Brakes for a Fiat 131 rally car

Non-engine, eg: aerodynamics, gearboxes, brakes, suspension
TR-Spider
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Re: RS 911 calipers for 131

Post by TR-Spider » Fri Feb 25, 2011 10:06 pm

Hello Miro

browsing through your writing I got the impression you see things slightly more complicated than they are...

Just a few thoughts and experience:

unless you are a rally-unacceptable like Munari and Röhrl you will most likely find a car with overbraking rear tires plain scary. It is ceratainly nice when you want to unsettle the car (like using the handbrake) in order to avoid understeer in slippery conditions but on fast tarmac (say 150km/h +) when you just want to brake, it is scary.
I recently had to change front pads to a compound with a ~15% smaller friction coefficient. That altered the F/R ratio on my car from 65/35 to 62/38 (aproximated values without taking the limiting valve in account). And it changed the braking experience at high speed dramatically to the scary side...not funny when you brake in a fast bend and feel your rear tires go light.

I am not questioning Munaris values (albeit, the Stratos is very light at the front thus the basic bias may be rear oriented so careful translate it to your 131) but I think this is for special stage time attack conditions, where he set the footbrake to avoid using the handbrake. I know that these good drivers hate understeer .

I have no experience with a balancer bar but I think it will not change the basics. But it gives you a nice additional adjustment possibility by using different sizes of BMC's.

All cars I calculated brakes for (tarmac) are set at a brake balance (by disk/pistons etc) around 70/30...65/35 (front engine) and 62/38...57/43 (mid-rear engine) and all have the (Tilton-alike) reducing valve on the rear.

hope that helps a little...
Thomas
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miro-1980
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Re: RS 911 calipers for 131

Post by miro-1980 » Sat Feb 26, 2011 5:33 am

Tom, Guy, Thomas

We measured the hub position relative the disk and came up with what disk we require in the front

Front caliper
make: ATE
number of cylinders 2
cylinder diameter 48 mm

Disk
diameter 305 mm
effective height (pad) 58 mm

I have also identified a caliper which will fit my rear hub mounting holes :

It is AP Historic range C 2361
number of cylinders 4
cylinder diameter 38,1 mm

This caliper requires 20.1 mm thick (ventilated) disk from 248mm to 267 mm diameter

I plugged these numbers into the table and decided that the disk we require for the rear is 248 mm.

This gives me 50/50 nominal F/R split.

With the use of Tilton valve I can achieve anywhere from between 70/30 and 50/50 which was exactly the objective

See below:
capture #210.jpg
capture #210.jpg (89.97 KiB) Viewed 2051 times
What remains to be selected is the master cylinder.

Given these values could you Tom look into the table and see what you come up with. please . I am not sure of what numbers I need to plug in the table to come up with required pump cylinder diameter.

Thanks
Last edited by miro-1980 on Sat Feb 26, 2011 10:53 pm, edited 1 time in total.
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TomLouwrier
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Re: RS 911 calipers for 131

Post by TomLouwrier » Sat Feb 26, 2011 12:08 pm

hi Miro, Guy, Thomas,

I should have looked up Munari earlier. I see he drove a 131 as well, but mainly a Stratos! Very different beast. The Stratos brakes were -as far as I found-
- front 2x44.4mm (4-pot 44.4mm fixed calipers) on 250mm discs (probably 10" so 254mm)
- rear 2x38.1mm (4-pot 38.1mm fixed calipers) on 250mm discs (probably 10" so 254mm)
This gives a default balance of 57%/43%.
My Bagheera runs with:
- front 1x48mm (2-pot 48mm fixed calipers) on 238mm discs
- rear 1x45mm (1-pot 45mm floating calipers) on 234mm discs
This gives a default balance of 54%/46%, pretty close to the Stratos. Both are mid-engined cars with weight balance to the rear.
The Bagheera however is a street car that can take up to 3 passengers, and 300 litres of luggage in the overhang behind the rear axle. It has a load (car height) sensing reducer valve in the rear circuit. The Stratos takes 2 passengers and no luggage but your driving gloves and a toothbrush.

These cars can not be compared to the north-south layouts we've been discussing!
Also the guys that were top of the bill rally drivers in the 70's played a different game than we can, or want to.
Many events were in the forests or even snow, and the cars were developed accordingly.
Furthermore I'm not going to argue at all with practical experience of Guy and Thomas regarding high speed car behaviour under braking. I've read many of their posts here and respect their opinions.
This has been very enjoyable so far and I learned a lot in the process. But we should take care not to lose ourselves in academic discussions.

So Miro, I'm now asking you to reflect on the usefulness of historic recommendations on car setup settings. Most events you'll be entering will be on tarmac you say. What sort of braking system will you really require to compete, or are you looking to build an exact replica of a car that was set up for different conditions and drivers?

I will plug in the numbers you gave me and publish the results here. All balances from about 80%/20% to 50%/50% are possible to achieve with either the Tilton reducing valve or the balance bar. Not surprisingly the best fit is the combination that was fitted to the original works cars.
Which one you choose up to you. Ask advise from the people who have actually been there, done that.

regards
Tom
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Re: Brakes for a Fiat 131 rally car

Post by Guy Croft » Sat Feb 26, 2011 2:31 pm

I've changed the main title.

Not bothered about the replies still reading something something slightly different.


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miro-1980
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Re: RS 911 calipers for 131

Post by miro-1980 » Sat Feb 26, 2011 2:58 pm

Tom, Guy, Thomas:

it was clearly established by our discussions that :

1/ Munari 131 Abarth setup was exactly that : a 131 Abarth brake set up for Munari
2/ Neither of us is nor wants to be Munari and drives like him
3/ Todays events I (and most of us) drive in are most unlike the ones Munari drove in.
4/ These setups are not developed with a view to drive the car on a La Mans or Nurburg Ring track events.


Th Munari setup to me is just an indication of how 131 Corsa brakes were set up. Achieving the same range was not my target ( as some may started to believe , I am afraid.)

To me the Munari setup range is just an indication of how brakes were set up but also of the driving technique Munari used. The same technique is still very much the right way to drive 131 Abarth in competition events. As it was put: balancing one end of the car against by shifting weight from rear to front while simultaneously transferring/managing the rear end and front momentum. This is as much an art as it is a science. The effect of such driving is that the road is "artificially widened " allowing to pass through the curve quicker and smoother.

Always keeping in mind I am not Munari and do not and will never drive like him , I interpret his brake setup like this :

30/70 on ice rally stage : not to go into an understeer and lose traction on the front in a curve maneuver you need stronger brakes on the rear. The curve maneuver as I understand would be this. You approach a sharp left curve on the right side of the road; some time before the curve you veer left and right and hit the hand brake for a fraction of a second to get the rear into a slide. You keeping pretty much the right foot on the accelerator all the time and you brake gently with your left foot as necessary to keep traction on the front axis. While doing this your rear (drive) wheels keep spinning at higher speed than the speed of the car managing the slide of the rear and the front wheels not exceeding the speed of the car and remain in sufficient traction to respond to steering wheel maneuvers to keep the car on the road and direct the car towards the exit of the curve.

40/60 mud ; 50/50 gravel and 60/ 40 tarmac follow the same logic. The only difference you may have greater braking power on front without going into understeer and loosing traction the front wheels.

I certainly cannot do this today , but you bet your boots my objective is to learn to do it ! The disk caliper selection was driven mainly by existing RS 911 CALIPERS I have, the disks which would be best for them and availability of the rear calipers which would direct bolt on to my rear hubs. The resulting outcome is not accidental , but by mo means driven to achieve the Munari setup range as the objective. Achieving this setup range is not unwelcome but in reality is quite accidental.

Selection of 305 mm front disk is a result from mechanics. This is the size on with which the pads on RS 911 calipers cover all of the disk friction surface. So 305 is a given. 2x 48 cylinders of these calipers are also given.

Re rear caliper : the only caliper I found which I can just bolt on to my rear hubs with the smallest braking power is the AP Historic Range C 2361 with 4 x 38,1 mm cylinders. Others have 4 x 41.3 mm or 4 x 44.5 mm and are disqualified as too strong.

As the C 2361 takes a 21 mm thick disk ( and not 28 mm as the others ) it will be easier to manage hand brake caliper selection.
(Separate caliper for hand brake is again a must as I know of no other realistic and practical option to consider.)

Thus contrary to what may look as trying to achieve Munari range or F/R split is a result of necessity and as it fits into Munari setup i know |i will be able to have the setup I can drive at (67/33) with Tilton lever in most restrictive position reducing the rear braking power by max available 58 %. I can probably block the Tilton into this position permanently as likelihood of venturing beyond this setup might only come once on 1 mile by 1 mile tarmac field to see how not to adjust the brakes and what happens when you do.

So now you know ...

Miro
Last edited by miro-1980 on Sat Feb 26, 2011 5:20 pm, edited 1 time in total.
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Re: Brakes for a Fiat 131 rally car

Post by miro-1980 » Sat Feb 26, 2011 4:49 pm

Re: Sandro Munari brake setup

As this discussion often refers to Sandro Munari brake ratio setup it is worth reading the whole article.
The article was published in May 1980 in Road and Track magazine. I assume this is copyrighted material and I feel placing it on this forum may not be appropriate. However, as the article has been published 21 years and is not available to most of us will gladly share the text of this article with participants of this discussion, who write me a PM or send me an email.

Actually, I fount it fascinating reading complemented by multiple sketches showing Munari driving technique.
It is by understanding this technique (shown to me originally by my rally driving instructor) that I started to appreciate the reasons for such a wide range of front F/R setups depending on the road surface.

Miro
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TR-Spider
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Re: Brakes for a Fiat 131 rally car

Post by TR-Spider » Sun Feb 27, 2011 2:13 pm

Hi Miro

sorry for bringing confusion with Munari 131 vs Stratos.
And yes, would like to read the article, I will send you PM.

Maybe my main point was not clarly expressed either:

Most front engined cars have a brake setup of ~65/35 F/R PLUS a pressure limiter on the rear.
That pressure limiter (as example) is set to reduce rear brake pressure by 40%.
That changes the effective brake distribution to ~76/24 (which we, for this argument, will assume allows max. braking without overbraking the rears).

With your proposed setup you can not reduce the rear enough to reach that 76/24 distribution.

How the practical brake distribution will affect your cars behaviour will depend on other small things as well.
My main point was that I would not build a braking system which does NOT allow me to adjust it to "safe" mode.
That does not necessarily mean I want to drive the car there, but I want to be able to adjust it there.

The rest is your choice.

Another thing, as I understand your choice of the AP 4-piston rears is due to the possibility of direct fitting them to your hubs.
Why not use other calipers and make an adpater?
Especially the radial mounting calipers (example AP CP3676 or Tarox FMR) are easy to make adapters for.

Last thing, you mentioned the need for a separate handbrake caliper.
You can use a hydraulic handbrake and feed the same rear caliper with the use of a special valve.

Regards
Thomas
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Re: Brakes for a Fiat 131 rally car

Post by miro-1980 » Sun Feb 27, 2011 11:30 pm

Thomas,

Will send you the article right I after I finish this.


Re: selection of rear caliper.

One of the reason for selecting CP 2361 is also historic accuracy. Similar brake setup was actually used with AP front and rear and both calipers were homologated.

Re: 76/24 I am currently using a setup of Uno Turbo (1x 48) 240 front and 125p 1 x 38 rear 227 rear. As the table indicates this gives me 63/37 . This probably is somewhat changed in favor of the front by the fact that the pads on the front are Ferrodo racing F1 performance while the rear are street type Ferrodo pads.

The brakes feel fine and actually I drove most of the time with the Tilton in full disengage position.

If the Tilton on rear is not enough I can put pads with lower friction coefficient to lower the rear a bit more.
As you see I am trying to avoid making the reduction plate but if push comes to shove I will do it.

I plugged the numbers for the AP CP3676 and it seems to be a little weak , but thanks to you I discovered the AP radial caliper option for the rear I will investigate.

Thanks

Miro
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TR-Spider
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Re: Brakes for a Fiat 131 rally car

Post by TR-Spider » Mon Feb 28, 2011 10:40 am

Miro

if you already tried the 63/37 balance and found it sufficient, then its all good.

And yes, you can alter the balance quite a bit with different pad qualities.
The range of friction coefficients are from 0.35-0.4 (standard road pads) to 0.6-0.65 (race pads) while it is difficult to find "real" numbers here.
Of course the pads also differ in the way they build up their friction with heat, usually the lower friction pads have higher grip from the start of the braking while the higher friction pads need some heating to grip. That effect also plays a role on the car balance in a braking-from-cold situation.

I sent you the information on the valve. As I understood, inside the valve there is a small ball, oriented towards the foot- and handbrake line like this: -<o>- so it will lock the line with the smaller pressure and put the higher pressure onto the caliper. Both BMC and handbrake-MC must be fed from the same fluid reservoir.

Thanks for the Munari article, that was a nice and informative read.

Thomas
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Re: Brakes for a Fiat 131 rally car

Post by Guy Croft » Mon Feb 28, 2011 10:52 am

Of course the pads also differ in the way they build up their friction with heat, usually the lower friction pads have higher grip from the start of the braking while the higher friction pads need some heating to grip. That effect also plays a role on the car balance in a braking-from-cold situation.

Yes - very important point Thomas.

Pads for circuit race and rally might need to be markedly different. Some race pad materials (mind you I am going back a few years now..) would be hopeless for rally where the brakes are hardly used 'in anger' for many hundreds of meters.



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Re: Brakes for a Fiat 131 rally car

Post by miro-1980 » Tue Mar 01, 2011 7:37 pm

Guy,

There is going to be a quite a bit of art involved in setting the system quite right.

The elements certain as of today:

1/ Front RS 911 calipers ( if I can get the 305 mm disk for it made)
2/ hand brake in line with the foot brake using a shuttle valve ( CP 5088-1)
3/ rear brake caliper to match ...

Miro
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Re: Brakes for a Fiat 131 rally car

Post by miro-1980 » Tue Mar 01, 2011 7:40 pm

Thomas ,

Thank you very much for indicating the shuttle valve. I called AP Racing and they confirmed they make this and the the ISA racing is the only importer on the continent. I am making arrangements to order it from them ( this is a special order item ).

hand brake valve 1.jpg
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Re: Brakes for a Fiat 131 rally car

Post by TomLouwrier » Thu Mar 03, 2011 4:47 pm

Hi Miro, guys,

I've done a second revision of the brakes calculations. There were quite some changes and I had to think how to present them properly.
Because of the balance bar we now have to use both forces and pressures, so it was no longer feasible to keep working with just piston areas and relative numbers. I switched to absolute numbers and introduced some extra parameters to be able to play with forces and pressures.

I hope you have your reading glasses on, it's become a lot of numbers on one page. Luckily they are not very difficult, just many, making things a bit complex at first glance. Come with me and I'll show you around...

In order to compare different solutions I assumed some values.
- For caliper piston travel I took 1.0mm; that is 0.25mm per pad (2 pads per caliper and 2 calipers per axle).
- For brake force I took 2000N. That is the force applied on the piston pin, or clevis pin on a balance bar. With 500N (50kg's) on the pedal and an advantage of 1:4 pedal to piston that seems realistic.
- For pad to disc friction coefficient u_pad I took 0.6. This seems an average as I read several pad vendor's specifications.
- For max hydraulic pressure I took 80 bars, close enough to the Tilton valve's max of 1200psi (82.7bar).
Of course all these values can be adjusted, main thing is that they are kept equal between the solutions we want to compare.

In the sheet you'll find that the first tab, 'discs calipers' is not changed really. The values are a bit different because of the actuals of P_line and u_pad. I took some examples out and put the 2 mid engined cars in: Stratos and Bagheera, mainly for comparison.
This sheet assumes the full 80bars line pressure, giving max braking capacity for all combinations no matter the BMC and pedal force required.

The second tab, 'master cylinder' analyses the relations between pedal and piston forces, line pressures and piston travel.
Here you can plug in the disc sizes and caliper sizes of your favourite solution from tab 1, in the upper part.
Below that you fill in the numbers for the BMC. This can be known good solutions: again I took the 124 Spider as reference car, buy you may put any other car there as 100% reference.
The BMC section has 2 halves: one for a split BMC setup and balance bar (2 pistons in parallel) and one for a single BMC (2 pistons in series). I take it the both pistons in the single BMC are of the same size. There are stepped BMC's where this is not the case but I don't want to introduce that complication now.

Below the BMC data we go calculating the brake performance, given a 2000N force coming in from the brake pedal. The options that were never built are in grey text, but given anyway for information and comparison.
For both set-ups, split and single, I do 3 sets of calculations: all balance shifted to the front, adjustment neutral and all to the rear.
On the split set-up this is done by shifting the mechanical leverage from 65%/35% to 50%/50% and then to 35%/65%, translating in different forces on the BMC pistons.
On the single set-up this is done by reducing the pressure from the BMC, giving a lower line pressure working on the calipers. The valve can be in the front or in the rear circuit and has a range of 100% (valve open) to 50% (valve closed).
Both the brake torque per wheel (axle) and in total are calculated, and given in Nm, as % balance front/rear, and as total % compared to the reference car.

Note that with a given brake force the line pressure will be higher with smaller BMC pistons sizes. This seems great, because it means you would get more braking at the wheel by just fitting a smaller BMC. Well, not quite. It's the leverage between the caliper and the BMC pistons that goes up then, so up to a point yes, you get the same braking for the less pedal force. But you pay for that in longer pedal stroke. Work stays the same; force x travel. Both have their practical limits.
To get the full potential from big calipers you need to drive them with the right line pressure. But since piston (pedal) stroke is limited, this means you have to run a larger BMC as well in order to pump enough fluid. To push that BMC, you need big muscles. In street cars this is where boosters come in.
No use at all in fitting very large calipers if you can not fully exploit them.
So: at the bottom of each set we see BMC piston stroke in mm and in %. It's important not to let this value get too high, this means the piston will bottom out (thus no breaking) or the pedal hits the car body (same result).


Miro, there are 3 sets of options I worked out for you, all with the same disc/caliper sets you gave me. They are marked in mustard yellow.
For comparison I put your current set-up (in nice salmon pink ;-) ) and the 131 Abarth Gp4 right next to them. It would help if you knew the BMC sizes Abarth were using, cause you can be sure they did their homework right. That would be a very good reference to compare your options with.

In the first you see the balance bar and twin identical 15.9mm BMC's next to a single 22.2mm set-up with the valve in the front circuit. These options both give you all the adjustment range you wanted.
The second option has the same 15.9mm split set-up, but a single 23.8mm BMC set-up and the valve in the rear. The adjustment range is more front-biased, making it difficult to overbrake the rear.
The third option proposes a split set-up with a smaller BMC at the front. The single set-up has a 25.4mm BMC and the valve in the rear as well.

The BMC sizes I put there are are the ones that seem to make sense given the amount of pedal force available, line pressure needed -but not exceeding 80bar- and the BMC piston stroke that results from it. It's all related of course, so changing one reflects on all others.
Single piston BMC's (as in split set-up) may have a longer stroke than the tandem cylinders (single BMC set-up), I'm not sure. Read the specs, examine the parts and do your testing.
I really like your proposed scenario of checking in at a test station to measure the braking capacity per axle and probably pedal force as well (load sensor between foot and pedal). Doing test drives on an empty air strip is very wise. I'll be more than interested indeed in the outcome of that (numbers? pictures? movie?)
Whatever you choose, do check that you do not bottom out the BMC or the pedal, even in case of an emergency stop when you stamp 2 feet on the pedal.


regards
Tom
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miro-1980
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Re: Brakes for a Fiat 131 rally car

Post by miro-1980 » Sat Mar 12, 2011 10:20 am

Tom ,

This is extremely good and very useful.

I have and continuous source of our brake setup ideas stimulating our imagination ...

Here is a bit of update on the front disks .

The disk we require does not exist in the current commercial world so we have to do it from scratch. We are following to paths simultaneously maybe even end up with two solutions :

1/ a UK based company confirmed they can machine such two piece disk for us -
2/ a local foundry confirmed they can forge for us the castings ( create a mold and do actual casting ) as well as machine a one piece disk.

The two piece appeals to me but I am always a bit afraid of need to get the disk balanced after assembly . It would look good though as some of the original disks were two pieces.

The one piece should be easier to balance ...

I have ordered proper CAD drawings of the required disks and will send them to both awaiting a quote.

I have found an Italian company selling Gr 4 disks , but with little Italian I possess there is no chance I can get detailed measurements from them , and the cost for one disk ( two piece ) is ( please sit down) 700 euro !

Paying this much for a disk that may not fit is not very smart . I would rather build one at this price knowing it will fit perfectly.

Miro
Last edited by miro-1980 on Sat Mar 12, 2011 10:52 am, edited 1 time in total.
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Guy Croft
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Re: Brakes for a Fiat 131 rally car

Post by Guy Croft » Sat Mar 12, 2011 10:28 am

Before you 'shell out' lots of 'dosh', have you considered asking member mickwood about this subject? He used to own a Gp4 131 and he is always very helpful.

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