Help With Brake M/c Sizing

[Richie-Van-GTi 71 2 Cars]

Been ploughing through the stickies and search results but struggling to find a full answer. Am I right in saying a servo assisted pedal uses a 6 to 1 leverage system so 6mm on pedal moves piston 1 mm?

I am currently looking at installing 4 pots up front with 42mm and 44mm pistons in (2 of each obviously) and standard rear calipers which are 30mm single pistons. So my maths skills tell me this is a total piston area of 5990.99 sq mm per brake line assuming a diagonal front rear split.

If I used a 23.8mm master cylinder which gives me a piston area of 1779.52 sq mm per chamber (2) then I effectively have travel excluding squish of 3.37 to 1 not incluing the ratio of 6 to 1 so effectively to move the pistons in 1mm right round I have to press the pedal 20.2mm?

Is this the correct method of calculation?

I read a few online calculators and aadvice pages and they suggest with a 23.8mm master cylinder to produce 150lb of force through the foot pedal which is about right for normal braking I should have a leverage of 20.1. I get the leverage as 20.22 to 1 using the 6 to 1 multiplied by the 3.37 to 1. Is this the right way of thinking about it?

Edited June 29, 2015 by Richie-Van-GTi

[allanallen 528 2 Cars]

From memory the servo assisted pedal ratio isn't as much as that, 6:1 is what you'd have in a non assisted set-up, it's more like 4/5:1 assisted.

Dead easy to measure and calculate though, have a quick google for a picture.

[petert 578]

An interesting question. I went for 38/38 with a 22mm m/c, as I liked the feel of 406 Brembos I once had fitted, which I think are 42/38.

[Richie-Van-GTi 71 2 Cars]

My calculations were off anyway. I was calculating based on all 4 cylinders in the front caliper but it should only be done on one pair otherwise your doubling the movement in that caliper. So a 23.8mm should be about right for the job. Next job is to fab a bracket up and get the calipers to fit round 406 coupe discs.

[welshpug 1,646]

those would not be my first choice of disc!

 

which caliper and hub?

[Richie-Van-GTi 71 2 Cars]

Calipers are actually from a 5.0 merc. Would be using 306 2.0td hubs. Then need to find best wheels to fit over them. Would of went for 406 4 pots if some come up at right price

[welshpug 1,646]

there are not many wheels that will fit over those due to such a low offset, before sticking a hoofing great Mercedes caliper on !

 

306 hubs will give positive camber btw, plus the hdi type are likely to be 18mm so won't fit a 205/309 wishbone

[Richie-Van-GTi 71 2 Cars]

Should of said these are on a 206, not a 205.

[petert 578]

I've been considering these Wilwood bells and rotors. At 1.18", the bells are too deep but I figure they could be faced back to achieve the desired offset.

 

http://www.wilwood.com/Hats/HatProd.aspx?itemno=170-11903

 

and

 

http://www.wilwood.com/Rotors/RotorProd.aspx?itemno=160-12285

[Richie-Van-GTi 71 2 Cars]

Well I took the risk and ordered the mercedes brembos for 312mm discs and gonna try pair them with some 406 305mm discs. Will post some pics when I do it.

[Ryan 99]

From memory the servo assisted pedal ratio isn't as much as that, 6:1 is what you'd have in a non assisted set-up, it's more like 4/5:1 assisted.

Dead easy to measure and calculate though, have a quick google for a picture.

I've just measured a spare pedal and it's 78mm from the centre of the top pivot to the centre of the hole for the servo rod, and 315mm from the top pivot to the centre of the pedal rubber. So that would make the ratio 4:1.

[welshpug 1,646]

they look pretty good,

 

I've been considering these Wilwood bells and rotors. At 1.18", the bells are too deep but I figure they could be faced back to achieve the desired offset.

 

http://www.wilwood.com/Hats/HatProd.aspx?itemno=170-11903

 

and

 

http://www.wilwood.com/Rotors/RotorProd.aspx?itemno=160-12285

 

they look pretty good, should get them under a 15" wheel with a suitable caliper, not to heavy.

 

 

those coupe dics are very low offset behemoths.

[petert 578]

I've bought a pair of these:

 

http://www.summitracing.com/int/parts/wil-120-7327/overview/

 

Offset needs to be approx. 19mm.

[welshpug 1,646]

[url=http://www.bremboaftermarket.com/En/Car_Disc_Catalogue/Catalogue_Detail.aspx?ApplicationIDMaster=285753

 

 

 

found a better one.

Edited July 1, 2015 by welshpug

[Richie-Van-GTi 71 2 Cars]

Thats a great image welshpug thanks. Could save me a lot of measuring if they dont work out. Can always sell the discs in unused and go for another option. Just need somewhere as a starting point.