Nope, we’ve got 2-pot rear calipers.
Frank, the rear caliper has always been a twin piston design, the 3 piston is only used on the front.
Any way, to answer your question, the 1.69" rear piston caliper seems to be an Industry defacto standard. When I was researching available rear calipers I was amazed at the number of manufacturers that used that size.
If we look at the S1 XJ, it had 2.25" front and 1.69" rear piston sizes, which achieved an approx. 60/40 F/R brake ratio.
When the V12 engine was introduced,the much higher available performance required improved brakes.
Unfortunately only the front brakes were uprated with the 4 piston calipers paired with ventilated discs
Apart from the fact that the rear brakes could now experience brake fade under extreme conditions, there was no safety issue as such, because if the front brakes lock, the vehicle will go straight. If the rears lock the vehicle can rotate 180 deg. which is extremely dangerous for an inexperienced driver.
As part of my fitting of ventilated discs to the XJ IRS, I include the option of an uprated rear caliper that uses the front piston of 1.89" from the 4 spot caliper so as to restore the original brake balance, as well as improving the overall brake performance by nearly 25%
Cool! Is this accomplished by simply boring and remachining the OEM Girling caliper, or is it an entirely new caliper? Does the handbrake still work?
True enough, but the front end also got heavier.
I just bore out the exhisting caliper so everything else is unchanged
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How would that be with the old 3-pot fronts versus the 4-pots, Norman…?
As an aside; I sure agree with the danger of rear wheel lock-up - though on slippery roads it a choice of going off the road head or tail first. I’m a bit surprised that Jaguar didn’t fit a load sensor at the rear for more optimal braking…
Frank
xj6 85 Sov Europe (UK/NZ)
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Frank, don’t understand your question, 3 versus 4?
Fitting a load sensor to the rear would achieve nothing, as the rear brakes will never lock before the front
I’m guessing he wants to know how the switch from 3-pots to 4-pots affected the brake balance. Do the 4-pots apply more grip? Because that ain’t necessarily so. If the 2 pots on one side were the same size as the pots on the 4-pot, then I’d presume the single pot on the other side is a good deal larger to have approximately the same total piston area – meaning the calipers would apply the same clamping force to the pads. That would only leave the question of pad and disk differences for braking power.
That 3-pot caliper is just weird. I do wonder why it was developed.
[quote=“Frank_Andersen, post:26, topic:383863, full:true”]
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As Kirbert says, Norman - the brake balance on the early cars…
As it is; the overall braking effect is less than it could be. With calipers configured according to the normal weight distribution - adding a load sensor would allow varying rear brake pressure according to load.
Given everything else equal; weight on the wheels dictates how much brake effort that can be applied before breakaway. To avoid rear wheel lock-up by fixed 60/40 force distribution instead of actual weight distribution, they have reduced overall braking capacity - which may not be good with a fully laden car…
It also means that the front brakes are unnecessarily overworked - which increases the possibility of overheating brake fade (though vented front discs is a suitable counter to that). After all; most braking are carried out well below lock-up - and even on ice it would be useful to have all wheels brake to capacity…
Frank
xj6 85 Sov Europe (UK/NXZ)
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When ABS was introduced, the brake calipers themselves did not change, correct? So if the rear brakes were so weak, presumably the channel controlling the rear brakes virtually never operated because the rear wheels never locked up?
[quote=“Frank_Andersen, post:30, topic:383863, full:true”]
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As it is; the overall braking effect is less than it could be. With calipers configured according to the normal weight distribution - adding a load sensor would allow varying rear brake pressure according to load.
Frank, Manufactures do not build the best vehicle they can, they build to a price and to the minimum std, as required by LAW. A Law that does not require any vehicle to achieve a 1G. stop, it’s a lot less, more like 0.5G. So the fact that the rear brakes do not do a “fare share” is irelivant.
And most brake systems are over boosted to catour for people of small stature so any idiot can lock the brakes creating a dangerous situation, it’s the main reason ABS was introduced.
On a dry road extreme braking will only activate the front ABS, it takes wet or icy conditions to activate the rear.
Most Commercial vehicles are fitted with load sensors on the rear because loads can vary from zero to many tons.
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I’m not sure about retaining the original brake force distribution, Kirbert…?
It would be counterproductive; equalising the caliper brake force would not interfere negatively on ABS function - and give ‘best’ braking at all times and loads…?
Frank
xj6 85 Sov Europe (UK/NZ)
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I agree; that was the point I was getting at. Oddly enough, with ABS I guess it becomes less important. Apply the brakes to the point where the front tires are at the limit of adhesion, and the rears are still far from breaking loose. But continue to apply more force to the pedal and the fronts would lock up except the ABS kicks in and begins modulating – but the additional force applies to the rears, increasing the total braking effect. So with ABS operating, one would expect that brake balance is less important. I still think it’d be nice, though; it’d be better if you could bring all four tires to the limit of adhesion before any of them break loose and engage the ABS.
Frank, you’re confusing two issues, ABS and braking force are not directly related. ABS is activated in a low friction situation, it has nothing to do with braking force.
Before ABS, all vehicle brake balance was a compromise that each manufacturer made.
Some were able to get very close to perfect balance, of which I believe Jaguar did with it’s use of the Dunlop system which allowed the use of varying dia. pistons that were interchangeable.
For example, when I purchased my first S1 3.8 E type in 1966, it was in my opinion, the most perfectly balanced system on the market.
It had 2.125" front and 1.875" rear pistons, and one up with half a tank of fuel would just “chirp” both front and rear tyres in a panic stop. You couldn’t lock the wheels as it had a very low boost “Kelsey Hayes” booster.
Then the MK 2 had 2.125" front and 1.5" rear and the MK X had 2.25" front and 1.625" rear.
And if you go back to the MK IX, it had 2.5" front and 1.875" rear.
But all this was only possible because of the bolt on style of piston used by Dunlop.
When Jaguar started using Girling calipers on the S type, the calipr was a mombloc design so you no longer had the flexability of piston choice and from then on, very brake system balance was a compromise.
Jaguar chose to use a 1.69" caliper on ALL vehicles, just varying the front piston.
Then you had a 2.125" piston on S2 E, 3.8 S Type, and 2.25" on 420G, XJ6.
Not one of these combinations was any where near the perfect balance, but as I have previously stated, as the front wheel was always going to lock first, there was no safety issue.
Norman,
sorry to dissent, but I can see only one argument why Frank would not be right.
After all, you want to achieve two things with the braking system: (a) all four brakes doing their best for a quick stop. The “best” always is to have sufficient braking power at the wheel to exceed the friction between tire and tarmac. (b) You want to have no wheel locks and in particular no rear wheel locks. That is why from during the run of SII cars there is a Pressure Difference Warning Actuator (PDWA) installed in XJ Jags - just as in my Spitfire.
As a consequence of the dynamic load shift during braking the useful braking power of the rear brakes is considerable lower than that of the front wheels. Most rules of thumb I’ve heard so far say around one half, making it desirable to dispatch 2/3 of the braking power to the front wheels and 1/3 to the rear. Most cars pre-ABS do this simply by an arrangement of caliper size as you describe. You may applaud the E type SI setup - many people then rather criticized the braking power of such a fast car and the brake upgrade for SII E types confirms that the Jaguar engineers had to follow a need.
On modern cars braking power always exceeds friction, even despite better traction of low ratio and wider tires. I just found figures for Volkswagen Golf GTI. Mk I cars stopped from 50 kph in 53 m, Mk III cars in 39 m. Only much later pressure distribution systems were introduced. The latter development had as a consequence that in particular mildly used +250 kph cars typically seem to suffer from frozen rear calipers, because their braking power is designed to stop 2 tons quickly from that speed, but - used in light traffic and without serious braking situations - they never get any work to do.
In my car the front wheels block quickly, but I’m not sure whether I could lock the rears, even though my biennal brake test shows decent results. With the advent of ABS you could overpower the rear brakes without any safety issue - that’s what Frank was saying - , but in fact it would be pretty annoying to feel the pulsating every time ABS is activated. So even today it is typically the front brakes that lock up first and trigger ABS control.
Good luck
Jochen
75 XJ6L 4.2 auto (UK spec)
Jochen, you can’t have a) and b) at the same time. The “best” is to have the tyres just below the slip point, at both front and rear for maximum retardation, which is what I said the S1 3.8 E had.
The trouble is that the 3.8 E had insufficient boost to suit the average driver, which is why very few women were comfortable driving the 3.8. it was improved on the 4.2 with the Lockheed system, but this allowed wheel lockup for most drivers.
As I stated before, the Manufacturers had to make compromises, what suits you will not necessarily suit some else.
But ABS came to the rescue and provided the same maximum brake effect for ALL drivers, regardless of physical attributes.
It’s like when I go from My XJS to my wives Focus, I find it over boosted and I over brake until I adjust to the lighter pedal.
But for her it’s perfect.
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Braking force is still a matter of weight and tyre friction coefficient to the road, Norman…
And it is still the pads applied to the rotors that brings about the braking effect. The ABS can reduce pad pressure, but cannot increase it.
The relationship between the pedal force and pad pressure is relevant for brake ‘feel’. With large enough pistons you can reach lock-up point the instant you touch the pedal - and with small enough pistons you may never reach lock-up, and the ABS stays passive. Unless the road surface is slippery enough…
As you say, prior to ABS the car manufacturers made compromises, they had no choice - leaving it to the drivers to get ‘best’ braking without getting into trouble. Which on slippery or variable road surfaces was a big ask…
It’s a bit ironic; in my early days of driving, in snow and ice, gravel and whatever, when we really needed ABS - there was none. Nowadays; we must remember whether we have ABS or not in a critical situation…
Frank
xj6 85 ov Europe (UK/NZ)
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My point exactly, Kirbert…
The point is that before lock-up the braking distance with, say 60/40, is longer than necessary - as the rear wheels do not fully participate. Which may lure a driver into a situation where more drastic methods are suddenly required. Which is avoidable with more equal brake force - and not compromising the ABS function…
There is also the added complication that the front wheels use part of their traction to steer the car. We all now the danger of steering inputs while braking - but some extra help from the rears may be welcome…
By all means; I sure ain’t saying that ‘our’ Jaguar braking is unsatisfactory - I’m basically nit-picking principles…
Frank
xj6 85 Sov Europe (UK/NZ)
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Heh. All this may be moot if you haven’t upgraded to vented rear rotors. The OEM solid rotors typically get a nice coating of oil from the diff output shaft seals on the inboard side, rendering the inboard pads virtually frictionless and effectively reducing the rear braking force by half. If Norm is right that it’s low to begin with, it’ll be really low then!
“But my output shaft seals don’t leak!” I suppose that’s possible, but it doesn’t seem common. They may not be dripping, but apparently they allow just enough past that the inner pads stop wearing. If your outboard pads wear out before the inboard pads do, the inboard pads aren’t doing anything.