Do larger rotors affect braking performance?
As an engineering student who might be studying abroad next year, I thought it could be beneficial to start practicing engineering terms through my blog. Hence, I will be writing this kind of physics related posts more in the future. I think I am above average in physics, but if you spot mistakes/wrong terms in my posts please let me know! I’ll get them fixed as soon as possible because spreading false information is definitely not what I want.
In simple physics
I know that most of you are not interested in hearing the physics behind my calculations. Hence, I will start with simplified explanation with no math included, just the results. If you don’t trust my calculations, don’t worry, just scroll down and you can see how I got these numbers.
Do larger rotors affect braking performance?
We wanted to know how much switching to a larger rotor would affect braking performance in theory. In order to eliminate most of the variables, we have to assume that the only changing thing in the braking system is the rotor. The brake lever, caliper, pads, friction, they are all expected to stay exactly the same. The results were calculated for 160, 185, and 203 mm rotors, expecting them to be identical except for the size difference. Brake pad and piston diameters were expected to be 10 mm.
Through the calculations, we found out the following:
- Switching from 160 mm to 185 mm rotor increased the braking power by roughly 17%
- Switching from 160 mm to 203 mm rotor increased the braking power by roughly 29%
- Switching from 185 mm to 203 mm rotor increased the braking power by roughly 10%
Another way to think this is by how much less lever pressure is required to acquire the same amount of braking power.
- 185 mm rotors require roughly 19% less lever pressure compared to 160 mm rotors
- 203 mm rotors require roughly 32% less lever pressure compared to 160 mm rotors
- 203 mm rotors require roughly 16% less lever pressure compared to 185 mm rotors
In real life, these numbers are not 100% accurate. There are several reasons for this:
- In the calculations, we used a value of 10 mm for the brake pad width. Brake pads are not made equal, and hence different sized pads will throw the results off slightly.
- We had to assume that the tire could provide us with enough grip to brake at full power.
- We also had to assume that the rear tire wouldn’t lift off the ground and the rider wouldn’t get thrown over the bars.
- In real life, there are so many variables that accurately comparing brake rotors to one other would be very challenging.
Despite the numbers being fairly inaccurate, we can conclude that larger rotor offers better braking power. Other benefits of larger rotors are:
- Better thermal attributes. A larger rotor will, generally speaking, heat up slower and dissipate heat more effectively. This leads to reduced brake fade under prolonged braking. With that said, the rotor design has more of an effect on this than the sheer size of the rotor itself.
Larger rotors have some downsides too:
- Larger rotor might offer too much braking power for some situations. Skidding with the wheel locked up is not effective and controlled braking.
- Weight. Not a significant factor for most people, though.
EDIT: I got some feedback on my points about larger rotor offering too much braking power for some situations and how the tires may not be able to provide enough grip for maximum braking. To elaborate, this is simply because modern hydraulic disc brakes are easily powerful enough to lock the tire/throw the rider over the bars even with 160 mm rotors. Changing the rotors will not change the modulation of the brakes themselves, but it will change the “useful” modulation. In other words, the brake lever travel before the tire locks is reduced. That’s why the brakes might feel “too powerful” with over-sized rotors and cause unwanted locking of the tires.
Furthermore, it’s often the available traction from the tires/ground that limits how quickly the bike can stop, rather than the brakes not being powerful enough. Therefore, switching to larger rotors may not shorten the stopping distance at all.
For more about this topic, see this document. Great info there.
The first step was to calculate the effective diameter of the rotors. This is measured from the rotor center of rotation to the center of pressure of the caliper piston. The caliper piston and brake pad diameters are assumed to be 10 mm, so that’s why 5 mm is decreased from the radius of the rotors.
When calculating the torque [T(r)] increase from the comparison, the friction [F(friction)] is expected to remain constant. Thus, it can be eliminated from the equation.
When calculating the actual braking force [F(tire)] the tire size is expected to be the same, thus it can be eliminated from the equation.
In order to calculate how much less force is required to compress the brake lever while achieving the same amount of brake force, we needed to use the graph from the document made by the University of Applied Sciences Technikum Wien. It’s not exactly accurate because we cannot pinpoint the exact maximum value, but it’s close enough for this purpose.
In real life however
I know, I know. This is not the most accurate way to calculate how larger rotors affect braking performance. There were simply too many factors I didn’t take into account in order to simplify the calculations. In the real world, there would be numerous variables (such as disc design, heat, wear, friction, etc) and in order to gain even someone accurate calculations, some serious testing would need to be done to get the required data. We’d need an accurate test bench for that.
That’s exactly what they built at the University of Applied Sciences Technikum Wien, see the document here. Interesting test and results. What’s important is the fact that they used a well-made test bench to get the data for their calculations. Hence, the numbers they got should be quite accurate. That’s why I used the graph from their document in one of my calculations.
Have you got personal experiences on how larger rotors affect braking performance?
If yes, please let me know!
If you spotted a mistake or feel like I didn’t reply to the question fully, again, please let me know! Feedback is what makes writing these posts worth it, so don’t hesitate to let me know what you think about it. Even negative feedback is appreciated, my goal is to improve after all. Thanks for reading!