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Wurzel

Uprated Front Brakes, Mastercylinders,

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cybernck
You could make it equivalent to a 1.9 setup by fitting a 1.9 compensator in series with each of the rear brakes (these would have to be after the T piece).

 

couldn't he just replace the 1.6 one with a 1.9 one? :ph34r:

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pug_ham
couldn't he just replace the 1.6 one with a 1.9 one? :ph34r:

A single 1.9 compensator probably wouldn't allow enough flow to power two calipers efficiently, to restrictive IMO, especially if you have to brake hard.

 

Graham.

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jonah
If the brake line develops a leak the brake pedal will usually go straight to the floor, following the path of least resistance IMO.  You will still have some braking effort but not very much, the only benefit over front rear is that the rear brakes do so little as standard that it'll feel like you have no brakes anyway but the diagonal might seem to have slightly more due to a front disc still getting some pressure.

 

It shouldn't go straight to the floor (unless Peugeot have completely screwed up the pedal linkage design)! The master cylinder is designed so that if one circuit fails, the pedal will go half way to the floor, but beyond that the resistance comes back and any further force on the pedal will still generate pressure in the remaining circuit. So you can still brake with full force at two wheels. I would certainly hope that the linkage mechanism is designed to allow the pedal to operate the full stroke of the master cylinder, otherwise it defeats the whole point of having a dual circuit brake system...

 

With a diagonal split you will always still have one front brake working so you can still brake at about 0.5g. With front/rear split, if the front circuit fails, you're left with only rear brakes, which will have limited stopping power due to weight transfer. (although the car will still brake in a straight line unlike the diagonal system).

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cybernck
A single 1.9 compensator probably wouldn't allow enough flow to power two calipers efficiently, to restrictive IMO, especially if you have to brake hard.

 

well if it limits/scales the pressure (as discussed), then i can't see why not really :).

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jonah
well if it limits/scales the pressure (as discussed), then i can't see why not really :).

It depends how the thing works. If it is a hard pressure limit, then yes a single one should be fine. If it scales the pressure (above a certain threshold), then it would need an internal piston mechanism to do this, and this would limit how much volume it could shift in its scaling mode. Running two calipers off one compensator would require twice the volume flow through it, so the piston might reach the end of its travel under some conditions (or it might not :D )

 

If it just contains a fluid restriction as some people have reported (a perforated gauze or a porous ceramic material), then changing the volume flow through it would certainly affect its operation... although if that really is all that's inside you'd be better off throwing it away IMO!

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pug_ham
well if it limits/scales the pressure (as discussed), then i can't see why not really :).

But surely it'll be trying to flow twice the amount of fluid than under normal circumstances which might physically be impossible for it to do.

 

Graham.

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shepherdfte

One interesting little factoid to add to this most interesting party. The discussion about why the rears locked easilly with the 1.6 rear compensator, but not so with the 1.9 compensator(s) during heavy breaking is, I believe, due to the following:

 

The 1.9 is a simple pressure restrictor like most cars (AIUI). So if you stamp the pedal to (say) 1000psi, it may limit the rear circuits to (say) 600psi (like a Tilton valve, but unadjustable). So even if you press the brakes hard while parked in the garage, you will still get 1000 front/600 rear (say).

 

However, the 1.6 compensator restricts the pressure by allowing the cars deceleration rate to close the sliding ball. So if the car is not decelerating (e.g. it is parked) you will get 1000/1000 front and rear.

 

NOW. If you stand really suddenly on the pedal (like you do when passing your braking point on the track), I think what happens is this: Sufficient fluid gets past the ball to lock the rears before the car has had time to start slowing. Once the ball slides it's too late - sufficient fluid movement has occured to build pressure and lock the rears.

 

I tried some experiments with this. I found if I braked initially firmly, and then stood on the anchors once the car was already decellerating, I could not lock the rears, no matter how hard I pressed (the fronts would eventually lock first). BUT, if I just stamped the pedal, the rears would lock, even if the fronts did not. And this is despite the fact that when the pedal was stanmped, more weight would have been on the rear wheels (if you think about it).

 

So there you go.

 

Mind you, I've just fitted a 1.9 rear beam and brakes, along with a Tilton valve, and for some reason the brakes are now wooden, and the rears don't seem to be working well. Any ideas?

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Guest loddonway

couldn't be @rsed to read through as most of this is the same.

 

Being a race car designer myself its pretty simple physics.

 

The force exerted on the brake pedal by your foot creates a pressure in the mc.

All the brake lines comin out of the mc will all have to have equal pressure, lawsof physics.

 

The larger the the piston area of the caliper the larger the force created at the same pressures, pressure * area = force.

However the larger the piston area in the caliper the more fluid is required, increasing pedal travel, so a larger mc is used.

 

Note: it is the formula pressure*area=force for the calipers which makes the difference of braking force, the master cylinder has no bearing of braking power but is a leverage of fluid movement.

 

which split to use, in theory both should create exactly the same braking forces.

 

The compensators work where by when the max pressure is reached the valve closes to the rear brakes allowing highr fluid pressures to the front while the rears remin at maximum until the pedal is realesed, simple!

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Guest turnip
NOW.  If you stand really suddenly on the pedal (like you do when passing your braking point on the track), I think what happens is this:  Sufficient fluid gets past the ball to lock the rears before the car has had time to start slowing.  Once the ball slides it's too late - sufficient fluid movement has occured to build pressure and lock the rears.

 

 

Interesting stuff.

 

When I brake hard in my (bog standard) 1.6 the compensator buzzs away under the bonnet. I had assumed it was the sound of the little ball blocking and then unblocking the flow of fluid to the rear. Is this what it should be doing (the buzzing that is)?

 

Cheers

Matt

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shepherdfte

Buzzing? Are you sure that is what it is? You've not got ABS have you? Ball certainly does not buss on mine.

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Guest eddscruff

I'm a hydraulic engineer and loddonway's reply best somes it up, bigger pistons at same pressure gives more force. So more pistons gives even more force to clamp disc.

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Pugnut
One interesting little factoid to add to this most interesting party. The discussion about why the rears locked easilly with the 1.6 rear compensator, but not so with the 1.9 compensator(s) during heavy breaking is, I believe, due to the following:

 

The 1.9 is a simple pressure restrictor like most cars (AIUI). So if you stamp the pedal to (say) 1000psi, it may limit the rear circuits to (say) 600psi (like a Tilton valve, but unadjustable). So even if you press the brakes hard while parked in the garage, you will still get 1000 front/600 rear (say).

 

However, the 1.6 compensator restricts the pressure by allowing the cars deceleration rate to close the sliding ball. So if the car is not decelerating (e.g. it is parked) you will get 1000/1000 front and rear.

 

NOW. If you stand really suddenly on the pedal (like you do when passing your braking point on the track), I think what happens is this: Sufficient fluid gets past the ball to lock the rears before the car has had time to start slowing. Once the ball slides it's too late - sufficient fluid movement has occured to build pressure and lock the rears.

 

I tried some experiments with this. I found if I braked initially firmly, and then stood on the anchors once the car was already decellerating, I could not lock the rears, no matter how hard I pressed (the fronts would eventually lock first). BUT, if I just stamped the pedal, the rears would lock, even if the fronts did not. And this is despite the fact that when the pedal was stanmped, more weight would have been on the rear wheels (if you think about it).

 

So there you go.

 

Mind you, I've just fitted a 1.9 rear beam and brakes, along with a Tilton valve, and for some reason the brakes are now wooden, and the rears don't seem to be working well. Any ideas?

 

Ok , sorry to dig this one out again. Shepherdfte is explaining exactly what i used to experience with big front brakes and the standard 1.6 gear are the back.

 

progressive braking( planned braking so to speak!) was all good but any reactional braking or emergency stops meant you'de lock a rear up (nae gooood!)

 

So what i'm wondering is that if anyone has experienced the same with the 1.9 compensators ?

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cybernck

actually, thanks for digging it up as you've reminded me to post some new findings.

 

 

this was in mid Jan/2005, with rear left compensators drilled through (removed).

 

a few days ago i got my brakes tested as a part of the MOT test, the results were:

 

FL 2.65 kN, FR 2.12 kN, difference 20%, total front brakes 4.77 kN

RL 1.91 kN, RR 1.03 kN, difference 46%, total rear brakes 2.94 kN

HBL 1.58 kN, HBR 1.03 kN, difference 35%, total handbrake 2.61 kN

 

what i'm willing to bet my money on is that when i drill through the other compensator

(don't want to buy new ones as i'll be installing a bias valve soon) the left-to-right

brake balance will restore to 5-10% max on BOTH front and rear brakes.

 

and this is exactly a year after, but with the rear right compensator removed too this time:

 

FL 2.19 kN, FR 2.01 kN, difference 8%, total front brakes 4.20 kN

RL 1.74 kN, RR 1.54 kN, difference 11%, total rear brakes 3.28kN

HBL 1.15 kN, HBR 1.11 kN, difference 3%, total handbrake 2.26 kN

 

most interesting results... and nearly exactly what i predicted!

 

 

*only* other differences between the two tests are changed rear pads and handbrake cables.

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jonah

cybernck this argument's been going on for years now! :(

 

I know what you're getting at... but it doesn't change the laws of physics! The only thing that this proves is that either the measurement was inaccurate, or (quite likely) the surface condition of the pads and discs has changed in the mean time. If you still believe that a change to the rear brakes will alter the force at the front brakes (for the same force on the brake pedal), then I challenge you to find a physical law or formula that states this...

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DaveK

sorry to bring this back up but ive spent a good hour and a half reading through this trying to understand it, so.....do i dare ask this?

 

??
So even if you press the brakes hard while parked in the garage, you will still get 1000 front/600 rear (say).

 

If you increase the braking force of the front brakes you then need to increase the pressure to the rear slighty (but not more than the front) so that they have correct ratio of braking force??

 

Since less pressure is needed to create the same force on the front, more pressure is going to be needed to create the same force on the back, assuming the back brakes are standard.

 

so instead of 1000/600psi you get 900/700psi (eg)

 

Also this means that if you upgrade the brakes u effectively reduce the amount of pressure required to creat the same stopping force, thus leaving more pressure in reserve for even more braking force. so instead of 900/700psi you can end up with 450/350psi (exaggerated i know but gets my point across, hopefully)

 

And is this the point in adjustable brake bias compensators?

 

Apologies for the 20 questions, just wud be good to get a grasp of this

 

cheers folks

dave

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jonah
If you increase the braking force of the front brakes you then need to increase the pressure to the rear slighty (but not more than the front) so that they have correct ratio of braking force??

Yes

 

Since less pressure is needed to create the same force on the front, more pressure is going to be needed to create the same force on the back, assuming the back brakes are standard.

Not sure what you mean here... obviously if the rear brakes are unchanged then they will need the same pressure to create the same force.

 

so instead of 1000/600psi you get 900/700psi (eg)

Well if 1000/600psi were the pressures you needed before to lock the front and rear brakes, and you uprate the front brakes, then the fronts will need less pressure to lock up but the rears will need the same. e.g. you would need 900/600 for optimal braking (using your example).

 

Also this means that if you upgrade the brakes u effectively reduce the amount of pressure required to creat the same stopping force, thus leaving more pressure in reserve for even more braking force. so instead of 900/700psi you can end up with 450/350psi (exaggerated i know but gets my point across, hopefully)

Well "reserve" pressure should never be an issue unless your leg runs out of strength! (You can always increase the pressure just by pushing the pedal harder.) So I'm still not sure what your point is...

 

And is this the point in adjustable brake bias compensators?

Yes you could use it to compensate for brake upgrades... but I think their intended purpose is to optimise brake bias for different grip conditions (you need more rear bias in lower grip conditions) and to suit driver preference (whether you prefer understeer or oversteer in braking).

It's more complicated than that anyway because brake compensators do not cause a linear pressure reduction... the bias changes depending on the pedal force. The car will have been set up from standard for near optimal brake biasing under most conditions... changing it in any way (including uprating the front brakes) unless you really know what you're doing will almost always INCREASE stopping distances!

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DaveK

The car will have been set up from standard for near optimal brake biasing under most conditions... changing it in any way (including uprating the front brakes) unless you really know what you're doing will almost always INCREASE stopping distances!

 

 

so basically if i upgrade to 307 front brakes i need to change the bias? can this be changed with standard set-up or do i need a new bias controller?

 

cheers

dave

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jonah

Ideally you would want to scale up the rear braking force by the same amount as the front... but that's not possible just by changing the bias valve. You could probably get an adjustable bias valve and adjust it for the correct bias on dry tarmac... but the rears would still be undereffective in the wet. There's no way of getting it perfect without upgrading the rears as well.

 

Out of interest how much larger are the caliper pistons and discs on the 307 front brakes? And, why do you want to upgrade the front brakes anyway?

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DaveK

247mm (from 205 standard) vs 283mm (307) discs

 

The pistons in the 205 GTI brakes (1.9) are 48mm diameter. I believe the caliper pistons from both donor cars are 54mm

 

took that from the guide on the main website

 

well im making my 205 a track car, and decided that stopping would be better done first than the engine. would rather stop in a hurry than speed up in a hurry.

 

unless its better to keep standard and change the discs and pads on front and rear with ones more suited to heavy use?

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jonah

Change in front braking force:

 

283/247 = 1.146 times due to disc size increase

(54/48)^2 = 1.266 times dues to piston size increase

1.146*1.266 = 1.45 times total (45% increase)

 

Or in other words 1/1.45 = 0.69 times original hydraulic pressure for same braking force at front wheels (31% decrease)

 

So the rear brakes will be approx 31% down in effectiveness.

 

Plus due to the resulting decrease in weight transfer, the fronts will lock slightly sooner than before (probably only 1-2%) as well so both front AND rear wheels will be doing less braking than before.

 

This is neglecting the effect of the rear brake compensator which will help balance things out to some extent, especially under heavy braking on dry tarmac, but on lower grip surfaces (e.g. wet tarmac) there will be a definite decrease in overall brake performance.

 

So yes, I would stick with standard brakes and use higher temperature pads if you're getting fade.

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Pugnut
Ideally you would want to scale up the rear braking force by the same amount as the front... but that's not possible just by changing the bias valve. You could probably get an adjustable bias valve and adjust it for the correct bias on dry tarmac... but the rears would still be undereffective in the wet. There's no way of getting it perfect without upgrading the rears as well.

 

Out of interest how much larger are the caliper pistons and discs on the 307 front brakes? And, why do you want to upgrade the front brakes anyway?

 

An old argument this but if you increase the braking force at the front you are moving more of weight to the front when braking , this obviously means you have less weight on the back meaning you need less braking force at the rear to avoid locking the rear wheels . In my opinion if you are going to be messing with braking forces over and above standard calipers anywhere on the car you need to fit an adjustable bias valve and get the braking set up properly front to back .

 

From experience with fitting big front brakes to a 1.6 gti , emergency braking can easily and quickly turn nasty , this is a seperate issue regarding the type of the 1.6 compensator but if you apply the scenario to a 1.9 set up with uprated front brakes , hit the pedal hard and send more weight forwards than normal the rear is going to be lighter.

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Emmy Seize

ERRR, well....

 

There is a decrease in weight transfer if I increase the stopping power at the front ?

 

I need to uprate the rear brakes as weight on the rear axle is decreasing (under braking) ?

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Pugnut
ERRR, well....

 

There is a decrease in weight transfer if I increase the stopping power at the front ?

 

I need to uprate the rear brakes as weight on the rear axle is decreasing (under braking) ?

 

Sorry , i dont quite understand what you are trying to say?

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Emmy Seize

Just trying to get the discussion going a bit.

 

A lot of what has been written here is spot on, some statements are utter nonsense, though.

 

Of course you don´t want to uprate or "scale up" the rear brakes once bigger brakes have been installed up

 

front and of course there is no such thing as a decrease in weight transfer with bigger brakes fitted to the

 

front axle.

 

Yes pugnut, you are pretty right about less weight on the back axle so if anything, (you´re right again) you

 

will want to install an adjustable bias valve to lower the pressure even more, if needed.

 

For 1,6ses with drums at the rear setting up can get really tricky as the wheel cylinders themselves have

 

built-in pressure limiters, depending on the type of OE fittment. But who really leaves the drums in and changes the front setup ?

 

After converting 4 cars to the gti-6 setup, the only reasons for not working propperly were

 

shot pressure valves and blocked brakelines.

Edited by Emmy Seize

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