Sprint rear brake bias valve.

[GrahamFountain]

Trying to redo the calculations, I've been back and looked for what I have and find this drawing. Can we confirm that it's at least close to correct in the dimensions for the placement of the pads and pistons for the Sprint and Sierra front brakes?

I suspect the difference between the centroid of the pad and the centre of the Sierra piston isn't important. So a measure from hub to piston centre, which should be easier to make, will probably be enough at the level of accuracy that's needed here.

Graham

I've studied both pics and both seem to contain the same error expressed different ways as regards the Sierra piston placement.

In the first drawing, the Sierra pad/piston placement is correct, there is a slight overlap of the piston to the body of the pad at the outer edge (in real life, pads have a small crescent bulge to accomodate this, not shown in the drawing, but its lack is immaterial to our calcs) but the pad/disc placement is wrong, showing the body of the pad overlapping the disc by maybe 5mm or so, this is not correct, the body of the pad starts around 1mm IN from the edge of the disc.

In the second drawing, the disc/pad placement is correct, but the pad/ piston placement is incorrect meaning that in both drawings the piston centre for the TJs is too far out by 5mm or so (I can't be more accurate than that without scaling the drawing and measuring, something my computer skills arent up to, i'll try to calculate it from available data for my next post)

However, I agree that once that error is corrected, we should have a basis to calculate from, everything else seems satisfactory.

I also have some used Puma pads in my garage which were marked in service by the piston on the piston side face and clearly show the relationship between pad and piston placement. I'll take a pic tomorrow, too cold, wet and windy to go out there tonight.

Steve

Will that give the distance between the centre of the piston and the centre of the hub? There may be a small correction needed if the centre of the piston isn't exactly aligned with the centroid of the pad, but I don't think it could ever be much.

Graham

[Carledo]

The KA and Fiesta I looked up where both 54mm
I didn’t go thru all the ones listed.

But dare say there is other sizes.

Um, but do all these Ford calipers have only one piston, like half the effective hydraulic area you'd normally expect?

Graham

Graham you just flunked caliper design 101! The Ford calipers, based on the Girling/ATE designed "Diana" caliper is a single piston FLOATING caliper, meaning that the single piston applies pressure in BOTH directions, so the piston counts twice!

Steve

[jikovron]

I'm theory the pads have the same running clearance and such travel the same distance to meet the disc before pressure builds, such that the fluid volume used in each case remains the same, also the fluid pressure of each will be equal for the same brake force.

Yes, the one piston uses the same fluid as two at the same hydraulic pressure. But to get the same brake force, it needs twice the hydraulic fluid pressure on that one piston to apply the same force to both of two pads. That's because it has half the area of two pistons and the force is the hydraulic pressure times the total piston area. So there's more give in the system at twice the pressure. It may not be twice as much, indeed I think its less. But it must more at the same brake force.

Graham

2 hydraulic pistons each generating 1000kg at a given pressure when opposed will add up to a total force of 1000kg , the same size piston in a sliding caliper will generate the same 1000kg of clamping force as the caliper body reacts with the equal and opposite force.

[GrahamFountain]

In the first drawing, the Sierra pad/piston placement is correct, there is a slight overlap of the piston to the body of the pad at the outer edge (in real life, pads have a small crescent bulge to accomodate this, not shown in the drawing, but its lack is immaterial to our calcs) but the pad/disc placement is wrong, showing the body of the pad overlapping the disc by maybe 5mm or so, this is not correct, the body of the pad starts around 1mm IN from the edge of the disc.

In the second drawing, the disc/pad placement is correct, but the pad/ piston placement is incorrect meaning that in both drawings the piston centre for the TJs is too far out by 5mm or so

I still can't find the original data from which the drawings were made, just the picture of the pad. It's possible I was given the disc diameter and distance between the piston and hub centres and was trying to reconcile the two when one of them is wrong. If either the disc is about 10mm larger or the piston hub centre spacing is about 5mm less, it all would make sense.

Graham

[GlenM]

I have got the standard GTI trackerjack kit for my car, which I can see I am going to have to upgrade at some point. It's a Nissan CA18DET going into it with a hybrid T25 turbo, so it should be about 230 BHP. My plan is to get the car up and running with this brake set up, then upgrade the brakes/wheels/tyres later on.

Does anyone know if the larger Passat discs are a big improvement over the GTI ones?

The point of bigger dia discs (as Graham will tell you) is to move the piston centres outboard. My mod only moves these centres an extra 8.5mm further out which doesn't seem like a lot, but the effects are not linear, they are exponential. I haven't worked it out exactly, but it's about the same again as going from stock to TJs. So I expect a decent return for my investment. As it's only travelled a few hundred yards under it's own power so far (making sure the ecu controlled trans shifts properly) I can't yet say exactly HOW much better. But as Graham says and I agree, you CAN make the brakes TOO good! I'd try the car with the normal 239mm discs first and see how you get on with it. Car weight is also a factor, are you running it fully trimmed or as a stripped out racer? The Carledo is down to a kerb weight of 760kg (+82kg of ME) to make the most of it's miserable 140odd horses, but the standard 239mm TJs are fine with that, no fade at all and consistent stopping, from a ton or a bit more, for a full days track use at Castle Coombe. The MOT brake testing weight (including driver, spare wheel, tools, half a tank of gas and "accumulated road dirt" for a standard, fully trimmed Sprint is something like 1270kg IIRC. That's a big difference!

Steve

Steve,

Yes, understood about the principle of moving the caliper further out on a larger diameter disc. I do have a larger bore tandem master cylinder to fit, so that should help with the pedal travel. Will definitely be trying the standard TJ brakes first, if they are good enough then I will stick with them. I guess the CA engine is a similar weight to the Sprint engine but I would imagine that the 5 speed TX1 'box will be quite a bit lighter that the Sprint OD 'box, so there may be a bit of a weight saving. I haven't decided on what to do with the interior yet, although I do like the idea of stripping it out and perhaps just having a pair of bucket seats.

Glen

[killysprint]

Glen

I’ve had a TJ kit on my sprint for more than 10 years now. Has the Sierra callipers and golf discs.
My car has a modded sprint engine, no where near your 230hp but last time on the dyno more than 165.

The car has a standard single line master cylinder, and the std valve for the rear brakes - with the TJ kit there is a longer pedal travel, but it brakes much harder, quicker and feels better than std. It gets driven hard when I have it out, and I feel it’s certainly been a worthwhile change.

When I was younger, a lot younger, I frequently used to have the brakes smoking and fading badly on a std sprint. Has never happened with the TJ kit, in a quicker more powerful car.

[Carledo]

I've studied both pics and both seem to contain the same error expressed different ways as regards the Sierra piston placement.

In the first drawing, the Sierra pad/piston placement is correct, there is a slight overlap of the piston to the body of the pad at the outer edge (in real life, pads have a small crescent bulge to accomodate this, not shown in the drawing, but its lack is immaterial to our calcs) but the pad/disc placement is wrong, showing the body of the pad overlapping the disc by maybe 5mm or so, this is not correct, the body of the pad starts around 1mm IN from the edge of the disc.

In the second drawing, the disc/pad placement is correct, but the pad/ piston placement is incorrect meaning that in both drawings the piston centre for the TJs is too far out by 5mm or so (I can't be more accurate than that without scaling the drawing and measuring, something my computer skills arent up to, i'll try to calculate it from available data for my next post)

Steve

Will that give the distance between the centre of the piston and the centre of the hub? There may be a small correction needed if the centre of the piston isn't exactly aligned with the centroid of the pad, but I don't think it could ever be much.

Graham

I've looked at the data contained in the drawings and come up with an answer (slow, but I get there!)

If you take the 4.75mm distance from pad centroid to piston centre (correct in the 1st drawing) and add it to the hub centre to pad centroid distance of 93.75mm (correct in the 2nd drawing) Then by my calculation, you get 98.5mm which is what I reckon is the correct hub centre to piston centre figure for the TJs, a difference from standard of 12.25mm rather than the 18.5mm the drawings both imply.

I'm happy to accept that if you are, as a basis for calculation. I'm also happy to accept 107mm as a hub centre to piston centre figure (98.5mm + 8.5mm extra disc radius) for my improved 256mm disc/Puma caliper jobbie if you'd like to calculate that too!

Steve

[GrahamFountain]

I'm theory the pads have the same running clearance and such travel the same distance to meet the disc before pressure builds, such that the fluid volume used in each case remains the same, also the fluid pressure of each will be equal for the same brake force.

Yes, the one piston uses the same fluid as two at the same hydraulic pressure. But to get the same brake force, it needs twice the hydraulic fluid pressure on that one piston to apply the same force to both of two pads. That's because it has half the area of two pistons and the force is the hydraulic pressure times the total piston area. So there's more give in the system at twice the pressure. It may not be twice as much, indeed I think its less. But it must more at the same brake force.

Graham

2 hydraulic pistons each generating 1000kg at a given pressure when opposed will add up to a total force of 1000kg , the same size piston in a sliding caliper will generate the same 1000kg of clamping force as the caliper body reacts with the equal and opposite force.

Yes, I was wrong. There is the same area with one piston and one caliper body on a sliding mount. The other half is the area of the back face of the cylinder in which the piston sits. It would only be half the area where the caliper body was fixed and the disc slid on the hub.

Though 2 pistons each applying 1000kg give 2000kg total and one piston applying 1000kg and one caliper body applying 1000kg also give 2000kg.

I guess, assuming the caliper slides easily, the pedal length is also the same.

My apologies, I'm not that familiar with single piston calipers.

Graham

[Carledo]

I guess the CA engine is a similar weight to the Sprint engine but I would imagine that the 5 speed TX1 'box will be quite a bit lighter that the Sprint OD 'box, so there may be a bit of a weight saving. I haven't decided on what to do with the interior yet, although I do like the idea of stripping it out and perhaps just having a pair of bucket seats.

Glen

When building the Carledo, I was gobsmacked to discover that my 2 litre Vauxhall 8v and Omega box combo was more than 40kgs LIGHTER than the Sprint engine-with-O/D setup. It's even marginally lighter than the car's original all iron 1300 and non O/D box. Your power unit may be a tad heavier than mine with 16 valves and a couple of kilos of turbo added on and my gearbox (which is very compact and light) is probably a bit lighter than your TX1.

I still suspect that if you fit the engine and box to a car with stock suspension, it'll sit like a monster truck, mine did with standard used Sprint coilovers all round, it was also almost undriveable, the back end was so hard, it BOUNCED round corners!

Steve

[GrahamFountain]

I've studied both pics and both seem to contain the same error expressed different ways as regards the Sierra piston placement.

In the first drawing, the Sierra pad/piston placement is correct, there is a slight overlap of the piston to the body of the pad at the outer edge (in real life, pads have a small crescent bulge to accomodate this, not shown in the drawing, but its lack is immaterial to our calcs) but the pad/disc placement is wrong, showing the body of the pad overlapping the disc by maybe 5mm or so, this is not correct, the body of the pad starts around 1mm IN from the edge of the disc.

In the second drawing, the disc/pad placement is correct, but the pad/ piston placement is incorrect meaning that in both drawings the piston centre for the TJs is too far out by 5mm or so (I can't be more accurate than that without scaling the drawing and measuring, something my computer skills arent up to, i'll try to calculate it from available data for my next post)

Steve

Will that give the distance between the centre of the piston and the centre of the hub? There may be a small correction needed if the centre of the piston isn't exactly aligned with the centroid of the pad, but I don't think it could ever be much.

Graham

I've looked at the data contained in the drawings and come up with an answer (slow, but I get there!)

If you take the 4.75mm distance from pad centroid to piston centre (correct in the 1st drawing) and add it to the hub centre to pad centroid distance of 93.75mm (correct in the 2nd drawing) Then by my calculation, you get 98.5mm which is what I reckon is the correct hub centre to piston centre figure for the TJs, a difference from standard of 12.25mm rather than the 18.5mm the drawings both imply.

I'm happy to accept that if you are, as a basis for calculation. I'm also happy to accept 107mm as a hub centre to piston centre figure (98.5mm + 8.5mm extra disc radius) for my improved 256mm disc/Puma caliper jobbie if you'd like to calculate that too!

Steve

The area difference from 48mm to 54mm pistons is 1:1.266. The moment difference (from piston centres) is 1:1.142. Together they give a total increase of 1:1.445, or about 45% increase.

I still think that's a lot.

For the Puma disc & caliper, that's the same area ratio, but the moment difference goes up to 1:1.241 and the total is 1:1.570, or 57% more effort.

If I think that 44% is a lot, I wonder if you can guess what I think of 57%.

What does that do to the pedal length is, I suppose the next question.

Also, if you could increase the grip from the tires enough to take advantage of either of those increases in front brake forces and actually get 40 or 50% greater maximum deceleration, what does that do to the ideal brake balance (where front and rear wheels all lock together), how much less rear brake force would you need to get to the same safety margin from it, and does the LSV do anything to save you - when it works?

Graham

[cliftyhanger]

Re the single piston and slider.
I think we need to remember the laws of physics. The force on the piston pushes the piston towards the disc. But at the same time the equal and opposite force pushes the slider towards the disc too. So the single piston does act the same as 2 pistons. The amount of fluid moved is teh same as 2 pistons (as the piston moves twice as far)

I think that makes sense. But I have been on a zoom call and had a few beers. Feel free to pick holes in my idea.

[jikovron]

Yes, the one piston uses the same fluid as two at the same hydraulic pressure. But to get the same brake force, it needs twice the hydraulic fluid pressure on that one piston to apply the same force to both of two pads. That's because it has half the area of two pistons and the force is the hydraulic pressure times the total piston area. So there's more give in the system at twice the pressure. It may not be twice as much, indeed I think its less. But it must more at the same brake force.

Graham

2 hydraulic pistons each generating 1000kg at a given pressure when opposed will add up to a total force of 1000kg , the same size piston in a sliding caliper will generate the same 1000kg of clamping force as the caliper body reacts with the equal and opposite force.

Yes, I was wrong. There is the same area with one piston and one caliper body on a sliding mount. The other half is the area of the back face of the cylinder in which the piston sits. It would only be half the area where the caliper body was fixed and the disc slid on the hub.

Though 2 pistons each applying 1000kg give 2000kg total and one piston applying 1000kg and one caliper body applying 1000kg also give 2000kg.

I guess, assuming the caliper slides easily, the pedal length is also the same.

My apologies, I'm not that familiar with single piston calipers.

Graham

The 1000kg total still stands, the forces are in equall opposition as the 1000kg of one piston only exists due to the 1000kg reaction force of the other piston/caliper body. Newton would be able to explain it better than I.

[cliftyhanger]

2 hydraulic pistons each generating 1000kg at a given pressure when opposed will add up to a total force of 1000kg , the same size piston in a sliding caliper will generate the same 1000kg of clamping force as the caliper body reacts with the equal and opposite force.

Yes, I was wrong. There is the same area with one piston and one caliper body on a sliding mount. The other half is the area of the back face of the cylinder in which the piston sits. It would only be half the area where the caliper body was fixed and the disc slid on the hub.

Though 2 pistons each applying 1000kg give 2000kg total and one piston applying 1000kg and one caliper body applying 1000kg also give 2000kg.

I guess, assuming the caliper slides easily, the pedal length is also the same.

My apologies, I'm not that familiar with single piston calipers.

Graham

The 1000kg total still stands, the forces are in equall opposition as the 1000kg of one piston only exists due to the 1000kg reaction force of the other piston/caliper body. Newton would be able to explain it better than I.

Oops, I missed that... looks like there is agreement amongst us.
If you want to see a really quirky caliper setup, look at the saab 96. Absolutely nuts!

[jikovron]

Yes, I was wrong. There is the same area with one piston and one caliper body on a sliding mount. The other half is the area of the back face of the cylinder in which the piston sits. It would only be half the area where the caliper body was fixed and the disc slid on the hub.

Though 2 pistons each applying 1000kg give 2000kg total and one piston applying 1000kg and one caliper body applying 1000kg also give 2000kg.

I guess, assuming the caliper slides easily, the pedal length is also the same.

My apologies, I'm not that familiar with single piston calipers.

Graham

The 1000kg total still stands, the forces are in equall opposition as the 1000kg of one piston only exists due to the 1000kg reaction force of the other piston/caliper body. Newton would be able to explain it better than I.

Oops, I missed that... looks like there is agreement amongst us.
If you want to see a really quirky caliper setup, look at the saab 96. Absolutely nuts!

Robust debate is an enriching part of forums ,,otherwise the tumble weeds set in !

[soe8m]

Robust debate is an enriching part of forums ,,otherwise the tumble weeds set in !

To add, most Dolomites I have, have a 100liter LPG tank above the rear axle. It really benefits the roadholding and cornering and most important, no rear lock up.

Jeroen.