Fun with math - calculating MC bore sizes

[Stangrob]

Evening guys,

OK, I'll admit it - I've gotten caught up in the whole Wilwood MC conversation. Call me a fan of hard anodized surfaces and steel pistons, but since I can still easily access my pedal box without being a gymnast, I'm going to use Tilton MCs for my brakes and hydraulic clutch. If nothing else I'll sleep a bit better :)

The question for me (and as posed by others) is - which sizes? For the clutch I'm just going to swap out the Wilwood unit that Mike Forte sent with the same size Tilton (still debating 75 or 76 series). But the front and rear brake MCs are a different matter. I decided to do some research and figure out the math vs. just ordering the .75" MCs from Tilton (although believe me, I was tempted to do so). Since I was going through the effort I figured I'd share and let everyone grade my math. If I'm off I'm a big boy and can make adjustments (and take criticism as well). And BTW, my setup is using the default Ford brakes, which are essentially '04 Mustang GT front calipers and '88 T-Bird rear calipers.

So, for this exercise you need a few pieces of information:
- Total piston area of one (1) front caliper
- Total piston area of one (1) rear caliper
- Pedal ratio.

Since my car is still in its early stages of construction, I still have my calipers off of the car. As it turns out both the front and rear calipers use 1.75" pistons. I verified my numbers by checking out the calipers on Rock Auto, where the technical specs for the calipers are listed. The area per piston comes to 2.41 in2. For the front caliper there are two pistons so the total area is 4.81 in2, while the rear is a single so it's 2.41 (yes, some rounding is going on). BTW, if you don't recall how to get area from diameter, it's:
((Piston Diameter/2)2)x3.14 or Pi R2

(Quick PSA - everything I read said to only use a single caliper's piston area for the calculations. I thought that strange, but the numbers do work, so at this point I'll trust that approach).

For the pedal ratio I broke out my handy ruler and measured the following on my Wilwood pedal box:
- Distance from the pedal pivot to center of the brake pedal: 10.5"
- Distance from the pedal pivot to the MC/clevis pivot: 1.75"
- Pedal ratio: 6:1

So the goal of this exercise is to find a MC with a bore size/piston area that, when divided into your caliper piston area, is close to your pedal ratio. If your number is too high then you'll have a hard time stopping - if the number is too low then you'll lose modulation.

For my front calipers the math looks like this:
- Master cylinder piston area=Caliper piston area/Pedal ratio
- Master cylinder piston area=4.81 in2/6
- Front master cylinder piston area=.80 in2

For the rear brakes the math is:
- Master cylinder piston area=2.41 in2/6
- Rear master cylinder piston area=0.40 in2

At this point you take the numbers and find a MC that's in the range of what you need. It's unlikely that you'll find a perfect match but I think close is "good enough". I borrowed this chart from Speedway Motors (they have a good article that I'll provide the link to below).

225469 (sorry, you'll have to click on it to read it...)

So, for my setup it appears that I need the following:
- Front MC: 1" bore (0.79in2 piston area)
- Rear MC: .75" bore (0.44in2 piston area)

A few other small items to share:
- I really don't want to order the entire Tilton kit - just the MC. But I do need the remote reservoir cap. It turns out that it's available separately if you want - Tilton part #74-200. Summit has them - I assume others do too.
- Speedway had a really good article that helped me a lot. Here's the link: https://www.speedwaymotors.com/the-toolbox/effects-of-master-cylinder-bore-size-on-your-brake-system/30895?gad_source=1&gad_campaignid=21930090154&gbraid=0AAAAAD8-Mcn0Y-mYK82XGkayimqHUC7yP&gclid=Cj0KCQiA7rDMBhCjARIsAGDBuEAxZQoaTqsf_qrKxZ0c B80jagbtFS-ilyTdJmi2cKhChwSnT_qzrN0aAg9SEALw_wcB
- Summit had an article as well but I'm pretty sure their formula is off. The only reason their example worked is because their piston area and pedal ratio were very close, so that the results were a 1.0 in2 master cylinder.
- Sorry for the novel. I tend to write a bit too much.

I hope this helps guys. Feedback is welcome!

Rob

[Jphoenix]

I found this calculator handy: https://www.tceperformanceproducts.com/dual-bias-calc/

[StangRacer]

Evening guys,

OK, I'll admit it - I've gotten caught up in the whole Wilwood MC conversation. Call me a fan of hard anodized surfaces and steel pistons, but since I can still easily access my pedal box without being a gymnast, I'm going to use Tilton MCs for my brakes and hydraulic clutch. If nothing else I'll sleep a bit better :)

The question for me (and as posed by others) is - which sizes? For the clutch I'm just going to swap out the Wilwood unit that Mike Forte sent with the same size Tilton (still debating 75 or 76 series). But the front and rear brake MCs are a different matter. I decided to do some research and figure out the math vs. just ordering the .75" MCs from Tilton (although believe me, I was tempted to do so). Since I was going through the effort I figured I'd share and let everyone grade my math. If I'm off I'm a big boy and can make adjustments (and take criticism as well). And BTW, my setup is using the default Ford brakes, which are essentially '04 Mustang GT front calipers and '88 T-Bird rear calipers.

So, for this exercise you need a few pieces of information:
- Total piston area of one (1) front caliper
- Total piston area of one (1) rear caliper
- Pedal ratio.

Since my car is still in its early stages of construction, I still have my calipers off of the car. As it turns out both the front and rear calipers use 1.75" pistons. I verified my numbers by checking out the calipers on Rock Auto, where the technical specs for the calipers are listed. The area per piston comes to 2.41 in2. For the front caliper there are two pistons so the total area is 4.81 in2, while the rear is a single so it's 2.41 (yes, some rounding is going on). BTW, if you don't recall how to get area from diameter, it's:
((Piston Diameter/2)2)x3.14 or Pi R2

(Quick PSA - everything I read said to only use a single caliper's piston area for the calculations. I thought that strange, but the numbers do work, so at this point I'll trust that approach).

For the pedal ratio I broke out my handy ruler and measured the following on my Wilwood pedal box:
- Distance from the pedal pivot to center of the brake pedal: 10.5"
- Distance from the pedal pivot to the MC/clevis pivot: 1.75"
- Pedal ratio: 6:1

So the goal of this exercise is to find a MC with a bore size/piston area that, when divided into your caliper piston area, is close to your pedal ratio. If your number is too high then you'll have a hard time stopping - if the number is too low then you'll lose modulation.

For my front calipers the math looks like this:
- Master cylinder piston area=Caliper piston area/Pedal ratio
- Master cylinder piston area=4.81 in2/6
- Front master cylinder piston area=.80 in2

For the rear brakes the math is:
- Master cylinder piston area=2.41 in2/6
- Rear master cylinder piston area=0.40 in2

At this point you take the numbers and find a MC that's in the range of what you need. It's unlikely that you'll find a perfect match but I think close is "good enough". I borrowed this chart from Speedway Motors (they have a good article that I'll provide the link to below).

225469 (sorry, you'll have to click on it to read it...)

So, for my setup it appears that I need the following:
- Front MC: 1" bore (0.79in2 piston area)
- Rear MC: .75" bore (0.44in2 piston area)

A few other small items to share:
- I really don't want to order the entire Tilton kit - just the MC. But I do need the remote reservoir cap. It turns out that it's available separately if you want - Tilton part #74-200. Summit has them - I assume others do too.
- Speedway had a really good article that helped me a lot. Here's the link: https://www.speedwaymotors.com/the-toolbox/effects-of-master-cylinder-bore-size-on-your-brake-system/30895?gad_source=1&gad_campaignid=21930090154&gbraid=0AAAAAD8-Mcn0Y-mYK82XGkayimqHUC7yP&gclid=Cj0KCQiA7rDMBhCjARIsAGDBuEAxZQoaTqsf_qrKxZ0c B80jagbtFS-ilyTdJmi2cKhChwSnT_qzrN0aAg9SEALw_wcB
- Summit had an article as well but I'm pretty sure their formula is off. The only reason their example worked is because their piston area and pedal ratio were very close, so that the results were a 1.0 in2 master cylinder.
- Sorry for the novel. I tend to write a bit too much.

I hope this helps guys. Feedback is welcome!

Rob

Rob, I definitely appreciate the work you put into the calculations, but I took the lazy way and just filled out the Tilton master cylinder worksheet. The following day I received an email from Tilton with the appropriate master cylinder sizes.

[Stangrob]

Rob, I definitely appreciate the work you put into the calculations, but I took the lazy way and just filled out the Tilton master cylinder worksheet. The following day I received an email from Tilton with the appropriate master cylinder sizes.

You know, I was looking for that darn worksheet but I couldn’t remember which post had it. Plus I couldn’t find it on Tilton’s website. It would have been easier! If you’re running the Ford brakes did my numbers line up with what you got?

Rob

[Dgc333]

If you are using the Mustang GT front calipers/rotors a 1" master cylinder bore will require an extremely high amount of leg effort to comfortably stop the car. Even the .75" bore that FFR supplies takes more effort than I find comfortable, that is why I went with the Mustang Cobra brakes. They are 13" rotors and the Cobra calipers have a slightly larger bore. Going any smaller on the MC bore you run the risk of not having enough stroke in the pedal to move the needed amount of fluid.

[CraigS]

I can't figure the 'use only one caliper' in the calculations theory. Seems to me that, although the fluid doesn't compress, the caliper has flex, the pads need to move slightly before they contact the rotor, and those need fluid movement to overcome. Another way to look at it, say you had a valve in one front caliper line so it could be shut off at will. And say you could ignore any pulling to one side etc while braking w/ just one front caliper. And say you can apply exactly the same pedal pressure during all your testing. Isn't the car going to stop faster w/ both front calipers than with just one? So how would ignoring one of two calipers work out? IDK. OTOH, as DGC33 points out your resulting 1" MC would probably give a heavy brake pedal. I like your efforts to not just accept the standard stuff but maybe it's a good idea in this case. Also keep in mind that pads like Hawk HP+ will require a lot less pedal effort than generic pads that usually come in a loaded caliper. Still this is a fun and useful exercise and discussion.

[Stangrob]

Craig, I totally agree with your one caliper comment. My thought at this point is that the math is an approximation, and that the results get us in the range we need. That's the best I can do at this point ;)

While I'm waiting on additional feedback, I'm going to reach out to Tilton today and get their recommendation as well so I can see if I'm in the ballpark. FWIW in going through this exercise I did find it strange that FF spec'd out .75" MCs front and back, despite the front calipers having twice the piston area as compared to the rears. I'm still a newbie when it comes to brake system engineering, and I get it that you'd want to bias the greater braking force to the front wheels, but this seems a bit much.

Rob

[Mike.Bray]

Rob, I definitely appreciate the work you put into the calculations, but I took the lazy way and just filled out the Tilton master cylinder worksheet. The following day I received an email from Tilton with the appropriate master cylinder sizes.

Same for me:cool:

225488

[Mike.Bray]

I did find it strange that FF spec'd out .75" MCs front and back, despite the front calipers having twice the piston area as compared to the rears.

The 0.75" bore cylinders are as close to universal as they can get. They will "work" with most setups, just not optimum.

Glad to see you're actually doing it right.

[Stangrob]

Same for me:cool:

225488

Thanks Mike - that's the form I was looking for :D

I guess the engineer in me wants to understand the how in addition to the what. If nothing else I'm hoping the conversation gives some insights into how these components are sized.

I'll be sending in my form to Tilton shortly!

Rob

[StangRacer]

You know, I was looking for that darn worksheet but I couldn’t remember which post had it. Plus I couldn’t find it on Tilton’s website. It would have been easier! If you’re running the Ford brakes did my numbers line up with what you got?

Rob

IIRC, I ended up with two 13/16 and one 3/4. I am also using the Tilton hydraulic HRB.

[defilade]

It’s only one data point but my coupe has the same Mustang GT brakes that you have, with ‘03-‘04 Cobra brakes in the rear. When it was built (I’m the third owner) the builder started with .75 MCs front and rear but changed to .625 for the rear because he couldn’t get enough braking power on the rears either the .75.

[Stangrob]

FYI, I did send off my MC sizing form to Tilton, and they got back to me yesterday. They recommended 7/8" front and rear, so I'll be ordering those today as well as the new clutch master. I'd have liked to have talked about the process of picking these sizes, but if you've submitted the MC form to them you know that they ask for a lot of specific data, so I'm very confident that these will work fine. In hindsight, I suspect the formula that I was using originally is as I thought - an approximation to get you in the ballpark. That being said, if you can ask for a more accurate recommendation then going to your manufacturer of choice seems to be the right move ;)

Rob

[defilade]

Btw I would also recommend looking into overtravel stops for your master cylinders. The reason is that with a balance bar setup, a failure anywhere in your braking system will result in losing all your brakes. More info in this video (jump ahead to 4:10 to see hear about the overtravel stops):


https://youtu.be/L_EWceoApj0?si=XfxeyLNeR5_-_FEP

[Stangrob]

Btw I would also recommend looking into overtravel stops for your master cylinders. The reason is that with a balance bar setup, a failure anywhere in your braking system will result in losing all your brakes. More info in this video (jump ahead to 4:10 to see hear about the overtravel stops):


https://youtu.be/L_EWceoApj0?si=XfxeyLNeR5_-_FEP

Thanks for the reminder - I was following the thread but forgot to watch that video. I definitely want to apply that info to my setup.

Rob

[RobHartley]

I dug out my Wilwood pedal box and looked up the part number stamped on the unit 340-11299, Rob, not sure this is the same one you got with the 289 kit as this is with the Mk5
https://www.wilwood.com/Pedals/PedalProd?itemno=340-11299

Their web pages states the pedal ration 6.25:1
225777

[Stangrob]

Interesting - I’ll have to check. My measurements came right to a 6:1 ratio but it’s good to confirm. Thanks!

Rob

[RobHartley]

I submitted my form to Tilton, they were super responsive (I forgot the pedal ratio, I had an email back in less that 8 hours!)
Wilwood 12.88s

Front Pistons: 1.62, 1.12, 1.12
Rear Pistons: 1.12, 1.12
Pistons on both sides
Pedal Ratio: 6.25:1
Weight: 2500
Weight Dist: F:48 R:52
90" wheel base
Front tires: 255/35 ZR18 ~ 25"
Rear tires: 315:30 ZR18 ~ 25.5

With everything completed they came back with Front: 7/8 Rear 7/8. Thinking of Mike Bray's car he has front 7/8" and rear 1". Why such a change?

[RobHartley]

Interesting - I’ll have to check. My measurements came right to a 6:1 ratio but it’s good to confirm. Thanks!

Rob
Looking at the Wilwood diagram they appear to be measuring to the bottom of the pedal maybe? Even though their instruction page states differently https://shop.wilwood.com/blogs/news/pedal-ratios-and-how-to-find-them
https://cdn.shopify.com/s/files/1/0326/6353/5755/files/pedal_ratios.jpg?v=1620252232

[Mike.Bray]

With everything completed they came back with Front: 7/8 Rear 7/8. Thinking of Mike Bray's car he has front 7/8" and rear 1". Why such a change?

My rear caliper pistons are 1.38" diameter. I'm running the Wilwood 140-7146 rear brake kit.

[RobHartley]

Thanks Mike, I missed that, must read deeper into your build.

[Stangrob]

Looking at the Wilwood diagram they appear to be measuring to the bottom of the pedal maybe? Even though their instruction page states differently https://shop.wilwood.com/blogs/news/pedal-ratios-and-how-to-find-them
https://cdn.shopify.com/s/files/1/0326/6353/5755/files/pedal_ratios.jpg?v=1620252232

Possibly. What I read indicated that you want to measure from the pivot to the center of the brake pad, then pivot to the MC. I'm pretty sure my measurements were good - I was only drinking coffee that night ;)

So it looks like we ended up with the same MCs. I should have my new Tilton MCs on Friday along with a bunch of brake line and fittings so I can start plumbing next week :D

Rob