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  • #31
    Whilst I've presently got the front brakes dismantled on my MC, I've been doing some measuring and calculations.

    Since the pressure applied to the brake pads is the hydraulic force in the brake pipes x the area over which the hydraulic force acts then the effective total surface of area of piston pressure is one critical measurement in assessing brake efficiency. The OEM single pistons measure 60mm diameter, therefore the area of the piston is 3.14159 (pi) x 30mm x 30mm (radius squared) = 2827 square millimeters. When the single piston presses against the inside of the brake disc, there will be an equal and opposite force exerted by the brake pad on the outside of the disc, and as such, the effective total surface area of piston pressure on the brake pads will be 2827 x 2 square millimeters = 5654 square millimeters.

    I've managed to find some dimensions of piston sizes for AP Racing's 4-pot calipers sold for the MC and in a single caliper, one pair measure 38mm and the other pair measure 41mm. This means that the combined area of the 38mm pair of pistons is 3.14159 (pi) x 19mm x 19mm (radius squared) = 2268 square millimeters and the combined area of the other pair is 3.14159 (pi) x 20.5mm x 20.5mm (radius squared) = 2640 square millimeters, giving an effective total surface area of piston pressure of (2268 + 2640) = 4908 square millimeters. Therefore the AP Racing 4-pot calipers using the same hydraulic pressure in the MC's braking system are capable of producing only 87% of the pressure on the brake pads that the OEM calipers can produce: that's 13% less than the MC's OEM brakes!

    The OEM brakes work on a 315mm x28mm disc and the AP Racing brakes work on a 330mm x 28mm disc. Since the braking torque is proportional to the radius of the disc, then the OEM brakes work on a lever arm equivalent to 157.5mm and the AP Racing brakes work on a lever arm equivalent to 165mm. These figures give the AP Racing brakes a mechanical advantage of 4.55% more than the MC's OEM brakes.

    However, since the AP Racing calipers can only create a pressure on the brake discs which is 13% less than the OEM calipers working on 315mm discs, it would appear that, overall, the OEM brake set up should actually be a more effective braking set up than the AP Racing 4-pot kit.

    The AP Racing 6-pot kit has pistons measuring, 27mm, 31.7mm and 38mm and they work on a 343mm x 32 disc. This gives the piston areas of 1145mm2, 1578mm2 and 2268mm2, respectively, which gives a total area of 4991mm2. That's still 11.7% less than the area of pressure in the MCs OEM system. However a 343 mm disc gives a lever arm equivalent to 171.5mm which gives a mechanical advantage of 8.8% more than the MCs OEM brakes.

    Since the effectiveness of a car's brakes is totally reliant upon the conversion of kinetic energy to heat energy, then for brakes to be the most effective, they need to create lots of friction to generate that heat energy. On the face of it, you would expect that bigger brake pads would generate more friction/heat. However, although a larger area of contact between two surfaces would create a larger source of frictional forces, it also reduces the pressure between the two surfaces for a given force holding them together. Since pressure equals force divided by the area of contact, it works out that the increase in friction generating area is exactly offset by the reduction in pressure; the resulting frictional forces, then, are dependent only on the frictional coefficient of the materials and the FORCE holding them together. This means that the greater pressure of the MCs OEM one-pot caliper is more effective than than larger brake pads operating under less pressure with an AP Racing 4-pot or even 6-pot kit.

    Therefore, from doing these calculations, it seems to me that multi-piston "big brake" kits are not necessarily the "upgrades" you might imagine them to be. I must admit that I'm rather surprised by my findings from doing these calculations. I expect that any improvement that these big brake kits might give to overall braking performance would be from the greater mass of the discs over the OEM 315mm x 28 disc, where a larger mass of disc can act as a better "heat sink".

    From the information I've worked out above, I think this confirms that the basic stopping power of the OEM brakes is as good, if not better than aftermarket brake systems.

    I think that improving the duty cycle of the brakes by the provision of ducting is the most effective upgrade you can make to the MC's brakes and the cheapest of all.

    If you want to improve the basic stopping power of the OEM brakes then you could do this in several different ways:
    1. Fit brake pads with a higher coefficient of friction.
    2. Fit bigger discs.
    3. Fit one-pot calipers with larger pistons than 60mm diameter.

    Can anyone see any flaws in my calculations or logic on this?
    /// Exdos ///
    "Men who try the impossible and fail spectacularly are infinitely superior to those who reach for nothing and succeed" --Napoleon Bonapart

    Comment


    • #32
      for exodos,
      But you try to drive on track fast whit oem disk?
      I dont know whic is the best laps of yours and of various lotus and gt3 on uk track,but normaly i to an good driven exige s modded take 3-4 second to mugello,from an gt3 from 7-10 second,consider i am not a really great driver and best z3m-m3 run 2-3 fastes of me..
      But anyway in italy,the fastes m3 and z3m had all big brake kit..
      www.fastdrive.org
      S50 black/black-red In Rome
      Front BBK Stoptech Red+Eibach Sway Bars+Kw Clubsport Whit Camber plates+Weichers Strutbar+Agk Urethane front Silent Block & differential Bushing+Tuner Motorposrt Alluminium Rear Mount+Hamann Exaust

      Comment


      • #33
        Originally posted by exdos View Post

        Can anyone see any flaws in my calculations or logic on this?
        well sort of - not thats its much help but without having a good think there are probably some aspects you have missed i.e. is the pressure distibuted in the same way across the pad in the different caliper designs etc need an expert to calculate these sorts of things.
        Ex 2001 S54
        New cars:
        Lotus Carlton
        350Z

        Comment


        • #34
          fastdrive,

          I'm not trying to knock any aftermarket big brake kits. All I have done is provide some calculations which IMHO make for very interesting comparison.

          I've experienced firsthand the braking capacity of a racing V8 Chevrolet Supercar and also a race prepared M3 CSL with bespoke uprated brakes and I know just how awesome some brakes can be. However, in these two instances the brake kits were MONSTER brakes not just 4-pot Big Brake upgrades.
          Last edited by exdos; 08-07-2008, 03:00 PM.
          /// Exdos ///
          "Men who try the impossible and fail spectacularly are infinitely superior to those who reach for nothing and succeed" --Napoleon Bonapart

          Comment


          • #35
            Originally posted by MGR View Post
            well sort of - not thats its much help but without having a good think there are probably some aspects you have missed i.e. is the pressure distibuted in the same way across the pad in the different caliper designs etc need an expert to calculate these sorts of things.
            I'm sure that I've missed quite a bit. I claim no expertise on this subject at all. All I've done is use my elementary knowledge to do some relatively simple calculations. I'm actually looking for someone with real expertise to prove why the OEM brakes are not as good as the AP Racing 4 pots, and explain to me why. The forum has been a bit quiet of late so I thought I'd make a posting on something technical.

            The Red Stuff pads and OEM discs which have just come off my car show pretty even wear on both sides of the car and on both sides of the same disc. I think this proves the point (to me at least) that the forces on a one-pot caliper are equal and opposite on both sides of the disc and that the force is applied across the full face of the pads (no pad flexing). There's no balancing of piston pressures required in one pot designs, because of this equal and opposite principle in the design.
            /// Exdos ///
            "Men who try the impossible and fail spectacularly are infinitely superior to those who reach for nothing and succeed" --Napoleon Bonapart

            Comment


            • #36
              Originally posted by exdos View Post
              I'm sure that I've missed quite a bit. I claim no expertise on this subject at all. All I've done is use my elementary knowledge to do some relatively simple calculations. I'm actually looking for someone with real expertise to prove why the OEM brakes are not as good as the AP Racing 4 pots, and explain to me why. The forum has been a bit quiet of late so I thought I'd make a posting on something technical.

              The Red Stuff pads and OEM discs which have just come off my car show pretty even wear on both sides of the car and on both sides of the same disc. I think this proves the point (to me at least) that the forces on a one-pot caliper are equal and opposite on both sides of the disc and that the force is applied across the full face of the pads (no pad flexing). There's no balancing of piston pressures required in one pot designs, because of this equal and opposite principle in the design.

              I agree - I'm no expert either but I'll have a better think about it tonight see if I can think of anything.

              The actual amount of force generated doesn't seem to be the issue with the OEM brakes though - well they seem to work well to me, but its the fade as you have said. Maybe the AP design is better at removing the heat and so they are operating at normal temps for longer and so can resist fade for longer and so appear to (and do) work better.
              Ex 2001 S54
              New cars:
              Lotus Carlton
              350Z

              Comment


              • #37
                Originally posted by exdos View Post
                Can anyone see any flaws in my calculations or logic on this?
                How about the strength of your right leg?

                Current - BMW M6
                Previous - Estoril S50 '98

                KW V3's, ACS Splitters, H&R ARB's, Rogue Engineering rear top mounts,
                Carbon Induction kit, Supersprint Exhaust, BBS 19" CH alloys, Strong Strut

                Comment


                • #38
                  The main benefits of multi pot calipers is that they give much better feel as they are rigidly mounted compared to the one-pot sliding OE calipers which twist on their sliders which is why generally the pads wear very slightly tapered under very heavy use. They are also much better for heat dissipation, generally being made of aluminium instead of iron and this also means they are loads lighter. They also tend to be easier to change pads in a race situation being "open back".

                  A big disc is always an advantage with either setup as it physically means more leverage means less pedal effort for the same braking forcing. Having a wider disc is just as important though IMO for resisting fade and disc durability - luckily the OEM discs are a decent thickness although they still fade easily which I think is due to poor OE pad compoound and inadequate cooling. I remember cracking some discs at a trackday years ago in my 205, they were big discs but weren't thicker than standard at 285mm by just 20mm wide. Now I run 26mm thick discs of the same diameter and they've not had to be replaced yet (over 7 years use) and generally a lot tougher.

                  Pad compound is the one of the biggest factors though IMO, you could fit DS3000 pounds to a standard setup and could be persuaded to think a big brake kit has been fitted such is the mega friction pads like that offer. The downside to that is they are harder to modulate. Pads like Yellowstuff are easier in this regard as the friction level is closer to factory but they resist fade, and most important for me is that they run "clean" on the disc.

                  Master cylinder sizes (and perhpas servos) are normally factored in when racing as this greatly affects pedal effort/travel.

                  I'm happy with my brake setup on track which is the best test of brake durability. I think a BBK is the BEST setup to get up but for the money as previously mentioned the OE setup can be modified to provide nearly of they have to offer for a fraction of the cost. But I think also a lot of people think they need the most expensive brakes, the best track tyres etc when there is a lot more for the OE stuff and road tyres to give.
                  Last edited by c_w; 08-07-2008, 03:48 PM.

                  Comment


                  • #39
                    As there is a greater speed differential between the pad and the disk at the edge of the disk, due to it being further from the origin (the hub), then I would think that bigger disks have this as a principal advantage. Especially, as friction is (broadly) proportional to speed squared. So with something equally "sticky" (same co-efficient of friction) you want to be further from the point where it spins, to get maximum effect.

                    Point about pressure well made though. No point in increasing area without increasing overall system pressure.

                    Jez
                    Scientist at college, lacking in any knowledge of brakes!
                    1998 Cosmos Black - Not quite standard ...

                    Comment


                    • #40
                      I've already put solid brake bushings on my OEM brakes all round and this mod definitely gives a firmer feel.

                      I think that the actual pedal "feel" gives a very strong subjective take on how good you think your brakes are. To me, the brakes on BMWs are very "progressive" whereas when I use my wife's Citroen C3, the car pitches heavily which makes many people think that the brakes are "better" when this happens. But I don't want that kind of behaviour from my brakes on the MC.

                      I think that aluminium construction of Big Brakes is a major factor in the dissipation of heat, and hence increases the duty cycle, whereas cast iron OEM brakes retain heat. If only BMW would make some nice one-pot aluminium calipers. The advantage of considerable weight savings at each corner to unsprung weight is a major advantage too. Perhaps we should approach a manufacturer to make some direct replacements in aluminium for the MC OEM discs???????? I'll make some enquiries.

                      I think the point about higher speed at the edge of a wider rotating disc is also contributory factor to increasing friction at a given pressure, thus improved braking capacity of big brakes. Interesting subject.
                      /// Exdos ///
                      "Men who try the impossible and fail spectacularly are infinitely superior to those who reach for nothing and succeed" --Napoleon Bonapart

                      Comment


                      • #41
                        Originally posted by exdos View Post
                        fastdrive,

                        I'm not trying to knock any aftermarket big brake kits. All I have done is provide some calculations which IMHO make for very interesting comparison.

                        I've experienced firsthand the braking capacity of a racing V8 Chevrolet Supercar and also a race prepared M3 CSL with bespoke uprated brakes and I know just how awesome some brakes can be. However, in these two instances the brake kits were MONSTER brakes not just 4-pot Big Brake upgrades.
                        naturally!
                        I see the 8 pot brake by brembo on an m3 e46 and brake 30mt after me at 230km/h...
                        But for our car first off need 19" wheels,after it's cost 5000 euro!
                        The your calculation,it's really intrest but it's normal see the really problem is the caliper not the braking disk,all racing cars ride whit multipot caliper and think it's the best solution,the bmw M whit only 1 pot caliper all people tell an insufficent brake sistem,whit few mod can improve but if you compare whit an boxster s of my brother it's really another planet whit stock brake sistem of my car..
                        Anyway for give an idea of the improve i brake to same point to best lotus driven,naturally the ride fastes on the curves,but you can had an idea of the improvement whit the bbk had my car..
                        www.fastdrive.org
                        S50 black/black-red In Rome
                        Front BBK Stoptech Red+Eibach Sway Bars+Kw Clubsport Whit Camber plates+Weichers Strutbar+Agk Urethane front Silent Block & differential Bushing+Tuner Motorposrt Alluminium Rear Mount+Hamann Exaust

                        Comment


                        • #42
                          Originally posted by fastdrive View Post
                          Anyway for give an idea of the improve i brake to same point to best lotus driven,naturally the ride fastes on the curves,but you can had an idea of the improvement whit the bbk had my car..
                          fastdrive,

                          What we need to know is what is the stopping distance of your car from, say, 120mph, with OEM brakes and then with your 4-pot brakes? Times around a track and comparison to other cars isn't proof of your brakes deceleration capabilities.

                          The other thing that really affects braking that we've not yet mentioned is tyres.
                          /// Exdos ///
                          "Men who try the impossible and fail spectacularly are infinitely superior to those who reach for nothing and succeed" --Napoleon Bonapart

                          Comment


                          • #43
                            It shouldn't be surprising that a BBK is, on paper, no more powerful than OEM brakes. Unless it's a 4-wheel kit, ideally with a new master cylinder, a more powerful kit would upset the front/rear balance, which would adversely affect stopping distance. See StopTech's site for more information. I think the benefits that c_w lists are accurate:

                            The main benefits of multi pot calipers is that they give much better feel as they are rigidly mounted compared to the one-pot sliding OE calipers which twist on their sliders which is why generally the pads wear very slightly tapered under very heavy use. They are also much better for heat dissipation, generally being made of aluminium instead of iron and this also means they are loads lighter. They also tend to be easier to change pads in a race situation being "open back".
                            In addition, a larger pad will improve heat dissipation from the pad - which is the bit you really want to keep cool.

                            The ability to swap pads more easily is a real one: trying to get a car that works on both road and track is always going to be a compromise and this is very much the case with the brakes. I've got the yellowstuffs and they're great once they're warmed up, but they definitely lack bite at colder temperatures.

                            Paul

                            Comment


                            • #44
                              As far as the feel goes with a BBK, ZZZEMMCO has previously written, when referring to his Stoptech kit: "Another tip--you Must use them, not just a touch touch, like I usually do in driving, foot planted and pressure stop.!!"

                              The brakes on the V8 Chevrolet Supercar had amazing stopping power but with absolutely no feel or feedback. Admittedly it was a totally different system to the MCs though.

                              I think that what's coming out of this discussion is that cooling is probably the most effective part of improving brake performance without changing all the rest of the hardware.
                              /// Exdos ///
                              "Men who try the impossible and fail spectacularly are infinitely superior to those who reach for nothing and succeed" --Napoleon Bonapart

                              Comment


                              • #45
                                I have the Stoptech 332mm kit and it is in every way better than oem brakes, except price :)
                                Stopping distance with Stoptechs is propably the same, but brake feeling is much better and they last on track without extra cooling.

                                With oem brakes I had bimmerworld cooling set, solid brake guides and i tested numerous pads. I would have tested performance friction discs and pads with oem calipers, but both my brake cylinders were in bad shape so it was a good time to buy bbk.

                                Performance-wise there is no reason to buy bbk if you only drive on street.
                                S50 - '99 Estoril/Estoril-Black
                                Mods - KW 3 Coilovers, H&R ARBs, Raid steering wheel, Wiechers Sport half-rollcage & strut brace, Simota CAI, Powerflex front and beam bushes, Beyern Mesh 18".

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