Alfa Romeo Forum banner

21 - 40 of 41 Posts

·
Registered
Joined
·
938 Posts
Discussion Starter #21 (Edited)
If the standard 4 piston calipers for 156/147/GT have 40mm dia pistons (I don't know the exact dimension, but it's close to that) and you have fitted calipers with 60mm pistons, then you have more than doubled the piston area of the front brakes (Area = pi R squared). Assuming you haven't changed the rear calipers, you must still be close to doubling the total piston area, so the brake pedal travel is also going to be nearly doubled (excluding any slop or free play in the system, which won't be much).
Stanard 156 brakes on the v6 have a single 55mm piston. These calipers have 4x 40mm pistons [I've just looked it up again]. Obviously that's a big increase, but these calipers are just the same as the standard 330 brakes which will work off a standard master cylinder [i'm not the first v6 owner to have done a big brake upgrade]. Is the GTA master cylinder any different to the 2.5 V6?

I'm confused. It should be fine. Lots of others have upgraded to Brembo 4 pots without swapping the master cylinder or having excessive travel...
 

·
Registered
Joined
·
276 Posts
I think you have air in the abs unit, that can make pedal travel very long. Needs bleeding electronically. The procedure is to put your foot firmly on the brake pedal, then they run the ABS pump and open an NC valve. You release and reapply the brake, and they open the next valve etc... takes a minute or so to cycle through all the valves.
 

·
Registered
Joined
·
571 Posts
The difference in clamping force (at a given fluid pressure) between four 40mm pistons and four 60mm pistons will be simply huge. I wouldn't think that you could use 60mm pistons in place of 40mm pistons without having substantial issues (maybe I'm wrong, but I doubt it).

On the other hand, while a calipers' clamping force will be different with a single piston of 55mm diameter (Bosch caliper) vs four 40mm pistons (Brembo, as fitted by AR), it won't be nearly as great a difference as might appear likely at first glance. It should be kept in mind that the Bosch caliper is a 'floating' or 'sliding pin' design, and the Brembo is a 'fixed' design, and this affects the clamping force vs piston size comparison substantially.

With a 'fixed' caliper there is one or more pistons located on each side of the disc. Each piston exerts a force on the back of only one brake pad, and exerts no force on the brake pad located on the opposing side of the disc. The fluid pressure also acts with equal force in the opposite direction to the piston motion, but the caliper body is 'fixed' and very rigid, so the pressure has no influence on the clamping force exerted on the brake pads, other than the force of pressure that is acting directly on the back of each piston.

With a 'floating / sliding pin' caliper pistons are located only on one side of the disc. As with a fixed caliper, each piston only acts directly against one brake pad on one side of the disc, but the fluid pressure also causes the caliper body to slide on the 'slider pins'. This causes the other brake pad to be clamped against the disc by the caliper body, using the same fluid pressure that acts on the pistons, and to the same degree as determined by the piston area. So with a floating caliper, a given fluid pressure acting on the piston(s) AND on the caliper body has an equal clamping affect on the brake pads on each side of the disc, whether or not the pad is in direct contact with a piston. On one side of the disc the fluid pressure acts directly via the pistons, on the other side it acts through the caliper body.

Back of envelope calculations:

A piston with a 55mm diameter has an area of 3.66 square inches (apparently the piston diameter in the Bosch front calipers for the V6, the pistons in my Twin Spark front calipers are a bit smaller). The AR fitted Brembos have 40mm diameter pistons, so each piston has an area of 1.95 square inches. There are four pistons in the Brembo, so the total piston area is 7.8 square inches in the Brembo vs 3.66 square inches in the Bosch. This suggests that the clamping force of the Brembo will be nearly twice that of the Bosch, but this isn't so.

For example, if we had an arbitrary 100psi in the system, then with a 40mm piston (i.e. 1.95 square inches) the linear clamping force from that single piston would be 195 pounds. If there are four 40mm pistons in Brembo caliper, then 195 X 4 = 780 pounds of clamping force acting on the disc.

The Bosch 'floating' caliper has a single 55mm piston with an area of 3.66 square inches. With 100psi in the the system the linear force acting on the piston will be 366 pounds. However, since the pressure not only moves the piston, but also moves the caliper body on the slider pins (or at least force acts via the pins, even if nothing were to actually move), the clamping force will be 366 pounds at the piston and 366 pounds acting along the slider pins. So, the total clamping force will be 732 pounds.

So according to the envelope, there is a reasonably significant difference in clamping force between the Bosch and Brembo calipers, but it isn't huge.

At our arbitrary 100psi of fluid pressure, the Brembo caliper exerts about 780 pounds of clamping force on the disc, and for the Bosch it is about 732 pounds. In other words the Brembo caliper exerts about 17% more clamping force, give or take. Unless I'm missing something...

Regards,
John.
 

·
Registered
Joined
·
571 Posts
If the standard 4 piston calipers for 156/147/GT have 40mm dia pistons (I don't know the exact dimension, but it's close to that) and you have fitted calipers with 60mm pistons, then you have more than doubled the piston area of the front brakes (Area = pi R squared). Assuming you haven't changed the rear calipers, you must still be close to doubling the total piston area, so the brake pedal travel is also going to be nearly doubled (excluding any slop or free play in the system, which won't be much).
The HUGE leap from 40mm diameter to 60mm way more than doubles the piston area.

Single 40mm piston area = 1.95 square inches, so 4 X that = 7.8 square inches total area.

Single 60mm piston area = 4.38 square inches, so 4 X that = 17.52 square inches total area.

This will add substantially to any existing 'free' motion at the pedal, as caused by running clearances between the discs, pads and pistons in the front calipers. But this isn't the real problem.

It will also severely disrupt the fundamental braking balance front to rear, by substantially increasing braking effect at the front wheels with no change at the rear. ABS would possibly help to limit the severity of this probable disaster, but even if so it can't be good. The car will likely want to lock the front wheels easily with little provocation, the ABS will then have to work overtime, and probably often

IMO the only saving grace here might be that with a 'fixed' caliper the clearances between the discs, pads and pistons may be somewhat less than is likely to be the case for a sliding caliper, so an increase in unwanted free pedal motion may not be as bad as it otherwise could be.

For all the reasons I have stated, IMO the Jaguar calipers (with their much larger pistons) are most probably quite unsutable for the Alfa, unless a lot of other things are also changed to suit. But, I've been wrong before...

Regards,
John.
 

·
Registered
Joined
·
938 Posts
Discussion Starter #25
Just to clarify, the pistons are 40mm not 60mm. I made a mistake earlier in the thread and looked up the wrong brembo caliper.

Excellent and very useful post though

Sent from my Mi 9T using Tapatalk
 

·
Vendor
Joined
·
45,953 Posts
At our arbitrary 100psi of fluid pressure, the Brembo caliper exerts about 780 pounds of clamping force on the disc, and for the Bosch it is about 732 pounds. In other words the Brembo caliper exerts about 17% more clamping force, give or take. Unless I'm missing something...
6.5% more clamping force. (780/732*100)

You're also moving the clamping force further away from the fulcrum, from a disc with 286mm diameter to 330mm diameter. That'll increase the braking effort also.
 

·
Registered
Joined
·
2,648 Posts
Not that it matters at all, Valinco having found his calipers have 40mm pistons after all, but John, your own numbers for the increase in piston area show it to be 2.25 times greater. I'm happy that my generalisation of "...more than doubled..." is correct.

Good luck bleeding the brakes Valinco.
 

·
Registered
Joined
·
276 Posts
The difference in clamping force (at a given fluid pressure) between four 40mm pistons and four 60mm pistons will be simply huge. I wouldn't think that you could use 60mm pistons in place of 40mm pistons without having substantial issues (maybe I'm wrong, but I doubt it).

On the other hand, while a calipers' clamping force will be different with a single piston of 55mm diameter (Bosch caliper) vs four 40mm pistons (Brembo, as fitted by AR), it won't be nearly as great a difference as might appear likely at first glance. It should be kept in mind that the Bosch caliper is a 'floating' or 'sliding pin' design, and the Brembo is a 'fixed' design, and this affects the clamping force vs piston size comparison substantially.

With a 'fixed' caliper there is one or more pistons located on each side of the disc. Each piston exerts a force on the back of only one brake pad, and exerts no force on the brake pad located on the opposing side of the disc. The fluid pressure also acts with equal force in the opposite direction to the piston motion, but the caliper body is 'fixed' and very rigid, so the pressure has no influence on the clamping force exerted on the brake pads, other than the force of pressure that is acting directly on the back of each piston.

With a 'floating / sliding pin' caliper pistons are located only on one side of the disc. As with a fixed caliper, each piston only acts directly against one brake pad on one side of the disc, but the fluid pressure also causes the caliper body to slide on the 'slider pins'. This causes the other brake pad to be clamped against the disc by the caliper body, using the same fluid pressure that acts on the pistons, and to the same degree as determined by the piston area. So with a floating caliper, a given fluid pressure acting on the piston(s) AND on the caliper body has an equal clamping affect on the brake pads on each side of the disc, whether or not the pad is in direct contact with a piston. On one side of the disc the fluid pressure acts directly via the pistons, on the other side it acts through the caliper body.

Back of envelope calculations:

A piston with a 55mm diameter has an area of 3.66 square inches (apparently the piston diameter in the Bosch front calipers for the V6, the pistons in my Twin Spark front calipers are a bit smaller). The AR fitted Brembos have 40mm diameter pistons, so each piston has an area of 1.95 square inches. There are four pistons in the Brembo, so the total piston area is 7.8 square inches in the Brembo vs 3.66 square inches in the Bosch. This suggests that the clamping force of the Brembo will be nearly twice that of the Bosch, but this isn't so.

For example, if we had an arbitrary 100psi in the system, then with a 40mm piston (i.e. 1.95 square inches) the linear clamping force from that single piston would be 195 pounds. If there are four 40mm pistons in Brembo caliper, then 195 X 4 = 780 pounds of clamping force acting on the disc.

The Bosch 'floating' caliper has a single 55mm piston with an area of 3.66 square inches. With 100psi in the the system the linear force acting on the piston will be 366 pounds. However, since the pressure not only moves the piston, but also moves the caliper body on the slider pins (or at least force acts via the pins, even if nothing were to actually move), the clamping force will be 366 pounds at the piston and 366 pounds acting along the slider pins. So, the total clamping force will be 732 pounds.

So according to the envelope, there is a reasonably significant difference in clamping force between the Bosch and Brembo calipers, but it isn't huge.

At our arbitrary 100psi of fluid pressure, the Brembo caliper exerts about 780 pounds of clamping force on the disc, and for the Bosch it is about 732 pounds. In other words the Brembo caliper exerts about 17% more clamping force, give or take. Unless I'm missing something...

Regards,
John.
I don't believe this is correct John. When a floating caliper is seized on it's pins, and it only used 1 brake pad for braking, this can't be seen on a brake roller tester. The one brake pad just does double the work. Had a car in the shop today that btested fine, but at the rear 2 brakepads did double duty and 2 din't do anything (had to hammer them out), the 4 up front were underperforming aswell. 9 years of neglect... of the 8 braking surfaces only 2 looked like they should.

In fact the only issues you can see on a tester are a stuck piston, if both brakepads are resisting the brake pressure (for whatever reason or if the disc or pads have overheated. And then only if that condition is not present on both wheels. And it makes sense, as it's the piston movement that moves the caliper on the sliders.

Fixed calipers are much easier to diagnose with a brake tester, with any fault giving lower readings on that wheel.
 

·
Registered
Joined
·
571 Posts
I don't believe this is correct John. When a floating caliper is seized on it's pins, and it only used 1 brake pad for braking, this can't be seen on a brake roller tester. The one brake pad just does double the work. Had a car in the shop today that btested fine, but at the rear 2 brakepads did double duty and 2 din't do anything (had to hammer them out), the 4 up front were underperforming aswell. 9 years of neglect... of the 8 braking surfaces only 2 looked like they should.

In fact the only issues you can see on a tester are a stuck piston, if both brakepads are resisting the brake pressure (for whatever reason or if the disc or pads have overheated. And then only if that condition is not present on both wheels. And it makes sense, as it's the piston movement that moves the caliper on the sliders.

Fixed calipers are much easier to diagnose with a brake tester, with any fault giving lower readings on that wheel.
Jetronic,
I don't know why your brake test machine is not detecting significant loss of braking effect with siezed caliper pins. My best guess would be that even if the pins are siezed, so long as the static gap between the disc and outer pad is not yet too big, then some outer pad 'use' could still occur due to disc flexure. This might occur if the piston pushes hard enough against the inner pad? If so then this could press the inner pad against the disc as per normal, but with strong enough piston force then it might conceivably 'lean' on the disc hard enough to flex it into at least reasonably strong contact with the outer pad? Just a speculative thought bubble...

My attempt at understanding this is coming from first principles (as I think I understand them);

So, it is my understanding that none of the pads can ever do 'double the work' that the pad would normally do, if by 'double' we mean to generate substantially more than the normal braking friction because the pad is being pressed against the disc twice as hard (or thereabouts).

Further to this, at X fluid pressure I don't think a pad (and the piston behind it) can ever exert a greater linear force against the disc than it would normally do (normal service is at good as it ever gets with respect to linear force exerted by a pad against a disc). This is because fundamentally no piston can ever act against a pad with more force than is being generated as pressure behind the piston. With Xpsi behind the piston, the linear force acting against the brake pad will always be Y unless there is a problem with the piston, never more than Y (Y of course being X fluid psi x the piston area).

With a floating caliper having the pins in good condition; The piston directly acts only against the back of the adjacent brake pad. Simultaneously, or after the first pad to move stops moving because it has encountered the disc, the fluid pressure also acts against the caliper body and causes it to slide on the pins, in the opposite direction to the piston motion. The force manifested at the piston is resisted by the rigidity of the brake disc, but also resisted by the same force acting indirectly in opposition to itself on the other side of the disc. The 'opposing' force 'arrives' at the other side of the disc via the caliper body and the sliding pins, and is a mechanical transference of force from one side of the disc to the other (which still occurs with a 'fixed' caliper, but as a hydraulic transference of force).

The caliper body moves inward until the small gap between itself and the outer pad closes, so 'snugging' the outer pad into proper contact with the disc. Once both pads are in good contact with the disc, movement of the piston and caliper body effectively ceases and contact pressure rises exponentially depending on any increase in fluid pressure.

The force created by the fluid pressure kind of 'splits' and acts in 'opposite directions' as it pushes the piston 'out' of the caliper bore and pushes the caliper body in the opposite direction, until the force is ultimately reacted against the two opposing faces of the disc. On one side / direction this occurs through the moving piston and inner pad, and on the other side through the sliding caliper body and the outer pad.

The 'split' force 'meets' at the incompressible disc, with each pad clamped with the same degree of force on each side of the disc. The force strength is determined only by the fluid pressure and the piston area. So if the fluid pressure is X, then the linear force exerted by the pads against the disc faces will be Y, on both sides of the disc.

But, if the pins are seized; The piston still acts as per normal against the back of the inner brake pad. Obviously the caliper body can't move on the pins, so in theory the caliper can't exert any force on the outer brake pad or against the outer disc face. It may be that (as mentioned above) the piston could force the disc to be flexed into contact with the outer pad, but the probably substantial force needed to flex the disc would be deducted from the strength of pad contact (i.e. the outer pad would not be clamped as strongly as the inner pad, at best).

If the fluid pressure is X then the linear force will still be Y at the inner pad / disc interface, but significantly less than Y and quite easily zero at the outer pad / disc interface. Rather than all the force being reacted equally against the both sides of the disc, instead some is reacted between the inner disc face and the caliper body via the seized pins, so ultimately against the caliper mount, or even against the suspension upright.

(This becomes more complex the more I think about it. With fully stuck pins the hydraulic force which pushes the piston and caliper apart appears to be reacted through quite a long chain of elements on each side of the hydraulic 'cylinder' in the caliper body. Thought experiment suggests that as the force pushes the piston out of the cylinder bore, the force is first passed through the piston, then the inner pad, then against the disc (which flexes), and finally tries to pull the axle out of the hub. On the other side the force reacts through the rigid non moving caliper body, through the stuck pins, then the quite stiff caliper bracket, then through the robust suspension upright, and finally tries to pull the upright off the hub. The force initiates at the caliper cylinder as hydraulic pressure, and is then reacted as a mechanical force through a chain of elements until the force 'meets itself' at some point in a 'loop of force'. There will be lots of minute flexures all along the chain.)

So, where was I before I became distracted with theoretical force chains and flexures;

With seized pins fluid pressure is trying to push only the inner pad against the disc (with Y force). But, instead of also pressing a pad against the outer face of the disc, there is up to Y force trying to flex the caliper bracket etc through the seized pins. It does this because the moving piston imposes a linearly expanding force between the inner disc face and the caliper bracket.

This force does not do this if the caliper body is free to slide on the pins. The slding action allows the force to be reacted against the outer disc face as intended. If the pins can slide then all the forces acting within the caliper are reacted within the caliper and between the caliper and the disc with no force 'escaping' this essentially closed system (much neater all round than what happens with stuck pins causing some 'fugitive' force to 'escape' the system).

Seized pins cause the expanding piston force to in effect 'lever' the caliper bracket against the rigidity of the disc and vice versa, causing both the bracket and disc to flex 'outward' in some degree. I think the disc is far more likley to flex significantly than is the quite robust caliper bracket...

So if the pins are seized the braking effect will be reduced in some degree because some 'clamping' force is being 'diverted' from pressing against the outer side of the disc, and is instead attempting to flex the caliper mounting bracket and the disc. This won't harm the bracket, but it won't be good for the disc. The unequal force will stress the disc (high force pressing against the inner side of the disc, with no supporting force acting on the outer side). The worst case would be fracturing the disc, possibly. This may also manifest as pedal sponginess as the disc is flexed outward by the piston.

==========

Lets try an analogy. Think of a simple drum brake which has a leading shoe and a trailing shoe operated by a single slave cylinder. The cylinder is solidly attached to the backing plate, and fluid pressure pushes seperate pistons outward in each side of the cylinder, individually acting on each shoe.

Hydraulic pressure (X) acts on both shoes, which are forced against the drum with equal force (Y, ignoring the 'self energising' effect of leading shoes...). The force is reacted against the opposing sides of the drum surface, in a not disimilar manner to how with a disc brake the energising force reacts force against the opposite sides of a brake disc.

Now, we deliberatley sieze one brake shoe so that no matter how much force is acting against it the shoe cannot move into contact with the drum. Does the other still working shoe now do 'double the work'? Or is the shoe, as before, still only forced against the drum commensurate with the pressure in the slave cylinder, which has not changed just because the other shoe is not working'?

As I understand it, the answer must be 'no'. The single still working shoe is not forced into stronger contact with the drum if the fluid pressure has not increased (it is still X). The working shoe does not do twice the work in the sense of creating a stronger braking effect, though it might do more work because the car will now take longer to stop, so that shoe will produce the same degree of friction as it did previously, but possibly for a longer time.

Regards,
John.
 

·
Registered
Joined
·
571 Posts
Not that it matters at all, Valinco having found his calipers have 40mm pistons after all, but John, your own numbers for the increase in piston area show it to be 2.25 times greater. I'm happy that my generalisation of "...more than doubled..." is correct.
Thanks sprint.
I noticed afterward that it appears the 60mm pistons never existed...

Your generalisation is fine. I had it in my head that someone had said "nearly doubled".

Regards,
John.
 

·
Registered
Joined
·
571 Posts
6.5% more clamping force. (780/732*100)
Pud
Thank you for the correction.

I just did a quick mental calculation, dangerous since I am so mathematically challenged...

You're also moving the clamping force further away from the fulcrum, from a disc with 286mm diameter to 330mm diameter. That'll increase the braking effort also.
Increase braking effort, or effect?

Increased leverage on a larger disc, some 15ish% bigger. So, with the Brembo front brakes there is theoretically a 6.5% increase in pad / disc clamping force, acting on a 15% bigger disc.

What does this actually mean on the road? Lighter brake pedal? Probably yes. More effective braking performance? Harder to say, but probably better resistance to pad fade due to the larger discs being able to get rid of heat more effectively. If harder pads are used then what is gained on the swings may be lost on the roundabout, and vice versa.

Or my current brake obsession, does the pedal feel better with less unwanted pedal motion than the Bosch floating caliper? Probably yes, but if there is any significant disc wobble (as can occur with heavy cornering loads and bearing deflection) then larger discs tend to exaggerate it, so pad knockback can be a bit worse, so erratic pedal motion may possibly increase to some degree under certain conditions. Maybe, probably a non issue on a road car...

Only a bit off topic, my 'spring modified' Bosch and Lucas floating calipers are still behaving absolutely superbly and I have no reason at all to need or want to swap any of them for something 'better'...

Regards,
John.
 

·
Registered
Joined
·
276 Posts
The force from pressure acting on the caliper body is completely opposed by the caliper body itself, same as any other pressure vessel until it bursts. The reason the piston can apply force on the brake pads is it can move, and it's the movement of the piston that pushes the caliper back. The opposition of the brake pads being clamped is what causes the pressure. Or: Pressure is a function of the clamp load and the piston area. Try to put force on any part of the caliper body, it won't create pressure in the fluid, only if you put force on the piston (and the hydraulic circuit is closed).

The brembo calipers produce about double the clamp load for the same pressure. This moves the brake bias forward quite a bit, it's a downside and a reason not to upgrade to brembo calipers. Definitely in low grip conditions the front bias is a downside. On high grip surfaces it doesn't matter as much because theres a lot more load transfer to the front. Thats one reason why at least on the giulietta alfa also uprated the rear brakes if you got the factory option brembos.

Also, if you want to calculate the leverage, you have to do so at the distance between the centre of the brake pad and the centre of the axle. Brembos have much wider brake pads than the orginal brakes so that brake force gain is much less than you think.

Brake rollers are very simple devices, they measure the current needed to keep the roller turning at the same speed. Therefor the current is a measure of the applied brake fore. If one pad isn't contacting the disc at all, with a sliding or floating caliper, the other pad is pressed twice as hard on the disc (same force, half the area). Same retardation, same heat production (going into the disc as it has a much lower resistance to heat flow) but double the wear on the one pad.... Nasty things will happen after a while though and in practive pads are never completely seized or I wouldn't get them out without cutting. Eventually the seized pad will move but it wont move back after brakes are released, keep generating heat until it wears off enough and the cycle repeats. If it's the pins seized or the pad, doesn't matter. Impossible to diagnose watching just brake force.

However a seized piston shows immediately, and if both pads are impeded it also shows.

I work for hyundai but they don't offer fixed calipers on any cars in my country otherwise we could take some measurements on the rollers and do the maths together vs sliding calipers. I don't have diagnostic software yet for my Giulietta (lucky me) or we could do the same there. Brake hydraulic pressure is measured by the ABS, and rollers show the brake force.
 

·
Registered
Joined
·
571 Posts
The force from pressure acting on the caliper body is completely opposed by the caliper body itself, same as any other pressure vessel until it bursts.
If the piston were seized then yes all of the internal pressure must be entirely opposed / contained by the rigidity of the caliper body (CB), because it has become a sealed pressure vessel in which there is no possibility for pressure to escape, other than for it to burst.

But if the piston is not seized, then pressure can escape by moving the piston. Force acting to move the piston must ultimately be “opposed” by something outside of the CB, or else the piston will be pushed all the way out...
-----------------------
The reason the piston can apply force on the brake pads is it can move,
Yes. With a fixed caliper, in response to cylinder pressure, all individual pistons move and so apply a direct force against the pad to which each piston is directly adjacent, but not against the other pad. With a fixed caliper only the pistons apply force to the pads, which is not the case with floating calipers.

With a floating caliper, in response to cylinder pressure, the piston moves and so applies a direct force only against the inner pad. With a floating caliper it isn’t the piston which applies a force against the outer pad, it is the movement of and force transferred through the CB that does this. Both the piston and the CB move in response to fluid pressure. It isn’t the piston which ‘pushes’ the CB, it is the fluid itself which moves both the piston AND the CB.

The reason that the body of a floating caliper can, in response to cylinder pressure, apply force on the outer pad is because the CB can slide on the pins (unlike a fixed caliper where the body cannot move at all). The only reason that the piston can apply force on the inner pad is because the piston can slide in the cylinder in the CB.

If we think of the piston only as an object which is moved by a particular force acting upon it, and also think of the CB only as an object that moves in the same manner (but opposite direction) when acted on by the same force, we can see that the piston and CB are very similar to each other in their action / behaviour, despite looking physically quite different to each other.

This is a dynamic littered with chickens and eggs, with interacting forces and resistances, all dependant on each other to work as intended.
-------------------------
and it's the movement of the piston that pushes the caliper back.
It is the fluid between the piston and the CB which moves the CB, not the piston. How does the piston ‘push’ the CB when it is moving away from the CB?

Cylinder pressure ‘pushes’ against the piston and moves it in one direction, while simultaneously ‘pushing’ against the CB oppositely so moving it in the opposite direction. In theory, if both the piston and CB had the same mass, and neither were in any way constrained by any outside impediment (such as any even minor resistance or a slight friction against some external surface), then in response to an internal pressure increase both the CB and the piston would move to an equal extent but in opposite directions.
----------------------------
The opposition of the brake pads being clamped is what causes the pressure. Or: Pressure is a function of the clamp load and the piston area. Try to put force on any part of the caliper body, it won't create pressure in the fluid, only if you put force on the piston (and the hydraulic circuit is closed).
Yes, the only ‘rigid’ part of the caliper is the body casting. I think you must be misunderstanding something I’ve said, but I don’t know what. I think it is my fault in explaining this rather clumsily, if I was more articulate it might be more concise and clearer...

System pressure is a function of force input at the master cylinder, MC piston diameter, caliper piston diameter, and the resistance to movement of the caliper pistons AND the caliper bodies. Of course with fixed calipers the bodies are by definition 100% resistant to being moved, so only resistance to piston motion is concerned (not counting caliper ‘spreading’ with higher forces acting in the caliper).
--------------------------
The brembo calipers produce about double the clamp load for the same pressure. This moves the brake bias forward quite a bit, it's a downside and a reason not to upgrade to brembo calipers.
I agree that at a given fluid pressure the Brembo fixed calipers must create more clamping force than the Bosch floating calipers. My numbers say 6.5% more clamping force based on relative total piston area of each caliper. The Brembo has 7.8 square inches of area, the Bosch has 3.7 (near enough). This does not IMO mean that the Brembo has near twice the clamping force of the Bosch, it is not that simple.

But, 6.5% more clamping may possibly be enough, in conjunction with larger OD discs, to cause the significant forward brake bias issues that you say exist when Brembo front calipers are fitted(?). I strongly suspect that if there were a 200% increase in clamping force at the front brakes only (when Brembo front calipers replaced Bosch calipers), then the adverse affect on the cars’ braking performance would be rather spectacular, especially on a slippery surface...

So for that and other reasons I cannot agree with the proposition that the Brembo calipers produce around twice the clamping force as the Bosch calipers. Yes, the Brembo piston area is very roughly about twice that of the Bosch calipers, but because of differences in the ways in which fixed vs sliding calipers work this does not equate to twice the clamping force with the Brembo caliper vs Bosch.

With a sliding caliper, fluid pressure not only pushes the piston out of the CB, the same pressure also pushes the CB in the opposite direction to the movement of the piston, and does so with an equal force. It is the CB sliding on the pins which allows this movement and directional application of force via the CB to occur.

Because the piston moves one way, and the CB moves the other way, both the piston and the CB exert an equal force but in opposite directions. The piston pushes against the inside disc face through the inner pad, and the CB ‘pulls’ the outer pad against the outside disc face.

"For every action there is an equal and opposite reaction", and this is an example of Newtons’ insight.

If the CB is free to move along the same axis as the piston, which it is only if the CB is mounted on sliding pins (or maybe some less common ‘pivoting’ arrangement), then the only thing which limits the distance it moves (opposite direction to the piston), are the two 'finger' projections at the very outermost part of the CB.

The piston moves outward, until it pushes the inside pad into disc contact. The CB also slides in the opposite direction until the CB ‘fingers’ pull the outer pad into contact with the disc. The piston and the CB now cannot move any further, so any further increase in fluid pressure now causes an increase in clamping force, equally applied on either side of the disc. The inner face of the disc is being clamped by the force of the ‘moving’ piston, and the outside disc face is being clamped by the force of the ‘moving’ CB. Both the piston and the CB are moving in response to increasing fluid pressure inside the caliper cylinder. The forces acting on both the piston and the CB are equal in strength (according to Sir Isaac).

So in effect the CB acts as an additional ersatz piston (even if it does not look remotely similar to a conventional piston...), because fluid pressure moves it in the opposite direction to motion of the actual piston, and does so with equal force. This is why X pressure acting inside a single cylinder of a given diameter will in a floating caliper create twice the total caliper clamping force as X pressure creates in a single cylinder of a given diameter in a fixed caliper.

So for example;

Say we have a floating caliper with a single 55mm piston and cylinder on one side of the disc (obviously for a floating caliper). At X fluid pressure, the clamping force created by this caliper will be the same as that of a fixed caliper with a 55mm piston and cylinder on BOTH sides of the disc.

This is because in a fixed caliper, in each cylinder there is only one piston and no ‘in effect’ second ‘piston’. However each cylinder in a floating caliper has in effect two pistons, a real piston and an ersatz piston that looks deceptively like a caliper body (well, it is that too). The CB is a vector for movement and force transfer as is to an equal degree the ‘real’ piston, even though the CB isn’t an actual piston per se.

Does that make any sense? It's geting hard to tell through the headache...

---------------------------
Brake rollers are very simple devices, they measure the current needed to keep the roller turning at the same speed. Therefor the current is a measure of the applied brake fore. If one pad isn't contacting the disc at all, with a sliding or floating caliper, the other pad is pressed twice as hard on the disc (same force, half the area). Same retardation, same heat production (going into the disc as it has a much lower resistance to heat flow) but double the wear on the one pad....
Sounds to me like; with a defective caliper operating only one of its’ pads (vs a good caliper operating both pads), to keep the roller rpm the same, that the pedal force must be increased? That is, to create the same “applied brake force” (i.e. retardation at the brake) the brake fluid pressure would need to be raised if the caliper is faulty. If I’m understanding this correctly, I think I’m seeing an orange being compared to an apple...
--------------------------
However a seized piston shows immediately, and if both pads are impeded it also shows.
I don’t know what exactly you are seeing or why. There may be more things going on than you are observing or intuiting. There might be good reasons why the results on the brake roller are not similar between faulty ‘fixed’ vs faulty ‘floating’ calipers?

Regards,
John.
 

·
Registered
Joined
·
571 Posts
So, trying to pull this all together into something less rambling, and hopefully more coherent.

Fixed caliper:
The operation is fairly straightforward and easy to understand. Not much can go wrong other than one or more pistons can become seized. If a piston completely seizes then the clamping force normally created by that piston ceases to exist. Of course the piston seal can leak, but that is outside the scope of this discussion.

Floating caliper:
First, don’t forget Newtons’ Third Law of Motion; “for every action there is an equal and opposite reaction”. This means that if a piston exerts a force in one direction then there is an equal force which acts in the opposite direction, and acts on the body in which the piston is housed, which in turn exerts an equal force on whatever the body is rigidly attached to or loosely ‘leaning’ against, etc...

I’ve seen floating calipers with one and two pistons, and I’m sure there will be some with more than that. To help keep this simpler, lets’ assume a caliper with only one piston (the same principles still apply to multi cylinder calipers).

Fluid pressure causes the piston to move outward (away from the car body), sliding in the CB cylinder bore. The same pressure in the same fluid also causes the entire CB casting to move inward (toward the car body), sliding along the ‘slider’ pins.

The fluid pressure easily moves the piston outward, which in turn pushes the inner pad outward, until the pad firmly contacts the inner disc face. The fluid pressure also easily moves the CB inward, and in turn the CB ‘pulls’ the outer pad inward until the pad firmly contacts the outer disc face. From this point on, any significant increase in fluid pressure causes an exponential rise in clamping force, and so in braking effect.

According to Newton the force with which the piston moves outward must be of equal magnitude to the force which moves the CB inward. This is why the clamping force is normally equal on each side of the disc.

But if the CB cannot move due to seized pins, then at face value the CB cannot move so is unable to ‘pull’ the outer pad into contact with the disc (it isn’t quite that simple though).

The question in this discussion seems to have been; When the pins become seized, does the inner pad ‘step up’ and exert substantially more force against the inner disc face than it normally does when the pins are not seized, and if yes does this mean that braking effect is not greatly affected?

I cannot see any mechanism that makes this a possibility. The piston can only exert linear force commensurate with the fluid pressure pushing the piston out of the CB. An increase in piston force means that the fluid pressure must increase, and if we are accepting a rise in fluid pressure, then it’s apples and oranges time...

So what can go wrong with a floating caliper;

The piston can seize, ranging from a partial to complete seizure. If the piston completely seizes then regardless of how high the fluid pressure becomes the caliper will completely cease to function. If the piston only partially seizes, clamping force will be reduced equally at both pads, to whatever degree depending on how badly seized the piston is.

And / or the sliding pins can seize, partially or completely. For simplicity lets’ assume both pins are seized and the piston works perfectly. If the pins completely seize then the caliper function will be impaired but it will still work with some degree of reduced retardation, ranging from slight to fairly severe. Despite seized pins the caliper still has significant function because at the very least the piston still works.

But, even if the pins become completely seized, it is still possible for the outer brake pad to still be moved into relatively forceful contact with the disc. This is possible due to flexure of various parts.

Regardless of pin seizure the piston can still move freely. Fluid pressure pushes the piston outward against the inner pad, and so the pad against the inner disc face. This will happen with no loss of force since the piston is still free to move without internal or external hindrance until the pad contacts the disc. The piston will move with a force commensurate with its’ diameter and the fluid pressure acting behind it, and so the inner pad will be forced against the inner disc face as per normal.

But if the pins are seized then this will cause some parts to flex under the pressure of the piston being forced outward from the CB. The CB is not free to slide on the seized pins, and so the CB ‘fingers’ cannot freely pull the outer pad toward and against the outer disc face. The piston is still working normally and so forces the inner pad against the inside disc face. This causes the disc to flex outward because there is no equally strong force pushing against the disc in the opposite direction (because the CB cannot slide on the pins).

Now note that the static gaps between the disc and pad faces are usually not huge, keep that in mind...

The laterally unsupported disc will be flexing outward under the potentially considerable force exerted by the piston, so the outer disc face can move into contact with the outer pad (even if the outer pad doesn't move inward). This is opposite to normal service whereby the outer pad is pulled inward into contact with the disc. Despite seized pins the disc flexure means that the outer pad can still come into reasonably strong contact with the disc face, and thus generate some significant degree of friction and so retardation.

But the disc is not the only part that may significantly flex due to seized caliper pins.

The fluid pressure also pushes laterally and strongly against the CB, but just because CB cannot slide on the pins doesn’t mean this has no consequence. Because the pins are seized this force will cause at least some degree of stressed flexure in the caliper bracket (rather than the CB slide freely). This slight bracket flex will cause the CB to move at least slightly inward, in a somewhat similar way as it would move on the pins, but more ‘strained’ and restricted.

When this occurs, the parts of the CB that are positioned to the inside of the disc will move slightly further inward and so slightly further away from the disc. The parts of the CB that are to the outside of the disc will also move slightly inward, but this means those parts of the CB will move slightly closer to the disc. This includes the CB ‘fingers’ which pull the outer pad into contact with the disc (the ‘fingers’ being the outermost parts of the CB that ‘hook’ over the outer pad).

Because of this caliper bracket flexure, the CB ‘fingers’ will pull the outer pad slightly inward, which will move it toward and likely into significant contact with the outer disc face. This effect will be added to the effect of the piston causing the disc to flex, so there may be coincidental significant contact between the outer pad and outer disc face from more than one flexure.

So when the pins have become seized, the inner pad acts pretty much as per normal (because the piston still works), and there are at least two points of stressed flexure that have the potential to allow the outer pad to still exert a significant force against the outer disc face. The flexures are the disc flexing outward, and the caliper bracket flexing inward.

Given enough time and (ab)use, the outer pad will still wear thinner, until the various flexures are no longer great enough to allow the outer pad to come into significant contact with the outer disc face. After this the outer pad ceases to create friction, but the inner pad will still be working because the piston is still able to push it into disc contact. I think that once the outer pad stops contacting the disc, braking effect will at best be only half of normal, because only one pad is working.
------------------

I don’t know if I‘ve succeeded in explaining this much better, but at least I’ve had a crack at it.

I think this (above) might explain why you (Jetronic) have found that the brake dynamometer easily detects loss of braking capacity due to piston seizure but not so much with pin seizure. IMO it is probably because piston seizure is far more likely to cause a substantially greater loss of braking effect than is pin seizure. As simple as that...?

Regards,
John.
 

·
Registered
Joined
·
938 Posts
Discussion Starter #35 (Edited)
Just to update the original post. I am still having issues getting a pedal. I've heavily bled all four corners and the master cylinder banjos, but the pedal just goes to the floor.

What is interesting is that when I pump the brakes they go hard temporarily, then lose resistance in fairly short order...

I have spoken to my father, who is pretty good on things like this and he reckons I might have a dodgy seal in the master cylinder. The next step is to recover the car down to my local mechanic [non-specialist sadly] and see if he can bleed them on his hydraulic system. Failing that, I will try a new master cylinder.
 

·
Registered
Joined
·
571 Posts
So to be clear, the pedal can be pumped up, but if you maintain foot pressure then it slowly sinks to the floor?

If that is what is occurring then I tend to agree with your father, the MC is probably leaking. Assuming a correct diagnosis, fluid (and pressure) leaks internally past a seal, from the cylinder into the reservoir, so there is a leak but with no external sign of one.

Regards,
John.
 

·
Registered
Joined
·
938 Posts
Discussion Starter #37 (Edited)
So to be clear, the pedal can be pumped up, but if you maintain foot pressure then it slowly sinks to the floor?

If that is what is occurring then I tend to agree with your father, the MC is probably leaking. Assuming a correct diagnosis, fluid (and pressure) leaks internally past a seal, from the cylinder into the reservoir, so there is a leak but with no external sign of one.

Regards,
John.
Yes, if you pump the brakes, pressure, or resistance is generated on the pedal, but if you then maintain pressure you can quite easily push the pedal down to the floor.
 

·
Registered
Joined
·
938 Posts
Discussion Starter #38
Confession time. I think I have been a bit of an idiot and overlooked the obvious.

I've just spent all afternoon, bleeding the ABS, then the master cylinder banjos... then it occurred to me - these Brembos have a second bleed nipple...[inboard and outboard] I've always just bled through one - I mean its all the same system, right?!

It would seem not.

So, I gave the other nipple a good bleed and I now have a pedal!

I haven't driven the car yet - its got no wheels on, but I hope this will be okay. I'll report back later in the week.
 

·
Registered
Joined
·
938 Posts
Discussion Starter #39
As i suspected, it's tight in there. Maybe 2mm of clearance. That said the wheels fill the arches nicely IMO. I have an Eibach Pro kit with B4s.

938684
938685
 
21 - 40 of 41 Posts
Top