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  1. #1
    Join Date
    Sep 2017
    Location
    Southport
    Posts
    20

    Advice on brakes

    Hi Gents, I hope you’re staying safe and well!
    I have an early GD mk3 (chassis no 10) which I bought some years ago and completely rebuilt.
    It has the usual Jag running gear and inboard rear brakes.
    The front callipers had been replaced by the previous owner for mot so I retained these together with the four discs plus pads which were all in excellent condition. I rebuilt the rear callipers and GD supplied a dual circuit Jag master cylinder, GD also suggested I wouldn’t need a servo.
    Fast forward a few years and the car is back on the road and I’m well pleased with how it drives but I would like to improve the brakes. It pulls up well enough and without any sign of fade but it does need a heavy right foot, so much so that I can only lock the brakes in the wet!
    The obvious answer is a dual circuit remote servo but space is tight and I don’t know if it might give me too much power and the wheels would just lock up during one of those occasional bum twitching moments! (maybe I’m overthinking this?)
    The alternative is to alter the pedal ratio and/or replace the pads so given I don’t know the spec of the pads that I have already I’d appreciate any feedback you have on alternatives and how they perform.
    I don’t use the car for track days and I don’t tend to be heavy on brakes so I want to avoid the ones that only work well when they’re really hot!
    thanks in advance
    Alan

  2. #2
    Join Date
    May 2004
    Location
    Finchampstead, Berkshire, UK.
    Age
    56
    Posts
    3,858
    Chassis #10, blimey finally found one older than mine! (Mines #34).

    Easiest option will be softer pads (either standard Jag ones or some EBC pads (whatever their softest compound is currently) - I'm running AP disks/calipers and Pagid Blue pads, so zero comparison sorry.

    Next step would be an MC change.

    Might just be worth checking that you're using the GD pedal box (Mechanical ratio etc) given how early your car is.

    Regards

    Jim

  3. #3
    Join Date
    Jul 2012
    Location
    NW England
    Posts
    759
    I was really disappointed with the brakes on mine initially, tried swapping the front callipers for some Brembo items but no help. Eventually realised that looking as the front brakes was pointless as these would lock up OK (with enough leg, luckily a few decades of cycling have left me with quite in-weak legs) but not the rears, so in a moment of clarity I realised that the only way to improve them was to up-spec the rears, difficult while they were located inboard and I had no easy way of lifting the car to get at them. To enable any chance of working with them I decided to modify the axle to get them outboard using XJ40 or later (can’t recall which) halfshafts and uprights. I then made up some brackets to allow mounting Wilwood radial mount calipers. Liked the setup so much I decided to mount similar calipers (larger pistons) to the front too.
    still needs a big of leg power but like I say I’ve got that covered for now at least.

  4. #4
    Join Date
    Nov 2012
    Location
    Lanarks
    Age
    65
    Posts
    701
    Quote Originally Posted by Alan M View Post
    I would like to improve the brakes.

    It pulls up well enough and without any sign of fade

    but it does need a heavy right foot, so much so that I can only lock the brakes in the wet!
    So the braking system does not work as it should
    in a lightweight Cobra - not locking up the wheels.

    What is different from your original donor Jaguar braking system?
    Pedal ratio , master cylinder bore or no servo….. (All 3)

    So what is your best option to increase the line pressure to get them
    to operate effectively.

    What is your pedal ratio ?
    What is the Master Cylinder Bore Size ?

  5. #5
    Join Date
    Jul 2012
    Location
    NW England
    Posts
    759
    Quote Originally Posted by drammyboy View Post

    What is different from your original donor Jaguar braking system?
    Pedal ratio , master cylinder bore or no servo….. (All 3)
    That's a really good question to kick off with, I'd say the biggest difference is 'no servo' as I've highlighted (presuming the OP is using the standard, Jaguar, mastercylinder which I believe is standard for GD), although the (mechanical) pedal ratio may also be different to Jaguar.

    All things being equal a non-servo system will allways require more pedal pressure than an assisted system due to the pressure in the caliper cylinders and therefore between the pads and the disk will be lower (stating the obvious I know!).
    For this reason it's probably best to get pads which offer as much friction as possible to give higher braking force for lower pad/disc pressure, probably best to avoid 'race' pads which need to get up to temperature (and stay there) to give maximum braking effect, and will probably glaze more.
    May also be worth looking at grooved disks which are supposed to prevent the pads becoming glazed (or worse still double glazed), you'll most likely get more brake dust and shorter pad life but is that really a concern on this type of car?

    One problem with changing ratios (either hydraulic or mechanical) is that increasing the force at the pads will require longer movement at the pedal, possibly making it feel spongy, all depends on how much movement you have now.

  6. #6
    Join Date
    Sep 2017
    Location
    Southport
    Posts
    20
    Thanks for your replies guys, so in answer to the points you’ve raised, yes it is a GD pedal box, the paperwork I got with the car included two GD invoices to the original builder from1990, one of which details the pedal box, it’s a good solid pedal so there is some scope to improve the ratio without risking excessive travel.
    The original m/c was an oddball single circuit effort with an integral reservoir, very strange given the set up on a GD so I went to GD for the the correct master cylinder that they use.
    As GD Cobra has pointed out pad life isn’t really a concern so I think the way forward will be softer pads and alter the pedal ratio to see what effect that has before looking for a smaller diameter m/c.
    thanks again
    Alan

  7. #7
    Join Date
    May 2016
    Location
    Hockley ,Essex
    Age
    58
    Posts
    492
    For my SR V8 I have the old xj6 four pot system with inboard rears and standard Jag pads.
    The servo/master cyl (booster) that I have for the last twenty years is a Ford Fiesta van one.
    I only have a light weight 8 up front so therfore gives me about a 50 - 50 weight split, this
    will give me the option of locking up the fronts quite easy if to much pressure is applied at
    the pedal.
    The mot's have always been positive with the brake testing and the inspectors have sometimes
    commented on how good the hand brake performs, although that has nothing to do with the
    hydraulic circuit.
    Chris.

  8. #8
    Join Date
    Jul 2012
    Location
    NW England
    Posts
    759
    Quote Originally Posted by Alan M View Post
    I think the way forward will be softer pads and alter the pedal ratio to see what effect that has before looking for a smaller diameter m/c.
    thanks again
    Alan
    I'd sort out the softer pads first as this will be straightforward (as long as you can access the rears) and relatively cheap to do and easily reversible if it doesn't work out as you expected.
    The padal arrangement is proven to be good (although obviously may have been modified since #10) so "shouldn't" give any issues so probably best left alone until other avenues explored.

    Hey, I see you are over in Southport, not too far from me. If you need any help/advice once this lockdown nonsense is over we could arrange a meet up.

  9. #9
    Join Date
    Jul 2012
    Location
    NW England
    Posts
    759
    Quote Originally Posted by Chris1 View Post
    The servo/master cyl (booster) that I have for the last twenty years is a Ford Fiesta van one.
    Chris.
    Is that a mechanical servo (acting on pedal) rather than hydraulic?
    Unfortunately space is limited on the GD for a servo inline with the master cylinder. I don't think it is practical to squeeze a servo in there, going from memory the end of the master cylinder sits within about 1" of the (removeble) bulkhead behind the front wheel, it may be possible to add a "bulge" to this bulkhead to get a little more space but I'd be surprised it this would work out.

    That leaves hydraulic servos, where the assitance is applied to the hydraulic line(s), problem here (apart from being more complicated) is space, particularly bearing in mind it will need a servo per circuit, so 2 of them to locate. I guess it may be possible to locate them ahead of the engine or even ahead of the radiator, think I'd exhaust other possibilities first though.




    Quote Originally Posted by Chris1 View Post
    will give me the option of locking up the fronts quite easy if to much pressure is applied at the pedal.
    Chris.
    That's good as long as you are getting good overall decelaration, all wheels contributing to the slow down as much as possible.
    In my case I felt the front lock-up was premature. To prove this out I used an accelerometer device and found I was pulling around 0.7G when the fronts locked. A back to back comparison with my daily drive on the same piece of road showed I was getting just over 0.9G when the ABS kicked in. This is what prompted me to up-spec' the rear brakes.
    But that doesn't seem to be Alan's (immediate) problem, he is struggling to get any wheels to lock up in the dry so that needs sorting first before worrying about the front-rear balance.

    I put this 'issue' down to the way the weight is distributed on the Cobra compared to the Jaguar doner. The centre of gravity is lower and most of the weight is more centrally located (only bodywork, radiator and fuel tank outside of wheelbase) therefore rear->front weight transfer under braking is reduced. This car is effectively mid engined.



    Quote Originally Posted by Chris1 View Post
    The mot's have always been positive with the brake testing and the inspectors have sometimes commented on how good the hand brake performs, although that has nothing to do with the hydraulic circuit.
    Chris.
    Never has a problem with my Jaguar inboard handbrake at MoT either. I know a lot of people knock the Jaguar handbrake but I think as long as all parts are working correctly this can work fine.
    I've recently swapped the handbrake to outboard also, not had it MoT'd yet but hope this is at least as effective.

  10. #10
    Join Date
    May 2016
    Location
    Hockley ,Essex
    Age
    58
    Posts
    492
    Will get back to you tomorrow with dimensions and a part no of the unit if required.
    The brake/servo is vacuum operated from the inlet manifold and the pedal drives the
    hydraulic master cylinder via a push rod.
    If you need any other info will talk tomorrow, not much help at present after a good
    pummelling at the dentists, drugs are wearing off and have started on the whiskey.
    Chris.

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