brake system that works for you. I fortunately have access to FEA and CAD programs and equipment to manufacture what I need to ensure a safe upgrade, so please don’t assume that with all this information, you too will also be safe to upgrade your brakes. If you really need detailed calculations, just ask but this post is already long enough.
I have two driving forces for the brake upgrade – 1. To take the already great stopping ability (120ft from 60mph) and make it better for everyday and track use – 2. Brembo calipers look pretty nice behind the wheels. I also need to make these fit behind my 16” winter tires.
A few definitions to keep in mind:
Force – The capacity to do work or cause physical change – e.g. pound
Pressure – A quantity of force applied over a given area e.g. PSI = Pounds/In²
Area - The extent of a planar region or of the surface measured in square units – e.g. mm²
Lever – e.g. A rigid bar pivoted on a fixed point and used to transmit force – the caliper provides a force of a distance – the lever arm.
To start, let’s look at the basic principles of brakes: A hydraulic system is really quite simple – push a fluid at the master cylinder with a small piston and large distance, and move a larger diameter slave cylinder (the caliper piston(s)) a smaller distance with greater force. There are essentially two types of calipers – fixed and floating. The probe has a floating caliper, which means that there is one large inboard piston, with a floating bracket that reaches to the outboard side of the rotor and squeezes the rotor with the pad. The advantage to this system is ease of manufacturing and reliability. This type of rotor is also self centering, which is why most guys on this site can still personally make a bracket work for a larger rotor. The disadvantage to this is uneven pressure distribution to the pad/rotor surface, and a less stable clamping platform. The fixed caliper design has 2 to 8 pistons, equally distributed on both sides of the rotor/caliper. The only part that moves on these calipers is the piston(s). The advantage to these calipers is the rigidity, resulting in a firmer application of pressure to the rotors via the pads. There is also better pedal modulation. With 4+ piston calipers, there are often multiple sized caliper pistons, with the leading piston smaller to reduce the uneven pressure across a pad (the front side of the pad often wears down first on single piston calipers). The disadvantage of these calipers is the size and cost. These calipers are not self centering, and therefore require a higher precision in making relocation brackets. Fixed calipers do not allow for rotor run out and require higher tolerances. Floating calipers are typically axially mounted on the spindle, and fixed calipers can be either radial or axially mounted on the spindle (the probes are mounted axially – radial mounting involves vertical instead of horizontal bolts into the caliper). Neither is necessarily better, but I believe in our case the radial mounted options provide for a larger (and safer) bracket.
Now on to brake pads – This has been discussed in many posts, but it comes down to physics. The piston applies a pressure on the pad – based on the fixed surface area of the piston, the piston will apply a force to the pad, which in turn is applied to the rotor. The size of the pad does matter, but is beyond the scope for this post, and can be assumed for now to be irrelevant – except that a larger pad will not heat up as much, due to a larger surface area and mass (yes, brake pad stopping power is dependent on the friction coefficients etc, but we can again leave that for another time). Many of the fixed calipers will allow for a larger pad.
Now to look at the physics of the caliper pistons – the pistons are imbedded in a cylinder bore, and will move based on the pressure and amount of fluid behind it. The pistons will retract based on the rotors pushing back on the pads, and in turn back on the pistons. The more fluid and pressure, the farther and harder the pistons can move. The only way that we are concerned about the volume of the pistons, is whether they will stay within the caliper bore, or pop out – if we choose too large of a caliper for the rotor thickness, the pistons will pop out. We are of course very concerned with the area of the pistons. Remember the formula for area is A = 3.14*Radius² or 3.14*Diameter²/4.
Many people out there will try to calculate the “Effective Piston Area”, based on the sizes of the piston(s) of only ONE side of the caliper. To me, it makes more sense to take into account all the pistons in the caliper, since they are all being acted upon by the fluid. So if you have 4 piston calipers, for THIS post, we will assume the Effective Piston Area (EFA) to be the sum of the area for the four pistons. Now to complicate things, we have a single piston floating caliper – to account for this we need to realize that our single caliper piston actually has to move twice as far as the fixed caliper pistons, to accommodate for pads on both sides of the rotor – this means that for OUR EFA for single floating calipers, we will use 2*Piston Area. This is not to be confused with the actual caliper area, but that can be assumed to be the same for this article.
I have two driving forces for the brake upgrade – 1. To take the already great stopping ability (120ft from 60mph) and make it better for everyday and track use – 2. Brembo calipers look pretty nice behind the wheels. I also need to make these fit behind my 16” winter tires.
A few definitions to keep in mind:
Force – The capacity to do work or cause physical change – e.g. pound
Pressure – A quantity of force applied over a given area e.g. PSI = Pounds/In²
Area - The extent of a planar region or of the surface measured in square units – e.g. mm²
Lever – e.g. A rigid bar pivoted on a fixed point and used to transmit force – the caliper provides a force of a distance – the lever arm.
To start, let’s look at the basic principles of brakes: A hydraulic system is really quite simple – push a fluid at the master cylinder with a small piston and large distance, and move a larger diameter slave cylinder (the caliper piston(s)) a smaller distance with greater force. There are essentially two types of calipers – fixed and floating. The probe has a floating caliper, which means that there is one large inboard piston, with a floating bracket that reaches to the outboard side of the rotor and squeezes the rotor with the pad. The advantage to this system is ease of manufacturing and reliability. This type of rotor is also self centering, which is why most guys on this site can still personally make a bracket work for a larger rotor. The disadvantage to this is uneven pressure distribution to the pad/rotor surface, and a less stable clamping platform. The fixed caliper design has 2 to 8 pistons, equally distributed on both sides of the rotor/caliper. The only part that moves on these calipers is the piston(s). The advantage to these calipers is the rigidity, resulting in a firmer application of pressure to the rotors via the pads. There is also better pedal modulation. With 4+ piston calipers, there are often multiple sized caliper pistons, with the leading piston smaller to reduce the uneven pressure across a pad (the front side of the pad often wears down first on single piston calipers). The disadvantage of these calipers is the size and cost. These calipers are not self centering, and therefore require a higher precision in making relocation brackets. Fixed calipers do not allow for rotor run out and require higher tolerances. Floating calipers are typically axially mounted on the spindle, and fixed calipers can be either radial or axially mounted on the spindle (the probes are mounted axially – radial mounting involves vertical instead of horizontal bolts into the caliper). Neither is necessarily better, but I believe in our case the radial mounted options provide for a larger (and safer) bracket.
Now on to brake pads – This has been discussed in many posts, but it comes down to physics. The piston applies a pressure on the pad – based on the fixed surface area of the piston, the piston will apply a force to the pad, which in turn is applied to the rotor. The size of the pad does matter, but is beyond the scope for this post, and can be assumed for now to be irrelevant – except that a larger pad will not heat up as much, due to a larger surface area and mass (yes, brake pad stopping power is dependent on the friction coefficients etc, but we can again leave that for another time). Many of the fixed calipers will allow for a larger pad.
Now to look at the physics of the caliper pistons – the pistons are imbedded in a cylinder bore, and will move based on the pressure and amount of fluid behind it. The pistons will retract based on the rotors pushing back on the pads, and in turn back on the pistons. The more fluid and pressure, the farther and harder the pistons can move. The only way that we are concerned about the volume of the pistons, is whether they will stay within the caliper bore, or pop out – if we choose too large of a caliper for the rotor thickness, the pistons will pop out. We are of course very concerned with the area of the pistons. Remember the formula for area is A = 3.14*Radius² or 3.14*Diameter²/4.
Many people out there will try to calculate the “Effective Piston Area”, based on the sizes of the piston(s) of only ONE side of the caliper. To me, it makes more sense to take into account all the pistons in the caliper, since they are all being acted upon by the fluid. So if you have 4 piston calipers, for THIS post, we will assume the Effective Piston Area (EFA) to be the sum of the area for the four pistons. Now to complicate things, we have a single piston floating caliper – to account for this we need to realize that our single caliper piston actually has to move twice as far as the fixed caliper pistons, to accommodate for pads on both sides of the rotor – this means that for OUR EFA for single floating calipers, we will use 2*Piston Area. This is not to be confused with the actual caliper area, but that can be assumed to be the same for this article.
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