MC bore, MC stroke, pedal ratio

[ksagis]

I’ve been reading a lot of the great posts on the forum about disk brake upgrades that have big caliper pistons and folks ending up going to bigger MC bore to get back to decent pedal travels. I’m not super happy with my pedal travel and based on all my reading, I was thinking of going to a new MC with 1.25” bore. But was also looking at pedal ratio (travel) as a possible improvement.

My general thinking is that upgrading to large diameter pistons in disk calipers versus drum brake wheel cylinders obviously requires more brake fluid to flow which can be solved by bigger bore MC as many have done. But MC stoke seems like it plays a pretty big factor so I’ve been trying to determine what pedal ratio I want if I modify my pedal.

I picked up a pedal that I could measure fairly closely ( thanks @904Bronco ) and the pedal ratio is about 5.8. Given a roughly reasonable pedal height of 5 inches and maybe 4 inches of travel before bottoming on floorboard says I’ll get about 0.68” of MC stroke at full travel. That seems way too low amount of MC stroke to me.

Anybody have any info on typical max stroke for MCs? I know most say to not stoke more than 1” when bench bleeding (by the way, anybody know where that comes from)? (best I can figure out is most domestic MCs have a full stroke of 1.5 to 1.75”)

Clearly there’s a lot to sort out like maintaining good pushrod alignment with moving the attach point on pedal, but wanted to see if I was barking up the wrong tree on getting more MC stroke as a possible contributor to getting better pedal versus only MC bore.

 

[DirtDonk]

Never got a chance to read this in detail. In the meantime, back to the top.
Hopefully you get your answers.

 

[ksagis]

I more closely measured the candidate pedal, looks like the factory pedal ratio is closer to 6:1 or maybe 6.1:1 (versus the 5.8:1 I mentioned above).

-Brake pushrod to pivot: 2.197”
-Middle of push pad to pivot: 13.375”
-Pedal ratio: 6.08”

To get to 5:1 pedal, I’d need to move the attach pin down another 0.478” which would give me a total stroke of 0.8” or 18 percent more fluid volume (roughly same as going to 1.25” bore MC which is 23 percent increase in flow)

Mainly spitballing and looking for constructive feedback of the idea. The only negative I see so far is possible pushrod binding, but the angle is only changing about 3 degrees which seems pretty darn small.

 

[DirtDonk]

Right. You do have to be careful about rod angle going through the firewall and into the master.

Are we talking about using a booster of any kind, or just directly to a master cylinder?
Each of the things you’re talking about, changing the pedal ratio, or increasing the size of the master, will increase your pedal effort by quite a bit.
I’m sure there is a chart for that too, but perhaps it’s a simple as a percentage increase?
I do know that even going to a 1.125 inch bore master cylinder makes it prohibitively hard to push the pedal. Not impossible, just not feasible for day-to-day driving.

So in either of your scenarios, adding a booster, sounds like a must do.
If there’s not already one present.

Your measurements seem to prove out what has been discussed here over the years. That a general pedal ratio is 6 to 1.
Not just for Broncos but for many vehicles. Seems like a generic ratio for the masses.

All this head scratching might be why, when it came to the Bronco, Ford decided use the smaller piston calipers, and to keep the same master size between drum brake, disc brake, manual brake, and power brake setups.

 

[ntsqd]

For non boosted brakes you will want the hydraulic ratio times the pedal ratio to be about 95:1 If you like a pedal that is very firm you may be able to get down to 93:1, but I doubt many will like that.
In the other direction you can go out to about 100:1 before the brakes will feel "mushy" to just about everyone.

Notes on calculating the hydraulic ratio:
With floating calipers you're sort of in luck because there is only one side that the piston(s) are on. With fixed calipers that have pistons on both sides of the rotor you only use the piston areas from one side of the caliper.
Total caliper piston area is two times the piston area of one side of the calipers unless you only have one total front caliper. And I don't know wtf to do.....


Add in a booster and none of this applies.

 

[ksagis]

I have hydroboost so not too worried about increase in leg pressure by going to a decrease in pedal ratio or a bigger bore MC

@DirtDonk, the angle I was referring to is the pedal to hydro input since moving the pedal attach point down 0.478” result in about a 4 degree off axis push .

@ntsqd, I’m not sure what you mean by hydraulic ratio, can you elaborate? (the ratio of my total piston area to MC area is around 16 so you must mean something else)

 

[ntsqd]

Hydraulic ratio is caliper piston area (X2) divided by M/C area. Example: GM D52 calipers are Ø2-15/16" or 6.78in^2 per caliper; so 13.55 in^2 for both calipers. A Ø1-1/16" M/C has a piston area of 0.89 in^2, resulting in a hydraulic ratio of 15.4:1 Multiply that by the 6:1 nominal pedal ratio and I get 92.3:1 That's going to be a pretty stiff pedal w/o a booster.

With H-B those ratios that I posted above are useless. For observation H-B's can usually run at least a Ø1.25" M/C with those calipers.

 

[Apogee]

Hydraulic ratio is caliper piston area (X2) divided by M/C area. Example: GM D52 calipers are Ø2-15/16" or 6.78in^2 per caliper; so 13.55 in^2 for both calipers. A Ø1-1/16" M/C has a piston area of 0.89 in^2, resulting in a hydraulic ratio of 15.4:1 Multiply that by the 6:1 nominal pedal ratio and I get 92.3:1 That's going to be a pretty stiff pedal w/o a booster.

With H-B those ratios that I posted above are useless. For observation H-B's can usually run at least a Ø1.25" M/C with those calipers.

FWIW, this example would have come from the factory (lots of manual disc brake cars and trucks in the 60's and 70's from GM for comparison) with a Ø1" bore master cylinder and ~6:1 pedal ratio. Some of my customer prefer running a Ø15/16" bore master cylinder for the 11% lower pedal effort, especially if they're not running a particularly aggressive brake pad. While I agree with ntsqd for the most part, GVW comes into play, as what "works" on a 3400# roadster may not inspire confidence on a 6000# rig.

As for increasing caliper piston area in order to increase brake torque, that only works if you stick with a smaller bore master cylinder to apply more pressure than you would otherwise with a larger bore unit. If I increase my caliper piston area by 25% and then increase my master cylinder area by 25%, I haven't changed my overall brake performance in any way, as they cancel each other out. If you want more brake torque, the single most effective method to get there would to first add in a brake booster, since that applies a gain/boost factor between you and the master cylinder. Of course, some of that is cancelled out by a numerically lower pedal ratio, but the general statement is still sound. Hydroboost typically generates the most gain of any boost option, though units do vary and not all units are equivalent. The Bosch units have three common power piston diameters for various truck and other high GVW applications, but units like those from the 2000-era Mustang GT have much smaller power pistons for obvious reasons. The gain factor provided by vacuum assist brake boosters will depend on the booster configuration (diaphragm area) as well as the vacuum levels used to power it. A dual-Ø7" unit is not going to offer the same level of performance as a dual-Ø8" or dual-Ø9" unit, where bigger is generally better. We don't tend to have many issues with 15 inHg+ vacuum levels at warm idle, but that doesn't mean you can't work with less, especially if you can tolerate a little pedal inconsistency, particularly during low-speed manuevers.

The next easiest way to achieve more brake torque for any given pedal effort would be to go to a higher CoF brake pad. This a pretty easy to do if you're running the GM brakes with the D52 spec pads, as you should have most of the options under the sun available to you...not so much if you're running Ford spec brakes, as they're not as well supported.

After that, reducing your master cylinder bore size would be the next best opiton IMO.

The last thing I would mess with is my pedal ratio, as that tends to be the most difficult thing to change in the system, unless you're fabricating a new pedal box from scratch.

HTH, Tobin

 

[ntsqd]

snipped........
As for increasing caliper piston area in order to increase brake torque, that only works if you stick with a smaller bore master cylinder to apply more pressure than you would otherwise with a larger bore unit. If I increase my caliper piston area by 25% and then increase my master cylinder area by 25%, I haven't changed my overall brake performance in any way, as they cancel each other out. .....

HTH, Tobin

Well said. This is the part that most people either don't get or don't understand, and I see it done a lot on all of the vehicle forums that I frequent.

 

[ksagis]

Thanks for all of the useful info, much appreciated.

Gonna start with swapping to a 1.25” bore MC and see how I like things.

 

[Madgyver]

when I built these brake set up, it was all by what works. i didn't factor in ratios and all those numbers. I just went it. All I remember was a 1.25" throw on the astroboost unit with corvette MC 1-1/8" bore. I did have some trial and error involved and you will encounter those challenges. It make you build better.