What is "brake torque"?

[Danib]

Hi Guys,

I am sure this is a stupid question, but what is "Brake Torque"? Some colleagues were talking about brake system system upgrades the other day, and they were talking about this.

[treks]

Brake torque is an important aspect of the physics of brake systems, and it must always be borne in mind when brakes are upgraded, or if you want to change to bigger wheels.

The best way to explain brake torque is to compare a car wheel to a torque wrench. With a torque wrench, the amount of force required to torque a bolt is dependent on the lenght of the tool, since torque is defined as "Force Multiplied by Distance".

If for instance you need to apply a force of say, 10 kg to a torque wrench that is say, 600 mm long to torque a bolt, but you lengthen the tool to 1200 mm, you will only need to apply a force of 5 kg to torque that same bolt to the same value. Similarly, if you make the tool only 300 mm long, you will need to apply a force of 20 kg to torque the bolt. 

Much the same thing happens with a car wheel. The distance from the center of the wheel to the point of contact with the road can be compared to the length of the torque wrench. The distance from the certer of the brake disc to its outside diameter can be compared to the force required to torque a bolt, but in this case, the torque works in reverse to stop the wheel. Nonetheles, the principle of "Force Multiplied by Distance" remains the same.

"Force" as it applies to a rotating wheel is generated by the motion of the car (for our purposes, we will ignore the kinetic energy of the wheel) , and "Distance" is the relationship between the diameters of the wheel and the disc. The difference between the diameters of the wheel and the brake disc can be compared to the length of atorque wrench.

Thus, the bigger the difference between the diameters of the wheel and brake disc, the more brake force is required to stop the wheel, since the effect is the same as making the torque wrench shorter, while  "Force" remains the same. Fitting smaller wheels reduce the difference in diameter between the wheel and the brake disc, which is the same as making the torque wrench longer, which in turn reduces the brake force required to stop the wheel. 

Brake upgrades often involve fitting slighty larger discs, which has the same effect as changing (reducing) the wheel size. Therfore, upgrading brakes by fitting bigger discs only works if the wheel size remains the same, because if you fit bigger wheels at the same time, you lose the advantage of the bigger discs that reduce the difference in the relative diameters of the wheel and the disc.

 

[salmanahmed]

On 7/1/2016 at 0:00 PM, treks said:

Brake torque is an important aspect of the physics of brake systems, and it must always be borne in mind when brakes are upgraded, or if you want to change to bigger wheels.

The best way to explain brake torque is to compare a car wheel to a torque wrench. With a torque wrench, the amount of force required to torque a bolt is dependent on the lenght of the tool, since torque is defined as "Force Multiplied by Distance".

If for instance you need to apply a force of say, 10 kg to a torque wrench that is say, 600 mm long to torque a bolt, but you lengthen the tool to 1200 mm, you will only need to apply a force of 5 kg to torque that same bolt to the same value. Similarly, if you make the tool only 300 mm long, you will need to apply a force of 20 kg to torque the bolt. 

Much the same thing happens with a car wheel. The distance from the center of the wheel to the point of contact with the road can be compared to the length of the torque wrench. The distance from the certer of the brake disc to its outside diameter can be compared to the force required to torque a bolt, but in this case, the torque works in reverse to stop the wheel. Nonetheles, the principle of "Force Multiplied by Distance" remains the same.

"Force" as it applies to a rotating wheel is generated by the motion of the car (for our purposes, we will ignore the kinetic energy of the wheel) , and "Distance" is the relationship between the diameters of the wheel and the disc. The difference between the diameters of the wheel and the brake disc can be compared to the length of atorque wrench.

Thus, the bigger the difference between the diameters of the wheel and brake disc, the more brake force is required to stop the wheel, since the effect is the same as making the torque wrench shorter, while  "Force" remains the same. Fitting smaller wheels reduce the difference in diameter between the wheel and the brake disc, which is the same as making the torque wrench longer, which in turn reduces the brake force required to stop the wheel. 

Brake upgrades often involve fitting slighty larger discs, which has the same effect as changing (reducing) the wheel size. Therfore, upgrading brakes by fitting bigger discs only works if the wheel size remains the same, because if you fit bigger wheels at the same time, you lose the advantage of the bigger discs that reduce the difference in the relative diameters of the wheel and the disc.

 

you have earned my Respect buddy. thank you for such a detailed explanation. I have be enlightened