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  • Brake Upgrades Defined

    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.
    Last edited by tazwid; January 12, 2006, 10:58 PM.

  • #2
    what exactly would you define as unsafe upgrades?
    /X\ipoc - Chipoc Black 1994 Probe GT - R.I.P.
    1993 honda civic hatch - K20
    1991 honda civic hatch - 12.3:1 C/R balanced for 10,000rpm b16 -Gone-
    1995 Acura integra GSR 4dr - stuff -Gone-

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    • #3
      Now on to the rotors: The rotors are what rotate with the wheel. The larger the rotor, the larger the mass rotating, and the harder it is to slow down. This is similar in principle to why formula race cars run 16” wheels – the more mass located away from the center of rotation, the harder it is to slow. So what is the advantage to increasing the rotor size? The larger the rotor, the larger the mass – the larger the mass, the more energy it will take to heat up the rotor. Heat is an enemy of the braking system, and what causes warpage and brake fade. The wider the rotor, the more area between the pads (the heat generators) and the better chance of cooling. With a larger rotor, there is also an increase in the effective lever arm that the caliper can work against – think of it this way – the bigger the wrench, the more force you can put on a bolt. But remember, going from a 11” rotor to a 12” rotor is NOT a 1” increase in break leverage, but rather a 0.5” increase (rotors are dimensioned in diameters, but levers work on the distance from the center of leverage – the center of the rotor).

      Rotors come in either solid or vented versions. The front rotors on the probe are vented (think two discs separated by vanes), and the rears are solid. Vented rotors are needed where larger forces are applied, to help with cooling (only one side of each disc is touching a pad). Directionally vented rotors are the most efficient at dissipating heat, as they actually draw in air similar to a centrifugal pump. Now on to drilled/slotted rotors. The main reason behind both of these is to reduce the build up of gases between the rotor and pad surfaces. Nowadays, with a good brake pad, most of these gases are not an issue for everyday driving (gases are created when parts are heated by friction). Slotted rotors will also surface the pad each revolution, providing a slightly better coefficient of friction – making the pad more “grippy”. Drilling rotors will also help to reduce some of the rotational mass of the rotors. The problem with drilling rotors is that the Probe’s rotors were never designed to be Swiss cheesed. The rotors on many supercars have been designed to accommodate the additional stress points around these holes – but not ours – that is why you will often see crack propagation from drilled holes on many non-factory rotors. I personally will only run a slotted rotor.

      As far as brake lines, there are three kinds: the stock solid lines – these are the most efficient at holding pressure, but of course we can’t use them across a moving distance. The second kind is the stock rubber hoses (they do have some metal in them) – although these are sufficient for everyday use, the will expand under larger brake loads. The last kind is Stainless Steel braided lines. These are the most preferred kind of brake lines, due to the higher resistance to expansion under pressure.


      Now let’s look at our stock braking system:

      Master Cylinder i.d. 23.81mm, 0.937"
      Front Disc - Cylinder Bore - 57.15mm, 2.250"
      Front Disc - Pad Dimensions - 4800mm² & 10mm thick
      Front Disc - Dimensions - 258mm x 24mm (Probe/MX6 can be 264mm)
      Rear Disc - Cylinder Bore - 30.16mm, 1.187"
      Rear Disc - Pad Dimensions - 2900mm² & 8mm thick
      Rear Disc - Dimensions - 261mm x 10mm
      Power Brake Unit - MTX - 239mm, 9.4"; ATX - 188+215mm, 7.4+8.5"
      Brake Pedal Lever ratio - 4.1, Max Stroke 125mm, 4.92"
      Bolt Pattern: 5x114.3mm (5x4.5”) This is the diameter of the circle the bolt holes make, with 5 bolts.

      So for every 10mm our master cylinder piston moves, our front slave cylinder pistons will move 1.74mm (a ratio of areas – NOT diameters). Now to add in the brake pedal mechanical advantage of 4.1:1, for every 100mm our brake pedal moves, our master cylinder will move 24.4mm, and the caliper piston will move 4.24mm. The same principle can be applied to force – for every 10 pounds of force on the pedal, the master cylinder sees 41 pounds and the caliper piston sees 236.2 pounds of force. The larger the master cylinder piston the LESS pressure is developed in the caliper piston, but the MORE fluid is moved to that piston. To accommodate for the floating caliper, our master cylinder will move twice the fluid as a similar single piston fixed caliper, but at the same pressure. This is why we can still look at upgrading our calipers to multiple pistons. Our master cylinders are adequate for a larger caliper upgrade, but read on.


      Now for some comparisons
      , let’s find out what options are out there (yes, there may be many other options for non-Brembo calipers, but I want these to look the part as well – Wilwood, etc can work just as well). I looked mainly towards Porsche, as their calipers are manufactured by Brembo and information is somewhat readily available


      Comparing Caliper Pistons:

      Our calipers have an EPA of 57.15²*pi/4*2 (x2 because of sliding design) which gives 5130mm². When we compare this to the EFA of a Porsche 993 Twin Turbo – 2x44mm + 2x36mm = 5076mm². So our stock front caliper does have potential, but again remember that the Porsche systems have a larger stroke on the master cylinder, a higher pressure, they move more fluid, and the fixed caliper is more efficient.


      Model Front Piston 1 Front Piston 2 Rear Piston 1 Rear Piston 2
      964 C2 ('92-on)/C4/RS America 40mm 36 30 28
      964 C2 Turbo-Look 44 36 34 30
      964 C4 Turbo-Look 40 36 34 30
      964 Turbo 44 36 34 30
      993 C2/C2S 44 36 34 30
      993 Carrera RS 44 36 36 30
      993 C4/C4S 44 36 30 28
      993 Turbo 44 36 30 28
      996 Carrera 40 36 30 28
      996 C4S 44 28/32/38 30 28
      996 Turbo 44 36 30 28
      996 Turbo w/ PCCB or GT2 32/38 28 30 28
      Boxster/Boxster S 40 36 30 28
      Subaru STI 46 40 36
      Lancer Evo 46 40 36
      Probe GT 57.15 30.16
      Integra Type R 57.15

      So to choose a caliper that displaces roughly the same amount of fluid as our stock ones, we should choose a caliper with 2x44mm and 2x36mm pistons – too large of EFA, and our master cylinder may not be able to displace enough fluid = no braking power. To small of EFA, and our front brakes will actually have less stopping power, and the bias will be shifted to the rear = dangerous rear lockup. Again, a proportioning valve can help. The only catch with going to a caliper with these sizes of pistons, is whether it will fit under our rims, and if we have the wheel offset to allow our rims to still turn safely away from the caliper.
      Last edited by tazwid; January 12, 2006, 10:59 PM.

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      • #4
        Comparing Master Cylinders:
        Mazda Millennia: 25.4mm diameter
        Porsche 930: 23.1mm
        Porsche Boxster: 23.8mm
        Porsche 993&996: 25.4mm
        3000GT: 26.99mm
        Ford Probe GT 23.81mm

        Now at first glance you may say that we have the same master cylinders as some Porsches, and although we have the same piston bore, they may have longer stroke, develop higher pressures, or have a higher mechanical pedal advantage (for example some Porsches have a 7:1 mechanical advantage on the brake pedal, meaning they can develop a higher pressure). If we are to assume the same stroke, mechanical pedal advantage, and final pressure as the Millennia, the extra 2.3mm diameter equates to a 17% increase in fluid moved but a 12% decrease in the pressure at the caliper piston. This means that we have to balance the distance the pistons in the calipers need to move, versus the pressure to squeeze the rotors. If we move too much fluid to the front pistons, there will be not enough pressure, and the rear brakes will be too strong for balanced braking. We can correct for this by adding a different proportioning valve than stock, but I will leave this calculation up to you for the many possibilities of rotors, calipers, and Master Cylinders.

        Comparing Rotors:

        Rotor Sizes F/R Pad area/sq-cm
        911T 69-73 282x20 / 290x20 210
        911S 69-73 282x20 / 290x20 261
        Carrera RS 282x20 / 290x20 261
        Carrera 74-77 282x20 / 290x20 261
        911SC 78-83 282x20 / 290x20 261
        930 Turbo 78-89 304x32 / 309x28 376
        Carrera 3.2 84-9 282x24 / 290x24 261
        Carrera 4 89-94 298x28 / 299x24 344
        Carrera 2 90-92 298x28 / 299x24 284
        Carrera 2 92-94 298/28 / 299x24 344
        Carrera RS 92 322x32 / 299x24 422
        C2 Turbo 3.3 322x32 / 299x28 422
        C2 Turbo 3.6 322x32 / 299x28 474
        993 304x32 / 299x24 422
        993 Twin Turbo 322x32 / 322x28 552
        993 C4S 322x32 / 322x28 552
        993 RS 322x32 / 322x28 552
        996 Carrera 318x28 / 299x24 450
        996 Twin Turbo 330x34 / 330x28 552
        Boxster 298x24/292x20
        Boxster S 318x28/299x24
        Lancer Evo 320x32.0/300x22.0
        Subaru STI 323x30/ 312x20
        Sentra SE-R 305x22.1/278x9
        Probe GT 264x24/261x10

        There are two types of rotors out there: fixed and floating. The Probe has fixed rotors, and the floating rotors generally consist of an aluminum hat (to reduce rotational mass) and a steel rotor. The floating design is superior, as it allows more for thermal expansion, but it is of course more expensive. Floating rotors can be custom made, and if you can afford it, I would go with this option. Upgrading rotors is probably one of the easiest ways to upgrade our stock braking system, without sacrificing too much Front/Rear bias. With larger rotors, there is also a need for custom machined relocation brackets, because we need to move the calipers away from the center. This is where my caution comes in: if you don’t do proper calculations on the stresses involved with this bracket, deadly result can occur – it’s not worth it!

        So to come to some conclusion?: There are many options out there, all dependent on what you really want to accomplish, and what you can afford. I will be personally selecting a Brembo caliper based on the size of rotor that I can fit under my rims, what thickness of rotor the caliper can handle, and the compatibility of our stock system to this new caliper. If you’re in doubt about safety or reliability, I would personally go with one of the big brake relocation packages out there – they are the least expensive, they come with machined larger rotors and brackets, and use stock calipers – this way your bias is relatively the same, and it’s safe. One such system uses a 12.0x1.1” front rotor, and an 11.0” rear rotor (that’s a 13% and 6.6% breaking torque increase F&R) for around $600. Remember, FWD cars do 70-90% of their stopping with the front brakes, so invest up there first.

        I could have done a very large number of calculations to prove everything, and to try every example, but that was not the point of this post – I am trying to lay the foundation down for a better understanding. I will add to this post as required, and am more than happy to entertain differences.

        Dimensions were obtained from the following sites:
        http://www.evolutionm.net
        http://www.nissanhelp.com
        http://www.pelicanparts.com
        http://www.europeancarweb.com
        http://homepage.ntlworld.com
        Last edited by tazwid; January 13, 2006, 09:24 AM. Reason: Automerged Doublepost

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        • #5
          yow! if ever there was a thread that needed a stickey ....

          thanks for all the research dude
          1989 Orig Owner 221K+miles not mine anymore GT

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          • #6
            The accuracy of your numbers and pressures is a bit "overdetailed" IMO. You didn't add in any fudge factors for expansion of the softlines in the system and general compliance such as flexing of the brake pedal, caliper, brake pad etc. And that'll change your numbers, especially when you're using hundreths of a millimeter in some of your numbers.
            -Jesse
            "Racing makes herion addiction look like a vague desire for something salty."
            -93 MR2, #129 ES, SCCA ProSolo Season: 2nd place: 2004 & 2005 & 2006.

            Comment


            • #7
              Most of this has been said on PT, thanks for summing it up in one nice peice, however I would argue that a larger caliper and pad is really not the most direct route to better braking as fade is the biggest issue, simple mechanical advantage is proven and in expensive.
              Last edited by pgt95; April 18, 2006, 09:34 AM.
              MazdaSPEED6

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              • #8
                Originally posted by neofreak
                what exactly would you define as unsafe upgrades?
                I guess that depend on everyones confort levels. For me, the only thing that I don't take chances with is the brakes, because failure means possible serious injury. As far as brake systems go, I will only install items that have been either throughly engineered, or tested extensively, with conclusive and positive results. That includes brackets "made to fit". If people realized the stresses and faults with some of these brackets, they'd think twice. Calipers, and rotors usually are a pretty safe bet, it's just the mounting that sometimes worries me.

                Originally posted by PseudoRealityX
                The accuracy of your numbers and pressures is a bit "overdetailed" IMO. You didn't add in any fudge factors for expansion of the softlines in the system and general compliance such as flexing of the brake pedal, caliper, brake pad etc. And that'll change your numbers, especially when you're using hundreths of a millimeter in some of your numbers.

                You are correct, I did not include the "fudge factors" because I assumed them to be fairly consistent - if you have the same brake lines, pedals, etc, then assuming the relative same pressures and flow (which we are trying to match) then the "fudge factors" then become a consistent non-issue. I would also have to start a seperate post on the deformation of materials under stress - and this is far beyond the scope or concern of most. If you're really interested, please feel free to contact me and we can discuss.

                Originally posted by pgt95
                Most of this has been said on PT, thanks for summing it up in one nice peice, however I would argue that a larger claiper and pad is really not the most direct route to better braking as fade is the niggest issue, simple mechanical advantage is proven and in expensive.
                You are correct, a larger caliper is not always the best option, and that was one point I was trying to convey. Mechanical advantage is good, but supplementing this with a more efficient caliper is even better. Larger pads help to dissipate this heat, as well as a wider and larger rotor, which sometimes you have to go with a bigger caliper to acheive.
                Last edited by tazwid; January 15, 2006, 10:42 PM. Reason: Automerged Doublepost

                Comment


                • #9
                  Originally posted by tazwid

                  You are correct, I did not include the "fudge factors" because I assumed them to be fairly consistent - if you have the same brake lines, pedals, etc, then assuming the relative same pressures and flow (which we are trying to match) then the "fudge factors" then become a consistent non-issue. I would also have to start a seperate post on the deformation of materials under stress - and this is far beyond the scope or concern of most. If you're really interested, please feel free to contact me and we can discuss.
                  Eh, I just found humor in making calculations to the hundreth millimeter given unknown deformations.
                  -Jesse
                  "Racing makes herion addiction look like a vague desire for something salty."
                  -93 MR2, #129 ES, SCCA ProSolo Season: 2nd place: 2004 & 2005 & 2006.

                  Comment


                  • #10
                    Originally posted by PseudoRealityX
                    The accuracy of your numbers and pressures is a bit "overdetailed" IMO.

                    But that allows you to choose your own number of significant digits and still be fairly confident in the results.



                    I'll have to do my brake upgrade calculations this week then. I based mine on the severe testing conditions under which they were developed and tested(NASA wheel-to-wheel racing).
                    90 Mazda 323 - KLZE, fender-flared, right hand drive, 2350 lb fully loaded sex machine. || Pic Thread ||- SOLD
                    93 PGT FRANKENPROBE - 10.24 @ 139.9 mph ||545 whp @ 20 psi || Timeslip || Dyno slip|| Build Thread - GONE
                    97 GTS - Rear Wheel Drive KLZE, 6-speed, 3.90 Torsen LSD, 2650 lb, daily driver! - Build Thread - GONE
                    90 Ghettocet KLiata - forever WIP

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                    • #11
                      Very nice posts, but I am concerned about the use of areas for the calcuation of the piston movements. You were right about it not being diameters, but the issue is really volume, not areas. The hydraulic oil is incompressible (if without entrained air), so the amount of oil moved by the master cylinder must be the area times the stroke i.e. volume. The resulting volume moved out of the cylinder is divided between two caliper cylinders i.e. driver's side and passenger side.

                      Using the information you provided:

                      Originally posted by tazwid
                      ........Now let’s look at our stock braking system:

                      Master Cylinder i.d. 23.81mm, 0.937"
                      Front Disc - Cylinder Bore - 57.15mm, 2.250"

                      So for every 10mm our master cylinder piston moves, our front slave cylinder pistons will move 1.74mm (a ratio of areas – NOT diameters). .
                      Area of master cylinder: 445mm2
                      Volume displaced at 10mm stroke: 4450mm3

                      Area of caliper cylinder bore:2565mm2
                      times 2:5130mm3 (again, because the oil is split between LH & RH calipers)

                      caliper piston movement= volume of oil moved from master cylinder/piston area:
                      =4450/5130
                      =0.87mm piston displacement

                      exactly 1/2 of what you calculated.

                      regards,
                      Mac

                      Comment


                      • #12
                        Hey Mac, you are correct on your calculation, but volume will only factor in when we try to determine piston extension. If there is not enough fluid displaced by the master cylinder, the brake piston will not extend out far enough, and your brake pedal will bottom out before you acheive full clamping force. My comparisons were geared more for the overall clamping force, remember that force = pressure x area, not volume. We need to make sure the same or greater clamping force is obtained. If we match the stock brake piston diameter, we will therefore also match the brake piston displacement (for the same master cylinder displacement). Based on this, it appears that a dual 44mm and 40mm setup would give us roughly the same brake piston area as stock, and therefore displace the brake pistons the same as stock. Also remember that our calipers are the floating design, and must displace twice the fluid that the fixed calipers displace, bringing your calculations inline with mine.

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