General Chat for advice

[GrahamFountain]

I went back and looked at what Carledo wrote for the dimensions of the callipers, just to check, and it looks like I have another big admission of error to make.

Because we were, I thought, talking about upgrades, with the emphasis on "up", I didn't notice that Carledo had written that the Sierra calliper has only the one piston, not two like the Doly. I've never seen one (never had a Sierra), and didn't realize - though I have seen GM, etc., callipers like that.

So I've been assuming the change in force on the pads is 54 squared divided by 48 squared (1.27), or about 27 percent increase. But if it's one 54 mm piston verses two 48 mm pistons, its 54 squared divided by twice 48 squared (0.633), which gives about 37 percent less (100*(0.633 - 1)) force on the disc than a pair of 48 mm pistons, for the same pad material and pedal pressure.

There would still some increase from moving the pressure centre -15 percent more according to the above, which is not affected by the number of pistons or their size. And that would give an overall front brake effort of only about 27 percent (0.633 * 1.15 = 0.727) less than the doly set.

That does not affect the effects on fade, etc., but as that's not what bothers me. And, were I to upgrade, for preference I'd want more, not less effort from the front brakes.

Graham

[Carledo]

I would say you are underestimating the parrelax effect, in pic 3, the lining edge (which you cannot see) is flush with the edge of the disc. I think pic 4 may have been taken at a larger angle (I did try for consistency but I had to move) The pad in pic 4 is in fact the "other" pad from the same caliper so I didn't have the big clip in the way, the backplates are slightly different but the pad area and distribution is the same.

All I can say about this is, however you look at it, from the figures, the TJ kit gives an approximately 50% improvement over the original system. Which I would not have believed possible in the restricted space available.
The fact that this DOESN'T make the TJ system undriveably good is a sad condemnation of just how BAD the originals are!

This MAY not be quite fair however, since my brain does come into the equation. It tells my foot how much pressure to exert on the pedal to give the required stopping effort. And because I am well used to modern brakes which don't require massive pedal pressure to stop the car effectively, my brain can cope with the TJ brakes just fine. Pre conversion, I also had the pleasure (?) of driving the car on the standard setup and though it DID stop the car, I felt like I needed 2 feet on the pedal to get a result! And they faded, badly, under anything more than the most sensible road use, even with Greenstuff pads and Dot 5.1 race fluid.


So thanks for doing all the sums, it is great to know the figures to back the claims and I will be sticking with the TJ conversion! On my forthcoming Dolomite Sprint Auto, with 2.2ltr 147bhp Omega twincam, I am going for even bigger discs from Jon at 255mm, they won't fit under Sprint wheels but they don't need to!

Incidentally I have run the same front tyres with both setups and they are nothing special, just 185/55/15 Colways (not remoulds) a fairly soft compound budget tyre.

Cheers and a Happy New Year to you and yours, Steve

[Carledo]

I went back and looked at what Carledo wrote for the dimensions of the callipers, just to check, and it looks like I have another big admission of error to make.

Because we were, I thought, talking about upgrades, with the emphasis on "up", I didn't notice that Carledo had written that the Sierra calliper has only the one piston, not two like the Doly. I've never seen one (never had a Sierra), and didn't realize - though I have seen GM, etc., callipers like that.

So I've been assuming the change in force on the pads is 54 squared divided by 48 squared (1.27), or about 27 percent increase. But if it's one 54 mm piston verses two 48 mm pistons, its 54 squared divided by twice 48 squared (0.633), which gives about 37 percent less (100*(0.633 - 1)) force on the disc than a pair of 48 mm pistons, for the same pad material and pedal pressure.

There would still some increase from moving the pressure centre -15 percent more according to the above, which is not affected by the number of pistons or their size. And that would give an overall front brake effort of only about 27 percent (0.633 * 1.15 = 0.727) less than the doly set.

That does not affect the effects on fade, etc., but as that's not what bothers me. And, were I to upgrade, for preference I'd want more, not less effort from the front brakes.

Graham

Ah, right, well I don't really know what to say about that, since, in practice, the TJ upgrade IS much better than the original system, not just on the fade front, but in needing less pedal effort to achieve the same stopping power as well. so either the figures are cockeyed or there is some factor we have not taken into account! Or me and several dozen other people are imagining it!

Steve

[GrahamFountain]

Ah, right, well I don't really know what to say about that, since, in practice, the TJ upgrade IS much better than the original system, not just on the fade front, but in needing less pedal effort to achieve the same stopping power as well. so either the figures are cockeyed or there is some factor we have not taken into account! Or me and several dozen other people are imagining it!

Well it's simple hydraulics really: the pressure on the pads is entirely determined by the hydraulic pressure and the surface area of the pistons to which that pressure applies. There'll be some differences in the losses in the bearing surfaces, etc., but I can't see how they will make a significant difference, considering the sizes of the forces involved. There'll be some dynamic effects as the pistons move, but once they stop, it's just the pressure and area.

If the rest of the system (master cylinder, servo, pad material, fluid, etc.) is unchanged, the pressure on the pistons must be the very nearly the same for the same foot pressure (though not necessarily at the same point in its travel). And if both sets of callipers are in much the same condition, and the surface on the Sierra piston that the hydraulic pressure bears on is 54 mm diameter (2290.22 sq mm) and the equivalent surface on the Sprint calliper is 2 x 48 mm diameter (3619.11 sq mm), then the pressure on the pads with the Sierra calliper must be only 63 percent of that with the Sprint calliper. The rest is down to how far from the hub that pressure is applied, which can't be more than a 20 percent gain. And it's not like with tires - the area of the contact between solid surfaces makes no difference to the ratio between the forces, i.e. the coefficient of friction between them.

So, if these callipers really do give more rather than less force for the same pad material and foot pressure, there's something wrong with one or more of the data. Is it possible that the diameters of the back of one of the pistons that the hydraulic pressure is applied to is not what I've used in the calculations?

Graham

[sprint95m]

Is this still going on?
Yet again I am loathe to get involved.


On page 2 I informed you that the Ford calipers are the single piston sliding type.


You are only using part of the equation.....
none of your calculations include the vented disc.
This is a crucial thing. (That has already been pointed out as well.)




Consider the ST brakes....they used widened standard calipers over vented discs.........
Furthermore, it should be worthwhile to read Chris Witor's articles where he (at considerable expense) actually
tested different set ups (on the same car).




Ian.

[GrahamFountain]

I would say you are underestimating the parrelax effect, in pic 3, the lining edge (which you cannot see) is flush with the edge of the disc. I think pic 4 may have been taken at a larger angle (I did try for consistency but I had to move) The pad in pic 4 is in fact the "other" pad from the same caliper so I didn't have the big clip in the way, the backplates are slightly different but the pad area and distribution is the same.

Problem is, I can see that the Doly pad edge lines up with its disc edge. So why does the Sierra pad look so overhung? Pic 4 is a bit blurred, which don't help.

There's also another measurement that would be useful, and that's the diameter of the boss at the centre of the disc. I think it should be 134.8 mm, but it looks a little bigger if the disc really is 239 mm dia. And one more question, which is whether the pad on the other side overhang inside the edge of the disc's eye, which I believe will be 143 mm in dia? If it aligns with that, that gives the outer to inner radius edge dimension of the pad as 119.5 – 71.5 = 48 mm. The one on the drawing is about 50 mm.

I have changed the drawing to move the pad down so that the outer edge aligns with the disc and worked out its centroid to give the drawing below.



With that the centroid is moved about 9 percent from the Doly's. However, if the two pads are the same vertical height, then the parallax or whatever has made the Sierra pad look wider, which would make the centroid appear near the hub. So it could be a percent or three higher at 10 to 12 percent.

I still think the misalignment between the centroid and the piston centre is a problem and that the pressure centre is a bit further out than the centroid of the pad, and so it's moved more than 9 percent. But it still can't have moved more than the piston centre, so it must be less than 20 percent.

So from what I've got, the gain from moving the pressure centre is somewhere between 9 and 20 percent increase (93.75/87 to 104/87).

Graham

[Carledo]

Is this still going on?
Yet again I am loathe to get involved.


On page 2 I informed you that the Ford calipers are the single piston sliding type.


You are only using part of the equation.....
none of your calculations include the vented disc.
This is a crucial thing. (That has already been pointed out as well.)




Consider the ST brakes....they used widened standard calipers over vented discs.........
Furthermore, it should be worthwhile to read Chris Witor's articles where he (at considerable expense) actually
tested different set ups (on the same car).




Ian.

Yes Ian, it's still going on! If only as a conversation between Graham and myself!

It's become like that nasty itch between your shoulder blades to me, I just have to scratch it even though I can't reach it easily!

As, I think, you are a fellow TJ user, you are as aware as I am how good they are, all we are trying to do is quantify this which I think is fairly harmless and keeps us out of the pub on these cold winter nights.

Steve

[Carledo]

So, if these callipers really do give more rather than less force for the same pad material and foot pressure, there's something wrong with one or more of the data. Is it possible that the diameters of the back of one of the pistons that the hydraulic pressure is applied to is not what I've used in the calculations?

Graham

A couple of things occur to me, though whether they have enough bearing to matter is problematic (at least for me) The first is that the Sierra caliper is a sliding one which allows the force of one piston to be transmitted to 2 pads. My brand of theory says this should make no difference but i've been wrong before! The second is the standard pistons, I've been operating on the 48mm figure that you use but is it correct? If you look at the pics of the stock pad, the mark left by the piston is obviously smaller than the pad swept radius (which I HAVE measured at 47mm) It's centre is also displaced outward from the centre of the lining area and the piston APPEARS (parrelax again?) to overhang the disc slightly. I've not been down to the workshop since the 23rd but may be in breifly tomorrow so will do some more measuring and take some more pics then.

Steve

[sprint95m]

As, I think, you are a fellow TJ user, you are as aware as I am how good they are, all we are trying to do is quantify this which I think is fairly harmless and keeps us out of the pub on these cold winter nights.

While this may well be keeping you out of the pub of an evening Steve, it is not going to quantify how good TJ brakes are, since the effect of the vented disc is not included.
All you have here, thus far, is an attempt to establish the effect of Ford calipers on a solid disc....said effect being small when compared to the inclusion of a vented disc.




Ian.

[GrahamFountain]

On page 2 I informed you that the Ford calipers are the single piston sliding type.

Well I did already apologize for being a complete tit and missing that fact. And you weren't alone in not noticing I was still comparing the pressure area for two 54 mm pistons with that for two 48 mm pistons, afore I 'fessed up.

You are only using part of the equation.....
none of your calculations include the vented disc.
This is a crucial thing. (That has already been pointed out as well.)

Well the use of vented discs may be crucial to you, but (as I believe I have also pointed out, a couple of times) I'm not interested in it. ALL I am interested in is the gain in brake effort, from the various kits that are about, relative to that from the standard Sprint set; all within their normal operating temperature ranges, i.e. not including fade conditions.

But to address the issue again, but in more detail: There will/can be terms for the temperatures of, and or heat flows into, the components, in the equation relating the effort at the pedal to the force on the road. These will affect the CofF between pad and disc and the pressure applied to the pad as a function of force on the pedal (constrained by travel limits), etc. But I expect them to be highly non-linear, like absolute temperature to the fourth power, or related to abrupt changes to compressibility due to fluid state transitions. I also expect that they will be largely irrelevant, i.e. invariant, well within the normal operating temperature ranges for each component, and largely determine the limits of these linear ranges. Hence, oh lucky me, I get to ignore them.

So, even if you get a form of the equation that includes the temperatures of and flows into the components from somewhere, you'd have to relate the thermal energy generated in braking the car to the temperatures of the components – which would also mean specifying some braking profile, e.g. deceleration, time, repetitions, etc., to use as a standard in any comparisons of different sets. And the last time I did something to relate thermal energy flows (brake effort and time), component thermal capacities (disc and calliper compositions and masses, pad sizes, etc.), thermal dissipations (venting, cooling flows, etc.), and thermal conductivities (disc to pad to piston to calliper to fluid), to component temperatures; it did involve a second order differential equation. This situation will, I expect, involve several.

So you are right I don't include the venting. However, the reason for that is I don't have to, powered, to some extent, by the fact I probably couldn't, without at least the support of a sizable University Physics Dept.

Graham

[GrahamFountain]

All you have here, thus far, is an attempt to establish the effect of Ford calipers on a solid disc

Exactly! And well within the normal operating temperature ranges, a vented and solid disc will respond to the applied pressure on the pads in exactly the same way - venting (and all the other thermodynamic effects) only increse the heat input (level and duration) required to get beyond those normal ranges.

Graham.

[GrahamFountain]

I probably couldn't, without at least the support of a sizable University Physics Dept.

Okay, deriving and solving the thermodynamic equations for the brake system would be hard-ish.

But it occurs to me that someone could, perhaps, measure the fundamental difference in effort for a fixed hydraulic pressure between the Sprint brakes on their disc and the Sierra calliper on a VW disc, if they've got some suitable hydraulics (pump and gauge), a suitable frame, and a torque wrench. They could also assess the effect on pedal length by measuring how many pumps to reach a specific pressure.

It needs the sliding friction values to compare. Which means they'd have to keep turning the wrench's torque setting up till the wrench just doesn't unclick when the disc starts to turn. It still should click over before the disc turns, because static friction is normally greater than sliding.

Yes, I know there'll be no heating effect. But for a measure of the ratio between the forces they apply against the rotation of the disc, I simply don't care. Though I'd be very pleased if I've got it wrong in the end, and it's 1:1.2 or so, against the Sprint set.

Graham

[soe8m]

A man needs a new table in his house but isn't sure the new one he's standing for will fit through the kitchendoor. He grabs a tape measurer and holds it on one side of the table. He takes a trip around the world to the other side of the table. 40.075km minus the outcome minus the extra lenght what is gained by going over mountains and the temperature differences what caused expanding and shrinking of the tape measurerer and he knows the size of the table. Or just tryfit.

Jeroen

[GrahamFountain]

What's the difference between a simile and an allegory?

The allegory is the one where you can still see the teeth when the mouth is shut.

[GrahamFountain]

I've been doing a bit more research on the Net and I've found the discrepancy between the calculated change in effort with the 1 pot Sierra calliper and what the TJ kit gives. And it turns out, embarrassingly, that when I said I was wrong I was in fact mistaken (LOL). So please ignore the previous apology and accept this free of charge replacement: Sorry.

I thought you just use the area of a piston multiplied by their number – it seemed logical at the time. But I'd missed that the cylinder end face in the slider acts in the same way as the back of another piston - the hydraulic pressure, as a scalar, pushes in both directions at once. So there's the full force pushing the piston onto one pad and the same from the other end of the slider pushing onto the other pad (less any extra stickiness in the slider mechanism). So its not the force on one piston split between two pads - it would only be that if the disc were floating between two pads, one pushed by the piston and one fixed to the suspension. So you calculate the area the fluid acts on as twice that of the piston's, just as with a two pot calliper. (See Escort RS Turbo Series 2 piston area calculation at https://www.rallydesign.co.uk/pdf/desig ... brakes.pdf)

Anyway, t3hat means that the earlier calculated values of something like 39 (1.27*1.09) to 51 (1.27*1.2) percent overall increase in front brake effort from the TJ set (depending on where the pressure centre is between the pad centroid and piston centre), should be right.

I go back to calling that a "massive gain", in relation to what you can take advantage of with 10 or 12 percent grippier road tires. It's therefore rather too much for what I would want, were I to upgrade my road car.

The TR8's 54 mm piston callipers on the 220 mm solid disc should give about 22 percent more effort, which seems a much more reasonable figure. From what I remember, it's a fair bit bigger pad too - same as the Stag's I think. So there should be some improvement in fade as well – not near as much as a vented disc would give, but some. But I've no easy way of seeing if they'd fit and the only set I have belong to the TR7 Sprint.

Graham