I'm in the "pre brake upgrade stage" right now and I'm sick of these gawd awful 87' rear calipers! I'm taking off the 2nd set of rebuilds in 3 years right now, (inner and outer pistons locked up AGAIN)
I didn't know Rube Goldberg worked in the brake engineering department at GM
I know that the front calipers fit just fine on the rear, but other than the obvious loss of the parking brake are there any other drawbacks or problems I should be aware of? (Since I have an automatic and no inspection to worry about, the loss of the parking brake doesn't worry me all that much right now.)
I read on another old thread something about the rear brakes being on the larger volume circuit of the system. Also, as I understand it, the front brake pistons are 2 inch dia. and the OE rears are 1.75 dia. Is the larger rear piston diameter being on the, (rear), larger volume portion of the circuit likely to cause problems?
Seems like it would be easy enough to switch the front and rear circuits right at the proportioning valve if I absolutely need to, but I'd rather just hang some OE front calipers on the rear and be done with it for now if thats possible?
Thanks. I assume that you're referring to the piston diameters being the same front and rear on your set-up?
I'm looking at several brake upgrade options, but right now I have new front OE calipers mounted on the front and a "like new" set of front calipers that I'd like to hang on the rear for now if it's not going to cause any unforseen problems. I just can't bear the thought of buying yet another set of OE rear calipers that will end up seized-up junk like the last 2 sets.
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12:23 AM
Marvin McInnis Member
Posts: 11599 From: ~ Kansas City, USA Registered: Apr 2002
I didn't know Rube Goldberg worked in the brake engineering department at GM
Yes, there are some bad engineering ideas that make it into production, but ithey are more often the result of conscious decisions to avoid possible patent infringement. Our cars (all cars) are full of patent-driven compromises.
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... as I understand it, the front brake pistons are 2 inch dia. and the OE rears are 1.75 dia. Is the larger rear piston diameter being on the, (rear), larger volume portion of the circuit likely to cause problems?
Yes. Using calipers with 2 inch pistons in the rear will significantly alter brake balance toward the rear. As a first approximation, tire braking force at the rear will increase about 31% while front braking force will remain the same. At best, you will be a test pilot.
The normal design rule is that you want the front brakes always to lock before the rears ... under all conditions ... as a stability criterion. If the fronts lock before the rears, the car will travel in a more-or-less straight line because the rears are still rolling and thus still provide directional control, while the center of gravity of the car will be ahead of the center of braking force. Conversely, if the rears lock first the car will not be directionally stable and it is likely to swap ends. The reason is that the maximum force available from the tires' rolling friction is always significantly greater than the sliding friction.force available from the same tires.
[This message has been edited by Marvin McInnis (edited 05-04-2009).]
Yes. Using calipers with 2 inch pistons in the rear will significantly alter brake balance toward the rear. If fact, a first approximation is that rear brake effort will increase about 31%. You will be a test pilot.
The normal design rule is that you want the front brakes always to lock before the rears ... under all conditions ... as a stability criterion. If the fronts lock before the rears, the car will travel in a more-or-less straight line because the rears are still rolling and thus still provide directional control, while the center of gravity of the car is ahead of the center of the braking force. Conversely, if the rears lock first the car will not be directionally stable and it is likely to swap ends. The reason is that the maximum force available from the tires' rolling friction is always significantly greater than the sliding friction.force available from the same tires.
Thanks Marv. I'm curious how you came up with that 31% delta.
Again, my understanding is that the rear hydraulic circuit is larger than the front circuit which supposedly helps bias to the front brakes locking first. I'm aware of the need to bias the brakes to the front, especially on the Fiero, so before I "test pilot" this I'd like to have an idea what I've done. I knew the larger piston diameter might bias more to the rear, but I'm still not sure by how much.
By the way, bad patent "work-around" engineering, is still bad engineering. Although he possibly didn't know it, Ogre makes quite a case in his cave article on the stock OE rear brakes having needed a serious FMEA before production, but then GM seems to have always allowed their cost benefit analysis and warranty cost predictions to run rough shod over proper engineering practice.
[This message has been edited by randye (edited 05-04-2009).]
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10:22 AM
Marvin McInnis Member
Posts: 11599 From: ~ Kansas City, USA Registered: Apr 2002
Simple piston area ratio: (2.0 / 1.75)2 = 1.31. This assumes that braking torque (and thus braking force) is directly proportional to normal force between the pads and rotors, and that the normal force is directly proportional to hydraulic pressure times piston area.
I'm pretty sure that there is a F/R proportioning valve in the Fiero brake circuit, but it is not field adjustable and there is no technical documentation available that I know of.
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By the way, bad patent "work-around" engineering, is still bad engineering.
I agree completely ... but at big companies the line engineers seldom get to participate in those decisions. Personal experience with unacceptable technical compromise is one of several reasons that I ended up starting my own company back in 1972. That way I got to make my own technical decisions, but I also had to assume full technical responsibility for them, along with the accompanying financial risk. When you are small, all it takes is one bad decision to destroy the accumulated result of years of good decisions.
[This message has been edited by Marvin McInnis (edited 05-04-2009).]
I assume the 31% figure was obtained by comparing the rear caliper piston area 1.75^2*pi/4 to the front caliper piston area 2.00^2*pi/4 For a given hydraulic pressure, this is accurate.
However, the effect of the proportioning valve must be taken into account. The proportioning valve is a simple spring-loaded piston with a certain amount of preload. The spring preload will affect the cracking pressure. A combination of the rear circuit fluid volume and the spring rate will determine how fast the rear circuit pressure rises compared to the front.
With a greater rear circuit volume compared to before, the proportioning valve's effect will be reduced. So below the cracking pressure, the 31% figure will be 100% accurate. However, above the cracking pressure, the rear braking force will be higher than predicted, because of the increase in volume.
Don't be fooled that a heavily front biased braking system will be safer than a balanced one. Would you rather spin out? Or, would you prefer long stopping distances, and plow into some kid?
GM really went overboard on the front bias, aside from the parking brake issue, I think you'll see an improvement in stopping distance. I would try the front calipers on the rear.
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11:04 AM
Marvin McInnis Member
Posts: 11599 From: ~ Kansas City, USA Registered: Apr 2002
Don't be fooled that a heavily front biased braking system will be safer than a balanced one.
I never said that a "heavily front biased" system is desirable. What I did say was, "The normal design rule is that you want the front brakes always to lock before the rears ... under all conditions ... as a stability criterion." Tire rolling friction is very different than tire sliding friction, both in magnitude and in dynamic characteristics.
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With a greater rear circuit volume compared to before, the proportioning valve's effect will be reduced.
I disagree that volume has anything to do with brake proportioning, except as the volume may depend upon piston area. First, consider that the proportioning valve regulates pressure, not flow volume. Second, consider that a fundamental property of static hydraulic systems (disregarding initial pad clearance and elasticity of components) is that pressure is communicated equally throughout that system with a flow volume of zero. As a thought experiment, you could add a one gallon tank to the rear brake circuit in a Fiero and, assuming an inelastic and properly bled tank, the net effect on brake operation would be zero.
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The proportioning valve is a simple spring-loaded piston with a certain amount of preload. The spring preload will affect the cracking pressure.
I agree that many braking systems are designed with what you call a "cracking pressure" threshold. The primary reason for such designs is to guarantee that, to meet stability criteria, the rear brakes are always applied and developing some minimal force before the front brakes begin developing any force. Then, after that initial threshold is reached, the proportioning valve regulates a preset pressure ratio, front vs. rear, not volume. This design feature is particularly common in brake systems with discs in the front and drums in the rear.
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A combination of the rear circuit fluid volume and the spring rate will determine how fast the rear circuit pressure rises compared to the front. With a greater rear circuit volume compared to before, the proportioning valve's effect will be reduced.
Again, I disagree that volume has any significant effect in a properly functioning static hydraulic system.
Originally posted by Marvin McInnis: I never said that a "heavily front biased" system is desirable. What I did say was, "The normal design rule is that you want the front brakes always to lock before the rears ... under all conditions ... as a stability criterion." Tire rolling friction is very different than tire sliding friction, both in magnitude and in dynamic characteristics.
This wasn't particularly directed at you, it was a somewhat general comment.
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Originally posted by Marvin McInnis: I disagree that volume has anything to do with brake proportioning, except as the volume may depend upon piston area. First, consider that the proportioning valve regulates pressure, not flow volume. Second, consider that a fundamental property of static hydraulic systems (disregarding initial pad clearance and elasticity of components) is that pressure is communicated equally throughout that system with a flow volume of zero. As a thought experiment, you could add a one gallon tank to the rear brake circuit in a Fiero and, assuming an inelastic and properly bled tank, the net effect on brake operation would be zero.
I agree that the pressure is equal throughout the rear circuit. I agree that the proportioning valve regulates pressure and not fluid flow. Like you, I'm discussing static no-flow situations.
The thing is, real-life brake systems are elastic. Real-life brake fluid is compressible.
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Originally posted by Marvin McInnis: I agree that many braking systems are designed with what you call a "cracking pressure" threshold. The primary reason for such designs is to guarantee that, to meet stability criteria, the rear brakes are always applied and developing some minimal force before the front brakes begin developing any force. Then, after that initial threshold is reached, the proportioning valve regulates a preset pressure ratio, front vs. rear, not volume. This design feature is particularly common in brake systems with discs in the front and drums in the rear.
I'm not talking about a residual pressure valve or anything.
The proportioning valve does not regulate a fixed front/rear pressure ratio. A proportioning valve is a device that changes volume, based on pressure.
Look at any aftermarket proportioning valve, or even the Fiero proportioning valve for that matter. They don't connect to the front and rear circuits, they only connect to the rear circuit. They normally have just two ports, an "in" and an "out", which really is just a tee fitting integrated into the same block.
Proportioning valves work, because they cause a change in front/rear volume, which in turn causes a change in front/rear pressure.
A proportioning valve does not and should not regulate a fixed front/rear pressure ratio.
Now I've searched a little, and these numbers may not be accurate, of course depending on the exact car, but for discussion's sake, lets say a Fiero has a 43f, 56r static weight distribution. Assuming the 19.5" CG height figure I read somewhere is close to reality, with the forward weight transfer under 1 g of braking, the dynamic weight distribution should be about 63f, 33r.
Notice how the brake bias requirements change with the deceleration. The more deceleration (dry asphalt), the more weight transfer, the more the brakes need to be front-biased.
That is the purpose of a proportioning valve. It's not to regulate a fixed front/rear ratio. It's to adjust the front/rear bias based on the deceleration, which is affected by the hydraulic pressure.
Say you attach a 10000 gallon brake fluid tank to the rear circuit. Or even better, say you open a rear bleeder screw.
With the rear bleeder screw open, the bias will become 100f, 0r. That's the best analogy I could give on the function of a proportioning valve...Anyway, this is what the spring loaded piston does, to a lesser extent.
[This message has been edited by pmbrunelle (edited 05-04-2009).]
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12:49 PM
daveg Member
Posts: 193 From: Barrie, Ontario, Canada Registered: May 2004
I have this setup, and have had it for 6 or 7 years. I compete in solo2 (aka slalom and autocross) and could appreciate the self adjusting and I too was frustrated with the costly factory rear calipers that failed quickly.
In every situation my brakes have worked very well. I have the same pads front and rear, 215/40-17 front an 235/40-17 rear tires. For competition I use 225/50-15 on all 4. My front still lock up first.
daveg
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01:26 PM
PFF
System Bot
Marvin McInnis Member
Posts: 11599 From: ~ Kansas City, USA Registered: Apr 2002
A proportioning valve does not and should not regulate a fixed front/rear pressure ratio.
... the purpose of a proportioning valve [is not] to regulate a fixed front/rear ratio. It's to adjust the front/rear bias based on the deceleration, which is affected by the hydraulic pressure.
Good information, but I'm still not convinced. As time allows, I'll do some more research and report back. Can you cite some good technical sources? How does the Fiero proportioning valve sense deceleration?
But back to the original question, increasing the rear caliper pistons from 1.75 to 2.0 inches, without changing anything else, is going to result in a significantly more rearward brake bias. That's probably not a good thing for a street-driven Fiero.
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Originally posted by daveg:
I have this setup, and have had it for 6 or 7 years. I compete in solo2 ...
In every situation my brakes have worked very well. ... My front still lock up first.
Also a good data point. But I hope you will agree that almost everything about tires, suspension setup, and brakes is different for autocross competition than for normal street driving. At the very least, during autocross you seldom have to be concerned with brake performance on oily, rain-slicked asphalt.
[This message has been edited by Marvin McInnis (edited 05-04-2009).]
Originally posted by Marvin McInnis: Good information, but I'm still not convinced. As time allows, I'll do some more research and report back. Can you cite some good technical sources? How does the Fiero proportioning valve sense deceleration?
It doesn't detect deceleration on it's own. It's hydraulic pressure sensitive. Keep in mind that the deceleration can be calculated from the hydraulic pressure if other factors such as weight, wheel diameter, pad coefficient of friction, etc...are known.
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Originally posted by Marvin McInnis: But back to the original question, increasing the rear caliper pistons from 1.75 to 2.0 inches, without changing anything else, is going to result in a significantly more rearward brake bias. That's probably not a good thing for a street-driven Fiero.
Eh that's a subjective thing...although the autocrosser says the fronts lock up first on good traction (read high deceleration) situations.
In slippery situations, the maximum deceleration is less than in dry situations. There will be less deceleration, so less weight transfer when it's slippery than when it's dry. Therefore, when slippery, the ideal brake bias will be more rearwards than what would be ideal for dry situations.
Assuming a constant bias, if the fronts lock up first when it's dry, then the fronts will also lock up before the rears when it's slippery.
With the proportioning valve, that makes things a little more complicated. The bias curve is not a smooth curve.
I do not have any information on the Fiero prop valve's spring rate, preload, or piston diametre. This is a somewhat made up chart. This is an excel spreadsheet I made, I don't have accurate numbers, but the point is to show the nature of the ideal (green) versus actual (blue) brake bias. Notice the "kink" in the graph. This is at the cracking pressure of the prop valve. This is where it might be possible to lock up the rears before the fronts, even if the fronts lock up before the rears in dry conditions.
The spreadsheet uses these as input parameters, it's not terribly sophisticated, but I think I covered the essentials.
Front effective rotor radius (in) Rear effective rotor radius (in) Front wheel radius (in) Rear wheel radius (in) Front caliper piston area (in^2) Rear caliper piston area (in^2) Brake pad coefficient of friction
What I called the valve pressure slope, is how after the cracking pressure, the rear pressure rises in comparison to the front. This is dependent on volume, flexibility of parts, etc...
Also a good data point. But I hope you will agree that almost everything about tires, suspension setup, and brakes is different for autocross competition than for normal street driving. At the very least, during autocross you seldom have to be concerned with brake performance on oily, rain-slicked asphalt.
Marvin is correct in taking into account the entire *system*, and I do agree that there is a lot about how this particular autocross car is equipped or set up that I don't know. I have a steadfast rule for my engineers; "Best by test". My present set up is 18 X 8 wheels at all 4 corners shod with P235/40/18 tires. The car is lowered 1.8 inches all around on Intrax, (progressive rate), springs. All 4 brake hoses are new , (rubber), and all 4 rotors are new. Shocks are Monroe SensaTrac on all 4 corners, also new. The master cylinder is new, (aftermarket OE replacement) and the hard lines are in good condition.
I'm very interested in the theory behind calculating brake bias and performance, so I don't want this thread to die, as there seems to be much more to learn. Since we have at least one case of anecdotal evidence that my intended set-up works, I'm compelled to live up to my mantra and subject it to test.....clear the roads! I'll report back with findings soon.
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04:38 PM
Marvin McInnis Member
Posts: 11599 From: ~ Kansas City, USA Registered: Apr 2002
From the '88 GM/Helm Factory Shop Manual, page 5-2:
"Operation of Combination Valve --
"To prevent early rear wheel lock-up under heavy braking loads, the proportioning section of the combination valve proportions outlet pressure to the rear brakes after a predetermined rear input pressure has been reached.
"The valve has a by-pass feature which insures full system pressure to the rear brakes in the event of a front brake system failure. Similarly, full front pressure is retained in the event of a rear brake pressure failure."
Thus the proportioning function is dependent upon hydraulic pressure, not deceleration, and above some threshold it proportions pressure between front and rear. That proportioning with pressure will occur even when the car is standing still.
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Originally posted by randye:
Since we have at least one case of anecdotal evidence that my intended set-up works, I'm compelled to live up to my mantra and subject it to test.....clear the roads! I'll report back with findings soon.
Nothing wrong with that, as long as you proceed methodically and understand what you may be getting yourself into. Still, I would hope that your initial limit testing will be conducted somewhere other than public roads.
FWIW, I have been testing (without incident) a relatively simple mod to '88 rear brakes for more than three years now, but I haven't yet published the details because I know what the risks are and I want to be sure that it's a sound long term solution before anybody else tries it.
[This message has been edited by Marvin McInnis (edited 05-04-2009).]
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06:21 PM
FTF Engineering Member
Posts: 710 From: Near Philadelphia PA Registered: Sep 2001
From the '88 GM/Helm Factory Shop Manual, page 5-2:
"Operation of Combination Valve --
"To prevent early rear wheel lock-up under heavy braking loads, the proportioning section of the combination valve proportions outlet pressure to the rear brakes after a predetermined rear input pressure has been reached.
"The valve has a by-pass feature which insures full system pressure to the rear brakes in the event of a front brake system failure. Similarly, full front pressure is retained in the event of a rear brake pressure failure."
Thus the proportioning function is dependent upon hydraulic pressure, not deceleration, and above some threshold it proportions pressure between front and rear. That proportioning with pressure will occur even when the car is standing still. Nothing wrong with that, as long as you proceed methodically and understand what you may be getting yourself into. Still, I would hope that your initial limit testing will be conducted somewhere other than public roads.
Thanks again for the data on the proportioning valve Marvin. I'm *guessing* that a proper functioning prop valve might allow this to work.
I do indeed know what I'm getting myself into and my "clear the roads" comment was meant to be tongue in cheek. I'm obviously going to proceed very cautiously with this and my initial "test protocol" will include a series of stopping distance and handling tests from incrementally increasing speeds... in an appropriate area of course. I spoke with the crew chief of my brother's Indy Car team and , while he knows nothing of the Fiero's particulars, he seems to believe that the proportioning valve function is the key to this working appropriately. The Dallara Indy Lights chassis uses the same basic caliper and disc arrangement on all 4 corners and has a driver controlled prop valve. While I obviously don't have the luxury of an adjustable valve, and my Fiero certainly isn't a race car, I'm a bit more optimistic about the success of this.
After successfully installing a new master cylinder, rotors, hoses, calipers and pads, *including* installing OEM front brake calipers on the rear of my 87' GT, I'm happy to report that it all seems to be working great.
The OEM front calipers simply mounted on the rear with NO modifications whatever. The only difference is that the right and left front calipers had to be swapped to opposite sides in order to connect the OEM brakes hoses. Pretty simple!
Along with others I had the same concerns about early locking of the rear brakes, but after testing this set-up under hard braking, in increments of 10 MPH, from 15 MPH up to 80 MPH, I have noted ZERO problems. We didn't bother with measuring stopping distances, but I know they have improved marginally.
Apparently the proportioning valve does exactly what it's supposed to do, as the front brakes have consistantly locked before the rears in every hard stop.
Now the obvious question; Is the braking performance much better than the original set-up?
NO, it isn't dramatically better, but the brakes have a more "solid" feel to them and stop the car reliably and predictably without the "mushy" feel I previously had. A good bit of that "mush" could have been due to the 20+ year old brake hoses on the car though. It still takes a good amount of pedal effort to lock the brakes up, but I can now do that at will. Replacing the brake booster with the S10 Blazer upgrade is the next step as pedal travel is only a slight bit improved over stock.
The "up side" to this small modification is that I no longer have to worry about the complexities and resultant unreliability of the OEM rear calipers and I no longer have to utilize the emergency brake to adjust the rear brakes to operate properly.
The "down side" is that I no longer have an operable emergency brake, but since my car is an automatic and I have no state inspection to worry about, it isn't something that I particularly miss at this point.
I did keep the e-brake cable from the lever connected and simply stuffed the now free end of the cable inside the rear fascia in the event that I someday upgrade the brakes to a set-up that allows the use of the e-brake, such as Seville rear calipers on a "big brakes" mod. The other 2 cables were removed from the cradle and stored away in a box in the garage.
I did forget to disconnect the warning light from the e-brake handle which obviously resulted in the brake warning light being on which gave me a momentary fright, but the single wire plug is now disconnected from the lever mechanism and all is well as far as dash warning lights are concerned.
[This message has been edited by randye (edited 06-04-2009).]
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07:25 PM
Jun 4th, 2009
FTF Engineering Member
Posts: 710 From: Near Philadelphia PA Registered: Sep 2001
After successfully installing a new master cylinder, rotors, hoses, calipers and pads, *including* installing OEM front brake calipers on the rear of my 87' GT, I'm happy to report that it all seems to be working great.
The OEM front calipers simply mounted on the rear with NO modifications whatever. The only difference is that the right and left front calipers had to be swapped to opposite sides in order to connect the OEM brakes hoses. Pretty simple!
Cool. Glad that worked out.
So...... Did you measure the piston diameters while everything was loose? Can you confirm the info from earlier in this thread?
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The Ogre shows charts in his cave in the brakes section and describes the OE front piston diameter as 49mm and the rear at 45mm.
Yes, the front caliper pistons are indeed 4mm larger diameter than the rear caliper pistons. I didn't try to measure it, but the rear caliper internal fluid volume of the casting also appears to be larger than the front calipers. Not sure that would matter though once the system is fluid filled and fully bled of any air.
Interesting thread. My back calipers are locked up and I need to replace them. The fronts were locked up and I am doing the Grand Am brake upgrade on the front. How big a difference doing the GA in the rear versus the front OEM in the rear?
Yes, the front caliper pistons are indeed 4mm larger diameter than the rear caliper pistons. I didn't try to measure it, but the rear caliper internal fluid volume of the casting also appears to be larger than the front calipers. Not sure that would matter though once the system is fluid filled and fully bled of any air.
Cool. Thanks for checking that. I'd heard differently. Glad to know for sure.
Now makes me wonder about the 88's... I'd always heard the same thing about them as well (that all four corners were the same piston diameter) but I've never verified it for myself. Heh... I wonder if I heard that from the same source that said the pre-88's were the same.
Thanks again for verification.
FTF E
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08:33 PM
Jun 5th, 2009
jaskispyder Member
Posts: 21510 From: Northern MI Registered: Jun 2002
88 fronts fit on 88 rears. You will have to grind off part of the caliper so the hose can rotate to the correct position (easy). Otherwise, they work great this way.
J.
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02:12 PM
FTF Engineering Member
Posts: 710 From: Near Philadelphia PA Registered: Sep 2001
88 fronts fit on 88 rears. You will have to grind off part of the caliper so the hose can rotate to the correct position (easy). Otherwise, they work great this way.
J.
Thanks J, but that's not what we're after here. The question isn't whether the 88 front calipers will FIT on the back... The question is are they the same piston diameters on both front and back?
Thanks though!
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09:30 PM
Oct 19th, 2009
Brembo-Fiero Member
Posts: 111 From: Reutlingen, Germany Registered: Nov 2006
The OE 84-87 Front pistons are 49mm and the Rear pistons are 47,5mm diameter. This is the result after measurements i have done today. I measured the pistons of 2 different Rear Calipers.
------------------ Jürgen Martin white GT 87 Germany
