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Open Track Racing
Venom F14 '03 Cobra Brake Kit - Installation
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<blockquote data-quote="Jack Hidley" data-source="post: 9841938" data-attributes="member: 17703"><p>Greg,</p><p></p><p>Tom is of course correct that for a given initial and final vehicle velocity and vehicle mass, the amount of work done by the brakes is a constant. However, there are some other considerations.</p><p></p><p>When you brake 10-20' later, the vehicle has been accelerating that much longer and is traveling slightly faster (higher Vi). </p><p></p><p>When you leave your braking to the last possible instant, you are going to compromise your entry line into the corner some. This is going to require you to be going slightly slower entering the cornering phase to not miss the apex, so Vf is slightly slower. </p><p></p><p>These two factors make the delta V slightly greater. You are also spending less time under braking, so more work is done by the brakes, resulting in slightly higher average temperatures.</p><p></p><p>However, the biggest difference is this. All of the thermal energy that is generated during braking occurs at the brake pad to rotor interface. The rotor has 3-5 times the thermal mass that the caliper does. So for a given amount of energy put into the system, the rotor temperature is going to rise more slowly. Bodies with large mass give you high thermal capacitance. This means they filter out the peaks and dips in the temperature as the energy input to the system goes up and down.</p><p></p><p>Since the caliper has much less thermal capacitance, its temperature goes up and down much more, following the varying energy put into the system. So when your total braking time only lasts 3 seconds instead of 3.5 seconds, the energy/time (power) goes up by the ratio of 3.5/3 (1.16). This means your caliper, brake fluid and brake pad temperature is going to go up an amount proportional to this ratio nearly instantly, while the brake rotor temperature is only going to increase in temperature a much smaller percentage. Similarly, if your total braking time is constant, but the delta V is the same, the energy/time ratio is higher and the same thing occurs.</p><p></p><p>Once the brake fluid gets too hot, no more brakes. The gas doesn't magically reform into a fluid once the temperature drops 3 seconds later. If the pads overheat during this transient, they may recover 5 seconds later if they are good track pads (while you are parked in the tire wall), but if they are organic street pads, the binding agent may overheat and they will start falling apart never to return to normal behavior. </p><p></p><p>Granted I'm ignoring a lot of things going on in the system (rotor cooling, brake pad thermal resistance, etc), but none of that changes the underlying laws that govern thermal behavior in the system.</p></blockquote><p></p>
[QUOTE="Jack Hidley, post: 9841938, member: 17703"] Greg, Tom is of course correct that for a given initial and final vehicle velocity and vehicle mass, the amount of work done by the brakes is a constant. However, there are some other considerations. When you brake 10-20' later, the vehicle has been accelerating that much longer and is traveling slightly faster (higher Vi). When you leave your braking to the last possible instant, you are going to compromise your entry line into the corner some. This is going to require you to be going slightly slower entering the cornering phase to not miss the apex, so Vf is slightly slower. These two factors make the delta V slightly greater. You are also spending less time under braking, so more work is done by the brakes, resulting in slightly higher average temperatures. However, the biggest difference is this. All of the thermal energy that is generated during braking occurs at the brake pad to rotor interface. The rotor has 3-5 times the thermal mass that the caliper does. So for a given amount of energy put into the system, the rotor temperature is going to rise more slowly. Bodies with large mass give you high thermal capacitance. This means they filter out the peaks and dips in the temperature as the energy input to the system goes up and down. Since the caliper has much less thermal capacitance, its temperature goes up and down much more, following the varying energy put into the system. So when your total braking time only lasts 3 seconds instead of 3.5 seconds, the energy/time (power) goes up by the ratio of 3.5/3 (1.16). This means your caliper, brake fluid and brake pad temperature is going to go up an amount proportional to this ratio nearly instantly, while the brake rotor temperature is only going to increase in temperature a much smaller percentage. Similarly, if your total braking time is constant, but the delta V is the same, the energy/time ratio is higher and the same thing occurs. Once the brake fluid gets too hot, no more brakes. The gas doesn't magically reform into a fluid once the temperature drops 3 seconds later. If the pads overheat during this transient, they may recover 5 seconds later if they are good track pads (while you are parked in the tire wall), but if they are organic street pads, the binding agent may overheat and they will start falling apart never to return to normal behavior. Granted I'm ignoring a lot of things going on in the system (rotor cooling, brake pad thermal resistance, etc), but none of that changes the underlying laws that govern thermal behavior in the system. [/QUOTE]
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Venom F14 '03 Cobra Brake Kit - Installation
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