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Uprated Front Brakes, Mastercylinders,


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#1
Wurzel

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Once again, the discussion about the effects of larger front brake calipers etc has raised its head. This time though, there appears to be more constructive answers as people have had time to think about/research the effects. I thought it a good idea to try and air some more views on th subject and (maybe) solve or at least come to a satisfactory conclusion. Alternatively, it could degenerate into a complete slanging match ;)

So, here is my theory, remembering I'm talking specifically about the 205 although I'm sure hyrdraulic physics are the same world wide.



I reckon that the type of braking split from the master cylinder can effect the way the front/rear brakes work. The diagonal split on the later cars and 1.9 GTi's means that you have a longer pedal travel (to fill the extra void when using larger front calipers) but the pressure will remain the same for the front corner and rear corner brake caliper. The compensator will still act as a max pressure valve but will only be utilised if you press the pedal harder. As people have already said on other threads, the pressure applied at the pad is no greater when using larger or smaller mastercylinders for a given pedal pressing force. It's the extra leverage from using larger discs and the greater frictional area between pad and disc that gives the extra braking force. I think that it may be these two separate factors that are confusing some people (me included occasionally).

If the braking system used is a simple single line supplying the rear of the car coming from the rear end of the master cylinder, and the front brakes utilising the two front ports of the master cylinder, then when using larger front calipers, it will take a longer pedal travel again to fill the calipers. However, the rears will fill more quickly and the pedal will begin to stiffen. Applying more force on the pedal to pump more fluid into the fronts will mean a greater line pressure in the rears to the point where the rear compensators allow no more and the pressure between the compensator and rear caliper will (in theory) not increase. The line pressure between mc and compensator will though. In this instance, the rears could be at their maximum operating pressure before the front calipers?

The thing I'm not totally convinced by is the age old argument about 'weight transfer'. Larger brakes cause a greater weight transfer from the rear to the front and therefore the rears need to be down rated, to me doesn't sound right. There is a set amount of weight both before and during braking on the rear of a 205. How can you cause a 'greater' weight transfer? Once the fronts have locked, maximum weight transfer has occured (in fact it is slightly less than max). Front wheel skids can be obtained on standard calipers as well as uprated ones. The only effect on weight transfer I can see is the speed at which it occurs. Uprated brakes due to frictional and leverage properties, will retard the front disc rotation with more force from higher speeds. Surely this is where the 'more powerful' braking feeling is conjured from? Maybe though, I'm forgetting the momentum factor which perhaps plays a part?

With regards to the rear brakes and their effects, applying the rear hand brake whilst driving slowly causes the rear to squat as we all know. Is this a weight transfer to the rear? It would seem to me, up to a point, the rear brakes need 'uprating' to create a greater rearward weight transfer at a greater speed to help negate the effects of the accelerated weight transfer to the front?

I'll leave it there for now, let the arguments begin :D

#2
Rob Thomson

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I'll start the ball rolling!

Before I start I should perhaps mention that although I am a degreed engineer I'm far more au fait with concrete than I am with hydraulics. But having said that, physics is a piece of piss and all this is really very simple..... :ph34r:

I reckon that the type of braking split from the master cylinder can effect the way the front/rear brakes work. The diagonal split on the later cars and 1.9 GTi's means that you have a longer pedal travel (to fill the extra void when using larger front calipers) but the pressure will remain the same for the front corner and rear corner brake caliper.


I'm pretty sure (ie. 99.9% sure, but argue if you like....) that the way the MC is connected is totally irrelevent. The two chambers of the MC are split by a floating piston and thus the pressures in each chamber is the same unless one of the chambers fails and the MC adopts it's fail-safe mode - in which case the centre piston stops being floating. How it does that has been discussed elsewhere. But anyway, even if you had front/rear split with two small rear calipers connected to one chamber and two larger front calipers connected to the other, the pressure in each brake line would be the same.

Also, remember that the fluid transfer really is tiny. You shouldn't really think of the fluid as being dynamic. The tiny movement of the pad onto the disc is amplified into a large movement at the pedal because the size of the MC piston is tiny compared to the 4 slave cylinders and because the pedal has a leverage ratio of about 1:6. Really, there really is sod all fluid movement.

The compensator will still act as a max pressure valve but will only be utilised if you press the pedal harder.


Is that definitely how the (1.9 GTi) compensators work? I have to admit to not having a clue. I am a little suspicious, though. If they really are a max pressure valve how do they work? Where does the excess pressure 'blow off' to?

Think of the engine oil pump. It has a pressure relief valve to maintain near constant pressure in the engine oil galleries. The excess pressure produced by the pump is bled off by oil being allowed to flow back to the sump. That can't happen with brakes because the compensators are sealed units.

I reckon they're much more likely to be sprung valves - which would result in a linear pressure reduction relative to the pressure produced by your foot on the pedal.

As people have already said on other threads, the pressure applied at the pad is no greater when using larger or smaller mastercylinders for a given pedal pressing force.  It's the extra leverage from using larger discs and the greater frictional area between pad and disc that gives the extra braking force.  I think that it may be these two separate factors that are confusing some people (me included occasionally).


If the pressure on the pedal and the size of the calipers is constant, changing the diameter of the MC will effect the force generated at the pad. Smaller MC = greater force and vice versa (sp?).

Larger pad area has no real effect other than to reduce fade. Braking force is barely effected by pad area.

Disc diameter is important because bigger discs provide greater leverage which gives a greater braking force for a given pad-on-disc pressure.

If the braking system used is a simple single line supplying the rear of the car coming from the rear end of the master cylinder, and the front brakes utilising the two front ports of the master cylinder, then when using larger front calipers, it will take a longer pedal travel again to fill the calipers.  However, the rears will fill more quickly and the pedal will begin to stiffen.  Applying more force on the pedal to pump more fluid into the fronts will mean a greater line pressure in the rears to the point where the rear compensators allow no more and the pressure between the compensator and rear caliper will (in theory) not increase.  The line pressure between mc and compensator will though.  In this instance, the rears could be at their maximum operating pressure before the front calipers?


If you assume that there is sod all fuid movement and also that what fluid movement there is doesn't require any real force on the pedal, you can assume that the above doesn't happen! Remember, the pressure in all parts of the system (apart from behind the comensators) is the same at all times.

The thing I'm not totally convinced by is the age old argument about 'weight transfer'.  Larger brakes cause a greater weight transfer from the rear to the front and therefore the rears need to be down rated, to me doesn't sound right.  There is a set amount of weight both before and during braking on the rear of a 205.  How can you cause a 'greater' weight transfer?  Once the fronts have locked, maximum weight transfer has occured (in fact it is slightly less than max).  Front wheel skids can be obtained on standard calipers as well as uprated ones.


You're right.

Weight transfer is merely a result of the decellerative force acting on the car. To create more weight transfer you need more decelleration. What creates and limits that stopping force and in turn the weight transfer? It's the grip between the tyres and the road - so that's ultimately what you have to improve.

If you ask most people how a Formula 1 car stops so quickly they'll say it's because of the carbon brakes. That's total utter bollocks. The reason is the daft levels of grip generated by aerodynamic downforce. Sure, they have to have powerful brakes to exploit that grip - but it's the grip that allows such incredible stopping forces (circa 5-6G when braking from high speeds).

A 205 doesn't have aerodynamic downforce working in it's favour so it has to rely on the grip generated by the weight of the car acting through it's tyres. It'll stop at about 1G. Fitting F1 brakes to a 205 wouldn't make it stop any quicker because the standard brakes are more than capable of generating enough force to lock the wheels - and as we all know maximum braking force is generated just as the tyres are starting to slip.

The only effect on weight transfer I can see is the speed at which it occurs.  Uprated brakes due to frictional and leverage properties, will retard the front disc rotation with more force from higher speeds.  Surely this is where the 'more powerful' braking feeling is conjured from?  Maybe though, I'm forgetting the momentum factor which perhaps plays a part?


I maintain that a lot of the illusion of 'more powerful' braking is the reduced effect of the rear brakes (caused by less hydraulic pressure being necessary to achive wheel lock threshold with more powerful front brakes) causing less squating of the rear suspension (see below).

But there is perhaps a benefit at higher speeds. Sure, the wheels/hubs/shafts/final-drive will all have rotational momentum and that needs to be braked too. So at higher speeds the brakes aren't just working to slow the car, they're working to stop parts of the car spinning. It's possible that standard brakes cope with this whereas uprated brakes can.

But, think how easy it is to lock a wheel on ice at any speed. All it takes is a dab on the brakes - so that's all the force that's required to stop all those bits spinning.

With regards to the rear brakes and their effects, applying the rear hand brake whilst driving slowly causes the rear to squat as we all know.  Is this a weight transfer to the rear?  It would seem to me, up to a point, the rear brakes need 'uprating' to create a greater rearward weight transfer at a greater speed to help negate the effects of the accelerated weight transfer to the front?


The rear squat is under braking is due to the geometry of the rear suspension. If you lock the wheel to the trailing arm (ie. by applying the brakes) and try to move the car forwards, the trailing arm tries to roll with the wheel - and it's that action that creates the squat, not weight transfer.


That'll do for the time being....

#3
markrnorton

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This thread is gonna be a good one:
let the bitching commence !

I'll start with this !

The only fluid movement in your system is in the MC when you put your foot on the brake, ie piston movement, which increases pressure in the whole system.

If you have fluid movment in the system you have a leak (or you are bleeding the system !)

Next !!!!!! :ph34r:

and before anybody says what about when the pads wear and the chambers behind the pistons refill , yes ok a bit of fluid movement ! - this is nominal in day to day operation

Edited by markrnorton, 15 January 2005 - 05:27 PM.


#4
Wurzel

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Right, that's great reading fellas. I do admit to not knowing how the master cylinder worked or works properly which is a reason for my 'guess work'.

I'm used to piston rams moving quite a distance compared to the brake caliper so I understand that there is practically no fluid movement, only a pressure increase.

Now I've re read what I wrote about the rear compensator, I guess it can't be a max pressure valve. This is something I need to look into. I know the old 1.6 type used a ball bearing to block off the flow as the pressure increased. The angle at which the compensator sat was critical as to what pressure this 'block' occurred. Lowereing a car used to cause problems if not done equally front and rear.

Good points though, and something I hope others will add to. Hopefully at the end, some conclusions can be made as to whether uprating the rear brakes is necessary when uprating the fronts?

:ph34r:

#5
cybernck

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I reckon that the type of braking split from the master cylinder can effect the way the front/rear brakes work. The diagonal split on the later cars and 1.9 GTi's means that you have a longer pedal travel (to fill the extra void when using larger front calipers) but the pressure will remain the same for the front corner and rear corner brake caliper.  The compensator will still act as a max pressure valve but will only be utilised if you press the pedal harder.

i agree to a point. i tend to argue that rear compensators, by limiting the pressure
to the rear brakes, actually increase the pressure to the front brakes (IF the brakes
run in a cross-diagonal arrangement). what backs my story and makes me think that
way in the first place, apart from lots of different arrangements and combinations we've
tried out, is that a while ago i had a blocked left rear brake compensator. as a result,
the rear left brake (as well as the handbrake at that wheel) hasn't been functional
at all and the front right brake was stronger than the left one (always first one to lock).
i then took the blocked compensator off and drilled through it as a first aid to get my
rear left brake working. from then, that wheel was the one that easily locks at hard
braking and front left brake became stronger than the front right one as well!

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. if this happens,
i will not allow anyone to condtradict me anymore :ph34r:, but if only the rear brakes get
affeceted by this - i'll shut up and stand corrected. in a corner as well :D.




As people have already said on other threads, the pressure applied at the pad is no greater when using larger or smaller mastercylinders for a given pedal pressing force.  It's the extra leverage from using larger discs and the greater frictional area between pad and disc that gives the extra braking force.  I think that it may be these two separate factors that are confusing some people (me included occasionally).

yeah i agree this causes a lot of confusion. larger disc will make the car stop faster,
but what about the same size disc with the calipers that have bigger pistons? autofive
big disc kit vs gti-6 setup. also, it's hard not to think that larger pads won't stop better,
just think of how wider tyres produce more GRIP.



If the braking system used is a simple single line supplying the rear of the car coming from the rear end of the master cylinder, and the front brakes utilising the two front ports of the master cylinder, then when using larger front calipers, it will take a longer pedal travel again to fill the calipers.  However, the rears will fill more quickly and the pedal will begin to stiffen.  Applying more force on the pedal to pump more fluid into the fronts will mean a greater line pressure in the rears to the point where the rear compensators allow no more and the pressure between the compensator and rear caliper will (in theory) not increase.  The line pressure between mc and compensator will though.  In this instance, the rears could be at their maximum operating pressure before the front calipers?

this is EXACTLY why i always tend to convince people to use rear compensators,
ESPECIALLY if they upgrade the front brakes to gti-6 setup. it's not just that the
rear brakes will lock easily, but the fact (or so i have found it to be on the road)
that the rear brakes will brake hard BEFORE the (uprated) fronts will. again, this
could be proved or denied by my next brakes test. i'm also going to test my gti-6
setup with rear compensators to see what figures i will get. before anybody points
out, i now understand that there's no fluid movement as such, but i tend to explain
the fluid PRESSURE distribution in the same manner.



The thing I'm not totally convinced by is the age old argument about 'weight transfer'.  Larger brakes cause a greater weight transfer from the rear to the front and therefore the rears need to be down rated, to me doesn't sound right.

i think the term weight transfer is very "mystic" as well. what's true though is that
if you have really soft suspension on the car, during braking the front end will tend
dive a lot which will try to lift the rear end and the rear brakes will then lock easily.

what tends to wind me up is when people say "my brakes are strong enough, they
can easily lock". as said, it's the grip between tyres and the road which dictates the
maximum stopping power (without locking) - with slick tyres you'll find standard
brakes not to be very effective and will have a hard time locking them. BUT, what
one also has to consider is that from 40-0 kph there won't be almost any difference
between various different brakes, but when braking from 200-100 kph (especially
with sticky tyres) there definatelly WILL BE.


keep the constructive comments coming - i really want us to get to the bottom of this!

#6
Rob Thomson

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i agree to a point. i tend to argue that rear compensators, by limiting the pressure
to the rear brakes, actually increase the pressure to the front brakes (IF the brakes
run in a cross-diagonal arrangement).


There is no way in the world that the rear compensators can effect the pressure in the front lines however the MC is connected. It just won't happen. That's really simple physics. If you want to argue that point I suggest you talk to Sir Isaac Newton.

Your evidence to support this is interesting. Your on-the-road evidence can easily be explained by the rear suspension geometry. As explained above, braking the rear wheels causes the rear suspension to squat. If that braking is assymetric the squat will be assymetric and thus the loading on the front wheels will change assymetrically. In other words, one shagged rear brake will cause one front wheel to lock before the other.

Your evidence on the rollers is more interesting, but seriously it has to be down to s*it instrumentation or your brakes being knackered in some other way. There is no way the front brakes can be effected by the rears. No way.

Also note your handbrake is out of balance by a similar amount to your rear brakes. That make me think your problem isn't hydraulic.

yeah i agree this causes a lot of confusion. larger disc will make the car stop faster,
but what about the same size disc with the calipers that have bigger pistons? autofive
big disc kit vs gti-6 setup.


In either case you're effectively increasing the size of the lever arm.

Turn your bike upside down and spin the front wheel. Now stick your little finger in the spokes near the hub. It hurts, doesn't it? And the wheel probably didn't stop very fast. Now spin it again and stick your finger in the spokes near the rim. Let me guess, the wheel stopped pretty chuffing quick and you aren't hopping about in agony. It's exactly the same with the ratio between wheel and ratio disc size. If the disc is large relative to the wheel it'll need less braking effort to generate the same braking force at the tyre.

Bigger pistons in calipers mean they will generate the same force against the pad for a smaller force at the pedal. Same as sticking your thumb into the spokes instead of your little finger.

also, it's hard not to think that larger pads won't stop better,
just think of how wider tyres produce more GRIP.


So how exactly do wider tyres create more grip?! To use another F1 analogy, how much cornering force can an F1 car generate at low speeds or if you removed the wings? 1.5G perhaps? Probably less. That's not a whole lot more than a 205 or other non-aerodynamically assisted cars, despite the tyres being hugely wide and incredibly soft. Does that make you think you're barking up the wrong tree?

Wider tyres on a road car generate more grip because that extra width does two things. Firstly, it enables the designers to use a softer (and thus grippier) compound without compromising durability. Secondly, wider tyres are more stable and having a stable contact area is very important in such a dynamic situation.

Think about the tyre or a brake pad. In the case of the tyre there is a certain force acting through it - the weight of the car or the force applied by the hydraulics. Now, the friction (let's call it G) generated between two objects is a product of the pressure across the contact patch (P), the co-efficient of friction for those materials © and the area of the contact patch (A).

G = P x C x A.

But, pressure is force divided by area (F/A). If you folow that through you're left with...

G = F/A x C x A.

The two 'A's cancel out and so you're left with...

G = F x C

So area is unimportant - the only things that matter are the force acting through the interface (the weight of the car or whatever) and the co-efficent of friction for those materials.

this is EXACTLY why i always tend to convince people to use rear compensators,
ESPECIALLY if they upgrade the front brakes to gti-6 setup. it's not just that the
rear brakes will lock easily, but the fact (or so i have found it to be on the road)
that the rear brakes will brake hard BEFORE the (uprated) fronts will. again, this
could be proved or denied by my next brakes test. i'm also going to test my gti-6
setup with rear compensators to see what figures i will get. before anybody points
out, i now understand that there's no fluid movement as such, but i tend to explain
the fluid PRESSURE distribution in the same manner.


I've played around with various brake set-ups on various cars over the years and never experienced anything like that - which is a relief because it just doesn't make any sense.

i think the term weight transfer is very "mystic" as well. what's true though is that
if you have really soft suspension on the car, during braking the front end will tend
dive a lot which will try to lift the rear end and the rear brakes will then lock easily.


The stiffness of the front suspension doesn't come into it. Weight transfer is related to where the car's centre of gravity is and how much braking force can be generated relative to the weight of the car. You won't get any more or less weight transfer with soft suspension.

what tends to wind me up is when people say "my brakes are strong enough, they
can easily lock". as said, it's the grip between tyres and the road which dictates the
maximum stopping power (without locking) - with slick tyres you'll find standard
brakes not to be very effective and will have a hard time locking them.


That's perfectly understandable.

BUT, what
one also has to consider is that from 40-0 kph there won't be almost any difference
between various different brakes, but when braking from 200-100 kph (especially
with sticky tyres) there definatelly WILL BE.


Do you care to explain why?

#7
markrnorton

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Rob

I'm glad you mentioned friction co-effiecents before I did, I wondered when somebody was going to bring it up.

Nice explanations.

#8
markrnorton

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Whilst on the subject(ish) of F1 brakes. Do people realise that they generate the huge braking forces under 13" wheels.

#9
DrSeuss

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I think we'll talk around in circles until someone discovers the true effects of the rear compensators. Perhaps rear compensators are just a restriction to fluid flow. Though fluid flow in a cars braking system is minimal a certain amount does move. Thus if the compensators are just a narrowing of the brake pipes they'll slow the rear brakes filling with fluid and allow the front and rear brakes to come on simultaneously.

Removal of the compensators would mean the rear brakes will start exerting braking force before the front callipers are fully exerting maximum braking effort.

If one were to start braking with the rear first it would affect the amount of weight transfer to the front of the car. IMO it would help prevent front end dive under deceleration. This would lead to a more stable car position under braking. If one minimises weight transfer to the front then the rears will still be effective. The only problem is when you start talking about suspension dynamics you don't take into account other factors such as uprated front springs or rear torsion bars.

#10
jonah

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Ah, this old chestnut again! :)

Everything that Rob has said sounds spot on to me. A few bits to add:

I think people are sometimes confusing volume with pressure in hydraulic systems. One theory that's been suggested is that "bigger front brakes require more fluid to operate, which means that the rears will also receive more fluid and therefore operate sooner". This is wrong because a master cylinder does not produce equal VOLUMES in each circuit, but equal PRESSURES. Pressure is the same at all points in a hydraulic system (ignoring effects of fluid movement), so the braking force at each wheel depends on the force you're putting on the pedal, not the distance you move it.

Something else that hasn't been mentioned is that weight transfer from rear to front is caused by both front and rear brakes. Therefore it is wrong to say that bigger front brakes cause more weight transfer. The amount of weight transfer is proportional to the total retardation force generated by both front and rear brakes.

If you fit more powerful front brakes, then you will need less pressure in the hydraulic system than before to lock up the front wheels. If the rear brakes remain standard, then at the point when the front wheels are about to lock, the rear brakes are doing less than before. This means that a) there is less weight transfer onto the front wheels, so the amount of grip they can provide is reduced, and :P there is still unused grip available at the rear wheels (assuming that the original setup already locked the fronts first). Both of these mean increased stopping distances.

As for the 1.9 rear compensator... it remains a bit of a mystery! I remembered reading something insteresting in this thread... It seems that there are two different types. The type with a hole in the body, I would assume, is a limiting valve. I guess the hole is necessary for some internal piston mechanism to compare the fluid pressure with atmospheric pressure. The other type mentioned apparently just contains a fluid restriction...

I don't believe that the compensator causes a linear pressure reduction - for two reasons. One, I don't see how it is possible to design something to do that in a unit that shape & size, and that still allows fluid through for brake bleeding. Two, because there would be no point - they could have achieved exactly the same effect by making the rear caliper pistons smaller.

#11
Rob Thomson

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Perhaps rear compensators are just a restriction to fluid flow. Though fluid flow in a cars braking system is minimal a certain amount does move. Thus if the compensators are just a narrowing of the brake pipes they'll slow the rear brakes filling with fluid and allow the front and rear brakes to come on simultaneously.


There has to be more to them than that. There really is very little fluid movement, so any pressure reduction due to a restriction in a pipe would be over almost instantaneously.

If one were to start braking with the rear first it would affect the amount of weight transfer to the front of the car. IMO it would help prevent front end dive under deceleration. This would lead to a more stable car position under braking. If one minimises weight transfer to the front then the rears will still be effective. The only problem is when you start talking about suspension dynamics you don't take into account other factors such as uprated front springs or rear torsion bars.


As Jonah says, weight transfer is not effected by front or rear braking but by the total braking force. It also has nothing to do with suspension stiffness or geometry. (You can have anti-squat geometry, but that doesn't effect the weight transfer, it just prevents that weight transfer from causing squat!).

I think I might take five minutes out of my lunch break to draw some nice little sketches to explain weight transfer. It's really simple, but difficult (for me) to explain just in words.

#12
Rob Thomson

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As for the 1.9 rear compensator... it remains a bit of a mystery! I remembered reading something insteresting in this thread... It seems that there are two different types. The type with a hole in the body, I would assume, is a limiting valve. I guess the hole is necessary for some internal piston mechanism to compare the fluid pressure with atmospheric pressure. The other type mentioned apparently just contains a fluid restriction...


I'd love to know how they work. I used to have the two I took off my car kicking about somewhere, but I think they might have been binned. If I can find them I'll cut one open and see what there is to see.

I don't believe that the compensator causes a linear pressure reduction - for two reasons. One, I don't see how it is possible to design something to do that in a unit that shape & size, and that still allows fluid through for brake bleeding.


The bleeding thing is something I hadn't thought of. Fluid gushes through the compensators during bleeding. I use a Gunson Easi-bleed that operates at about 20psi - which is sod all in terms of braking systems. If the easi-bleed can create decent flow then foot on pedal will too - and I think that throws the restrictor argument out of the window.

I eventually replaced my compensators with an adjustable bias valve - although things have moved on again since then. But I remember reading the instructions for that, and there being a graph showing a linear reduction in pressure. That valve is tiny, so if Tilton can do it I guess Peugeot could.

Two, because there would be no point - they could have achieved exactly the same effect by making the rear caliper pistons smaller.


But that calliper design was used on several different cars. It's cheaper to use a compensator than to manufacture a bespoke calliper for each application.

#13
DrSeuss

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After thinking about what i said further, i reckon most of it is total bollox and i'm still no closer to understanding it. I've got some 1.9 compensators that are doing nothing currently, not massively keen on cutting them up, but i might be able to rig a test rig to find out how much effect they have.

#14
Anthony

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this is EXACTLY why i always tend to convince people to use rear compensators,
ESPECIALLY if they upgrade the front brakes to gti-6 setup. it's not just that the
rear brakes will lock easily, but the fact (or so i have found it to be on the road)
that the rear brakes will brake hard BEFORE the (uprated) fronts will. again, this
could be proved or denied by my next brakes test.

Except that as I have said before, that is not the case in my experience - I run GTi-6 brakes up front, standard 1.9 GTi calipers at the rear, Mintex 1144 pads front and back, and no rear compensators at all... and my car still locks the front wheels first.

As far as I'm aware, this setup (GTi-6 up front, standard calipers rear, no compensators) is how Jonnie205 does all his GTi-6 brake conversions and to the best of my knowledge he's never had any problems. I'm sure he will conform this and add his comments when/if he reads this thread.

#15
buchanan84

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OK I think I know how the 1.6 compensator works (read it somewhere)

the reason the compensator is at a critical angle is because it has a ball inside it (at the angle) which moves forward under braking therefore limiting the pressure to the rears. This would prevent the rears from locking as the rear of the car gets light.

this is my basic understanding of what I read and it makes some kind of sense to me

#16
Rob Thomson

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Ok, this is probably hugely irrelevent but I've just spent a few minutes doing a couple of drawings on AutoCAD to explain this weight transfer thingy.

When a car is stationary (is that the right spelling - I never know which means pens and which means still?) or moving at a constant speed, it's mass acts vertically downwards from it's Centre of Gravity (CoG). The downwards force is due to gravity, so lets call that G.

As I mentioned somewhere in the pages of ramblings above, the grip provided by a car's tyres is the product of the force acting through them and the coefficient of friction for the rubber/tarmac interface. As it happens, rubber on tarmac has a CoF of around 1.0. s*it tyres will have a lower value, but 1.0 is in the right ballpark. So, the grip generated by the tyres is about 1.0 x G, which is G.

So, if a car has efficient brakes that utilise the maximum grip of all four wheels it can generate just about the same braking force as it's weight. In other words, it'll stop at about 1G. The same goes for cornering or whatever. To get above that you really have to start employing aerodynamics.

Now, when a car is braking the forces still act about it's CoG. Gravity is still acting vertically downwards and the braking force (lets call it B ) is acting forwards. If you work out the resultant force (that is what you get when you add those forces together - we'll call it R ) you see that the weight of the car is now acting diagonally, towards the front of the car. This is the weight transfer. Have a look at these diagrams of a proper car and see if you understand what I'm on about....

Posted Image

As long as R intersects the ground behind the front wheel contact point, the rear wheels will remain on the ground. As soon as that arrow passes in front of the front wheel, the car would flip. But of course that doesn't happen because there's just not enough grip from the tyres and the CoG is always somewhere sensible (in standard cars). But, you should be able to see that if a 309 could stop at 2G it's stability would be getting very marginal - assuming my guessed CoG is in about the right place!

The weight distribution at any time depends on where R crosses the ground. If it were exactly at the front wheel the car would have 100% of it's weight on the front wheel. If R were exactly at the rear wheel you'd have 100% of the weight of the car on that. If R crosses anywhere between the two wheels the weight distribution is the opposite of the ratio of the distances between the wheels and R. In the upper pic G (which is R because there are no other forces) is a third of the way along the wheel-base and so 66% of the car's weight is on the front wheels. In the lower pic, about 83% of the car's weight is through the front wheels. That change from 66% to 83% is of course the weight transfer that's being discussed in this thread.

Get the gist? Sorry if this reads like a page out of a school text-book!

#17
Wurzel

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OK I think I know how the 1.6 compensator works (read it somewhere)

the reason the compensator is at a critical angle is because it has a ball inside it (at the angle) which moves forward under braking therefore limiting the pressure to the rears. This would prevent the rears from locking as the rear of the car gets light.

this is my basic understanding of what I read and it makes some kind of sense to me

<{POST_SNAPBACK}>


That's for the 1.6 GTi models using drum brakes. The 1.9 Compensators are mounts transversely on the car. I thought that they may be some sort of differential pressure valve.

#18
cybernck

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great, healthy and productive discussion going on in this thread :D.


Your evidence to support this is interesting. Your on-the-road evidence can easily be explained by the rear suspension geometry. As explained above, braking the rear wheels causes the rear suspension to squat. If that braking is assymetric the squat will be assymetric and thus the loading on the front wheels will change assymetrically. In other words, one shagged rear brake will cause one front wheel to lock before the other.

makes sense :). but then again, all what i'm talking about is based on on-the-road
experience, and i drew my conclusions from that, that's why it all made sense to me,
without thinking about it on theoretical level :D. anyway, i think i found a flaw in
my logic (says the vulkan :P), while i'm still thinking on the "fluid movement and
quantity" level which makes me believe that calipers with larger pistons will require
more fluid and therefore more pressure (hence the longer pedal travel) which will
in turn produce too much pressure in the rear brakes, but what actually goes on is
that the fronts will lower the overall pressure in the system (or in that section of m/c
at least) to start with, which will affect the rear brakes in the same manner or in
other words - practically won't affect the way the rears behave in relation to fronts.
is that right? :)


Your evidence on the rollers is more interesting, but seriously it has to be down to s*it instrumentation or your brakes being knackered in some other way. There is no way the front brakes can be effected by the rears. No way.

rollers are brand new MAHA ones so i don't think they're knackered. on the other
hand my brakes could be. still i'm very eager to see the results after the retest.
regardless of rear compensators affecting the front brakes or not, we'll be able to
relatively precisely calculate the propotioning effect of them on the rear brakes :).


Also note your handbrake is out of balance by a similar amount to your rear brakes. That make me think your problem isn't hydraulic.

yeah but my handbrake on that wheel also didn't work at all when that compensator
was blocked ;). rear brake pads are new btw, but one of the rear discs is thinner,
though that shouldn't affect things much.


Turn your bike upside down and spin the front wheel. Now stick your little finger in the spokes near the hub. It hurts, doesn't it? And the wheel probably didn't stop very fast. Now spin it again and stick your finger in the spokes near the rim. Let me guess, the wheel stopped pretty chuffing quick and you aren't hopping about in agony. It's exactly the same with the ratio between wheel and ratio disc size. If the disc is large relative to the wheel it'll need less braking effort to generate the same braking force at the tyre.

that's perfectly understandable, but i haven't implied it wasn't anyway.


Bigger pistons in calipers mean they will generate the same force against the pad for a smaller force at the pedal. Same as sticking your thumb into the spokes instead of your little finger.

so it's only the disc size that affects the braking power? or back to my original
question, will a standard gti caliper spaced out to accept a 283 discs brake the
same as a full gti-6 setup with the same size discs? also, i still can't understand
why larger brake pads won't grip the discs better... i understand that the friction
will be less stressful on the pads with larger area, but doesn't that in turn mean
that they'll be able to provide more "grip" with the relatively same "stress" as
the smaller pads, if you see what i mean - excuse for the layman expressions
but it's nearly 4AM here. and what about uprated and race pads, will they only
be able to resist fade and operate at high temperatures but still have the maximum
braking power as standard pads? what i mean is - just as they don't brake well
when cold (i've got Pagid RS15 Grey's :D) they bite like crazy when hot, definately
better stoppers than standard pads at their normal temperature, so wouldn't that
mean they're ultimately producing better stopping power, more friction / grip on
the discs or whatever?


So how exactly do wider tyres create more grip?!

sorry, a lapsus. i meant to use word TRACTION. when my gti had a clutch on it's
way out, with bigger tyres the clutch was slipping yet was still perfectly fine with
the smaller ones. and isn't that the best part of the reason why rwd supercars
have the rear tyres considerably wider than the fronts? or is that the wrong tree
again? :)


what's good is that this time there are some guys like you on the forum, willing to
go in-depth with this matter (and know what they're talking as well :)). every time
i started topics like this one, i got next to no replies.

cheers.


p.s. topic pinned.

#19
jonah

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anyway, i think i found a flaw in
my logic (says the vulkan :D), while i'm still thinking on the "fluid movement and
quantity" level which makes me believe that calipers with larger pistons will require
more fluid and therefore more pressure (hence the longer pedal travel) which will
in turn produce too much pressure in the rear brakes, but what actually goes on is
that the fronts will lower the overall pressure in the system (or in that section of m/c
at least) to start with, which will affect the rear brakes in the same manner or in
other words - practically won't affect the way the rears behave in relation to fronts.
is that right? :)


No, not quite. The force (F) exerted by the caliper on the pads is:
F = P x A
where P is the hydraulic pressure, and A is the cross-sectional area of the piston. So, if you increase the size of the piston, you need LESS hydraulic pressure to generate the same force on the brake pads.

The volume of fluid that moves into the caliper when you press the brake pedal is:
V = d x A
where d is the distance moved by the piston (and A is the same as above). This distance is very difficult to estimate as it will only be a tiny amount - affected by the amount of "squish" in the brake pad material (and any anti-squeal backing), the amount of piston retraction caused by the fluid seals, the amount of flex in the caliper body, and whether the piston has been knocked back into the caliper since the last braking maneouvre by wobble on the brake disc. You could assume that if you compared two calipers that had different piston sizes but were otherwise identical, d would be about the same for both for a given amount of force on the brake pads. So for the same amount of braking force, d is the constant but A is larger with a larger caliper piston, so the volume of fluid required is greater. This gives you a longer pedal travel but with less resistance as the hydraulic pressure will be lower.

Since the pressure is equal throughout the whole braking system (unless the master cylinder is in its failsafe mode), the lower pressure at the front (for a given braking force) means that the pressure at the rear will also be lower, and this reduces the amount of braking force generated by the rear brakes.

rear brake pads are new btw, but one of the rear discs is thinner,
though that shouldn't affect things much.


Thinner = more worn. All the worn brake discs I've seen have had a slightly concave surface - i.e. the very edges of the pad contact area are less worn than the rest of the surface, creating a lip at the inner and outer edges of the contact area. Perhaps your new pads are not yet fully bedded in to the worn disc, and are not making contact over their full surface area? That could be affecting braking performance on the side with the worn disc.

so it's only the disc size that affects the braking power? or back to my original
question, will a standard gti caliper spaced out to accept a 283 discs brake the
same as a full gti-6 setup with the same size discs?


No, it's both disc size and piston area that affect brake behaviour. I assume that the gti-6 calipers have a larger piston than standard gti calipers? If so, they will require less hydraulic pressure and (probably) more fluid volume to exert the same braking force. (I say probably because we can't assume that the "d" above is the same). The larger disc diameter means that the force generated by the pad friction against the disc translates to a larger torque at the wheels, and therefore a greater deceleration of the car. This means that you need even less pedal pressure for the same deceleration, which is even more bad news for the rear brake effectiveness!


also, i still can't understand
why larger brake pads won't grip the discs better... i understand that the friction
will be less stressful on the pads with larger area, but doesn't that in turn mean
that they'll be able to provide more "grip" with the relatively same "stress" as
the smaller pads, if you see what i mean


Larger pads won't make a big difference to the braking force because the increase in contact area is counteracted by a proportional reduction in surface pressure. The force on the pad is distributed over its entire contact area with the disc, so the larger the pad, the lower the force between the disc and each unit area of the pad surface.

The pad area does become important at high speeds and high braking forces, because of the large amounts of heat generated. The larger the pad, the easier it is for the heat to be conducted away from the contact surfaces into the bulk material of the disc and pad, so the lower the instantaneous change of temperature at the pad surface. Temperature will affect the friction coefficient between pad and disc and therefore alter the braking performance. Competition pad compounds tend to actually increase their friction coefficient with temperature up to a limit, so a smaller pad may actually give higher braking performance in some conditions...

and what about uprated and race pads, will they only
be able to resist fade and operate at high temperatures but still have the maximum
braking power as standard pads? what i mean is - just as they don't brake well
when cold (i've got Pagid RS15 Grey's :D) they bite like crazy when hot, definately
better stoppers than standard pads at their normal temperature, so wouldn't that
mean they're ultimately producing better stopping power, more friction / grip on
the discs or whatever?


Sorry, I don't know... I think that's getting into quite advanced material physics. The basic theory of friction says that there shouldn't be any limit to how much frictional force can be generated - if you keep applying more force to the system (pressing the brake pedal harder), the friction force should continue to increase. In practice, the coefficient of friction between any two materials is probably not linear, and the high boundary temperatures will start to come into play...

sorry, a lapsus. i meant to use word TRACTION. when my gti had a clutch on it's
way out, with bigger tyres the clutch was slipping yet was still perfectly fine with
the smaller ones. and isn't that the best part of the reason why rwd supercars
have the rear tyres considerably wider than the fronts? or is that the wrong tree
again? :P


Same goes for tyres I think... the friction coefficient between rubber and tarmac is not constant / linear. This is even further from "ideal" than brake pads because tarmac is not a smooth surface, and the relatively soft rubber of the tyres actually moulds into the shape of the bumps in the tarmac surface. So I suspect that wider tyres really do give higher grip levels in most conditions.


Phew!! :)

Cheers,
Jonah

#20
TT205

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Cracking thread

I previously sprinted a 1.9 205 with the conventional diagonally split lines and two compensators - each located shortly in front of the rear wheels - If I braked extremely hard in this car I would lock the rears

I now race a 205 that started life as a 1.6 gti and despite fitting 1.9 hubs and drive shafts, it still has the 1.6 front callipers. At the rear it now has a 1.9 rear beam and disc brakes. The difference being that being a 1.6 it does not have diagonally split lines and it only has the one compensator which is situated at the front.
This car seems to have better overall stopping power compared to the 1.9, but to be fair it does have mainly new parts - discs/pads/callipers.
However, if I brake hard in this car it is the FRONT wheels that lock, entering a corner with a stalled engine when racing is not how I like to drive :rolleyes:

Is this what you budding physicists would expect or is this peculiar, what would be the likely effect of removing the single compensator altogether

Cheers

Dave