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  1. #11
    Join Date
    Sep 2017
    Location
    Southport
    Posts
    20
    Hope the “medication” worked after your trip to the dentist Chris!
    As GD pointed out fitting a mechanical servo isn’t straightforward, the m/c’s are mounted in a compartment behind the front wheel, it’s neat but space is very limited so two singles or a twin remote servo are the only realistic option.
    Your comment about the accelerometer got me thinking GD, my mot guy has an electronic version of the old Tapley meter which will be more accurate than an app so I might pay him a visit to see exactly what’s going on even though the brakes perform well during mot.
    Your suggestion of a meet up sounds good GD, whereabouts are you based?

  2. #12
    Join Date
    May 2016
    Location
    Hockley ,Essex
    Age
    58
    Posts
    492
    I didn't realise you were so pushed for space on a GD, thought they would have been similar
    to Dax and the SR's .
    The SR's used as mentioned previously, a Fiesta type or one from the MGB's back in the day.
    Both of these are relatively small unlike the huge booster of the Jag's.
    By the way both the drugs & the alcohol has worn of and will just need a recreational drink tonight,
    the dentistry has now settled down .
    Chris.
    Last edited by Chris1; 17-02-21 at 02:34 PM.

  3. #13
    Join Date
    Jul 2012
    Location
    NW England
    Posts
    759
    Quote Originally Posted by Alan M View Post
    Hope the “medication” worked after your trip to the dentist Chris!
    As GD pointed out fitting a mechanical servo isn’t straightforward, the m/c’s are mounted in a compartment behind the front wheel, it’s neat but space is very limited so two singles or a twin remote servo are the only realistic option.
    Your comment about the accelerometer got me thinking GD, my mot guy has an electronic version of the old Tapley meter which will be more accurate than an app so I might pay him a visit to see exactly what’s going on even though the brakes perform well during mot.
    Your suggestion of a meet up sounds good GD, whereabouts are you based?

    Hi Alan

    The accelerometer I used was something we had at work for measuring accelerations on machine tools, a metal box about the size of a pack of 20 but I had to link it to my laptop on the passenger seat.
    You could probably do just as well now with most (so called) Smart phones as these have multi-axis accelerometers built in, probably even more accurate than the one I had also due to the passage of time.

    The reason I decided to do that test was due to my feeling that the rate of deceleratoin may have simply been a perception issue due to the car not nose diving as much as a typical saloon car, partly due to harder suspension and partly due to reduced rear->front weight transfer because of the centre of gravity and weight distribution.
    It turned out it was low but not quite as bad as it felt, worth sorting out though.

    I'm based on the North side of Preston these days (just south of Garstang), I was on the Southport side at Tarleton when I built the car so still get around the Southport area visiting friends and family (when allowed!).

  4. #14
    Join Date
    Jul 2007
    Location
    peterborough
    Posts
    274
    Hi,

    Your main gripe is not that the brakes don't work but more how hard you have to push the pedal to actually stop. Having driven one of our cars with a faulty servo I can sympathise with you, not a great experience . Two options: Option 1 a remote dual servo. Option 2 remove current master cylinder and find a small compact servo/master cylinder that will fit in the limited space and adapt bulkhead etc to fit.
    Anything else will not make much difference (pedal ratio, pads etc)
    We use a rover25/200 MGTF servo and master cylinder. It's the smallest we could find and works perfectly, great feel and good even front to rear split. Always passes brake tests.
    You won't get to much power to the wheels, it will just mean less pedal force (a bit like driving your daily car)
    Hope this helps.

    Jon
    AK

  5. #15
    Join Date
    Aug 2004
    Location
    Christchurch, NZ
    Age
    60
    Posts
    2,522
    Is it feasible to go down a size (diameter) of master cylinder to improve the ratio rather than fitting a servo?
    Cheers

    Myles D-W

  6. #16
    Join Date
    Sep 2017
    Location
    Southport
    Posts
    20
    I’m probably suffering a bit from the same perception issue that you mention GD, the brakes are powerful enough so it’s just a case of reducing the pedal pressure. Out of interest I’m going to try the electronic Tapley meter before I change anything then I’ll have before and after readings.
    And I think a meet is definitely on the cards once were able to get out and about again so keep in touch!
    Thanks also for your input Jon (and Myles) I did look at a VW Polo servo as an option when I rebuilt the car so it might be worth another look now I’m older and supposedly wiser!

  7. #17
    Join Date
    May 2016
    Location
    Hockley ,Essex
    Age
    58
    Posts
    492
    Quote Originally Posted by Alan M View Post
    I’m probably suffering a bit from the same perception issue that you mention GD, the brakes are powerful enough so it’s just a case of reducing the pedal pressure. Out of interest I’m going to try the electronic Tapley meter before I change anything then I’ll have before and after readings.
    And I think a meet is definitely on the cards once were able to get out and about again so keep in touch!
    Thanks also for your input Jon (and Myles) I did look at a VW Polo servo as an option when I rebuilt the car so it might be worth another look now I’m older and supposedly wiser!
    Your on the right tracks as the small vw is similar weight to your GD ,
    good luck with what decision you come to .
    Chris.

  8. #18
    Join Date
    Sep 2017
    Location
    Southport
    Posts
    20
    Cheers Chris

  9. #19
    Join Date
    Jul 2012
    Location
    NW England
    Posts
    759
    Quote Originally Posted by Alan M View Post
    I’m probably suffering a bit from the same perception issue that you mention GD, the brakes are powerful enough so it’s just a case of reducing the pedal pressure. Out of interest I’m going to try the electronic Tapley meter before I change anything then I’ll have before and after readings.
    And I think a meet is definitely on the cards once were able to get out and about again so keep in touch!
    Thanks also for your input Jon (and Myles) I did look at a VW Polo servo as an option when I rebuilt the car so it might be worth another look now I’m older and supposedly wiser!
    The bit that is bothering me is that in your first post you say:

    "so much so that I can only lock the brakes in the wet!"

    I never had a problem locking the wheels in the dry so I'd have thought you should be able to do this which would indicate that the system is not very efficient (input force translated into braking force).

    Obviously locking up the wheels is not something you want to do but you do want to be able to apply maximum braking force (just prior to lockup) with "reasonable" pedal effort.

    Of course the variable there is the word "reasonable", what is OK for one user may be too much (or too little) for another. Personally I don't mind having to use a high(ish) input force to get maximum braking effect and I do quite like the limited pedal movement (makes heel/toe easier OMO) this gives.
    Having said that if I could improve the efficiency (reduce input force) without too much complexity I would be tempted. Been spoilt with modern cars and massively high brake assistance.
    If this was to be done via a servo that would mean it would have to be a mechanical one (between pedal and master cylinder) as a hydraulic one would fit into the "too much complexity" category for me.

    I'm assuming a VW Polo servo is the mechanical type? If so do you have any dimensions (depth and diameter)?
    As has already been mentioned the depth can be an issue but the diameter can also.
    Presuming you have a hydraulic clutch the master cylinder will be sitting alongside your brake master and not very far apart, if the servo diameter is too large it will intersect the clutch master so that also needs to be taken into account.

    Other ways of improving the efficiency would be
    1) Grippier pads - Increasing friction force between pad and disc for constant caliper cylinder force) - Simplest to experiment with, possibly also cheapest
    2) Larger discs - Increasing the leverage applied to the spinning wheel. - Reletively straightforward on front (will need to re-locate caliper) but difficult on inboard rear. Converting to outboard rear gave larger disc diameter "for free".
    3) Increasing the hydraulic ratio (calipers with larger cylinders, smaller dia' master cylinder) - This will mean greater pedal travel
    4) Increasing the the mechanical ratio, reduce the distance between pedal pivot and master cylinder pushrod - This will also increase pedal travel.

    I guess the starting point is to rig up the accelerometer and work out what kind of retardation you are getting for an acceptalbe pedal pressure, I found it good to compare this against my daily driver to take road surface out the the equation. If the result is OK then look no further, if not then it's decision time!

  10. #20
    Join Date
    Jul 2012
    Location
    NW England
    Posts
    759
    I went and did a bit of measuring in the master-cylinder area on my car.

    On the GD the pedalbox is mounted to one side of the firewall with the master cylinder on the other, this is in a compartment which is sealed on the front by a removeble cover which effectively makes up the rear of the wheel arch.

    The front cover runs at an angle to the firewall so I took measurments at the level where the master cylinder is mounted. Individual vehicles will vary a bit but should not be too much.

    From the firewall to the inside face of the cover plate is 240mm

    From the front of my master cylinder to the cover plate is 50mm.
    I have a 5mm thick aluminium plate between the master-cylinder and the firewall, probably not really needed so this could increase to 55mm

    To fit a tandem/mechanical servo in this area it would need to be no greater than 55mm (just over 2").

    It may be possible to increase this space by 'blistering' the cover plate but this would need careful investigation/measurement and I suspect no greater than 1" would be available.
    There may be shorter master-cylinders available (Jag' one is 185mm from mount flange to front face) which would free up some more space.

    On my car the pedals are spaced at 125mm (which should be standard) allowing about 105mm from the brake cylinder centreline and the side of the clutch master so the servo would have to be around 210mm maximum diameter.

    Also for reference:
    Master cylinder bore = 7/8" (22.225mm) acording to Haynes manual
    Front discs are 284mm diameter - I reckon these could be increased at least 50mm with my 16" wheels more (obviously) with larger wheels.

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