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Old 03-06-2003, 10:14 PM   #1 (permalink)
Zeus of the Sluice
 
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bypassing 4wabs

I am in the process of doing a sas on my 94 bronco and would like to go with the stock 79 bronco brake lines for the 44 in the front, necessitating all new plumbing.
If I run all new lines and bypass the 4wabs, will I need an adjustable proportioner if I am still running rear drums.
A little history, I replaced the master, booster and rear cylinders with F350 stuff, and noticed little if any gains. My 'theory' is that the 4wabs bottle necks everything and won't allow any better braking. I am sure my theory is prolly totally fubar, so can someone with the knowledge please set me straight?
thanks
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Old 03-07-2003, 12:19 AM   #2 (permalink)
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hey once you get it done post some pics.im getting ready to swap my 81.
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Old 03-07-2003, 01:28 PM   #3 (permalink)
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anyone?
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Old 03-08-2003, 10:07 AM   #4 (permalink)
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I'm confused. What is the exact problem that you are fighting right now? Are you having a problem (right now, without doing the soild axle swap) with it being too rear-biased? Is the vehicle braked pretty neutral, but the decel performance is quite limited? Is the prtal ratio poor in that it takes too much force to stop the vehicle, or does it have too much travel that you cannot generate enough pressure to stop the vehicle?

I think that you are over simplifying a braking system. You mentioned that you want to change to a solid axle, what are the caliper size and rotor diameter of the solid axle compared to the TTB axle? What is the bore diameter of the F350 master cylinder (m/c) that you installed? What diameter wheel cyinders did you put in? This is all important because a braking system is exatly that... a System. You have to understand what the effect these compnent changes will have, other than saying that "they are F350 components so they must be better".

The most important thing for you to figure out to answer your questions is to learn if the 94 Bronco uses a prop valve or uses EBD (Electronic Brakeforce Distribution). If you ABS unit uses EBD, then you will probably be better off bypassing it and running a prop valve. If the truck has a prop valve in it, you may want to run adjustable because you changed the m/c.

I could write alot more, and I can try to help you if you have more specific questions, but I will just refer you to some good tech info on brakes. It was written by good friend of mine.

http://www.stoptech.com/whitepapers/...ake_122701.htm

This is also pretty good:

http://www.stoptech.com/whitepapers/...ake_122701.htm

Later-
Ed
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Old 03-09-2003, 12:46 PM   #5 (permalink)
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thank you for the links. I will look into it.
My braking has always seemed fairly inadequate, especially with the 35's now. I tried the upgrade several people have done, whcih involves swapping the booster and m/c (up from 1 1/16 to 1 1/8 IIRC) to the f350 stuff, and then larger diameter rear cylinders. I don't know how to do the calculations for this stuff, so I really can't answer that. While everyone with RABs and 'less' reports great braking increases, I feel that mine has gotten worse if anything. The 4wabs is an electronic device, that as I understand it, cycles little pistons or something inside the unit to keep the brakes from lockin up. There is no other prop valve that I can detect.
If anything, I would say that currently my rears do not work as well as they should, though they shoes, etc are all fairly new. So I would say I am front biased. With the sas, rotor and pad size stay the same. I found a good link on using thunderbird calipers (ford car really) that have a larger steel piston, that should greatly improve front braking, while keeping most everything else the same as stock 79 ford.
So if I do bypass this thing, will I need a prop valve if I am running a single line to the front and one to the rear(standard solid axle style that splits along the axle, instead of two seperate lines in the front and one in the rear, like I have now) if I am running rear drum brakes. From what I have found on the subject, it seems to me I may not?????
Thanks again for the help Ed.
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Old 03-09-2003, 07:20 PM   #6 (permalink)
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There is a ABS cutoff writeup for the rear axle, is that what you are looking for?
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Old 03-09-2003, 11:17 PM   #7 (permalink)
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no, I am interested in removing it all together.
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Old 03-11-2003, 12:19 PM   #8 (permalink)
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Sorry, that second link was supposed to be:
http://www.stoptech.com/whitepapers/...ons_122701.htm

First, I would like to comment that I do not think that your ABS unit is not causing your problems with underbraking the rear axle. I think that there are other fundamental problems with the braking system. Of course I could be wrong. The best way to tell this is to shut off the power to the ABS unit and re-test. You can do this by pulling the ABS fuse or pulling off the connector at the ABS unit. When a ABS unit has no power, it does nothing to limit pressure in the system, it is pass through. If you still have poor braking from the rear axle, it is some other fundamental problem.

The math to calculate your brake proportioning is really simple, but it just takes some serious thinking and data to do the calculation. It is all just simple geometry and statics. You don't have to do the whole calculation, you just need to understand how your changes are going to affect the end result. I'll go through it in some detail:

First you need to know your center of gravity to figure out what the weight transfer is under braking. You will also need the static weight bias of your truck. The weight is transfered to the front under braking, which means that you now have less normal force on the rear tires. This is why you need to have proportioning. If your weight bias was 50/50 and the COG was very low, you could run the same m/c diameter and calipers on the front and rear w/o proportioning. Trucks obviously have high COGs, so hence the need for heavy proportioning. BTW - you change your brake proportioning when you lift your vehicle (Higher COG = more frontal weight transfer under braking).

Second you need to look at the pressure in the front and rear system. You take the force that you apply on the brake pedal and multiply that by the pedal ratio and that then is the force into the m/c. You take this force divided by the area of the m/c piston and that is the pressure in the line. The pressure in the line is then converted back to force at the caliper. This is calculated by taking the pressure and multiplying it by the area of the caliper piston. That is then the force on the rotor. As you can see then, if you increase the size of the caliper, or wheel cylinder piston (at the wheel), you increase the force at the rotor or drum.... HOWEVER (this is a big misconception) if you DECREASE the size of the m/c piston you INCREASE the force at the rotor (assuming the caliper diameter stays the same). If you are having problems with the rear axle being too heavily biased, you could increase the size of the m/c piston going to the rear axle and vice-versa.

The rest of the picture is just torques, but this is the most complicated step. (Calculations with drum brakes are really complicated, so I will ignore them for now) You need to know coefficients of frictions between the friction pads and rotor (which is HIGHLY temperature dependant) and the coefficient of friction between the tire and road. The caliper creates a torque which is the rotor radius times the caliper force (from the pressure) times the coef of the pads. This torque divided by the tire radius is the force at the tire-road interface. If this force exceeds the force that the tire can support, the wheel starts sliding. Ice can support much less force than dry pavement can. Normal force (weight on that tire) changes the force that the tire can support also.

Basically, you need to balance all of these components to have a well-designed braking system. A prop valve in the rear circut is going to LIMIT pressure at the rear axle, which will not help your poor braking performance at the rear axle. What you need to do is change system components to fix your problem. You could do something simple like put really high coef pads or shoes on the rear axle (less pressure will give more torque) or even put crappy pads on the front. You could even change m/c bores, or wheel cylinder bores.

I am not trying to be a know-it-all. I just think that there should be more written on this subject because there is a general lack of knowledge about brakes. One of these days I will do a complete write up and post it on a web page somewhere. I could even generate a web based calculator for braking... I just need more hours in the day!

BTW - I wrote this up pretty fast, so it may be confusing. Ask questions if you don't understand a part of it.

Later-
Ed

Last edited by munkee; 03-11-2003 at 12:21 PM.
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Old 03-11-2003, 05:25 PM   #9 (permalink)
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[QUOTE]Originally posted by munkee
HOWEVER (this is a big misconception) if you DECREASE the size of the m/c piston you INCREASE the force at the rotor (assuming the caliper diameter stays the same).

Thank you for all the info. I will try and digest it all and check it against my vehicle. But based on the snipit of info above that you gave me, does that then mean that if I went from a 1 1/16 stock mc to a 1 1/8 f350 that I have then decreased my force at the rotors, and would that not then help explain why my braking is poor?
Would that contribute to high breake heat/fade, lending to my squishy brake pedal (though it seems equally squisher hot or cold)
I have been running with the abs fuse pulled for the last year, and noticed no effect at all on the braking. Is it possible that the larger rear wheel cylinders is the major culprit in the poor braking?

Thanks again Ed.
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Old 03-11-2003, 09:05 PM   #10 (permalink)
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And keep in mind that the stock calipers and pads AND rotors are pretty damn small. The 78/79 Bronco stuff is even worse. (In other words, don't bother with stock calipers)

There's a writeup on ProjectBronco.com somewhere in their forums that I can't find at the moment. It basically boils down to this: use mid-70s Thunderchicken calipers with the straight D44 axles. The caliper is maybe 3-1/2" dia vs 2-1/2" dia for the stock Bronco stuff.

I'm also not really sure why you'd need to swap to D44 brake lines, either. You could actually keep your 4WABS with the straight D44 (and your stock brake lines) IF you wanted to.

The prop valve on the earlier RABS-only Broncos is a 2" long 7/8" dia cylinder mounted on the rear M/C output. I don't know if the 4WABS trucks still use these or not.
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Old 03-11-2003, 10:07 PM   #11 (permalink)
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no the 4wabs dont have the rabs. That is a good article about the tbird calipers, I have seen, but was going to hold off in the anticipation of a 8 lug, sterling swap in the near future. I like the simplicity of the solid 44 brake lines, and its non limiting factor for wheel travel if I should go to a coil over setup with the wristed arm I am going to use. Not sure the brake lines would be a factor, but figured I would only do this once.
I want to bypass the 4wabs due to the inability of bleeding the unit without some 1500 dollar tool from mother ford, and since it wont work with the sas, I figured ditch the bitch.
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Old 03-12-2003, 10:44 AM   #12 (permalink)
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Do you already have the D44?

I'll tell you this much flat-out (and this really has nothing to do with your question, but it's going to help, I promise):

Don't incrementalize. Save your money. If you're holding off on putting larger calipers with the straight D44 because "you're planning" on going with 8-lug fronts and a 10.25 out back, you're only a stone's throw from yanking the D44 in favor of a D60. A few broken u's or busted ball joints will help you make up your mind, I'm sure.

How long was that straight D44 under Dave Blue's truck? Can't have been much longer than 3 or 4 days by my estimates.

And yeah, I agree about ditching the 4WABS. Rob Ellis did it on his truck (351/E4OD 94ish model) with no ill effects that I have seen or heard. I just wasn't sure what your game plan was.
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Old 03-12-2003, 12:58 PM   #13 (permalink)
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yeah, it didn't last long under daves rig, but he is a lot of right foot where I am not.
The 44 came cheap with everything that swaps over from the ttb. Figured I would run this first, swap the e40d for a zf and then start going bigger next year.
our buddy with the chevy is also in the process of going to a 95 60 with radius arms, so well see.
Time and money!
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