38mm rear calipers - maybe not my best idea

The forum for all of your technical questions on your classic FIAT autos
User avatar
miker
quattromila club
Posts: 4149
Joined: Sun Jan 24, 2016 7:55 pm
Last Name: Richmond
First Name: Mike

Re: 38mm rear calipers - maybe not my best idea

Post by miker » Fri Apr 12, 2019 2:06 am

The adjustment procedure in the shop manual is a bit opaque, which is one reason I gave up on it, I could never tell if I had it right or not. Now that I have lowered the car, I'm also suspect of the "measure from body to here" diagram. How do you adjust it?

My plan is to first see if I can sort out what's happening without the compensator. One idea I had is that if one side is locking then the other side might be more likely to have air in it. If a side is locking, it's likely air-free. The other idea I had is to make sure the handbrake cable is engaging both sides equally, by measuring the displacement of the levers on the calipers when the handbrake is disengaged. If one caliper piston has to travel farther than the other, that would cause uneven braking.

I do want to point out that even with these issues, the car hauls ass to a stop in a way that I never experienced with a stock setup.
MikeR (mirafiori.com since 1995)


1977 Fiat 124 Spider
Previously owned:
2012 Fiat 500 Prima Edizione #236 (now owned by my son David)
'86 Bertone X1/9
'81 Fiat Spider 2000 #236
'78 Fiat 131 four door
'76 Fiat 128 4 door
'74 Fiat 128 4 door
fp55scca
duemila club
Posts: 2627
Joined: Sun Jan 24, 2016 11:34 am
Last Name:

Re: 38mm rear calipers - maybe not my best idea

Post by fp55scca » Fri Apr 12, 2019 11:14 am

miker wrote:
Fri Apr 12, 2019 2:06 am
...My plan is to first see if I can sort out what's happening without the compensator. One idea I had is that if one side is locking then the other side might be more likely to have air in it. If a side is locking, it's likely air-free.
...The other idea I had is to make sure the handbrake cable is engaging both sides equally, by measuring the displacement of the levers on the calipers when the handbrake is disengaged. If one caliper piston has to travel farther than the other, that would cause uneven braking.
----------------------------------------------------------
--Mike, let's leave the compensator alone for now, and consentrate on your brake calipers.

--I'm confident that you are on the right track with this new setup. You have air in the system causing uneven braking. When you do your checks, I would still use the same sequence, and start at the RR. The sequence is based on distance from the MC, with RR being the furthest, and LF the closest, etc. So the idea of bleediing is to remove "downstream" air first.

--With regard to the parking brake, mis-adjustment should not effect brake pedal action, unless you are so mis-adjusted that parking brakes are not releasing. In that case your brakes might be dragging. By design, the parking brake cannot be mis-adjusted to one side or the other, because the cable passes through a "cable equalized" at the pivot point under the parking brake lever. (the horseshoe looking bracket that holds the cable).

--When doing your brake testing after bleeding, 30-40 mph is sufficient to check for equal braking initially. Just hold the steering wheel lightly and apply the brakes firmly, assuring that the car does nose down properly, and does not pull to one side or the other; and of course, you don't want the rear brakes to lock during this test. Given your new setup on all 4 corners, your brakes should be pretty darned nice!
Jim Scurria
Norfolk, VA

1972 Fiat 124 Spider
1971 Spider - SCCA FP-24
1974 CSA Abarth Replica
1981 Fiat Spider Ratrod
User avatar
kmead
Posts: 78
Joined: Sat Aug 20, 2016 8:33 am
Last Name: Karl
First Name: Mead

Re: 38mm rear calipers - maybe not my best idea

Post by kmead » Sat Apr 13, 2019 12:20 pm

I would go back to the smaller rear pistoned calipers and put in a pressure reducing valve.

The rear locking up is due to weight transfer. You have a number of things you changed that all add up to trouble in this area.

Consider that on front wheel drive cars until the advent of ABS the front brakes would do @80% of the braking due to front weight bias and dynamic load transfer. Rear brakes were along for the ride (some of you may remember the GM X Cars having a big hullabalu over rear lockup and cars spinning in the early 80’s) and were quite undersized and often had pressure limiters on rear lines either as line limiters or compensator valves that were vehicle load dependent.

You changed a number of things that affect the dynamics of the car:

The car was lowered. This changed the front suspension geometry and the way it works relative to braking.

The spring rate was likely changed when you changed springs. This in combination with how the front geometry has changed reduces the front dive to some degree even though more weight is being transferred onto the front wheels.

Added larger, more capable brakes on the front. These increase the amount of possible braking at the front which has increased front weight transfer. Keep in mind that the whole car is trying to rotate around the front brakes as you clamp down hard. So the harder you brake, the more transfer of weight off the rear axle, thus less traction and thus increased rear locking possibility.

Changed to larger rear pistons which apply more clamping force and increase the possibility of locking.

Removed the rear pressure limiting device for the rear brakes. Although this is problematic, the existing solution might not have been able to do its job given the other changes to the front brakes and the changes in the suspension of the car. Removal of the function of this device is likely more problematic than the device itself. The OE solution may not have been able to be adjusted to a point where it would ever be effective due to the suspension changes you made.

Fiat chose the rear calipers on the 124 based on the dynamic loading of the car under braking and added a pressure limiting device to reduce the chance of rear wheel locking in an emergency braking application. The emergency braking application could be in the dry or in the wet. Wet braking always has greatly reduced traction and as the weight transfer increases the higher the likelihood of rear locking so the system was optimized around this scenario to rarely lock. This means that most of the braking on a 124 which has a front weight bias anyway also has front brake bias intentionally.

You have somewhat increased your front brake bias and then moved bias to the rear with larger pistoned calipers. On the face of it it makes sense if all else was the same but as stated above this is not the case. Right now you have more rear braking bias than it needs which is evidenced by rear locking. It is normal to have one brake lock before the other due to the manner in which the rear axle is located and suspended, the Panhard rod will tend to unload one side of the axle more than the other and thus why you have one brake locking first.

One brake locking is a recipe for disaster (ie the car will at the least turn or worse case spin, particularly in the wet).

You always want a street car set up so that in all the conditions it may face that the driver can maintain control of the vehicle with reasonable driving ability. Right now that is not the case.

So if it was my car, I would go back to the OE rear braking ability (ie the 124 rear calipers) and add an inline pressure limiting device which can be adjusted for either all conditions or have two marked positions one for dry and the other for wet. You could keep the bigger calipers and just add the pressure limiting device as another alternative.

I don’t think the OE rear compensator can be made to work properly here if the car is not diving forward or rising as much in the rear as it did from the factory. An inline valve which is cheap and easily tapped into the single line I think is the right solution in this case.

There is a lot at play here, but first and foremost is making sure your car will be safe in an emergency braking situation where you are trying to avoid hitting a child or hitting another object which won’t be as kind to you given that you are driving a 40 + year old car.
Last edited by kmead on Sun Apr 14, 2019 8:35 am, edited 1 time in total.
1969 850 Sport Coupe
1970 124 Sport Coupe
1985 X1/9, ugly but fun to drive. Olive trees are hell on paint and interiors apparently. Who knew?
1987 X1/9 slowly coming back
And a few appliances for day to day
User avatar
atruscott
Posts: 645
Joined: Sat Mar 19, 2016 10:40 pm
Last Name: Truscott
First Name: Andy

Re: 38mm rear calipers - maybe not my best idea

Post by atruscott » Sun Apr 14, 2019 7:02 am

I agree with Karl’s logic path here.

As an interim step, I would simply place a proportioning valve in the rear brake line up by the MC, and restrict the fluid flow slightly - not a lot - right up until you get no locking.

A
1960 Autobianchi Bianchina Transformabile
1970 238 Camper OHV
1974 124 Wagon TC
1974 124 Special TC
1975 124 Sport Coupe
1976 124 Sport Spider (The Racer)
1981 Spider 2000 (The Resurrected)
1985 Pininfarina Azzurra
2017 124 Spider Abarth Elaborazione
User avatar
kmead
Posts: 78
Joined: Sat Aug 20, 2016 8:33 am
Last Name: Karl
First Name: Mead

Re: 38mm rear calipers - maybe not my best idea

Post by kmead » Sun Apr 14, 2019 8:34 am

atruscott wrote:
Sun Apr 14, 2019 7:02 am
I agree with Karl’s logic path here.

As an interim step, I would simply place a proportioning valve in the rear brake line up by the MC, and restrict the fluid flow slightly - not a lot - right up until you get no locking.

A
This unit would be ideal, inexpensive and as Andy suggests could be plumbed in near the master cylinder. The accessibility would allow reasonable adjustment without having to crawl around.

You could set it up an adjustment for Dry or Wet and mark the knob to be able to maximize braking ability for the conditions. This would like require a fair amount of testing to get it right.

https://www.wilwood.com/MasterCylinders ... =260-12627

The OE solution was optimized around wet weather, thus the reduced rear braking.

As an aside, the Coupe and Sedan benefited the most from the rear proportioning valve, both vehicles would have the greatest additional load in the rear due to rear passengers reducing rear rise and the added weight would provide more traction. The Spider rarely would be so loaded (except when we were all in college) and generally would rarely enable the rear brakes to deliver on their potential.

If you do go to the smaller rear pistoned calipers, you will have slightly less travel at the brake pedal.

Good luck and safe driving.
1969 850 Sport Coupe
1970 124 Sport Coupe
1985 X1/9, ugly but fun to drive. Olive trees are hell on paint and interiors apparently. Who knew?
1987 X1/9 slowly coming back
And a few appliances for day to day
User avatar
friedman
mille club
Posts: 1838
Joined: Wed Aug 24, 2016 8:13 am
Last Name:

Re: 38mm rear calipers - maybe not my best idea

Post by friedman » Sun Apr 14, 2019 4:43 pm

An Xweb member asked me to pass this on since the use of these rear calipers was discussed at great length. TonyK noted that the lash was different from right to left calipers and had to shim the pads to get it right.

I know nothing about this, just passing it on as requested. If you want more discussion, buzz over to the Xweb and ask.
Carl in Virginia
75 spider
83 Bertone
fp55scca
duemila club
Posts: 2627
Joined: Sun Jan 24, 2016 11:34 am
Last Name:

Re: 38mm rear calipers - maybe not my best idea

Post by fp55scca » Sun Apr 14, 2019 7:00 pm

--Carl, I tried to search the topic of 38mm rear calipers, under author "TonyK", on the Xweb and I came up empty handed. Maybe someone else may know their way around that site better than I? I'm not a Lancia Scorpion guy either, but I think these 38mm calipers were used on the Lancia Scorpion, and the Pininfarina 85.5 Spider as well? Not sure about the Volumex?

--Mike, some interesting discussion here about compensating for the use of these calipers in conjunction with larger front rotors (57mm) and improved front calipers (Wilwood). I can't say I disagree with the rationale proposed for installing a brake proportioning valve in the rear circuit, or for re-installing a new rear compensator; but, before you consider making any of these modifications, I think you need to sort a few things out.

--(1) Mike, were the front Wilwood calipers you installed P/N 120-6807? Do yours have 4 bleeder screws on each front caliper? If so, did you bleed all 4 ports of each front caliper, beginning with the outboard ports, and then the inboard? Did you bleed the front calipers upon installation, before you proceeded to the 4-corner bleeding process?

--(2) When you installed the braided brake hoses, did you install all five hoses, including the center-rear, and the rear calipers?

--(3) You mentioned Autozone Duralast brake pads in the rear calipers, but what brake pads are you running in front?

--(4) Is your replacement brake booster an "OEM reproduction" provided by our vendors?

--Mike, sorry for the "20 questions" but I don't think you can press forward with this until you sort out your basic setup.
Jim Scurria
Norfolk, VA

1972 Fiat 124 Spider
1971 Spider - SCCA FP-24
1974 CSA Abarth Replica
1981 Fiat Spider Ratrod
User avatar
miker
quattromila club
Posts: 4149
Joined: Sun Jan 24, 2016 7:55 pm
Last Name: Richmond
First Name: Mike

Re: 38mm rear calipers - maybe not my best idea

Post by miker » Sun Apr 14, 2019 7:33 pm

I found the Xweb thread, thank you! https://xwebforums.com/forum/index.php? ... all.31288/

It looks like Tony got some old calipers and rebuilt them, and he got pretty deep in the weeds compared to what I'm willing to do (resleeving brake pistons with TIG welding...whew!).

I'm getting much better idea of how this all works from reading the debate back and forth.

I am intrigued by the Wilwood valve, which would be pretty easy to install, but if I do, it would be find a setting and leave it. I always try to engineer things in my car so that if I get hit by a bus the next owner doesn't have to think about things like wet/dry.

I have a new rear proportioning valve, and I'm going to put it back in, but I'm not sure I'll enable it yet with the bar. In particular, I want to make sure I get all the air out and the handbrake really right (taking up all the slack) to ensure I'm happy with the pedal height. If I'm not, I might have to take a loss on these 38mm calipers and go back to stock. Right now the handbrake is at four clicks. I'm a little paranoid about rear drag (based on the old calipers).

I really appreciate the observation about the panhard rod leading to one wheel locking before the other. Very interesting.

I understand and agree with the point that the compensator is more important for the sedans. But my car isn't any lower than the stock cars from the early 1970s that had the compensator. I have this nightmare about coming down Indian Memorial Road near Ashland and standing on the brakes when a deer runs out, having the rear come around and goodbye Mike and Sue over the edge. What I'm realizing is that I never had correctly functioning or setup rear brakes since I lowered the car. And I'm realizing that if I use the compensator, it's going to take some careful testing to make sure it actually works on a hill. I have a 14% grade not too far from here that I can use. Fortunately, it's straight down with curbs on either side!

BTW, I'm running 13 x 6 wheels with 185/70-13 rubber.

I'm a little concerned that with the Wilwood calipers and 10" rotors I have not been able to lock the front brakes yet.

Just read the latest post...1) yes, that the Dynalite calipers from the Whoa brakes kit. Yes, bled all four screws, but not before bleeding the rears. But at this point, the fronts are not disturbed and the rears have to be done again, so OK? 2) yes, all the hoses. 3) pads in front are Wilwood BP-10, I think, but I'll pull a wheel and check. 4) booster is Volumex-style with bigger M/C from Autoricambi.
MikeR (mirafiori.com since 1995)


1977 Fiat 124 Spider
Previously owned:
2012 Fiat 500 Prima Edizione #236 (now owned by my son David)
'86 Bertone X1/9
'81 Fiat Spider 2000 #236
'78 Fiat 131 four door
'76 Fiat 128 4 door
'74 Fiat 128 4 door
fp55scca
duemila club
Posts: 2627
Joined: Sun Jan 24, 2016 11:34 am
Last Name:

Re: 38mm rear calipers - maybe not my best idea

Post by fp55scca » Mon Apr 15, 2019 12:53 am

miker wrote:
Sun Apr 14, 2019 7:33 pm
...I am intrigued by the Wilwood valve, which would be pretty easy to install, but if I do, it would be find a setting and leave it.

...I have a new rear proportioning valve, and I'm going to put it back in, but I'm not sure I'll enable it yet with the bar. In particular, I want to make sure I get all the air out...

...I'm a little concerned that with the Wilwood calipers and 10" rotors I have not been able to lock the front brakes yet.

...Yes, bled all four screws, but not before bleeding the rears. But at this point, the fronts are not disturbed and the rears have to be done again, so OK?
...pads in front are Wilwood BP-10, I think, but I'll pull a wheel and check.

...booster is Volumex-style with bigger M/C from Autoricambi.
----------------------------------------------------------------
--Mike, I'll be the first to admit that I don't always have much memory, but a quick search reminded me that you've been having some of these same brake issues for many years now. You reported the left rear brake lock-up problem (circa 2011) long before you installed the rear 38mm calipers. As discussed recently, you need to complete the brake bleed process, until you have clear brake fluid flowing from each of the screws. If you have SpeedBleederes installed, ditch them until you get your brakes bled properly.

--When I piece together your brake system, you essentially have duplicated a Volumex, or 85.5 Spider, less the OEM Fiat front calipers that go with this system. You have substituted Wilwood 120-6807 calipers, which are equipped with four 1.38" pistons: You have the Volumex booster and MC, 257mm rotors on the front, 38mm calipers on the rear, and braided brake lines thoughout. The only thing you are missing now is the rear compensator, which was original equipment with both the Volumex and the 85.5 Spider. You could install that now, and be done with it.

--I would like to make one more comment about upgraded Fiat braking systems. You can definitely get your Fiat to stop much better than original, but no matter what you do to upgrade your Fiat brake system, it will NOT stop with he same reliability, efficiency, and consistency as a modern ABS system. ABS brakes compensate for ambient conditions, weather and road conditions, and even tire pressure/wear, etc. A proportioning valve or a rear compensator can do none of that, and never will. Each of these can only aid in eliminating rear brake lock-up under "emergency braking" conditions, and the adjustments will only be nominal. Change road conditions, temperatures, tire wear, etc., and you can throw those nominal adjustments out the window.

--Let's be realistic, 99% of our driving and braking is under less than "emergency lock-up conditions". The front and rear brakes work in proportion to stop the vehicle. When serviced and setup properly, this is why the addition of the 38mm calipers will definitely compliment your 257mm front rotors and Wilwood calipers, and aid in stopping your Spider.
Jim Scurria
Norfolk, VA

1972 Fiat 124 Spider
1971 Spider - SCCA FP-24
1974 CSA Abarth Replica
1981 Fiat Spider Ratrod
cstorry
Posts: 111
Joined: Sun Jan 24, 2016 11:02 am
Last Name:

Re: 38mm rear calipers - maybe not my best idea

Post by cstorry » Mon Apr 15, 2019 12:21 pm

FYI - it's true that he volumex used the larger rear calipers but the fronts were different from the 85.5 model

The volumex front brakes were larger vented discs and fixed caliper mounting with 1 piston each side of the rotor/disc.

While the 85.5. did use the larger piston rear caliper and larger front rotor with sliding pin calipers I think the booster was same as earlier cars.

... more data but sorry no conclusions or recommendations
Post Reply

Who is online

Users browsing this forum: No registered users and 6 guests