Okie doke -- Time to start on the brakes!
And, yes a very long winded post!I'm planning on going disk in front and disk in back. I've considered the classic full-size GM 4x4 disk setup. But I keep going back to the GM thing... nothing against GM, but I'd like to stay Nissan as much as possible. So, I'm thinking later Frontier trucks... just like mine! Turns out that the 4,426 lbs. curb weight of the Frontier 4x4 crew cab is just about the same as the Patrol's 4,575 lbs gross weight -- Additionally, the Patrol tops out at 70mph (with some serious white knuckles) where as the Frontier, even at is 5,816 lbs gross weight, will still probably top out over 100 mph! With that in mind, I'm pretty sure the Frontier brakes will have plenty of stopping-power and thermal capacity. My plan is to change everything... except the actual brake pedal! Even so, I'm finding two big challenges in the conversion from drum/drum to disk/disk.
Nissan Frontier front caliper and rotorDesign Challenge 1The Patrol is short and tall. This means that in a maximum effort stop, the dynamic weight transfer is significant... meaning it is really easy to lock up the rear wheels!
Design Challenge 2The Patrol's brake pedal is long and yields a 7:1-ish pedal ratio. According to the internet, vehicles with manual brakes generally use a 6:1 pedal ratio. I'm guessing since the Patrol is a truck, they went a little high on the ratio to keep the brake pressure manageable at a gross weight, overland expedition trek! Vehicles with power brakes often have a 4:1 or even a 3:1 pedal ratio. The problem with too long a ratio is that the brakes may turn out to be sensitive and touchy!
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So, to tackle the first challenge I need to calculate how much the dynamic weight transfer actually occurs at a maximum effort stop (think panic stop).
First off, I did all the calcs in metric units... Sorry! So, it turns out that at minimum weight, the transfer is 455 kg from the back axle to the front axle. Since the weight on the rear axle only started with 740 kg, there's only 285 kg remaining on those back wheels... that's not a lot left pushing down on those tires. They'll be quite easy to lock up. At gross weight, the transfer is even more! At gross, the rear axle starts at 845 kg -- during that panic stop, 604 kg moves forward leaving 626 kg pushing those rear wheels down - which is good... except that the additional 510 kg of mass that we have to slow down really wants to keep going full speed!
That same dynamic weight transfer goes the opposite direction when talking about the front axle. The min front axle weight goes from 825 kg to 1,280 kg. At gross weight the transfer is even bigger -- from 845 kg on the front axle at gross weight to 1,449 kg on that axle during a max stop!
This means that those back tires are really easy to get skidding long before the fronts are out of grip! This can often be addressed using an adjustable rear brake proportioning valve... but there just may not be enough adjustment in the valve to compensate for the large difference in the front and rear braking needs. Another way to adjust the F/R balance is to make the rear brakes smaller. By choosing a matched set of brakes from a truck, the under sized rear disks are already part of the package!
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The second challenge is is that pedal ratio -- weird, right? Using the 295.6 mm rotor diameter (just under 12"), an average tire grip factor, the rolling tire radius, and the weight on that front axle means that the brakes must generate 2,395 Nm of torque (1,766 ft.lbs) to keep those front tires stopping at their best rate. Next, I had to figure out what kind of hydraulic pressure the caliper needs to generate that kind of torque -- Turns out to be 9,778 kPa of pressure is required! (A.K.A. 1,418 psi --
again, sorry about the metric units!)
During the same stop, the rear brakes only need 6,453 kPa (936 psi) to achieve the needed 1,014 Nm of torque (748 ft.lbs.) to contribute to the emergency stop without skidding. At min weight, the braking split is 75% front, 25% rear. At gross weight, it's a 60% front, 40% rear split.
To generate this kind of force, we have to back-through the system and figure out how hard our foot needs to press. The internet suggests that number should be in the 75 pound range. Using the master cylinder diameter of 25.4 mm (1") and the two-per caliper slave cylinders on the front (123 mm x 2 per caliper) and rear slave cylinder piston diameter (118 mm x 1 per caliper), and the pedal ratio, the pressure achieved is only 4,608 kPa (668 psi) -- no where enough pressure for a max stop. To get the 1,481 psi needed, we'd need to press on the pedal with over 150 pounds - And that is way, way too much effort to get to max stopping power!
Note that the classic GM 4x4 front caliper has more piston area, so it will work at lower pressures but then it wouldn't have Nissan brakes!But, if we add a small vacuum booster to the system, we can multiply our foot pressure a whole bunch. Vac boosters range from a factor of about 2:1 to 8:1... but none publish this number! I'm assuming I'll only need a small-ish one, so I used a 3:1 factor in my calcs. With that booster, 35 kg (77 lbs) of force on the pedal will yield a little over 2,000 psi in the system! So we'd have room to spare! Enough that I may tweak the pedal enough to lower it's ratio to 6:1 or so to keep the brakes from being too sensitive. At 6:1, that same 77 lbs push still yields a respectable 1,769 psi in the system... which still has some room to spare!
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So, in a nutshell, that is my plan and these are my calcs -- Honestly, I suspect that my calcs have errors. There were a ton of assumptions so I'll likely be refining my calcs before ever needing to stop the Patrol from rolling! Here's my calculation spreadsheet if you were so inclined to see the data:
Check out this page:
https://www.pirate4x4.com/threads/the-b ... t-44988271It's not the only resource I used for my calcs... but I did lean on it for most!
And, again, sorry about switching between metric and SAE units so freely!
John
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