This Subaru STI caliper seems to have the same problem (for Fiero use) as most parking-brake calipers; small piston area (40 mm bore).
Concur. That's likely not enough rear caliper. Even with an adjustable prop valve, you'll back the valve all the way off and still not have enough rear braking.
Doesn't work like that. The two piston caliper's effective area is the piston on one side of the rotor. A fixed 2 piston caliper with 40mm pistons will deliver the same clamping force at a given line pressure as a floating caliper with one 40mm piston. Also, the leverage increase of a 13" rotor over a 9" rotor is 2".
What we were informing you of that you ignored is a lot of community experience that Fieros like a brake setup that's very close to "square". That's why I said what I said about your prop valve.
What we were informing you of that you ignored is a lot of community experience that Fieros like a brake setup that's very close to "square". That's why I said what I said about your prop valve.
I understand that a stock Fiero has a brake bias of about 55/45 50/50 depending on what that brass brake controller / prop valve is doing. My real wonder is why you think its un-achievable with my setup. Right now my bias is along the lines of 73/27 due to the stock rear setup. Even with that wonky setup it stops better than it ever did. For the record a stock BRZ has .73 bias.. With the STI calipers I estimate it will be around 63/37 or 10% more in the back. For reference that is the exact same bias as a Lamborghini that is also a mid-engine car. If I put racing pads on the rear and keep the front stock I can achieve a estimated bias of 54/46 standalone. I agree I probably will wind up switching my prop valve to the front, as my goal here is to have a setup that can be dialed in. My car has more weight in the front than stock, as well as I have lightened the weight past the rear axle. But who knows what the 3800 swap did to the balance. I can sit around doing math all day, I can run some simulations, or I can just send it and see what happens. Make corrections, then once I have it dialed in let everyone know what I did to make it work.
I have not seen a single full technical thread besides people either saying they did it, or wondering if they should do it. Even within those threads most people intended to leave on the stock system and only replace the rotors and calipers. I took the approach of taking a high performance modern system off an existing car, and placing it on the Fiero wholesale. Lets be real, even in 87' the Fiero did not have top of the line brakes. I'm not just adding Brembo calipers to my car, im adding 30 years of technological advancements in brake systems.
On another note, if I could go back in time I would upgrade the Fiero to have a 114.3 bolt pattern. But I am upgrading the rear bearings to something bigger and wont blow out every summer. So stay tuned for some info on that, it will give the rear disks 3mm offset for better clearance.
[This message has been edited by Dan W (edited 12-30-2022).]
My real wonder is why you think its un-achievable with my setup.
Because your front calipers have 42mm pistons. You have literally more than twice the clamping force on the front as you have on the rear. Your brake torque bias is >66/33, and you can't get more pressure to the rear brakes from a prop valve.
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Originally posted by Dan W: I agree I probably will wind up switching my prop valve to the front,
Just because some people do that doesn't make it a good idea.
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Originally posted by Dan W: But who knows what the 3800 swap did to the balance.
Sounds like numbers you should have before you try to argue what your ideal brake bias should be.
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Originally posted by Dan W: I can sit around doing math all day, I can run some simulations, or I can just send it and see what happens.
Or you could learn how to do the very basic level of modeling required to get into the ballpark with a BOTEC and waste less time ungedorking a wonky WAG for your first setup.
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Originally posted by Dan W: I took the approach of taking a high performance modern system off an existing car, and placing it on the Fiero wholesale.
You didn't though. You pieced things together from different manufacturers. The context of front-engine performance cars is very different with regard to braking that mid-engined cars, Lamborghini and their generally ham fisted clientele notwithstanding.
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Originally posted by Dan W:
On another note, if I could go back in time I would upgrade the Fiero to have a 114.3 bolt pattern. But I am upgrading the rear bearings to something bigger and wont blow out every summer. So stay tuned for some info on that, it will give the rear disks 3mm offset for better clearance.
Swap to either A-body JA2 hub carriers or U-body minivan carriers and bolt-in 5x115 or 5x4.75 hubs in the rear.
[This message has been edited by Will (edited 12-30-2022).]
Originally posted by Dan W: I have not seen a single full technical thread besides people either saying they did it, or wondering if they should do it.
There are several highly technical threads with actual math and calculations if you search for them...
Some of the data below was posted (by me) in one such thread evaluating the impact of the S10 booster upgrade and how it changed the front/rear brake bias. As part of that evaluation, an air cylinder was used to apply known force to the pedal while the brake line pressure was measured at the caliper. The data below is from a stock 88 fiero with a 2.5L engine. Since the 88s run 48mm calipers front/rear, the brake line pressure bias = overall brake bias.
Now, the big thing to also notice is that the bias shifts from 57/43 to 64/36 as pedal load is increased. You want this shift in bias, especially in a Fiero with a light front end. You want the rears to help transfer some weight and load the front and allow the fronts to really dig in w/o locking up. Once the fronts are doing most of the braking, the rear suspension will unload and as it unloads the rear braking reduce some to avoid lockup. Locking up the rear wheels on a Fiero is a very, very bad idea. The device that provides this changing bias based on brake pedal force is the Fiero combo/proportioning valve.
Tossing the stock Fiero combo/proportioning valve is a bad idea in my opinion, but some people do it trying to get past other self inflicted issues
Now, lets put some math to your rear brake issue... Using your front caliper with a range of rear brake calipers, assuming removal of the Fiero combo/proportion valve and no other proportioning valve.
Since you drove the car with the stock rear caliper (call it 48mm) and the Fiero combo/proportion valve removed, you should have had a 60.5% front bias which is right is in the middle of the 57% to 64% front bias range that GM shipped the cars with.
Why did you feel you needed more rear brakes? If the stock rear calipers were not working well, or they had too much of a gap between the rotors, they wouldn't work well, but it would have nothing to do with sizing. If the rears were working properly and pad clearance adjusted right, then I suspect the desire for more rear brakes is because it is too easy to lock up the fronts w/o even trying.
As you can see going to a 40mm caliper from a 48mm one is the wrong direction if the issue you are trying to address is not enough rear brake bite. To get to the 57% front bias and more rear brakes, you would need to run a 52mm rear caliper. If you want to run an adjustable rear proportioning valve, then you need to run a larger rear caliper than 52mm.
Basically you installed 2019 tech to get: Rotors that give you 35+% more braking leverage Calipers that give you 52% more clamp force Booster that gives you 10-15% more vacuum assist So something in the combined range of 100% increase in braking leverage/force...
Just for reference, brake upgrades that are considered phenomenal night/day improvements in useable braking performance are in the 20-40% total improvement range from stock.
There is nothing wrong with using newer more readily available parts, just choose the right ones that are properly sized to work well with the Fiero.
There are cheap Brembos from OEMs that range in 67% larger than stock all the way down to 0% larger than stock... Picking the right one to start with saves a lot of headaches, rework and $$$.
[This message has been edited by fieroguru (edited 12-30-2022).]
The X axis is pedal load (lb). The blue is front psi and the orange is rear psi.
The first thing to notice is that at 53 lb of pedal load and 1200 psi front line pressure, the booster is done. That's where the booster is overdriven, which is why the front line pressure goes up only slowly at higher pedal loads.
The second is that the rear pressure is linear up to that point, but essentially flat beyond it. The combined action of the booster maxing out and the prop valve operating results in rear line pressure only increasing from 675 to 775 (15%) as pedal load doubles from 53lb to 106lb.
Even with an appropriate booster, you do *NOT* want that behavior from your front brake line pressure. Which means you should never put a proportioning valve on your front brake circuit. .
I went through the trouble of modeling that data. What i dont understand is how, given the same sized master cylinder, and same sized caliper pistons, is the rear pressure less than the front pressure. The prop valve should only reduce when it hits its knee point. Before that it should be a 1-1 linear increase, then it should still be linear but reduced. Either way, i can only assume the brass brake distribution block is doing something..
I had to make a lot of guesses in my model to match the real data he has. Then i applied the exact same model to the 87 fieros stock setup. Then i applied the same model to my new setup. Fieroguru's data is pressure per caliper, my model calculated torque per wheel to take into account rotor size changes. The model is probably still out since my booster is different, as well as the pad sizes are also different.
In my model i calculated the leg input through the booster to get the PSI force out of the booster taking into account an approximation of the booster ratio and its cutoff. I took that master cylinder PSI force and estimated the force output the front/rear calipers taking into account the prop valve and its reduction amount I had to add a magic number of reduction to maintain that PSI difference in the data i mentioned above, otherwise the front/rear PSI would be equal I tuned it until my model had similar front/rear PSI to the real data I then calculated the torque based on the caliper size and static pad friction coefficient Finally i calculated the bias, compared it to the real data and its in the zone
88 Fiero / Real Data
I then updated the numbers to match the 87 fiero master cylinder/caliper information while keeping everything else. This is a big assumption considering the differences between the 87 and 88. Then i did that again with the setup i have now.
87 Fiero
My Fiero
I took the model data and instead of using leg force I made a standard spread from 100 PSI at the master cylinder to 375 PSI at the master cylinder and graphed that. The end result shows im not that far off stock and it will probably be fine. You can see how my willwood prop valve is a .75 reduction while the stock fiero seems to be less.
I cant comment if i am going to put a reduction on the front, or if im going to swap the rear calipers, install ABS, two master cylinders, or if its just going to be good out of the box. Like i mentioned many times before there are too many variables here. The only similarities right now are that these hypothetical cars all have a Fiero badge on them. So instead i ordered some brake bias gauges that i will set up so that i can better give people who care some actual, factual, information about how it turned out. Also it will replace the functionality of the 'brake warning' light i removed.
**Edit: I talked to a guy who specializes in Lotus performance in the EU. His shop does engine swaps with bigger heavier audi blocks. He thinks my bias will be good enough for anything besides serious competition purposes and that if its track-bound to install two master cylinders with a balance bar.
[This message has been edited by Dan W (edited 01-31-2023).]
As I mentioned before, part of upgrading the rear brakes is also a good time to upgrade the rear bearings. It will give the rotors 3mm more clearance, while unnecessary is a nice to have. It also solves the problem of the bearings blowing out every summer due to the increased engine weight. Credit goes to -->this<-- post by still around Steven Snyder who documented his process (much like I’m doing here) of Alan Hamilton’s method / part list. I went over it as you would have guessed, and I cannot see an alternative upgrade option so if you want to fix the Fiero’s stock bearings this is it.
Specs: Original hub pilot ID: 71mm J-Body hub pilot ID: 73.5mm
Original bolt circle: 102mm J-Body bolt circle: 98mm
Attempt #1 The initial attempt was to do it the same way they showed in the linked post, and that was to set up a line bore and bore it out. I did not have a big CNC machine, but I did have experience line boring, a lathe, and vices and clamps. I set up the hub, clamped it down, aligned it all.
The issue I had was the notch. Every time the cutting bit went over it, it would create a vibration that would eventually throw the whole thing out of alignment. No amount of clamping helped this. The end result was me giving up attempting to line bore this hub as it created some gnarly results.
Attempt #2 I called up a few machine shops, they wanted anywhere between 2-3 hours a side. End result would have been between $600 and $800.
Attempt #3 I decided to take another approach at it. I designed a wooden jig in Fusion 360 that would allow me to use a wood router rocking a carbide burr bit to cut out the circle with a decent amount of accuracy. Its important that the size be close to the stock J-Body, as well as round. If you get it too far off, or an oval your in for a bad time.
Creating the jig The jig was designed to be made from 1/2” MDF and 1/4" MDF pinned together using 1/4" dowels. I created it so that its possible to make easily with hand tools and a drill in case any other DIYers want to upgrade their bearings and don’t have a CNC mill or $800.
I used my Maslow CNC to cut it out, its +- 1mm so I made it add a tolerance so I could use sand paper to get the right diameters. The end result turned out ok, looked like some kinda wood spirograph.
Mounting the jig To aid in this process, I also cut out a guide that is the same size as the stock Fiero hub, and the same size as the hole in the jig. This coupled with the bolt holes in the jig help align everything. It is important to get it on center. I put painters tape on the inner circle to give myself some leeway in case I had a measurement off, once you cut the hole too big there is nothing you can do but hit the junkyard for another part.
The carbide burr bit is a 3/8” bit with a 1/4" shaft that’s about 5” long.
I mounted the guide circle to the router, the guide circle has a 1/4" hole in the center. This allows you to put a dowel in there and mount the dowel in the router (that also accepts 1/4" tools). This well center up the guide circle perfectly. I glued mine in place with hot glue, but you want to attach it somehow.
The jig was designed so that the first cut is into the wood. This allows for the hole to be measured before cutting into the hub itself. It is still possible to make a minor adjustment or abandon the attempt at this point.
Making the cut Once it was all mounted and test cuts made, it’s a quick and easy process. The carbide bit chews through the steel like butter. Just make a cut, plunge the router down a bit, make another cut until you are done.
You can take the router off after each cut, inspect it, measure it.
I wound up making more cuts than I needed, measured, took the tape off the guide circle. Cut again. This time it was the exact OD of the bearing. So I took the outer ring off and sanded it. its aligned with dowels and not glued for this reason. Made a another set of cuts, measured, took the ring off, sanded it again.
Final size was 73.25mm, stock is 73.50mm, the bearing slid in no problem so I decided to stop there with slightly tighter tolerance than stock.
I cleaned it up with a cylinder hone and called it a success. The nice thing is now that my rings are dialed in, its easily repeatable on the second hub.
Final Results
Final fitment was mint, was able to do it for about $60 in materials and tools. Getting it within .25mm without a CNC mill was more than I expected. I will post the schematic for my jig if anyone wants it.
Cool DIY solution to open that up. I have not had an issue with my rear bearings yet but I will keep this in mind if mine fail on me.
If you do find you need more rear brakes, have you looked at Wilwoods offering for calipers? I have been looking for second hand brembos from Camaros and Subarus and they are all more expensive than what Wilwood has to offer.
Alternatively any reason to not just run the same ATS calipers on the rear and use a bias valve to reduce their clamping force? I run the same calipers and rotors front and rear with the stock proportioning valve and it works great.
If you do find you need more rear brakes, have you looked at Wilwoods offering for calipers? I have been looking for second hand brembos from Camaros and Subarus and they are all more expensive than what Wilwood has to offer.
Alternatively any reason to not just run the same ATS calipers on the rear and use a bias valve to reduce their clamping force? I run the same calipers and rotors front and rear with the stock proportioning valve and it works great.
Partly I wanted to see if it was possible to put Subaru Brembo's on a Fiero. I have seen people talk about it, some claim to have done it. But i have never seen any decent details. I went with the ATS Brembo's because they are actually the same as the Subaru just GM branded and $100 cheaper each. I also upgraded the master cylinder and booster so I decided I should just keep it matchy-matchy and went with the ATS master and booster.
I considered putting the same front brembos on the rear, but then I fell into a free set of STI rear calipers. Technically the same rear calipers matched up with the front calipers on the STI. So I decided to go with that. After being coerced to do more research I found that basically all modern mid-engine cars have a bias closer to what I'm gonna have for safety reasons so I figured I would see how it goes. I mean if they have 67% front bias in a lambo, mid-engine corvette, Mr2, lotus... then I'm sure the Fiero will be ok with it. If I die ill leave it in my will to update this forum with those results.
The other reason I went with it was because I like the look, the wheel gap was a little large with stock fiero-sized rotors when you have larger more open wheels. Its nice having a setup where i can just go to auto-value and grab new rotors, pads, whatever. They have it all in stock.
Can I ask where you got the GM Brembos? And I don't think you will die lol, but you might find that the back feels unstable and wants to come around during hard braking if it does end up under-biased. I am definitely curious to hear how it goes though, it would be nice to run a smaller rear caliper and save some unsprung weight. As a WRX owner myself I have always wondered about the STI Brembos on a Fiero, especially with the Subaru rotors also being 5x100 and having and offset that works in a Fiero.
Edit to say I looked up Ferrari 488 brake specs and did some calculations. Looking at piston sizes from caliper rebuild kits, the caliper piston area is 55:45 and the brake rotor diameter is 53:47, single piston master cylinder so around 54:46 overall in terms of torque applied to front and rear wheels with no biasing. Front and rear wheel overall diameter roughly the same. Don't think pad size has anything to do with friction available since it's a coefficient and we are looking at pressure, pad size probably only to deal with heat and to balance lifespan of pads front to rear. The front pads are way larger, the front calipers are 6 piston while the rears are 4. Not sure how the ABS biases it but I assume the fronts take over after some amount of braking once weight has transferred. Obviously the front does way more work from how the pads are sized, but during initial braking before the fronts are loaded up, they seem to want the ability to load the rears up with close to as much torque as the fronts eventually get applied to them.
I was just curious, not hating.
[This message has been edited by zkhennings (edited 01-31-2023).]
The Cadillac in the first post has them, a lot of Subaru guys in my area buy them instead of STI replacements (although they are made fun of in Subaru circles)
I know about the Ferrari bias, I think the difference is customer. Cars meant to be driven by trained individuals will have closer to 57/43 bias. The ones geared towards joe-blow who will never take it on the track are more 65/35. The reason I read in multiple places including an interview with the dude who engineered the mid-engine corvette bias is that joe-blow doesn't know how to handle the rear end coming loose during hard braking. So they would rather sacrifice performance to make it stop in a straight line every time. People who race Fieros expect the rear to slide out and know what to do.
I'm in no way saying the bias I will have will be optimal for winning races. But for my purpose, and that is occasional auto cross and weekend drag racing it should be alright. If you want a racing setup get the two master cylinders with the balance bar. But that being said, once the snow melts I will test it and let everyone know if it was a bad idea or not. I think I said before and ill say it again, there are too many variables so the best bet is to send it and see what happens.
I agree on sending it. You seem to have plenty of ability to fabricate and modify, I am sure you will end up somewhere good, and the documentation is appreciated.
Originally posted by Dan W: Either way, i can only assume the brass brake distribution block is doing something..
The brass combination valve has two functions: shuttle valve and proportioning valve. If the pressure in the front and rear circuits is too different, the shuttle valve operates, closing off the low pressure circuit to prevent the pedal from going to the floor and operating the switch that lights the brake light. The prop valve you know about.
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Originally posted by Dan W: I had to make a lot of guesses in my model to match the real data he has. Then i applied the exact same model to the 87 fieros stock setup. Then i applied the same model to my new setup. Fieroguru's data is pressure per caliper, my model calculated torque per wheel to take into account rotor size changes. The model is probably still out since my booster is different, as well as the pad sizes are also different.
At this point in writing, I haven't looked through your screen caps in detail, but following up on your previous posts... In order to calculate caliper clamp load from line pressure, you use the piston area on ONE SIDE of the brake rotor. IOW, for a fixed 4 piston caliper with two pistons on each side of the rotor, you use the area of two pistons to calculate clamp load. For a 2 piston floating caliper with both pistons on the same side of the rotor, you use the area of the two pistons to calculate clamp load. Thus, at a given line pressure, a 4 piston fixed caliper with 4x 40mm pistons generates the same clamp load as a 2 piston floating caliper with 2x 40mm pistons.
As I mentioned before, part of upgrading the rear brakes is also a good time to upgrade the rear bearings. It will give the rotors 3mm more clearance, while unnecessary is a nice to have. It also solves the problem of the bearings blowing out every summer due to the increased engine weight. Credit goes to -->this<-- post by departed Steven Snyder who documented his process (much like I’m doing here) of Alan Hamilton’s method / part list. I went over it as you would have guessed, and I cannot see an alternative upgrade option so if you want to fix the Fiero’s stock bearings this is it.
Steven's car is an '88, so there weren't any other options.
An '84-'87 car will accept the hub carriers from Aa FWD A-Body (Chevy Celebrity, Pontiac 6000, Olds Ciera, Buick Century) with the large brakes (JA2?). This lets you bolt in 27 spline 5x115mm hub bearings. Another option is the U-Body Dustbuster minivans, which use 33 spline 5x115mm hub bearings. Those carriers bolt into the Fiero suspension as well, but require swapping ball joints, which also bolt in.
Second Bearing Update I thought I would post some pictures of the other hub/bearing upgrade. The second time went much faster and had the same result.
Second bearing not thrashed by an attempted line bore
I cut out a new jig, made some slight tweaks such as adding notches in the inner circle to help with visual alignment
Mounted and aligned with the alignment tool (just hammer it in). Then verified with lots of measuring to be center.
After the cut, looks better than the last one. Lesson learned that day, only cut 1/4" at a time, I tried 1/2" and the bit heated up and tried to jump out of the router.
Caliper bracket I added a bar on the caliper bracket, seemed like a good idea.
Note the new 12.9 bolts w/ a little lock tight torqued to 95ftlb
On the car Back on the car, without the 3mm spacer noted in the bearing update forum. I am going to make them since the axel is on the very edge of falling out.
Should be noted, one of my two stock bearings had an unreasonable amount of play in it. I just replaced them last year...
[This message has been edited by Dan W (edited 02-06-2023).]
Results! I did not want to jump the gun on this one, so I took all summer to collect my results. First things first, I'm not dead
I put about 5,000km on it driving around on the highway/city/mountains/back roads. I took it to a couple of events and a couple show-n-shines.
What went wrong? Originally the first test before last winter I noted the rear brakes were weak. It sparked a debate about bias and I did a LOT of research on the subject. Besides coming up with a whole model based on real world data and theoretical numbers, I compared it to modern cars with mid-rear engines. Early spring, after installing brand new rear rotors/calipers and trying (and failing) to bleed the system for longer than a few hours... Then spending way too much time diagnosing what/where in the rear system could be clogged or letting in air considering I just loosened and replaced everything...
I found it was the Wilwood adjustable prop valve, that I guess deiced to be junk right out of the box. The way the valve was miss-manufactured meant I had basically zero rear brakes. I just removed it
On the road They don't feel like Fiero brakes, it feels like a modern sports car. I can lock all 4 wheels on a hot day before the booster hits its limit. Before if I had to stop in a hurry, I had to put my foot into it and hope for the best. With the new system I more of just deciding how quickly I want to stop and applying a little more pressure. I never had any issues in the rain or gravel. You cannot lock the rears without the fronts already sliding. It would be really hard to send it spinning on the highway.
On the track Taking it out to a few events I put in some super gripy rear pads, that I can install in 5 minutes without taking the wheel off, and that's cool. That gives the rears a noticeably more bias. Driving it is a lot easier, before I had to brace myself for the hard brake. Now I find I can just casually out stop a Miata.
The one thing that is now an issue is the amount of rear travel in the rear suspension is a problem. On a hard stop the whole back end lifts up way too high. I need to find a way to limit the travel so that it can only lift up maybe 1/3 of what it can now. If anyone has any solutions let me know.
*On a side note* To the guy who figured out how to upgrade the rear bearings, you nailed it. Not a single issue. The car will fall apart before those bearings fail.
Show & Shine Everyone who didnt own a Fiero thought it looked really slick, those that did I believe were jealous.. probably. The rears are larger than the C5 and I think if you are running larger more open wheels takes it to the next level. On the down side, these rotors rusted over almost over night. So if you park it outside and want to take it to a show, become friends with a wire wheel.
Conclusions When it comes to replacing a brake system, this is definitely an option. Nothing done here cant be done by someone else. All the parts can be purchased from most part stores. All of the brackets and spacers can be made with regular garage tools. All the parts bolt together easy, very little clearance issues. Biggest problem was getting wheels that could clear the big calipers. But that being said, this is not for beginners. I may come back with pictures and videos if I get a chance.
The one thing that is now an issue is the amount of rear travel in the rear suspension is a problem. On a hard stop the whole back end lifts up way too high. I need to find a way to limit the travel so that it can only lift up maybe 1/3 of what it can now. If anyone has any solutions let me know.
A few of use have installed anti-dive spacers in the front suspension. They work great. The actual Indy Pace Car had modified cradle mounts to rotate the cradle a little bit in order to reduce pro-squat in the rear suspension. Pro-squat also gives lift under braking.
Designed for 3/8" burr bit. Anything else will not work.
