Bike rotors come in a variety of shapes and sizes, but how much does size really matter when it comes to your brake discs?
In this article, we’ll look at the pros and cons of bigger rotors and try to answer the question – Does rotor size matter?
Are Bigger Brake Discs Better?
A larger disc brake will provide more stopping power than a smaller disc when the same force is applied to the brake lever as a larger diameter disc gives more leverage for stopping. Beyond a certain point when a disc provides enough stopping power, using a larger disc just adds unnecessary weight.
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Do Bigger Rotors Stop Better?
Bigger brake rotors provide more stopping power than smaller rotors as a greater distance between the outer edge and the center of the disc gives the larger disc more leverage. A larger disc gives more stopping power than a smaller disc when the same force is applied to the brakes.
A bigger brake disc will give more stopping power than a smaller rotor when the same force is applied to the brake lever. This is true regardless of whether you’re mountain biking, road cycling, or touring on a hybrid.
That’s because a larger rotor means the brake has greater leverage since the stopping force is acting further away from the center of the wheel. In other words, a larger rotor means more leverage.
It is important to note that getting enough braking power is not a problem for most riders. Ultimately once the wheel is locked out, maximum braking has been reached regardless of the size of the rotor.
Provided the rotor is big enough to reach that locked-out position of maximum braking, we can’t then say that a bigger rotor would provide more stopping power.
What we can say is that a bigger rotor would provide the same stopping power to the wheel as the smaller rotor, whilst requiring less force to be applied to the brake lever.
This is the beauty of increasing rotor sizes, it really takes the pressure off your hands, wrists, forearms, and shoulders when you don’t have to squeeze the brake levers as hard as you physically can whilst heading down a long, steep descent.
All that said, if your brake rotors are undersized and aren’t capable of locking the wheels out during your typical rides, then a larger rotor would actually increase total stopping power.
Pros and Cons of Larger Disc Brakes
Whilst there are a number of pros to larger brake discs, there are a few drawbacks that are worth mentioning. Here’s a summary of the pros and cons of larger brake discs vs. smaller ones:
|Less pressure on brake levers for the same stopping force.
|Will increase wear on brake pads & disc as the disc rotates faster through the brake pads the larger it is
|Provide greater overall stopping force (in cases where the smaller rotor can’t lock the wheel)
|Larger rotors are more prone to distortion and damage
|Increases the weight of the brakes – more material used
We’ve already considered the pros in the section above, so here let’s just take a look at the main drawbacks of larger rotors and any mitigations to them:
Increases Wear on Pads & Discs
Larger brake discs travel through the pads quicker, resulting in the pads and sometimes discs wearing out faster than you might like. Consider this diagram to understand why this happens:
Imagine a line drawn on a brake disc from the center to the outer edge. Two points are drawn on the vertical line, a red point on the outer edge of the disc and a green point closer to the center.
Now imagine rotating the disc so that the vertical line moves from position A to position B. The line is fixed, so both of the points on the line move through the same angle in the same amount of time.
However, as we can see by the arcs drawn on the diagram, the distance traveled by the red point is much greater than the distance traveled by the green point.
The red point has traveled a greater distance in the same time compared to the green point. In other words – the red point has traveled faster.
This is the effect seen when increasing rotor size, the part of the rotor that is clamped by the pads is moving through the pads faster the bigger the rotor is.
This can cause problems as any grit or dust trapped between the brake pads and the disc will be more abrasive the faster the disc is traveling.
You can avoid or at least reduce this effect by simply using good-quality brake pads and discs made by trusted manufacturers like Shimano or SRAM.
Reducing wear on disc brakes is of interest to both brake manufacturers and cyclists alike. The holes in brake rotors serve a number of purposes, including helping to minimize wear. Check out this article we wrote on ‘why disc brakes have holes’ here if you’d like to learn more.
More Prone to Distortion & Damage
A larger disc means a larger target that you can potentially catch on a rock whilst out on the trail.
As well as being a bigger target, a longer part is more prone to bending.
In the same way that you would find it easier to bend a long metal bar than a shorter one if the material and thickness are the same, a larger rotor is more easily bent or distorted as impacts near the outer edge of the disc apply a greater moment force than that which could be achieved with a smaller disc.
This is less of a factor with modern discs, particularly for mountain biking where going to the ends of the earth to find a way to save 10 grams in weight isn’t all that common.
Shimano and SRAM discs come with a weight penalty of around 20 grams for moving up one disc size – a small price to pay if it means enough stopping power to avoid that tree!
If you’re a road cyclist or a mountain biker particularly keen on saving any extra weight, just make sure you aren’t oversizing your discs (which of course makes them heavier) or using discs from poor-quality manufacturers. They won’t last as long and their design won’t be well optimized for maximum performance at minimal weight.
One way manufacturers reduce the weight of brake discs is by drilling holes through the discs. Weight saving is only one of a number of practical applications for the holes in disc brakes. If you’d like to learn more about why holes in brake discs are so important, check out the article we wrote on it here.