Direct 'Dear Dave' tech letters to dave@eyesoreracing.com.Coleman will share mind-numbing details, earth-shattering revelations, and technical nerdisms in this space each month.
Q Mensa ManI want to point out a common error in your analysis of the Wilwood calipers on Project Sentra SE-R. You stated that: "pushing four 34mm pistons takes about 60 percent more fluid [than the 54mm single-piston NX2000 calipers]." With sliding calipers, the piston area is accounted for twice, since it moves twice as far as in a fixed caliper. Dave must know this already, because he wrote about it correctly in Technobabble, Dec '02: "Third thing to remember: sliding calipers act like they have twice their actual piston area."
The effective piston area of the Wilwood calipers is only about 80 percent of the NX2000 caliper, 3,630 mm2 versus 4,580 mm2. Any pedal feel issues he had must have been related to air in the system. Increasing from a 7/8-inch to 15/16-inch master cylinder isn't enough of a change to account to go from bottoming out to a firm pedal.Cory LongLong Beach, WA
A I think you can get a Mensa membership just for using my own four-year-old quote to prove me wrong. Luckily for me, I wasn't quite as dumb as you think I was.
What Mr. Long is rightfully pointing out is a fundamental difference between fixed and sliding caliper designs. On a four-piston caliper, moving the pads 2mm closer together, for example, requires the inboard and outboard pistons to each move 1mm. With each 34mm piston having an area of 908 mm2, 1mm of movement takes 908 mm3 of fluid. Four pistons means 3,632 mm3 of fluid in that caliper.
On a single-piston sliding caliper, moving the inboard pad also means moving the piston 1mm. The outboard pad, though, is moved by the big claw that reaches around the rotor. That claw is moved when the cylinder moves backward, relative to the piston. Moving the 54mm piston 2mm takes 2,290 mm3 of fluid from piston movement, and another 2,290 mm3 from the cylinder pulling on the claw, for a total of 4,580 mm3.
The math club guys already know 3,632 is not 60 percent more than 4,580. So what was I smoking? A little too much brake geekery, that's what.
Turns out that although it takes twice as much fluid to move a sliding caliper a given distance, the distance it has to move in the first place is only half as far. In a perfect world, with dead straight rotors, and completely rigid hubs and wheel bearings, the distance a brake pad has to travel from its rest position to the rotor surface is determined by the piston seals. When the piston slides toward the rotor, the seal first sticks to the piston and just stretches out about 0.1mm, then the piston starts sliding through the seal. When the pressure behind the piston is released, the seal pulls back by that same 0.1mm. Assuming there's nothing else pushing the piston back, that 0.1mm is how far the piston has to travel next time before the braking starts.
On a four-piston caliper, the pistons have to squeeze 0.2mm, since there are seals pulling back from both sides of the rotor. On a sliding caliper, there is only one seal pulling the piston back 0.1mm, so the free travel of the caliper is shorter.
So, the free play on the 54mm Nissan caliper came from moving the piston and cylinder 0.1mm apart, taking 229 mm3 of fluid. On the 34mm Wilwood, it takes 363 mm3 of fluid to move all four pistons 0.1mm. The grand total, 62 percent more fluid, just like I said.
Only problem is, when I initially made my bold, 60 percent proclamation, I didn't actually know how far the piston seals pulled back. I had assumed it was far more than 0.1mm, and that it was the main contributor to brake free play. Turns out that wavy rotors and flexy mounting will cause most of the piston knockback that occurs between brake applications, so my numbers, in the real world, were complete rubbish.