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Any way to adjust the front/rear brake bias?

Discussion in 'A3/S3 Forum (8L Chassis)' started by Westy, Sep 25, 2012.

  1. Westy
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    Westy Double Dark Side! Diesel & 8P

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    [Sep 25, 2012]
    Just wondering if there was any way at all to adjust the brake bias? Basically the rears don't appear to do a thing. I changed my rear discs and pads 2 weeks ago as they were nearly 3 years old and the components I removed were like brand new!

    Is there a bias lever attached to the rear beam? For some reason I thought the bias was adjusted by the height of the car. So the more passengers/weight in the back, the lower the car and the bias lever would adjust similar to the xenon lever on the front?

    I know some other VAG cars have a screw type thing to adjust the bias but is there anything we can do at all to our cars?
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  3. jojo
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    jojo Looking for Boost! Staff Member Moderator

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    [Sep 25, 2012]
    Not sure on our S3's but my mk2 has a brake bias valve, which is height dependant on the rear. Our S3's have brake force distribution, not sure what it exactly does, but it's suppose to spread the braking forces, when the ABS kicks in or something?

    Anyways, on my S3, when I put the ECS stage1 kit on, I could see a line of brake dust on the rear wheels after a track session, so they are definitely doing something on my car.
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  4. StaceyS3
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    StaceyS3 Well-Known Member

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    [Sep 25, 2012]
    Strange because I've gone through 1 set of redstuff on the rear and over half way through a set of pagid RS2's which still got as spare and now got ds2500's on there but not sure how they're holding up as not checked them yet?

    Maybe got ultra hard pads on the rear?
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  5. Prawn
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    Prawn My other car is a MINI!!!!

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    [Sep 25, 2012]
    are you using standard rear Pads Westy?

    I've been using std rear pads up until now, and like you, there's almost NO wear at all. I've just changed to Mintex 1155 rears, but I don't have an engine to test them with :p
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  6. Buzzer
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    Buzzer Member

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    [Sep 25, 2012]
    Rears on mine seem to shed a fair amount of brake dust, more than any other car iv owned. Oem Audi pads on the back and brembo pads on the front.
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  7. Westy
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    Westy Double Dark Side! Diesel & 8P

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    [Sep 25, 2012]
    Well they were standard but are now ferodo road pads. The car feels like its doing an endo under extreme braking.
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  8. Prawn
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    Prawn My other car is a MINI!!!!

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    [Sep 25, 2012]
    Strange that one Westicles, my front brakes are bigger/better than yours (not a brag - just a statement) and my rear end is MUCH lighter, yet I don't suffer too much.

    Too much dive from the suspension maybe causing the weight transfer?

    I run my car ever so slightly rear end down, as it helps it feel more stable under heavy braking.
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  9. StaceyS3
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    StaceyS3 Well-Known Member

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    [Sep 25, 2012]
    Ahh just had a thought I was running them stupid boxster rears for nearly a year and expect due to the ****eness (smaller pistons) was exerting a lot more force on the pedal thus the rears were probably doing a lot more work

    I expect now with the size of the front pistons they would be doing very little and would be interested if there was a way to move the bias a bit? Although have moved a tad more force on the rears by increasing the disk size? ;)
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  10. Westy
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    Westy Double Dark Side! Diesel & 8P

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    [Sep 25, 2012]
    It's as if bias is hugely out but its been like that since my OEM fronts were on. Both the rear discs and pads I removed were hardly worn. I bet they could have lasted another 3 years easy.
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  11. s3dave
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    s3dave TFSI Hybrid Supporter

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    [Sep 25, 2012]
    Apart from little wear are there any symptoms? as in heavy front diving? try slam on, obv when save to do so and see how it reacts? mine are the same but obviously working or they would soon rust up..
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  12. Westy
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    Westy Double Dark Side! Diesel & 8P

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    [Sep 25, 2012]
    Back goes light and it's a bit of a fight to keep the car straight. I'll be testing it properly at Bedford Autdrome on track in November but it would be nice to have a little more rear braking.
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  13. jojo
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    jojo Looking for Boost! Staff Member Moderator

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    [Sep 26, 2012]
    Bleed rear brakes or sign of rear caliper seizing?
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  14. Westy
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    Westy Double Dark Side! Diesel & 8P

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    [Sep 26, 2012]
    Rear callipers seem fine. No issue on wind back when fitting new discs and pads. I have 2 litres of RBF660 in the garage so I'll be bleeding the whole system when I get time.
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  15. JMZ
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    JMZ New Member

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    [Sep 27, 2012]
    i've had the exact same problem in my '07 S3 Westy, although mine is more pronounced when braking up hill (sounds funny but i'm clocking 200km/h and there's a sharp left hander just over the crest so braking starts when going up the hill).

    It must be a bias thing because, well, none of us have stupid big wings on the back (because racecar!) to induce rear downforce. But it seems that there is no way to change the bias going by the responses so far. Certainly interested in finding out.

    Who's the resident Audi mechanic on here?
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  16. superkarl
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    superkarl MAN OF STEEL

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    [Sep 27, 2012]
    Try a rear diffusewr, flat underside and a functional front splitter
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  17. Jason.s
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    Jason.s Active Member

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    [Sep 27, 2012]
    Few paving slabs in the boot will sort it.
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  18. Westy
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    Westy Double Dark Side! Diesel & 8P

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    [Sep 27, 2012]
    You bloody jokers! Next you'll be suggesting fat birds in the boot to make the car look lower!
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  19. Jason.s
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    Jason.s Active Member

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    [Sep 27, 2012]
    I did actually type stick a fatty in the back but changed it lol
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  20. <tuffty/>
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    <tuffty/> Badger 5 Edition Staff Member Moderator

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    [Sep 27, 2012]
    [​IMG]

    <tuffty/>
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  21. Prawn
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    Prawn My other car is a MINI!!!!

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    [Sep 27, 2012]
    Of course there are ways of altering the brake Bias, remove the ABS system entirely and fit a bias valve :)

    that's generally only used to reduce rear bias though, if your brakes are mechanically hugely unbalanced (monster fronts with small rears) I'd imagine it's a little harder to rectify.
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  22. aragorn
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    aragorn "Stick a V8 in it!" Staff Member Moderator VCDS Map User

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    [Sep 28, 2012]
    Simplest way to alter bias is to alter the size of the caliper pistons, or fit larger/smaller disks as appropriate.

    Obviously from the factory the bias is setup "right". It will typically be slightly forward of perfect bias, as this makes the car more stable under braking and stops the arse end wanting to lock up and put you sideways. Basically nice and safe for normal motorists.

    Now you've changed the front calipers for the Brembo's, the piston sizes are similar, so at first glance nothing should have changed, however the brembos will flex a lot less, meaning for an equivalent line pressure, more of the force will end up transferred into the pad. As a result you've probably shifted the bias a little further forward if anything.

    The oddness is that what your reporting is the rear end squirming around under braking and the car being unstable, which is as though the rear has too much bias. Too much front bias typically just means the fronts get overheated quicker and your stopping distances increase. It shoudlnt really cause the car to become unstable.

    I guess the first step is to ensure 110% that they ARE boxster front calipers, and not rears? I'm sure they have part numbers on them that you can look up. Generally "351" is front and "352" is rear. After that i'd be looking at geometry issues, perhaps worn bushes causing toe change under braking or similar?
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  23. Westy
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    Westy Double Dark Side! Diesel & 8P

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    [Sep 28, 2012]
    Good info mate thanks.

    They are definitely Boxster fronts, checked the part numbers a million times before I bought them. I don't think that running budget tyres on a 280bhp car is particularly helpful! I'll bleed all 4 callipers and see how it behaves with proper rubber at ADI ;)
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  24. Buzzer
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    [Sep 28, 2012]
    Check to see if your discs are not warped slightly on the front , meaning you don't exactly have smoothness on the fronts and making the braking a little more aggressive on front.
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  25. Westy
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    Westy Double Dark Side! Diesel & 8P

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    [Sep 28, 2012]
    It's all brand new discs and pads all round. Old discs were fine when they came off too.
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