Rear Brakes Locking before front JZA80

[1jz68u]

hi guys,


ok so here is my problem my rear brakes are locking before my fronts, simple enough.

car is a S1 JZA80 TT
small rear brakes (could this be the problem)
Big front brakes
different pads front to rear.
EBC dimpled and slotted rotors on the front
Slotted rotors on the rear.

now a bit of background. at the EC track day i went on had some dramas with the ABS going crazy and i couldnt work it out (just had standard pads in the rear at this stage)

put QFM a1rm's in the rear.

went to wakefield park track day, same problem, had a bit more time to work on it this time. it seemed to be when i was down shifting the rear was locking then ABS came on, so i disconnected the ABS. the problem went away but could still hear the rear chirping as i was down shifting.

now i have changed the brake lines and fluid and took it for a test drive yesterday and the rear is locking before the front.

now could it be the smaller rear brakes?
or is the bias out?
something wrong with the calipers?

i have no idea, i have changed everything in the braking system other than the calipers

please help

thanks.

[midnight_purple_GA70]

does it only happen on downshifting?

or will it happen when you just apply the brakes?

if its only on downshifting then it means you are getting compression lockup rather than the rears locking under braking...

if anything the setup you describe would have more front bias than rear

[JustCallMeOrlando]

It sounds like compression lockup if it only happens when downshifting...

[1jz68u]

thats what i thought it was but yesterday when testing the brakes rolling down a hill i would apply heavy brakes and the rear would lock.

under basically any heavy braking situation it locks the rears first

[1jz68u]

if anything the setup you describe would have more front bias than rear

thats what i thought too

[mattysshop]

"Big frront brakes" are they stock front brakes?

wondering if it is a master cyl size issue? and a bias valve may be needed?

[1jz68u]

Front Brakes - type PE45T/ S12W
Front Brakes - pot diameter 44.4mm / 42.8mm
Front Brakes - pot number 2 pot / 4 pot
Front Brakes - pad area 59 cm2/ 64 cm2
Front Brakes - rotor size 296.0mm x 32.0mm/ 323.0mm x 30.0mm
Front Brakes - rotor minimum 30.0mm thick/ 28.0mm thick
Rear Brakes - type PE43R/ S11
Rear Brakes - pot diameter 42.8mm/ 40.4mm
Rear Brakes - pot number 1 pot/ 2 pot
Rear Brakes - pad area 33 cm2/ 35 cm2
Rear Brakes - rotor size 307.0mm x 16.0mm/ 324.0mm x 16.0mm
Rear Brakes - rotor minimum 15.0mm thick/ 15.0mm thick


above is what supras came with and in red is what i have.

so standard brakes just not the same...

[mattysshop]

do all JZA80's have the same size master cyl bore?

[oldcorollas]

Front Brakes - pot diameter 44.4mm / 42.8mm
Front Brakes - rotor size 296.0mm x 32.0mm/ 323.0mm x 30.0mm

Rear Brakes - pot diameter 42.8mm/ 40.4mm
Rear Brakes - rotor size 307.0mm x 16.0mm/ 324.0mm x 16.0mm

i'm gonna assume rear pots are opposed twin pots?

to make it real easy to compare, you multiply area of one side of pistons, by the rotor diameter...

old front = 916cm^2.cm
new front = 929cm^2.cm

so with the same line pressure and pad friction, your new front brakes apply 1.5% more force to the tyres.

old rear = 415cm^2.cm
new rear = 441.7cm^2.cm

so with the same line pressure and pad friction (and non-sliding, opposed pots), your new rear brakes apply 6% more force to the tyres.

but if the original rear are 2 pots on a sliding caliper...
old rear = 830.7 (doesn't seem likely)


what pads are ont he front?

[Draven]

no, big and small brake setups have different maso, but small bore works with big brakes (that's what I have on mine)

[JustCallMeOrlando]

Works fine on mine too. Stu, stock rears are single-pot sliders.

[1jz68u]

yeah single sliders on the rear. Pads on front are these www.intima.net.au/sr_pads.php

[oldcorollas]

a1rm are abotu same friction (0.40 - 0.48) as those SR's (0.4~0.5) so probably not a big factor

edit.. i can't read red

swapping them around... are the stock ones oppsed twin or same side twin? looks like opposed based on the nunbers?


so... 1.5% increase at front, 6.4% increase at back....
not a huge difference, but could be the reason? 5% extra rear brake bias and downshift = too much for the tyres?

[AndyTTR]

I'd say that your problem is due to incorrect proportioning...

The factory options were 2 piston fronts, 1 piston rears OR 4 piston fronts, 2 piston rears.

You currently have a mix-and-match setup of factory 4 piston fronts with factory single piston rears yeah?

So when you hit the brakes, the fluid is split ~80% to the front, ~20% to the rear.

Because you've got more volume in the 4 piston front calliper, it requires more fluid to apply the same force than the original 2 piston calliper did.

If you had the matching 2 piston rear callipers this wouldn't be a problem because they also require more volume than the original single piston calliper did, so the brake bias is maintained (albeit with a marginally longer pedal throw I've been told).

If you look at the specs:
Rear Brakes - pot diameter
Single piston: 42.8mm
Twin piston: 40.4mm

The rear twin piston callipers have 2 x 40mm pistons, so for the same piston displacement you're getting twice the volume of fluid into the caliper. By running a single ~42mm piston you're effectively getting the same amount of pressure, but applied to only a single piston.

This means that when you brake, the rears are applying a lot more force than the fronts, hence they're locking up before they should be. This will be more apparent when you're downshifting as the weight of the car will shift more towards the front axle. Less weight on the rear wheels = less brake pressure required to lock the wheels.

What you could to to fix it:
- Fit the appropriate 2 piston rear brakes.
- Downgrade the rear pads and rotors.
- Fit an after-market brake bias adjuster and re-tune the front / rear split. I would advise against this though... the rear brakes provide stability when the car is turning while braking. Too much rear brake force will cause your car to oversteer, too little means you're not getting the full braking power available. Getting it wrong could be disastrous on a street-driven car...

By the by, MC size is the same across all JZA80 and ##Z30 models. People who have fitted the 4/2 brakes to a car that originally came with the 2/1 setup often say that the pedal throw is marginally longer compared to a car that came from the factory with the 4/2 brakes.

A possible reason for this is a change in front / rear proportioning. The bias with the larger brakes should be a little more towards the front compared to the 2/1 setup?

I haven't fully researched it yet, but from what I gather the 4/2 brakes didn't become available until the midlife update which also included changes to the configuration of the ABS unit fitted to the car. This also meant a slight change in the way front / rear proportioning was done. AFAIK you can't directly swap the post-update MC for a pre-update MC as the port configuration is different (post update has a return line from the ABS unit to the MC IIRC).

[oldcorollas]

andy, you have to think abotu pressure and volume seperately.
you won't double the line "pressure" in the rear just from reducing piston area.

the propotioning valve is based on pressure, not volume yah?

if no propotioning valve and seperate, parallel master cylinder bores, then for same MC displacement you may get whacky line pressures,
but then you have to look at pad knockback and how far the pistons really need to move (ie 0.5mm, 1mm etc), since once the pads are in contact with the discs, you really don't need any large volume to increase the braking force.. ie just need volume to counter any expenasion or flex in the parts..