38mm rear calipers - maybe not my best idea

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

Post by miker » Mon Apr 15, 2019 3:51 pm

Jim-

Your memory is better than mine! But it doesn’t surprise me since 2011 is around the time I lowered the car and I suspect the rear valve setup stopped working then, because I used the shop manual spec to set the lever, and that spec is keyed to the car at the stock height.

I’m going to have to experiment to see if I can get it adjusted at least close to right with the lowered car; maybe an older manual will have a different spec for that adjustment. (Haynes manual: 3.74" for the Spider, 5.787" for the coupe. Later shop manual: 5.787" for the Spider!)

Yes, for all intents and purposes I’ve replicated the 85.5 brake setup. I understand it won’t be like my BMW X1 :), but even now the car stops quicker and straighter than before.

I can tell you aren’t a fan of the speed bleeder screws, I would be curious to know why.

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

Post by fp55scca » Mon Apr 15, 2019 5:16 pm

miker wrote:
Mon Apr 15, 2019 3:51 pm
Jim-

Your memory is better than mine! But it doesn’t surprise me since 2011 is around the time I lowered the car and I suspect the rear valve setup stopped working then, because I used the shop manual spec to set the lever, and that spec is keyed to the car at the stock height.

I’m going to have to experiment to see if I can get it adjusted at least close to right with the lowered car; maybe an older manual will have a different spec for that adjustment. (Haynes manual: 3.74" for the Spider, 5.787" for the coupe. Later shop manual: 5.787" for the Spider!)

Yes, for all intents and purposes I’ve replicated the 85.5 brake setup. I understand it won’t be like my BMW X1 :), but even now the car stops quicker and straighter than before.

I can tell you aren’t a fan of the speed bleeder screws, I would be curious to know why.

Thanks again!
----------------------------------------------------
--Mike, Chuck Story is absolutely correct about the differences between the Voulmex and 85.5 front calipers. In our case, that makes little difference, because we are substituting the Wilwood caliper for these Fiat calipers anyway, we are already mixing apples and oranges. No worries....we can do that, and still make it work safely, and better than original.

--I am very much a fan of Speed Bleeders. (it's the rear brake compensator that I'm not fond of). I run Speed Bleeders on all my Fiats, including the racecar, in order to be able to bleed brakes by myself. It's just when doing an initial setup, the Speed Bleeders will not allow a final "gravity visual check", which (after all the bleeding is done) is the best test of success. I suppose you could have an assistant press and hold the brake pedal, while you crack each bleeder slightly to verify that you get fluid immediately. Not quite as good as gravity alone, and I don't always have an assistant handy. :(

--Mike, if reinstalling the compensator, I would use the 3.74" figure from the Haynes manual as a starting point, which also agrees other early manuals. You may have to do some trial and error testing. Personally, I would do complete emergency braking testing before installing the compensator. Brake proportioning wise, you won't know where you are until you actually test, because you've changed ride height, front calipers/pads, rear calipers and pads, and brake hoses. You might find that you are already proportioned for an emergence stop.
Jim Scurria
Norfolk, VA

1972 Fiat 124 Spider
1971 Spider - SCCA FP-24
1974 CSA Abarth Replica
1981 Fiat Spider Ratrod
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Re: 38mm rear calipers - maybe not my best idea

Post by atruscott » Tue Apr 16, 2019 4:05 am

A couple of loosely connected thoughts in reading this thread.

- I’ve found a power bleeder using the shop air line to be very effective when solo bleeding

- Similarly, a long (1.5m) piece of clear hose to a catch can (the can isn’t really needed but keeps my caution gene in check) running vertically from the caliper nipple let’s you easily see when you’ve pumped clean fluid through - when you’ve still got a foot on the brake pedal

- the best front brakes I’ve installed are using vented, drilled and slotted rotors from the modern Abarth 500 coupled with wilwood 6 pot front calipers combined with stock rear brakes with no compensator, a proportioning valve on each circuit, and the booster removed. The car will stop on a dime and has excellent pedal feel. This might not be everyone’s cup of tea though!
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
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Re: 38mm rear calipers - maybe not my best idea

Post by fp55scca » Tue Apr 16, 2019 10:32 am

atruscott wrote:
Tue Apr 16, 2019 4:05 am

...the best front brakes I’ve installed are using vented, drilled and slotted rotors from the modern Abarth 500 coupled with wilwood 6 pot front calipers combined with stock rear brakes with no compensator,

...a proportioning valve on each circuit...
---------------------------------------------------------
--Good stuff Andy. Fiat OEM, 4-wheel disc brakes were good in their day, but are a bit of a weak spot for today's higher speed driving.

--Is your brake kit something you put together yourself? Sounds a bit like the AR kit with 10" rotors, but 6 pot Wilwood's instead of 4? You might also add the wheel size you used to fit this kit?

--Andy, maybe you could clarify "a proportioning valve on each circuit"?
Jim Scurria
Norfolk, VA

1972 Fiat 124 Spider
1971 Spider - SCCA FP-24
1974 CSA Abarth Replica
1981 Fiat Spider Ratrod
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Re: 38mm rear calipers - maybe not my best idea

Post by SimcaBertone66 » Tue Apr 16, 2019 12:02 pm

How about a simple , cockpit mounted, manually adjusted prop valve ...Tilton makes one... Installed one in my spider in the early 1990's ... after chucking the factory setup … worked great with a totally stock braking system …
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Re: 38mm rear calipers - maybe not my best idea

Post by atruscott » Tue Apr 16, 2019 2:50 pm

fp55scca wrote:
Tue Apr 16, 2019 10:32 am
---------------------------------------------------------
--Good stuff Andy. Fiat OEM, 4-wheel disc brakes were good in their day, but are a bit of a weak spot for today's higher speed driving.
I've always found that when built up / rebuilt correctly they're pretty good. Bleed them right with zero air, and they very good. As I've approached the 200hp mark in the spider, I kind of thought I'd like something a little more efficient though!
fp55scca wrote:
Tue Apr 16, 2019 10:32 am
--Is your brake kit something you put together yourself? Sounds a bit like the AR kit with 10" rotors, but 6 pot Wilwood's instead of 4? You might also add the wheel size you used to fit this kit?
Yes. I based it on the Whoa! approach, but trying to use stock (ish) parts. The brackets are custom though, but don't require hub removal, and basically bolt onto the stock mount holes, and support the Wilwood caliper. It all fits under 15" wheels (just), although that's not so much of a problem for me as I'm running larger offsets on the widebodied spider. There are some rear brackets that have a second caliper location for a handbrake caliper too... just incase I feel that I'd like to migrate the rear end to a beefier setup.
fp55scca wrote:
Tue Apr 16, 2019 10:32 am
--Andy, maybe you could clarify "a proportioning valve on each circuit"?
One proportioning valve on the rear circuit, and one on each of the front circuits. Front are matched (obviously) but I couldn't find a dual circuit valve :)
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
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Re: 38mm rear calipers - maybe not my best idea

Post by fp55scca » Tue Apr 16, 2019 3:51 pm

atruscott wrote:
Tue Apr 16, 2019 2:50 pm
...As I've approached the 200hp mark in the spider, I kind of thought I'd like something a little more efficient though!

...One proportioning valve on the rear circuit, and one on each of the front circuits. Front are matched (obviously) but I couldn't find a dual circuit valve :)
--------------------------------------------------------------
--Now imagine if your car only weighed 1965 lbs Andy, you'd have the beginnings of a race car! :lol:

--Interesting on the proportioning valve setup, but not sure I understand it. With stock rear brakes, seems like you would already be very under proportioned on the rear? Are you reducing pressure to your front brakes with your proportioning valves?

--This was one of the points I tried to make to Mike, and why Fiat added 38mm calipers to the rear of the Volumex and the 85.5. When you significantly improve the effectiveness of the front brakes, you can definitely benefit from more braking on the rear, in fact you need it to keep the car in balance.
Jim Scurria
Norfolk, VA

1972 Fiat 124 Spider
1971 Spider - SCCA FP-24
1974 CSA Abarth Replica
1981 Fiat Spider Ratrod
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Re: 38mm rear calipers - maybe not my best idea

Post by kmead » Thu Apr 18, 2019 6:58 am

fp55scca wrote:
Tue Apr 16, 2019 3:51 pm
atruscott wrote:
Tue Apr 16, 2019 2:50 pm
...As I've approached the 200hp mark in the spider, I kind of thought I'd like something a little more efficient though!

...One proportioning valve on the rear circuit, and one on each of the front circuits. Front are matched (obviously) but I couldn't find a dual circuit valve :)
--------------------------------------------------------------
--Now imagine if your car only weighed 1965 lbs Andy, you'd have the beginnings of a race car! :lol:

--Interesting on the proportioning valve setup, but not sure I understand it. With stock rear brakes, seems like you would already be very under proportioned on the rear? Are you reducing pressure to your front brakes with your proportioning valves?

--This was one of the points I tried to make to Mike, and why Fiat added 38mm calipers to the rear of the Volumex and the 85.5. When you significantly improve the effectiveness of the front brakes, you can definitely benefit from more braking on the rear, in fact you need it to keep the car in balance.
On a race car there is often an adjustable proportioning valve the driver can adjust while driving the car to deal with track conditions, fuel load change and tire change in the course of the race. This is generally not the way a street car would be dealt with. Alternatively if using two masters, a balance bar is used to apportion the motion to each of the masters, sometimes also in combination with the inline adjustable to ensure no rear locking.

As he hasn’t been able to lock up the fronts yet, he has too much rear bias. Balance on a street driven car is not the same as balance for a race car.

On the late Spiders they may have changed the brakes but they also kept a means of reducing rear brake pressure under heavy braking.

The Wilwood adjustable pressure limiting valve is just for the rear brakes (in this case) because he has removed the factory solution and changed many other factors. It would only go on the rear line.

What Andy has done is quite different and I am not sure of the overall why’s of what he had done as I don’t know the specs of the system.

On a street driven non ABS car, the brakes need to be set up to lock the fronts reliably first. The rears just cannot do as much braking due to less weight on them to start with and the unloading of the weight they have due to load transfer to the front brakes as a function of the dynamics of the chassis under braking. Going to a rear biased braking system will ultimately increase braking distances if the fronts can’t get to maximum braking (meaning the limits of traction of the front tires) if the rear brakes reach maximum braking first (as in exceeding the traction the rear tires can attain). This is the balance you are trying to attain, not parity of the front to the rear.
1969 850 Sport Coupe
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Re: 38mm rear calipers - maybe not my best idea

Post by fp55scca » Fri Apr 19, 2019 10:37 am

kmead wrote:
Thu Apr 18, 2019 6:58 am

...As he hasn’t been able to lock up the fronts yet, he has too much rear bias.
-------------------------------------------------------
--Karl, rear brake bias could be the case, but we don't know that to be true, and even if true, we don't know the cause of it. I don't favor installing a proportioning valve in the rear circuit until we know the cause; and I really don't favor doing that on a street car in most cases.

--To solve Mike's brake issues, we need to establish "what we know", and "what we think we know". Mike installed the Volumex brake booster, with a larger MC than original (22.2mm vice 19.05mm). We can be pretty sure that the larger MC bore will reduce line pressure, but we don't have a value for that. He also installed the larger piston, rear clipers (38mm vice 34mm). These will generate greater clamping force. Mike is also using 10" rotors with Wilwood 120-6807 fixed-calipers with four 1.38" pistons, vice the single piston floating-caliper that Fiat used. The Wilwood caliper should generate greater clamping force.

--Without knowing line pressure and calculating clamping force of the front calipers using piston area, etc., I still believe it is reasonable to believe that the 4 pot, Wilwood fixed-calipers will generate at least the same (but most likely more) clamping force than the single piston floating-caliper that Fiat used with a similar system on the Volumex. Piston area aside, we know that a fixed caliper generates more clamping force than a floating caliper; and we know that four 1.38" pistons have a greater area than a single 48mm piston.

--In view of the above, I think it's reasonable to conclude that (if installed and serviced correctly), Mike's system should generate enough clamping force to bias his brakes to the front. If not why not? Once this issue is resolved, Mike can then get on to evaluating the system under emergency braking conditions, and install a compensator if needed.
Jim Scurria
Norfolk, VA

1972 Fiat 124 Spider
1971 Spider - SCCA FP-24
1974 CSA Abarth Replica
1981 Fiat Spider Ratrod
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