Page 1 of 2 12 LastLast
Results 1 to 10 of 17
  1. #1
    Join Date
    Jul 2018
    Location
    france
    Age
    48
    Posts
    19

    Brake bias question

    Hi everyone.
    My question is about brake bias on our Cobras as i have to replace my rusty calipers and so planed to go with aftermarket calipers.
    My setup will use a single double M/C with a little (compact) booster assist.
    I assume that :
    - weight repartition will be near 50/50% with a low COG
    - XJ6 calipers / disk give a brake balance of 63/37
    - some DAX (and RAM) user seems to place a front brake proportionning valve

    So my guess is that a setup with a 55/45 bias (and so with a rear proportionning valve) is a good start point ?

    Thanks, have a good day.

  2. #2
    Join Date
    May 2016
    Location
    Hockley ,Essex
    Age
    58
    Posts
    380
    You won't get too much braking on the rears in the first place if you are using standard inboard xj discs.
    So you will need to keep that in mind, you should be able to get sufficient but will lock up if you try.
    Just enough is what you need.
    Chris.

  3. #3
    Join Date
    Jul 2018
    Location
    france
    Age
    48
    Posts
    19
    Quote Originally Posted by Chris1 View Post
    You won't get too much braking on the rears in the first place if you are using standard inboard xj discs.
    So you will need to keep that in mind, you should be able to get sufficient but will lock up if you try.
    Just enough is what you need.
    Chris.
    Front rotors are updated to 12.19" vented rotors and rear by 11.75" vented rotor. 55/45 bias is based on these rotors and with front total piston aera of 4.8in² and rear total piston aera of 4.12in². My question is more about "good" initial brake balance numbers : no doubt that standard inboard non vented discs are the limiting factor when keep XJ brakes. My goal is to have enought available brake power on the rear and to adjust it with a prportionning valve. I was curious if anyone have experience with such mods and can feedback to me if i make a mistake in my approach ?

  4. #4
    Join Date
    Jul 2012
    Location
    NW England
    Posts
    690
    Different car as mine's a Gardner Douglas but still a Cobra with low CoG, centralised mass and Jaguar XJ S3 running gear. Originally not happy with braking, mainly fronts locking up way before rears so not getting enough contribution from them.
    There was some advice to fit a pressure reducing valve into the front line but that just didn't seem like good practice to me, up spec'ing the rears seemed more like 'good practice', possibly could have got the inboard system working better but struggled finding any better available brakes and too difficult to work on (didn't have a lift back then) so converted to outboard using ( I think) X300 discs which are just over 300mm diameter and vented, though only about 21mm thick, this allowed me to get much better balance although still need more pedal force than I like so currenlty looking at fitting servo.

    What (compact) servo/booster have you used out of interest?

  5. #5
    Join Date
    Jul 2018
    Location
    france
    Age
    48
    Posts
    19
    Quote Originally Posted by GDCobra View Post
    Different car as mine's a Gardner Douglas but still a Cobra with low CoG, centralised mass and Jaguar XJ S3 running gear. Originally not happy with braking, mainly fronts locking up way before rears so not getting enough contribution from them.
    There was some advice to fit a pressure reducing valve into the front line but that just didn't seem like good practice to me, up spec'ing the rears seemed more like 'good practice', possibly could have got the inboard system working better but struggled finding any better available brakes and too difficult to work on (didn't have a lift back then) so converted to outboard using ( I think) X300 discs which are just over 300mm diameter and vented, though only about 21mm thick, this allowed me to get much better balance although still need more pedal force than I like so currenlty looking at fitting servo.

    What (compact) servo/booster have you used out of interest?
    Thanks for your information that confirm advantage to add rear brake bias. Do you know your brake bias with your setup now, or have you calipers infos (pistons diameter and numbers) ? For the servo/booster i've planned to use an an AYGO/107/C1 because of relativly small assisted and very compact. Another question, have you add an adjustable rear prop valve ?

  6. #6
    Join Date
    May 2004
    Location
    Cowbridge, United Kingdom
    Age
    63
    Posts
    10,277
    I've built three Cobras with Jaguar brakes.
    All three have required a pressure reducing valve to the rear circuit.
    Don't forget if you have to go through IVA you cannot use an adjustable valve.
    I've used Ford Sierra / Mondeo ones previously.
    The Hawk has a bias pedal box so is different.
    Still need to lock the adjustment for IVA.
    Kev Davies
    South Wales Area Rep. UKCC Membership Secretary
    DAX Mk4, 383 Chevy Stroker, Tremec. SOLD
    Contemporary CCX 3-4028, 445ci Big Block FORD FE,TKO 600, Old School IVA'd and SOLD
    Dax De Dion LS2 and T56 IVA'd June '17 SOLD
    1965 Ford Mustang 289 Convertible SOLD
    In build- Hawk 289 Sebring Awaiting IVA

  7. #7
    Join Date
    Jul 2012
    Location
    NW England
    Posts
    690
    Quote Originally Posted by sunday View Post
    Thanks for your information that confirm advantage to add rear brake bias. Do you know your brake bias with your setup now, or have you calipers infos (pistons diameter and numbers) ? For the servo/booster i've planned to use an an AYGO/107/C1 because of relativly small assisted and very compact. Another question, have you add an adjustable rear prop valve ?
    Bear in mind I only added the bias valve to the rear line because I was up-spec'ing the rear brake capability and I wanted this capability 'in my back pocket' in case I went too far. As it has turned out I've never really needed to do much with it.

    Once I had gone to outboard brakes I fitted Wilwood radial mount Dynalite 4 pot calipers with 1.25" diameter pistons, wanted same caliper types both ends to added same calipers to front but with 1.5" pistons (although weight transfer is reduced fronts will still need to be stronger).

    Worth mentioning that rear disks are now (IIRC) 306mm dia, fronts are standard XY6 S3 which I believe are 284mm giving rears a small advantage.

    Thanks for the information on your servo/booster. Best information I can find on that is that the diameter is 225mm, perhaps you can confirm (I use tape measure to measure circumference then work out diameter, easier/more accurate than trying to measure over the unit), I'm currenlty working with an MX5 unit which is similar in size but slightly larger than I want due to space requirements. If I can find something smaller I'll go down that route.
    Last edited by GDCobra; 13-04-21 at 11:50 AM.

  8. #8
    Join Date
    Jul 2012
    Location
    NW England
    Posts
    690
    Quote Originally Posted by kdavies3 View Post
    I've built three Cobras with Jaguar brakes.
    All three have required a pressure reducing valve to the rear circuit.
    Don't forget if you have to go through IVA you cannot use an adjustable valve.
    I've used Ford Sierra / Mondeo ones previously.
    The Hawk has a bias pedal box so is different.
    Still need to lock the adjustment for IVA.

    Maybe you got lucky or I got unlucky but my fronts would always lock up before the rears limiting how much rears could contribute. I tested this with accelerometer, managed 0.7G when fronts locked, daily driver managed 0.96G on same piece of road a few min's later so overall braking capability was significantly limited. Obviously different cars with different CoG and weight distribution would have a positive or negative effect on that.

  9. #9
    Join Date
    Jul 2018
    Location
    france
    Age
    48
    Posts
    19
    Quote Originally Posted by kdavies3 View Post
    I've built three Cobras with Jaguar brakes.
    All three have required a pressure reducing valve to the rear circuit.
    Don't forget if you have to go through IVA you cannot use an adjustable valve.
    I've used Ford Sierra / Mondeo ones previously.
    The Hawk has a bias pedal box so is different.
    Still need to lock the adjustment for IVA.
    I'am ok that many Cobras use donor parts and so donor brake bias. Nevertheless these brake bias are targeted to totally different weight and cog vehicles. I've also read posts about putting some rear brake upgrades or front valve to help improve overall braking efficiency (front brake valves used on RAM and some DAX ?). Others Cobras Users make same observations (see this FF5 thread : proportioning valve on front | Factory Five Racing Forum (ffcars.com)). With taking onto account safety (ie not too much rear brake), a better front/rear bias from original 63/37 to somethink near 55/45 seems to be logic ?

  10. #10
    Join Date
    Jul 2018
    Location
    france
    Age
    48
    Posts
    19
    Quote Originally Posted by GDCobra View Post
    Maybe you got lucky or I got unlucky but my fronts would always lock up before the rears limiting how much rears could contribute. I tested this with accelerometer, managed 0.7G when fronts locked, daily driver managed 0.96G on same piece of road a few min's later so overall braking capability was significantly limited. Obviously different cars with different CoG and weight distribution would have a positive or negative effect on that.
    Thank you for that info, have you keep a rear prop valve or direct plumbed ?

Page 1 of 2 12 LastLast

Tags for this Thread

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •