Bigger Brakes and Stock Prob Valve

[Bazda]

Ok, so i'm deciding on upgrading my brakes.
Currently have Wilwood 4-pot 35mm pistons.

From stock calipers to the wilwoods the pedal became much stiffer and higher. I thought it would be the other way around but I was wrong.

I can only think of 2 reasons to why.
1. Piston needs to move less to make good braking power
2. The stock prob valve is distributing the fluid differently in some strange way.

So basically all this has caused me to have alot less rear brakes.

I want to try sort this out by changing the piston size in the front calipers. But dont know weather to go bigger or smaller.
Since going to a larger piston surface area the pedal is harder and higher, which in theory is not correct. The way it has worked now it says I should be going smaller in piston size to get more pedal travel to how it was when in stock form...

Someone have any input on this?

[mjrstar]

sounds to me that option 1 is right:


an opposed piston caliper will not need as much fluid displacement as a sliding caliper of the same piston diameter. as the pads only have to move 1/2 the distance back to the disc. I had a lengthy thread on another forum with a member on this very subject. I was more looking at master cylinder sizes vs pedal effort..


here is a snippet.

Matt:
A single piston sliding caliper with a 1 inch bore piston will use the same fluid as a twin pot caliper with opposing pistons the same size? It will use half the fluid of a twin piston sliding caliper with pistons the same size. The twin piston sliding caliper with 1 inch bore pistons will use the same fluid as a 4 piston fixed caliper with the same size pistons. This is because on a sliding caliper the the piston must travel twice the distance as it accounts for the travel of the pads on both sides if the disc, so a single piston caliper will travel 2mm to take up a 1mm "gap" on each side of the disc but for a twin piston fixed caliper each piston only moves 1mm?

When I say bore i'm talking total piston area, sure we all went to school for a bit but how much attention during maths, it wasn't all that exciting.

I think I have missed something here...

and the reply:


You are right about the comparison between sliding calipers and opposed-piston calipers.
When comparing effective areas (piston bores), the sliding caliper's total piston area is compared to ONE SIDE of the opposed piston caliper, not all of them.

The difference in knock-off succeptibility between the two types has more to do with the fact that the opposed caliper is fixed than the number of pistons, although this does increase it even more.

For those that don't know what pad knock-off is:
When cornering hard there is always some flex in the wheel bearings, this causes the brake disks to deflect a bit. (since they are clamped to the wheel)
This deflection causes the brake pads to be pushed back, thereby pushing the pistons back into their bores a bit.
The next time you jump on the pedal, the hard point is lower than normal since the master cylinder has to push the pistons back out.
(This is pretty much the only time that 'fluid volume' comes into the equation, all the rest is pressure, force, and areas)
Sliding calipers are largely immune to knock-off since they just slide with the disk deflection.

A sliding caliper with a 1" piston vs a 2-pot opposed caliper with 1" pistons will require the same pedal effort to achieve a given braking force. (All else being equal of course)

Lith might know which forum this from...

In the end i just gutted the preportioning valve internals and it's given me a pretty good balance, although a tiny bit too much rear end bias if the brakes are cold.
I also installed a large master cylinder, 1'' up from 3/4''

[Bazda]

Just the info I needed.

Therefore the pistons I run now in area are 3848mm^2 (4-piston)
Vs stock is around 4247mm&2 (doubled being pull type)

Explains why I have a harder pedal.
Might move up to the next size piston which brings it to 5281mm^2 as i'm also moving up in a bigger pad.

[mjrstar]

Nothing wrong with a nice high hard pedal when approaching the hairpin down the back straight at pukekohe.

I'm sure you have a bit more pace on than I do. (245ish).

I'm thinking about some form of anti-dive kit like what is available for escorts to see if that will allow for more rear braking performance before wanting to lock the rear wheels.
Or maybe a bigger fuel tank for more weight/grip

[Bazda]

Nothing wrong with a nice high hard pedal when approaching the hairpin down the back straight at pukekohe.

I'm sure you have a bit more pace on than I do. (245ish).

I'm thinking about some form of anti-dive kit like what is available for escorts to see if that will allow for more rear braking performance before wanting to lock the rear wheels.
Or maybe a bigger fuel tank for more weight/grip

I'm only doing that speed down the straight as well!
Well last track day before it blew a piston used a gps and said 240kph, but car felt alot slower than usual, something was going on and thats why the piston had a melt down!

Its only geared to 260kph max! on the limiter in 5th.

[jondee86]

Regarding the proportioning valve. Its purpose is to progressively reduce
the percentage of braking on the rear as braking effort goes up. This
stops the rears locking up as weight is transferred to the front during
hard braking. If you are now having to press harder on the pedal to get
the same amount of braking, the proportioning valve is effectively
cutting in earlier, and reducing rear braking more than it should.

Cheers... jondee86

[allencr]

Take a look at what happens to a piston when you stop putting fluid pressure into a caliper - it retracts/moves back into the caliper a slight amount because the seal rolls back into its groove & pulls the piston along with it.

2 pistons of the same OD as a single piston, will require 2x the amount of fluid to push on the disk as a single piston would - IF there are no mickeymouse shims, no pad taper, the caliper is parallel to the disk and it doesn't twist/spread/distort,

The valve for the rears shuts off any more fluid and limits it at a set pressure, nothing progressive or proportional at all IMO.

[GOLDAE86]

Les hunter has a programme that will be able to calculate what size master cylinder to piston size you need for your vehicle. Have used him many times for this.

[mjrstar]

The valve for the rears shuts off any more fluid and limits it at a set pressure, nothing progressive or proportional at all IMO.

the older hilux's and hiaces had a lever operated preportional valve, basically the more weight in the back the further a level moved which gave more bias to the rear.

It makes good sense.

[matt dunn]

the older hilux's and hiaces had a lever operated preportional valve, basically the more weight in the back the further a level moved which gave more bias to the rear.

It makes good sense.

We mounted one of them in the dash of our old celica and made a lever for it for brake bias, worked a treat.

[allencr]

the older hilux's and hiaces had a lever operated preportional valve, basically the more weight in the back the further a level moved which gave more bias to the rear.

Yes, many cars have it, the more weight in the rear or the less the back of the car lifts during heavy braking, the higher the pressure before it gets cutoff.

[Bazda]

I figured it all out.
Ordered my new brakes and they are on the way.

[cat007]

I figured it all out.
Ordered my new brakes and they are on the way.

I'm also happy about this

[jondee86]

The valve for the rears shuts off any more fluid and limits it at a set
pressure, nothing progressive or proportional at all IMO.

http://www.centricparts.com/files/Centric%20White%20Paper%20D4-Proportioning%20Valves.pdf

Quite an informative little article for anyone interested in brakes

Cheers... jondee86

[matt dunn]

I figured it all out.
Ordered my new brakes and they are on the way.

what did you get? Brakes are next on my list now the gearbox is ok.

[Bazda]

I figured it all out.
Ordered my new brakes and they are on the way.

what did you get? Brakes are next on my list now the gearbox is ok.

I got Wilwood superlites for the front end.

[allencr]

http://www.centricparts.com/files/Centric%20White%20Paper%20D4-Proportioning%20Valves.pdf

Thanks jondee86.
I've never gauged them but when bleeding the rears, high pedal pressure shuts it off completely, and I'd thought that it was the weight transfer/front end dive lockup that caused the need for re-adjustment not a continued rise in pressure to the rear.