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Uprated Front Brakes, Mastercylinders,


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117 replies to this topic

#21
pug_ham

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Dave, you can adjust the percentage of the rear compensator on the 1.6 to alter the limiting amount by changing the anlge it is mounted at or remove it completely.

My STDT doesn't feel to have much restriction to the rear brakes because when you brake hard you can feel the back end dipping as well instead of going light due to weight transfer so I think less restriction of flow to the back brakes is better under heavy braking even with standard sized front discs.

Graham.

#22
jonah

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The difference between your original 1.9 and your 1.6 with disc rear brakes is probably down to the compensator - although it could also be affected by pad material and condition. The type of hydraulic split (diagonal or front/rear) makes no difference to brake bias but diagonal is arguably safer if one of the front lines develops a leak. Also there's no big difference in performance between 1.6 and 1.9 front calipers as they both use the same size discs and both have the same piston diameter.

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). Or experiment with the 1.6 compensator as Graham suggests - basically tilt it further towards vertical to raise the rear brake pressure limit.

#23
pug_ham

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The type of hydraulic split (diagonal or front/rear) makes no difference to brake bias but diagonal is arguably safer if one of the front lines develops a leak.

<{POST_SNAPBACK}>

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.

Graham.

#24
sorrentoaddict

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it is perfectly true that the smaller the pad surface, the better the stopping power is, but the heat dissipation is worse. Also pad life depends largely on the size of the pad - small pads stop better, but last shorter (and vice versa).

as for the ineffectiveness of rear brakes on heavily uprated front brakes, it is also true - the rears simply stop to work, and, to be even worse, this is triggered by two reasons, not one:

1) the sharper, faster weight transfer towards the front causes the rear brake compensators to decrease pressure to the rear calipers much sooner than ideal

2) the lower pedal pressure needed for a given deceleration rate when you have huge front discs and/or bigger calipers, makes a situation where the pedal travel IS longer, but the PRESSURE your foot exerts on the pedal is actually smaller, and the car starts to decelerate with a lower "momentary system pressure" than with std. front brakes - and that makes the rear brakes less effective in itself.

These 2 reasons are the main reason why 283mm brakes have usually their "side effects" to the overall sweetness of the car, and hence such brakes require some serious modifications to the compensators, or adding a bias-command valve, in order to be fully effective.

The 266mm set-up, on the other hand, is not THAT sharper to render the rear brakes "dead" - it decreases their effect, but only slightly - that's why if you do not want to go into expensive & tricky modifications, it is best to stick with the 266mm set-up (with all other advantages it has - as I listed in a thread several pages above this one).

That is unless you are running a MI-16 Turbo on nitrous, of course, where even a combination of 283mm + CX 4-pot calipers would be only a decent one I am afraid :ph34r:

Cheers

AG

#25
sorrentoaddict

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I forgot to add:

and on trailing rear-arm suspensions, like the Peugeot & Citroen, the effectiveness of the rear brakes is especially important, as they ensure the rear-squat under heavy braking, therefore keeping "on leash" the brake-transfer-induced body movement & ride-height disturbance.

This, in turn, makes the car very predictable in extremely late braking,
and makes the turn-in very repeatable and precise, as the rear end of the car is not "disturbed" when you finish with your braking and aim for the corner (turn-in).

That is why, if a Peugeot 205 is fitted with "too" big brakes on the front, without other proper mods that would restore the rear-brake efficiency, you will actually lose one of the best "weapons" of this chassis type - the "poise" and "sharpness" in turn-in (when driving on the limit, i.e. heavy braking etc..).


Cheers

AG

#26
cybernck

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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:

#27
pug_ham

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

<{POST_SNAPBACK}>

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.

#28
jonah

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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.

<{POST_SNAPBACK}>


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).

#29
cybernck

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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 :).

#30
jonah

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

<{POST_SNAPBACK}>

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!

#31
pug_ham

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

<{POST_SNAPBACK}>

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.

#32
shepherdfte

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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?

#33
Guest_loddonway_*

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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!

#34
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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.

<{POST_SNAPBACK}>


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

#35
shepherdfte

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

#36
TVH

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Any recommendations for uprated master cylinder?

Would anything like this http://www.rallydesi...roducts_id=8064
fit bolt-on?

#37
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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.

#38
Pugnut

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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 ?

#39
cybernck

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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.

#40
jonah

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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...