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Unusual brake question by Knight
Started on: 12-25-2014 06:55 PM
Replies: 3 (240 views)
Last post by: Knight on 02-04-2015 02:29 PM
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Report this Post12-25-2014 06:55 PM Click Here to See the Profile for KnightSend a Private Message to KnightEdit/Delete MessageReply w/QuoteDirect Link to This Post
Until I upgrade to the same size rotors all around, I have a question related to a 88 rear swap on a 86 GT.
I need not opinions but actual experiences or fact based answers or theory grounded ones.
And yes I am aware that the Fiero already has too much front bias.

What is the f/r bias on a Fiero with 11.26 inch Lebaron front and 12 inch upgrade 88 rear swap with 88 calipers.
1) 2.5 inch piston caliper front
2) 2 3/8 inch calipers
3) 2.25 inch calipers (sprint track calipers)

Thanks. Have a held drop spindle/Lebaron braked front I want to put on an 88 GT with a 88 swap with 88 brakes.
Really hope to keep the stock rims for now.

Also what's the current bias with stock 86 front and 88 rear brakes?

Again thanks.
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Report this Post12-25-2014 08:58 PM Click Here to See the Profile for fieroguruSend a Private Message to fieroguruEdit/Delete MessageReply w/QuoteDirect Link to This Post
Your question is quite complicated...

I do not think anyone has ever measured the brake line pressure vs. varying input pressures to measure the actual line pressure bias on the 84-87's, but I did do this on an 88 as part of this thread:

Here is a summary of the data and what it shows is that at low pedal input pressures the front brakes start out with about 57% of the line pressure and it increased to 64% once the combination valve has reached it bias limit (very early on). The 88's use the same diameter rotors and same caliper piston diameters, so this table is also the overall brake bias for a stock 88 Fiero.

Caliper Line Pressure Comparison Stock 88:

2.5L 2.5L
Front Rear %
Stock Stock Front
Input Air Booster Booster Bias
(psi) (psi)
5 n/a n/a n/a
10 400 300 57%
15 600 400 60%
20 800 500 62%
25 925 600 61%
30 1200 675 64%
35 1225 700 64%
40 1250 700 64%
45 1300 725 64%
50 1325 725 65%
55 1375 775 64%
60 1400 775 64%

As you change the caliper piston areas and the line pressure remains the same, the clamp load on the brake pads increases, increasing the stopping rate.
As you change the rotor diameters (caliper position from wheel center) you change the mechanical advantage of the caliper, which can also increase the stopping rate.

Impact of Caliper Piston Area:
Stock 88 = 48mm = 1,809 sqmm (Centric lists the 84-87 caliper pistons to be 48mm as well)
2.5" Caliper = 3,156 sqmm (75% larger than stock)
2.375" caliper = 2,900 sqmm (60% larger than stock)
2.25" caliper = 2,564 sqmm (42% larger than stock)

As you can see, all of your caliper options for the front GREATLY increase the caliper area and overall clamp load and will significantly alter the brake bias to the front.
Add using these larger calipers on a larger rotor and the change is much more significant.

Impact of the Rotor Diameter (actually, caliper placement further from the wheel center) vs. stock 88 Fiero:
Stock 84-87 rotor: 9.69" (246mm). Estimated to be in the 3.8 to 3.9" range (based off the 88 caliper geometry) or about 11% less caliper leverage than a stock 88.
Stock 88 rotor: 10.43" (265mm). Center of caliper piston to center of wheel: 4.3" = caliper lever arm for braking force.
12" C 4 rotor: 12.01 (305mm). Center of the caliper piston to center of wheel: 5.2" or a 21% increase in the level arm. (this is slightly more than the rotor diameter change as the caliper must sit higher on the larger rotors for proper bridge clearance).
I have never done a LeBaron swap, but the caliper piston will likely be in the 4.8" range or about 14% increase in the lever arm length vs the stock 88.

So if you run the stock 84-87 front brakes and the stock 88 rear brakes... the piston diameters stay the same, so the only change is the impact of the larger rear rotor which would be 11% increase in rear braking leverage.

Using the Lebaron rotors and the 2 1/4" caliper would increase the front braking by 42% due to the caliper size & about 25% due to the increase in rotor diameter. In the rears even if you up-size to the 12" rotors, you only see a 35% increase in braking leverage vs. stock 84-87, so this system will still have a major shift in brake bias to the fronts. To get this system back close to the stock brake bias would require replacing the combination valve with one that would send more equal line pressure to the rear brakes.

[This message has been edited by fieroguru (edited 12-25-2014).]

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Report this Post12-26-2014 01:53 PM Click Here to See the Profile for thesameguyClick Here to Email thesameguySend a Private Message to thesameguyEdit/Delete MessageReply w/QuoteDirect Link to This Post
There are two things that have really been troubling me about all the bias discussions on PFF -

1. A lot of reviews of the Fiero from back in the day comment on poor brake bias - too much rear bias that makes stopping unpredictable.
2. There is no "perfect brake bias" - the bias you want will vary with driving conditions, tires, and even driver preference.

Based on reading here I had a lot of concerns with my setup - '87 GT, '88 cradle swap, a Blazer MC, C5 front brakes, 225s up front and 245s out back. I shifted a lot of bias to the front and a lot of grip towards the rear - and it's fantastic.

I feel like on a mid-engine car you really don't want a factory-level of rear brake bias as weight transfer under heavy braking will tend to reduce rear traction and increase the likelihood of rear lockup. In my very limited experience the stock bias was far too rear-heavy for stable braking - my stock car was not at all confidence inspiring. With a shift towards front bias and brakes big enough to handle the demand the car became a lot more chuckable and fun to drive.

Again, I've got limited experience - a year's worth of driving pre-upgrade and a year post-upgrade, but I've got no regrets. Wherever I ended up has yielded a very drivable car. Personally, I would not stress about shifting a lot more brake to the front, and frankly I feel like the Fiero emerged as it did not by design but by cost cutting. If the budget had been there, I think the Fiero would have shipped with a lot more front bias originally.
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Report this Post02-04-2015 02:29 PM Click Here to See the Profile for KnightSend a Private Message to KnightEdit/Delete MessageReply w/QuoteDirect Link to This Post
Thank you both very much. The wealth of knowledge and experience of the members of this forum is amazing and invaluable for a Fiero enthusiast.
So for now the only two options would be to get a 88 specific bracket for the rear to complete the 11.26 inch upgrade or use use a adjustable bias valve to decrease rear line pressure. The adjustable valve would be a good idea anyway since I plan on using a staggered tire setup with (205/215) on stock rims for now.
The adjustment valve goes after the factory bias valve, correct?
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