New brakes - is my thinking right
Ivan - 14/7/12 at 07:38 AM
Bit of a story but here goes.
My Cobra has Jag XJ6 suspension and braking system and I always suspected that it was under-braked at the rear as the weight bias is a lot more to the
rear than the original Jag - also rear tyres are a lot wider than the front now.
Another indicator is that when on gravel and putting it into gear with brakes on the locked front wheels get dragged along by the rear (even when
idling or just touching the accelerator 
which is interesting in confined areas - it's an auto box by the way)
So now to the question - I am fitting Willwood callipers on the front (Leaving the front discs and rear brakes & MC standard) that have roughly
20% more piston area so the same pedal depression will cause the pistons to move 20% less but provide 20% more pressure on the disc - will this
translate into extra pedal travel for the same stopping force and if so will it mean more braking force at the rear brakes?
I suppose pad area comes into it as well so maybe as the new pads are significantly larger than the old maybe the pad pressure will be lower.
I am hoping that the changes will up bias to the rear but don't know what to expect - needless to say I will go carefully when testing making
sure there is plenty of room for sudden end swaps.
jollygreengiant - 14/7/12 at 08:10 AM
quote:
Originally posted by Ivan
Another indicator is that when on gravel and putting it into gear with brakes on the locked front wheels get dragged along by the rear (even when
idling or just touching the accelerator which is interesting in confined areas - it's an auto box by the way)
First of all it sounds like the idle is set a bit TOO high.
Secondly as I remember from my jag owner days/driver days the rear brakes on a Jag ALWAYS needed more TLC than the fronts because they were always
weaker and always did a LOT less work than the fronts, they always became less able to work than the fronts. I suspect that if you do replace them
they will work better because they have been serviced as much as they have been replaced.
JMHO
JGG/Clive
snapper - 14/7/12 at 08:40 AM
Also fitting better front pads will add efficiency.
I would also look at a bias pedal box as this will allow you to adjust
britishtrident - 14/7/12 at 09:45 AM
Your changes are likely to achieve exactly the opposite from what you are trying to achieve i.e. the front brakes will tend to lock earlier.
Reading your post if it were me my first action would to check the pistons and pads in the rear callipers are moving freely and that the rear
discs are free from oil contamination from the differential.
If your rear brake can't hold the car stationary at idle something is wrong, as even the hand brake should be able to do this.
Test the car brakes at slow to moderate speed while driving in a straight line on a good clean dry road surface, the front brakes should lock
before the rears.
On a front engined rear wheel drive car on a good dry surface the rear brakes only supply 20% to 33% of the brake effort
If setup properly bigger callipers really shouldn't need a bigger master cylinder because the calliper pistons should only move a tiny
ammount. the explanation is that the calliper pistons are pulled back purely by the flexing of the rubber seal ie the piston doesn't slide in
the rubber seal.
However when people do fit after market all alloy callipers they do seem to run into problems because of a variety of reason, incorrect alignment
of calliper to disc ( pad is no presented "square" to the disc surface) or trapped air or flexing of the alloy calliper (ie the
calliper "bows" and opens out under pressure from the opposing pistons), nb aftermarket light alloy callipers flex significantly more
than steel callipers.
To clear up a point with disc brakes pad area has no major effect on braking (force) efficiency (pad area only comes into the equation
when heat effects and pad wear rate are considered). School physic Coulomb friction isn't a perfect model for the friction of disc
brake but close enough.
The braking effect from one corner essentially comes down to calliper piston area, pad friction coefficient and the effective radius at which the
brake pad acts on the disc and tyre rolling radius.
Ivan - 14/7/12 at 03:33 PM
Thanks for the comments - idle is at 750 revs so not too fast. Also never could get the hand brake to work properly but it was never very important to
me on an auto car.
When you jack up rear and have someone brake it is impossible to turn the rear wheel by hand so don't think there is problem with rear brakes,
anyway they were re-built recently and there is no oil contamination. What I will do some time is jack it up and see at what revs the rear brakes stop
holding the wheels (with the right chassis stands of course.)
But still think that the bigger front pistons will mean that the pedal will move down further before the front pads make proper contact with the disc
than it would with smaller front pistons, so the back brakes will see more pressure and the piston will have moved out more than before at the point
at which the front starts contributing to stopping the car - or am I wrong. Also think that any extra flex mentioned by British Trident in the AL
calliper might add to the extra engagement of the rear brakes which are the original Cast Iron Girlings.
Just what might be a dumb question - won't it be more difficult to achieve a decent pedal pressure and travel for a dual master cylinder system
on a 1100 kg car than a 600 kg one.
jacko - 14/7/12 at 04:16 PM
You say the brakes have just been re-built is there any air in the rear pipes ?
Not on a jag, but i had this on my mk indy
Jacko
Ivan - 14/7/12 at 04:31 PM
quote:
Originally posted by jacko
You say the brakes have just been re-built is there any air in the rear pipes ?
Not on a jag, but i had this on my mk indy
Jacko
No - this has been a problem for 23 years and never been sorted - through countless rebuilds and bleedings - although it doesn't bug me that much
otherwise I would have done something about it before like check line pressures and then sorted a better MC sizing. Just wondered if the current
changes of front callipers would help.
In fact on a cold start when you are keeping revs up a little bit to keep motor running (lumpy cam makes that necessary) and foot hard on the brakes
(essential) there is always a chirp from the rear tyres when putting it into gear (guess the torque multiplication from the Torque Converter
doesn't help here). Rather fun to see bystanders reaction. (There's seldom a lack of bystanders when Cobra starts 
)
[Edited on 14/7/12 by Ivan]
ed1801 - 14/7/12 at 04:32 PM
I agree with this. Larger front piston area effectively moves your bias rearwards. (I think) 
quote:
Originally posted by Ivan
But still think that the bigger front pistons will mean that the pedal will move down further before the front pads make proper contact with the disc
than it would with smaller front pistons, so the back brakes will see more pressure and the piston will have moved out more than before at the point
at which the front starts contributing to stopping the car
daviep - 14/7/12 at 04:56 PM
In myy opinion the problem will worse with the new calipers, you are correct that you will have a longer peda lBUT nothing will happen with the rear
brakes until pressure begins to build in the front system.
When on the move the system pressure required provide the same ammount of stopping power to the front wheels will now be lower due to the bigger
piston area and so the pressure to the rears will also be lower and doing less work than previously, so the result is now the rears are doing less
work compared to the fronts.
Cheers
Davie
Ivan - 15/7/12 at 01:24 PM
quote:
Originally posted by daviep
In myy opinion the problem will worse with the new calipers, you are correct that you will have a longer peda lBUT nothing will happen with the rear
brakes until pressure begins to build in the front system.
When on the move the system pressure required provide the same ammount of stopping power to the front wheels will now be lower due to the bigger
piston area and so the pressure to the rears will also be lower and doing less work than previously, so the result is now the rears are doing less
work compared to the fronts.
Cheers
Davie
Davie I think you are right 
- I was forgetting that the two pistons in the MC are uncoupled so the front (Secondary) one is driven by hydraulic
pressure from the rear (Primary) piston so they will both only make pressure once the bigger primary piston at the rear of the MC makes pressure. So I
can expect the rear brakes to be less effective and will definitely need to re-look at the MC sizes.
Methinks a visit to our local brakes expert is on the cards to find the right MC size or preferably slightly smaller secondary pistons that will work
in the existing MC after a re-line of the secondary bore.
Maybe it will be easier to fit dual cylinders!!!!
Fred W B - 15/7/12 at 04:33 PM
quote:
Maybe it will be easier to fit dual cylinders!!!!
Absolutely Ivan, I always think that it you are going to do something do it properly!
I will send you a spreadsheet I found on here that helps with brake calculations
Cheers
Fred W B
Ivan - 15/7/12 at 05:07 PM
Thanks Fred - of course you are right - I have a spare brake pedal assembly with booster etc so when I am free I will strip off the booster and
measure pedal ratio etc. and see what I can use in a dual system or design up a new system. In the mean time I will drive the car as it worked for 23
years like it is even on track days.
britishtrident - 15/7/12 at 06:59 PM
quote:
Originally posted by Ivan
Thanks for the comments - idle is at 750 revs so not too fast. Also never could get the hand brake to work properly but it was never very important to
me on an auto car.
When you jack up rear and have someone brake it is impossible to turn the rear wheel by hand so don't think there is problem with rear brakes,
anyway they were re-built recently and there is no oil contamination. What I will do some time is jack it up and see at what revs the rear brakes stop
holding the wheels (with the right chassis stands of course.)
But still think that the bigger front pistons will mean that the pedal will move down further before the front pads make proper contact with the disc
than it would with smaller front pistons, so the back brakes will see more pressure and the piston will have moved out more than before at the point
at which the front starts contributing to stopping the car - or am I wrong. Also think that any extra flex mentioned by British Trident in the AL
calliper might add to the extra engagement of the rear brakes which are the original Cast Iron Girlings.
Just what might be a dumb question - won't it be more difficult to achieve a decent pedal pressure and travel for a dual master cylinder system
on a 1100 kg car than a 600 kg one.
Excessive flex in any brake part is very bad news, flex in the front brakes won't magically add extra hydraulic pressure to the rear
brakes.
Normal*tandem brake master cylinders work on hydro static pressure not movement as soon as pressure is applied to the piston the resulting hydraulic
pressure rise is instantly applied to the other circuit.
*Unless considering one of the very few tandem master cylinder with different bore pistons for front and rear circuits
To get an idea how little fluid gets pumped in a disc brake system pump the brakes in a non-abs car then try and put a thin feeler gauge between the
pad and disc or between pad and piston................... if you can get a gauge in it will be very thin indeed. The amount of fluid pumped is
almost zero so even if you doubled the the calliper piston are double is still almost zero. The brake pedal movement you find a modern tin top is
99% down to lost motion in the servo and flexing of the pedal box and bulkhead.
Big heavy cars have always depended on high pedal leverage and some form of brake servo assistance (in the old days a gearbox driven servo) to
generate the required pressure.
Your problem is down down to brake effort distribution the bore size of the rear callipers and diameter of the inboard rear discs are small
for the job.
britishtrident - 15/7/12 at 07:01 PM
quote:
Originally posted by ed1801
I agree with this. Larger front piston area effectively moves your bias rearwards. (I think)
quote:
Originally posted by Ivan
But still think that the bigger front pistons will mean that the pedal will move down further before the front pads make proper contact with the disc
than it would with smaller front pistons, so the back brakes will see more pressure and the piston will have moved out more than before at the point
at which the front starts contributing to stopping the car
[/quote
In a word no,,,,
ed1801 - 15/7/12 at 08:13 PM
I stand corrected
Ivan - 18/7/12 at 09:09 AM
Have decided what to do after some feedback on here and on this thread
http://www.locostbuilders.co.uk/viewthread.php?tid=172528
I will swap the rear brakes to the secondary portion of the master cylinder and the front brakes to the primary - this will shorten pedal stroke and
give a bit more hydraulic leverage to the rear whilst reducing leverage to front which might well be overbraked. Then some careful testing in the
middle lane of an empty bit of road followed by a run on a brake dyno should tell me what I need to know - if rear is still underbraked I will then
sleeve down the secondary bore of the MC.