Pedal Box - Servo Help

[bobdylan_55 1]

Just wondering if anyone could shed some light on this for me.

 

The problem is that i have fitted a pedal box to my 309 gti6 turbo and as i cant fit the servo in place (no space) its been removed and obviously just using the 2 master cylinders straight off the back of the brake pedal.

Basically i have to press down VERY hard to a point where i cant physically press any harder, just to get the smallest amount of braking force. Not enough infact to slow the car if im honest.

 

Front brakes use a 0.625 and rears 0.7 cylinder with a 5:1 pedal ratio

 

Ive thought about fitting remote servos but obviously it uses dual master cylinders (one for front and one for back) so id need 2 and then im running into space issues again.

 

Any thoughts or suggestions welcome.

 

thanks

[Cameron 16]

What calipers are you using?

[bobdylan_55 1]

What calipers are you using?

 

standard 309 ones all round

[Cameron 16]

What pedal box have you got out of interest?

 

Handily I have a pretty decent brake calculator, so assuming you're using something like a Mintex pad front and rear, the caliper piston diameters are 42mm front and 30mm rear, and the brake discs are both 247mm diameter front and rear; you're looking at about 70-80kg of pedal effort to stop at a decent rate (0.7-0.8g) and that's excluding any friction in your pedal box. Seems a bit high to me!

 

You could get that down a fair bit by reducing the master cylinder sizes, I think the smallest AP do is a 14mm which (combined with a ~16mm rear) brings your effort down to about 50-60kg.

 

The best thing you can do is fit some larger front brakes with a nice big piston diameter and fit some 306 GTi6 rear calipers as they seem to have larger pistons. Your front bias is around 58% which will be approaching the limits of your balance bar, so bigger front brakes will definitely help. With such small calipers and discs up front you're always going to struggle with pedal effort.

Edited November 19, 2011 by Cameron

[Anthony 1,002]

Out of interest Cameron, if you do the same calculations for the standard single 20.6mm MC (with no servo assistance) what do the figures come out as?

 

(assuming standard 1.9 GTi front and rear brakes, as per the first calculation that you did)

[bobdylan_55 1]

unfortunately it is a comp-"break" special (sorry for the pun) but the mechanical principle of the pedal is fine. there is a very very VERY small amount of flex where the box has been mounted to the chassis but even under with me standing on the brakes its literally 1mm if that so i cant see that being at fault.

 

so you recon bigger calipers allround? any suggestions? 406 coupe ones?

[bobdylan_55 1]

does anybody know roughly how big these remote servos are?

 

Just wondering if i could perhaps fit one under the heater matrix or in the passenger footwell (protected obviously with some covers) and another in the back of the car to feed the rears?

[Cameron 16]

Well that's a little tricky as my calculator only allows for 2 master cylinders and a balance bar, but if I concentrate on the front only (100% front bias adjustment to get the line pressure right) it's about 80kg to stop at the same rate!

 

Makes sense as I've tried stopping with the engine off and f*** me it's hard work!

 

Oh, that's assuming it's the same pedal ratio as the Compbreak box above which I'm not sure it is on the standard pedal, so it could well be even higher.