# Thread: Surface Area effects on braking performance?

1. ## Surface Area effects on braking performance?

I know that the surface area of the rotor has no effect on stopping power or modulation because Kinetic Friction = (Coefficient of Kinetic Friction)*(Normal Force applied). Well at least it shouldn't, but it would have an effect on heat dissipation and perhaps the structural integrity of the rotor. has anybody used some of the skimpier rotors like the alligator ones and if you have, what did you feel about fading, modulation, and how easy they were to knock out of true? Thanks.

2. What you are describing (surface area not having an effect on stopping power) is true only for an instant before the rotor starts to heat up. In real life this is not the case.

In real life surface area does have an effect on stopping power because the coefficient of friction is dependent on the rotor's temperature. Brakes work by transforming kinetic energy to thermal energy, and if a rotor has less surface area it isn't as efficient at performing this transformation.

I'm using alligator wind cutters on one of my bikes, and stopping power and fade are noticeable compared to less skimpy rotors. It doesn't take much to cause fade, so I constantly have to be careful about braking technique.

The only time I've kocked one of the rotors out of true was when I bashed it on a rock.

3. ## Actually

The pad contact is the same regardless of the size of the rotor, BUT the larger rotor does dissipate heat better due to its larger surface AND the distance of the larger rotor from the center of rotation (AKA Radius) of the wheel allows more torque to be applied using the same pads, calipers and pressure.

The heat dissipation goes up with the square of the radius, but the torque goes up in a linear fashion.

4. Originally Posted by b4 stealth
I know that the surface area of the rotor has no effect on stopping power or modulation because Kinetic Friction = (Coefficient of Kinetic Friction)*(Normal Force applied).
The Coefficient of Kinetic Friction is seldom a constant in real life situations. It varies with temperature, certainly. At limit states between many materials, it varies with the force applied also. I'll caveat that statement by saying that although I know it is true of, say, tyres in contact with the ground, I do not know whether it is applicable to disk brakes pads on rotors.

5. ## Actually surface area does matter

Surface area (or contact patch) is part of the normal force applied to the system so it absolutely matters in calculating kinetic friction. (force is measured in newtons which are in kg* m/s^2). I think that some of you are confusing is the calculation for friction and the Co-Efficient of kinetic friction. The coefficient is definitely independent of surface area but the total calculation takes the surface area into effect during the energy calculation.

Also dont forget that a larger rotor will pass more material through the caliper per revolution so a greater energy dissipation will occur per revolution. The braking system works by converting rotational energy into heat according to this formula. You can see that this is dependent on position (x)dx and if you integrate with respect to time dx/dt you will end up being able to figure out total energy dissipated over a time interval.

6. Also check out this link regarding how temperature affects the coefficient of friction:

http://www.labthink.cn/service/show558.html

As can be seen in the data on that page, the coefficient of kinetic friction increases with temperature until it peaks at a certain temperature. After passing this peak, the coefficient drops with increasing temperature. This drop is what's felt as brake fade (the kind where the brake lever is still firm but braking effectiveness is reduced).

By increasing the surface area, the rotors are able to dissipate heat more effectively and prevent reaching temperatures past the peak of the temperature-friction curve.

7. ## Don't ignore the function of tires in braking

Friction force is a function of the coefficient of friction and the normal force. Surface area does not come into play for friction force. The braking force is the cross product of the radius (rotor size) and the friction force.

Friction force equals the product of the coefficient (static or dynamic) of friction and the normal force.

Ff=u*Fn

braking force= R cross F where both R and F are vectors.

The disc brake converts kinetic energy of the system into thermal energy thereby reducing velocity by the conversion of momentum into heat via the friction of the pads on the disc. One reason brakes can fade is that the pad material can outgas when overheated which has the pad floating over the rotor on a thin layer of gas instead of making direct contact with the rotor which drastically lowers the coefficient of friction. But one critically important factor in being able to slow down/stop is the coefficient of friction at the tire/ground interface. If you don't have good "traction" at the tires you will not be slowing down effectively. The radius (R cross F) of your tire is significantly larger than that of the brake rotor on a bike! Your tire performance is a critical part of the braking efficacy and by changing tires you can noticeably alter braking performance with no change to the brakes themselves.

8. Weight issues aside, wouldn't you want more mass in a given diameter rotor? This was why I picked my BB7s with the nearly solid rotors. You can get those aligator things in 8" with about as much mass as a 6" solid rotor and sure, you get more leverage but is fade resistance really increased much?

9. I can say from experience that my 8" alligator wind cutter rotor starts fading much much sooner than my 6" Hayes V6 rotor.

10. Originally Posted by OneTimeCRX
I can say from experience that my 8" alligator wind cutter rotor starts fading much much sooner than my 6" Hayes V6 rotor.
Thanks for being honest. This question has been bugging me for a long time.

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