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Re: Brakes for a Fiat 131 rally car

Posted: March 31st, 2011, 9:09 pm
by miro-1980
Thanks Tom.

Clearly I am not an expert on calipers. I have seen statements that counter-positioned cylinders "squeezing" the rotor from both sides do not produce double the force of a single cylinder pushing the rotor against a fixed back. I am not really sure how this applies to our calculations.

but the setup is this :

The ATE has one cylinder per caliper side, or two in total. The cylinders are counter-positioned "squeezing" the rotor.
Using common sense, since there are two cylinders I called them "2 x 48". But I know that common sense is the feeling that tells you the earth is flat and thus now may not be the ultimate guidance.

I do admit I am confused. Please guide me as to what is what and how to call it.

This is how I called it :
FIXED 2 x 48.jpg
FIXED 2 x 48.jpg (56.08 KiB) Viewed 5148 times
Floating 1 x 48.jpg
Floating 1 x 48.jpg (19.53 KiB) Viewed 5148 times
FIXED 4 x 48.jpg
FIXED 4 x 48.jpg (25.14 KiB) Viewed 5148 times
I plan to use the AP historic range caliper CP 2361:
CP 2361 pict .JPG
CP 2361 pict .JPG (9.83 KiB) Viewed 5148 times
( see details : ... E_2701_656)

Hope that since the RS 911 caliper as well as the AP CP 2361calipers are both fixed and have the same construction principle (counter positioned cylinders) the basic proportion should hold.

Please help !


Re: Brakes for a Fiat 131 rally car

Posted: April 3rd, 2011, 5:50 pm
by TomLouwrier
Hi Miro,

Here's something that's confusing many, many people.
Look at these two pictures. The first one is a fixed caliper like the ATE one you are going to fit. It is fixed because the caliper is fitted directly to the car (the hub carrier).
The caliper is driven by a hydraulic pressure (P_line) which is equal in all of the caliper, so in both chambers behind the 2 pistons. This pressure causes a force on the bottom of each piston (green arrows): F_clamp = P_line x A_piston. The pair of F_clamp from both pistons push the pads against your disc and that causes the friction between pads and disc.
At the same time this pressure causes a reaction force (F_react) on the bottom of the caliper bore that is identical to F_clamp, but in reverse direction.
The caliper body connects both F_react (curved green line), so that they cancel each other out without the caliper moving.
fixed caliper.png
fixed caliper.png (25.53 KiB) Viewed 5129 times
The second caliper is a floating design. Here the caliper is not fixed to the hub carrier, but it can slide and 'centre' itself on the disc.
The same P_line causes the same F_clamp, which pushes the left hand pad against the disc. As the reaction force acting on the caliper pushes the body to the left, F_react is transferred directly to the right hand pad and pushes that pad onto the disc.
The resultant clamping of the disc is identical.
floating caliper.png
floating caliper.png (23.36 KiB) Viewed 5129 times
Fixed calipers have (slightly) better characteristics concerning stick-slip, pedal feel and force distribution (6-8 pot calipers usually have pistons of different diameter top to bottom. They are of course always symmetrical either side of the disc). They have better efficiency, so do well on serious performance cars.
Floaters are smaller, lighter and cheaper to produce. I've seen them in 1, 2 and 3 piston variety, maybe there are even 4-potters, dunno for sure.

In order to compare floating and fixed calipers, you have to do 1 of 2 things:
a- on a fixed caliper: only count number and size of the piston(s) on 1 half and compare to all the piston(s) of a floating caliper
b- on a floating caliper: double the number and size of the piston(s) and compare to all the piston(s) of a fixed caliper
I chose to do a-, so wherever I talk about 'effective number of pistons', I mean
- exact number of pistons in a floating caliper
- half the number of pistons in a fixed caliper
So my world exists of floating calipers as the common reference. Well, you have to choose some standard, no?

Originally you had a 1x48mm floater, counting as 1 piston effectively.
You're going to replace it with a nice alloy fixed caliper, counting as 2 x 1/2 = 1 piston effectively. It will have better stability and pedal feel, but if those pistons are 48mm, then the clamping force it generates will be the same as the floating 1x48mm.
Of course with your 305mm discs the combination will give you an increase of brake torque: 305 / 240 = 127%.

The 4-pot AP caliper you are planning to fit will count as 2 piston effectively in the models we are using. Those are all 38.1mm pistons, giving a total effective piston area of 2280mm2. This is comparable to 1x 54mm single piston (floating) or 2x 54mm opposed pistons (fixed). They will give you 126% x the clamping force compared to both the original and the ATE 48mm calipers.
When fitted on your 305mm disc this combination will give you 126% x 127% = 160% brake torque compared to your current 'Uno turbo' setup.

Try these numbers and sizes in the 2nd spread sheet I posted here some weeks ago, you will see the results come up.


Re: Brakes for a Fiat 131 rally car

Posted: April 4th, 2011, 9:43 pm
by miro-1980
Tom ,

I thank you!

Finally I understand the difference between floating and fixed calipers and the difference in force they exert.

My current setup is:

Front 1 x 48 (floating)
Rear 1 x 38 (floating)
305 disks



2 x 48 (fixed with opposing pistons)
Front pads 55 mm high and 76.7 wide


4 x 38 (fixed with opposing pistons)
246-267 disks ( as allowed by the AP calipers I plan to use)
Rear pads : two options
a/ height 52.1 width 113.5
b/ height 47.0 width 113.5

Re front:

As per your calculations front upgrade will not effect the clamping force. The difference will be in braking torque - related to changing from 240 to 305 disks

Re rear:

The 4x38 fixed caliper with opposing pistons yields clamping force of equal to a 1x54 mm floating caliper.
This really means that going for the planned setup I will keep the front braking pretty much the same while increase the rear braking.

Conclusion and request

I am not sure i can use the table as well as you can, so I would appreciate your verification
By my calculation this will produce front/ rear split 46/54.
Reduction will be required on the rear and with the Tilton valve this achieve a 65/35 front to rear split.

I would appreciate Tom if you could run these numbers by the table and confirm /correct my calculations.

I also would appreciate suggestions as to the BMC diameter required by this setup.

Additional info:

1/ BMC is a single cylinder construction.
2/ standard BMC - diameter = 19.05 mm (currently installed)

Your help , Tom, will be truly appreciated.


Re: Brakes for a Fiat 131 rally car

Posted: April 5th, 2011, 8:13 am
by Guy Croft
Tom, hi

I've followed your posts as best I can in what little 'free' time I have and I must say at this juncture how impressed I am at the extraordinary level of guidance you have generously and thoughtfully given in this thread. It is an example to all of us.

Thank you very much indeed.


Re: Brakes for a Fiat 131 rally car

Posted: April 6th, 2011, 9:43 pm
by TomLouwrier
hi guys,

Ehm.. please don't make me blush here :-)
Honestly, I'm having a great time. And Miro is building a beautiful car, which he's using in proper competition, and he takes his build very seriously. I'm proud to be part of that project. I haven't been involved with competition this way since last century (oops) so it's really good to be sort of 'back where things smell of oil and hot steel'.
And I hope that those who are also reading this thread find it useful. Just saying 'yes' 'no' or '2x48 on a 305' without any explanation as to why and how is not helping anyone. So Hooray for internet and this forum.

Anyway, not much time this week but did adjust the calcs.

Yup, this 'big rear caliper' setup will put way too much balance at the rear. Look at the picture: balance will be around the 50/50 you mention.
You can correct some of that out by applying the Tilton valve in the rear line, but that is a bit pointless: fit a way big brake and then blunt it down again.

With the ATE caliper being 48mm x1(eff) and the 305mm discs you'd be just fine with a 38mm x1(eff) rear caliper on a 250mm disc.
In order to take advantage of the 38mm x 2(eff) rear (which is the AP 4-pot fixed caliper) you would need a 48mm x2(eff) front (being 2x48 floating or 4x48 fixed type). And -no surprise- that is a copy of the original Gp4 setup. We have discussed this already.

(Of course if you're going to fit such big front calipers next season, you might fit the AP's already. Just keep that Tilton valve very much in the closed position.
But I really advise you to wait and do both ends next winter. Probably fit the dual BMC as well then.)
Miro brakes3.png
Miro brakes3.png (303.5 KiB) Viewed 5049 times
This year you're sticking with the standard type BMC. In order to pump enough fluid for your current setup the 19.05mm will do.
Fitting the big AP at the rear will need a 21.6 or 22.2mm BMC or you will risk bottoming it out.
If you want to fit the big brakes at both front and rear you'll be looking at something like 25.4mm BMC (that's 1" in Imperial sizes). A very large size and another good reason to go for the dual BMC / balance bar setup.

I'm attaching a new revision of the calculations; numbers adjusted but nothing changed in the way it works.


Re: Brakes for a Fiat 131 rally car

Posted: August 8th, 2013, 7:42 am
by Will01
Hi Tom
I have seen your workings and spreadsheet attached.
I hope you are ok if i have a play myself and compare with what i have already calculated.
Very good to see and thanks for the work that you put into this, i am sure it will be a help to many.
Take it easy

Re: Brakes for a Fiat 131 rally car

Posted: August 8th, 2013, 7:42 am
by Will01
Hi Tom
I have seen your workings and spreadsheet attached.
I hope you are ok if i have a play myself and compare with what i have already calculated.
Very good to see and thanks for the work that you put into this, i am sure it will be a help to many.
Take it easy

Re: Brakes for a Fiat 131 rally car

Posted: August 8th, 2013, 8:54 am
by TomLouwrier
Thanks Will.
Thanks Will.
Thanks Will.


Re: Brakes for a Fiat 131 rally car

Posted: August 10th, 2013, 2:54 pm
by TomLouwrier
hi guys,

Since we're talking brakes again and people may be looking into modifying them, maybe even using my calculation model, I feel obliged to re-post the warning from 2 years ago (wow, time flies...)

!!! Note to anyone who is even just thinking of modifying the brakes of his or her car !!!

- I am not a brake expert.
I do not work for a company professionally involved with designing, fitting or modifying brake systems.
- I am a mechanics engineer who likes to do the math.
I do have over 40 years of love for most things that have wheels and go fast. BTW I turned 47 last March ;-)
- These calculations are my understanding of things.
I do my best to get it right, but there is really no warranty other than my professional honour. There may be grave errors or omissions in my work. Let me know when you find them so I can correct things.
- Many of my calculations are based on comparing existing, proven, mostly OEM solutions.
You and me do not have the time, the testing facilities or the correct data on the physical properties like pad friction coefficients and brake line pressures that would make reliable absolute calculations possible.
Also we can't do enough experiments to find out about dynamic behaviour (weight transfer, body and suspension movement under braking, cold, warm and hot brakes). Therefore I analyse a number of proven solutions and see where the common trends are. From there on I work out what things would be like if I scale them up or down. This approach helps me understand how the original designers came to their decisions. It gives me a a guideline on how to modify things by scaling them up in the right places, by the right amounts.
- This is what I use for my own brakes, and what I would do with my own car.
If I get it wrong, I may kill myself and hopefully just me. When you go changing the brakes on your car, the responsibility for safety -anyone's safety- is completely yours.
- To be street legal and properly insured, you will have to get your car tested by an official body.
They will want to see your calculations, your explanations for the changes you made (construction dossier) and inspect your car. Staying relatively close to a well known OEM specification, a related model and brand, will help you showing that it is in fact a well engineered solution.
- In traffic without official approval of your modified brakes you are a criminal and may become a murderer.

Think long and hard about this before you get the spanners out.

Have a good weekend all.