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Thread: Brake Bias setting

  1. #1
    Registered User ahk068's Avatar
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    Brake Bias setting

    Upgraded the front brakes on the xr6t. Brembo 6 piston, all 40mm (yes all the same) on 355/28 rotors, rears are territory calipers, single 45mm on 328/28 rotors.

    First time out with stock booster gave me a really long pedal inspiring no confidence. Came home and bought a wildwood 7:1 pedal that came with 3 x 7/8th cylinders which I figured were a good place to start (with no technical support to prove this being correct). I'm about half way through mounting and realised getting brake lights to work wasn't simple so picked up a wildwood proportioning valve with a light sender. My logic is the rears will have higher line pressure due to the smaller caliper so I can reduce this to even out bias and then use the balance bar to fine tune things.

    Does all of this sound like it will work ok? I was going to set line pressures in the shed to get them close but what is a good starting point given falcons are nose heavy?

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    Pressure has nothing to do with the caliper
    Its to do with the master
    Smaller master... higher pressure
    So if your running the same front and back... same pressure
    Though... balance will determine when the light comes on
    You'll need to play with master cyl sizes when you get it working to get the balance right

    What caliper... never seen a Brembo with 6 pistons the same size
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  3. #3
    Down with ma homies Greg Rust's Avatar
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    long pedal sounds like air in the system,
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    Registered User ahk068's Avatar
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    ran a litre of fluid through the system and it never came up. abs plumbing might have been being difficult so dunno. car stopped, just in the last 20% of movement

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    Registered User ahk068's Avatar
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    Quote Originally Posted by dattoman1000 View Post
    Pressure has nothing to do with the caliper
    Its to do with the master
    Smaller master... higher pressure
    So if your running the same front and back... same pressure
    Though... balance will determine when the light comes on
    You'll need to play with master cyl sizes when you get it working to get the balance right

    What caliper... never seen a Brembo with 6 pistons the same size
    jeep srt caliper.
    I was thinking with balance bar level the rears will build pressure quicker and therefore potentially lock before fronts get to that pressure level. does that make sense?

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    Last edited by ahk068; 08-07-19 at 09:52 PM.

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    Long pedal is because the front master cylinder is now too small, and it can't displace enough fluid for the increased piston area surface in the new front calipers.

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    Registered User ahk068's Avatar
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    Quote Originally Posted by Sprinkles View Post
    Long pedal is because the front master cylinder is now too small, and it can't displace enough fluid for the increased piston area surface in the new front calipers.
    Thanks, volume was creeping into my thought process along the same lines you've described.

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    Į\_(ツ)_/Į burn is weird's Avatar
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    I'm prepared to fuck around with many things without knowing what i'm doing, but brakes are not one of those things.

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    Racer 36's Avatar
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    Have a look at the sizes used on the SRT - will give you a starting point for what size master/rear should be. You can spend a lot of time farking around with this.

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    People like you scare the fuck out of me, not the simplest understanding of what you are doing.

    There is no such thing as a "Brake Proportioning Valve" it is simply a pressure limiting valve it sets a maximum pressure for that circuit.

    It's going to be a pretty radical set up you end up with, with 12 x 40mm caliper pistons on the front and 2 x 45mm pistons on the rear.

    Good Luck

    P.S. how much does the Falcon weigh?
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    Down with ma homies Greg Rust's Avatar
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    Quote Originally Posted by Sprinkles View Post
    Long pedal is because the front master cylinder is now too small, and it can't displace enough fluid for the increased piston area surface in the new front calipers.
    Not really but letís work it out so we can solve this. The total volume of the MS isnít displaced each time you apply as the system is closed. Itís actually the ratio of the MS to the caliper that provides the increased force.

    OP; whatís the three piston diameters and whatís the previous caliper two pistons diameter.

    I still reckon thereís air in the system
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    Registered User ahk068's Avatar
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    Quote Originally Posted by Roy928tt View Post
    People like you scare the fuck out of me, not the simplest understanding of what you are doing.

    There is no such thing as a "Brake Proportioning Valve" it is simply a pressure limiting valve it sets a maximum pressure for that circuit.

    It's going to be a pretty radical set up you end up with, with 12 x 40mm caliper pistons on the front and 2 x 45mm pistons on the rear.

    Good Luck

    P.S. how much does the Falcon weigh?
    I understand the logic behind the setup and the bias isn't great with what I have. I enjoy figuring it out and making what I have work. However I also understand there is some maths involved here and asking advice from people with more experience fast tracks my learning. Yes I may be brave but I'll test on a quiet track day before going nuts.

    Weight would be around 1500-1550kg

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    Last edited by ahk068; 09-07-19 at 10:03 PM.

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    Registered User ahk068's Avatar
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    Quote Originally Posted by Greg Rust View Post
    Not really but letís work it out so we can solve this. The total volume of the MS isnít displaced each time you apply as the system is closed. Itís actually the ratio of the MS to the caliper that provides the increased force.

    OP; whatís the three piston diameters and whatís the previous caliper two pistons diameter.

    I still reckon thereís air in the system
    brembos run 6x40mm pistons, yes they're all the same.

    Google won't tell me what the stock twins are but I reckon about 40mm also

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    Quote Originally Posted by Greg Rust View Post
    Not really but letís work it out so we can solve this. The total volume of the MS isnít displaced each time you apply as the system is closed. Itís actually the ratio of the MS to the caliper that provides the increased force.
    To be precise, we are talking about the ratio of the area of the master cylinder piston to the combined areas of the caliper pistons. A bigger master cylinder area displaces (pushes) more fluid for the same pedal stroke, and exerts less pressure. Conversely, a bigger master cylinder diameter for the same piston area will have a shorter pedal stroke.

    That's the technical explanation, the non technical explanation is what 36 said. Have a look at the diameter of the master cylinder from the original vehicle the calipers came off, that would be your guide for the starting point for the master cylinder diameter for your car. I expect it will be bigger than what you have now.

  15. #15
    Registered User ahk068's Avatar
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    Quote Originally Posted by Sprinkles View Post
    To be precise, we are talking about the ratio of the area of the master cylinder piston to the combined areas of the caliper pistons. A bigger master cylinder area displaces (pushes) more fluid for the same pedal stroke, and exerts less pressure. Conversely, a bigger master cylinder diameter for the same piston area will have a shorter pedal stroke.

    That's the technical explanation, the non technical explanation is what 36 said. Have a look at the diameter of the master cylinder from the original vehicle the calipers came off, that would be your guide for the starting point for the master cylinder diameter for your car. I expect it will be bigger than what you have now.
    jeep is 1" or 1 1/8th depending on year so bigger than the original falcon one.

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    Quote Originally Posted by ahk068 View Post
    jeep is 1" or 1 1/8th depending on year so bigger than the original falcon one.

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    That would be my suggestion for the next thing to try.

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    I'm with Greg Rust on this, the difference in the MC sizes isn't enough to correlate to such a big change in the pedal throw. Do you have the balance bar in the centre?
    I'm a big fan of pushing brake fluid back up from the caliper to the MC, bubbles like to go up. I don't know how this would work with ABS though.

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    Quote Originally Posted by ahk068 View Post
    I understand the logic behind the setup and the bias isn't great with what I have. I enjoy figuring it out and making what I have work. However I also understand there is some maths involved here and asking advice from people with more experience fast tracks my learning. Yes I may be brave but I'll test on a quiet track day before going nuts.

    Weight would be around 1500-1550kg

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    If you don't understand the mathematics you don't understand the logic.
    Are you optimistic or pessimistic about the end of the world?

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    Ellis Juan mizone's Avatar
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    Grab the MC from a SRT 300 or Jeep. They should be cheap enough.

    That will fix the fronts. Then use your PV to set the rear bias. Empty open space with lots of stops and dial it in.

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    Registered User ahk068's Avatar
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    Quote Originally Posted by Komdotkom View Post
    I'm with Greg Rust on this, the difference in the MC sizes isn't enough to correlate to such a big change in the pedal throw. Do you have the balance bar in the centre?
    I'm a big fan of pushing brake fluid back up from the caliper to the MC, bubbles like to go up. I don't know how this would work with ABS though.
    Car didn't have balance bar fitted when last time out.

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    Registered User ahk068's Avatar
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    Quote Originally Posted by mizone View Post
    Grab the MC from a SRT 300 or Jeep. They should be cheap enough.

    That will fix the fronts. Then use your PV to set the rear bias. Empty open space with lots of stops and dial it in.
    Pedal box and master cylinders already mounted so I'm kind of committed to this path.

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  22. #22
    Ellis Juan mizone's Avatar
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    All I'm gunna write is all the boys and girls in Auburn Hills, and those in Italy, spent a LOT of time and effort getting the Brembos to work on the 300 SRT. Hence the piston sizes and complex maths involving such things as the ratio of MC size to caliper piston size You might be flogging a dead horse trying to figure anther way around it.
    Last edited by mizone; 12-07-19 at 11:58 AM.

  23. #23
    Į\_(ツ)_/Į burn is weird's Avatar
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    Quote Originally Posted by Komdotkom View Post
    I'm a big fan of pushing brake fluid back up from the caliper to the MC, bubbles like to go up. I don't know how this would work with ABS though.
    not sure i agree with this approach. the bleed points are in the top of the caliper and the fluid entry point is down usually in the centre of the caliper. much more difficult to push air down a wide piston bore than through a tiny tube. any having air in the caliper is MUCH worse than air further up the system, where it is 30 degrees C instead of 300 degrees C.

    I do like Vac bleeding though, as it enlarges any air pockets and makes it easier to get out.
    Last edited by burn is weird; 12-07-19 at 10:34 AM.

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  24. #24
    BOOSTFARKIN Morcs's Avatar
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    Probably find original mc on jeep has a stepped bore or some shit whilst easy enough to figure out mathematically would still take some fine tuning on the other end.
    As above system could have air before or in the abs module could be causing issues to bleed there is a procedure to do so may need to go to dealer or specialist to plug in service tool...

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