Att: Owner of AE86 with big brake

[lexsmaz]

If the fronts are locking too easy, the front brakes are maybe too big for your car setup ...
if you ran a smalller m/cyl or bigger booster, you would find that they locked up even easier ... What size front tyres are you runing, & do you have a lot of negative camber setup in the front end, which will make it worse ... you might have to go for a softer tyre compound & some bigger / wider front wheels, unlesss you are already runing some big rubber up front .. basicallly the front brake package you have is to suit a 1700+ kg falcon with maybe a 1000 kg over the front wheels versus your 1000 kg total weight of your car & the falcon runing at least a 205 minimum width tyre ..

[RobertoX]

without changing the proportioning valve you will need more braking effort - you've doubled the front piston size without changing the amount of fluid going to the rears.

It does depend on a lot of things on what front rear balance you will achieve after a conversion. (rotor size, pad friction, tyre size, piston area etc) If you increase the front piston area you will generally get more front bias (line pressure acting on a larger area = more clamping force). Also with the larger discs up front you will obviously have more front bias.

Also what 30psi_4agte said about the master size is right on the money!

So maybe try a smaller master of different booster, also I would get an aftermarket proportioning valve.

[takai]

What i would do.

[RobertoX]

Would?
......

[takai]

Did .................

[WDE_BDY]

if you ran a smalller m/cyl or bigger booster, you would find that they locked up even easier ...

A smaller master means more travel for the same clamping force, this will allow you to modulate your pedal pressure easier (preventing lockup) than the on/off switch you tend to get when the master is too big. Too big a master means more pedal effort and once you push hard they will grab easier. Too small and your travel increases and gives a spongy pedal.

Callum

[takai]

FWIW The theories posted in this thread are all sound. Bigger master == more pressure for less travel, smaller master == more travel for same pressure.

Also FWIW i like unboosted brakes, but my car cares not about being road legal anymore.

[lexsmaz]

The last two guys have got it a bit topsy turvy,
Bigger m/cyl = more volume displacement but less clamping pressure on the callipers for the same amount of X pedal effort ..
Smaller m/cyl = less volume displacement but more clamping pressure on the callipers for the same amount of X pedal effort, down side being longer pedal travel ...
Smaller mcyl would in my opinion be harder to modulate due to less pedal effort required to get front brake lock up ....

[takai]

Smaller master is MUCH easier to module in my opinion, and what both Callum and I have said is exactly what you are proposing, just controlling for different variables.

[RobertoX]

What lexsmaz says is correct and the best way to approach thinking about the problem IMO...

However, Tom does say that he needs to press quite hard to get them to work, which implies that he wants less force at the pedal for the same braking force. The drawback with this is more pedal travel, whether or not this will be a problem should be figured out through some experimentation I think.

[Tom86]

Hey Takai,

I'm running a similar Master as the one you've got. Are you running the proportioning valve on the front or the rear? because I think the front outlet on the master I've got goes to front brake and outlet at the back goes to rear brake.

So are you running proportioning valve on the front? and nothing on the rear? (according to the pic)

Ok my original question was about the feel of the brake, but since we're talking about brake balance and bias. I will start from the beginning.

Originally this is how the brake used to behave. After I did the rear disc conversion the rear would always lock before the front. I know that ideally you would want the front to lock up before the rear, so after I put the big brake in front the front would always lock up first. So thought I fix the problem. Now you guys have got me thinking that there might not be enough fluid going to the rear.

Is there a way to actually measure this? after I saw the pic of Takai's setup I thinking of having a T piece for the front and an adjustable proportioning valve for the rear that way i can control how much fluid goes to the rear and improve overall braking power?

Takai, I wish I have that much room in my engine bay and how did you bend your brake lines so neatly?

[takai]

On mine the front res is the rear brake, and the rear the front brakes. Proportioner is on the rear brake line. The T-piece which you see there in the middle splits the front brakes, while the rear is a single line to the diff.
Its quite common to get rear lockup when you convert to discs, you need much less pressure to lock the discs than the drums due to the piston designs and rotational force.

I think the lines are bent nastily when i took that photo, mind you i havnt neatened it up much since. Those were bent with a cheap ABW pipe bender.

The other thing to think about is the stock T-piece/passthrough on the firewall has a decent sized bias limiter in it. I would ditch this as fast as possible and run an adjustable bias valve. But then again that AE86 isnt about legality.

[NFI]

Im having the same issue on my aussie 86,have s13 suspension with the 4 pots and borg warner rear with its factory drums (im told slightly larger\better than the s series brakes). Brakes are running thru the factory proportioning valve but have r32 master. The front wheels are 205\40\17s with a decent amount of camber,its zero on the tops but still looks like the delorian. Havent had it on a aligner.
Front pads are a cheapo brand but new and discs i just machined. Braked a bit better and actually got the smell of burning brakes which never hapen before,still not rite tho.
In traffic i have too push a crap load on the pedal too stop the car and its sorta worying cos sometimes you think it wont pull up in time. And being a GTE it gathers speed quick.Just doesnt have that bite a factory set up of any car does.
It will lock up the front but it wont be a constant lockup,more like abs which i spose is a good thing but it doesnt wash off speed all that well.
I know you can buy adjustable proportioning valves but they cant be in the cabin plus they have a locking nut on them,legally fitted that way.
I was thinking the only standardish item left on the brake setup was the prop. valve and it being made for tiny brakes the ports in it would be small ,i think,so wouldnt allow much or enuf fluid through. If it was the main problem and once that was changed itd be sweet. And cos the master and calipers are of the same car logically ud think itd would work like it would on a skyline.
My daily is a peugeot 306 and have fitted brembo 4 pots on it and they have great bite,like head butting the wheel bite and wash's of speed real nice. No other mods too braking system.
Sory for essay guys. haha

[EVOSTi]

is there a question in that?

try an aftermarket proportioning valve, might help things out a bit for not alot of cost compared to what you have already spent.

[WDE_BDY]

NFI, what R32 master are you using? R32 from turbo/4WD models is disc front and rear so will cause problems with a drum rear. Have you changed the residual pressure valve to a drum brake one for the rear?

Callum