Rear brake bias

[pshooter]

Having a problem with rear brake bias. Cant really get to late hard braking without the rears locking.


Massive fronts PFC11
Stock rears PFC08
All the ABS hardware is there but no wheel sensors or brain.


Rear pads too aggressive?
Install an adj proportioning valve in the rear?


Looking for pearls of wisdom


TIA
Shoe

[TobyB]

Wow- that don't seem right.
You doubled the front piston area, so you should be all fronty and stuff.

But wait!
What master are you using? The later ABS cars have that whackadoodle 22/17 split
master, and if the 17 bore's on back, maybe that's doing it.

The earlier cars are 22/22 and the early 318 had a neat 20.5/19 split, so those are
2 options.

If'n it was me, I'd yank all the ABS plumbing so you can get it to bleed completely,
and then go from there.

I had some trouble with a special Canadian- market bias valve that'd seized, so I'm
pretty anti- anything- fancy in the brakes.

t

[ForcedFirebird]

^^^actually, what happens in the case of increasing the front pistons, the rears will lock earlier than the front, without a doubt. Since fluid pressure will be spread across more area in the front, the new front calipers will have LESS movement than stock if all else is equal.

Lee has helped me over the years with wacky brake systems I cobbled together on race cars and such. Basically you want a 2:1 F:R caliper piston ratio (with the factory bias valve removed). So, when people upgrade the front brakes with larger calipers/more pistons, you are effectively creating LESS piston travel while increasing pad to rotor contact.

[FredK]

The ratio between brake master cylinder and wheel cylinder (brake piston diameter) is the determinant of braking force. Increasing brake piston diameter while keeping the master cylinder diameter the same results in having a larger hydraulic advantage and therefore mechanical advantage at the axle in question.

Think about it the other way around: People often "upgrade" to a larger 23.8 mm master cylinder which reduces pedal travel and perceived braking system "stiffness". The way by which is reduces pedal travel of course is because per unit length of stroke, the amount of travel at the wheel cylinders is increased. However, the brake pedal effort also increases.

One of the effects of increasing wheel cylinder size is that you are now more susceptible to the effects of pad knockback. As ForcedFirebird said, the new front calipers will have less movement (per unit pedal stroke) than stock if all else is equal. This same property makes smaller amounts of knockback more detectable at the pedal.

Putting it all together, when you apply the brakes, the initial amount of travel in braking systems feels "dead" because the brake pads are retracted a small amount relative to the rotor surface. As the hydraulic circuits push the wheel cylinders relative to the master, the pads are then brought into contact with the rotors and you feel the pedal start to ramp up in pressure.

In an ideal world, the pads touch the rotors at the same time and the braking force also ramps up linearly. However, this isn't always the case; if there is more knockback on one axle vs. the other, the way the brakes ramp up in braking torque will be different. This is why when you watch race car drivers approaching a corner in a race, they often tap the pedal so that when they do apply the brakes that they apply squarely and predictably.

Because the front brakes are potentially more susceptible to knockback, if they are indeed knocked back, the rears will ramp up in pressure quicker and the fronts won't, which would result in the car feeling squirrely on braking and the rears locking before the fronts.

The other possibility is that there is air in one circuit vs. the other, which would affect line pressure.

[1990m3]

I had a similar problem when I upgraded to wilwood brakes. The rears locked so badly I spun really hard into turn 1 of my first qualifying race. Turned out my front calipers weren’t working properly. I pulled them apart, rebuilt with new o-rings and lube and it solved my problem. I also added a proportioning valve so I could micro adjust at the same time.

[TobyB]

John, I may have to eat my words here-
you might have been wrong, once!

Fred's explanation is what happens on my cars- the hydraulic ratio determines
the braking force ratio.
Smaller front MC= more front brake.
Bigger front caliper pistons= more front brake.
Reason being, it's not travel that generates braking force,
it's pressure. The relative pressure on the pads sets the relative braking force.

And blessedly, pad area is NOT a factor, as when pad area increases,
unit pressure on the pad decreases. and friction remains, all else being equal, the same.

Fred, the only time I have experienced 'grab' from the opposite
end of the car due to knockback was when the knockback
was so bad that the front M/C ran out of travel. THAT is, agreed, terrifying.
You have one brake system, and all it can do is lock your rears.
If it's just travel, the pressures involved to move the pistons back
out are 'usually' so low that you might notice, but it doesn't upset the car.

t

[pshooter]

Thanks for the input guys.


As this car has been in the "build" process for a couple years I've tried to address all the big things . As I'm shaking it out its the growing list of all the other stuff.


I'll go thru the rear calipers next week.
Question is for the short term I'm thinking of putting a Wilwood adj valve in the rear line. Leaving the stock valve in place should not make a difference should it?


In the off season I look into a proper dual or single boosterless system.


Thanks
Shoe

[FredK]

If I were you I would delete the stock valve. The Wilwood adjustable valve and the stock E30 valve are both pressure reducing valves, with the Wilwood obviously being an adjustable unit.

Deleting the stock valve is pretty easy, you can replace it with a bubble flare union that you can get at Autozone for around $5.

[ForcedFirebird]

John, I may have to eat my words here-
you might have been wrong, once!

Fred's explanation is what happens on my cars- the hydraulic ratio determines
the braking force ratio.
Smaller front MC= more front brake.
Bigger front caliper pistons= more front brake.
Reason being, it's not travel that generates braking force,
it's pressure. The relative pressure on the pads sets the relative braking force.

And blessedly, pad area is NOT a factor, as when pad area increases,
unit pressure on the pad decreases. and friction remains, all else being equal, the same.

Fred, the only time I have experienced 'grab' from the opposite
end of the car due to knockback was when the knockback
was so bad that the front M/C ran out of travel. THAT is, agreed, terrifying.
You have one brake system, and all it can do is lock your rears.
If it's just travel, the pressures involved to move the pistons back
out are 'usually' so low that you might notice, but it doesn't upset the car.

t

I'm 180° out, just like when I hooked up the new 208v 3ph compressor motor today (or is that considered 120° out lol).

[TobyB]

You know, the best part about 3 phase is that it's never wrong, it's just sometimes backwards...

...not that I've ever done that, of course...

t

[1990m3]

Thanks for the input guys.


As this car has been in the "build" process for a couple years I've tried to address all the big things .

Sounds like my situation. Caliper pistons just got stuck/fused to the o-rings. New o-ring, clean up the pistons and it works perfectly. Should be an annual service with these parts

David

[moatilliatta]

Why not make the ABS work?

[pshooter]

everything on the electrical / electronic side for the ABS no longer exists.


As this car progresses I may entertain adopting a newer or aftermarket ABS if the right opportunity arises.


I will say that getting back into a completely analog track car is a good refresher. You quickly realize how much some of these newer cars do for you.


Shoe

[Beej '86 325es]

I replaced my OEM bias valve over the winter with a Wilwood bias valve.

You can purchase it with 10mm threads and it fits right in the OEM valve’s place. It’s a little inconvenient to reach there, though, so I bought the Wilwood remote adjuster knob, removed the fitting that came on the end of the bias valve, trimmed the cable to length, and it works great!

Here’s a photo of the valve before I removed it and modified it for the cable.


Sent from my iPhone using Tapatalk

[1990m3]

I did the exact same thing with the exact same proportioning valve.