Sprint rear brake bias valve.

[Carledo]

I set a lot of this off asking about what brake effort the single piston sliding caliper gives and saying I wasn't familiar - with an apology for and admission of getting rather wrong. So I hope those who are familiar with these devices will understand if there's some error of detail and correct me. But this drawing is what I understand of its general operation, intended as a schematic not a detailed design and done without too much modification of what I've already drawn for the 2 pot fixed caliper it's equivalent to.



Graham

Assuming the upper horizontal bar in the drawing represents the slider, then I understand the drawing and it's an accurate representation of how a floating caliper works. Trying to accurately draw the sliders where they really are would have been confusing at best when portrayed from that angle.

Steve

[GrahamFountain]

If you were to have a 4 pot caliper, with 4 separate pads, it would give exactly the same brake effort as a four pot one with two pads. Also, two, two pot calipers, acting on the same disc, give you twice the brake effort of one.

Question for Steve:
Should I be shot for mentioning two two calipers or does that needs a round of applause.

Graham

Shot no, but no applause either, I simply understood what you meant. An extra comma between the two twos might have helped, however i'm not getting into grammar too. Just for fun i've tentatively punctuated that sentence in the quoted section above this text! No criticism implied or intended

Steve

It's not just applause asked for the appalling pun with .22 caliber, but the additional contextual play between "shot" and "round".

Also, I still got a 1st ed of Fowler's Modern English Usage.

Graham

[Carledo]

If you were to have a 4 pot caliper, with 4 separate pads, it would give exactly the same brake effort as a four pot one with two pads. Also, two, two pot calipers, acting on the same disc, give you twice the brake effort of one.

Question for Steve:
Should I be shot for mentioning two two calipers or does that needs a round of applause.

Graham

Shot no, but no applause either, I simply understood what you meant. An extra comma between the two twos might have helped, however i'm not getting into grammar too. Just for fun i've tentatively punctuated that sentence in the quoted section above this text! No criticism implied or intended

Steve

It's not just applause asked for the appalling pun with .22 caliber, but the additional contextual play between "shot" and "round".

Also, I still got a 1st ed of Fowler's Modern English Usage.

Graham

Whereas I only got an "O" level! 50 years ago! The pun was too subtle for me, missed it entirely, sorry!

Steve

[jikovron]

I mentioned unbolting the caliper but as I was concentrating on strut compression I was thinking of a disc purely clamped with no rotational torque.

Your example , if the second pad of the fixed single piston caliper is also fixed to the suspension leg then the suspension leg and bracketry is the replacement system that force is transmitted through instead of the the sliding caliper body, just the sliding disc moves instead , so is basically the same.

I may setup a load cell and bottle jacks in a frame , well if I get out of hospital haha

[GrahamFountain]

then the suspension leg and bracketry is the replacement system that force is transmitted through

No it's not. It's the frame.

Graham

[jikovron]

Brackety/ frame,,,it does the same job, the hydraulic piston presses the pad into the disc,which slides along till it meets the other pad, then as the sandwich of parts meets resistance the force grows as per the sliding caliper surely.
You could even say the disc from its frame of reference stays still, and it's the fixed caliper, other fixed pad and whole car that slides
Looks like I'll be setting up load cells and bottle jacks as a practical test.

[GrahamFountain]

Brackety/ frame,,,it does the same job, the hydraulic piston presses the pad into the disc,which slides along till it meets the other pad, then as the sandwich of parts meets resistance the force grows as per the sliding caliper surely.
You could even say the disc from its frame of reference stays still, and it's the fixed caliper, other fixed pad and whole car that slides
Looks like I'll be setting up load cells and bottle jacks as a practical test.

Hmm, I need to think about that a bit.

Graham

[GrahamFountain]

So it's clearer, these are the various different arrangements I think have been discussed so far, shown A to E all in one diagram:



I think we've, finally, reached agreement that A and B are different, and that, for the same individual piston area, distances between piston to hub centres, and hydraulic pressures, the two pistons in B give twice the brake effort of the single piston in A. That's because the force between the disc and the one pad in A is half then total force between the disc and the two pads in B, not because there's any more area of pad or number of pads in B. That two pistons pressing separately on the disc gives twice what one piston on its own gives is what I always thought was obvious and didn't understand why it was being argued against.

B is, of course, the two piston fixed calliper as per the Sprint, and C is the single piston sliding calliper, as per the Ford Sierra, etc., but normalized to the same sizes as the Sprint one. I think that there is and nearly always was agreement that B and C are equivalent and, for the same individual piston area, distances between piston to hub centres, and hydraulic pressures, give the same brake effort.

In case there's any doubt (please God no), C gives the same as B because the piston and cylinder both act to apply force to compress the pad-disc-pad sandwich and so that the one piston in C gives twice the force of the one piston in A, and thus the same as B.

D is what happens to B if one of the piston seizes and to C if the slider seizes. That is, only one piston is able to act on the fixed disc. I hope it's clear, it's the same as A, which gives half the brake force of B and C.

E is a peculiarity. It is the single piston fixed calliper sliding disc I introduced (spuriously it turns out) when trying to excuse why I got confused by the Sierra calliper. Though I did rapidly realized why C gives the same brake effort as B. I don't know that E exists anywhere outside this thread (but I hope not). My guess is, that if it does exist, the disc rattles about something horrible when not under braking.

I have to admit I was wrong in thinking that E was different from C. And I now realize they are the same. However, it was never intended as a suggestion that such an option as E could ever be a good idea for a brake system, any more than A, and frankly, I so wish I'd never mentioned it.

What may have been part of the confusion is what happens to B if one of the pistons seizes and to C if the slider seizes. Assuming the piston/slider seizes so it's pressing the pad that's no longer operated close up to the disc, and there's more end float in the hub bearing than needed to close the disc against it, then, initially at least, E works more or less like C. However, as the pad that's not operated wears away, the compliance in the bearing won't be enough to allow the disc to press on it, this setup will become like D, and give half the brake effort of B or C, like they do when they're part seized that way.

Graham

[Carledo]

Quick notes on floating caliper slider failures/seizures, whilst it's not universal, nearly every floating caliper in my experience has had 2 separate sliders placed roughly above and below the piston, some are identical and some differ, there are several different designs too, some being more prone to seizure than others. Original and best is the Girling/ATE "Diana" caliper with it's fixed stainless pins that rubber tubes in the caliper body slide on, worst is probably a Japanese design used on Shoguns and other Jap SUVs with a metal pin in a blind metal tube with only a tiny rubber ring to seal it and that on the inner (blind) end of the pin. Horrible! And the only design that regularly seizes BOTH sliders.

But what happens most often is a single slider seizes. This doesn't make a lot of difference to brake effort till pad wear starts to play a part. Mostly, the pads just wear unevenly at an angle. Until the pad wear gets bad enough that the caliper reaches the limit of angle inherent in the design there are no real symptoms. Often nobody notices 'till some poor mechanic has to strip it down and then the fun starts!

Steve

[new to this]

Quick notes on floating caliper slider failures/seizures, whilst it's not universal, nearly every floating caliper in my experience has had 2 separate sliders placed roughly above and below the piston, some are identical and some differ, there are several different designs too, some being more prone to seizure than others. Original and best is the Girling/ATE "Diana" caliper with it's fixed stainless pins that rubber tubes in the caliper body slide on, worst is probably a Japanese design used on Shoguns and other Jap SUVs with a metal pin in a blind metal tube with only a tiny rubber ring to seal it and that on the inner (blind) end of the pin. Horrible! And the only design that regularly seizes BOTH sliders.

But what happens most often is a single slider seizes. This doesn't make a lot of difference to brake effort till pad wear starts to play a part. Mostly, the pads just wear unevenly at an angle. Until the pad wear gets bad enough that the caliper reaches the limit of angle inherent in the design there are no real symptoms. Often nobody notices 'till some poor mechanic has to strip it down and then the fun starts!

Steve

Ive had a couple of cars where caliper sliders seize both vauxhauls as you say dont really notice it on braking,the one i always think of cavalier SRI the disc glowed red

Dave

[jikovron]

So it's clearer, these are the various different arrangements I think have been discussed so far, shown A to E all in one diagram:



I think we've, finally, reached agreement that A and B are different, and that, for the same individual piston area, distances between piston to hub centres, and hydraulic pressures, the two pistons in B give twice the brake effort of the single piston in A. That's because the force between the disc and the one pad in A is half then total force between the disc and the two pads in B, not because there's any more area of pad or number of pads in B. That two pistons pressing separately on the disc gives twice what one piston on its own gives is what I always thought was obvious and didn't understand why it was being argued against.

B is, of course, the two piston fixed calliper as per the Sprint, and C is the single piston sliding calliper, as per the Ford Sierra, etc., but normalized to the same sizes as the Sprint one. I think that there is and nearly always was agreement that B and C are equivalent and, for the same individual piston area, distances between piston to hub centres, and hydraulic pressures, give the same brake effort.

In case there's any doubt (please God no), C gives the same as B because the piston and cylinder both act to apply force to compress the pad-disc-pad sandwich and so that the one piston in C gives twice the force of the one piston in A, and thus the same as B.

D is what happens to B if one of the piston seizes and to C if the slider seizes. That is, only one piston is able to act on the fixed disc. I hope it's clear, it's the same as A, which gives half the brake force of B and C.

E is a peculiarity. It is the single piston fixed calliper sliding disc I introduced (spuriously it turns out) when trying to excuse why I got confused by the Sierra calliper. Though I did rapidly realized why C gives the same brake effort as B. I don't know that E exists anywhere outside this thread (but I hope not). My guess is, that if it does exist, the disc rattles about something horrible when not under braking.

I have to admit I was wrong in thinking that E was different from C. And I now realize they are the same. However, it was never intended as a suggestion that such an option as E could ever be a good idea for a brake system, any more than A, and frankly, I so wish I'd never mentioned it.

What may have been part of the confusion is what happens to B if one of the pistons seizes and to C if the slider seizes. Assuming the piston/slider seizes so it's pressing the pad that's no longer operated close up to the disc, and there's more end float in the hub bearing than needed to close the disc against it, then, initially at least, E works more or less like C. However, as the pad that's not operated wears away, the compliance in the bearing won't be enough to allow the disc to press on it, this setup will become like D, and give half the brake effort of B or C, like they do when they're part seized that way.

Graham

The only detail missing with A and D (not a criticism by any means, just a technical note for clarity only) is that the red reaction force is via the brake disc which I can understand isn't easy to annotate given the space available.
Example A certainly gives half the braking effort as does D when as you rightly say the pad from the other side wears enough that compliance in the disc no longer reaches it.
C,B and E are all the same braking effort wise.

The difference between the 2 groups is that the reaction force for frictional interface of the second pad is missing in A (and gradually is lost through pad wear in D) as the strut compression from the reaction force is stopped and returned by the disc
A way to replicate the same half braking effect with C,B and E would be to add a frictionless roller in place of the second pad as the force would travel through the disc and be contained by the caliper body, but only one frictional interface would exist, the roller 'pad' basically replacing the role of the wheel bearing in Example A (and worn out D)
I think there is a general sense of wavelength alignment.

[Carledo]

Quick notes on floating caliper slider failures/seizures, whilst it's not universal, nearly every floating caliper in my experience has had 2 separate sliders placed roughly above and below the piston, some are identical and some differ, there are several different designs too, some being more prone to seizure than others. Original and best is the Girling/ATE "Diana" caliper with it's fixed stainless pins that rubber tubes in the caliper body slide on, worst is probably a Japanese design used on Shoguns and other Jap SUVs with a metal pin in a blind metal tube with only a tiny rubber ring to seal it and that on the inner (blind) end of the pin. Horrible! And the only design that regularly seizes BOTH sliders.

But what happens most often is a single slider seizes. This doesn't make a lot of difference to brake effort till pad wear starts to play a part. Mostly, the pads just wear unevenly at an angle. Until the pad wear gets bad enough that the caliper reaches the limit of angle inherent in the design there are no real symptoms. Often nobody notices 'till some poor mechanic has to strip it down and then the fun starts!

Steve

Ive had a couple of cars where caliper sliders seize both vauxhauls as you say dont really notice it on braking,the one i always think of cavalier SRI the disc glowed red

Dave

Think you'll find that a glowing disc isn't a symptom of slider problems, but rather a partial seizure of caliper piston(s) creating bind and therefore heat. A drum brake has springs to pull the pistons back to rest, a caliper doesn't, instead relying on some sort of physics effect that, if the caliper is working properly and the piston is free, retract a few thou from the disc naturally. However, if sufficient friction exists between the piston and cylinder of the caliper to inhibit this natural tendency to retract the piston, the result is brake bind and a hot disc. The amount of friction in the piston bore correlating with the amount of bind experienced. This is ALWAYS the result of a partial seizure which happens when the car is in regular use as the pressure your foot can put in is massively more than the trick of physics that pulls the piston back a tad when you take your foot off. It's very rare to get a full piston seizure in a floating caliper on a car in regular use. That little gem is reserved for cars that have stood a few years!

Steve

[soe8m]

Think you'll find that a glowing disc isn't a symptom of slider problems, but rather a partial seizure of caliper piston(s) creating bind and therefore heat. A drum brake has springs to pull the pistons back to rest, a caliper doesn't, instead relying on some sort of physics effect that, if the caliper is working properly and the piston is free, retract a few thou from the disc naturally. However, if sufficient friction exists between the piston and cylinder of the caliper to inhibit this natural tendency to retract the piston, the result is brake bind and a hot disc. The amount of friction in the piston bore correlating with the amount of bind experienced. This is ALWAYS the result of a partial seizure which happens when the car is in regular use as the pressure your foot can put in is massively more than the trick of physics that pulls the piston back a tad when you take your foot off. It's very rare to get a full piston seizure in a floating caliper on a car in regular use. That little gem is reserved for cars that have stood a few years!

Steve

The seal inside the caliper does retract the piston.

Jeroen

[GrahamFountain]

I said that I realized I'd made a mistake about case E and it would give the same effort as B and C (even though it's still one of Bloody Stupid Johnson's ideas for a brake, and even mentioning it was a mistake in its own right).

However, in saying that, I missed explicitly apologizing to Marshman: More or less as he said, if the one force from the hydraulic pressure on the area of the piston in case E, is applied against the other side of a fixed caliper through 2 pads in series, either side of a disc free to move in the direction of the force; then the combined force of friction from the two pads on the disc between them is equal to that applied force times the sum of the coefficients of friction for the interfaces between disc and the two pads.

So the drawing for case E should look like this:


So that describes how, in case E, the single force from the one piston is multiplied by the two interfaces and it gives twice the brake effort of A. But that is quite different from how case C comes to give twice the brake force of A, even though it is the same outcome: In case C, two separate and opposing forces are applied to the fixed disc through the two pads, one from the piston and one, through the slide(s) the calliper hangs on, from the cylinder. In which case, the lateral compression applied to the disc is equal to the sum of the two forces, because in that case, the forces through the interfaces between the pads and the disc go in opposite, not the same, directions.

Graham