Carbon Brakes

[m3_lover]

I noticed that a lot of times the brakes have to heat up to work in there optimal range, why is it that you have to heat up the brakes to get them working properly.

Also is the reason why the tires need to heat up is because the more heat generated in the tire, the bigger the contact patch is and more grip is given by the tire because it becomes sticky?

[Reca]

I noticed that a lot of times the brakes have to heat up to work in there optimal range, why is it that you have to heat up the brakes to get them working properly.

It’s the characteristic of carbon (disc) + carbon (pads), need high temp to “bite”.

I’ve a graph friction coeff (mu) vs surface temp for a carbon disc of some years ago, mu grows from 0.3 to 0.4 between 50 and 200 °C, then it oscillates between 0.35 and 0.42-0.43 in the range 200->500 °C, over 500 °C then the mu grows, roughly linearly, up to 0.5 at 700 °C.

What usually drivers say about carbon brakes is that in the very first ms after they press the pedal it almost looks like nothing happens, that’s because the first moments are needed to build temp on the disc.
Consequently braking technique with carbon brakes is to press as hard as possible on the pedal in the first moments.
Obviously with cars heavily aero dependant, that’s the moment you also have the maximum grip on tyres thanks to downforce, so to reduce as much as possible that initial “dead” time and exploit the most potentially effective phase of braking is fundamental.

BTW, the main reason MotoGp riders use steel brakes on the wet is they have to be lot smoother with brakes in that condition hence carbon discs would never reach the operating temp.

Also is the reason why the tires need to heat up is because the more heat generated in the tire, the bigger the contact patch is and more grip is given by the tire because it becomes sticky

I don’t know how much it has to do with the contact patch getting bigger, pressure is certainly influenced by tyre temp and pressure has a bit of influence on the contact patch; nevertheless I would tend to think, but I could be wrong, that pressure influence on the spring rate and slip stiffness could be more relevant than that on contact patch area.
What is sure is that the tire surface needs a given temp to become stickier, while cold it’s not.

Sunday after the WTCC races I collected a few rubber marbles from the track, just warming up them up with an hair drier is enough to feel they become lot stickier.
[IMG:152:130]http://img313.imageshack.us/img313/5314 ... 8yu.th.jpg[/img]

[DaveKillens]

Racing tires have totally different properties when hot or cold. When cold, they have the consistency of regular ties, and not much grip. But when heated, they become almost like bubble gum, and each bit of rubber flows into and around every road surface irregularity. That is what gives that incredible grip, the rubber just doesn't sit on top of the road, the rubber almost melts into the microscopic surface of the road, and interacts with every miniscule ridge, hole, or bump.
I have been able to examine racing tires when hot, and they actually feel like soft bubble gum. Very soft, and pliable, and you would swear that if not careful, it would stick to your finger and come off the tire. There is a small temperature range where the tire has this property, and too hot or too cold would not give the tire it's maxumim grip.

[RacingManiac]

I've been fortunete enough to have visited Calspan's Flattrac tire testing machine(think of a giant arm with a tire mounted on it, rolling the tire against a massive belt sander, with strain gauges and various sensor attached to the whole assembly), and after a test the racing tire's surface is rediculously soft and sticky, if you just put it on the floor you'll have trouble picking it back up because it literally stuck to the floor....