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Old 03-11-2010, 06:11 PM   #1
diamondnik
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Default Master cylinder sizing.

Ok, long time ago I installed an e36 M3 master cylinder because I hated the squishy feel of the stock brake pedal, and I knew that I would upgrade my brakes at some point. Except that instead of M3 fronts (60mm piston) I installed e46 330 front brakes (57mm piston). I've been driving around like this for some time now and have mixed feelings about how well this arrangement works. I have larger rear calipers on the way (38mm piston) because I noticed that the rear just wasn't working well at all; pads and rotor would never really warm up and it was just causing galling on rotors (new rotors, brake lines and rebuilt calipers). Also, I don't think its normal to get over 100 000km out of same set of rotors and brakes...

So here is what I found regarding this. The M3 master cylinder is stepped; it's ID is 25.40 Front and 20.64 Rear, while on the TI it is 23.81 F/R. I looked at the 330 master and it is also stepped (23.81/22.20). So I looked into this reason, and figured that it must have something to do with ATC or DSC (on e46+). For this reason rear lines have different pressure, presumably to soften/dull line pressure when traction control kicks in. And since my Ti doesn't have traction control, I don't need a stepped master cylinder? And was the reason I got so much life out of my rotors and pads because rear was 20.64 ID?

So to recap:

e36 M3 60/40 caliper pistons and 25.40/20.64 MC
e36 Ti 54/34 caliper pistons and 23.81/20.64 MC
e46 330 57/44 caliper pistons and 23/81/22.20 MC

I can see how the M3 needs a larger diameter MC for front pistons, given their size, but if the 330 is fine with 23.81 front ID, then maybe I need to switch back to my original Ti master cylinder. Another thing concerning 'feel' generated by MC sizing. Originally I wanted the larger MC to stiffen the pedal and give better feel, like what you'd get in a competition car without power assist. But I think that the large MC just gives, what we'd call in motorcycle circles 'wooden lever' feel, in other words, you need lots of force to push fluid and get same result that you'd get from smaller MC and less needed force (it also dulls feel of line pressure). The Ti MC would give a softer pedal and still same braking power. The thing is, I can't even remember what the original Ti brake pedal felt like. On my one motorcycle I found that original MC made braking a chore and I'd need 4 fingers to modulate brakes, while two fingers with smaller diameter MC. I wonder how things would feel with the original MC.... Any thoughts on this issue?

Last edited by diamondnik; 11-01-2010 at 09:15 PM. Reason: Paragraph Spacing all wrong after submitting
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Old 03-12-2010, 01:02 AM   #2
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if you decide to switch out, i may be interested in the m3 MC for my 318i drift car..
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Old 03-14-2010, 05:37 AM   #3
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Ive driven mine with m3 front brakes, oe pads, ti MC, and rubber brake lines. There was a very slight change in pedal from the stock ti solid setup, the pedal would travel ~1/4" further than before to apply pressure fully.

After installing SS lines and changing the fluid, i regained that ~1/4" plus another 1/4". There is no mush in the pedal, modulation is great, and the stopping power is always there, though can still be better.

Checking into valving and sizing, my next setup is Porsche 996 turbo front and rear 4 piston calipers and E46 M3 CSL rotors. Using the E36 m3 spindles, MC, EBC green stuff pads, and possible parking brake delete (havent made it there yet), it will have proper brake bias as a whole and the 245/285 BFG tires will help slow this car beyond any of the Mono-piston brake systems and wheel package offered to the e36.
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Old 03-15-2010, 04:03 PM   #4
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just completed installing stainless lines, abs delete, parking brake delete, and hydro handbrake fabrication on the e36 318i drift car. first test is this weekend, havent driven it yet... i like a firm pedal, almost like manual brakes, where modulation happens as hard as i can push. i prefer the feel, so i may go to the m3 mc.. depends on what i find out this weekend.
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Old 03-17-2010, 01:53 PM   #5
e36 323ti
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Quote:
Originally Posted by diamondnik View Post
Ok, long time ago I installed an e36 M3 master cylinder because I hated the squishy feel of the stock brake pedal, and I knew that I would upgrade my brakes at some point. Except that instead of M3 fronts (60mm piston) I installed e46 330 front brakes (57mm piston). I've been driving around like this for some time now and have mixed feelings about how well this arrangement works. I have larger rear calipers on the way (38mm piston) because I noticed that the rear just wasn't working well at all; pads and rotor would never really warm up and it was just causing galling on rotors (new rotors, brake lines and rebuilt calipers). Also, I don't think its normal to get over 100 000km out of same set of rotors and brakes...

So here is what I found regarding this. The M3 master cylinder is stepped; it's ID is 25.40 Front and 20.64 Rear, while on the TI it is 23.81 F/R. I looked at the 330 master and it is also stepped (23.81/22.20). So I looked into this reason, and figured that it must have something to do with ATC or DSC (on e46+). For this reason rear lines have different pressure, presumably to soften/dull line pressure when traction control kicks in. And since my Ti doesn't have traction control, I don't need a stepped master cylinder? And was the reason I got so much life out of my rotors and pads because rear was 20.64 ID?

So to recap:

e36 M3 60/40 caliper pistons and 25.40/20.64 MC
e36 Ti 54/34 caliper pistons and 23.81 MC
e46 330 57/44 caliper pistons and 23/81/22.20 MC

I can see how the M3 needs a larger diameter MC for front pistons, given their size, but if the 330 is fine with 23.81 front ID, then maybe I need to switch back to my original Ti master cylinder. Another thing concerning 'feel' generated by MC sizing. Originally I wanted the larger MC to stiffen the pedal and give better feel, like what you'd get in a competition car without power assist. But I think that the large MC just gives, what we'd call in motorcycle circles 'wooden lever' feel, in other words, you need lots of force to push fluid and get same result that you'd get from smaller MC and less needed force (it also dulls feel of line pressure). The Ti MC would give a softer pedal and still same braking power. The thing is, I can't even remember what the original Ti brake pedal felt like. On my one motorcycle I found that original MC made braking a chore and I'd need 4 fingers to modulate brakes, while two fingers with smaller diameter MC. I wonder how things would feel with the original MC.... Any thoughts on this issue?
Are you sure the e36 m3 has different bore front and rear? According to ATE's web site the master cylinder for the e36 m3 (s50b30 and s52b32 (us-model)) has 25.4mm both front and rear.

Your findings regarding the handling of the car with your setup with e46 330 up front and stock ti-rear fits within my expectations. The stock ti has a brake bias of 72.62%. With your current setup this is changed to 77.05% (assuming equal MC-bore front and rear), which in my opinion is too much front bias.

When replacing brake parts, doing the bias calculations would indicate how the balance in the car changes. How to do the calculations could be found in the following link: http://www.318ti.org/forum/showthread.php?t=12852

My philosophy is to choose brake parts to keep the bias close to stock or maybe a tiny bit less front biased.

Then assume:
Fb = The force applied the brake pedal by the foot (typical 170 N).
FbAmp = The amplification of the brake leveler and the vacuum system (typical 4*6.9)
uPad = Coefficient of friction (typical 0.5)
dcal = diameter caliper piston (mm)
dmc = diameter master cylinder (mm)
dbd = brake disk diameter (mm)

The front brake torque (TF) can then be calculated as

TF= Fb*FbAmp*2*F.uPad*(F.dcal/F.dmc)^2*((F.dbd-F.dcal)/4)/1000 (F. means front)

The equation can be used for the rear torque as well (TR), using the appropriate data for the rear brake system.

What is interesting to analyze is how the changes in the brake master cylinder affects TF, i.e. the term (F.dcal/F.dmc)^2. You have the data to do the exercise yourselves.

Hence comparing to systems where one only changes the master cylinder, one should expect to use more force on the pedal to achieve the same brake torque when increasing the diameter on the master cylinder.

However, since increased diameter on the master cylinder means higher liquid volume pr. mm-master-cylinder-piston-travel, the pedal travel should decrease since the amount of brake fluid 'requested' by the calipers are the same in both cases (assuming a stiff/incompressible system).

Last edited by e36 323ti; 03-18-2010 at 10:14 PM.
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Old 03-19-2010, 06:02 AM   #6
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wow. my head hurts.

engineer much? good info i'm sure but i make no sense of it.
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Old 03-19-2010, 02:15 PM   #7
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My dad's M Roadster has over 120k miles on it with the original brakes and rotors. No idea how that happened, but those things just wear very slowly.
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Old 03-23-2010, 12:35 AM   #8
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Regarding the stepped-bore size for e36 m3 MC, I got information from ATE North America (http://www.showmetheparts.com/ate/). I got my 38mm rear calipers today (91 525i) so as soon as I get them refurbished they're going on and I'll see what difference that makes.

I noticed that Audi A4 cars come equipped with 57 front and 38 rear pistons as well as 23.81 MC... I know that sedans tend to be at least 200 pounds heavier over the rear wheels because of trunks, but wider tires on my Ti will compensate for increased caliper pistons.
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Old 03-23-2010, 10:45 AM   #9
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Quote:
Originally Posted by diamondnik View Post
Regarding the stepped-bore size for e36 m3 MC, I got information from ATE North America (http://www.showmetheparts.com/ate/).
I wonder why this do not show in the European web site? There must be an error some place...
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Old 03-24-2010, 12:32 AM   #10
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The website I used coincides with realoem information, for example e34 525i uses a 25.4 MC up to 9/91, then it switches to 25.4/20.66.

So after crunching some numbers it seems that when I installed the 25.4/20.64 MC with otherwise stock brakes (54/34) brake bias shifted considerably - in a bad way. With same amount of force, compared to standard 23.81 the new MC caused decreased line pressure in front and considerably increased pressure in the back.

The brakes I plan on using now (57 in front with 325 disc, and 38 in back with 272 disc) will offer almost same as stock brake bias if used with 23.81 or 25.4 master cylinders, but when used with stepped 25.4/20.64 brake bias changes; theoretically the change would increase rear line pressure but I'm not sure that is happening because rears never lock up right now. For those rears to get more line pressure the MC piston would also have to move farther but if the pedal doesn't move more, rear pistons aren't getting more pressure. What I mean is, the large bore responsible for line pressure creates stiff feeling and modulation is affected it.
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Old 03-27-2010, 06:10 PM   #11
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Quote:
Originally Posted by diamondnik View Post
The website I used coincides with realoem information, for example e34 525i uses a 25.4 MC up to 9/91, then it switches to 25.4/20.66.

So after crunching some numbers it seems that when I installed the 25.4/20.64 MC with otherwise stock brakes (54/34) brake bias shifted considerably - in a bad way. With same amount of force, compared to standard 23.81 the new MC caused decreased line pressure in front and considerably increased pressure in the back.

The brakes I plan on using now (57 in front with 325 disc, and 38 in back with 272 disc) will offer almost same as stock brake bias if used with 23.81 or 25.4 master cylinders, but when used with stepped 25.4/20.64 brake bias changes; theoretically the change would increase rear line pressure but I'm not sure that is happening because rears never lock up right now. For those rears to get more line pressure the MC piston would also have to move farther but if the pedal doesn't move more, rear pistons aren't getting more pressure. What I mean is, the large bore responsible for line pressure creates stiff feeling and modulation is affected it.
I am a bit confused.....
The master cylinder for our ti's (318ti/323ti) is said to be 23.81mm. The same master cylinder is used by several other BMW's as well. On my master cylinder it can be read the numbers 23/20 (see the circle in the picture below). I suspected that this meant a diameter of 23.81mm front and 20.64mm for the rear although this is not read by the datasheets.

I had an old master cylinder in my garage and decided, in the name of research, to dismantle it and do some measurements.




As can be seen from the picture, the results of the measurements for the front are 23.8mm (i.e. 23.81mm) and the rear is 20.6 (i.e. 20.64mm) and the master cylinder for our ti's are definitively stepped. Is there a possibility that the standard rear diameter used for the BMW master cylinders is 20.64mm if not otherwise noted?
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Old 03-27-2010, 10:40 PM   #12
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Last week I looked at the car's original master cylinder, it is also stamped 23/20. I assumed that number pertained to something else, but now I see that my original 318ti MC was stepped as well (Canadian market Ti). I wonder what the relationship is between front and rear line pressure, and how it changes when increasing only front piston size...
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Old 03-28-2010, 02:29 PM   #13
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Quote:
Originally Posted by diamondnik View Post
Last week I looked at the car's original master cylinder, it is also stamped 23/20. I assumed that number pertained to something else, but now I see that my original 318ti MC was stepped as well (Canadian market Ti). I wonder what the relationship is between front and rear line pressure, and how it changes when increasing only front piston size...
I have asked ATE for an explanation. I hope they respond....

Assuming our latest discoveries are correct (i.e. =23.81/20.64mm) a stock 318ti/323ti has a static front bias of 66.59%. Replacing the stock master cylinder with one with diameters =25.4/20.64mm (e36 m3 3.0 MC) the bias changes to 63.65% - i.e. the static front bias is moved approx. 3% to the rear.

The e36 m3 3.0 (assuming =25.4/20.64mm) has a static front bias of 61.21%...

With e46 330i disc (325mm) and caliper (=57mm) up front, stock ti' rear and m3 MC the static front bias of a ti' is 68.92%...
With e46 330i disc (325mm) and caliper (=57mm) up front, stock ti' disc rear but caliper =38mm rear and m3 MC the static front bias of a ti' is 63.97%...

Last edited by e36 323ti; 03-28-2010 at 10:36 PM.
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Old 03-30-2010, 02:58 AM   #14
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323ti, thanks for crunching those numbers.

i installed the 38mm calipers in the rear today and my first seat of the pants impression is that things work much better now; the car is stopping so much nicer and crisper. it feels like the brake pedal is softer now, but that was to be expected anyway.
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Old 03-30-2010, 02:55 PM   #15
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Quote:
Originally Posted by diamondnik View Post
323ti, thanks for crunching those numbers.

i installed the 38mm calipers in the rear today and my first seat of the pants impression is that things work much better now; the car is stopping so much nicer and crisper. it feels like the brake pedal is softer now, but that was to be expected anyway.
I expected that a change of approx. 5% in bias would be noticeable. It is interesting to hear that you noticed it.
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