My 131 abarth (never ending) story - 2017 season updates

[Rallyroller]

Hi Tom

I have started to read through the other thread. Respect for your calculations. When I get around to fitting the brakes I will look at the subject again. Back to Miros problems, one extra comment. As stated before , I swapped the setup from my Spider to the 124ST. It worked well with only the smallest of adjustment on the balalnce bar. I then changed the front springs ( slightly longer and much stiffer) and that really upset the brake balance. It took quite a while to get it back to where I wanted. Most of my rallying was on gravel, but I did always alter it slightly when doing tarmac stages.

Nigel.

[TomLouwrier]

Hi Nigel,

Changing the springs does not affect the car when everything is moving at constant speed (that includes standing still): both weight distribution and brake balance are static and predictable. Hitting the brakes however means everything changes. The weight shifts to the front, the nose dives, putting more force on the front wheels, and the rear comes up, taking force off of the rear wheels. While this is happening you're losing speed, so likely you'll release the brakes again and everything has to settle again.
The harder the springs, the faster this goes. Your harder springs will have shifted weight to the front so abruptly it may have caused the fronts to lose traction and lock up.

On bikes you learn the best way to brake hard is to apply gently, then build up the pressure. As the bike goes down on its nose you'll have more force pushing on the front wheel so it can take more braking. If you get it right you'll do a 'stoppie', lifting the rear wheel off the ground. Slam them on hard and the front will lock up and slide; down you go. Ouch.

This suspension dynamics stuff is something I'm not even touching as far as calculations are concerned! (Just in case anybody was about to ask. No!)
You really need to find out by testing. Basic setups to get in the ballpark can be done 'on paper' with quite complex models and software, but even then factories and racing teams take a lot of time to go out there and prove out their alternatives. You can catch some interesting ones sometimes at places like the Nurburgring. That's still one of the biggest milestones for demanding tracks.

regards
Tom

[miro-1980]

131 abarth brake update.

We have tried a 22.2 and 25 mm BMC .

The former did not work for this setup.

The 25 mm BMC was good , but only for the first push on the pedal. On any additional braking attempts the rear brakes did not get enough fluid to create required pressure.

So we went to square one.

The setup will involve original gr 4 pedal-box with twin pump and a balance bar but no servo.

AS it will be some time before this setup is actually installed ( these parts are not easy to come by) we will use 25 mm BMC but the rear will have smaller calipers ( from Polonez front (and a smaller disk of 227 mm ).

To my surprise this works well !

Front /rear brake balance seems ok. but we will put the car into MOT tests to find out numerical details of the R/F balance.

I plan to do one rally driven lightly with this setup ( to test how it goes in real life ).

PS as the car was not ready for the 1st Poland Hally Historique we drove as 124 spider. It is practically almost civilian the only modification was 13 inch 205 x 60 tires , 7 " front and 8" rear Cromodora wheels and Koni shocks on front while Bilstein shocks on rear ( both adjustable) , half cage and a halda ...

I was truly surprised that on these shocks effected the driving - absolutely fantastic.

Any way ...

Few pictures from the rally ....

124 Poland Rally Historique 8..jpg (85.57 KiB) Viewed 12641 times

124 Poland Rally Historique 5..jpg (70.43 KiB) Viewed 12641 times

124 Poland Rally Historique 3.jpg (59.7 KiB) Viewed 12641 times

124 Poland Rally Historique 2.jpg (92.67 KiB) Viewed 12641 times

Miro

[Guy Croft]

"setup will involve original gr 4 pedal-box with twin pump and a balance bar."

What can I say MM??!!


Ho hum.

G

[miro-1980]

If you do not know what to say keep quiet !

( I don't want to hear: "I told you so !" )

M


(Crofty is smirking 2000 miles away...!)

[TomLouwrier]

hi Miro,

Although I know you intended to rally the 131, it's nearly as good to see you having fun in the Spider.

...but only for the first push on the pedal. On any additional braking attempts the rear brakes did not get enough...

Call me a cynic, but this still bothers me. What is going on that upsets your braking after one attempt? This may very well be something in the wheels, not the MBC and if so it will come back to haunt you after you've built and fitted a beautiful twin setup.

regards
Tom

[miro-1980]

hi Miro,
Call me a cynic, but this still bothers me. What is going on that upsets your braking after one attempt? This may very well be something in the wheels, not the MBC and if so it will come back to haunt you after you've built and fitted a beautiful twin setup.
regards
Tom

Tom,

This element is exactly the thing which bothers me as well. What is going on that upsets your braking after one attempt?

This is a problem we encountered with a small 19.5 pump and fully installed big brakes.

When we installed 25 mm pump the front big calipers work every time , first second , third , etc. ... equally very well.

The problem is wth ATE 2x 38 gr 4 calipers in the rear. On the the second attempt I loose braking power in the rear.
What the heck is going on ?

When we installed back the Polonez floating calipers 1x 34(?) mm
the brakes work perfect every time .

Our analysis point to capacity of the rear ate . They are really front calipers installed on rear hubs. We measured the volume of brake fluid they require and the difference between the Polonez and rear ATE is massive ( many times over ) . Could this be the reason.

Could the in line Tilton be the problem ( fully opened ) ?

It just does not make sense that we can have a 25 mm pump and 300 front disks with RE 911 2x 48 calipers work well with Polonez small calipers 227 disks and not the ATE 2x 38 with 252 disks.

Could this really be caliper capacity ? Maybe there is something wrong with the calipers?

If my thinking is correct a BMC must strike a delicate balance. It must be big enough to hold enough fluid that when brake pedal is depressed the fluid is pushed into the caliper cylinder filling it out fully and create pressure against the pads and the disk.

capacity vs pressure 1 .jpg (28.44 KiB) Viewed 12618 times

Miro

[TomLouwrier]

Miro,

Just a quick note as I'll be off for a couple of days and need to pack my stuff now.
The volume you need to drive the caliper is simply the total piston area times piston travel. Travel is the same as the total disc-pad clearance (for both pads). The rest of the fluid is just there to fill up space. The disc dimensions are of no influence, we're just looking at the hydraulics here.

If it works the first time but not the second time, it should not be related to the MBC. Something seems to be pushing your pistons too far back into the calipers and you must 'pump out' that distance before you can brake again. This may be a warped disc, rotten bearing (especially if you get this after cornering), malformed spacers (faces not parallel causing the disc to wobble) or a problem in the caliper itself.
If it's related to the Tilton valve I would expect dragging (residual line pressure) rather than losing braking at the rear.

When you pull the hand brake slightly, what do you feel? (You have separate calipers for hand braking now, right?) A shudder points to a warped or out-of-true disc.
Have you measured the rear discs on the hub for running true using a dial indicator?
Does this happen as well when you're not driving at all (pedal stays hard)?
Of course you have the bleed nipples correctly facing upwards, all air is bled and caliper alignment is good (centred and parallel to the disc). Right?

Please double check the piston sizes and numbers you mention, it is not really of big influence right now, but can confuse us later.
As far as I know the original rear was 34mm floating caliper on 227mm disc, the Polonez used a 38mm on the same 227mm disc (nice bolt on upgrade btw) and the 'big break' set is 2x38mm fixed caliper on a 252mm disc.
NOTE: just as with the 48mm caliper at the front, the 38mm fixed caliper at the rear is hydraulically speaking the equivalent of the 1x38mm floating caliper you got of the Polonez. Same clamping force, same total piston travel. Either 2 pistons both travel 1x pad clearance each (fixed) or 1 piston travels 2x pad clearance, moving the caliper body in opposite direction (floating).
So effectively you have the same pistons front and rear you already had (48mm / 38mm). Something tells me it's in the components (defect), not in the system capacity (design).

regards
Tom

[timinator]

Hi Miro, To add to Tom's list : Is the rotor chamfered to clear the register ( radius or step) of the hub? Are the wheel studs stepped and proud of the hub? Tightening the lug nuts to torque can cause excessive runout with these conditions.

[miro-1980]

As you wrote :

"This may be a warped disc, rotten bearing , malformed spacers (faces not parallel causing the disc to wobble) or a problem in the caliper itself."

One element you mentioned we noticed during the work. The rear caliper pistons are drawn too far back after the brake pedal is released.

It is interesting though that the problem is both when the car is stationary as well as when it is moving. Wobbling disk, bad bearing or malformed spacer would cause this effect when the car was moving. The fact is that the effect is present when the car is both moving as well as when it remains stationary discounts everything except for the caliper itself and/or its positioning on the disk.

So we have identified the suspect : the caliper. There is either something wrong with the caliper ( which pulls the pistons to far back the the pressure is released or the caliper is not centered centered on the disk ( different distances between the pads and the disk of the same caliper).

As certain gent form 221 b Baker street wold say: "It is elementary...".

We have a suspect ...

We will inspect the pads position relative to the disk for any differences in distance, and if this proves to be correct we will turn to calipers themselves.

Thanks Tom again, you have been of great help !!! You have narrowed the suspect list to two related items ( eliminating the BMC capacity as possible reason).

Miro

PS: I seem to recall that some old ATE calipers had internal springs retreating the pistons .

PS: The front brakes we used before were Uno Turbo on a floating caliper.

[miro-1980]

Just one more element.

There is a small solid metal block between the BMC and the calipers. All 3 brake lines ( two for the front calipers and one to the rear calipers) leading from the BMC to the calipers go through it. The rear brake line is later split in two feeding two rear calipers.

This block (as I understand it) sets a fixed balance between front and rear brakes. Does it effect both volume and pressure delivered to the rear and the front calipers?

Any reason this should stay on /be removed ?

Miro

[Guy Croft]

it is not unheard of for the Tilton device to fail. The AP Racing one is not as pretty but in my exp more reliable.

G

[miro-1980]

Thanks Guy ,

We will look into this, but how come it works with Polonez calipers and not the ATE ones

Miro

PS this soon will be past as we are planning to go for full gr 4 brake setup on genuine parts including twin BMC and balance bar as well as genuine pedal box.

M

[Guy Croft]

ok - it won't be the adjuster then.

It's not just the pad material is it? You need a rear pad that offers good friction from cold. That could maybe explain why one caliper was more effective than another.

Sorry - I am way behind the thread on this..

G

[miro-1980]

G,

Both rear caliper have the same problem - the rear calipers backing the pads to a distance of ca 1,5 mm while it should stray say at .7 mm.

Incidentally I ma using race pads a bot hotter on the front and a bit cooler on the rear.
It is not only effective , I have to pump twice every second time i use the brakes for the rear to be affective ...
and the pedal drops ...

We are working on it .

If push comes to shove I will send the calipers to ATE for testing and adjustement and rebuilding if required)

M