38mm rear calipers part II

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miker
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38mm rear calipers part II

Post by miker » Mon Jun 10, 2019 6:36 pm

I am starting a new thread on 38mm rear calipers because the last one zeroed in on improving my brake bleeding skills (which helped, thank you!). There are also some interesting points made on the relative merits of rear compensator vs. none, and brake balance.

Here I just want to focus on one question: are 38mm rear calipers appropriate for Spiders.

Recall I’m running a 3/4” MC (vs. stock 5/8”) with a bigger booster to compensate for the larger effort required with a larger diameter booster. In theory, the larger MC should mean less pedal travel. But that’s not been my experience.

Here is a theory for what’s happening. First recognize that in our cars, the MC gets pushed on by the actuating rod coming out of the booster. Free play can be adjusted with the acorn nut on the end of the rod (I currently have that nut extended one turn more than it was supplied to me, probably 2mm vs. 1mm spec).

The rod pushes the MC piston and the first fluid compressed is going to the rear brakes, but at the same time the rear piston acts on the front piston which goes to the front brakes. So if the rear brakes have a lot of movement to take up, front braking will be delayed. If the brakes in back fail altogether, the piston in back will make mechanical contact with the piston in front and you will get some braking in front with the pedal on the floor. If the fronts fail, the same mechanical compression will eventually get you some rear braking.

Question 1: is this correct?

Question 2: how much more brake pedal travel will larger calipers require?

The area of a 34mm piston is (17mm2 x 3.14 = 907 mm sq) or about 1.4 sq in. The area of a 38mm piston is (19mm2 x 3.14 = 1134mm sq) or about 1.75 sq in.

But we have to double these numbers because there are two calipers: 1814mm2 (2.8 sq in) for the stock ones, 2268mm2 or about 3.5 sq in. for the 38mm calipers.

We could calculate the volume of fluid needed to move these pistons and compare that to the volume pushed by the MC, but we don’t need to. We just need to compare the areas of the pistons. If the pistons on each end have the same area, they will move the same amount. If the MC has half the area, it will have to move twice as far. But the larger MC will require twice the effort. In my case, that’s OK, the bigger MC is mated to a bigger booster.

A stock MC is 15.87mm (5/8”). Mine is 19.05mm (3/4”). Area of these is (15.87mm/2, squared, times pi) = 198 mm2 for the stock one and 285 mm2 for mine.

Let’s suppose we need 1mm of piston travel. How much MC travel is that? The 38mm caliper piston has 7.958 times the area of the large MC I am using. So to get 1mm at the caliper, I need 7.96 mm at my big MC. With stock pistons at the calipers and my large MC, I need 6.36mm. So I need 1.6mm more MC travel to move the bigger rear calipers over the stock rears with my big MC.

There are about 60mm from the pedal hinge point to the booster yoke. The pedal continues another 200mm or so, for a total of 260mm from the hinge. That’s about 4.3:1. So my 1.6mm increase at the MC to move the larger calipers translates to almost 1.1 cm more movement at the pedal.

With a stock MC and the bigger calipers, it’s worse: 11.45mm needed in the stock MC for 38s, vs. 9.16 for the stock calipers and stock MC, a 2.29 mm difference translating to 1.4cm more movement at the pedal.

More movement isn’t just a pedal feel issue. If too much stroke is used up top, you will run out of stroke if one of the brake circuits fails.

Of course this begs the question #3: if I have larger rear pistons and a larger MC, why do I have a problem? Is 1.1cm at the pedal that big a deal?

Well, I have Wilwood calipers up front Wilwood pistons are 1.58 sq in or 1019 mm2 and there are two (actually two on each side, but each side only moves half as much as stock) – 2038mm2 total for one caliper. 48mm stock *front* calipers have a piston area of 1808 mm2. 2038 – 1808 = 230mm2 more area to move.

With my big MC, that’s 7.15mm of movement in the MC for the Wilwood calipers to move 1mm vs. 6.34 mm of big ass MC to move the stock pistons. About .8mm more, or another 3.4mm at the pedal.

So, when I went from Wilwood w/stock MC to Willwood + 38s plus bigger MC, I expected I would net out with less pedal travel. Wrong, apparently.

How much piston movement in the caliper is actually required? My math is for 1mm of movement. What if it’s 2mm?

Comments welcome.
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 part II

Post by lanciahf » Mon Jun 10, 2019 7:32 pm

Mike,I thought stock BMC diameter was 19mm and your upgraded one was 22mm?
Ralph DeLauretis
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1984 Pininfarina Spider
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Re: 38mm rear calipers part II

Post by miker » Mon Jun 10, 2019 8:15 pm

oops yes. I'll redo the math and repost. Please hold off comments until then, folks....
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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atruscott
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Re: 38mm rear calipers part II

Post by atruscott » Tue Jun 11, 2019 8:57 am

... and while the calculator is out...

I'm not sure improving the rear brakes is going to matter that much, in fact in my experience of messing with such things - you might want them slightly less efficient. Here's why: we all know that the front of our cars dip somewhat under heavy braking, and even with suspension upgrades the stiffened vehicle transfers a lot of weight to the front end. Both of these effects have the impact of causing a degree of "lifting" at the rear, reducing the contact the rear wheels make with the ground. Fiat recognized this and included the rear brake compensator which did an admirable job of reducing pressure to the rear brakes under heavy braking.

Now consider the modifications being made to the front brakes. They are (by design) improving the braking of the vehicle. This should increase the weight shift effect to the front (otherwise, what's the point of the brake upgrades?), and exacerbating rear lift. If we improve the rear brake efficiency then we increase the likelihood of rear end lock up....

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
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Re: 38mm rear calipers part II

Post by davedecker4 » Tue Jun 11, 2019 1:39 pm

Mike,

The answer your first question is "about yeah much"
Dave Decker
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miker
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Re: 38mm rear calipers part II

Post by miker » Tue Jun 11, 2019 4:37 pm

Please post all further comments in "38mm rear calipers Part II, revised"
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 part II

Post by vandor » Fri Jun 14, 2019 12:06 am

atruscott wrote:
Tue Jun 11, 2019 8:57 am
... we all know that the front of our cars dip somewhat under heavy braking, and even with suspension upgrades the stiffened vehicle transfers a lot of weight to the front end. ...
Just quick unrelated note: weight transfer is what causes the car's front to dip, or the car to lean in corners. One can do things to lessen the physical effects of weight transfer (stiffer springs, swaybars, etc), but that does NOT change the amount of weight transfer.

The only way to decrease weight transfer is to lower the car (thus shortening the lever arm between the car's center of gravity and the tire's contact patch on the road), of putting the tires farther apart (increased track or wheelbase).
Csaba
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