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| Anyone else doing brake proportioning valves? (Page 1/4) |
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Raydar
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FEB 08, 01:25 PM
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My 88 has a very pronounced front-bias to the brakes. From talking with other folks, I understand that this is pretty typical.
This was aaron88's mall thread.
http://www.fiero.nl/forum/Forum4/HTML/064690.html
It would appear that he no longer is making these for whatever reason.
I was wondering if there is another solution for this situation, or if someone else might be inclined to produce these.
I would have posted this request in the mall, but there are obviously technical aspects to this that are more involved than simply posting a "want to buy" thread.
Thanks.
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Raydar
88 Formula IMSA Fastback. 4.9, NVG T550
 
Praise the Lowered!
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Gall757
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FEB 08, 01:44 PM
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Don't miss the Better Braking thread over in General Chat. It covers a bunch of issues including front bias (when it's not off-topic about modern cars  ).
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Raydar
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FEB 08, 01:48 PM
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cmechmann
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FEB 08, 02:43 PM
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Has anyone made up a data base for bias over years.
I haven't had much exposure to 88s. This is also going to have variables with drive train, suspension, condition.
To me it seemed as follows:
84 heavy rear.
85 heavy front.
86 neutral, but wide variables.
87 tad heavy rear.
It boils down to finding that happy medium. Seems the best would be heavy rear until the rear unloads from heavy cornering and panic stops. Had my share of a little wet, light turn, hit the brakes, goes straight.
Has anyone installed rear weight differencial valves?
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jmbishop
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FEB 08, 02:49 PM
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| quote | Originally posted by Gall757:
Don't miss the Better Braking thread over in General Chat. It covers a bunch of issues including front bias (when it's not off-topic about modern cars ). |
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It's a great thread but this is a different issue. Changing tire size and running a staggered setup changes things and the only way to fix it is by changing wheel and tire sizes otr tuning the bias to your setup.
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Gall757
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FEB 08, 03:09 PM
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If the proportioning valve is different for different years, I have not heard of it. People swap them all the time, and think nothing of it. It's more likely that the adjustment mechanism in the rear calipers is not working, and what little stopping power that is assigned to the rear by the proportioning valve goes unused as the rear adjuster fails.
edit...The Ogre is going to update his cave about rear e-brake adjusters...apparently GM has blown a lot of smoke about them over the decades.[This message has been edited by Gall757 (edited 02-08-2015).]
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Raydar
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FEB 08, 04:07 PM
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| quote | Originally posted by Gall757:
If the proportioning valve is different for different years, I have not heard of it. People swap them all the time, and think nothing of it. It's more likely that the adjustment mechanism in the rear calipers is not working, and what little stopping power that is assigned to the rear by the proportioning valve goes unused as the rear adjuster fails.
edit...The Ogre is going to update his cave about rear e-brake adjusters...apparently GM has blown a lot of smoke about them over the decades.
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My adjusters (and parking brake) work fine. My brakes work fine, in general, except for the front bias, under hard braking.
I had a conversation with fieroguru. He seemed to share my opinion that the 88 brakes are biased towards the fronts. I asked him about increasing my front tire size. His opinion was that it probably wouldn't help much. Would just bandaid the issue, and probably not that well. (That's what I took away from the conversation, anyway. Not trying to put words in his mouth.)
Aside from that, 88 GTs and Formulas (which mine is) started out with staggered tires, anyway.
I have not explored aftermarket proportioning valves.[This message has been edited by Raydar (edited 02-08-2015).]
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olejoedad
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FEB 08, 04:12 PM
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88 proportioning valve is different. 88 caliper bores, front and rear are the same. 84-87 caliper bores are different sizes, f/r.
I use an 88 valve when doing 4 wheel G/A swaps.
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Larryinkc
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FEB 08, 07:05 PM
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WCF and Aaron both suggested disabling the 88 proportioning valve for the C4 brake mod on my 88 Mera.
Here's the explanation:
It looks like I have some bad news for you. By my calculation you will have about 5097 lb (at max braking effort), front caliper force and only 1266 lb rear caliper force with the stock proportioning system. That's only 20% on the rear system. Depending on how soft your tires are you will need about 40% of your system force at the rear caliper in order to make use of my proportioning adapter. If you completely remove the proportioning system you still only have about 2825 lb at the rear caliper. That's 36%
If you use the stock 88 rear caliper (which is about 45.5mm if I remember right) then you could get 42% on the rear, but that's only if you remove the proportioning. But even with my adapter installed and cranked all the way in you will be in abouts 2394 lb, which is 32% rear braking. Unless you are running slicks that's not enough. and running 42% (no proportioning) might be too much rear, therefore causing a probelm with the back end swinging out.
There is a big problem with brakes when using a modern ABS designed system. They are basically designed to fail safe (if the abs module stops working) and are therefore always undersized on the rear. I'm not sure what to advise here. You could try installing your calipers as purchased and remove the proportioning system rubber regulator (not sure what to call it). And just see how the balance is. Make sure the fronts lock up first.
If I sold you a proportioning adapter you would just crank it to full and still end up with too much rear proportioning (with my adapter installed the system will start proportioning the rear at about 800psi system pressure, if fully cranked).
Sorry for the bad news, but your brakes will be much better than stock anyway. I'm just afraid you haven't left any room for proportioning. However there are a lot of other factors that come into play such as front to rear tire to ground contact patch, tire compound, and driving style. It's too hard to calculate before hand which is why we like to have adjustable proportioning. I'm not sure what to recommend in your case though. You would basically need a rear caliper that measures 47 mm to 52 mm I think. Nothing larger but maybe 46 would work (that's stork Vette rear).
Maybe what I would try first is using your stock 88 calipers on the rear, and remove the proportioning. Then go from there. This would give you a gauge to start from.
When you disable the proportioning in you car. Do not remove the spring. Instead remove the rubber washer looking seal at the end of the plunger (not the shaft o-ring seal).
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fieroguru
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FEB 08, 07:24 PM
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As you stagger tire width to better match weight bias front/rear, you will want more rear brake bias.
If you run shorter wheels/tires up front and taller in the rear, you will want more rear bias.
The C4 world has stiffer springs they can swap into their combo valves to increase rear bias, but Aaron88's setup is the only one I am aware of that provides linear adjustment. How it works is the combo valves start with the front/rear brakes seeing close to the same pressure at very low pedal pressure. At the "Knee Point" the combo valve starts to reduce the rear pressure as it assumes the cars weight is shifting forward. What the C4 stiffer springs and Aaron's adjuster do is move the knee point to a much higher pressure - so the rear line pressure remains closer to the front line pressure up to a much higher pedal input pressure.
All off the shelf brake bias adjusters can only reduce rear brake pressure (and therefore bias), not increase it.
The oddity of the Fiero is the combo valve controls 100% of the bias front/rear because it uses the same diameter caliper pistons front year. So to create any bias, it must limit the rear brake line pressure.
Most other cars use smaller caliper pistons in the rear which builds in less rear braking even with the same line pressure as the front. So the rough bias is due to caliper piston diameters, then it can be fine tuned within the combo valve with line pressure reductions. A combo valve from one of these other applications will likely result in more rear brake line pressure, which will increase rear bias substantially, then you probably could dial it back in with an off the shelf bias adjuster to reduce rear brake line pressure.
So you can better "see" what I am talking about, here is a chart where I used an air cylinder to apply various levels of pedal pressure and measured the caliper line pressure in the front and rear on an 88 2.5L Fiero with the engine running (stock brake booster). As you can see, the rear line pressure starts out at 75% of the front pressure at very low pedal pressure and drops to around 56% at 30 PSI input pressure and stays there as pedal pressure continues to increase. Since the caliper pistons are the same front/rear (48mm = 1809 mm^2) and the brake rotors are the same front/rear, this pressure difference front/rear is the brake bias front/rear as well.
code:
Caliper Line Pressure Comparison Stock 88 Booster
Input Air 2.5L 2.5L Rear vs.
1 ½” air cylinder Front Rear Front
(psi) (psi) (psi)
5 n/a n/a
10 400 300 75%
15 600 400 66%
20 800 500 63%
25 925 600 65%
30 1200 675 56%
35 1225 700 57%
40 1250 700 56%
45 1300 725 56%
50 1325 725 55%
55 1375 775 56%
60 1400 775 55%
Now let's look at an 88 Corvette (non-HD, so it uses the same diameter rotors front/rear)...
Front Caliper: Dual 38mm pistons = 2267 mm^2
Rear Caliper: Single 40mm piston = 1256 mm^2
So with the exact same line pressure front/rear the rear calipers will have 55% less braking force than the fronts and end up with about the same final bias as the Fiero (unless its combo valve further reduces rear line pressure), but does so via caliper size vs. reducing line pressure.
Unfortunately the combo valve for the C4 Vettes is built into the brake master cylinder, so you can't just swap it into a Fiero. Plus the C4 brake master cylinder bore is smaller and the line fittings are true metric vs. the fiero ones being standard... so it isn't a good option for a retrofit (but is a good example showing how things are different). Just as an FYI, the C4 world can buy stiffer springs for their combo valve to increase rear bias as well, but those are not adjustable like Aaron88's).
It would be a fun experiment to measure caliper line pressures on several GM cars with similar style combo valves in an effort to find one that sends more pressure to the rear than the stock fiero ones do... Once you have too much rear line pressure, reducing it is quite simple with many off the shelf options to pick from.
EDIT: Just saw Larryinkc's post...
From my example above, the stock 88 rear bias limits at the high end with the rears seeing about 56% of the braking that the fronts do.
The C4 setup with the SAME line pressure to the front & rear will have the rear brakes at 55% of the fronts, which is very, very close to the 56% seen with the stock brake setup. The easiest way to ensure front/rear see the same pressure is to eliminate the combo valve, and with it gone the brake bias will be very close to stock, but you lose the additional rear bias at lower pedal inputs (the function of the combo valve).[This message has been edited by fieroguru (edited 02-08-2015).]
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