Musings on front brake upgrades (WARNING: actual math)

[Littlebeefy]

Hi guys (and gals),

I have been "imagineering" how I am going to improve my brakes for what seems like 3 years now and have really gotten into the weeds. Obviously there are a lot of "opinions" and "anecdotes" online about it, but I got sick of the "he said, she said" of it and sat down to do some real math. The results were both shocking and extremely satisfying. I'd be interested to hear what others have to say about this because I had to make some simplifying assumptions. Ultimately I compared the stock D44 disc brakes to the Tbird calipers, the Wilwood 6 piston Big brake kit (with both 12.88 and 14" rotors) and the Wilwood D8-4 Truck setup. The Tbird calipers will fit the TTB D44 but the knuckles require some clearancing to fit the banjo bolts. Wilwood does not sell kits that fit the TTB or HP D44, but there is a company called Little Shop MFG (littleshopmfg.com) that manufactures and sells custom mounts and hats with the Wilwood kits to make them fit; I have not used them but they are broadly recommended and I have yet to find anyone online who said they couldn't get it installed and working. My point is, all of these are potential upgrades for the D44 so I compared them.

The biggest assumption that I had to make in order to simplify the math was where the center of force exerted by the piston on the rotor is. I don't have the exact measurements for the distance of the piston from the edge of the rotor. I also didn't do the trigonometry to figure the exact center of clamping force relative to the center of the piston (i.e. I ignored the fact that a point on the rotor that passes across the exact center of the piston will not actually bisect the piston). Basically, I just assumed that the force is centered at the midpoint of the piston and the edge of the piston is somewhere between the edge of the rotor and .5" from the edge of the rotor. So my math is not absolutely correct but I provide a range in my force estimates to account for this fudge factor. (more on mathematically centering the piston-disc force can be found here for those of you with nothing better to do: https://www.sensorprod.com/news/white-papers/2006-01_mdc/wp_mdc-2006-01.pdf)

Second, I made the assumption that it doesn't matter what is moving the piston. The master cylinder will ultimately move the pistons if bore size and stroke capability are selected appropriately.

Third, I ignored the Coefficient of Dynamic Friction completely. I know this is a big one to leave out, but in my opinion it is only making a difference "on the margin". Pad surface area is different, but so is the piston surface area making contact with the back of the pad. Also, most pistons are concave so only the rim of the piston is making contact with the back of the pad, so circumference of the piston is more crucial in that regard than the "surface area" of the piston. My point is, I am really looking only at two factors when comparing the calipers: clamping force applied by the piston and distance of that force from the center of rotation. By multiplying those two I get a Moment that I can use to compare the relative stopping force of the caliper and rotor combo.

Lastly, I treat 2 opposing pistons as equivalent to a single floating piston. This is a fact, not an assumption. I am staying out of boring people with how hydraulics work so you will just have to trust me on that.

Now that I have absolved myself of any responsibility for accuracy, here is what I cam up with (note that the measure of net stopping force (moment) is unitless but linear, meaning that a moment of 30 has 25% more "stopping power" than a moment of 24):

Stock Caliper: 2.87" bore (6.48"sq), 11.72" rotor, 3.805" to 4.305" from axle centerline = 24.656 - 27.896 moment.

Tbird caliper: 3.11" bore (7.60"sq), 11.72" rotor, 3.805" to 4.305" from axle centerline = 28.918 - 32.718 moment.

Wilwood Forged Narrow Superlite 6R Big Brake Front Brake Kit: 4.86"sq (provided by Wiwood), 12.88" rotor, 5.065" to 5.565" from axle centerline = 24.616 - 27.046 moment.

Wilwood Forged Narrow Superlite 6R Big Brake Front Brake Kit: 4.86"sq (provided by Wiwood), 14" rotor, 5.625" to 6.125" from axle centerline = 27.337 - 29.767 moment.

Wilwood D8-4 Truck Front Brake Kit: 5.56"sq (provided by Wiwood), 12.19" rotor, 4.655" to 5.155" from axle centerline = 25.88 - 28.66 moment.

Just for shits and grins, and to sanity check my above results, I calculated a D60 front axle with the twin piston calipers: 2x2" bore (6.48"sq), 12.56" rotor, 4.78" to 5.28" from axle centerline = 30.02 - 33.16 moment.

What this says to me is that the Wilwood upgrades simply are not worth the money. A stock D44 caliper/rotor is roughly equivalent to a Wilwood 6R on 12.88" rotors and the D8-4 is just barely better. To upgrade the front's with Wilwood you have to go to 14" rotor and 18" wheel, at the minimum. Alternatively, if you upgrade to the Tbird calipers, you get almost as much stopping force as the D60 twin piston configuration and it is as or more powerful than anything Wilwood can fit under a 18" rim. Still, if I had to put a Wilwood kit on the front, it would definitely be the D8-4 given the price difference.

What about rear brakes, you say? Rear brakes, even drum brakes, are rarely the problem when it comes to actual stopping power, but I like the disc upgrade so I looked into it. I know a lot of people like to use the Explorer brakes because of the nifty drum parking brake so I compared those to the Wilwood package that Moser sells with a similar style of drum parking brake. Due to the fact that it has both a larger piston surface area and a larger rotor, the Wilwood is clearly more powerful. I estimate the Wilwood rears to be about 20-25% more powerful than the Explorer setup. You can get even more stopping power out of the more expensive Wilwood Big Brake kits, but that juice is not worth the squeeze given that you can only do so much at the front.

Musings over... for now.

Cheers from Atlanta -Chad

 

[Gary Lewis]

Cool! I like the analysis.

And the results are interesting. Looks like the Tbird calipers would be the way to go. And I can tell you that a D60 dual-piston combo will stop a truck quickly, so if they are roughly equivalent to that then they should be good.

By the way, you do know you can post spreadsheets on here, right? Two way: as a picture thereof, or embedded.

[Rembrant]

What about rear brakes, you say? Rear brakes, even drum brakes, are rarely the problem when it comes to actual stopping power, but I like the disc upgrade so I looked into it. I know a lot of people like to use the Explorer brakes because of the nifty drum parking brake so I compared those to the Wilwood package that Moser sells with a similar style of drum parking brake. Due to the fact that it has both a larger piston surface area and a larger rotor, the Wilwood is clearly more powerful. I estimate the Wilwood rears to be about 20-25% more powerful than the Explorer setup. You can get even more stopping power out of the more expensive Wilwood Big Brake kits, but that juice is not worth the squeeze given that you can only do so much at the front.

Great read Chad. When you asked about the 9" rear end offsets, I figured you were looking into rear disk brake swaps. I like seeing the math of "upgrades". Most of the time people neglect to say (or admit) what percentage a specific upgrade is. I get it that something can be superior...but how superior is it? A modification or an upgrade can be 5% better...but like you say, is the juice worth the squeeze?

I'm about to try something radical with regards to braking in a few months with my 1952 project truck: Front drum brakes and rear disk brakes. Now, before anybody tells me that I'm going to kill myself and a thousand other motorists my first time out, I know and understand all the risks. I installed a firewall mounted power master cylinder, and a 2001 Explorer disk brake rearend. I don't want to get too deep into the weeds here, but this was never really my original intention...it just ended up this way for a couple reasons. I did come across a couple others that ran this exact setup for many years without issue.

If it doesn't work, I'll do one of two things...install a disk brake kit on the front, or swap the rear disks for drums.

I was going to do all the math on it like you did, but like I said I have some reasons for trying this. I'll be doing plenty of road tests on the 1.2 mile road I live on before venturing out into the real world;).

Stand back non believers!

 

[ArdWrknTrk]

I'd have to say that master/slave relationship has a LOT to do with how the brakes are going to stop.

Yes, as long as the master can move the pistons (and especially if you have rear drums that are out of adjustment) but this says nothing about pedal effort and sensitivity.

I can't think of a situation where the brakes in one of these trucks is going to be the limiting factor, locked steering wheels pushing or severe body pitch causing a complete loss of tracking when the rear comes off the road.

Maybe the big dual piston calipers and the amount of weight I carry have me jaded?

I can not speak directly to whatever calipers fit a high pinion D44.

Gary, I think you need to compare apples to apples, and with hydroboost your truck has a distinct advantage over those of us with a spongy vacuum booster.

[Gary Lewis]

I'd have to say that master/slave relationship has a LOT to do with how the brakes are going to stop.

Yes, as long as the master can move the pistons (and especially if you have rear drums that are out of adjustment) but this says nothing about pedal effort and sensitivity.

I can't think of a situation where the brakes in one of these trucks is going to be the limiting factor, locked steering wheels pushing or severe body pitch causing a complete loss of tracking when the rear comes off the road.

Maybe the big dual piston calipers and the amount of weight I carry have me jaded?

I can not speak directly to whatever calipers fit a high pinion D44.

Gary, I think you need to compare apples to apples, and with hydroboost your truck has a distinct advantage over those of us with a spongy vacuum booster.

Jim - You are right, I have hydroboost. But that doesn't change the braking capability does it? It does change the time to engage as they come on very quickly, and it certainly changes the feel of the brakes as it takes little pressure to make the truck stop.

[Littlebeefy]

I'd have to say that master/slave relationship has a LOT to do with how the brakes are going to stop.

Yes, as long as the master can move the pistons (and especially if you have rear drums that are out of adjustment) but this says nothing about pedal effort and sensitivity.

I can't think of a situation where the brakes in one of these trucks is going to be the limiting factor, locked steering wheels pushing or severe body pitch causing a complete loss of tracking when the rear comes off the road.

Maybe the big dual piston calipers and the amount of weight I carry have me jaded?

I can not speak directly to whatever calipers fit a high pinion D44.

Gary, I think you need to compare apples to apples, and with hydroboost your truck has a distinct advantage over those of us with a spongy vacuum booster.

Of course you are right, Jim. I left the MC out as an assumption because it is independent in installation, but it is fundamental. But I also have hydroboost, but all that is really is "pedal" boost. It doesn't make the calipers work better, it just assists in pressing in the MC. Why does my master cylinder suck? It’s fancy and it has a 1.125 bore AND I’ve got a hydroboost. So why does my pedal go all the way to the floor without my brakes being able to actually lock up my tires? It’s because I’m using an oversized single piston caliper from the Tbird. I’m finally starting to understand why calipers have changed so much over the years. There is a massive difference between 1 piston and multiple pistons.

A single cylinder floating caliper with a 2” bore has the same piston surface area as a dual piston floating caliper with 1.414” bore each. The surface area in both cases is equal to Pi square inches. In both cases the clamping “pressure” is the same. The benefit to going with the twin piston design is that it moves the center of the clamping force further away from the axis of rotation on the same net size disc, thus increasing the “stopping” power out of the same amount of “clamping” force. Just add a few more moving parts and Bob’s your Uncle! You can do even better if you make it 3 pistons at 1.149” which still gives you the same clamping force but even further out from center. As you try to game the system, though, you run into diminishing returns from this tactic because the ratio of the area of the piston to the diameter is exponential. That’s why you don’t really see more than 3 rows of pistons on calipers.

The total surface area of the Tbird calipers is 7.6”sq. Compare that to the twin cylinder D60 that has a total surface area of 6.28”sq or the stock Bronco calipers with 6.48” sq SA. ow math:

Volume of a cylinder = length x Pi x Radius squared

So regardless of the number of cylinders, the total surface area of the cylinders times the length (assuming they are all the same length) will give you the volume. Since all of the pistons have to go roughly the same distance to reach the disc (both within and between caliper models), then you can circumvent all of the math and simply say that the volume of fluid required to displace the pistons and engage the rotors is directly and linearly related to the total surface area of the pistons. So going back to my example, the single piston Tbird caliper require 15% more fluid to travel the same distance. Where is that fluid coming from? Clearly it has to come out of the master cylinder, right?

This is where I get really annoyed because variables build up and factors compound. I can get more fluid out by increasing the bore size on the MC. The original MC on the Tbird had a bigger bore than the TTB calipers, so I know I have to go up in size, but who knows how the brake rod was situated relative to pedal hinge and drivers foot in the Tbird? Now I’ve got to figure out the stroke in both applications? Aluminum universal MCs pretty much stop at 1.125” bore. This is why my MC sucks. It’s not my caliper’s fault that they are thirsty. They work twice as hard and they do a good job and they deserve the brake fluid. But my MC is a Millennial. It is lazy and entitled and puts out only enough to scrape by. And the sneaky bastard let’s me go on thinking that it is my calipers and rotors faults. SHAME!

 

[ArdWrknTrk]

Jim - You are right, I have hydroboost. But that doesn't change the braking capability does it? It does change the time to engage as they come on very quickly, and it certainly changes the feel of the brakes as it takes little pressure to make the truck stop.

Edit: you answer this question while I think about this post.

Don't forget that the much longer pad of the twin piston caliper has more area. (Even if the track was the same)

Then it comes down to mu, and how the material responds to heat. (Friction vs adhesion)

Gary, And as I noted we both have dual piston calipers compared to what is going to be found on a Bronco.

Your hydro boost came from a truck with discs all around. 💡

What do you get if you put the larger bore 250/350 master in a truck with single piston brakes?

When the ratio changes you have more movement but less force.

Though I think that the booster has to swap as well, because the stud centers differ.

This thread is about maths and not 'so and so said' but as long as you can lock the brakes it's going to veer from that objective fact and go to effort and the very subjective experience of how well you can modulate them before breaking free

Would the *T-bird brakes offer objectively better stopping power? Or would they just be awkward and mismatched?

I get the fact that we live in a world 40 years later where you can flip a switch and completely change the driving dynamics of a vehicle.(brakes, steering, ride, engine response and shifting)

[Littlebeefy]

Edit: you answer this question while I think about this post.

Don't forget that the much longer pad of the twin piston caliper has more area. (Even if the track was the same)

Then it comes down to mu, and how the material responds to heat. (Friction vs adhesion)

Gary, And as I noted we both have dual piston calipers compared to what is going to be found on a Bronco.

Your hydro boost came from a truck with discs all around. 💡

What do you get if you put the larger bore 250/350 master in a truck with single piston brakes?

When the ratio changes you have more movement but less force.

Though I think that the booster has to swap as well, because the stud centers differ.

This thread is about maths and not 'so and so said' but as long as you can lock the brakes it's going to veer from that objective fact and go to effort and the very subjective experience of how well you can modulate them before breaking free

Would the *T-bird brakes offer objectively better stopping power? Or would they just be awkward and mismatched?

I get the fact that we live in a world 40 years later where you can flip a switch and completely change the driving dynamics of a vehicle.(brakes, steering, ride, engine response and shifting)

I'll tell you what "they" say, but I can't vouch for it myself. "They" say that to get the performance out of the Tbird calipers you need to switch to the F350 MC and rear wheel cylinders. "They" aren't very clear on what year F350 or other specifics so that could be a 1.0625, 1.125, 1.25, or 1.3125 MC and as big as a 1.125 wheel cylinder. I don't know if "they" have tried it themselves.

But, let's look at that with maths. The 97 F350 has a dual front caliper that's 56mm so that nets out to 7.64"sq of piston surface area. Almost identical to what the Tbird comes out to. I'm not sure on the advice around the rear drums (assume you can proportion it correctly), but in theory what works on the F350 front brakes should perform similarly on the Tbird calipers. So what is the right size MC for Tbird calipers given that the F350 used anywhere from 1.0625 to 1.3125 MC? In theory, there is no difference between theory and practice, but in practice there is!

I'm going to swap to discs in the rear anyway and then I am going to reassess. In reality I'm fairly certain that I will need a 1.25 or 1.3125 MC.

[ArdWrknTrk]

I'll tell you what "they" say, but I can't vouch for it myself. "They" say that to get the performance out of the Tbird calipers you need to switch to the F350 MC and rear wheel cylinders. "They" aren't very clear on what year F350 or other specifics so that could be a 1.0625, 1.125, 1.25, or 1.3125 MC and as big as a 1.125 wheel cylinder. I don't know if "they" have tried it themselves.

But, let's look at that with maths. The 97 F350 has a dual front caliper that's 56mm so that nets out to 7.64"sq of piston surface area. Almost identical to what the Tbird comes out to. I'm not sure on the advice around the rear drums (assume you can proportion it correctly), but in theory what works on the F350 front brakes should perform similarly on the Tbird calipers. So what is the right size MC for Tbird calipers given that the F350 used anywhere from 1.0625 to 1.3125 MC? In theory, there is no difference between theory and practice, but in practice there is!

I'm going to swap to discs in the rear anyway and then I am going to reassess. In reality I'm fairly certain that I will need a 1.25 or 1.3125 MC.

Are you doing a disc conversion or swapping out the rear axle?

I have no idea who "they" are...

I don't hang out on any of the 4x4 boards.

You're absolutely right, without saying "you need a master from an '05 F-350 part# xyz123" it's pretty meaningless.

I went round and round like that when I did my Saginaw swap.

In the end I bought a hose for the '86 E350 Club Wagon I got the pump and brackets from and just bent it around to fit.

[Gary Lewis]

I'll tell you what "they" say, but I can't vouch for it myself. "They" say that to get the performance out of the Tbird calipers you need to switch to the F350 MC and rear wheel cylinders. "They" aren't very clear on what year F350 or other specifics so that could be a 1.0625, 1.125, 1.25, or 1.3125 MC and as big as a 1.125 wheel cylinder. I don't know if "they" have tried it themselves.

But, let's look at that with maths. The 97 F350 has a dual front caliper that's 56mm so that nets out to 7.64"sq of piston surface area. Almost identical to what the Tbird comes out to. I'm not sure on the advice around the rear drums (assume you can proportion it correctly), but in theory what works on the F350 front brakes should perform similarly on the Tbird calipers. So what is the right size MC for Tbird calipers given that the F350 used anywhere from 1.0625 to 1.3125 MC? In theory, there is no difference between theory and practice, but in practice there is!

I'm going to swap to discs in the rear anyway and then I am going to reassess. In reality I'm fairly certain that I will need a 1.25 or 1.3125 MC.

Yikes! I got a bit lost as I'm not sure exactly what I have. I know I have '95 F350 front calipers, but I'm not sure what the master is nor the size thereof. Jim - What is it off of? Do you remember the bore size?

But I have the '85 F250 rear drums, and the MPC shows them to have a 1" bore.

So I have a hodge podge system, but it works. And it works very well.