Brake Bias Calculations and Master Cylinder Selection

[$iriu$black]

Originally Posted by Jeb_

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

Thanks for the source, I knew i saw it from here before. And I agree, working w computers on a car is always stressful as I want to make sure I don’t brick it.

So, just to be sure, you used the M3 MC and the Reservoir for this swap, correct?

[Jeb_]

Correct. The master cylinder and reservoir came from an E9x M3. The clutch and brake master cylinders share a reservoir so I made sure the donor car I got mine from was a manual like my 128i. The reservoirs did seem to be the same between the 128i and M3 but I used the M3 one just to be sure.

[$iriu$black]

Originally Posted by Jeb_

Correct. The master cylinder and reservoir came from an E9x M3. The clutch and brake master cylinders share a reservoir so I made sure the donor car I got mine from was a manual like my 128i. The reservoirs did seem to be the same between the 128i and M3 but I used the M3 one just to be sure.

Thanks mate. Cheers!

[chris_flies]

Update regarding brake bias concerns: pffft. The brakes are absolutely PERFECT. The car is wayyyyyyyyyyyyy more stable in threshold braking, the pedal height is perfect for downshifting, and the effort isn't that high. I was absolutely dumbfounded how much better the car was stopping last weekend.

[iminhell1]

Originally Posted by chris_flies

Update regarding brake bias concerns: pffft. The brakes are absolutely PERFECT. The car is wayyyyyyyyyyyyy more stable in threshold braking, the pedal height is perfect for downshifting, and the effort isn't that high. I was absolutely dumbfounded how much better the car was stopping last weekend.

Just curious what brake fluid you're using (blue is my gut feeling)?

I know when I swapped the DOT4 for DOT5.1 (I'm on the Bosch stuff) the peddle got softer. But the reduction in high speed fade was worth it. In time I'd like a little more solid peddle feel.

[Driven5]

Originally Posted by chris_flies

The car is wayyyyyyyyyyyyy more stable in threshold braking

This is telling, but in hindsight perhaps not entirely unexpected.

The more grip you have, the more weight transfers forward, and the more the 'ideal' brake bias moves similarly. So your stickier tires have taken whatever the bias was for stock grip, and made it effectively more rear biased at max brakingl. So moving the brake bias forward with the the mild 'BBK' only on the front may have actually ended up being more of a bias correction in this case.

Then again, a tire a mass longitudinal grip loses lateral function. So even for with the improved feel, the increased stability could actually be a sign of reduced total braking capacity from now under-utilized rears.

What the physics says is largely irrelevant when out up against the stomach though. So even if that added stability is a net takeaway but gives you the confidence to brake later, it's still a win anyway.

[chris_flies]

Originally Posted by iminhell1

Just curious what brake fluid you're using (blue is my gut feeling)?

I know when I swapped the DOT4 for DOT5.1 (I'm on the Bosch stuff) the peddle got softer. But the reduction in high speed fade was worth it. In time I'd like a little more solid peddle feel.

DOT 4, ATE Type 200. No fade at all, but it wasn't really a very brake-heavy circuit.

[chris_flies]

Originally Posted by Driven5

This is telling, but in hindsight perhaps not entirely unexpected.

The more grip you have, the more weight transfers forward, and the more the 'ideal' brake bias moves similarly. So your stickier tires have taken whatever the bias was for stock grip, and made it effectively more rear biased at max brakingl. So moving the brake bias forward with the the mild 'BBK' only on the front may have actually ended up being more of a bias correction in this case.

Then again, a tire a mass longitudinal grip loses lateral function. So even for with the improved feel, the increased stability could actually be a sign of reduced total braking capacity from now under-utilized rears.

What the physics says is largely irrelevant when out up against the stomach though. So even if that added stability is a net takeaway but gives you the confidence to brake later, it's still a win anyway.

Doing the "BBK" made no difference in braking stability or perceptible bias. It was still twitchy and slightly terrifying blasting into the braking zones, stickier tires did *slightly* improve it (a hair better, maybe), but, it was after doing this master cylinder that I actually felt confident and in-control entering into braking zones.

I'll re-evaluate when I return to less sticky tires after I burn through this set of RE71R's (going back to Indy 500's for their street manners). The only other change is going from an MFactory 1.5-way LSD to a DiffsOnline 35/60 LSD, but the DiffsOnline LSD has far less decel lockup, which would tend to reduce stability under braking.

[Driven5]

So you noticed a difference in threshold braking from a master cylinder operating the same size piston under those conditions? Interesting. Off hand I'm struggling to come up with a technical reason you should feel any difference once the master cylinder bypass opens up under pressure.

Perhaps it's additional control (technical softer pedal) after the calipers are fully engaged, that was technically already there since in F30 brake install, combined with the firmer pedal and faster caliper take-up from the new matter cylinder large bore just making it feel more natural to get there? I'll have to mull on that one a bit.

Dual bore masters sizing may be more interesting than I had previously given them credit for. I learn something new every day.

[chris_flies]

Originally Posted by Driven5

So you noticed a difference in threshold braking from a master cylinder operating the same size piston under those conditions? Interesting. Off hand I'm struggling to come up with a technical reason you should feel any difference once the master cylinder bypass opens up under pressure.

Perhaps it's additional control (technical softer pedal) after the calipers are fully engaged, from the master actually being smaller relative to the caliper pistons than stock, combined with the firmer pedal and faster caliper take-up from the new matter cylinder large bore? I'll have to mull on that one a bit.

Dual bore masters may be more interesting than I had previously given them credit for. I learn something new every day.

I think you're thinking the DSC module in these cars is way more advanced than it actually is. The brake feel, through the entire length of the pedal, is different. The take-up isn't different. It isn't pre-loading the brakes with the big piston. (It does that anyways with the internal pump) Nothing weird or complicated is going on. Big piston flows more fluid in less stroke making the pedal shorter.

As far as I could see while I was in there installing it, the front and rear fluid channels are isolated, which makes perfect sense for safety's sake, perfectly explains both my observations on the foot dyno, and my intentions with this little endeavor when buying the parts to do the modification.

I'll have to do some research on this specific DSC module, it would be good to know some stuff about it...

EDIT: It's the same basic DSC Mk60 as in the turn-of-the-century BMW's (actually DSC Mk60E5). Nothing special is going on here. You're probably thinking about F-chassis cars.

Another Edit: https://www.newtis.info/tisv2/a/en/e...system/1tHNoSj

There! The front and rear are two entirely separate hydraulic systems. BAM! Hunch confirmed.

[Driven5]

Originally Posted by chris_flies

Another Edit: https://www.newtis.info/tisv2/a/en/e...system/1tHNoSj

There! The front and rear are two entirely separate hydraulic systems. BAM! Hunch confirmed.

Nice find!

I was only able to locate one technical reference and a few comments, that all had to be inferred from, which had me inclined to believe it was a diagonal split system. That's exactly the type of resource I was looking for! I'm still trying to get up to speed on the intricacies of these BMW's as fast as I can, but obviously still have a ways to go yet. LOL

So the fact that it's a F/R split rather than diagonal, means that it is possible the stepped bore master could be applied to large front/small rear as well, but would not necessarily be so either. However, at this point I'm not going to even venture a guess, as there are more questions than answers, and indicators pointing both directions.

Regardless, thanks for the correction.