UPDATE: THIS IS NOW 5 REASONS TO HAVE BIGGER ROTORS!!
Now that we have learned more about braking, there are a number of things we can do to improve the performance of our brakes.
One of the best things we can do to get better performance from our brakes is to have larger rotors.
The reason for this is that every time we use our brakes, we are removing energy from the combined kinetic energy of ourselves, our bike and our speed.
This energy is transformed *mostly* to heat, which builds up in our rotors and pads. Too much heat and we lose performance of our brakes, since beyond a certain point, the coefficient of friction between the pads and rotors will reduce. Beyond this, we get a boiling of the pad material or even brake fluid. These are obviously not ideal!
Larger rotors have a greater mass and allow for greater storage of heat. This is good because if there is a greater capacity for heat in the rotors, your brake pads and hose fluid can stay at operation temperatures.
Furthermore, the larger rotors have a greater area, which means that heat can also dissipate more quickly.
See below for now FIVE reasons to try larger diameter rotors.
Below are 5 Science-Based reasons you might want to consider bigger rotors:
REASON #1 TO HAVE BIGGER ROTORS
Very fast riders have greater kinetic energy than slow riders. In fact, greater speed increases your kinetic energy more than what your weight does!
Kinetic energy= 1/2*mass*velocity^2
To remove the kinetic energy, we use the brakes, and this is measurable as brake energy. This means that for the fast riders to slow down as much as we do, they will need to remove more energy (and thus build more heat!).
Having a larger area for the heat to be transferred means that less heat builds up on the braking surface itself, thus saving our pads and keeping the temperature of our brake oil within an operational range (i.e. less brake fade).
If fast riders want to ensure the performance of their brakes, they might want to look at larger rotors. 203mm is really the only option for enduro and DH racers.
Lately there has been more and more interest in even BIGGER rotors. Here are some Galfer rotors on test with the BrakeAce in 223 and 246 on this Giant Reign 29er, and then a 246mm rotor on the front of a Giant Trance 29er.
They work amazingly!
REASON #2 TO HAVE BIGGER ROTORS
There is a sad fashion that has existed since disc brakes were invented for bikes, which stipulates that many bikes come with a larger front rotor than in the rear. However, we’ve learned that most riders brake more (and thus remove more energy and build more heat) in the rear brake.
The argument for this was always the one that says, ‘you get 70% of your braking power from the front’.
This might be true in motorcycles on a road, but for a lot of riders the skills to dump a handful of front brake just doesn’t exist, and so buildup of heat in the front is less likely to occur. This is not to say that braking in the front is not effective–because it is! But on trails with variable conditions, things are just not the same.
Try having at least the same size rotors…and if you put a larger one in the rear you wouldn’t be the first person. After testing one of our very first brake power meter participants, we showed him how much rear braking he was doing and he swapped rotors and put the big one on the back.
It’s highly likely that you are braking more overall with the rear brake, so having a higher heat capacity in the rear will improve your control of the bike.
REASON #3 TO HAVE BIGGER ROTORS
Another benefit of having larger rotors is that the brake has a greater mechanical advantage over the wheel.
The brake itself converts line pressure to clamping force between the pads and rotor. This clamping force is a torque. When this toque is at a greater distance from the center of the wheel, you are gaining a greater mechanical advantage over the rotor, which is attached to the wheel.
This means that at a given pressure, you are able to generate a higher torque…
Or similarly, that at a lower pressure, you can have the SAME torque!
I have seen it wrongly quoted many times that larger rotors have more power. This is not true since power is equal to the torque times angular velocity of the wheel, and that the power needed is equal to the change in kinetic energy of the rider slowing down. So unless you are planning to stop more quickly, the larger rotors won’t have more power.
And I say BUT, once you have larger rotors you will be able to get the same power at a lighter lever squeeze.
Believe it or not, this feels pretty darn good on your fingers and arms as you can relax a little bit more.
REASON #4 TO HAVE BIGGER ROTORS
If you regularly ride on big hills, you’ll notice that you cannot ride the whole time without using your brakes. I mean this is obvious!!
And don’t forget–riding faster isn’t about not using the brakes at all–it is about using the brakes more efficiently!
Hills are giving us free speed in the form of potential energy. This is handy because it could be possible to coast down a hill and save energy!
Potential energy= mass*9.81*height
As we travel, this potential energy becomes kinetic energy, with the result being us on a bike bouncing down the hill.
Since we need to slow down to maintain control, we will be braking. Therefore, the bigger the hills we ride, the more kinetic energy we will have. Thus, more energy and heat overall will need to go in to the brakes.
On longer descents, you might find that your brakes pump up or fade. This happens when either the coefficient of friction changes between the pad and rotor surface or when the brake fuid becomes too hot and start to boil.
Why not go a little bit larger to gain some more area for heat build up?
REASON #5 TO HAVE BIGGER ROTORS
A 20mm larger rotor rotor can weigh as little as 20 grams more.
My guess is that you won’t notice the additional weight, but you WILL notice the additional performance!
Time and time again when I get riders out testing with the brake power meter, one of the first things they notice is how powerful the brakes are. I am just using standard brakes, but ALWAYS use at least a 203mm rotor that is pretty close to new.
And for a pro tip:
I always replace my rotors regularly–even more often than my brake pads. The rotors wear fast, and once the etching in them is smoothed over by the pads too much, they are pretty much useless! We noticed this a lot when setting riders up to test with the BrakeAce and using new rotors for the sessions. Riders noticed a difference right away, even without new pads–it was only the rotors.
Unless you are doing a style of racing where braking performance matters less than the weight of your bike (such as climbing up a single hill and hitching a ride in a car to the bottom), it’s probably worth experimenting with larger rotors.
What do you have to lose?
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Hopefully this makes your next ride more fun!