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Uprated Front Brakes, Mastercylinders,


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117 replies to this topic

#41
DaveK

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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

#42
jonah

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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!

#43
DaveK

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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

#44
jonah

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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?

#45
DaveK

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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?

#46
jonah

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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.

#47
Pugnut

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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.

#48
Emmy Seize

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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) ?

#49
Pugnut

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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?

#50
Emmy Seize

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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, 06 March 2006 - 08:42 PM.


#51
jonah

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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 .

Sorry, this is wrong. Don't forget that the amount of weight transfer is proportional to the TOTAL braking force from BOTH axles, not just the front!

Uprating the front brakes alone does not change the grip between tyres and road, so at the point that the front wheels are beginning to lock up, there will be no more weight transfer than before. So you still need just as much rear brake force as before to make the most of the rear grip that's available.

there is no such thing as a decrease in weight transfer with bigger brakes fitted to the front axle.

You're taking what I said out of context. What I'm talking about above is the MAXIMUM amount of braking force available, which is where the front wheels are just on the brink of locking up. Yes, for a given pedal pressure, bigger front brakes means more braking force and more weight transfer. BUT this is only true up to the point that the front wheels lock, which does not change when you uprate the brakes. So the peak amount of weight transfer for an emergency stop situation does NOT increase. With larger front brakes this point will occur at lower hydraulic pressure, so there will be less braking force at the rear (provided that the compensator valve is not limiting the pressure), therefore less total braking force!

So, indirectly, this decrease in TOTAL braking force means a slight reduction in weight transfer, this is simple physics!!

I'm putting together a spreadsheet that calculates some figures to show the effect, will post up later...

#52
Rob Thomson

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F*ck me - someone (Jonah) actually understands how brakes work.

#53
DaveK

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nice one jonah, will be good to see this

#54
jonah

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Ok, the forum won't let me upload a .xls file as an attachment, I'll have to put it on my webspace when I get home. Meanwhile a couple of screenshots...

Friction coefficient = 1.14 (dry tarmac):
Attached File  brakedry.jpg   77.95KB   75 downloads

Friction coefficent = 0.7 (wet tarmac?):
Attached File  brakewet.jpg   87.5KB   58 downloads

What it shows is that in high grip conditions, the compensator valve is at its pressure limit with both types of front brakes, so the peak g force is the same (big brakes still do not give any improvement!). But when there's less grip (I've guessed at a value for the tyre friction coefficient for wet tarmac), the lack of rear brake pressure comes into effect and the peak g force is about 9% lower with the larger front brakes.

More later...

Edited by jonah, 07 March 2006 - 02:26 PM.


#55
jackherer

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sorry if you've already said, but which compensator(s) is that data describing?

#56
Pugnut

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Sorry, this is wrong. Don't forget that the amount of weight transfer is proportional to the TOTAL braking force from BOTH axles, not just the front!

Uprating the front brakes alone does not change the grip between tyres and road, so at the point that the front wheels are beginning to lock up, there will be no more weight transfer than before. So you still need just as much rear brake force as before to make the most of the rear grip that's available.
You're taking what I said out of context. What I'm talking about above is the MAXIMUM amount of braking force available, which is where the front wheels are just on the brink of locking up. Yes, for a given pedal pressure, bigger front brakes means more braking force and more weight transfer. BUT this is only true up to the point that the front wheels lock, which does not change when you uprate the brakes. So the peak amount of weight transfer for an emergency stop situation does NOT increase. With larger front brakes this point will occur at lower hydraulic pressure, so there will be less braking force at the rear (provided that the compensator valve is not limiting the pressure), therefore less total braking force!

So, indirectly, this decrease in TOTAL braking force means a slight reduction in weight transfer, this is simple physics!!

I'm putting together a spreadsheet that calculates some figures to show the effect, will post up later...


OK , so i get what you're saying and all that (i'll get my coat blah blah)but at the same time i'de be very wary of of having uprated rear brakes on such a light/tail happy car.


F*ck me - someone (Jonah) actually understands how brakes work.


a very constructive post. Thanks for that.

#57
Rob Thomson

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OK , so i get what you're saying and all that (i'll get my coat blah blah)but at the same time i'de be very wary of of having uprated rear brakes on such a light/tail happy car.

But all you'd be doing is maintaining the standard brake bias - rather than moving it to the front which is what you do if you only uprate the fronts. Uprating the rears without doing anything to the fronts would be very daft, but that's not the scenario we're talking about here.

a very constructive post. Thanks for that.

No problem.

Edited by Rob Thomson, 07 March 2006 - 05:13 PM.


#58
Anthony

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I'm sure this will become apparent when you upload the spreadsheet, but with larger front calipers/disks (eg GTi-6) would removing the rear compensator and thus allowing more pressure to the rear brakes help matters?

I'm assuming it will in the dry as you're exceeding the pressure rating of the compensator and (if I've understood what you're saying correctly) it's the lack of extra rear braking that's holding back the bigger front brakes? But in the wet, you're locking up before reaching the compensator limit and thus removal will have no effect.

On a heavier car, does the increased weight mean increased weight transfer and thus the front brakes doing a higher proportion of the work? Just wondering, as obviously stuff like 306 GTi's have the same size rear brakes with the same calipers as 205 GTi's, and one assumes Peugeot wouldn't have made a complete mis-match with the front and rear brakes...

#59
Rob Thomson

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I'm sure this will become apparent when you upload the spreadsheet, but with larger front calipers/disks (eg GTi-6) would removing the rear compensator and thus allowing more pressure to the rear brakes help matters?

Yeah - absolutely. That's the easist way to upgrade the rear brakes. I'd like some means of regulating the pressure to the rears, though - an adjustable bias valve is preferable to no pressure reduction at all.

I'm assuming it will in the dry as you're exceeding the pressure rating of the compensator and (if I've understood what you're saying correctly) it's the lack of extra rear braking that's holding back the bigger front brakes? But in the wet, you're locking up before reaching the compensator limit and thus removal will have no effect.

This really depends how the standard compensators work - whether they're pressure limiting or proportioning valves. I don't know.

On a heavier car, does the increased weight mean increased weight transfer and thus the front brakes doing a higher proportion of the work? Just wondering, as obviously stuff like 306 GTi's have the same size rear brakes with the same calipers as 205 GTi's, and one assumes Peugeot wouldn't have made a complete mis-match with the front and rear brakes...

Although the calipers may be the same, I'll bet anything that the compensators are different. Making bespoke compensators is much less expensive than making calipers with different sized pistons for each model.

Edited by Rob Thomson, 07 March 2006 - 07:16 PM.


#60
Anthony

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Yeah - absolutely. That's the easist way to upgrade the rear brakes. I'd like some means of regulating the pressure to the rears, though - an adjustable bias valve is preferable to no pressure reduction at all.

Thought as much - I run GTi-6 fronts, standard rears, and no compensators, and I've always found that setup to be very good. Was just starting to wonder if that was some kind of halo effect in light of the above...