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38mm calipers, part II revised

Posted: Tue Jun 11, 2019 4:36 pm
by miker
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 7/8" MC 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 the larger calipers I am running eats up that capacity, and more.

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.

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?

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.

OK, here are the specific numbers. I went first from column B to Column C. I'm currently at Column D, contemplating a return to stock rear calipers, Column E. If I did, I would get a 5mm higher pedal.

Note to all: if you use Wilwood calipers up front, and 38mm rear calipers with a stock MC, it could be dangerous.
spreadsheet.jpg (441.27 KiB) Viewed 1942 times
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. So if you used Wilwood calipers up front, and 38mm rear calipers, it could be dangerous.

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

In my last attempt to make brakes feel better, I loosened the acorn nut 1 turn, which I think corresponds to 1mm. It definitely helped. If someone has a brake booster on the shelf and can measure the thread pitch of the adjusting rod, we could nail this down.

The thing I can't nail down in my head is the effect on "feel" that having rear calipers travel farther has on the way the front brakes feel.

Comments welcome.

Re: 38mm calipers, part II revised

Posted: Tue Jun 11, 2019 8:35 pm
by miker
There is a big discussion about these calipers over on Xweb. ... 288/page-3

One interesting idea was the use of a brake line residual pressure valve. ... =260-13783

Re: 38mm calipers, part II revised

Posted: Tue Jun 11, 2019 10:08 pm
by atruscott
I refer to my previous comments on the two prior threads ;) and add an additional one. This only really makes sense in my mind with extra weight in the rear to compensate for the enhance front end dip.

Re: 38mm calipers, part II revised

Posted: Tue Jul 23, 2019 8:00 pm
by miker
OK, let's review.

Wilwood brakes (Whoa kit), with two opposing sets of two 1.25" pistons. 10" rotors in front, 9" in rear. 38mm rear calipers (new aftermarket, not OEM). 22.2mm MC with bigger booster as on a Volumex Spider.

Symptoms: mushy brakes, can't lock fronts.

Tried: use a 10x1mm brake bleeder screw to plug up rear circuit of the aftermarket MC. Definitely less mushy, fronts feel like they are 'biting'. Did not try stops faster than 20mph due to safety concerns.

Fix: excessive disk runout (as much as 10 thou more than it should be) traced to a bad wheel bearing. Fixed, not a big change in feel, but, I'm glad it's fixed anyway.

Try: measured front caliper deflection, Wilwood says, "not a concern".

Fix: replaced MC with German-made FTE unit, same diameter. Noticeable improvement in 'feel'.

Rear calipers are suspect because of brake balance F/R (see spreadsheet) and because XWeb reports excessive lash in the adjusters on some aftermarket rear 38mm calipers.

Front calipers are suspect because they are smaller than they could be.

For $300 I can upgrade the front calipers from 1 1/4" pistons to 1 3/8" pistons, or buy a set of high-quality rebuilt OEM rear calipers in the stock 34mm piston size.

Or I could do both, for $600 which puts me far more in the hole than I ever expected to improve the brakes on my car.

I think my math is right, all units in mm.

Re: 38mm calipers, part II revised

Posted: Tue Jul 23, 2019 8:33 pm
by beek
my 76 spider has a set of the scorpion rear calipers on it with stock front brakes. It works perfectly fine. I am going to upgrade the fronts soon to vented disc but same diameter piston calipers just different ones. 3/4" oem size master cylinder.

Re: 38mm calipers, part II revised

Posted: Tue Jul 23, 2019 10:38 pm
by miker
Randy - Fiat OEM Scorpion calipers or pattern aftermarket?

Re: 38mm calipers, part II revised

Posted: Wed Jul 24, 2019 5:30 pm
by lanciahf
Mike I used a residual pressure valve on my custom brake set up from TCE Brakes.

Todd at TCE brakes said the residual pressure valves are helpful when brake master cylinders are mounted low like in a Scorpion.

If I were you I would give all your specs to Todd at TCE brakes and see what he thinks. He really loves his brakes.


Re: 38mm calipers, part II revised

Posted: Thu Jul 25, 2019 11:37 pm
by beek