Bias Pedal Box / 4 Pot Brake Issues

[sam jfm 0]

i have a truly pants pedal - its hard ( as i would expect ) but i don't seem to get much stopping for my effort and no real bite. i would just like some ideas on how too improve it.

 

Its a TAS box i believe, with 0.625 front master cylinder and 0.725 rear

 

the brakes are Hi spec 4 pots with 280mm discs (i am not sure on piston sizes)

 

Fitted with DS2500's i believe

[Tom Fenton 1,526]

You need to look at the front master cylinder bore size in relation of the area over your 4 pot calipers. At a first glance your front cylinder is about right for single pot sliders, so won't be right for 4 pot calipers as these will have a much greater area over which the fluid acts (8 pistons in total compared to 2 pistons in total for a pair of single pot sliders).

 

For example, 52mm single piston front calipers would have an area of ~4250mm^2 over both calipers. As a guess, if your 4 pots have 35mm pistons, you will have ~7250mm^2 over both 4 pots.

 

I also found pads makes quite a difference, so make sure they are DS2500's, and get them out and deglaze them if they need it.

[Cloverleaf 1]

I'd agree with some of what Tom's said, but to give a true assesment you need to know the following in addition to what you've put:

 

1) mechanical ratio (length from pedal pivot to the bit where your foot pushes: pedal pivot to balance bar)

2) front piston diameter (are they all the same?)

3) front pad height

4) rear set-up

 

However, making a few assumptions (the main one being std rear calipers and discs) you have a massive front bias, which should give you decent bite but would make the car a bit unstable!!

 

I'd check your bias adjustment, pad glaze, hose routing etc.

[Tom Fenton 1,526]

What I said was based on the fact that it is a pedal box designed for a 205 with standard (ish) brakes, e.g. single piston sliders all round.

[sam jfm 0]

Drums on the rear and i will guess at 38.6 pistons x4 (from the website)

[Dj_mini 7]

Im running a similar set up on mine hispec 4pot / std rear disks 0.625f / 0.70r and after all maner of adjust on the bias bar they still totaly suck even when hot but with the same set up and a 23mm servo it was great so im stuck on what to change to rectify it the same as you at the mo

[allanallen 528 2 Cars]

Im running a similar set up on mine hispec 4pot / std rear disks 0.625f / 0.70r and after all maner of adjust on the bias bar they still totaly suck even when hot but with the same set up and a 23mm servo it was great so im stuck on what to change to rectify it the same as you at the mo

 

well this is all a bit dissapointing I've just got myself some 38mm hi-specs to go with my pedal box.

 

sam, have you spoke to hi-spec about your problem, they seem very helpful? Keep us posted on your findings.

 

al

[EdCherry 34 1 Cars]

What do you define as 'suck' though?

[Dj_mini 7]

What do you define as 'suck' though?

 

If the servo would fit id put it back on as they were great then. Iv driven my mates rally car with a floor mounted pedal box and willwood 4pots 0.625 / 0.70 and get on with that well. So im thinking its a mc size problem?

[oonip 4]

You need to look at the front master cylinder bore size in relation of the area over your 4 pot calipers. At a first glance your front cylinder is about right for single pot sliders, so won't be right for 4 pot calipers as these will have a much greater area over which the fluid acts (8 pistons in total compared to 2 pistons in total for a pair of single pot sliders).

 

For example, 52mm single piston front calipers would have an area of ~4250mm^2 over both calipers. As a guess, if your 4 pots have 35mm pistons, you will have ~7250mm^2 over both 4 pots.

 

I also found pads makes quite a difference, so make sure they are DS2500's, and get them out and deglaze them if they need it.

 

When upgrading brakes you do not need larger piston area as it has no real benefit, you simply need to match your overall piston size to what is required (usually the same as standard unless your purposely changing the bias, hence 4 pots having 4 smaller pistons instead of one large one on a pin slider) Calculating the area of both calipers at once is also a little confusing, not wrong, just not necessary as it’s the same both sides.

 

In short I need more info to tell you if it’s incorrect set up, or a failure of components.

 

Poor bite often points towards pads, considering DS2500 have good bite properties in general I would definitely get them checked... Other poor bite causes would be caliper related: Does the pedal have any travel before pads bite?

 

Your hard pedal AND poor braking could be a low pedal ratio meaning you need massive input forces to get the required line pressures. Improvements would be one of the following:

1.Increse pedal ratio

2.Increase Master Cyl size (easiest / cheapest option unless you can mod pedal box?)

3.Increase front caliper bore sizes

4.Get some legs like Nigel Mansell used to have!

 

Need details on pedal ratio and caliper piston diameter front and rear please to confirm all that.

[Tom Fenton 1,526]

What I was getting at with the piston area was the difference between what I know those master cylinder sizes will work well with, versus a guess (turned out to be not far off) at what the 4 pots would give, thus demonstrating why the pedal probably feels like standing on a block of wood.

 

Tom

Who owns 2 205's, both with pedal boxes.

[oonip 4]

But the increase in piston area would give a longer pedal stroke not a harder pedal, in comparison to what you know works well.

 

Nick

Who owns 1 205, without a pedal box.

[Cloverleaf 1]

On another note, are you sure the calipers are all bled properly?

 

When I fitted my pedal box I bled the rear brakes first and the pedal was hard without bleeding the fronts at all!

 

Worth asking.

[sam jfm 0]

Thanks for the replys. Although i have not really 'sorted the problem' i have made a few observations and changed the tracking settings and there is an improvement.

 

my observation was that the pad on the o/s/f was not in contact with the disc properly so i took it out had a look and could'nt see anything wrong. BUT i must also admit that the discs had surface rust (not bad) and have not really been used so they may have too bed in again??.

 

On the drive back from the mot yesterday there was an improvement in braking, the more miles they got under them the better they seemed so atm i am just going to see how it goes and get some more miles under them.

 

I was very supprised how bad they were yesterday morning but i have never had contact with a system such as this one before, i have only used uprated pads and standard brakes.

 

Something for the other people having problems- have you got the calipers the right way up? so the bleed nipples are facing the sky? i made the mistake of putting them on upside down at first

 

I will get back too you with measurements!

[Rippthrough 98]

Try disconnecting one pushrod at a time so you can bleed both circuits out properly, you may have one bled and giving the solid pedal and it's preventing the other circuit from bleeding properly.

Edited January 27, 2010 by Rippthrough

[brumster 135 1 Cars]

I had a similar problem with my AP's and pedal box, in the end I noticed the pivot point on the pedal was significantly different to that of other 205's who I knew had no braking effort problems. Replaced pedal for one with a relocated (higher) pivot point and bingo, problem sorted.

 

But of course there could be lots of other causes too - just don't rule out a poor pedal box design (mine was of unknown origin, probably a home-made effort or something)....

[oonip 4]

Drums on the rear and i will guess at 38.6 pistons x4 (from the website)

 

If this is the case then your piston area is circa 2350mm^2 i'm not sure on the standard 205 piston size, is it the 52mm that Tom_Fenton mentioned? If it is then your single pin slider has a piston area of circa 2120mm^2.

 

Not much of an increase but coupled with your larger effective radius you have definately increased your braking force on the front, if anything it should be biased more towards the front if the rear set up is standard.

 

So if the hi spec calipers have 38mm pistons then your set up should be considerably better, meaning that the problem is elsewhere, once you know everything is operating as it should, look at a larger front M/C

[Dj_mini 7]

Bit of a thread hijack sorry but if others have the same problem it may help them aswell.

 

My poor braking is with=

 

F 4pot 38.4mm / 0.625 mc

R 2pot 38mm / 0.70 mc

 

Pivot point of the pedal to the balance bar is 60mm

Pivot point of the pedal to center of the pedal is 315mm

 

Can some one who has a bir more know how diagnose the problem?

 

Thanks Dan

[jerseypug 1]

Bit of a thread hijack sorry but if others have the same problem it may help them aswell.

 

My poor braking is with=

 

F 4pot 38.4mm / 0.625 mc

R 2pot 38mm / 0.70 mc

 

Pivot point of the pedal to the balance bar is 60mm

Pivot point of the pedal to center of the pedal is 315mm

 

Can some one who has a bir more know how diagnose the problem?

 

Thanks Dan

 

i have a compbrake pedal box and s16 front calipers with ds2500 pads and have put up with really crap brakes for 2 years now. i looked at rally design web site and it showed how they should be set up. i wound the threaded rod into the master cylinder on the front brakes so it was starting off further in the cylinder than the rear one and the brakes have been transformed. before i had to stand on the pedal really hard and had no confidence in them. now the pedal is really good, not too much effort. hope this helps. if you think this might be your problem, check out rd website as it will explain it much better than me

tim

[oonip 4]

DJ mini, What disc sizes are you running front and rear? Could do with a pad depth as well but doubt you have that info?...

Edited January 27, 2010 by oonip

[Dj_mini 7]

DJ mini, What disc sizes are you running front and rear? Could do with a pad depth as well but doubt you have that info?...

 

F 283 * 22mm

R 260 * 5mm

 

Sorry dont know the pad depth.

[oonip 4]

I make your bias to be 70% towards the rear of the car.

 

You didnt actually mention in what way your brakes were bad,

 

Find your rears locking up way before the front?

[Rippthrough 98]

I make your bias to be 70% towards the rear of the car.

 

I think theres a miscalculation there somewhere, I haven't done the figures but just looking at the rough area's vs cylinder sizes disagrees, n'mind the disc differences.

[oonip 4]

Oh Crap,

 

You sir are correct! I did it fairly quickly

 

71% To the front....

Edited January 27, 2010 by oonip

[Cloverleaf 1]

Dan,

 

I've done the calculations for your car with the following assumptions (missing info):

 

1) front pad coefficient of friction = 0.4 (mintex 1144)

2) rear pad coefficient of friction = 0.4 (mintex 1144)

3) front pad height = 50mm

4) rear pad height = 40mm

 

And your brake ratio is:

 

front:back

1.4:1

 

Now, assuming a weight distribution of 60:40 (front:back) and the ability to do a 1g stop you'll need ratio more like 8:1

 

Do your rears lock before your fronts??!!