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Thread: A survey of rear brake calipers

  1. #1
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    A survey of rear brake calipers

    Here we R&D stronger rear brakes for the E31, going to the BMW parts bin for a larger piston alternative to the stock.
    The mission is to boost the rears to match a similar uptick in the front due to a BBKoB -> See the E60 348x30 front rotor project.



    Z8 brakes on the E31


    A typical E31 rear is 324x12 solid and is clamped by a 1-pot with a 40mm diameter piston which will be refer to herein as the 40/12.
    Previously, the car featured here was retrofitted with the 328x20 rotor and caliper from the E38/750. See the E38 rear brake project.



    Although migrating from solid to vented improves the heat capacity and cooling, the change leaves the cold braking performance unchanged beyond the 1% due to mechanical leverage from the radius increase.
    A search was on for calipers with larger pistons that are compatible with the two rotors (324x12 & 328x20) that are known to fit the e31.
    Some guesses were made, some salvage units brought in, and a spreadsheet was constructed.
    Note: there is no 324x20 rear for the E31, until you make one.



    Definitions.
    Looking at the first two lines, the stock 840/850ci w/solid 324x12 rotor AND the e38/750 w/vented 328x20 rotor that can be retrofitted...
    - These calipers are from an ealier period in ATE caliper design. marked by a smaller 40mm piston and 75mm pin spacing and the dubious use of the rotor diameter in the caliper label.
    - The piston size is 40mm and the width of the caliper and stock caliper accepts a 12mm thick rotor, hence we'll call it a 40/12 caliper though its actually stamped "40/324" using the obsolete protocol.
    The e38 is 40mm and 20mm respectively and we'll call the caliper a 40/20 though actually stamped "40/328".
    - The height of the rotor is 61mm and is the distance between the wheel mounting surface (aka WMS) and the inboard surface of the rotor.
    Although this definition is industry standard, it is not an precise indicator disk of offset, unless the hat thickness is also standardized.
    A more directly useful definition would be hat seat to disk centerline but that's not how its reported.
    Note that any rotor that greatly exceeds this 61mm spec will press into the shield and other hub mechanisms and therefore is not a candidate for use on the e31.
    - Therefore compared to the e31 solid application, the e38 vented, maintains the same rotor height and adds the disk thickness to the outboard side.
    - The offset of the 40/12 is ~25mm (+/-0.5mm) and is defined as the distance between bracket's mounting surface and the centerline of the bracket's rotor gap.
    We are happy when the bracket's offset matches the centerline of the rotor.
    - Most radial measurements are taken with respect to the mounting bolt line (MBL), the imaginary line passing thru the two mounting bolt holes.
    - The bracket depth is what I have defined as the distance between the MBL and the extreme nose and tail of the bracket.
    Although not a precise indicator of rotor clearance (due to differences rectangular v polar), it is easily measured for comparison between brackets.
    - The pad edge location is what I have defined as the distance between the MBL and the outside radius of the friction material of the pad, piston side.
    We are happy if the pad sits entirely on the rotor without leaving any significant rotor unswept.
    This number can be imprecise because it depends on the the axial alignment of the bracket to the piston side caliper.
    While measuring, the bracket is forced into alignment just as the clip would do it.



    With the definitions layed out above, we compare the e31 40/12 against the e38 40/12, noting that the two apps (can) share the same pad.
    - The e38 radial offset from the MBL was found to be 3mm (+/-0.5) more than the e31, making (extra) room for the 328mm diameter rotor in place of the 324.
    - As the e38 appends disk thickness to the outboard side, the bracket offset grows by (roughly) half that amount.
    - The hose port on the e31 angles towards the hose termination on the chassis, whereas the E38 port comes straight out.
    Since the e31 hose was already at its limit, you'll need a longer hose when swapping in the e38 caliper, and btw any other caliper on this list.

    Looking at the remainder of the spreadsheet...
    - The mounting bolt spacing is 76mm for all the surveyed calipers.
    Therefore all of them will bolt onto the strut.
    Two successfully, as-is. One unsuccessfully, and hopeless.
    The balance can be made to fit by modifications.
    - A caliper normally paired with an excessively tall rotor could still be a candidate for use, because the caliper offset is a function of how it is mounted on the (donor) vehicle.
    Example: E53 original rotors with 80mm height.
    - Conversely, just because a rotor has a friendly looking offset, it still doesn't follow that the associated caliper has a friendly offset.
    - While we're on the topic, and for the same reason, a donor application's caliper can still bolt on even though the donor's rotor diameter is vastly different. Example: E90 42/20 with 300x20 rotor
    - A woefully unsuitable candidate is found in the E60 42/20. Reason: the offset is at 38 rather than 28.
    Milling this amount off the bracket mounting surface would not leave you with much bracket left.
    It suffers an addition problem of depth (67 versus 77), great if we were to mount say a 300x20 rotor (with 51mm height).
    As an exercise, the caliper side of the assembly could still be salvage and paired with a suitable bracket but this would not be our first choice.
    - The E53 42/12 looks like it should just bolt right up to the e31 with the stock 324x12 solid disk. It does.
    - The E53 has a banjo style port, 12mm btw, and it appears from my salvage units, that there are two styles of banjo.
    One style prevents rotation via a flatted tower, the other via a pin. So a hose termination would be either squared style or have what I call a jaw style that wraps the pin. See details below.
    - Considering the two E90 candidates (42/20 and 44/22), we might assume the mounting points on the chassis are unchanged between the two apps, seeing that the disk radius differs by 18mm and the bracket depth, caliper depth and pin offset differ by a comparable amount.
    Thus (amongst other details) the larger rotor was accomodated by growing the bracket inward with respect to the pad, and moving the mounting holes by this amount.
    - And this provokes the idea that if, we could only move the bracket mounting holes again, we would match the e38 bracket depth and therefore adapt the caliper for use with a 328 diameter disk.
    Which we do, step-by-step on the next post {ed: actually appended on this post}, to create the 44mm piston option.
    - These two E90 apps also sport larger piston offsets, the distance from MBL to the piston center, than do the other candidates.
    This is because these apps actually have the pad's metal backing plate bowed out (into the hollow of the caliper) on the piston side and the piston overhangs the friction material that necessarily stops at the rotors edge.
    The result is that the mean pressure radius of the pad is shifted outward, increasing the mathematical torque factor.
    - The E90 42/20 and the E53 42/20 both have depth and offset to fit a 324x20 (x61) disk, all mounted to the E31.
    - Unfortunately, there is no such disk in the BMW bin and it would seem we are left without a 42mm candidate. There are several options:
    1) Mill a 328 rotor down to 324, something trivial for a machine shop. This is the action I recommend if you want to go 42/20.
    2) Mod the bracket, weld and redrill the hole or perhaps cut and weld the bracket sides. For most people it would be easier to cut your 328 down at a machine shop for which you give up 1% of torque.
    3) Substitute a taller bracket of a suitable offset and arch and that just happens to create the target depth.
    This requires getting lucky, acquiring yet a second pair of calipers, then merging the two, discarding the remains.
    4) If you are not so lucky to find an exact bracket laying around, then mod one to do the job.
    In fact, we already did this when we relocated the holes on the E90 44/22 bracket.
    So for my research I've used this e90 44/22 mod'ed bracket to mount both the 44/22 and the two 42/20's to the 328x20 rotor.
    This route is also easy but it requires merging two pair.
    - The E52 46/20 looks like it should just bolt right up to the e31 with 328x20 disk.
    (After splurging on a pair of rebuilts,) I found out it does.
    Fyi the CSi, e38/750 and Z8 all share the same rotor dims but have different PN's. Likewise they can use the same pad.


    ****************
    Gallery and impressions
    Shown with 328x20 rotor.
    324x20 rotor coming soon.

    * E53 42/12 straight bolt on to stock 324x12 rotor
    This is my reference for a 42mm on stock rotors, with or without a front BBK.
    The math tells me the piston area for a 42mm is up by 10% over the stock 40mm.
    Still owe an install pic, on 324x12 rotor.



    * E53 42/20 adapted with mod'd 44/22 bracket (so I'll legitmately call it a E53 42/21 frankencaliper).
    Uses banjo end style lines.
    If you wish to retain the original bracket, you must take the 328 rotor down to 324.
    Check back in a week for painted, orig 42/20 bracket, on 324x20 rotor.



    * E90 42/20 adapted with mod'd 44/22 bracket (therefore E90 42/21 frankencaliper).
    Uses threaded end style lines.
    If you wish to retain the original bracket, you must take the 328 rotor down to 324.
    Check back in a week for painted, orig 42/20 bracket, on 324 rotor.


    Might be slightly grippier than the E53 due to piston offset.
    This was originally paired with a 300x20 rotor so the radial dims are a bit squished.
    Shown here with a used E31 pad and the 44/22 brkt, you can see the caliper tabs do not support the full radial height of the pad and also therefore a shorter clip is required.



    * E90 44/22 with mod'd bracket.
    This is my reference for a 44mm, when paired with the 348x30-Brembo BBK, and the current subject of long term testing.
    For this 22 wide caliper on a 20mm rotor, the pads should definitely be shimmed 1mm each, so that the piston doesn't exit before the friction material depletes.
    Leave dire sounding notes around for mechanics and subsequent owners.
    The math tells me the piston area is up by 20% over stock.
    The observed result is subtle, best described as a flatter stance during braking, with just slightly more G's versus pedal effort.
    There is little or no change in pedal travel (for the same G's), something I was concerned about at the start of this project.



    * E52 46/20 straight bolt on, good as gold.
    Obviously a limited supply.
    Might be nice with a E52 or CSi front, others including aftermarket Brembos.
    The math tells me the piston area is up by 30% over stock.


    - More -

    - - - Updated - - -
    ***** Mod'ing the E90 44/22 bracket for use with the 328 rotor *****
    The E90 44/22 bracket was originally paired with a 336mm rotor on a strut that also accepts a 42/20 caliper for a 300x20 rotor.
    Our target is an E31 with a 328mm rotor.
    Therefore we modify the bracket, creating a new pair of threaded holes, closer the the pads by 17.5mm (+/-0.5).

    After an initial warmup trial, this hobbyist took 60 minutes to perform the procedure seen below.
    A professional machinist would have taken the same time...but completed both sides.

    ******
    Vise the bracket by the guide pins. This makes the subsequent drilling ops (Z-direction) parallel to the pins.
    Also the Mounting Bolt Line (MBL) is now also aligned to the vise.


    The vise is bolted to an X-Y table, in careful alignment with the table's axe-eye.


    However, the top surface is sloped just slightly, not parallel to the (bottom) mouting surface, and rough also.
    Therefore to prevent the pilot drill from wandering, we level the surface with an end mill.


    Set the stage height (below the drill chuck) to accept the longest bit to be used (11/32").
    The pilot is referenced to the existing hole.
    Note: Imagine now trying to get a smooth paint finish on a cast iron surface. Hmmm.


    Note the "Pierre" Vernier dial and, since it is in "King George" imperial inches, calculate how many turns required to travel to the new hole location.
    Note: The proto trial shown used 17mm but, after finding rotor clearance a little high, the recommended target is now 17.5mm, or even 18.


    Machine using: pilot drill(s), final 11/32. It's a nice touch to chamfer/countersink, especially if you will be starting a tap from this side.
    Do not start with anything smaller than 5/32 for pilot, else it will jam and snap in the hole. A single 3/16 is better.
    If it so happens you have a chuckable tap chuck (I didn't), now's the time to get your threading started.










    Travel the X-Y stage by the required hole separation, 76mm, or 30 turns of the imperial dial.
    Then, because you might not have the vise bolted orthogonal to the X-Y stage, reference again 17.5mm off the existing bolt hole.
    Machine the 2nd hole.


    Flip the work, exposing the bracket's mounting surface.
    Note that some sort of large radius cutter was used by the factory.
    We'll machine with quite smaller end mill, taking back this edge by say 17-18mm.
    It was found important (by an attempted install) that the milling follow the original cutter radius rather than radiusing about the bolt hole.



    Now you can chamfer/countersink the new hole on this side, out to a diameter of 11.5mm.


    Start and/or complete the tap.
    If you started the tap using the drilling head, you MUST then continue on that side.
    If your are just now starting the tap, be absolutely certain the tap ends up perpendicular to the MOUNTING SURFACE, which is the bottom side in these pics, not the slanting top side.
    If you are using a tap handle, as shown, it's convenient to first remove the slider pins (M7 allen). You'll want the pins off anyway if you are subsequently painting.


    Dance, sing, pat yourself on the back.
    Last edited by Hyper; 08-14-2017 at 01:19 PM. Reason: Pic link repair, re PB affair

  2. #2
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    Thanks for doing the research, really good info! On the E52 caliper if the piston area is 30% up from stock that's already a huge improvement! It will be interesting to see the results on the E90 caliper performance with your mod. I have a 4 piston Brembo Porsche Cayenne caliper in development using 345 mm rotor to match monster front 360 mm set up.


  3. #3
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    Great R&D and Details. Fantastic design and engineering as always Hyper!

  4. #4
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    Check out the E46/M3 - 328x20mm rear rotor with 42mm caliper piston.
    Direct bolt on.
    IMHO, worth investigating..
    I calculate the 42mm caliper adds 2% to the rear brake bias.

    However, the brake line entry is 90 deg. out and I'm cautious about allowing the flex line to dip below the rear suspension.
    Could adapt to a hollow bolt with banjo to re-direct the brake line entry angle.
    I picked up a set last year on eBay for $150 but have had no time to explore adapting; a workshop & room addition/remodel eating my time and now I just has back surgery.
    Last edited by m6bigdog; 10-26-2016 at 09:23 AM.

  5. #5
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    Actually, these E39/M5 & E46/M3 42/20's along with the E46 330i 42/22 were on the short after the first pass thru the parts bin but were dropped early due the considerations you brought up, re: the need to retrofit banjo onto a port that is otherwise going the wrong direction. It wasn't impossible to work out, its just that there was an abundance of even lower hanging fruit. So these didn't get procured, they fell off the radar and didn't even make my report above.

    Retrofitting a banjo would involve cutting a mating surface and deciding on an anti-rotation strategy which I had assumed would need a flatted tower. However, since killing off the M3 & M5 & 330i, I was then unexpectedly sucked into banjo technology for the E53's and also got into the mod'ing of brackets, finding banjo'd lines, etc. One of my alleged E53's did anti-rotation using a dowel rather than flatted tower (the others did tower), and that seems rather doable on these others and would probably use the same (Audi A3?) banjo line. {ed: I recall now my ultimate reason for the kill: "Can just any owner/operator do the milling and tapping ops?"}

    So, tell you what...I'll attempt the mod and this might as well be done on yours, if you like.



    *******
    I've seen ~2% before, but it was only for the 2mm radius shift. How about the piston area, or were you already at 42mm?
    {ed: ah, I see now. 2% added to the "bias", not 2% to the rear torque.}
    Last edited by Hyper; 07-12-2017 at 11:04 PM. Reason: Pic link repair, re PB affair

  6. #6
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    Quote Originally Posted by Hyper View Post
    Actually, these E39/M5 & E46/M3 42/20's along with the E46 330i 42/22 were on the short after the first pass thru the parts bin but were dropped early due the considerations you brought up, re: the need to retrofit banjo onto a port that is otherwise going the wrong direction. It wasn't impossible to work out, its just that there was an abundance of even lower hanging fruit. So these didn't get procured, they fell off the radar and didn't even make my report above.

    Retrofitting a banjo would involve cutting a mating surface and deciding on an anti-rotation strategy which I had assumed would need a flatted tower. However, since killing off the M3 & M5 & 330i, I was then unexpectedly sucked into banjo technology for the E53's and also got into the mod'ing of brackets, finding banjo'd lines, etc. One of my alleged E53's did anti-rotation using a dowel rather than flatted tower (the others did tower), and that seems rather doable on these others and would probably use the same (Audi A3?) banjo line. {ed: I recall now my ultimate reason for the kill: "Can just any owner/operator do the milling and tapping ops?"}

    So, tell you what...I'll attempt the mod and this might as well be done on yours, if you like.



    *******
    I've seen ~2% before, but it was only for the 2mm radius shift. How about the piston area, or were you already at 42mm?
    {ed: ah, I see now. 2% added to the "bias", not 2% to the rear torque.}
    I'm game to have you mod my E46/M3 calipers if you think it would simplify the 42mm piston caliper upgrade path. PM your shipping address and I'll send them over to you.
    I'm no machinist so I can't tell you whether it is best to Banjo a set of misfit 42mm calipers or bore a set of 40mm E31 form factor calipers (E31/CSi or E38/750) for example and get the 42mm pistons from BrakeParts.co.uk or other brake parts supplier.

    My approx. 2% calculation is based on just the increase in rear caliper piston diameter from 40 to 42mm with a 40 & 44mm front caliper with 345x32mm front rotor. Rear caliper piston and pad sweep center line would remain the same with a E46/M3 rear caliper upgrade.
    IMHO, installing a 42mm rear caliper and adding 2% with a 324mm front rotor (single or 4-pod caliper) is just too aggressive a rear brake bias.

    Update:
    Added images of the E46/M3 caliper and side by side E31 & E46/M3 calipers.
    Attached Images Attached Images
    Last edited by m6bigdog; 10-28-2016 at 11:56 AM.

  7. #7
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    I recalled another method to change direction, although not as secure as a banjo.

    Application shown here: On my MBZ, coming off a CIS fuel distributor, thru an adapter, into a 3AN brake line with elbow and collar nut (aka swivel style), then to my fuel gauge.


    This particular adapter (TORQ-003TOM101.0SS) had a 90 degree flare, and our needs are likely a DIN bubble style. I didn't come across a DIN to 3AN.


    The adapter bottomed at the flare before it seated at the o-ring base, so there would be just enough length to re-profile the tip to DIN.


    Or skip the top profiling and use a sufficiently thick crush washer at the o-ring base.


    Another adapter is required on the other end, male 3AN to female DIN(?), at the hardline.
    Last edited by Hyper; 07-12-2017 at 11:06 PM. Reason: Pic link repair, re PB affair

  8. #8
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    Here a brake line with banjo end style, for adapting the e53 calipers to the e31.
    An original application is the 2010 Audi A3 w/272 rear rot, but it's also found on several VW's, others.
    Same line works with either the slotted tower or jaw&dowel style port.



    A slight mis-match between BMW spec and Audi & VW, but the anti rotation still works.



    Chassis end has a different shape than the e31 round style, requiring a slight mod to the mounting point to let it pass through.
    {ed: Hole in mounting bracket is opened up to match shape.
    Also this end can be secured to the mounting point using Audi/VW clip (PN 191611715) instead of the orig BMW clip method.}



    Shown patching into the e53-42/12 caliper on a e31 solid rotor.
    Plenty of clearance to M-Pars.
    This solution may (?) also work with e46 and e39 calipers, after they are mod'd to banjo style.


    Last edited by Hyper; 07-12-2017 at 11:10 PM. Reason: Pic link repair, re PB affair

  9. #9
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    excellent article Hyper.

    I am in the process of installing an 840 rear end into a hot rod truck project and would like to use a 15" wheel however with the reduced diameter at the inside of the wheel the outer diameter of the caliper will not clear. Are you aware of any ATE mounting bracket that would move the caliper toward the axle centre while maintaining the existing E31 setup. Looking at your spreadsheet it looks like a no so I would need to re drill the bracket and reduce the rotor diameter provided that it will still allow the pad to sit so that we have reasonable pad cover on the rotor face. I need to pick up approx 6mm. Any help here would be appreciated.




    Quote Originally Posted by Hyper View Post
    Here we R&D stronger rear brakes for the E31, going to the BMW parts bin for alternate ATE 1-pots with larger pistons.
    The mission is to boost the rears to match a similar uptick in the front due to a BBK.
    See the E60 348x30 front rotor project.




    Z8 brakes on the E31


    A typical E31 rear is 324x12 solid and is clamped by a 1-pot with a 40mm diameter piston which will be refer to herein as the 40/12.
    Previously, the car featured here was retrofitted with the 328x20 rotor and caliper from the E38/750. See the E38 rear brake project.



    Although migrating from solid to vented improves the heat capacity and cooling, the change leaves the cold braking performance unchanged beyond the 1% due to mechanical leverage from the radius increase.
    A search was on for calipers with larger pistons that are compatible with the two rotors (324x12 & 328x20) that are known to fit the e31.
    Some guesses were made, some salvage units brought in, and a spreadsheet was constructed.
    Note: there is no 324x20 rear for the E31, until you make one.



    Definitions.
    Looking at the first two lines, the stock 840/850ci w/solid 324x12 rotor AND the e38/750 w/vented 328x20 rotor that can be retrofitted...
    - These calipers are from an ealier period in ATE caliper design. marked by a smaller 40mm piston and 75mm pin spacing and the dubious use of the rotor diameter in the caliper label.
    - The piston size is 40mm and the width of the caliper and stock caliper accepts a 12mm thick rotor, hence we'll call it a 40/12 caliper though its actually stamped "40/324" using the obsolete protocol.
    The e38 is 40mm and 20mm respectively and we'll call the caliper a 40/20 though actually stamped "40/328".
    - The height of the rotor is 61mm and is the distance between the wheel mounting surface (aka WMS) and the inboard surface of the rotor.
    Although this definition is industry standard, it is not an precise indicator disk of offset, unless the hat thickness is also standardized.
    A more directly useful definition would be hat seat to disk centerline but that's not how its reported.
    Note that any rotor that greatly exceeds this 61mm spec will press into the shield and other hub mechanisms and therefore is not a candidate for use on the e31.
    - Therefore compared to the e31 solid application, the e38 vented, maintains the same rotor height and adds the disk thickness to the outboard side.
    - The offset of the 40/12 is ~25mm (+/-0.5mm) and is defined as the distance between bracket's mounting surface and the centerline of the bracket's rotor gap.
    We are happy when the bracket's offset matches the centerline of the rotor.
    - Most radial measurements are taken with respect to the mounting bolt line (MBL), the imaginary line passing thru the two mounting bolt holes.
    - The bracket depth is what I have defined as the distance between the MBL and the extreme nose and tail of the bracket.
    Although not a precise indicator of rotor clearance (due to differences rectangular v polar), it is easily measured for comparison between brackets.
    - The pad edge location is what I have defined as the distance between the MBL and the outside radius of the friction material of the pad, piston side.
    We are happy if the pad sits entirely on the rotor without leaving any significant rotor unswept.
    This number can be imprecise because it depends on the the axial alignment of the bracket to the piston side caliper.
    While measuring, the bracket is forced into alignment just as the clip would do it.



    With the definitions layed out above, we compare the e31 40/12 against the e38 40/12, noting that the two apps (can) share the same pad.
    - The e38 radial offset from the MBL was found to be 3mm (+/-0.5) more than the e31, making (extra) room for the 328mm diameter rotor in place of the 324.
    - As the e38 appends disk thickness to the outboard side, the bracket offset grows by (roughly) half that amount.
    - The hose port on the e31 angles towards the hose termination on the chassis, whereas the E38 port comes straight out.
    Since the e31 hose was already at its limit, you'll need a longer hose when swapping in the e38 caliper, and btw any other caliper on this list.

    Looking at the remainder of the spreadsheet...
    - The mounting bolt spacing is 76mm for all the surveyed calipers.
    Therefore all of them will bolt onto the strut.
    Two successfully, as-is. One unsuccessfully, and hopeless.
    The balance can be made to fit by modifications.
    - A caliper normally paired with an excessively tall rotor could still be a candidate for use, because the caliper offset is a function of how it is mounted on the (donor) vehicle.
    Example: E53 original rotors with 80mm height.
    - Conversely, just because a rotor has a friendly looking offset, it still doesn't follow that the associated caliper has a friendly offset.
    - While we're on the topic, and for the same reason, a donor application's caliper can still bolt on even though the donor's rotor diameter is vastly different. Example: E90 42/20 with 300x20 rotor
    - A woefully unsuitable candidate is found in the E60 42/20. Reason: the offset is at 38 rather than 28.
    Milling this amount off the bracket mounting surface would not leave you with much bracket left.
    It suffers an addition problem of depth (67 versus 77), great if we were to mount say a 300x20 rotor (with 51mm height).
    As an exercise, the caliper side of the assembly could still be salvage and paired with a suitable bracket but this would not be our first choice.
    - The E53 42/12 looks like it should just bolt right up to the e31 with the stock 324x12 solid disk. It does.
    - The E53 has a banjo style port, 12mm btw, and it appears from my salvage units, that there are two styles of banjo.
    One style prevents rotation via a flatted tower, the other via a pin. So a hose termination would be either squared style or have what I call a jaw style that wraps the pin. See details below.
    - Considering the two E90 candidates (42/20 and 44/22), we might assume the mounting points on the chassis are unchanged between the two apps, seeing that the disk radius differs by 18mm and the bracket depth, caliper depth and pin offset differ by a comparable amount.
    Thus (amongst other details) the larger rotor was accomodated by growing the bracket inward with respect to the pad, and moving the mounting holes by this amount.
    - And this provokes the idea that if, we could only move the bracket mounting holes again, we would match the e38 bracket depth and therefore adapt the caliper for use with a 328 diameter disk.
    Which we do, step-by-step on the next post {ed: actually appended on this post}, to create the 44mm piston option.
    - These two E90 apps also sport larger piston offsets, the distance from MBL to the piston center, than do the other candidates.
    This is because these apps actually have the pad's metal backing plate bowed out (into the hollow of the caliper) on the piston side and the piston overhangs the friction material that necessarily stops at the rotors edge.
    The result is that the mean pressure radius of the pad is shifted outward, increasing the mathematical torque factor.
    - The E90 42/20 and the E53 42/20 both have depth and offset to fit a 324x20 (x61) disk, all mounted to the E31.
    - Unfortunately, there is no such disk in the BMW bin and it would seem we are left without a 42mm candidate. There are several options:
    1) Mill a 328 rotor down to 324, something trivial for a machine shop. This is the action I recommend if you want to go 42/20.
    2) Mod the bracket, weld and redrill the hole or perhaps cut and weld the bracket sides. For most people it would be easier to cut your 328 down at a machine shop for which you give up 1% of torque.
    3) Substitute a taller bracket of a suitable offset and arch and that just happens to create the target depth.
    This requires getting lucky, acquiring yet a second pair of calipers, then merging the two, discarding the remains.
    4) If you are not so lucky to find an exact bracket laying around, then mod one to do the job.
    In fact, we already did this when we relocated the holes on the E90 44/22 bracket.
    So for my research I've used this e90 44/22 mod'ed bracket to mount both the 44/22 and the two 42/20's to the 328x20 rotor.
    This route is also easy but it requires merging two pair.
    - The E52 46/20 looks like it should just bolt right up to the e31 with 328x20 disk.
    (After splurging on a pair of rebuilts,) I found out it does.
    Fyi the CSi, e38/750 and Z8 all share the same rotor dims but have different PN's. Likewise they can use the same pad.


    ****************
    Gallery and impressions
    Shown with 328x20 rotor.
    324x20 rotor coming soon.

    * E53 42/12 straight bolt on to stock 324x12 rotor
    This is my reference for a 42mm on stock rotors, with or without a front BBK.
    The math tells me the piston area for a 42mm is up by 10% over the stock 40mm.
    Still owe an install pic, on 324x12 rotor.



    * E53 42/20 adapted with mod'd 44/22 bracket (so I'll legitmately call it a E53 42/21 frankencaliper).
    Uses banjo end style lines.
    If you wish to retain the original bracket, you must take the 328 rotor down to 324.
    Check back in a week for painted, orig 42/20 bracket, on 324x20 rotor.



    * E90 42/20 adapted with mod'd 44/22 bracket (therefore E90 42/21 frankencaliper).
    Uses threaded end style lines.
    If you wish to retain the original bracket, you must take the 328 rotor down to 324.
    Check back in a week for painted, orig 42/20 bracket, on 324 rotor.


    Might be slightly grippier than the E53 due to piston offset.
    This was originally paired with a 300x20 rotor so the radial dims are a bit squished.
    Shown here with a used E31 pad and the 44/22 brkt, you can see the caliper tabs do not support the full radial height of the pad and also therefore a shorter clip is required.



    * E90 44/22 with mod'd bracket.
    This is my reference for a 44mm, when paired with the 348x30-Brembo BBK, and the current subject of long term testing.
    For this 22 wide caliper on a 20mm rotor, the pads should definitely be shimmed 1mm each, so that the piston doesn't exit before the friction material depletes.
    Leave dire sounding notes around for mechanics and subsequent owners.
    The math tells me the piston area is up by 20% over stock.
    The observed result is subtle, best described as a flatter stance during braking, with just slightly more G's versus pedal effort.
    There is little or no change in pedal travel (for the same G's), something I was concerned about at the start of this project.



    * E52 46/20 straight bolt on, good as gold.
    Obviously a limited supply.
    Might be nice with a E52 or CSi front, others including aftermarket Brembos.
    The math tells me the piston area is up by 30% over stock.


    - More -

    - - - Updated - - -
    ***** Mod'ing the E90 44/22 bracket for use with the 328 rotor *****
    The E90 44/22 bracket was originally paired with a 336mm rotor on a strut that also accepts a 42/20 caliper for a 300x20 rotor.
    Our target is an E31 with a 328mm rotor.
    Therefore we modify the bracket, creating a new pair of threaded holes, closer the the pads by 17.5mm (+/-0.5).

    After an initial warmup trial, this hobbyist took 60 minutes to perform the procedure seen below.
    A professional machinist would have taken the same time...but completed both sides.

    ******
    Vise the bracket by the guide pins. This makes the subsequent drilling ops (Z-direction) parallel to the pins.
    Also the Mounting Bolt Line (MBL) is now also aligned to the vise.


    The vise is bolted to an X-Y table, in careful alignment with the table's axe-eye.


    However, the top surface is sloped just slightly, not parallel to the (bottom) mouting surface, and rough also.
    Therefore to prevent the pilot drill from wandering, we level the surface with an end mill.


    Set the stage height (below the drill chuck) to accept the longest bit to be used (11/32").
    The pilot is referenced to the existing hole.
    Note: Imagine now trying to get a smooth paint finish on a cast iron surface. Hmmm.


    Note the "Pierre" Vernier dial and, since it is in "King George" imperial inches, calculate how many turns required to travel to the new hole location.
    Note: The proto trial shown used 17mm but, after finding rotor clearance a little high, the recommended target is now 17.5mm, or even 18.


    Machine using: pilot drill(s), final 11/32. It's a nice touch to chamfer/countersink, especially if you will be starting a tap from this side.
    Do not start with anything smaller than 5/32 for pilot, else it will jam and snap in the hole. A single 3/16 is better.
    If it so happens you have a chuckable tap chuck (I didn't), now's the time to get your threading started.









    Travel the X-Y stage by the required hole separation, 76mm, or 30 turns of the imperial dial.
    Then, because you might not have the vise bolted orthogonal to the X-Y stage, reference again 17.5mm off the existing bolt hole.
    Machine the 2nd hole.


    Flip the work, exposing the bracket's mounting surface.
    Note that some sort of large radius cutter was used by the factory.
    We'll machine with quite smaller end mill, taking back this edge by say 17-18mm.
    It was found important (by an attempted install) that the milling follow the original cutter radius rather than radiusing about the bolt hole.



    Now you can chamfer/countersink the new hole on this side, out to a diameter of 11.5mm.


    Start and/or complete the tap.
    If you started the tap using the drilling head, you MUST then continue on that side.
    If your are just now starting the tap, be absolutely certain the tap ends up perpendicular to the MOUNTING SURFACE, which is the bottom side in these pics, not the slanting top side.
    If you are using a tap handle, as shown, it's convenient to first remove the slider pins (M7 allen). You'll want the pins off anyway if you are subsequently painting.


    Dance, sing, pat yourself on the back.

  10. #10
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    * E52 46/20 straight bolt on, good as gold.
    Obviously a limited supply.
    Might be nice with a E52 or CSi front, others including aftermarket Brembos.
    The math tells me the piston area is up by 30% over stock.
    Hyper, you have done a tremendous amount of work and research on so many 8 series projects! Thank you sir!

    I am wondering what brackets or carriers you used to mount the E52 46/20 calipers onto your car? Were you able to use the E38 750iL carriers or did you purchase the E52 carriers along with the calipers? The carriers don't appear to be common with any other models besides the 750iLS. Thanks,

  11. #11
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    Quote Originally Posted by JDP530 View Post
    Hyper, you have done a tremendous amount of work and research ...
    Times three, because I circulate in 3 parallel universes.
    Today I'm over in the Ford universe, scheduled for a helical LSD install (aka Eaton TrueTrac) because the V6 engine build got stalled, waiting on an M10-1.0 tap to cleanup a cross-threaded hole on the (reman'd) crank output flange.

    ****
    As the quoted notes say, the 46/20 is a straight bolt-on of the caliper assembly. No machining required, no adapters required.
    When you start talking "carrier", you are referring to the components that make up the assembly, and that's rarely the case that one starts by collecting the low level pieces.
    Certainly not here, because I bought the entire set (L/R) as a loaded reman from Rock.
    Lol, within hours of my original post on this matter, the last pair got snatched up, after it had been sitting there for at least months.

    The rotor is a standard issue E38/750 vented, which iirc is shared by the E52.

    ****
    Note again: One does not independently install the 46/20 without some counterbalance up front.

  12. #12
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    Hyper sorry to dig up a dead thread but wonderful work.

    Digging through my old notes I had scribbled Z8 rear as a target for further inquiry as prelim investigation told me it would fit, but I didn't know their piston sizes. I could have swore Ed and I chatted about this years ago. Looks like it is 46mm; neat.

    I have to do the calculations but on a napkin it appears a Z8/e52 caliper swap w using the OEM 750/CSI rear disk may be an option to rapidly correct the bias on the E31 down; I just dont remember the piston sizes in the fronts exactly...42/42? 40/42? It's been too many years.

    Also in a random aside. Another front engined V12 GT, the Ferrari 575M (itself a revision of the 550), also has a bias around 63-64%, though that I believe has a closer to 50-50 weight distribution.
    Last edited by Auraraptor; 04-04-2021 at 03:10 PM.

  13. #13
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    The e52 46/20 set shown in this thread went over to m6bigdog.
    For the highway, the e52 rear might be a bit much without building up your fronts with say the Euro CSI as done by m6bigdog or my BBKoB.
    Even with the BBKoB, I elected to stay within the 44/20 rears. {ed: 44/22, read the thread}
    This combo gives much flatter braking and I have found it to be safe, with the fronts sliding first under most / all conditions I've encountered.
    ***
    My memory can't instantly help with the Brembo's other than you are correct, that there are two piston sizes and iirc the trailing is larger.
    The data is around, but I'm busy as usual.
    Last edited by Hyper; 04-04-2021 at 06:01 PM.

  14. #14
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    Thanks Hyper, I agree I would want a front upgrade as well before ramping up the rear so much.

    Question:
    While exploring different front rotors, did you/do you have any information for various other (post-E31 era) OEM front rotors and their filament to E31s? In particular E46 CSL fronts (sorry if you answered this before; I am catching up on 10 years of posts)

    The CSL rotors are extremely light and can be had for <$500 a pair, but I am not sure their inside diameter/offset/shield clearance/retaining bolt location/etc.

  15. #15
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    I recollect a spreadsheet but can't locate it without more effort.
    A sequentially first source for dimensions would have been Rockauto.

    Let's say its E46 CSL, a car I'm not familiar with.
    Google says its a 2004 M3, so I go to Rock, find a vendor that has included dims in their pics.
    A random disk will fail some key dim.
    You gave a good list of parameters, although I would also include stud diameter (12 or 14) and obviously thickness.
    On the rears, you have added compatibility with shoes.
    You'll have a little flexibility since you have already signed up to do custom brackets.
    However, at some point a key dim could be just too big, too small and will be unworkable.

    Then just bring a candidate pair in and start noodling.

  16. #16
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    Pretty sure its the same diameter stud, but I'll add it to the list to check. Good point on thickness.

    Not as worried about the rears, that is a separate project.

    BTW casual research also points to F30 3 series having very close specs. Looks like a kit to mount on F30>E34s was available for a while.

  17. #17
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    Quote Originally Posted by Auraraptor View Post
    Pretty sure its the same diameter stud, but I'll add it to the list to check. Good point on thickness.

    Not as worried about the rears, that is a separate project.

    BTW casual research also points to F30 3 series having very close specs. Looks like a kit to mount on F30>E34s was available for a while.
    The M3 CSL rotor is 345x28mm and IMHO that is too light/thin a rotor for the E31. Euro 850CSi is 345x32mm.
    That I'm aware every 3 series rotors overall height is dimensionally too thin for the E31.
    CSL rotor overall height is only 52.5mm with the E31 front rotor at 77mm
    Even the F30-M3 rotor 370x30mm or 340x30mm (Slotted / Perforated) is only 73.1mm overall height. The E34 mounting kit would fit but the rotor is only 30mm thick.
    see attached rotor dimensions.

    You will want to select from the 5/6/7/8 series vehicles to find a compatible large rotor that is dimensionally compatible.
    The E60-M5 has a massive directional 376x36mm rotor (directional / perforated rotor) that should fit the strut but you will need to find/adapt a compatible dust shield and caliper.
    The E39-M5 front rotor is a 345x32mm and it should fit also but the E39 front caliper mounting holes are 2mm too wide and will not fit the E31 front strut.

    I will be posting a Rear Brake upgrade for the E31 that includes brake bias & temperature rise calculations and your enquire may help me get it posted.
    I also have the E52-Z8 rear 46mm calipers available that I got from Hyper. The price is right for new calipers.
    Attached Images Attached Images

  18. #18
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    Very cool. I completely agree the OEM 3 series rotors are too thin and lack the thermal capacity needed for the E31. Similar w the 1 series (further research points to all <30mm)

    I am using custom calipers and mounts so therefore the disk compatibility is key. In that vein, the E,G, and H measurements matching are most critical on your pictures. Looks like those are reasonably standardized which is reassuring. Trying to find a caliper the the correct piston sizes that maintains the bias around 65% when all said and done is the trick. Having the Z8 as an option is very helpful in the back pocket indeed

    PS the E60 M5 brakes are great but in IMHO far too large for the E31. I actually have a set on Brembo 6 piston 380mm Front and 4 piston 380mm rears originally off an M5 mounted on my E38 750il. Incidentally those brakes WILL clear OEM M-pars, but just barely. That said I think anything over 355mm is overkill and really limits your wheels to 19in+.

    Very excited to see what you find.
    Attached Images Attached Images
    Last edited by Auraraptor; 04-07-2021 at 12:26 PM.

  19. #19
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    Quote Originally Posted by Auraraptor View Post
    Very cool...

    PS the E60 M5 brakes are great but in IMHO far too large for the E31. I actually have a set on Brembo 6 piston 380mm Front and 4 piston 380mm rears originally off an M5 mounted on my E38 750il. ....
    The E38 and E31 are the same size chassis wheel track and weight vehicle (4500 lbs with driver & passenger) and have many similar suspension dynamics.
    Big Brakes are useful on a heavy performance vehicle but the tire needs to be upgraded (tread width & Max performance tread compound) also to meet the brake torque challenge since the tire stops the vehicle not the brakes.
    Large/heavy rotors (+8 lbs over stock) do have the drawback of wheel bounce due to added unspring weight but a heavy unspring weight on any vehicle causes the same problem.

    I suggest selecting rotors with an Aluminum Hat and use Aluminum Calipers where possible to keep the unspring weight as low as possible and that included light-weight forged wheels!!
    After that, bigger brakes is better only if you intend to use the potential capacity of the brakes.
    E31 front rotor 324x30mm, all cast = 21.5 lbs.
    E31-850CSi Euro front rotor 345x32mm, aluminum hat = 19.9 lbs
    E60-M5 front rotor 376x36mm, aluminum hat = 29.7 lbs

    I would say the E31 - sport coupe suspension geometry is more suited for more brake torque than the E38 - 4-door saloon.
    The only chassis design problem with the E31 suspension is the rear spring upper mount directly to the body; that generates road noise.

  20. #20
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    I agree hence why I’m building a set for the E31. Al hats and floating rotors are a given.
    I will likely go 355F/345R, but let’s first see what fits best. 380 All around would be much easier as I could use essentially the same mount we made for my E38 rear now, but it’s both a bit heavier and limits wheel diameter/selection.

    And tires are a given, RE71R or the equivalent level I use for all weekend cars and RE11 before that!

    Tires stop the car...The brakes just mean it will do it consistently.
    Last edited by Auraraptor; 04-07-2021 at 03:36 PM.

  21. #21
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    And for all the rest of you out there just trying to get down the highway sporting a cruise attitude,
    I was quite satisfied straight bolting onto Alex840 Red, the e53 42/12 around the solid stock 324 rear rotor.
    This "solved" the original problem while keeping weight and cost low.

  22. #22
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    Quote Originally Posted by Hyper View Post
    And for all the rest of you out there just trying to get down the highway sporting a cruise attitude,
    I was quite satisfied straight bolting onto Alex840 Red, the e53 42/12 around the solid stock 324 rear rotor.
    This "solved" the original problem while keeping weight and cost low.
    Hello Hyper

    Will e60m5 rear calipers and 370 mm rotors , height 61 mm or 345x24 rotors , height 63 mm fit to the e31 rear? Calipers are 46-24.

    Thank you in advance

  23. #23
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    I would not know, better to poll the audience.
    Find someone who's tried it.
    ***
    I did search and find my spreadsheet mentioned in post 15.
    Lists the E31 (12mm) solid rotor having a height of 61 and the (20mm) vented from the e38 or e52 also at 61.
    As (probably) mentioned at the thread start, the 8 mm additional thickness was added to the outside,
    thus growing the caliper width to the outside.
    The spreadsheet lists a caliper dimension, the bolting surface to the centerline of the rotor, which I call the "offset".
    Caliper offset = 25 mm for the stock solid and 25 + (20-12)/2 = 29mm for the e38/e52 vented.
    For a hypothetical 24mm thick rotor with 61mm height, we would want the offset of a bolt-on caliper to be 25 + (24-12)/2 = 31mm.
    ***
    Our bolt-on caliper has the mounting bolt spacing as 76mm. Sorry, I don't have the hole thread spec handy.
    ***
    Several more caliper info's required...
    1) Depth of the caliper. If and when mounted to E31 stock mtg points, does the caliper clear the outer edge of the rotor? With such a big increase in rotor diameter, there is no expectation that it would.
    2) Does a pad that is standard for the caliper clear the hat? Some pads designs are (radially) tall and therefore could collide with the outer extents.
    3) Are the ports conveniently located? Or are they pointed in the wrong direction, at an obstruction?
    4) Clears proposed wheels? At the rim and at the inside face.

    Several rotor info's required...
    1) Bolt pattern and bore compatible to the hub?
    2) Inside surfaces compatible to the hub?
    3) Any support for hand brake? Or are we talking integrated parking system with no shoes?
    4) Sure, the height is the same at 61mm, but we really want to know is the distance from the hub mounting surface to the rotor (inner) surface, which is 54mm on the e31. iow's hat thickness is 61-54=7mm. This impacts assumptions, caliper offset.

  24. #24
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    Good day!

    Here is what I found on the other forum

    http://www.bimmerboard.com/forums/posts/878049

  25. #25
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    Quote Originally Posted by Hyper View Post
    I was quite satisfied straight bolting onto Alex840 Red, the e53 42/12 around the solid stock 324 rear rotor.
    This "solved" the original problem while keeping weight and cost low.
    Hi Hyper,
    I assume you just needed to buy a longer (non- Bajo bolt) brake hose to connect, instead of using e31 factory designed brake hose? Or does the e31 hose screw connection differ from the e53 caliper screw connection (M10x1 to M10x1.5 perhaps) and therefore a new conversion piece added in-line, somehow?

    Those calipers 42/12 are so cheap to buy, I feel the brake hose would be costlier
    Could be a fun/easy upgrade to help take some strain off the front brakes, for sure, for daily driving.
    as others have said, I super appreciate your dedication/education to this subject
    Last edited by clockwork; 09-20-2021 at 12:08 PM.
    I swear, my cars are like a girlfriend.
    Sometimes its a rough ride, sometimes its smooth motorin'.
    Sometimes she doesnt like how i treat her and sometimes i dont like how she behaves.
    BUT at the end of the day, she loves it when I am inside her.

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