Rear Brakes Locking before front JZA80

[AndyTTR]

Indeed, you're correct. I think I oversimplified...

It may be wrong, but I thought that the pressure is proportional to the volume going into the calliper?

Eg, the small volume of fluid going into a single 40mm piston calliper will result in more piston displacement (and hence pressure applied to the pads) than the same amount of volume going into a twin piston calliper with 40mm pistons?

I'm thinking in terms of volume of the system because that's what he's changed here... Might have to crunch some numbers to get my head around it. :S

[big_zop]

You need to remember that when comparing a sliding caliper with an opposed poston type, simply saying that 2x40.4mm is more force than a 1x42.8mm isnt entirely correct. The nature of the sliding caliper as that it effectively acts on both sides of the caliper, otherwise how would the opposing pad to the piston perform work on the disc?

Andy, you are correct in assuming that approx half the piston area in the sliding caliper causes double the piston extension versus the opposed caliper. But because the body moves, BOTH sides of pads move together an equal amount, ie. half the piston movement which is similar to the opposed piston caliper.

It seems that oldcorolla's made the assumption of only using one side in his calculations without actually knowing the caliper configuration, which allowed a correct comparison.

[oldcorollas]

It may be wrong, but I thought that the pressure is proportional to the volume going into the calliper?

Eg, the small volume of fluid going into a single 40mm piston calliper will result in more piston displacement (and hence pressure applied to the pads) than the same amount of volume going into a twin piston calliper with 40mm pistons?

remember that you don't basically don't get any pressure until the volume stops...

ie, assuming brake fluid is incompressible, and you don't have flex in lines or calipers, once the pads touch the disc, there is no more fluid movement. volume becomes irrelevant.
once the pads touch, you can push harder and harder, but the pedal (ideally) won't move.. hence you have increase in pressure, but no change in volume.

in reality, you have line expansion and calipr flex, so the pedal does move a bit when you push harder, btu not heaps compared to the volume required to get the pads to touch discs in the first place (ie, take up the slack) which occurs with virtually zero line pressure..


now if you have a twin master system (ie parallel bores) which have the same displacement, and no proportioning valve (completely seperate front/rear circuits)
then you might have a case where the volume required to take up slack is different, and one side of the master runs out of slack and builds pressure before the other one finishes taking up slack..

but if you have twin MC's and a balance bar , than won't happen (unless the balance bar binds)
if you have proportioning valve, then it shouldn't happen either

[oldcorollas]

It seems that oldcorolla's made the assumption of only using one side in his calculations without actually knowing the caliper configuration, which allowed a correct comparison.

yup, twin opposed pots on the front or 4 pot slidign caliper does not make much sense (although old galants had twin opposed on the front).
on the rear, the numbers say twin sliding caliper would have fairly similar force to the front braks, which wouldn't work either.. would lock up all the time.


you can refine the numbrs by using the diameter at the centre of the pads, as that will change the actual values a bit..

black values, rear = 45% of the front braking, and 31.2% of total braking effort
red values, rear = 47.5% of the front braking, and 32.5% of total brakign effort
not a huge change, but maybe just enough

[AndyTTR]

Ahh, thanks for the volume / pressure explanation OC. That makes more sense now.

So, the problem is not caused by the distribution of pressure front to rear?

Just out of curiosity, how did you calculate those percentages for braking effort?

I'm a little confused... so because he's upgraded the front brake package, but not the rear... the rears are now doing more of the braking than they're supposed to?

Or am I reading it wrong, those percentages mean that the single piston sliding callipers are better than the twin piston opposed callipers?

[/ head explode]

Which values did you work out the percentages from OC? I think the way he posted the numbers up is a bit misleading.

Early model 2/1 Package
(as was fitted to the OP's car from the factory)

Front Brakes - type: PE45T (sliding calliper)
Front Brakes - pot diameter : 44.4mm
Front Brakes - pot number: 2 pot
Front Brakes - pad area: 59 cm2
Front Brakes - rotor size: 296.0mm x 32.0mm
Front Brakes - rotor minimum: 30.0mm thick

Rear Brakes - type PE43R (sliding calliper)
Rear Brakes - pot diameter: 42.8mm
Rear Brakes - pot number: 1 pot
Rear Brakes - pad area: 33 cm2
Rear Brakes - rotor size: 307.0mm x 16.0mm
Rear Brakes - rotor minimum: 15.0mm thick

Later model 4/2 Package
OP has fitted the front brakes only. Rears are single piston as above.

Front Brakes - type: S12W (opposed pistons)
Front Brakes - pot diameter: 42.8mm
Front Brakes - pot number: 4 pot
Front Brakes - pad area: 64 cm2
Front Brakes - rotor size: 323.0mm x 30.0mm
Front Brakes - rotor minimum: 28.0mm thick

Rear Brakes - type S11 (opposed pistons)
Rear Brakes - pot diameter: 40.4mm
Rear Brakes - pot number: 2 pot
Rear Brakes - pad area: 35 cm2
Rear Brakes - rotor size: 324.0mm x 16.0mm
Rear Brakes - rotor minimum: 15.0mm thick

Sorry if I've missed something obvious, please say so if I have!

Trying to get a good understanding of this so I can make an informed decision come upgrade time for the brakes on my car... Soarer guys upgrade to the 4 piston Celsior callipers without problems apparently... those have a smaller diameter rotor though, perhaps avoiding this bias issue.

[oldcorollas]

well..

the force transmitted to the tyre is a function of a chain of events.

pedal pushes MC
MC creates line pressure
Line pressure pushes caliper piston
piston pushes pad
pad causes slowing of the disc by friction,
pad friction on disc is function of pad coefficient of friction and the force applied by the piston.

pad friction x the moment arm of the disc = force applied to tyre.

so it comes down to:
pedal force x leverage / MC piston area = line pressure
piston area x line pressure = pad force (all pistons if sliding, half the pistons if fixed caliper with opposed pistons)
pad force x disc moment arm = braking force

but you can make a few assumptions and get it easier to work out, if just thinkign about caliper and discs, ie pad friction is same, line pressure is equal, force applied by pad to disc acts through centre of pad etc. you can factor them in afterwards.


anyway, it boils down to
area of pistons x disc moment arm {is proportional to} braking force.

so you can just use the piston area multiplied by the radius (or diameter) where the centre of the pad touches the disc, and you can then compare braking force betwen calipr/disc combinations.

you can do this cos the (near meaningless) numbers you get are directly proportional to braking force.


in the case above, i simplified futher, just using the total diamter of the discs, as i don't know the pad size.
(ideally you'd work out the geometric centre of the pad, fudge factored for the diferent velocity of the disc at different pad position, adn the change in friction with velocity, but thats not gonna change the answer much anyway)

[oldcorollas]

so..

for example.
original 2/1 package.

front:
44.4mm / (2 x 10) = 2.22cm piston radius
Pi x (2.22)^2 = 15.483cm^2 per piston, or 30.966cm^2 per caliper

29.6cm x 30.966cm^2 = 916.515 cm^3 is the meaningless number (MN) proportional to brake force

then do the same for each caliper.

percentage change = ((new MN / old MN) x 100) - 100

[oldcorollas]

I'm a little confused... so because he's upgraded the front brake package, but not the rear... the rears are now doing more of the braking than they're supposed to?

Or am I reading it wrong, those percentages mean that the single piston sliding callipers are better than the twin piston opposed callipers?

the rears of the 2/1 package have more braking force than the rears of the 4/2 package.

so if you think of it as putting the 2/1 rears into a 4/2 setup, you increase rear brake bias.

he has the higher force brakes from each setup, but that has changed the balance a bit to be more rear biased.

same as putign RX7 4 spots on an AE86. the AE86 single pot slider provides more pad force for the same line presure than the 4 pots do (don't think abotu volume yet) and so you end up with rear bias if used on stock diameter discs.

[1jz68u]

ok guys this mathematical conversation has been awsome.

so to fix the problem i need to get 2 pot rears or put a bias adjuster in or get rubbish rear pads?? Correct?

And all these numbers prove that i currently have more braking force in the rear than the front??

[oldcorollas]

lower friction coefficient rear pads, bias adjuster to reduce rear line pressure, or get the matching 2 pot rears which will reduce your rear braking force by 6%, even with the larger discs.

nah, front is still much greater braking force, but it is the balance between the front and back that could be the issue. (rears are still less than half of what fronts are capable of, but rear should have lower line pressure, but thats a different story)

to make it even more meaningless number, but easier to compare.. normalise to the 2 pot front brakes braking force...

4 pot front = 101.4
2 pot front = 100
2 pot rear = 45.3
1 pot rear = 48.2

how tall are all these pads? with the pad height, we can guess the radius of the brake disc at centre of pad, and make a better comparison (before cutting/changing anything...)

[1jz68u]

so the braided lines i also just fitted will also make thiis worse??

I will attempt to get pad dimensions tomorrow oc

So crownman hard pads should help??

[JustCallMeOrlando]

Just get the matching rear calipers and discs dude It's worth it for the lack of hassle.

[1jz68u]

not as easy as it sounds. Where am i going to find just rear calipers? And im not sure i want to buy a set just to get the rears then have a spare set of fronts??

Wonder if i can get just rears from toyota?

Just to clarify also the brake combo (small rear large front) was already on the car when i got it i have just changed everything else to do with the brake system.

Thanks again for the comments guys

[allencr]

so the braided lines i also just fitted will also make thiis worse??

NO, nothing at all to do with it if your OEM lines were half way decent, they just increase sensitivity by not expanding & requiring more volume.
Put in a limiting valve, usually called a proportioning valve and/or harder pads for the rear.
Every car should have one to keep the rear from locking and your ABS should accomplish the same thing even better, sorry, I don't know why it isn't doing its job. Don't know about ABS except the theory & advertising.

[midnight_purple_GA70]

rory at rutherford and bond should be able to hook you up with a set of rear calipers...

he is in nz but prices are generally very competitive/awesome

[email protected]

thats his email address... you will need either part numbers or vin/chassis of a car with the 4/2 pot brakes