# What are the limiting factors for braking performances?

In a car with disc brakes, the performances (how long it takes to brake, how many braking cycles can be performed without overheating, brake power before locking the tires, and so on) depend basically on tires, brake discs, brake pads.

In theory friction between two objects only depends on the friction coefficient and on the force, not on the contact pressure, so narrow and wide tires should be equal (the weight of the car is the same), but in fact the rubber in narrow tire (besides getting worn out faster) will heat more, potentially limiting the performances.

I think that bigger discs will cool down faster, but I don't know how and in which cases this affects braking performances.

Bigger or multiple pads... besides lasting longer, I'm not sure.

So, in what are each of these three elements limiting the braking performances? for a given car, what will I obtain if I mount wider tires? (I think the radius won't matter much). What about larger discs? or (hypothetically) bigger pads or multiple pads?

• A big factor is what speed are you slowing / stopping from? (Knowledgeable) Drivers going down mountains tend to take care of their brakes so they don't overheat. Why will bigger discs cool faster? They have more mass to heat and heat transfer is limited by temperature difference mainly. Forced cooling has the best effect - fitted that to a race car with a friend, made so much difference to braking later in the race compared to others... – Solar Mike Sep 3 '20 at 9:35
• In Switzerland brakes for Audi’s are significantly oversized due to hill braking and thermal wear for longevity and reliability. – Tony Stewart Sunnyskyguy EE75 Sep 3 '20 at 15:39

## 1 Answer

No, they do not. They depend mostly on SPEED and WEIGHT. The faster and heavier your car is, the bigger the energy your brakes are gonna have to dissipate in heat form. Just put the numbers on this little known formula: e=m*c² and you will find how much energy your system will have to dissipate. Tires matters, bigger diameter disks helps, because of the leverage factor, besides the friction material. They must be reliable for the amount of energy they gonna have to dissipate, otherwise, your car will not stop. The cooler your disks, the better is the friction, unless your car wears carbon disks. Disks internal cooling are design just like vacuum cleaner turbines, so, the fast they spin, the faster they dissipate heat. Forced vent helps a lot. Curiosity: At my shop, long time ago, when I was still working with modifications, we use to measure the brake power by measuring the speed we can get the ABS to act. Usually, 60-70MPH with stock brakes, to 100-120MPH with big brakes.

• "they do not" what? and e = mc^2 isn't the mass to energy conversion? not really relevant here – FarO Sep 3 '20 at 18:31
• Best way to maesure braking performance is to use a Tapley meter. – Solar Mike Sep 3 '20 at 19:36