Sign in to follow this  
Followers 0

Brake System Theory

3 posts in this topic

This is something I wrote for a BMW site's wiki, but it applies to all cars so I thought it might help!


Basic checks:

First off, ensure the callipers are in full working order (no sticking pistons, seals etc) and the brakes hoses are in good condition before embarking on any upgrade. Also check the servo is working properly if fitted (a duff servo normally gives you a hard pedal with a lack of braking force) and that the master cylinder seals are good (leaking seals give you a soft pedal, often slowly "sinking" if held under pressure).

What can be improved?

There are a lot of closely interrelated aspects surrounding brake system operation, many areas can be improved but at the expense of another area. This makes everything a compromise, here are the aspects with their effects on performance etc:

Brake "power"

ie torque at the wheel for a given brake fluid pressure. This can be improved in several ways:

1 - Larger discs, this in effect increases the leverage that acts on the wheel. Downsides are increased unsprung mass (and rotational mass due to the larger diameter), a slight increase in cost if it's a re-drilled one-piece disc or a larger increase in cost if it's alloy bells and rotors.

2 - Higher coefficient of friction in the pad material. Downsides are that in most performance pads they work better when warm/hot, and full race pads hardly work at all when cold, plus increased cost.

3 - Larger pistons in the caliper (and/or smaller master cylinder piston/s). This increases the mechanical advantage that the system gives your foot over the force applied to the back of the pads, but like all forms of increased leverage the longer the "lever" the longer the travel needed at the input end. In the case of brakes, the added clamping force at the pad is offset by a longer travel / softer pedal.

Unsprung mass,

This is the effect of the mass of the brakes fitted to the strut, where they are not properly "sprung and damped" by the suspension like the chassis is. The lighter a brake system is, the faster and more controlled the suspension action can be, giving the car more grip. The downsides to lighter brake discs, is that they heat up faster due to the lower "thermal mass", but lighter calipers, pads, alloy bells and mounting brackets all give you "free grip".

Rotational mass

The lighter and smaller the brake disc (and wheel/tyre/hub etc) the less energy is needed to spin it faster or slow it down. So a lighter and smaller brake disc will give the car better acceleration, economy and braking (if the brakes magically still worked just the same!). The downsides are reduced brake power, reduced cooling from the smaller disc vents and reduced thermal mass.

Fade (pad) resistance

This is the amount of heat the brake pads can withstand before they overheat into "brake fade", this feels like you have a hard pedal but no brake power (similar to a failed servo). In the olden days pads used to "gas", where the glue in the material boiled and producing a thin layer of high pressure gas that lifted the pad off the disc using the same principles as a hovercraft uses! This lead to people drilling (and grooving) their discs to release this gas. Modern brake materials no longer gas in anything like the same way, so drilled discs are of FAR less use than they used to, and given that they can crack the main reason people buy them is for the "look". Grooved discs DO still have a place as they scrub at the face of the pads, and with many race pads they are needed to stop the pad surface "glazing".

Fade (fluid) resistance

This is the amount of heat the brakes can withstand without the fluid boiling, when the fluid boils the bubbles created are easily squashed by the pressure created when the pedal is pressed, making the pedal sink straight to the floor and allying almost no pressure to the pads at all! This is probably the most dangerous form of brake problem and for the few seconds it happens the effect is as bad as a burst brake line. Fluid boiling can be reduced by:

1 - Running a fluid with a higher boiling point.

2 - Changing the fluid regularly as water is absorbed by most brake fluids, and of course only needs 100 degrees C to boil. Also once brake fluid HAS boiled it's resistance to boiling is actually reduced, and next time it will boil at a LOWER temperature!

3 - Using cooling air from the brake duct to cool the caliper.

4 - Using cooling air from the brake duct to cool a brake fluid radiator.

5 - Running a brake fluid recirculating valve. This clever gadget cycles the fluid into the caliper then back out and up into the main fluid reservoir, this keeps the caliper cooler and stops a stagnant volume of fluid from sitting in the caliper and getting very hot.

Heat reduction

This is mainly about keeping the disc cool to stop it "warping" and keep the pads cooler so they can operate properly. You can reduce disc temperatures by:

1 - Having a larger disc with bigger, more efficient vents and larger area to be cooled, the downside is added unsprung/rotational mass (although mass helps in another way, next....).

2 - Having a heavier disc, this provides more "thermal mass". Thermal mass works because it takes twice as much energy to heat twice as much mass to a given temperature, so if you double the mass while keeping the energy input constant you HALVE the temperature (before cooling efficiency etc starts to come into play). The downside is added unsprung/rotational mass.

Disc "warping" resistance

It should be noted that brake discs juddering through the pedal are not "warped" in the way many people think, in most cases it's because road pads (which are abrasive) have been overheated and a high-spot has formed. This high spot gets VERY hot and into a changes into a different and much harder form of iron. Of course the new hard area is more resistant to the pad's abrasion and so as the material around it is worn away it becomes higher, which makes it hotter, and therefore even harder....and so on. The discs DO warp, but only when you press the pedal and this one high spot gets much hotter, rapidly expanding and warping the disc. The things people miss are that (1) the disc returns to a mostly non-warped state as soon as you back off the brakes or take the disc off the car for inspection, and (2) machining the disc won't fix it as there is still a hard spot in the iron waiting to start the whole process again once the abrasion or use continues! Upgrade pads often work by "smearing" their own pad material onto the disc rather than abrading the disc, thereby dodging the high-spot vicious cycle.

Resistance to high temperatures by all components in a performance/race application

If you assume that the brake system will be used and a hard and sustained way (trackdays etc) then the components ARE going to get very hot no matter what cooling you use, but there are ways to allow them to survive the temperatures and work very effectively for a sustained period.

- External piston dust seals on performance calipers often burn off, but they can be omitted without causing any problems in most cases.

- Brake discs get very hot in the area that's in contact with the pads and expend with huge force. In a one-piece disc this makes the disc form cone-shape as the vented "rotor" part expands but the "bell" part stays cooler and unchanged, in (very rare) extreme cases the vented part can crack right off the bell! To reduce these forces there are two options, (1) fit an iron rotor to an aluminium bell (aluminium expends faster with temperature than iron, so the cooler bell partly "keeps up" with the hotter rotor as it expands) and (2) fitting a "radially floating rotor (there are radial slots in the rotor or the bell with sliding "bobbins" in them, this allows the rotor to expand totally freely, with no force applied to the bell)! Floating rotors are very expensive though and are almost exclusively for race-cars only - although BMW have recently used the principle on the M3, the discs have aluminium bells with radial spoke-like pegs, and the cast-iron rotor "floats" on these pegs! Also large light-weight rotors rely less on the effects of thermal mass, and more on effective cooling vents to cool the brakes on each straight section of track.

- Race pads will continue to work happily way after normal pads would have totally failed, and often get better the more abuse they get, and race brake fluid can withstand the continued high temperatures.

- Brake ducts are very handy as the high speeds on the straight pump much more cooling air into the brakes than on a road car.

Share this post

Link to post
Share on other sites
il move this to your section mate :D

Cool, it's not a promotional thing but it can live here it if makes better sense. :)

Share this post

Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this  
Followers 0