ABS Delete

E28 technical advice asked and given! Troubleshooting, modifications and more.

ABS Delete

Postby spacerace » Jun 17, 2012 6:36 PM

I took out the ABS unit from the front of the engine bay and now I'm trying to find a T union that can connect the master cylinder lines to the brake lines that go to the individual wheels but I cant find one that fits. I have bought three different sizes that have not worked and I am thinking it is a special size. Please let me know what size it is and possibly where I can get it. It doesn't seem like it should be that hard to find.

Thanks in advance!! :)
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Postby m535is » Jun 17, 2012 7:29 PM

This is something you need to be very careful with. The simple fact is that the brake fluid is under a lot of pressure so you need something rated for high pressures. I personally would go and get Ts from BMW.

http://www.realoem.com/bmw/showparts.do ... g=34&fg=15

I would use 2 of item number 3. I would also suggest that you connect the fronts together and the rears together. This way you can do adjustable bias if you wanted to.

Hope this helps.
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Postby mooseheadm5 » Jun 17, 2012 7:55 PM

The proper way to do this is to redo the entire system so that it has the same split braking system that the non-ABS cars have as seen here with right front and left rear on one circuit and left front and right rear on another:

Image

Image

This is so that you will not lose all of your brakes at once if one caliper goes out. The way that the ABS system is set up, there are two inputs and 3 outputs (FL, FR, and Rear.) If you just tee together the front lines and run them on one master output and run a union from the other master output to the rear line, you will have uneven braking force across the two circuits. You will note that the non-ABS E30s run a different type of master cylinder along with a brake pressure regulator for the rear circuit to balance out the brakes.
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Postby m535is » Jun 18, 2012 1:05 AM

Moose that is actually really incorrect.

We are dealing with a sealed system and pressure. The way the ABS system runs is that there is one output from the master cylinder for the front brakes and one for the rear. While it is true that BMW did a cross braking for Non-ABS cars, this is not necessary. In fact I personally would like that in an emergency I would have 1 axle of brakes still functioning rather than diagonals. Anyway moving on.

The simple fact is this, the master cylinder puts out pressure based off of how much force is put into the master cylinder and the diameter of the pistons inside of it. Then what you get out is line pressure. Regardless of how many cylinders (aka calipers) you drive the output pressure is always the same. Therefore the amount of force placed through the brake pad is determined by the given piston diameter in the caliper. This force comes from the line pressure and since the pressure is not lost by adding addition calipers your statement is wrong.

You state that the E30 with non-ABS runs a different master that requires a brake pressure regulator. The second half of that statement is also false. All E30s with 4 wheel disks have rear brake pressure regulators. This is because the E30 uses a stepped master cylinder with a much smaller rear piston diameter which leads to much higher line pressure. Therefore to keep the proper brake bias front to rear the rear pressure has to be limited.

Now on to some practical experience. I have personally designed many brake systems for BMWs both street and track. Here is a great example.
Take an E30 M3 stock brake setup. Replace the front calipers with E28 535i calipers, direct bolt on. Remove the pressure regulator. Now you have a 68% front bias instead of the stock 73%. Works great has no ill effects.
Take the E9 that we built. We use an E28 535i master cylinder. We use E23 front calipers and E9 rears. We have run 1 line to the rear of the car and T'ed it there. The front we have 1 line from the master that is T'ed and runs to both fronts. Works great has about a 69% front bias.
This does not include the 5 different brake setups I have used on my 5.
Stock, E28 M5 Stock, E28 M5 front 540i rear, E34 M5 with E28 MC and E34 M5 with E34 M5 MC. I have done the balance calcs on all and proven them in real world, including the fact that the 3rd iteration gives too much rear bias and locks the rears before the fronts.
E30 318i non-ABS to 4 wheel disks for race setup. This was fun because not only did we have to change the master to get the right base proportion so the bias knob would work, but we also had to redo a lot of the plumbing. That was fun.

Now more calipers per output or larger diameter pistons in those calipers have only one effect, which is known as compliance volume. This basically means the amount of fluid that has to be pushed out of the master through the lines to get the calipers to squeeze the pistons. This compliance volume has nothing to do with bias or safety. In fact this is the only reason people upgrade to E34 master cylinders. The E34 master has a larger piston and therefore requires less input travel to cover the compliance volume. The drawback is that you must push the E34 master harder to get the same line pressure and therefore same braking effect.

So all in all, Ting the front together and the rears together will be fine. In fact the ABS pump is neutral unless the ABS is cycling anyway, so this is just like pulling the ABS relay.
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Postby C.R. Krieger » Jun 18, 2012 10:12 AM

m535is wrote:While it is true that BMW did a cross braking for Non-ABS cars, this is not necessary. In fact I personally would like that in an emergency I would have 1 axle of brakes still functioning rather than diagonals.


Maybe you should rethink your preference. If you have 'axle' braking, especially with the rears only, you lose a lot of braking if the front goes out. With rear brakes only, you're also almost sure to spin. That's elementary handling dynamics. In addition, because about 70% or your braking force comes from the front, if you lose them, you're down to 30% - with poor directional control.

OTOH, if you have dual diagonal braking, the loss of either half takes only 50% of your total braking and you will still have good directional control because it will stop straight no matter which circuit has gone out. Wanna know how I know? ;)
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Re: ABS Delete

Postby Mark 88/M5 Houston » Jun 18, 2012 11:25 AM

spacerace wrote:I took out the ABS unit from the front of the engine bay and now I'm trying to find a T union that can connect the master cylinder lines to the brake lines that go to the individual wheels but I cant find one that fits. I have bought three different sizes that have not worked and I am thinking it is a special size. Please let me know what size it is and possibly where I can get it. It doesn't seem like it should be that hard to find.

Thanks in advance!! :)


Why? :?

I'm "old skool" and I wouldn't want to give up ABS on the M5 (or any other street driven vehicle) here in the Houston area.
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Postby mooseheadm5 » Jun 18, 2012 11:44 AM

It is not "really incorrect".

Diagonals is just the way they used to do it. A split system was required by law, and that is what they came up with. Because of brake bias, you were guaranteed overall better total braking if you lost one circuit vs. if you lost the front axle brakes on a front/rear split. Because the volume and total piston surfaces in each circuit are equal, you are guaranteed equal line pressure on both circuits.

You are thinking of this as a sealed system with only one master cylinder piston, but this is not the case. The system has two circuits. Because of this you can, in fact, have uneven braking pressure. There are two thing going on here. The first is fluid movement. At first, there is no braking force at all. The fluid moves through the system to move the pistons in their bores and take up all the slack. The rear pistons are smaller, so they move further with the same stroke of the pedal. They will contact their rotors first. At the point where the rear pads are pressing against the rotors, but the fronts are not yet, you will have pressure in the rear line, but not in the front. Depending on the difference in size between the front and rear and your MC diameter and stroke, you may actually run into a situation where the master is almost bottomed out by the rear calipers alone. This can result in the front brakes getting little or no pressure at all. Obviously, you have found this not to be the case on these cars in particular, but it is definitely a possibility, and I have seen someone run into this problem on an 02. Without actually doing the math, I stand by my initial statement that the "correct" way is to set it up the way the non-ABS cars were set up. If modifications are made like you have done in the past, you have proven out that there would be no problem.

As an aside, you typically move brake bias rearward when you do your brake setups. This generally means larger pistons, which pretty much removes the possibility of bottoming just the rears first. Using a larger MC with these setups makes it pretty much impossible to run out of stroke on one circuit.

As for the E30 stuff, turns out we are both partially incorrect. The MC size and configuration was determined by ABS or non-ABS, and whether it had Girling or ATE brakes installed. My 87 325e had ABS and a non-stepped master and no brake pressure regulator. According to the parts catalog, the BPR had to do with GVWR. The 318is I parted didn't have it.
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Postby C.R. Krieger » Jun 18, 2012 11:54 AM

If you really wanted to do this right, you'd do it the way Volvo used to. They had two three-wheel braking circuits. Each circuit had both front brakes (four piston calipers) and one rear brake. That way, if you lost one circuit, you had over 80% of your braking force left. Only one rear wheel would be left unbraked. I always thought that was simply brilliant.
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Postby m535is » Jun 18, 2012 12:56 PM

Moose I am still not with you on this. You do push fluid, but the amount you are pushing is TINY. The simple fact is if you are pushing a lot of fluid you have 1 of 2 problems, either you have a lot of air or you have a very very bad master cylinder to caliper piston diameter ratio.

But lets go through it logically so we can see where we differ.

So first paragraph. I agree that diagonal was the way it used to be done. There are a couple of issues with diagonal. First off as CR stated diagonal give 50% of total braking. The problem with this is as follows. Like CR said 70% or well it really depends on what setup you are running, of the braking is done by the front axle. If you then say half of that or 35% is being done by the left front and 15% by the right rear this creates a major force imbalance in the car. While this car be corrected via steering input this I find is not very stable. As for losing the front brakes and only having the rear, you will only spin if you are pushing the car hard and have lost the fronts. Personally I have boiled the fronts on track and found that the decreased effectiveness on the front axle has made it a challenge to drive, but not unsafe.

Paragraph 2. The system is 2 individually sealed systems. We use split master cylinders in these cars. Therefore while you can have uneven braking pressure between the 2 sealed systems you have to assume that the proper brake bleeding has been done which will result in a minuscule delta between the 2 systems. Without that assumption all is lost. Now here is where I get to the major issue. You said that the fluid moves through the system. The problem is this: No more fluid is added to the system, the reservoir is designed to only add fluid back to the system as the pads wear and take up the compliance volume. Therefore the actual movement of fluid through the system is TINY. You are correct that in most systems the rears contact first. This is caused because the compliance volume on the rear is typically less than the front. But, this is a good thing because this actually helps with braking stability. When the rear brakes start to stop the car first it actually helps to settle the car a bit. But let us be more accurate. The amount of movement that the caliper must make to start exerting force on the rotor is very very small. This is why when you rotate the wheel with the brakes off you can actually hear a little scraping. The gap between the pads and rotors is very very small. So that is why the amount of fluid and therefore stroke of the master is also small. I mean look at how far you move the brake pedal, and remember that the connection to the master cylinder is above that. The total input stroke on the master cylinder given by the pedal is very small. And then once you have taken up the compliance volume you actually do not push more fluid through, you are talking incompressible fluid and pads and rotors. And the idea that you could have reached full braking force in the rear without having the fronts working at all also goes back to the assumption everything is bled properly. This just doesn't happen. Now as for bottoming of the master cylinder. I have never had this happen on any braking setup I have used because there are ratios between MC bore and Caliper bore that are pretty much the norm. In fact unless you start buying aftermarket components I don't think you can buy BMW parts to get out of the correct ratio range. So I guess my point is this. If you have the system bled, the compliance volume is so small that you will never have what you are describing.
I understand about going the way BMW did it, and I fully respect that, in fact I am a champion of that more often than not, but in this case the ABS setup is exactly what OP is suggesting to do, just with the ABS pump removed. So therefore it is just like every E28 out there, only the ABS pump does not function. I can say this because the ABS pump on these cars does not provide any additional line pressure like versions from later cars that have traction control or EBD. And this also goes back to Paragraph one, if you lose an axle on an E28 with ABS, or any BMW with ABS for that matter you will have either front or rear.

Paragraph 3. As for rear bias, you are correct that you need to use a larger MC piston bore. But as you can research most BMW MCs have the same bore front and rear. This means that balance is all done by caliper piston bore diameter and rotor diameter.

Paragraph 4. Yeah there were a lot of variations. I guess the most experience with E30 braking systems I have are M3s and 325i. I do remember doing a rear disc conversion on a 318i and that required some new parts, but can't remember what we did.

Anyway, I suggest reading the following book.
http://www.amazon.com/High-Performance- ... 1932494324
I believe this is just the latest version of the one in my library, but unfortunately I don't have it here in Germany. This book gives the theory and the math to show what is going on.
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Postby C.R. Krieger » Jun 18, 2012 6:22 PM

m535is wrote:There are a couple of issues with diagonal. First off as CR stated diagonal give 50% of total braking. The problem with this is as follows. Like CR said 70% or well it really depends on what setup you are running, of the braking is done by the front axle. If you then say half of that or 35% is being done by the left front and 15% by the right rear this creates a major force imbalance in the car. While this car be corrected via steering input this I find is not very stable.

On the contrary, it's just as stable as if you had all four. I know this from the last time I crashed a car (that had broken a brake line) in the rain*. It stopped perfectly straight - with about as much braking force as you'd expect with half the brake system out of operation.

As for losing the front brakes and only having the rear, you will only spin if you are pushing the car hard and have lost the fronts. Personally I have boiled the fronts on track and found that the decreased effectiveness on the front axle has made it a challenge to drive, but not unsafe.


I think that's the assumption we have to make; otherwise we could all drive cars with only one wheel braked. We only learn about Bad Things because they happen at the limits of handling and, if your normal limits are halved, you can find it surprisingly easy to exceed them. For example, if your car normally stops at 1.0g on dry pavement, you'll find that it only does about 0.5g in the wet (I've measured it; that's what you get: half.). If you then lose your front brakes, you're down to about 0.15g stopping with the rears only. That really ain't much of a stop. Granny probably stops at 0.2 or so. The other physical fact is that the sliding tires have less friction than the rolling ones, so they tend to lead the car. If they're in the back, they'll want to pass the front - and the only way to do that is to swap ends. You always want to have at least one rear tire with traction so it keeps the rear end where it belongs: behind you.


* This was a stunningly educational experience for me in terms of putting all of these factors together and understanding what a diagonal braking system can do at the braking limits.
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Postby m535is » Jun 18, 2012 9:56 PM

CR I am not questioning you on your experience regarding the swapping ends thing or the stability, but BMW by way of the way every ABS car is plumbed from the master cylinder, has shown that it is a front or a rear circuit. So from that standpoint I have to say that having a front to rear split in the braking circuits cannot be all that bad since BMW has put it in every ABS equipped car they have ever built. I would think if this were such a large safety risk, BMW would have changed this a long time ago, but even today with modern ABS technology, there is a front and a rear circuit.
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