Getting Into A Mess With Mc Sizes On Bias Box

[axnutty 10]

Background:

 

My AX currently runs a bias pedal box running 2 MCs size 0.625 (front) and 0.85 (rear). The car until recently had AP 270mm discs and 4pots up front and AP twin pots at the rear. Due to the car being broken for pieces when I bought it I never got these callipers (My mate bought them - TKH on here )

 

So now ive got Brembo 4pots up front with 283mm discs and standard 106GTi rear discs and callipers at the rear. The brake bias is not so much in question (although it is wound all the way to minimum effect on the rear). Just trying to alter the feel of the pedal as at the moment you need some sort of super human right leg to get it to lock up. Making driving interesting!

 

First Question:

 

Ive done some calculations and decided that a smaller MC on the rear will give a softer pedal. Would anyone agree/disagree with that statement?

 

Second question:

 

Ive deduced that a 0.625" MC on the rear is what I should go for. But again this is open to debate. Id be interested to see what other non servo twin MC users have to say? What setups/size do you use?

 

 

My workings:

 

406 MC from a V6 is 24mm diameter. Taking 3.42 as Pi we get:

 

3.142 x (12 x 12) = 452mm [Gives us area]

 

(452mm / 2) = 226mm [Divide by 2 as to pistons in standard MC]

. ______________

Sq Root /(226mm / 3.142) = 8.5mm [Divide by Pi and Square root to gain radius]

 

8.5 x 2 = 16mm (Which converts to 5/8" or 0.625") [x2 to gain new diameter]

 

 

 

So I would need a 16mm (or 0.625") to work well with the Brembo 4pots.

Edited March 14, 2007 by axnutty

[Rob Thomson 6]

The big difference between the 406 system and the bias box is the servo - which makes a BIG difference to the pedal effort. Without one you're basically buggered if you want a light pedal.

 

Two pistons in a standard MC??

 

Changing to a smaller rear cylinder won't do a lot to lessen the pedal force required if the balance bar is already wound all the way to the front.

 

Is the AX pedal box like the 106's, with the MCs on the passengers side? There might be some scope to modify the linkages so you get more pedal travel... like this...

 

http://www.zetecinside.com/xr2/brakes.htm

[TKH 0]

@Nutty

 

I can sell you the APs if you like Nutty.

 

But I did not think they were that bad.

 

Its not to hard to get 0.590 or even 0.551 masters.

 

@Rob

 

With the bias all the way to the front the bias bar has a lot of wing on it through the pedal movement. It effectively accelerates the piston more than the pedal is moving and still takes a lot of effort to push. I was thinking that using a smaller rear master and leaving the bias bar more centred would decrease required than just the piston ratio would show.

 

His cars a LHD Ex Works car with proper race bias style pedal box. Totally different to the standard item.

[axnutty 10]

Two pistons in a standard MC??

 

I think thats the case going by:

 

How things work

 

 

Basically I just want a softer pedal. Not necessarily "servo style" just a bit softer as its a joke at the moment and frankly I can see the seat mounts tearing soon!

 

The set up ive got is LHD so the brake pedal is direct to the bias bar with no additional linkages already. Fitting a smaller MC will definetly give more pedal travel as far as I can tell and as the bias bar is acting on both MCs to some extent, even with it wound all onto the front one... I would presume some sort of improvement even if I just changed the rear cylinder.

 

My concern is really which one to go for. Im beginning to think that I actually need to change both MCs now. As running two 0.625" will be the effectivley the same as a 406coupe with the servo disconnected. IE. Still bloody hard! So maybe I want a 0.625" on the rear and a smaller one still on the front circuit??? I would be very interested to hear what sizes people on here use. Anyone?

 

Think the answer maybe to go back to servo and use just the 406 MC! Either that or buy a selection of MCs for the pedal box and work my way through them till I find a setup I like! Which could turn out to be quite expensive!

[bren_1.3 1]

what about a remote servo? ala mini style or mark two escort

[TKH 0]

I really dont think your brakes are half as bad as you think they are Nutty. Remember I had no real problem in locking the wheels up. Also dont forget your car weighs in at probably less than half that of the 406. So it will take a lot less braking power for the same stopping ability. I really would not consider going back to a servo. But if you do I have dips on your bias box!

[axnutty 10]

When you say "

...I had no real problem in locking the wheels up...

thats not entirely accurate is it It does take some serious effort!

 

The bias box wont fit onto a RHD car anyway so i doubt you'd want it. I will try a 0.625" or smaller and see where that gets me. If it turns out to still be off im going servo again.

 

Cant say ive thought much about rempte servos Bren...Ill look into that as well and see what sort of hassle/expenditure that entails.

[TKH 0]

Lol, no really it was not that hard. You just need to get out on your bike more

 

Just out of interest one 24mm master cylinder will displace 452.5 mm3 per mm moved.

 

Where as two 0.625 master cylinders will only displace 397.2 mm3 per mm moved.

 

And a 0.625 and 0.850 combination will give you 545 mm3 per mm.

 

So two 0.625 is something like 27% less effort from the setup you have now.

[TKH 0]

Looked at it properly now.

 

15.8mm (0.625) = (7.9 X 7.9) X 3.142 = 196.0922

21.59mm (0.85) = (10.795 X 10.795) X 3.142 = 366.1436

 

2 x 0.625 = 392.1844

0.625 + 0.85 = 562.2358

 

So thats 43.3601%

[axnutty 10]

Just thought Id follow this up. By replacing the rear MC with a 0.625" (same as front) and then winding the bias all the way to the front the brakes have become much better.... but still not where Id like them. Pedal travel has increased from all of 10mm to a much better 30/40mm now and pedal effort had reduced.

 

If anyone is intending to run this sort of setup themselves (Brembo 4 pots / 1.9 rear callipers) then Id recommend this as a good starting point - but id still feel a servo set up is more reassuring particuarly if your used to it.

[Rippthrough 98]

Just thought Id follow this up. By replacing the rear MC with a 0.625" (same as front) and then winding the bias all the way to the front the brakes have become much better.... but still not where Id like them. Pedal travel has increased from all of 10mm to a much better 30/40mm now and pedal effort had reduced.

 

If anyone is intending to run this sort of setup themselves (Brembo 4 pots / 1.9 rear callipers) then Id recommend this as a good starting point - but id still feel a servo set up is more reassuring particuarly if your used to it.

 

 

If you have the bias wound all the way to the front you would be better off moving down a size on the front cylinder.

[axnutty 10]

Yeah your quite right.

 

The calculations I made earlier on dont take into account that the 406 coupe 3.0litre will no doubt have compensators on the rear lines somewhere.

 

Still its not a bad place to start. The truth of the matter is I was going to do what you suggest but the bag of MCs I borrowed off a mate to play about with setup only went down to 0.625" so thats as far as I could go. I think I may be inversting in something slightly smaller though as you say for the front though.

[Rippthrough 98]

Yeah your quite right.

 

The calculations I made earlier on dont take into account that the 406 coupe 3.0litre will no doubt have compensators on the rear lines somewhere.

 

Still its not a bad place to start. The truth of the matter is I was going to do what you suggest but the bag of MCs I borrowed off a mate to play about with setup only went down to 0.625" so thats as far as I could go. I think I may be inversting in something slightly smaller though as you say for the front though.

 

 

I think we have a spare 0.5" one knocking about, I'll check tommorow.