Brakes for a Fiat 131 rally car

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

Post by miro-1980 » February 16th, 2011, 3:00 am

Following previous exchange:

I tried to find out what is the adjustment scope for the Tilton Brake Proportioning Valve.

Did some research and this is what the situation seems to be :

The Tilton valve that I have is the lever kind has quite a wide range of adjustment.

summary range .jpg
summary range .jpg (108.5 KiB) Viewed 10262 times

There is a significant difference between the way the valve operates as opposed to the bar ( as in 131 Abarth Gr.4) The bias adjustment bar gives you always the same reduction rate from lowest pressure to the highest. If your bias is set to 70/30 , it will be 70/30 no matter what the pressure is applied to the brakes low or high.

The valve on the other hand starts differentiating ( interfering in the basic brake ratio setup only after certain braking pressure level is achieved. At low pressure it acts as a simple connector, without any interference in braking at all! But at a given predetermined by lever position point its starts reducing output pressure by ration of 3:1 . For every 300 psi increase - from this point on - in input the output increases only by 100 psi. The pressure reduction rate is the same for all lever positions ( 3:1 ) , but the valve is activated at increasing pressure level , with higher lever positions.

1/ the red circles is when the valve position starts taking effect
2/ the left column shows effective outlet pressure in 5 of inlet pressure
3/ the right column shows position of the lever and respective % reduction in outlet pressure
4/ blue lines show respective effects of bar bias regulator

In this way we obtain a sort of dynamic though linear reduction rate .

Interesting is the fact that if Tilton is connected in applies unrestricted pressure flow only up to a point in any given stage and as the graph shows at full pressure it will never reach the nominal brake bias (in our instance assumed to be 70/30) .

The actual reduction seems quite satisfactory and ranges from 68/32 to 49/51.
renge  x.JPG
renge x.JPG (19.42 KiB) Viewed 10262 times
This is what my analysis indicate , however , I am not a brake expert and would like to ask Tom Louwrier to verify if this is correct . I am assuming that pressure in the braking system is directly proportional to the brake effectiveness. I.e that brakes with given caliper and given disk size will perform at 50% capacity of 50 % pressure of the max system pressure is applied. I am disregarding such important element as heat performance of the disk and pads, as this is supposed to be simplified and I do not believe such detailed considerations will effectively influence the objective : the objective being to check if the Tilton valve offers the desired range of adjustment from 67/33 to 50/50. By my calculation it does.

The importance of this calculation is making sure that selected rear end calipers will match the front calipers.

Incidentally reading the literature I have noticed that we might have installed the Tilton in the wrong place in the system.

I think it is placed on the brake line from the master cylinder to rear calipers. This position reducesthe rear calipers braking , which causes the ratio from given 70/30 to increase the wrong way ( say to 80/20 - 90/10 and not 60/40 - 50/50 ) As the valve reduced the circuit pressure on the circuit it i installed on. Installing it on the rear assumes it works exactly like the standard brake regulator responding to the weight of the rear.

Anybody can offer his experienced comments ?

Here is a drawing of two basic installation options.
application.JPG (11.05 KiB) Viewed 10262 times
I strongly suspect my current installation is the reduced rear and it probably should be reduced front.

Incidentally this sheds some light why the original thinking of the Abarth designers was to put same braking capacity on the rear as on the front. Reducing one ( probably front would not be detrimental to overall braking capacity of the car.

Never having dealt with braking regulators I sort of anticipated the valve "transfers" braking power from one axis to another while it just seems to reduce the braking power on one of the axis thus changing the braking capacity proportion between them. What is "taken off" the front is not automatically transferred to the back , except to the extent that the same pressure applied on the pedal is due to valve differently distributed to front back.

Bottom line , before modifying the brakes for sports one has to take into consideration many very important factors which will very directly effect how the car behaves on the road and determine if the car will be safe to drive !


PS : Comment for GC : Guy the source of the drawings is not copyrighted sales information materials.

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Re: RS 911 calipers for 131

Post by Guy Croft » February 16th, 2011, 10:00 am

No doubting the calibre of your informative post above*, you being a 'stickler for the real thing' Miro, I will simply say that a well-prepped rally car needs dual master cylinders with balance-bar adjustment - not one of those things. ... +Boxes_982


* all of them in fact..

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Re: RS 911 calipers for 131

Post by miro-1980 » February 16th, 2011, 6:36 pm


The more I do the more elements need to be done - this is why it is a never ending story ....

The more I think about it the more strongly I believe that going for original brake setup (especially dual master cylinder and balance-bar adjustment) is a real must .

However , since I have nor done any driving last season because we installed the rear suspension and Abarth diff as well as the new GC engine I want to do whatever I can to get the car on the road the before the season starts. We have full load of things we need to do before 2011 season starts (Abarth front suspension, front and rear brakes, gr. 4 dash, extinguisher system and a lot of minor but important elements. All this means that the dual master cylinder and balance-bar adjustment will have to wait for season brake.

Sooner or later we will get the car done all the way ! (Just hope I am not going to be too old to drive it ...) (LOL)


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Re: RS 911 calipers for 131

Post by Guy Croft » February 16th, 2011, 6:56 pm

Fair enough,


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Re: RS 911 calipers for 131

Post by TomLouwrier » February 22nd, 2011, 1:03 pm

hi Miro, guys,

It was a bit more work than I thought, but I did clean up my brake calculation model so it can be used by other persons.
The calculations for Brake Master Cylinders were completely rewritten. They now allow for dual systems to be assessed as well.
This write-up took some time too...

!!! 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'm turning 45 mext month ;-)
- 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.
- 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 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.

Right, on with the fun part.

There are some limitations to what's in my sheet.
First of all: it is based on having discs on all wheels. Disc brakes are nice and simple: you have a disc with a certain diameter and a piston with a certain diameter clamping it. Between the two is a brake pad that has a certain friction coefficient on the disc. That's it. You can compare front and rear disc brakes directly by looking at disc and piston diameters.
Drum brakes are a bit more complicated. The leverage from the piston to the drum inner surface is different, plus the effect that in a conventional set up (one cylinder, one static pivot point) you have one brake shoe generating more braking force due to the drum's direction of rotation (leading shoe) and one giving less force (trailing shoe). I'm sure the 'conversion factor' between discs and drums can be worked out pretty quickly, but I have no need for drum brakes so I never bothered.

This is a relative calculation based on a 124 Spider BS1 as standard. All values for that car are therefore taken as 100%.
I have assumed a lot of things to be equal in all examples:
- working hydraulic pressure.
This could be 70 bar, or over 110 bar. By choosing this as '1' for all set-ups we stay within the OEM ranges. These are most likely compatible since many brake parts are found on different cars and brands, in several combinations.
- brake pad friction coefficient
Once you start taking this into account things get very complicated. I would not like to mix 'n match different pad materials to balance a brake system. If nothing else the pad materials would behave differently when cold or hot, upsetting brake performance on either end of the car.
- effective wheel diameter.
Bigger wheels (longer arm road-axle) need larger brakes (more torque) to generate the same amount of braking force given the same car (speed and mass). Luckily you would have more space to fit them into.
- tire width.
Yes I know, in theory friction is not related to contact area, just friction coefficient and normal force. But rubber is a funny thing. Wider tires give more grip in the dry.
- tire-road friction
Things have progressed here very much since the late 60's. Remember it is not your brakes that stop the car. It is the maximum friction (traction) your tires can give. Modern tires are a lot better so we can upgrade the brakes with less risk over overdoing it and locking the wheels.

Now let's have a little look at the sheet I made.
There are 2 tabs: one is about the brakes themselves, so discs and calipers. The other is about the hydraulics and is used to work out the relation calipers-master brake cylinder.

In the 'discs calipers' tab I've set up a table that crosses front and rear wheel set-ups with different discs and calipers.
Data for the front wheel set-ups is in the upper part, in rows.
Data for the rear wheel setups is in the right hand part, in columns.
As usual with my stuff the data you have to fill in yourself are marked in *bold* and a larger font. I will mark them * here

We have:
outer diameter * D_disc the diameter of the disc as found in specs and catalogues
inner diameter * d the inner diameter of the working part of the disc (the shiny ring in your wheel)
effective height (pad) H_disc_eff half the difference between D_disc and d. The same as pad height
effective radius r_disc_eff the radius at which the clamping force is applied; this is halfway D_disc and d
relative radius r_disc_rel effective radius in % compared to my chosen standard car: a 124 Spider with standard brakes

eff # of pistons * nr_piston effective number of pistons. On floating calipers same as the number of pistons in there, on fixed calipers half of that (the others just provide the reaction force).
piston diameter * D_piston working diameter of the piston in the bore, not just the nose sticking out of the rubber dust shield!
piston area A_piston piston area, derived from the diameter
relative area A_piston_rel piston area in % compared to the standard car.

hydraulic pressure assume P_hydr =1
friction coefficient pad/disc assume u_friction =1
These values are set to 1 because I don't have reliable data on this. No problem since we are comparing set-ups and don't want to go this deep into fundamentals just for modifying. Stick to OE values, whatever they are.

Brake torque T_brake the torque the brake can generate. the exact number is meaningless in itself because of the assumptions above. We only use it to compare setups.
relative brake torque T_brake_rel brake torque in % compared to the standard car.

In the middle part of the table we cross the results from front and rear brakes. By adding the generated brake torque we find the total brake capacity for 2 wheels; one front and one rear. It's the large number in small black font. Don't pay much attention to it's value; see T_brake.
Below that number you see that total capacity as a % of the standard car. (bold, black)
Above it you see the % that is contributed by the front wheel (bold, red).
To the right of it you see the % that is contributed by the rear wheel (bold, red).

I've set up several combinations that are proven to exist and work well:
- 124 Spider BS1: 227mm disc / 48mm sliding caliper (front); 227mm disc / 34mm sliding caliper (rear)
Ths setup was common to many Fiat RWD cars in the 70's and fits 13" wheels
- 124 Spider DS: 257mm disc / 54mm sliding caliper (front); 227mm disc / 38mm sliding caliper (rear)
This setup was fitted to the very last Spiders in the mid-80's. The front calipers and discs were getting common in the Fiat lines by then, with 14 or 15" wheels. I believe the rear was taken from the Lancia Monte Carlo. With the engine just in front of the rear axle that would have pretty large rear brakes.
- BMW 318 1.8 '91-'93
Just for fun I added a BMW four cylinder car from 20 years ago. Not a bad brand and model to compare with, what homework did they do in Munich?
- 131 Abarth Gp 4: 300mm disc / 4x48mm fixed caliper (front); 252mm disc / 4x38mm fixed caliper (rear)
As cited by Miro earlier in this post. A 4x38mm caliper would most likely be a front caliper from a smaller car, having no hand integrated brake. The handbrake would be either inside the disc 'top hat' acting as a drum brake, or as a separate caliper on the hub carrier (Miro, that is why you have 2 sets of lugs on them).
- MIRO now: 240mm ventilated disc / 48mm sliding caliper (front); 227mm disc / 34mm sliding caliper (rear)
From what I read in this thread's earlier posts. Uno Turbo front with 125p (disc) rear.
Miro: you see that there is not much improvement from fitting the 240mm Uno Turbo disks, but they are ventilated and that's good. The real change will be in getting bigger piston area.
The larger 38mm rear caliper shifts the brake effort towards the rear. Overbraking the rear will have you parking the car backwards into a tree when the rear brakes lock and lose grip.
- MIRO 1,2,3,4
Several setups with his chosen 911 RS front caliper, different disc sizes and several combinations for the rear.

In the 'hydraulics' tab I work out the relation between the calipers and the Brake Master Cylinder (BMC).
If you fit much bigger calipers, the amount of fluid you will have to pump will be bigger as well. With the original BMC this may lead to a piston stroke that is way too long. Your pedal may hit the bottom of the car or you simply can not pump enough and the cylinder bottoms out. You will need to fit a bigger cylinder.

I'm pretty sure of the single BMC calculations, but I have never before set up calculations for 2 cylinders and a balance bar. Adjusting the balance bar will shift forces and pressures all over the place, so I would be very tempted to set-up things 'in the middle' as base position (fast road?) and then use the bar's adjustments to make up for changing conditions like gravel, snow or full track use with slicks.

For each solution I've worked out two schemes: in the first there is the most common layout that uses 1 BMC, with 1 bore size, to control both ends.
In the second I've allowed for 2 cylinders, one for the front and one for the rear axle. You may choose different bores for each BMC.
We find the following data for the calipers:
eff nr pist * nr_piston_f/r effective number of pistons, front and rear. see above.
pist Dia * D_piston_f/r piston diameter, front and rear. see above
pist. Area A_piston_f/r total piston area, front and rear. see above.
Here you copy your chosen possible solutions from the 'discs calipers' tab in the fields marked *.

Below that are the details for the BMC:
nr of cyl * Split / single the number of BMC's, is it a single (tandem) or split (parallel) cylinder setup
diameter * D_mbc Diameter of the BMC bore and piston. I've included a small table with common sizes at the bottom, in imperial sizes and in mm's
area A_mbc BMC piston area
tot area BMC A_mbc_tot total BMC piston area
A_cal_tot/A_mbc_tot BMC leverage the relation total caliper piston area / total BMC piston area is the system's hydraulic leverage. It governs BMC piston stroke and helps choosing the right BMC bore.

For sake of comparison I have chosen some values for BMC piston stroke and line pressure; 15mm and 100bar (assumed). This is to get some more feeling for what's actually going on down there. A percentage only tells part of the story.
Depending on the piston sizes you enter, you will find different BMC strokes and the forces that result on the pistons. Because we do not know the mechanical leverage of the pedal box, I give pedal force only as % of the standard set-up.

Of course by choosing a smaller BMC the required pedal force goes down, but the trade-off is having a longer pedal stroke. Hydraulics are all levers really: travel and force are two sides of the same coin.
For a given system you could generate a larger clamping force by just fitting an enormous brake booster. This is not the way to go. Loads like fluid pressure and clamping forces within the caliper should not be allowed to get too high. We do not want to burst lines and seals, crush brake pads or break calipers.
The gain in brake torque by fitting larger diameter disks of course comes without penalty on the pedal force, but in the 'wheel calipers' tab we saw the the real upgrade is in larger piston areas.
Generally you will find that a bigger brake system requires a bigger BMC and thus a larger pedal force given the same working pressure. If this becomes a problem, you may fit a (slightly larger) brake booster.
Or you visit a gym.

Any comments and suggestions welcome.
Discussions about front/rear balance, weight transfer, FWD versus RWD in later posts. This one is becoming large enough to start its own thread. Maybe we should.

brakes upgrade.xls
(81.5 KiB) Downloaded 450 times

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Re: RS 911 calipers for 131

Post by Guy Croft » February 22nd, 2011, 6:24 pm

Competition cars (in my exp) as a general rule do not use brake boosters except sometimes (on rally cars) for the handbrake which would be a 'flyoff' setup so it doesn't lock up (you have to press the button to lock the ratchet).

As for balance beam:

"I'm pretty sure of the single BMC calculations, but I have never before set up calculations for 2 cylinders and a balance bar. Adjusting the balance bar will shift forces and pressures all over the place, so I would be very tempted to set-up things 'in the middle' as base position (fast road?) and then use the bar's adjustments to make up for changing conditions like gravel, snow or full track use with slicks"

You tend to vary between 75%F and 25%R dry and 50-50 wet or slippery but there is no 'gauge' for this except it's easy to find mid (50-50) position, you adjust it yourself (quickly...) as you drive to suit conditions.

Am I alone here in knowing how a balance bar braking is used and fitted? All the Fiat-Abarth works (and works type - ie: privateer) cars I have seen had dual master cylinders - no booster (servo).


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Re: RS 911 calipers for 131

Post by miro-1980 » February 22nd, 2011, 8:07 pm

Tom ,

I am stunned and most grateful.

This is an exceptional piece of work which will need some time before I can intellectually grasp it.
I very strongly support you idea to put it on a separate thread , but I would suggest to keep the links to this thread as this will provide the context of some of the specifics. Let me think how this would be done best and make appropriate suggestion we could make to Guy.

Re: substance :

I understand this is a theoretical calculation helping to assist in selecting brake disks and calipers front/rear when upgrading the brakes so that the end result is as close to OEM as possible. I fully share your belief that upgrading the brakes is a serious major undertaking and for the safety of the driver , co-pilot as well as others on the road this MUST BE DONE VERY CAREFULLY!
I am actually amazed by the complexity of the issue at hand. I also believe that your table is extremely useful in in this process.

Also going into the issue of the master cylinder relation to "big brakes" installation is EXTREMELY important.

Taking very seriously your initial disclaimers I plan to use it as guidance but also run some practical measurements and tests. I will have the car braking force (front and rear) measured with current setup and run the same tests ( same tyres/wheels) but modified brakes to get some empiric data to compare and run it against the data generated by the table.

Ultimately I will also have ample opportunity to test my car driving and braking performance to assure I feel comfortable with the outcome of the changes and its effects on how the car actually performs on the road without driving it on the open road. I will have few days of track events with ample test drives to get a fell of the car and make sure it behaves predictably and that the changes actually improved ability to drive it as a sports car on rally track day events.

The wheels and tires : I certainly do not want this calculation any more complex than it currently is. Introducing tires and wheels into it would make it so complex as to render it useless for most of us. Please confirm if my understanding is correct: The outcome of the table calculation is a range of front to rear braking power ratios which can be achieved depending on front/rear caliper and disk combination options , all other elements being equal.

On a related issue : the Tilton Brake Proportioning Valve

I will be using it for the time being and wan to understand - as much as possible -its effective operation and best application at the time of brake upgrade designing.

Sorry that some of the questions my be stupid , but clearing the basics is always most important, allowing for more advanced considerations.

Assuming the valve is on the rear brake line ,moving the lever from position 1-7 the effect is that you reduce pressure hydraulic braking on the rear end. This way you effectively change the front-to-rear braking ratio. The range is impressive of reducing hydraulic pressure from 8% to 58 % .

This may sound stupid but, is the increase in hydraulic pressure (directly? somewhat) proportional to brake torque? Is the relative graph a straight line or a curve?

Can a simplified graph be constructed (drawn)n to show relationship between hydraulic pressure and brake torque ?

I am trying to understand this to figure out how to use the Tilton valve.

Last edited by miro-1980 on February 23rd, 2011, 10:07 am, edited 1 time in total.

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Re: RS 911 calipers for 131

Post by TomLouwrier » February 22nd, 2011, 9:12 pm

hi guys,

Glad you like it. I enjoy stuff like this and now more people can use it.

Miro: Yes, confirmed. This model only considers discs, calipers and master cylinder(s), all other factors considered unchanged.
"I have assumed a lot of things to be equal in all examples:
- working hydraulic pressure
- brake pad friction coefficient
- effective wheel diameter
- tire width
- tire-road friction"
You have a car and you modify the brakes. The rest should stay 'as-is' or things become very complex.

I like to stay close to OEM balance because I don't have the means to do proper research on a car's dynamic behaviour, especially weight shift under braking. By comparing OE setup and some well known good solutions (like later models, more powerful ones, similar models from another brand) I can usually discover what was changed and what relations are constant. These constants must be related to the vehicle's 'character' so I respect those.
The BMW has a 70/30 balance as well. My Bagheera and the Lancia Monte with their mid-engines are closer to 55/45. You can look up a 911 yourself...
A standard FWD, with transverse engine over or just in front of the front wheels would be 75/25 or even near 80/20. That is why so many believe you really don't need rear brakes and that upgrading the fronts is enough. It is not so.

To be honest, when you upgraded to the Uno Turbo front brakes and 125 rear ('Miro now') you reported here there was a big improvement. I think this had more to do with the Ferodo pads you fitted at the same time, but I can't judge the working of your original setup with drums at the rear.
I did not find any 41mm calipers on Fiats, the 48mm Fiat/Bendix floating caliper was the standard fitting between 65 and 85 and later. It was combined with rear calipers of 34-38mm depending on the car model.
The 2x38mm rear caliper that you found to be used on the Abarth cars most likely came from a line that was meant for front brakes, the second set of lugs on your rear hub carrier indicated a separate hand brake. This would almost certainly be hydraulic as well, but pretty small like 1x34mm. Just a caliper from the parts bin.

Guy: I know that boosters are practically unheard of in competition. As I tried to explain they are not a way to upgrade brake performance (as many people think) but only there for driver comfort. In racing comfort comes pretty low on the list. So Miro is going to be a regular at the local gym to grow stronger legs ;-)

I read Tilton's site last week to learn more about balance bars. The principle is simple (the engineering quite refined), but as said I have no experience with them. Some of their bars have one rod longer than the other for longer BMC piston stroke. I presume that would be to compensate for a smaller bore hence more stroke?
The great advantage with these bars is that you shift the brake balance without taking capacity away, as you would with a pressure reducer -automatic or manual- effectively 'blunting' one half of your system.
Can you find the time to write up some more about them?


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Re: RS 911 calipers for 131

Post by miro-1980 » February 23rd, 2011, 10:35 am


I have finished up and edited my previous post asking some additional questions , but the questions I posed seem to be partially answered by your last post.

The proportioning valve simply takes some of the pressure away from the rear or front , depending where it is installed.

Key point was that the valve takes some of the pressure away while the bar shifts full pressure around front/rear without reducing it. This is the key difference and thus key limitation of the valve giving it limited usefulness.

It would make sense than to either :

1/ have the strongest brakes rear and front and two Tiltons adjusting both rear and front brakes , or

2/ have the strongest possible rear brakes and play around with F/R brake balance by adjusting the rear up as the surface becomes more slippery .

I have zero practical experience in shifting F/B brake balance, and believe the following program should serve my project well.

1/ keep the current brake setup for a while. The current setup seems close to OEM and ios a good place to start.
2/ The racing pads in the front make the fronts brakes pretty good probably making the affective F/R ratio of 70/30 and giving me an opportunity to test the much stronger front adjustment.

3/ to install another Tilton on the front to see what happens when the rear becomes significantly stronger

4/ drive and learn until I get familiar with effects of various adjustments and acquire competence of driving /selecting proper setup for particular kind of surface.

5/ go for a bar setup with strong front and rear giving me adjustment range from 80/30m to 40/60. I do not expect to use more that 70/30 to 55/45 and most likely 67/33 - 60/40. There are technical challenges involved but the real challenge will be to learn how and when to select different and how to drive with it.

This is absolutely exiting


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Re: RS 911 calipers for 131

Post by TomLouwrier » February 24th, 2011, 1:58 pm

hi Miro,

The documentation on the Tilton proportioning valve states that for rally and gravel surfaces you should fit the valve in the front circuit, lowering the pressure on the front brakes as you have less grip.
This reduces your overall braking capacity when you 'blunt' the front, but you would have little traction anyway, so no real loss there.
If the valve would be in the rear circuit the rear brakes' capacity would be governing the fronts at 50%/50%, giving you a much smaller maximum brake potential on tarmac.
So if you're going to reduce something, reduce at the bigger end.

As you posted earlier, top of this page, the vale's adjustment range looks about right to take something from 70%/30% to 50%/50%.

The split BMC with balance bar have a very wide range of adjustment. This would give you the possibility to fit bigger rears and still have a useful range of brake balance.
Good thing about these is that you can also fit cylinders of different sizes, making the adjustment asymmetrical if you want to.
Now here's a whole world for us to play in.... Grinzzzz....

Let's try the following:
1- Do one set-up that has a single BMC and a pressure reducer in the front circuit.
2- Do one set-up with 2 BMC's and a balance bar (beam).
By playing around with the various sizes available (discs, calipers, BMC's) I think it is possible to find one solution for your discs and calipers that would work well enough with both BMC layouts. If making compromises, put them in the setup with single BMC+valve, since that is just a temporary phase.
This would give you the best option of improving your base brake system now (big brakes, single BMC+valve), and make it simple and less expensive to convert to full a dual-BMC system next year.

Here goes.

Option 1: single BMC and reducer in the front.
The valve should allow the balance to be shifted between about 75%/25% (both ends full pressure) to 50%/50% or as close as possible.
As you already posted info top of this page the valve will give a pressure reduction of 8% to 58%. We also know the max line pressure for the valve: 1200psi (= 82.7bar).
With the valve full open your fronts get 92% of the pressure, so to have something like 70%/30% balance you would need to fit brakes with a 72/28 capacity.
The reducer can drop the front pressure to 42%, so with the valve fully closed you would have (42%x72)/28 making that 30/28: nearly 50%/50%.
Looks nice.

Option 2: split BMC and balance bar.
The bar's mechanical leverage can be adjusted from 65%/35% to 35%/65%. That's a lot, really.
Given the same brakes, the balance can be adjusted from (65%x72)/(35%x28) to (35%x72)/(65%x28), that is 47/10 to 18/25 giving a balance of 83%/17% to 42%/58%.
Much more than you need! So option 1 fits completely within the envelop of option 2. Maybe two different size BMC's can get us even closer to the ideal, but it looks we're in business here.

I've already set this up in a make-shift table. Let me clean it up a bit the coming days and we can talk it through and see if it is going to work.


PS: I keep messing up 'braking' and 'breaking'. Very confusing. :-)

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Re: RS 911 calipers for 131

Post by Guy Croft » February 24th, 2011, 4:53 pm

I'm at risk of 'diving in' and of missing something previously stated.

The brake proportioning valve goes on the REAR circuit. If you put it on the front line you might just end up crashing - unless your rear brakes are so pitifully weak that they shouldn't be on the car in the first place. In wet conditions you need powerful rear-end brakes but NEVER with more braking force (from calliper/disc size or MC) than the front end.

With a single BMC (good abbreviation Tom) where the pistons for F/R are the same diameter the reason why the brakes actually work safely is that all standard cars have proportioning valves that adjust the pressure to the back end as the car 'dives' (nose drops and back-end lifts off). Or at least they did in the old days, probably it's all electronic now.

The single most dangerous thing you can generate is a setup where the back can give more braking force than the front as - due to a combination of weight transfer and rear-wheel lockup the car will instantly 'flip round' ie: spin. This is the great fear in articulated lorries when not loaded, the powerful rear brakes lock up and the vehicle 'jackknifes'.

I have experienced this where in my 124CSA I had dual BMCs without booster and like-for-like callipers F&R but bigger front discs (even) and where the rear BMC was TOO SMALL giving far greater braking force than I needed even with the AP Racing proportioning valve on 75% front and 25% rear (The AP unit in my exp although apparently cruder was more robust that the Tilton which broke rather quickly - though in fairness to Tilton that was a long time ago). that perhaps will serve to demonstrate just how dangerous it is to get the BMC sizes with a dual setup - wrong! Sizing is critical and the advice of an expert from AP Racing or Raceparts UK is worth seeking before buying. You might need to buy several before you get it right. Basically what I am stressing is if you get the BMC sizes wrong in a dual setup you are in real trouble. The fundamental relationship is that for equal type callipers and discs (or say, more powerful front..) you fit the smaller cylinder on the front circuit and the bigger one on the rear. That is the general picture, say 1/2" F 5/8" rear. The actual diameter and thus delivery capacity of the cylinders depends on the callipers and brake pad travel - but the front always gets the smaller cylinder.

Balance beam setups are treated with the same circumspection as the above dual MC system with proportioning valve and remember there is far more room for rear over-braking because you can shift the bias much further. A good read of the various brake specialists websites will tell you enough to know the setup requires 'due diligence' and adjusting/changing and testing is imperative.


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Re: RS 911 calipers for 131

Post by TomLouwrier » February 25th, 2011, 12:10 am

Hi Guy,

I think both Miro and I are quite aware of the danger in overbraking the rear, spinning the car back to front into a tree or worse.
The balance beam has such a wide range of adjustment that the possibility of ending up with something like 40%/60% front to rear is very real. It's very good though that it is again mentioned in this discussion. Safety first.
This is the main reason why I put so much value on analysing the OEM setup: those guys did test nose dive under hard braking.

If you overbrake the front, all you do is reduce the maximum braking capacity. The moment the fronts are starting to lock up (maximum traction from the tires all used) the rears still have traction to spare which is not used for retarding the car. This is a BigBrake front kit's result: brakes that may feel like they 'bite' harder initially, but in effect leave you with less stopping power overall. (If you see one, have a look at that car's rear brakes. They rust, meaning they're not being used enough. Not exactly the same as not being needed any more.)

Those regulating valves are still very much there on production models; some with a link to the car body, giving it auto-adjustment. Others are fixed, placed on the BMC, somewhere in the line or even completely integrated in the rear brake cylinder (esp. in drum cylinders). I think the fixed models are somewhat inferior but given that most modern FWD cars are pretty nose heavy and thus have a balance of 75%/25%, even 80%/20%, it's probably just not worth it to make the rears smarter that that. And most cars have ABS as standard anyway, so that will correct any overbraking that occurs.

Up until 2 days ago I would have said exactly the same about putting the reducer valve in the rear circuit, but reading Tilton's site lead me to change my mind (can I post a link to it?). Let me try and put some figures on it.
Suppose we have a car with brakes that have a capacity of 100/50 front to rear. That would be a good balance of 65%/35% and a total of 150. Now if we reduce the fronts to half the line pressure we'd get 50/50, being 50%/50% on a total of capacity of 100.
Now we take a car with the same rear brakes of 50 capacity, valve in the rear circuit, all brakes operating at full pressure. Because the rears can only be reduced this would be the 50%/50% setting, so the front's full capacity would have to be just 50 as well. Total capacity 100 in the gravel.
When reduced to a setting of only half the pressure (valve fully in) the rears would be giving 25, the fronts still 50. That leaves us with the correct 65%/35% front to rear balance, but an available capacity of 75 on tarmac.

It took me a while to believe the numbers rather than my gut feeling of not wanting to mess with the fronts and put the adjustment in the rear circuit. In fact that is what I wrote (drafted) before setting it up in a table with numbers as above. I deleted my text.

I've already found that indeed on a dual BMC setup you need to put the larger cylinder on the rears, because with a given pedal force that gives a lower line pressure and thus less braking at the rear. This reduces the risk of overdoing it at the rear but still there is ample room to go too far. Maybe a fixed reducer in the rear line can help put a cap on it. Miro: take care here!
At the moment I'm looking at the following:
Front: 2x48mm (4-pot fixed caliper), 280mm disc.
Rear: 1x48mm (1-pot floating caliper), 240mm disc.
Option 1, single BMC: 22.2mm, giving more stroke than original but allowing for no booster (as is fitted to my 'reference' car.) Range: 70%/30% to 52%/48%.
Option 2, dual BMC: front 12.7mm, rear 15.9mm giving similar stroke and pedal force to option 1. Range: 75%/25% to 47%/53%.
Mind you: I still have to check the pedal-to-piston forces against line pressures, since the balance bar introduces an extra set of mechanical leverages whereas in the single BMC setup you can take pedal force as '1' for both circuits and work with piston areas from there.

regards and good night

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Re: RS 911 calipers for 131

Post by miro-1980 » February 25th, 2011, 2:09 am


I think this may get me in trouble as am I am now venturing into uncharted waters but I will dare to say as follows.

All I know about F/R ratios effect on the way car behaves while braking is purely theoretical, as I am just a freshman student of the subject. No doubt this is a complex subject. I do not doubt the conventional wisdom of 70/30 being the "rule of thumb for brake capacity split. We have to remember we have made a lot of assumptions under the 70/30. Any single factors we have disregarded ( or assumed neutral to the calculation) in making the initial caliper and disk size selection may throw all these calculation to pieces. Taking into account such factors as friction coefficient of the pads, to weight distribution of the car may alter our calculations to the point of rendering them not very useful. I do not even want to go into such aspects as brake temp, rubber compound, road / air temperature, tire pressure , etc. Once we get into these we have can start working on multiple PhD's since the matter becomes so complex as to become a scientific problem, certainly beyond my capacity to grasp in its entirety not to mention solving.

Having said that let me share the following. Complexities involved are worth the time and effort of the R&D masterminds of the top brake manufacturers working for F1. However, selection of safe to drive front/rear brake caliper and disk size solutions may be achieved without necessarily resolving and theoretically describing effect of different factors involved.

Let us take my car setup as an example. The car has certain fixed characteristics ( i.e. weight distribution, center of gravity, power, etc). I use only one GC engine and either of the two diffs (3.9 and 6.14) so the range of torque on the wheels is limited. The fuel tank capacity is only 45 liters so the empty to full tank effect on weight distribution is limited. I have only two sets of 15" x 7" wheels - one set with "wet" tires and one set of "dry" tires so possible variations are also limited. My season starts in late April and ends in late October so the applicable air and road temperature range is moderate ) from teens to around 40 C deg.

I have 40 years experience in driving but only 5 years experience in yountimer rallying and racing , so I me neither inexperienced nor a professional rally race driver with matching driving abilities. This limits what I expect of the car performance and what I can utilize in practice.

With these limitations we have just eliminated a number of key factors that need not be of concern at this stage at all. The adopted approach is :

1/ select and install a combination of disk caliper sized front to rear that likely make the car behave normal ( as one would expect from OEM) under normal dry conditions using Tom's table as guide.

2/ test it on MOT type equipment to measure braking force of front and to rear to have some idea of proportions.

3./ test the setup in real life wide open environment ( like a tarmac airfield) for performance and driving characteristics , using variety of Tilton valve lever positions to :
a/ confirm the car performance and behavior while braking with different strength on a straight and curve under
typical surface conditions ( dry , wet, etc. )
b/ learn the effects of Tilton in variety of lever positions on car performance under conditions as above.
c/ if required introduce any modifications to F/R caliper/disk size as required to get it right, test each setup as
required it until satisfied with performance.

I believe this limited science empirical testing (trial and error) approach has all the chances to get it done the way I will feel comfortable with safe and feel the car to the point of being able to "push it a bit " in actual competition events.

As regards F/R brake ratio: I have no real experience in driving anything but OEM specs front and rear drive cars ( most likely all with close to 70/30 F/R split). I know of the rule of thumb that the front should be significantly stronger than rear to avoid jackknife effect. However I also know that many professional performance drivers would alter this depending on how slippery the surface is.

Let me quote the Sandro MUNARI recommendations again:

(front-to-rear )
tarmac :60/40
gravel : 50/50
mud : 40/60
snow : 30/70

His justification for changing from 60/40 (on tarmac) to to 30/70 (on ice) was to was to avert loosing control of the car due to front wheels blocking too early on a slippery surface. This logic appeals to me intellectually knowing what happens when you start loosing friction of the front wheels on a slippery curve. I thus disagree with Guy that it always must be set up with front wheels doing most of the braking. I agree with him however that going in the direction of more braking on the rear is very dangerous and should be done very carefully , only under rally stage conditions ( not normal daily conditions), matching the driver's abilities as car as on very slippery surfaces. The logic of the stage rally driving is that you really do not brake to stop/slow the car to avoid hitting an obstacle (the stage is closed to traffic) but to slow the car down from a straight to enter and go through a curve with the highest possible speed. This is the key difference why F/R ratio on rally cars can differ greatly from a normal road car setup. Normally our daily driving does not involve using the whole width of the road to slide through a curve.

The question is can I set up the my rally car car for such driving and do I have enough skills to use drive this way with reasonable safety under variety of slippery road conditions. I do not expect to ever acquire the kind of driving skills that would allow me to drive like Munari or Rohrl, and certainly not at their speeds. This exercise of selection brake setup for my 131 Abarth is exactly to see how much I cam push the car safely and setup the brakes accordingly.

I again agree with Guy that the likelihood I will ever get close to 50/50 is remote. So far I have taken part only in tarmac events, and the likelihood I would want to risk the car in a gravel event is remote. So likelihood I would actually use more than 70/30 to 60/40 range is similarity remote.

In most likely event the effective use of the Tilton valve would be compensate brake setup to bring it as close to 67/33 original standard 131 Abarth setup and effectively use one. Ideally I would like to have the ability to adjust the F/R from 70/30 to 55/45 to give me the possibility to test my abilities and understand the effects of different setting.

As the valve unlike the bar serves to limit the pressure and not to shift it. This can be achieved by installing the Tilton valve on the front or rear calipers. Question is which is the better option and what are limiting factors involved.

Here are the options:
options 1.jpg
options 1.jpg (104.52 KiB) Viewed 10203 times
The option two and four are out of the question for obvious reasons.

Options one and four are possible but which to select.
Both options are based on reduction / increase rather than shifting. While theoretically we achieve the easily achieve the required base ratio of 70/30 ratio but it is achieved by reverse action. Option 1 by decreasing front and in option 4 by increasing rear. Option 4 seems to be operating pretty much in reverse to a standard rear braking correction valve, which increases the rear pressure in line with increase of the rear weight. Option 5 seems more in line with manufacturers installation instructions.

My initial reaction to these options would be to go for option 1 and install largest possible brakes for both rear and front in nominal F/R ratio of 70/30.

What do you Gents think about it ?


PS: here is an excerpt from Tilton installation instruction:
Tilton application 1.GIF
Tilton application 1.GIF (18.94 KiB) Viewed 10203 times

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Re: RS 911 calipers for 131

Post by Guy Croft » February 25th, 2011, 9:26 am

"I thus disagree with Guy that it always must be set up with front wheels doing most of the braking"

this is not what I said - I said that with equal power F&R brakes you would be unwise to go more than 50% bias to the rear. The only possible reason to want to do that is to be able to deliberately 'unsettle' the car to force the rear end out - as with 'left foot' braking. That of course is a whole other 'ball game'.

As for ever using 50-50 bias on a setup with single BMC and equal power brakes at front and rear it is a 'life saver' in the wet on tarmac so yes you definitely MUST build that into the setup.


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Re: RS 911 calipers for 131

Post by Guy Croft » February 25th, 2011, 10:01 am

Numerous excellent links here that tell you 9/10 of everything about balance bar brake setups.

No point my trying to write & draw it myself. ... mages&tbs=


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