Brake Bias Calculations and Master Cylinder Selection

[Jeb_]

I recently installed F30 front brakes and 135i rear brakes on my 128i and am looking to now swap master cylinders to regain some pedal firmness and get a reasonable brake bias. I'm trying to decide between installing a master cylinder from an 135i or one from an E9x M3.

Looking around, It seems like people frequently neglect master cylinder piston sizes when doing their calculations for brake bias. This strikes me as odd since as far as I can tell, all BMW master cylinders have different diameter pistons for the front and rear brakes, meaning that with the same amount of force applied to each piston you end up with higher pressure in the line for the rear brakes. That being said I'm trying to wrap my head around how the force applied to the brake pedal is split between the pistons. Is it split equally, is it a fixed ratio based on piston sizes, does the booster equalize line pressure? I feel like I'm missing something.

[fe1rx]

Originally Posted by Jeb_

Looking around, It seems like people frequently neglect master cylinder piston sizes when doing their calculations for brake bias. This strikes me as odd since as far as I can tell, all BMW master cylinders have different diameter pistons for the front and rear brakes, meaning that with the same amount of force applied to each piston you end up with higher pressure in the line for the rear brakes. That being said I'm trying to wrap my head around how the force applied to the brake pedal is split between the pistons. Is it split equally, is it a fixed ratio based on piston sizes, does the booster equalize line pressure? I feel like I'm missing something.

Interesting idea, but I don't think this is correct. When I log front and rear brake pressures (car not moving) with my AIM Solo DL, they are the same, no matter how hard I press the brake pedal. I checked this some time back to see if there was a hydraulic proportioning valve built into the system somewhere (and discovered there isn't).

[Jeb_]

That's helpful information to know, thanks!

So either the force split between the master cylinder pistons is a proportion of piston area which yields equal pressure at both pistons, or there's some trickery going on.

In either case that means the master cylinder selection will effect pedal feel and likely line pressure, but should not effect the bias.

Using arbitrary values for force and the coefficient of friction and taking the average diameter of the master cylinders pistons here's what my calculations are looking like:



Master cylinder piston diameters highlighted in yellow I'm not 100% sure about.

[Jeb_]

So with my new set up the split is 66/34, just a little more front biased than a stock 135i at 65/35.

Since I have more piston surface area in the front than a 135i, a 135i master cylinder would result in a slightly softer pedal, but I'm not sure it would be noticeable. I am inclined to go with the e9x master cylinder though, just for a little bit stiffer of a pedal.

[$iriu$black]

Originally Posted by Jeb_

So with my new set up the split is 66/34, just a little more front biased than a stock 135i at 65/35.

Since I have more piston surface area in the front than a 135i, a 135i master cylinder would result in a slightly softer pedal, but I'm not sure it would be noticeable. I am inclined to go with the e9x master cylinder though, just for a little bit stiffer of a pedal.

I believe chris_flies has done this. He used IIRC an E9X M3 MC. Hopefully he sees your post and chimes in with his experience. I’m interested as well as I have the parts but have not installed them yet due to fear that the pedal feel will be really mushy. I have race pads on my 128i and love the feel of them and despite having “improved” braking with the F30 setup, I’m afraid losing that pedal feel is what gives me second thoughts.

Since I’m here, when swapping MCs, is it as easy that, just basically swapping the MC, and then bleeding it? Do you also include the booster?

Sorry if I sounded novice there, but I am for this kind of work.

[chris_flies]

Can confirm, I did swap to an M3 master cylinder, also with F30 front brakes, but still with stock 128i rears. Pedal feel is LIGHTYEARS better than before. The pedal grabs nice and high in its stroke, is firm and short, but still extremely easy to modulate. I haven't gotten on track yet to really test if there's any noticeable change in bias(for obvious reasons). Really happy with the change, but it was kind of a pain to do. Very small spaces and you need to undo bolts behind the pedals, which always sucks.

[Jeb_]

Thanks for the input guys.

Good to hear from someone that has already done it. The only difference there will be between mine and Chris' setup seems to be I have 135i rears. I don't expect there to be much difference in stock feel due to those though since it looks like all BMW master cylinders of the last generation have the 22.2mm rear piston.

It does seem like a pretty straightforward, although frustrating swap. my plan is to:
- Drain the reservoir
- Undo the lines to the master cylinder
- Contort myself to undo the master cylinder mounting bolts
- Swap master cylinder and install in reverse order
- Bleed brake system with INPA

I haven't looked to closely into the procedure of bleeding the system with INPA but I don't think it will be difficult.

[chris_flies]

Originally Posted by Jeb_

Thanks for the input guys.

Good to hear from someone that has already done it. The only difference there will be between mine and Chris' setup seems to be I have 135i rears. I don't expect there to be much difference in stock feel due to those though since it looks like all BMW master cylinders of the last generation have the 22.2mm rear piston.

It does seem like a pretty straightforward, although frustrating swap. my plan is to:
- Drain the reservoir
- Undo the lines to the master cylinder
- Contort myself to undo the master cylinder mounting bolts
- Swap master cylinder and install in reverse order
- Bleed brake system with INPA

I haven't looked to closely into the procedure of bleeding the system with INPA but I don't think it will be difficult.

Those are the steps. Just missing removing the hose connecting to the clutch master cylinder since the reservoir is shared. Be prepared to use an absolute TON of brake fluid doing the INPA bleed. I went through 2.5 liters of it before the pedal felt correct.

[Jeb_]

Good to know.

I just got 3 Liters of DOT 4 delivered so once the master cylinder gets delivered next week I should be good to go.

[Driven5]

The circuits are not separated front and rear, they are separated diagonally LF/RR and RF/LR. (Edit: No they're not...See Post #32)

Correspondingly, stepped bore (or quick take up) master cylinders do not have each bore feeding a different circuit. Rather, the large piston moves more fluid to take up the caliper piston retraction and build the initial pressure with less pedal travel, then the small bore pistons take over the high pressure use. Thus the brake balance equations should be using only the smaller bore diameter on both front and rear. (Edit: Maybe...But maybe the equations should only be using the larger bore instead. See Posts #32)

So if two master cylinders have the same size small bore, they will feel the same under moderate to severe braking. However, the one with the larger large bore with feel firmer and travel less initially. As the size and quantity (total area) of caliper pistons increases, so to does the need to increase the size of the larger bore.

[b1aze]

Originally Posted by chris_flies

Can confirm, I did swap to an M3 master cylinder, also with F30 front brakes, but still with stock 128i rears. Pedal feel is LIGHTYEARS better than before. The pedal grabs nice and high in its stroke, is firm and short, but still extremely easy to modulate. I haven't gotten on track yet to really test if there's any noticeable change in bias(for obvious reasons). Really happy with the change, but it was kind of a pain to do. Very small spaces and you need to undo bolts behind the pedals, which always sucks.

Is there a reason you didn’t use any of the other brake cylinder components from the M3? I just bought a full MC, reservoir and booster assembly from eBay and will plan on doing the whole thing unless I find a specific reason not to.

[chris_flies]

Originally Posted by b1aze

Is there a reason you didn’t use any of the other brake cylinder components from the M3? I just bought a full MC, reservoir and booster assembly from eBay and will plan on doing the whole thing unless I find a specific reason not to.

Just didn't think it was necessary, and didn't want to add any other variables. I wanted a firmer, shorter pedal and just doing the larger master cylinder would do that. Didn't want an different booster or whatnot to possibly detract from it. Either way, the brake feel is perfect now, and I'm very happy with the swap.

[Jeb_]

Originally Posted by Driven5

The circuits are not separated front and rear, they are separated diagonally LF/RR and RF/LR.

Correspondingly, stepped bore (or quick take up) master cylinders do not have each bore feeding a different circuit. Rather, the large piston moves more fluid to take up the caliper piston retraction and build the initial pressure with less pedal travel, then the small bore pistons take over the high pressure use. Thus the brake balance equations should be using only the smaller bore diameter on both front and rear.

So if two master cylinders have the same size small bore, they will feel the same under moderate to severe braking. However, the one with the larger large bore with feel firmer and travel less initially. As the size and quantity (total area) of caliper pistons increases, so to does the need to increase the size of the larger bore.

Learn something new every day. Thanks, that clears everything up!

[Jeb_]

Originally Posted by b1aze

Is there a reason you didn’t use any of the other brake cylinder components from the M3? I just bought a full MC, reservoir and booster assembly from eBay and will plan on doing the whole thing unless I find a specific reason not to.

The MC I have on the way also is coming with the booster so I was wondering about this too.

I'm not sure how the M3 booster differs from the 128i one. think I'm going to stick with the 128i booster. Not that it sounds like I'll ever feel motivated to do the job again, but I'll hang onto the M3 one in case I'm still not happy with the brakes.

[Jeb_]

Figured I'd upload the numbers I got when using the smaller MC diameter for all calculations in case anyone comes looking for them in the future:

[juld0zer]

Check the master cylinder part numbers first. The 135i master IIRC is used in many applications.
Have a read of the E90 thread also as we documented the differences in the master cylinders and concluded the numbers in the casting refer to the bore diameter.

I have swapped boosters and masters before and there was not much difference in everyday driving. If you are swapping just the master, I would suggest tilting the booster upwards as that will give you the most room to work in. You can also unmount the DSC from the chassis and push that aside a bit to give you more room.

Water is the best cleaner for brake fluid.

If your pedal is currently mushy, make sure it is bled properly (pressure bleeder & INPA or ISTA) before chasing something like a master swap.
I also read somewhere that our cars are rear biased and the EBD programming will strive to use the rear brakes as much as possible, up until the limits of grip.
There is a lot of computer control in how the car slows down/stops. This is also how BMW can use the same master in so many cars with different brake sizes. We also discussed the worn pad sensor effect on brake response.

F30 standard pads are shit in terms of response compared to E90/E8x brakes. The compounds are different. F30 Brembo discs also seem to be a harder metal. That could also impact your brake feel.

[Jeb_]

I have since stumbled upon the e90 thread I think you're talking about. But I wasn't sure if the casting was marked with the full diameter or just to the closest mm. From what I remember seeing the small piston on them is marked as "22" but other numbers I've turned up seem to indicate that the actual piston diameter is 22.2 mm. (Not that it will make much difference on the line pressure.)

Messing up the ABS is something I am a little bit concerned about. However, I am going to have the coding done for performance brakes and based off of my rough calculations the bias hasn't changed much so I don't think the ABS will respond too much differently. I guess I'll find out when autocross / track days start back up. If at that point the brakes seem sketchy I'll look into what can be done programming wise.

I'm giving Cool Carbon ST plus pads a try. Hopefully they'll give a better feel than the stock pads.

[Jeb_]

I got the master cylinders swapped today. Just have to reattached the booster to the firewall and then bleed the system. Got brake fluid everywhere and got frustrated with how little room there is to get to everything, but overall it wasn't too difficult. I guess I should wait to see if the MC is making a good seal with the booster before I say that though.

I worked through it a little faster than I usually like because I was losing sunlight, but it seems like it would be a lot more difficult to swap the booster too. Even with the booster unbolted I was only able to tilt it upward slightly before it hit on stuff. I did disconnect the brake lines at the master cylinder because I thought it might cause slightly less air to get into the system. If you disconnect at the DSC module that may make things a bit easier, but the lower nut on the MC is probably always going to be a pain to get too. I think the DSC module and maybe some additional lines would need disconnected to be able to get the booster out of there.

[$iriu$black]

Hey @Jeb_, how did you find this mod? I, too stumbled upon that thread about MC swaps being somewhat 50-50 in terms of improvement. I wanted to do the whole F30 front brake swap and actually have all parts except for the MC itself, but hesitant due to losing that pedal feel. The tiny front brake for the 128i has been sufficient for me so far with the way I drive on track and besides, I still have a very thick race pad on there that I can use this season. I’m looking tonsave some weight upfront with this upgrade eventually and wanted to everything right the first time. I’m not as technical as you but really try to do my research from others’ experiences and hopefully learn from it.

Thanks for your input.

Originally Posted by Jeb_

I got the master cylinders swapped today. Just have to reattached the booster to the firewall and then bleed the system. Got brake fluid everywhere and got frustrated with how little room there is to get to everything, but overall it wasn't too difficult. I guess I should wait to see if the MC is making a good seal with the booster before I say that though.

I worked through it a little faster than I usually like because I was losing sunlight, but it seems like it would be a lot more difficult to swap the booster too. Even with the booster unbolted I was only able to tilt it upward slightly before it hit on stuff. I did disconnect the brake lines at the master cylinder because I thought it might cause slightly less air to get into the system. If you disconnect at the DSC module that may make things a bit easier, but the lower nut on the MC is probably always going to be a pain to get too. I think the DSC module and maybe some additional lines would need disconnected to be able to get the booster out of there.

[Jeb_]

Originally Posted by $iriu$black

Hey @Jeb_, how did you find this mod? I, too stumbled upon that thread about MC swaps being somewhat 50-50 in terms of improvement. I wanted to do the whole F30 front brake swap and actually have all parts except for the MC itself, but hesitant due to losing that pedal feel. The tiny front brake for the 128i has been sufficient for me so far with the way I drive on track and besides, I still have a very thick race pad on there that I can use this season. I’m looking tonsave some weight upfront with this upgrade eventually and wanted to everything right the first time. I’m not as technical as you but really try to do my research from others’ experiences and hopefully learn from it.

Thanks for your input.

The brakes feel great after swapping in the M3 master cylinder. Pedal firmness wise it feels really close to the factory setup, maybe just a bit stiffer with a little less travel before the bite really starts to pick up. After the master cylinder was swapped I didn't notice any difference in feel before and after I had the car coded for performance brakes, but I suspect that's something that would only be apparent during heavy braking and track driving which I haven't done since the swap.

Before I swapped the master cylinder, the pedal did stay soft for noticeably more travel before the the initial bite, but then it felt pretty normal. That being said, I did very little driving of the car after swapping brakes but before swapping master cylinders, and I'm not entirely sure I got all of the air bled from the system. I remember reading somewhere that BMW recommends bleeding the f30 calipers with a DSC pulsing routine, like INPA, because the design of the calipers makes it hard to get all the air out without the vibrations. At that point I had only bled the brakes with a power bleeder.

So I'm not sure if my experience is completely comparable with other people that have done this swap. It looks like most swap the front brakes and leave the rears alone and don't have any complaint about the pedal feel. Even if all the air was out of my system, it could have been the additional volume added by the 135i rear brakes in addition to the f30 fronts that caused my pedal to get soft.

If you're just swapping the front brakes I think it would be worthwhile to do the swap and see if your happy with the pedal feel before going further. If at that point you do want to get a stiffer pedal feel, you aren't really adding any more work by doing it in two steps other than having to bleed the brakes again. Not that the master cylinder swap is incredibly involved, but to me it wasn't a fun job and it was frustrating.

[$iriu$black]

Originally Posted by Jeb_

The brakes feel great after swapping in the M3 master cylinder. Pedal firmness wise it feels really close to the factory setup, maybe just a bit stiffer with a little less travel before the bite really starts to pick up. After the master cylinder was swapped I didn't notice any difference in feel before and after I had the car coded for performance brakes, but I suspect that's something that would only be apparent during heavy braking and track driving which I haven't done since the swap.

Before I swapped the master cylinder, the pedal did stay soft for noticeably more travel before the the initial bite, but then it felt pretty normal. That being said, I did very little driving of the car after swapping brakes but before swapping master cylinders, and I'm not entirely sure I got all of the air bled from the system. I remember reading somewhere that BMW recommends bleeding the f30 calipers with a DSC pulsing routine, like INPA, because the design of the calipers makes it hard to get all the air out without the vibrations. At that point I had only bled the brakes with a power bleeder.

So I'm not sure if my experience is completely comparable with other people that have done this swap. It looks like most swap the front brakes and leave the rears alone and don't have any complaint about the pedal feel. Even if all the air was out of my system, it could have been the additional volume added by the 135i rear brakes in addition to the f30 fronts that caused my pedal to get soft.

If you're just swapping the front brakes I think it would be worthwhile to do the swap and see if your happy with the pedal feel before going further. If at that point you do want to get a stiffer pedal feel, you aren't really adding any more work by doing it in two steps other than having to bleed the brakes again. Not that the master cylinder swap is incredibly involved, but to me it wasn't a fun job and it was frustrating.

Thanks for that insightful response. I think I’m going to pull the trigger and get myself an M3 MC and reservoir. Unlike you I am only planning on a front upgrade at this time as the rears require more work and parts. I don’t know how track season this year is going to pan out but I’m hopeful i can put this to test.

I only have a pressure bleeder but have heard a lot of good things about INPA/software bleeding. TBH this I think is out of my league just because I have NOT done yet ever but I’m sure it’s worth it and makes the process more complete. What is a reliable source for this software?

Thanks again, good luck w your goals w the car.

[Jeb_]

Originally Posted by $iriu$black

Thanks for that insightful response. I think I’m going to pull the trigger and get myself an M3 MC and reservoir. Unlike you I am only planning on a front upgrade at this time as the rears require more work and parts. I don’t know how track season this year is going to pan out but I’m hopeful i can put this to test.

I only have a pressure bleeder but have heard a lot of good things about INPA/software bleeding. TBH this I think is out of my league just because I have NOT done yet ever but I’m sure it’s worth it and makes the process more complete. What is a reliable source for this software?

Thanks again, good luck w your goals w the car.

No problem. Yeah, at this point it looks like my first autocross back will be in mid July. My next track day will likely be after that, so it's going to be a while before I get a good feel for these new brakes.

I'm always a little daunted by any work on a car that involves a computer, but the INPA bleed is really straight forward and I had barely any experience with the program before. I had downloaded it as part of "Mike's Easy BMW Tools" when I was swapping instrument clusters. I downloaded it from the link in chris_flies' writeup:

https://www.1addicts.com/forums/show....php?t=1655093