Recent events made me think maybe some questions folks might have could simply be answered here, and if not there are many others who are more than willing & capable to help!
This piece is to give some perspective to what is really needed to change a brake system. Especially if you have made substantial modifications to the vehicle with respect to weight, height, CG, RC or major suspension mods! Not if you are just replacing parts. 

There are lots of articles & sites that push bigger brake systems & there is nothing wrong with going bigger, to a degree, but you are going to pay for the upgrade & is it really needed? Maybe just maybe what you have is really close to what you need with just some tweaking.

 

So I will use the process of my build to show the process! After designing my suspension system I knew I needed to move to the braking system as it was just a subpar performing system & the OEMs due diligence is not the best! Sorry just a fact. Did you know the real reason anti-locks came to be? Do you realize that a properly engineered set of brakes will almost never activate that system? 

Ask yourself this. Why would a car that is of comparable weight, but has more weight in the front than the comparable one & as a result a greater transfer of weight upon braking have smaller rotors? Smaller pistons in all of the calipers? Harder pads & by comparison softer rotors? The answer! Money! What do you do to offset this? You would install a cheaper system to overcome that short fall, a antilock module that’s what! 

 

This accomplishes several things! You no longer have to do all the math for each car line, but more relevant you save money on components. The ABS modules are an easy CNC mass produced part & interchangeable from car to car, but are they better? Are they safer? No & yes respectively.

 

So needing to gut the brake system, but only after I had acquired all the data I needed to determine where the car was at with all the physics, with respect to the change in characteristics of the car, since virtually everything about it had changed. Weight, weight distribution, CG, RC, tires & rims (we’re going to change), E.I. everything! That was going to require several hours of just math once all those measurements were made! This was not the worst system I had ever seen, but per usual the component selection was poor & fluid was typically marginal. 

I now needed to find or make better rotors that were the right metallurgy that would be compatible with the right pads & both of these number would be base on their coefficients of friction, with respect to each other, as well pad material.

So first up was rim & tire selection! These needed selected first as this is one of the very first things that’s needed to be known. If the tires don’t have the tire-pavement coefficient that you’re looking for with regard to what you desire in performance everything else is wasted effort. 

 

So I wanted a wider stance as well as a larger diameter rim. Yet while still keeping the same outside diameter for the tires. So I settled on a set of American Racing two piece that were somewhat unique. These rims are built backwards, I.E. as the outer ring is flipped so that the outside edge is inside, but the center section still faces outwards. This actually gives the 5 spoke center a larger look than a standard 5 spoke rim of that size. I also went with Michelin Pilot Sport A/S 3 after quite a bit of research I found these tire had a compound makeup that gave them a .68 tire-road coefficient, and yet a good ride, plus excellent cornering characteristics, so I now had my starting point.

 

Having removed about 175 lbs from the front & where it was in relationship to CG & RC, plus adding around 235 lbs to the rear & it’s relationship to CG & RC, changed the dynamics of not just the handling, but the transfer of the weight’s dynamic energy on braking. This is important because this impacts the whole brake system & it’s mathematical calculations, I.E. rotor size, caliper piston size & location, master cylinder piston bore size & brake pedal configuration, etc…

 

Stepping through the math, to see where the car had needs that might have changed. First up was master cylinder bore/piston size! Since what was needed in wall thickness for the master cylinder was there, the master cylinder was pulled & Bored it out to 1 1/8”. This is an increase of 27% increase in volume moved. That’s a lot! So this, under the same perimeters, I.E. same pedal pressure, same stroke, same components, would increase line pressure by 27%, but would also cause a harder pedal. Now with power brakes that’s not as big of an issue as it would be with manual brakes, but still harder. So I will help correct this with a change in pedal geometry, but I actually need to change that geometry because the stock system is quite inadequate! The original pedal ratio was 5 to 1. That means 150 lbs applied to the pedal for the original master cylinder & components, generated only 750 pounds of line pressure & at 200 lbs would net 1000. That is actually on the very low end of line pressure. My change of master cylinder piston bore size changed those figures to a respectable 953 & 1270 psi respectively, but now we are going to change the pedal ratio to 6.25 to 1. So with the bore size increase & pedal ratio changed, 150 lbs applied at the pedal now yields 1190 psi line pressure & 200 lbs yields 1587 lbs. that is into the realm of racing brake pressure. And no you are not going to blowout a line at that psi even in a panic stop. Now in of itself these two changes brings the W platform into a very respectable braking car, even without changing anything else, but I’m not done yet.

 

Again, stepping through the math to make a judgment on just where I was for rotor & caliper needs was now the next order of business. It turned out that the major fault with the braking system was in three areas. Master cylinder piston size, pedal ratio & coefficient of friction matching of pads & rotor materials! Not a big surprise actually. 

 

When you calculate the needs for the front brake system it looked like this. (T=R x A x F x N x P) T is total system torque/clamping ability, R is the radius or the rotor minus the outside edge of the rotor to the caliper piston center line,(which can be more that half the piston radius), F is the coefficient of friction number of the pads, N is the number of sides, for disc it should be two, & P is line pressure. 

So for the stock system those  numbers were not good! The front applying 150 lbs at the pedal yielded  9,797 lbs & the back was 4,314. Now with the new configuration of master cylinder modification & pedal geometry change I now could get 15,676 lbs at the front & 6,902 at the back when applying 150 lbs to the pedal. You can now see the multiplying factor that physics plays. I now have by far more than a 27% increase in stopping power. Yet this is still short of what I want for stopping power, but I’m very close. So now with all these measurements made I haven’t physically done anything yet. So now I’m going to backup to my pedal ratio & make it 6.5 to 1, because I have the room on the pedal arm & it will accommodate the angle that comes with the change. Now I redo the math & I now have what I want. There is still much more to this! For instance, how much braking do I really need? This can be determined, again through math.

 

So this calculation goes like this. T=W x R x D / N. T is torque required in inch/pounds, W weight of vehicle, R is rolling radius of tires (centerline of wheel to ground), D is deceleration rate, (this can be from .2 to 1 or more, but most MFGs use 1) for safety purposes. N is number of braking wheels. So in the case of the Indian that final number was 11,375 inch/pounds! Still there was  more to get to before the end of this whole process!

 

Now I need to determine weight transfer. This is the action of stepping on the brakes & the energy of weight (inertia) transfers weight energy forward. This drastically changes the requirements for braking especially the higher the CG of the car is. As well as the weight distribution. So the calculation looks like this. Weight transfer Wf=U x W x H / L. This equation goes like this. U is coefficient of traction, W is vehicle loaded weight, H is center of gravity & L is vehicle wheelbase. Here’s how the Indian calculation went. 1 x 3500 x 19 = 66,500 / by 110.5 = 602. With the Indian having a 54/46 weight distribution, due to all the changes to the suspension, the weight transfer needed corrected like this, traction coefficient 1 x 3500 lbs x 19 CG / 110.5 = 602. Divide 3500 by 2 = 1750 + 4% (this 4% represent the 4% over the 50% margin of weight distribution) = 1820 + add 4% to 602 = 625 = 2445 lbs for the front weight transfer. The rear, divide 3500 = 1750 – 4% ( likewise subtract 4% because the back is 4% under the 50% ratio) = 1680 - 625 = 1055 rear transfer weight.

 

Almost done! Now I need to see what the brake load per wheel is. Which looks like this Tf = Wf x R x D / N, Tf is torque force, Wf is weight transfer, R is wheel radius as before, D is deceleration as before. 

So the front, 2445 x 13 x 1 divide by 2 = 15,892 & the rear 1055 x 13 x 1 divide by 2 = 6,857. 

 

So with the new master cylinder piston size & pedal ratio change I have the following output available compared to my need. With 150 pounds applied at the pedal I will create 16,172 inch/pounds of force at each front rotor, but I only need 15,892 of torque & if I apply say 170 lbs at the pedal I will generate 18,327 inch/pounds of force at each wheel! The rear brake configuration is similar, with 150 lbs applied at the pedal I will create 7,121 inch/pounds of force, but I only need 6,857 inch/pounds of torque, yet if I apply 170 lbs of force at the pedal I will generate 8,070 inch/pounds of force at each rear rotor.

 

When I finally made these modifications to the brake system, which include a specific type of cross drilled rotors & a pad material that had a .35 coefficient rating, the Indian became a completely different braking car! The very first time out, without doing anything different, the car stopped more than 75 feet short of were it had before. Than just two days later, I had a guy slam his brakes on in front of me as a rescue squad answering a call pulled out of a station in front of us. I was closer than I should have been & this was at 40 mph, I hit my brakes hard. The car just flat stopped really quick! No skid, no ABS, nothing! Just stopped. All of this accomplished by master cylinder piston size increase, pedal ratio reconfiguration, different rotor & different pads & oh yeah different fluid! A DOT 5.

So I didn’t need to get into expensive calipers & rotors, wheel clearance issues, different mounting issues, etc.. Just a pretty straightforward non complicated modification.

 

Edited July 20, 20223 yr by Last Indian

🤯 well I for one am mind blown! Seriously, I had no idea what all went into designing a break system. Which could be why my brakes feel like crap! I'm sure I'm personally going to have a few questions after I read this 6 or 7 times  but thank you for writing this Last Indian and helping us understand justa what is going on under our foot.