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Old 05-23-2002, 09:17 PM   #1
romanom
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Unhappy For those that like pulling the ABS fuse

I have heard that Subaru is looking into mechanical brake assist for the future. Now only looking, many have looked and most have baulked. Keep reading and you'll see why they decided against the idea.

Now for those that don't know what mechanical brake assist (MBA) is here's a quick description:

Inside the booster is a purely mechanical component that when the brake pedal is applied, at a certain distance and at that distance there is a certain pedal velocity the MBA applies full brake force for you. The velocity and distance is tuned to manufacturers specifications. MBA is purely an ON/OFF switch with only one setting for when it comes on.

Basically it assumes you are in an emergency and applies max brake force.

Now this is on some cars already, Nissan, Peugot, Toyota and some others. I don't believe there are any here in the USA yet.

Electronic Brake Assist (EBA) is on lots of cars, BMW, Mercedes, Lexus and others.

Now with EBA it is computer controlled, usually part of the ESP systems. And if the ABS fails so does the EBA.

With MBA however, if the ABS fails the MBA has no way of knowing and just keeps working.

So, if you pull your ABS fuse and then slam on the brakes going into Turn 2 the MBA takes over and you go into full lock up.


MBA can work quite well but it requires a functioning ABS system and there is no way of disconnecting MBA.

Why MBA? It's MUCH cheaper than EBA and can be put on cars that don't have expensive, fancy driver's aids.


I drove a few mule vehicles with this and it's great until you flick the ABS OFF switch and then hard braking is nearly impossible.
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Old 05-24-2002, 12:49 AM   #2
Jon Bogert
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Quote:
there is no way of disconnecting MBA
No way? Where does it plumb into the system? Likely just a big ol' master cylinder somewhere with a mechanical actuator that pressurizes the whole system when it activates. Thinking about it, how could they make it completely non-electric?
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Old 05-24-2002, 09:01 AM   #3
romanom
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Jon,

I've held the thing in my hand and designed boosters using it, it's purely mechanical.

Here's how is works:

Basically it uses 2 springs, 3 ball bearings and a ramp. It's designed in 2 pieces. When you apply the pedal if the velocity is great enough it overcomes the spring and allows the rear piece (which has the ramp) to move up along the bearings and release. The second spring then forces the the other piece forward until it locks. This locking forces the booster into it's most forward applied position just as if you had used your foot to do so.

The mechanism sits right in the middle of the booster as an internal part and there is no way of getting to it without destroying the booster.

You can add this feature to any car with ABS, no need for a new MC or expensive sensors or software development. Just swap booster.


MBA works by simulating your foot! It's nothing more than a lock and a spring really.


Ingenious idea, but it has real world flaws.

You can't see it, but the component is just behind the center plate on this booster demo cut-a-way.


"The Nissan Brake Assist System, being adopted in a set with an ABS, is the world's first brake booster incorporating a two-stage mechanical assist mechanism that boosts braking force in emergency situations."
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Old 05-24-2002, 09:13 AM   #4
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I want to pull the ABS fuse....where exactly is it?

Thanks,

Tim
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Old 05-24-2002, 10:20 AM   #5
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Just swap booster.
You got that right! While you're at it, you can swap in a single stage booster for better pedal feel. Two problems solved at once.
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Old 05-24-2002, 11:41 AM   #6
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These are the only things that matter when changing boosters:

1. It actually fits in the engine bay. The studs and rod pass through line up. It's got to package.

2. The input rod attaches to the pedal without it moving the original starting point of the brake pedal pad.

3. And the overall stroke of the booster is equal to or greater than that of the MC.


Meet the above and you can fit any booster you like.

If the first two don't fit, then you can always fab a spacer/adapter to sit between the booster and dash (firewall). And the input rod can be cut and then re-joined. Both these technics are used on OE boosters when a booster already exists on the shelf but just doesn't package right.

The last one is non-negoitable. Where you buy the booster should be able to tell you the stroke of the booster.


If you can, fit an E46 (1999-current) BMW 3 series single 10" tie-rod with 38mm stroke. Best light passenger car booster EVER! And the price proves it!
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Old 05-24-2002, 11:46 AM   #7
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Quote:
Originally posted by Jon Bogert

You got that right! While you're at it, you can swap in a single stage booster for better pedal feel. Two problems solved at once.

Actually MBA can be fitted to any booster of any size.

And Subaru is inquiring not buying. If they do it it will be no sooner than 2004 and may never be used on USA cars due to our lawyer rich environment.
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Old 05-24-2002, 11:56 AM   #8
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Quote:
If you can, fit an E46 (1999-current) BMW 3 series single 10" tie-rod with 38mm stroke. Best light passenger car booster EVER! And the price proves it!
Tell me more. I don't think anyone has tried fitting a non-Subaru booster yet. I--like many people--swapped the mushy dual stage booster for the old-style L booster and MC for better feel.

What are the parameters that determine the amount of boost? Diameter? Something else?
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Old 05-24-2002, 12:58 PM   #9
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Simplified Diagram





Now brake assist ratio is determined by the ratio of the diameter of the valve piston and the diameter of the reaction washer.
If you look at the picture above, start at the point where the booster meets the pedal and follow that to the ball at the end of that rod. Attached to the ball is a piston. This piston contacts a rubber reaction washer directly opposite (not shown). The ratio of the diameters dictates the boost ratio. The washer is always larger. So it's Washer/Valve.

Boost ratio is what ratio of force in/ force out. The size of the booster does not matter in this. Example, ratio of 6.5:1, 10lbs in 65lbs out.

Now the volume of the chambers on either side of the booster determine pedal effort. As you can see there is resistance from springs, fluid, seals, washers, friction, etc. The size of the booster determines how much of this is "negated" at the pedal. In other terms the booster size assists on the drivers side (the input side) not the output side.

It's like having two legs push the pedal rather than one. It multiplies your force. It acts very much like pedal ratio, the greater the pedal ratio the greater the mechanical leverage.


So the ratio always stays the same, in/out doesn't change with booster size. It's only how much help you get on the in part.

Therefore, with a smaller booster you get less help, less assist, so you have to do more of the work.


The reason I put it in these terms is because the ratio is independent of the size and a larger booster may have a small ratio or vise a versa.


How much assist you get is hard to measure just on size. Because in the end it becomes difficult to separate boost ratio from vacuum assist. But it's safe to say that the smaller the booster the less assist.

Now the difference between tandem and single boosters. There is no real difference. Tandems have come about to increase the size of those chambers without going to boosters so large in diameter that they can't possibly fit inside an engine bay.

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Old 05-24-2002, 01:03 PM   #10
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Quote:
Originally posted by Jon Bogert

Tell me more. I don't think anyone has tried fitting a non-Subaru booster yet. I--like many people--swapped the mushy dual stage booster for the old-style L booster and MC for better feel.

What are the parameters that determine the amount of boost? Diameter? Something else?

That's for reminding me about the master cylinder.

Of course the the master cylinder needs to fit also. Either the stock to the new booster or the new master cylinder must accept the fitting from the brake tubes and be compatiable with the Fluid Level Indicator connector.

Also, VERY IMPORTANT, is the pre-load dimmension. If you look a the B&W picture above you will see a bold arrow at the front of the booster and one at the rear of the master cylinder. That rod (the push rod) goes into the push rod piston on the MC. If the rod is too long then it will actuate the MC...VERY BAD. If it's to short then it will add more lost travel, but be perfectly safe.
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Old 05-24-2002, 01:48 PM   #11
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Oh by the way:

The biggest reason for going to a larger booster is that it raises the "run-out" point.

Run-out is the maximum level of assist before the booster goes to a 1:1 boost ratio.


It basically raise the maximum force the booster can put out.

If you look at the graph above it's number 7.
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Old 05-24-2002, 02:46 PM   #12
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If any of you do any technical stuff, like re-engineering or race car construction and such you need this book:

Bosch Automotive Handbook (5th ed.)

It's an awesome little reference on all vehicle systems and it fit right in you jeans or overall pockets.

You can get at Amazon.com for $31.47

http://www.amazon.com/exec/obidos/AS...roduct-details


or Barnes and Noble for $35.96

http://shop.barnesandnoble.com/books...sbn=0837606144
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Old 05-24-2002, 03:04 PM   #13
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sounds like a book I could use...
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Old 03-11-2003, 11:55 AM   #14
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The problem with the book is... I can't read it at all because it's printed on see-through paper in a font so small it's unbelievable.

I have the book. It was opened for approximately 6 minutes. My eyes crossed, I got a headache, and put it on the shelf.

Why why why why why is there also not a "Normal Human Being" version of this book?

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