![](3D"http://www.vdubengineering.com/vdub-decal.jpg")
=20
=20
| ||
|
Custom Brakes - Guidelines MARCH/05
With custom brake kits in Type 1's (T1), it is = important=20 to take into consideration and understand all the factors. Why brake upgrades? In my opinion, there are three reasons; brake = fade,=20 changing wheel bolt pattern and bling factor. Brake fade is when your brakes are getting too = hot and=20 the brake compound in the pads are gassing out and no longer = effective.=20 The temperature of the brakes is exceeding the thermal properties = of the=20 friction pad. Your rotors are not cooling because they don't get a = chance=20 to dissipate the heat between brake applications. If your brakes = fade,=20 then you need more thermal mass via larger and thicker rotors. For = performance applications, vented rotors are a must. Solid rotors = are for=20 motorbikes and go-carts in my opinion. If you currently have solid = rotors=20 and are experiencing brake fade, switch to a vented rotor. If your = vented=20 rotor fades, then you need to improve the cooling or increase the=20 thickness of your disk. Brake fade can also occur from boiling the = brake=20 fluid. This condition is not typical of street vehicles. The = caliper=20 pistons need to be insulated to prevent this, via titanium = pistions or=20 'pucks' inside the existing pistons. With brakes from other vehicles, your choice = of wheels=20 increases dramatically. The 4-130mm bolt pattern isn't very = popular with=20 aftermarket wheel manufacturers. And let us all agree, that the = wheel=20 offerings available for the 4-130mm wheel pattern are = outdated. Got 17's on your car? Got stock drums? = Hmm...not very=20 sexy. Big brakes behind big wheels. Brake Hydraulics Basic hydraulic theory states that 1) liquids = are=20 incompressible and 2) the pressure of a given closed system is the = same=20 anywhere within the system. Pressure[P] (psi) is force [F] (lbs) over = area[A]=20 (square inches). If a 1 inch diameter piston is depressed with a = force of=20 150lbs, this will generate a pressure of (150/3.14*.5*.5) =3D = 190.98 psi. If=20 a hose is connected to another piston that has a 0.5" diameter = piston we=20 will see some force multiplication. The force would be the = pressure=20 multiplied by the piston area. (190.98psi*.25*.25*3.14)=3D37.50 = lbs. So we=20 see that a smaller piston will generate a smaller force given the = same=20 pressure. To summarize for a closed system: P1=3D P2 (pressure 1=3Dpressure 2) F1/A1=3DF2/A2 (same as above since = P=3DF/A) So far so good. According to the above, just = increase=20 the piston area and you can increase the piston force for the same = pressure. If only life were so simple. Since hydraulic theory states that liquids are = incompressible, the fluid must be displaced. A displaced volume = [D] is=20 equal to a movement [M] times ares [A]. So if the same 1 inch = diameter=20 piston is depressed by 1 inch, we will displace .785 inches cube = of fluid=20 (.5*.5*3.14*1). If a second piston of .5" diameter was connected, = we would=20 see the second piston move by 3.998 inches (.785/(.25*.25*3.14)). = So,=20 bigger pistons require more volume. To summarize for a closed = system: D1=3DD2 (displaced liquid 1=3D displaced = liquid 2) M1A1=3DM2A2 = (movement1*area1=3Dmovement2*area2). Understanding brake = torque Brake torque is what stops the wheels. The = brake torque=20 (ft-lbs) of a wheel is the clamping force (lbs) of the caliper = times the=20 disk radius (ft), times the coefficient of friction for the brake = pad. The=20 disk radius is measured to the centre of the brake pad. Clamping force (lbs) is the brake line = pressure (psi)=20 times the total piston area of the caliper (square inches). The = total=20 piston area of a fixed caliper is the total of all the pistons = areas. In a=20 sliding caliper, the total piston area is the total of all the = pistons=20 times 2. The brake line pressure (psi) is calculated = from the=20 force applied at the pedal. The pedal force (lbs) is multiplied by = the=20 pedal ratio, and divided by the master cylinder piston area = (square=20 inches). If you're following the story so far you will = notice=20 that rotor thickness has nothing to do with brake torque. The = rotor=20 thickness is related to the thermal capacity. Also, note that the = pad area=20 does not play a part either. Increasing the line pressure is = possible by=20 either stepping on the brake pedal harder, or decreasing the = master=20 cylinder size. The system in action A master cylinder is depressed which = pressurizes a=20 fluid. This pressure is transferred to the caliper pistons which = in turn=20 push the brake pad onto the disk. Adequate volume is required to = insure=20 that the master cylinder does not bottom out before the caliper = can=20 generate sufficient brake torque to stop the vehicle. The actual = movement=20 of the piston in a caliper is about 0.010"-0.015". When the force = is=20 removed, the square seal in the caliper retracts the piston to = remove the=20 clamping force from the rotor. The above two hydraulic laws are = ideal. In=20 practice other factors must be considered; swelling of brake = hoses,=20 caliper deflections, caliper mounting deflections, compression of = fluid,=20 air inclusions in fluid etc.
Brake Bias Also, some consideration must be paid to brake = bias,=20 that is the front to rear braking distribution. Depending on the = size of=20 your front and rear caliper pistons, you may alter the existing = brake bias=20 of your master cylinder (MC). If you understand brake torque, you = will=20 notice that rotor diameter, friction material, caliper piston size = all=20 factor into the brake bias equation. Other factors include weight=20 distrubution of the vehicle, center of gravity of the vehicle and = tire=20 size. Weight distrubution and center of gravity factor in the = dynamic=20 weight transfer during braking. Typical rear static brake bias is = only=20 about 20-35%. Remember regardless of all the brake factors, the = final word=20 is between the tires and the pavement. With that said, you can use = these=20 factors to fine tune your vehicle's braking characteristics. If needed, use a proportioning valve to tame =
your beast.=20
A prop valve is basically just a hydraulic pressure regulator, =
except that=20
it only kicks in at a certain pressure (knee point). Up to that =
point, the=20
proportioning valve does not exist (hydraulically speaking). Prop =
valves=20
only reduce existing pressure. You want to set up the brake system =
so that=20
under panic braking all wheels lock up the same. Then bias is =
SLIGHTLY to=20
the front so that the front brakes lock up a LITTLE sooner than =
the rear=20
ones. (Easier and safer to remedy understeer then Depending on your brake setup, it is possible = that you=20 will have to add a prop valve to the rear circuit if you have a = brake=20 system that is biased towards the rear (rear wheels lock up before = front=20 wheels in a panic situation). Prop valves only reduce existing = pressure.=20 Prop valves can be found at any hot rod shop. = Wilwood=20 makes a nice little unit that you can purchase directly from them. = You=20 will also need some adaptor fittings to accept metric bubble flare = lines=20 for installation. Please check local laws regarding prop valves on = front=20 circuits.
|