The Triumph Dolomite Club - Discussion Forum

The Number One Club for owners of Triumph's range of small saloons from the 1960s and 1970s.
It is currently Wed Apr 21, 2021 8:23 am

All times are UTC




Post new topic Reply to topic  [224 posts ]  Go to pagePrevious11112131415Next
Author Message
PostPosted:Tue Mar 02, 2021 1:53 pm 
Offline
TDC Member

Joined:Tue Oct 15, 2019 8:00 pm
Posts:214
Quote:
I don't think these strained analogies are moving us forward, though that's clearly an irony in a discussion about brakes. And my value of Young's modulus of elasticity isn't up to this level of strain - too much stress.

This is the best I can manage as a drawing of what I see as going on between a single piston and two pistons both in fixed callipers - remembering that the caliper is, in both cases, fixed to the same structure as the disc.
It shows the main pressures (blue), forces (red), and moments (green).

Image

As I hope can be seen, with the single piston caliper, there's unit force on one pad and a moment on the hub as a result of that. There's also a different moment on the caliper mount - same force but different length and opposite rotation. YES

But with the two piston caliper, there's the same force on two pads, i.e. twice the total. It's spread over twice the area, but Amontons' law says that area of contact between the two surfaces doesn't matter. Hence in that situation of twice the force, there's twice the brake effort. This is all that really matters in the analysis of brake effect - its the hydraulic pressure times the sum of the piston areas. YES

If there was a second fixed pad in the case of the single piston caliper and the hub bearing had enough movement to let the disc push up against it there would still be the same force between the two pads as there was on the one pad, i.e. half of what there was with two pistons (hence half the compression in the thickness of the disc), but over twice the surface area. And, as said, area not mattering, the brake effort would be the same as before. It would not give the same effort as the two piston caliper, because there would be half the force, so half the effort.Can't agree with this, you have added a second point of contact/friction - yes dear old Guillaume says friction is independent of area but I am certain he was referring to between 2 single objects, you have added another object/junction, so the brake effort is doubled - it is the same result as a floating calliper, except in this case you have a floating disc!

What happens in the single piston sliding caliper is the same as the two piston fixed caliper because, as said, the force from the back of the cylinder, which is free relative to the disc and suspension, is hooked around and applied through the slider to the second pad.

_________________
1975 Sprint Man O/D in Honeysuckle Yellow
1971 Stag Auto White

Too many cars, too little time!


Top
   
PostPosted:Tue Mar 02, 2021 1:57 pm 
Offline
Future Club member hopefully!
Future Club member hopefully!

Joined:Fri Jul 12, 2019 6:25 pm
Posts:79
And additionally adding a second fixed pad to the first example , you just described a sliding caliper, except the disc is the sliding component instead.


Top
   
PostPosted:Tue Mar 02, 2021 4:03 pm 
Offline
Future Club member hopefully!
Future Club member hopefully!
User avatar

Joined:Sun Sep 08, 2013 2:35 pm
Posts:1545
Location:St Annes on Sea, Lancs.
Quote:
In the second drawing , the green moments equal zero force all of the force is contained by the caliper body and if you unbolt the caliper it wouldn't move.
In the first example the caliper body is forced away from the disc and the reaction force comes from the disc itself.
Compression forces through the pads are the same for both examples, but braking effort is halfed in the first example as there only half frictional effort (equivalent to replacing one pad with one of zero co-ef in example 2.
If you unbolt the two piston fixed caliper from the suspension, it moves with the disc and smashes on the road!

But it's true that, in the second drawn example, the two pistons do produce pairs of moments on the hub and the mounting that cancel. But that means that, as the first piston has not changed effort and the second piston is (near enough) the same distance from the hub, the second piston must be providing a second force that is equal and opposite to the force from the first piston. That these two forces are applied through two pads and so twice the area does not matter. Amontons shows that. They are (both) reacted to by the compression of the pad-disc-pad sandwich, that compression being equal to their sum and uniform throughout.

And Pascal shows that there really is twice the force from the two pistons, because there is the same pressure acting on twice the area. You could, I suppose (or is it fear), try to argue that there really aren't twice the number of pistons or they don't really have twice the area. That might be entertaining.

And it is also true that in the first drawn example the reaction is from the compression of the pad against the disc on the bending moment on the hub. Whereas in the postulated example that there's a second pad on a fixed surface and enough free movement in the hub, the reaction is from compression of the pad-disc-pad sandwich between the piston and the fixed surface.

But Amontons explains perfectly well why the force from one piston gives the same brake effort whether its applied through one or two pads. That is, through one pad against the reaction from the bending moment on the hub or through two pads against the reaction from the fixed surface behind the second pad - that there's only one force from the piston is shown by there still being a moment on the caliper mounting, if the one on the hub is gone (not cancelled).

I don't actually see why it needs any further analysis than that.

I'm actually sure it does not need anywhere near that much, i.e. no more than twice the pistons = twice the hydraulic area = twice the force (at the same hydraulic pressure) = twice the effort.

It certainly does not need any peculiar analysis of the implications of Newton's third law or spurious analogies that show why there isn't really twice the force - there just so is.

Graham

_________________
The 16v Slant 4 engine is more fun than the 3.5 V8, because you mostly drive it on the upslope of the torque curve.

Factory 1977 TR7 Sprint FHC VVC 697S (Now all of, but still needs putting together)
B&Y 73 Dolomite Sprint UVB 274M (kids!)
1970 Maroon 13/60 Herald Convertable (wife's fun car).


Top
   
PostPosted:Tue Mar 02, 2021 4:06 pm 
Offline
Future Club member hopefully!
Future Club member hopefully!
User avatar

Joined:Sun Sep 08, 2013 2:35 pm
Posts:1545
Location:St Annes on Sea, Lancs.
Quote:
And additionally adding a second fixed pad to the first example , you just described a sliding caliper, except the disc is the sliding component instead.
If you had read all the thead, you'd see where I point out that the single piston fixed caliper with a sliding disc gives half the brake effort of a single piston sliding caliper.

Graham

_________________
The 16v Slant 4 engine is more fun than the 3.5 V8, because you mostly drive it on the upslope of the torque curve.

Factory 1977 TR7 Sprint FHC VVC 697S (Now all of, but still needs putting together)
B&Y 73 Dolomite Sprint UVB 274M (kids!)
1970 Maroon 13/60 Herald Convertable (wife's fun car).


Top
   
PostPosted:Tue Mar 02, 2021 4:11 pm 
Offline
Future Club member hopefully!
Future Club member hopefully!
User avatar

Joined:Sun Sep 08, 2013 2:35 pm
Posts:1545
Location:St Annes on Sea, Lancs.
Quote:
Quote:

If there was a second fixed pad in the case of the single piston caliper and the hub bearing had enough movement to let the disc push up against it there would still be the same force between the two pads as there was on the one pad, i.e. half of what there was with two pistons (hence half the compression in the thickness of the disc), but over twice the surface area. And, as said, area not mattering, the brake effort would be the same as before. It would not give the same effort as the two piston caliper, because there would be half the force, so half the effort.Can't agree with this, you have added a second point of contact/friction - yes dear old Guillaume says friction is independent of area but I am certain he was referring to between 2 single objects, you have added another object/junction, so the brake effort is doubled - it is the same result as a floating calliper, except in this case you have a floating disc!
There's nothing in Amontons law that restricts it to single interfaces.

See prev response about a single piston siding disc and single piston sliding caliper being different. But to add, there's no connection from the force on the back wall of the cylinder and the second pad. The sliding caliper is like Steves description of the rear slave where the piston does half the work and the cylinder the other half.

Graham

_________________
The 16v Slant 4 engine is more fun than the 3.5 V8, because you mostly drive it on the upslope of the torque curve.

Factory 1977 TR7 Sprint FHC VVC 697S (Now all of, but still needs putting together)
B&Y 73 Dolomite Sprint UVB 274M (kids!)
1970 Maroon 13/60 Herald Convertable (wife's fun car).


Top
   
PostPosted:Tue Mar 02, 2021 4:24 pm 
Offline
Future Club member hopefully!
Future Club member hopefully!
User avatar

Joined:Sun Sep 08, 2013 2:35 pm
Posts:1545
Location:St Annes on Sea, Lancs.
Quote:
Quote:

you have added a second point of contact/friction - yes dear old Guillaume says friction is independent of area but I am certain he was referring to between 2 single objects, you have added another object/junction, so the brake effort is doubled - it is the same result as a floating calliper, except in this case you have a floating disc!
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 as one.

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

Graham

_________________
The 16v Slant 4 engine is more fun than the 3.5 V8, because you mostly drive it on the upslope of the torque curve.

Factory 1977 TR7 Sprint FHC VVC 697S (Now all of, but still needs putting together)
B&Y 73 Dolomite Sprint UVB 274M (kids!)
1970 Maroon 13/60 Herald Convertable (wife's fun car).


Top
   
PostPosted:Tue Mar 02, 2021 4:47 pm 
Offline
TDC Member

Joined:Tue Oct 15, 2019 8:00 pm
Posts:214
Quote:

There's nothing in Amontons law that restricts it to single interfaces.

See prev response about a single piston siding disc and single piston sliding caliper being different. But to add, there's no connection from the force on the back wall of the cylinder and the second pad. The sliding caliper is like Steves description of the rear slave where the piston does half the work and the cylinder the other half.

Graham
1. There is also nothing in it that indicates that it holds for multiple interfaces either! All references that I can find refer to two surfaces or bodies, in your example adding a second pad makes it effectively four bodies or surfaces. A practical example is the old friction lever dampers which had a sandwich of multiple friction faces.

2. If "there's no connection from the force on the back wall of the cylinder and the second pad." then what is the cylinder pushing against?

Roger

_________________
1975 Sprint Man O/D in Honeysuckle Yellow
1971 Stag Auto White

Too many cars, too little time!


Top
   
PostPosted:Tue Mar 02, 2021 6:17 pm 
Offline
Future Club member hopefully!
Future Club member hopefully!
User avatar

Joined:Sun Sep 08, 2013 2:35 pm
Posts:1545
Location:St Annes on Sea, Lancs.
Quote:
Quote:

There's nothing in Amontons law that restricts it to single interfaces.

See prev response about a single piston siding disc and single piston sliding caliper being different. But to add, there's no connection from the force on the back wall of the cylinder and the second pad. The sliding caliper is like Steves description of the rear slave where the piston does half the work and the cylinder the other half.

Graham
1. There is also nothing in it that indicates that it holds for multiple interfaces either! All references that I can find refer to two surfaces or bodies, in your example adding a second pad makes it effectively four bodies or surfaces. A practical example is the old friction lever dampers which had a sandwich of multiple friction faces.

2. If "there's no connection from the force on the back wall of the cylinder and the second pad." then what is the cylinder pushing against?

Roger
1. The situation of changing a 4 pot caliper from two to four pads is exactly the same as you complain I'm doing without taking account of, i.e. adding more surfaces. If that doubled the stopping power of the caliper they would all be like that. Doubling the 2 pot caliper doubles the number of surfaces (and their total area) and force (number of pistons) and yet only doubles the effect - if it quadrupled the effect, they'd all be doing that rather than upping the number of pistons on the same number of pads and or making the pistons bigger and or moving pads and pistons further out.

2. If you look at the diagram, you will see that the cylinder is in the caliper and that generates a bending moment on the mounting at the top, which reacts equally to that to hold the caliper still - a spectacularly bad idea for a design, but it is just an example.

Graham

_________________
The 16v Slant 4 engine is more fun than the 3.5 V8, because you mostly drive it on the upslope of the torque curve.

Factory 1977 TR7 Sprint FHC VVC 697S (Now all of, but still needs putting together)
B&Y 73 Dolomite Sprint UVB 274M (kids!)
1970 Maroon 13/60 Herald Convertable (wife's fun car).


Top
   
PostPosted:Tue Mar 02, 2021 6:59 pm 
Offline
TDC Member

Joined:Tue Oct 15, 2019 8:00 pm
Posts:214
I wasn't complaining! It was an observation :)

Not sure what the two pot/ four pot conversation has to do with it and I am not sure I want/need to know :wink:

Yes I can see the cylinder is in the calliper and that there is a bending moment but what is the calliper attached to? One assumes at some point all the bits are connected together and the circle is completed otherwise there will be no force generated - i.e. what is effectively a sliding calliper or in your example a moving disc. You can't push against something and create a permanent (constant) force unless you are attached to it in some way otherwise it will simply be pushed away and in this case the disc will likely break as it runs out of ability to bend towards the second pad.

_________________
1975 Sprint Man O/D in Honeysuckle Yellow
1971 Stag Auto White

Too many cars, too little time!


Top
   
PostPosted:Tue Mar 02, 2021 8:24 pm 
Offline
Future Club member hopefully!
Future Club member hopefully!
User avatar

Joined:Sun Sep 08, 2013 2:35 pm
Posts:1545
Location:St Annes on Sea, Lancs.
Quote:
I wasn't complaining! It was an observation :)

Not sure what the two pot/ four pot conversation has to do with it and I am not sure I want/need to know :wink:

Yes I can see the cylinder is in the calliper and that there is a bending moment but what is the calliper attached to? One assumes at some point all the bits are connected together and the circle is completed otherwise there will be no force generated - i.e. what is effectively a sliding calliper or in your example a moving disc. You can't push against something and create a permanent (constant) force unless you are attached to it in some way otherwise it will simply be pushed away and in this case the disc will likely break as it runs out of ability to bend towards the second pad.
Sorry, as I said, my value of Young's modulus is a bit low and there's a lot of strain herein. No. I mean high: Stress = E x Strain.

I understand about not being interested in the number of surfaces/pads/objects/junctions counting to the force of friction. It's so clearly a none issue. But I didn't raise the issue. And if it did count (and there's a new law for you to write to the Roy Soc about) it wouldn't matter if they were on the same or different sides. So putting two pads one under each of the two pistons on one side of a 4 pot caliper, would double the number of surfaces/objects/junctions as you at least appeared to suggest would double the force of friction: "you have added another object/junction, so the brake effort is doubled". I was just pointing out the logical inconsistency in that statement. If you no longer see it as an issue, I don't care either.

I have not shown the sliding caliper in the drawings. I thought it was a step too far while there was so much misunderstanding of the two pot fixed caliper.

But I have said it works like the rear slave in Steve's description, the piston works one pad and the cylinder works the other, by what amounts to a hook, to double the effort from the same hydraulic pressure. In which case, the compression on the pad-disc-pad sandwich is (losses aside) exactly the same as the expansion caused between the piston and the back wall of the cylinder, i.e. twice the hydraulic pressure times the area of the piston. But because it works both pads, it takes as much energy to operate as the two piston fixed caliper set-up.

Graham

_________________
The 16v Slant 4 engine is more fun than the 3.5 V8, because you mostly drive it on the upslope of the torque curve.

Factory 1977 TR7 Sprint FHC VVC 697S (Now all of, but still needs putting together)
B&Y 73 Dolomite Sprint UVB 274M (kids!)
1970 Maroon 13/60 Herald Convertable (wife's fun car).


Top
   
PostPosted:Tue Mar 02, 2021 8:59 pm 
Offline
Future Club member hopefully!
Future Club member hopefully!
User avatar

Joined:Sun Sep 08, 2013 2:35 pm
Posts:1545
Location:St Annes on Sea, Lancs.
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.

Image

Graham

_________________
The 16v Slant 4 engine is more fun than the 3.5 V8, because you mostly drive it on the upslope of the torque curve.

Factory 1977 TR7 Sprint FHC VVC 697S (Now all of, but still needs putting together)
B&Y 73 Dolomite Sprint UVB 274M (kids!)
1970 Maroon 13/60 Herald Convertable (wife's fun car).


Top
   
PostPosted:Tue Mar 02, 2021 9:12 pm 
Offline
TDC Member

Joined:Tue Oct 15, 2019 8:00 pm
Posts:214
Quote:

But I have said it works like the rear slave in Steve's description, the piston works one pad and the cylinder works the other, by what amounts to a hook, to double the effort from the same hydraulic pressure. In which case, the compression on the pad-disc-pad sandwich is (losses aside) exactly the same as the expansion caused between the piston and the back wall of the cylinder, i.e. twice the hydraulic pressure times the area of the piston. But because it works both pads, it takes as much energy to operate as the two piston fixed caliper set-up.

Graham
??? Difficult to follow but I am sure that contradicts what you said a few posts back.

You said .... See prev response about a single piston siding disc and single piston sliding caliper being different. But to add, there's no connection from the force on the back wall of the cylinder and the second pad. The sliding caliper is like Steves description of the rear slave where the piston does half the work and the cylinder the other half.

I asked ..... If "there's no connection from the force on the back wall of the cylinder and the second pad." then what is the cylinder pushing against?

You said ....... If you look at the diagram, you will see that the cylinder is in the caliper and that generates a bending moment on the mounting at the top, which reacts equally to that to hold the caliper still - a spectacularly bad idea for a design, but it is just an example.

I asked .......... Yes I can see the cylinder is in the calliper and that there is a bending moment but what is the calliper attached to? One assumes at some point all the bits are connected together and the circle is completed otherwise there will be no force generated - i.e. what is effectively a sliding calliper or in your example a moving disc. You can't push against something and create a permanent (constant) force unless you are attached to it in some way otherwise it will simply be pushed away and in this case the disc will likely break as it runs out of ability to bend towards the second pad.

You said ....... But I have said it works like the rear slave in Steve's description, the piston works one pad and the cylinder works the other, by what amounts to a hook, to double the effort from the same hydraulic pressure. In which case, the compression on the pad-disc-pad sandwich is (losses aside) exactly the same as the expansion caused between the piston and the back wall of the cylinder, i.e. twice the hydraulic pressure times the area of the piston. But because it works both pads, it takes as much energy to operate as the two piston fixed caliper set-up.


So you have just said that there is a connection between the back wall of the cylinder and the second pad via "the hook". So It is the same as a sliding calliper !!!!!!

Entertaining though it is I am going to leave it there until I can think of a practical way of actually demonstrating that the compression force is not the sum of the individual forces on the pads on either side of the disc.

Roger

_________________
1975 Sprint Man O/D in Honeysuckle Yellow
1971 Stag Auto White

Too many cars, too little time!


Top
   
PostPosted:Tue Mar 02, 2021 9:15 pm 
Offline
TDC Member

Joined:Tue Oct 15, 2019 8:00 pm
Posts:214
whoops duplicate post!

_________________
1975 Sprint Man O/D in Honeysuckle Yellow
1971 Stag Auto White

Too many cars, too little time!


Top
   
PostPosted:Tue Mar 02, 2021 9:53 pm 
Offline
Future Club member hopefully!
Future Club member hopefully!
User avatar

Joined:Sun Sep 08, 2013 2:35 pm
Posts:1545
Location:St Annes on Sea, Lancs.
Quote:

So you have just said that there is a connection between the back wall of the cylinder and the second pad via "the hook". So It is the same as a sliding calliper !!!!!!

Roger
It's clearly getting confusing.

In the single piston fixed caliper, there is no connection between the caliper and the pad that can allow the force to be applied to it. Rather, the caliper is rigidly mounted to the suspension leg, as, in some way, the fixed pad would be. In that case (were this a sensible design) it would be the pad the piston bears on that would push the disc along the axle to press on the second, fixed pad.

Whereas, in the sliding caliper, the pad on the opposite side to the piston is mounted directly on the caliper (see https://forum.triumphdolomite.co.uk/vie ... 95#p336104) which can slide on the mounting to the suspension leg. So when the hydraulic pressure pushes the piston against its pad, the hydraulic pressure also pushes the caliper back in the opposite direction and with the same force, so that the pad on the other side, attached to the sliding caliper, presses on the other side of the disc with equal force to that with which the piston pushes its pad. So the compression in the pad-disc-pad sandwich is twice the force the piston pushes on its pad with and the same as the two piston fixed caliper.

As to "demonstrating that the compression force {I assume on the pad-disc-pad sandwich} is not the sum of the individual forces on the pads on either side of the disc." Good luck with that. However, I suspect it would require the smoking of a lot of something green and very smelly.

Graham

_________________
The 16v Slant 4 engine is more fun than the 3.5 V8, because you mostly drive it on the upslope of the torque curve.

Factory 1977 TR7 Sprint FHC VVC 697S (Now all of, but still needs putting together)
B&Y 73 Dolomite Sprint UVB 274M (kids!)
1970 Maroon 13/60 Herald Convertable (wife's fun car).


Top
   
PostPosted:Tue Mar 02, 2021 9:58 pm 
Offline
TDC Shropshire Area Organiser

Joined:Sun Aug 21, 2011 4:12 pm
Posts:6086
Location:Highley, Shropshire
Quote:

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 :lol: :lol: :lol:

Steve

_________________
'73 2 door Toledo with Vauxhall Carlton 2.0 8v engine (The Carledo)
'78 Sprint Auto with Vauxhall Omega 2.2 16v engine (The Dolomega)
'72 Triumph 1500FWD in Slate Grey

Maverick Triumph, Servicing, Repairs, Electrical, Recomissioning, MOT prep, Trackerjack brake fitting service.
Apprentice served Triumph Specialist for 50 years. PM for more info or quotes.


Top
   
Display posts from previous: Sort by 
Post new topic Reply to topic  [224 posts ]  Go to pagePrevious11112131415Next

All times are UTC


Who is online

Users browsing this forum: Google [Bot] and 2 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to:  
Powered by phpBB® Forum Software © phpBB Limited