Here in the mountains of VA disc brakes are a nice addition to any classic cruiser. Especially if you want to be cruising at the same speed at the bottom of the mountain that you were at the top. I've been doing a lot of research about adding disc brakes to my 1941 Ford 1/2 ton pickup and I have a question I can't find the answer to. When selecting a master cylinder bore and brake caliper bore combination, why do you want the MC/BC bore size ratio to match the pedal ratio? Why not higher or lower? What would be the effects of a higher or lower MC/BC bore ratio vs. the pedal ratio? These are the questions inquiring minds want to know.
Don't over think it. What is under your 41 at the moment? ACTUAL MC bore and Wheel Cylinder bores? Current pedal ratio? Disks have been added to Early Fords for DECADES. No need to reinvent the wheel
If your pedal ratio doesn’t at least equal what you’re removing the pedal will stay as you have it but fading will be less. A higher pedal ratio lets say a 7-1 vs a 6-1or 5-1 will make for less pedal pressure and easier to drive if all your other vehicles have factory power****ist brakes. It’s all a personal choice. If my car had a pedal under the floor board I would probably have power****ist. My car style necessitates a vacuum can on the firewall with what I think looks like s**t so I worked with MC bore size and also rear wheel wheel cylinder bore size.
If you have the original 1941 pedals, I'd start with the brake pedal ratio as it is for manual brakes. https://techtalk.mpbrakes.com/how-to-series/calculating-pedal-ratio The caliper and wheel cylinder and master bores are not tied to the pedal ratio, directly.
This is the info I was looking for and this is what I suspected. However the larger MC/BC bore ratio also requires a high volume MC to handle the long pedal travel. Thanks for the replies.
You need a smaller M/C bore to get decent clamping pressure without a booster. And a 6:1 to 7:1 pedal ratio. Most boosted pedal ratios are 3.8:1 to 4.0:1 The problem with smaller M/C bores and Longer pedal ratios is excessive initial pedal travel to take up clearances. You can alleviate some of this with a quick uptake M/C [aka "stepped bore"] A quick uptake M/C was originally used for "low drag calipers" in the USA but they work just as successful on normal calipers [the other way round is a nightmare] The Germans used stepped bore M/C's a lot earlier than the USA so there is plenty to choose from. On my Lotus Cortina with manual disc brakes and hard Carbon Metallic pads I used a 3/4" stepped bore M/C from a 1978 Mercedes 250 C. It had a nice high pedal and I could do decent late braking without sh**ing myself. Prior to the smaller bore Mercedes M/C being fitted I bent the brake pedal , so I had to weld 2 pedals together for strength [below] This car had a simple non-boosted brake system that could almost stand the car on it's nose If your calipers are higher than the M/C simply mount the reservoirs remotely higher up in the engine bay to prevent gravity syphoning back.
go to speedway motors .com scroll down and click on toolbox search for mastercylinder size the article gives formula to determine correct size m/c bore and correct pedal ratio
Actually, that's where my quest started. That article is all about matching the pedal ratio to the MC/BC ratio to create a manual brake setup that doesn't*****. I've been trying to find out what happens if you use a higher (or lower) MC/BC ratio than pedal ratio and the advantages/drawbacks of each.
It seems like the go to manual (not power) disk brake dual master cylinder for years was the 69/70 or so Corvette manual brake master cylinder with the 1 inch bore. That would take residual valves (especially on the rear) . Rock Auto listings. Note that they show both power and manual.
*UPDATE* I got the bright idea to call Wilwood yesterday and ask them what they recommend in MC bore size with one of the disc brake kits I'm looking at. They asked a lot of questions, like the weight of the truck, caliber bore size, rear brake configuration, how I drive the truck, pedal ratio, etc.. They came back with a recommended MC/BC ratio of 7.1:1. They said it would be almost as good as power brakes and provide plenty of stopping power for that size of vehicle in stop and go traffic and cruising up and down the mountains. By my calculations that would give the truck a total brake leverage of 42.1:1 (pedal ratio * MC/BC ratio). The disc brakes I want to use are reproduction finned Corvette calipers from Roadster Supply: These would look great on truck which has that 1950's hotrod look to it. These calipers have a BC bore of 2.125" which means I'll need a MC with a 13/16" bore to get a 7.1:1 MC/BC ratio. I haven't been able to find a MC with a 13/16" bore that will fit the truck yet. The smallest MC bore most companies make is 7/8" and that drops the MC/BC ratio to 5.89:1. I may have to go with a disc brake kit from speedway that uses calipers from late 70's GM cars with the larger BC bore to get the ratio closer to 7.1:1.
Maybe talk to a machinist or one of the companies that sleeve masters about a custom size mod? ****uming the rubber parts are available, one could turn a piston to the correct size.
One way, you’ll have too little travel. Brakes that feel like stepping on a rock. Requires a lot of leg effort. Too far the other way, it’s like stepping on a plum. Pedal goes down, no braking happens. But the pedal is really easy to push, not much leg effort needed. You need to be somewhere in the middle, using existing parts and geometry limits the pedal travel you can actually use. Size the bore to move enough fluid to make the system work. Size the bore also to allow for good system pressure, effective braking, with reasonable effort and pedal travel. If you can, follow a known, tested recipe. Otherwise, trial and error while you work out the details on brakes can be unpleasant.
- Now, if every car and driver had the same braking requirements and pedal feel preferences, we would never need to adjust anything. But, every car and every driver are unique and adjustments will get made. That was a quote from the article. Interesting, but frankly I found it a bit dry of a read. As I get older I realize, if things are relateable they become more easily understood. Taking a shot, and with my quoting the article... You asked... When selecting a master cylinder bore and brake caliper bore combination, why do you want the MC/BC bore size ratio to match the pedal ratio? You do and you don't. Car companies have found a good balance. But it's tailored to the masses not ones man's expectations of perfect braking. But if it wasn't a braking system but a movement of fluid to raise a hoist for instance, you might see how these relationships form. If it was a jack, how much it raises with each stroke when pumped. You asked - Why not higher or lower? I'm no expert, why not. Like a welder up five down five. A painter saying, I'm going adjust the pressure. The carpenter needing a bigger hammer. Sure, some of this is like blowing a balloon up, maybe, so the components while making a difference, it still comes down to potential. One can compensate for another. Packing more in the suit case just take extra push. Or does it need a bigger suit case? You asked-What would be the effects of a higher or lower MC/BC bore ratio vs. the pedal ratio? Seems to me, like the suit case, it will reflect the feel of the braking system. Don't confuse the feel of balance with clamping force. And remember surface area of the clamping pressure being applied comes into play. I think there is a whole lot of extra thinking going on and that extra thinking cost money, which is exactly what the industry needs to churn a profit. I'm sure if you plunk in 13 disc brakes up front in a light car with rear drums brakes with a master under the floor and stock pedals you'll have some tweaking to do. Not an expert.
Don't know where Wilwood's recommended ratio number would come out; but stock '41 pedal set-up (6.4 - 1 ratio) with a 1" bore master and big GM calipers work pretty well together. 15/16" bore master give a little easier push; but you may not be able to achieve full pedal travel in "failure mode" (but whoever checks that).
Assuming MC/BC ratio means Master cylinder to Brake cylinder ratio! To get your holy grail of 7.1:1 MC/BC ratio with Corvette C4 calipers [above] requires a 1-1/8" bore MC Corvette C4 calipers have 2 x 1.5" pistons on each side [3.53" swept area each] totally 7.068" total front piston area for both sides. [1.5" x 1.5" x 0.7854 = 1.767" piston area x 4 pistons total = 7.068" sq total] Divide 7.068 by 7.1:1 pedal ratio = 0.995" sq of MC area If you reverse the math for the MC 0.995 ÷ 0.7854 = 1.267 √ 1.267 = 1.125" MC bore or 1-1/8" to get your 42.1:1 PR to BC ratio , now divide this by the MC/BC ratio 42.1 ÷ 7.1 = 5.92: pedal ratio So Corvette calipers + 1-1/8" MC and 5.92:1 pedal ratio will get you there. Me personally who despises excessive pedal travel [coming from the "chicken***** school of braking" ] I would use a 7/8" stepped bore MC from a Chevy S10 To get the same 42.1:1 Pedal to BC ratio the math would be The MC is [0.875" x 0.875" x 0.7854 = 0.601"] 7.068" ÷ 0.601" = 11.76:1 BC to MC ratio 42.1 ÷ 11.76 = 3.58:1 pedal ratio [a 4:1 would be enough] with a high pedal from the stepped bore. This ^^^^ is what we did on a manual brake conversion on an O/T race car The S10 MC does require bracket fabrication and a pushrod retaining guide
This all makes my head hurt. I honestly have done a lot of brake systems on my car builds, and builds for others, and never spent much time calculating all this stuff, and yet they stopped great. I use master cylinders designed for either drum/drum, disc/drum or disc/disc, depending on what the cars have. Usually if I'm using GM calipers up front, I use a 70's Corvette master. If it's GM calipers at all 4 corners I use later 80's Corvette masters designed for 4 wheel disc brakes. On drum setups I use GM drum/drum masters, or for Fords I use late 60's Mustang masters in drum/drum or drum/disc. As for pedal ratios, I measure up the pedal ratios, and if the donor pedal****emblies aren't long enough to give me the correct manual ratio I simply cut the pedal arm, and make weld them up longer. If they're too long for power****ist, I cut the pedal off and shorten the arm to power****ist length. Both of my hotrods are disc/drum setups, and use early 70's GM calipers up front, with Ford 8.8" drum rear axle brakes on one, and old Pontiac late 50's rear axle drums on the other. Both cars stop as good as any new car I own, so seems to be working without the math.
It seems the general feeling around here is that "you shouldn't cut and weld a brake pedal". Well I've done the same as you and have close to 80K on the pedal in the cpe. It's been cut multiple times as when I switched to 4 whl. disc I had problems. Turned out it was a faulty booster which the company wouldn't admit too. Shawn descending in the hills do you downshift? That would help a great deal also
That math is only for calculating the perimeters of what @Shawn Wildman provided [These were based on a phone call to Wilwood who gave him an educated generic answer] In reality the math is much more complicated [We start with a driver sitting in a seat with a scale under his foot] Then there is Weight, Wheelbase ,CGH calculations for weight transfer. Then Wheel Diameter to Rotor diameter for brake torque, Then the fun starts with Coefficient of Friction, Clamping pressure, Then all the above Calculations for Fluid Mechanics and Leverage. The Wilwood generic formula/equation supplied to Shawn Wildman [42.1:1 BC/Pedal ratio] never factored in the rear brakes. Willwood would recommend an adjustable proportioning valve for fine tuning. The reality is the above math would be closer to a 60:1 BC/Pedal ratio with a 70% Bias to the front brakes There is too many variables and my advice to a novice is to stay away from "Re-inventing the wheel" with brakes. I have always recommended for non academic types to find a donor vehicle of similar weight / wheelbase and cannibalize the total brake system. If somebody could get their hotrod to stop as good as a 1985 Honda Civic they would be ahead of the game. These scaremongers are probably the same people that weld pitman arms and steering arms. I've welded pedals on many occasions ,And at the worst part [between the M/C pivot and Pedal Pushrod] If a person can exert 110 lbs on the pedal [ 1/2 of performing a 220lb squat in the gym] it would only exert 660 lbs force on the pushrod and pivot [cantilever effect] I've done birdshit welds on engine lift plates that have lifted more The worst thing about a pedal is they're made from flat steel and have poor torsional strength but good beaming strength. A pedal will start twisting if it is too weak.
On my 40 pu, call it everyday cuz that’s how it rolls, an old 60’s ford 8 inch rear with drums, I think a TCI front disc kit gm rotors and small metric calipers. Stock pedals and a mid 60’s ford drum/disc non power Master. Stops great up hill or down. Master cylinder available at any real parts house.
VW did non power master cylinders in 13/16" in the 80s. Also a lot of 11/16s 1980 Westmoreland PN E150073 Carquest/Weatherly number Rockauto https://www.rockauto.com/en/parts/eis,E150073,master+cylinder,1836
The GM front calipers and a 1" bore MC from either a corvette or Mustang would work fine. Some of the GM front calipers had bore diameters as large as 2.81". With a 1" MC this setup would produce plenty of stopping power with a MC/BC ratio as high as 7.9:1. I actually found a picture of the corvette finned brakes mounted on a hot rod. Considering where the calipers are mounted and how low my truck sits in the front, there is no way anyone would ever see them, so the "cool" factor kinda goes out the window after that. So, I guess I'll probably use the GM front calipers and a Mustang disc/drum MC. I even found a Ford MC adapter kit for my truck: Thanks for all the posts. I learned a lot from the information everyone shared, which is half the fun of working on these old cars.
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