Disc brakes have become pretty much ubiquitous on modern road bikes, and many riders will be asking a few questions when they finally wear through the rotor: what size do I have, what should I actually be using, is there a benefit to going bigger? We’ve spoken to Shimano, SwissStop, Hope and SRAM to get you the answers.
If you’ve bought a road bike in the past few years with disc brakes on it, then you’ve probably got either 140mm or 160mm rotors. You might also have a combination with the smaller size on the rear.
Take a look at your rotors, the size should be printed somewhere in the centre of the rotor.
Simple physics dictate that yes, larger rotors provide more stopping power. The very basic premise is that, as you increase the size of the rotor, you increase the leverage acting on the wheel, thus it is easier to stop.
Simple, right? Well, it isn’t always quite that easy. SwissStop’s Christian Heule says that “there are several limiting factors that can actually affect how quickly a bike stops.”
The main barrier to braking power is grip. At some point, the strength of the brake will overcome the friction between the tyre and the road and you'll lock up, causing a skid. Shimano’s Ben Hillsdon points out: “If you use a rotor that is bigger than your riding style or than the terrain calls for, you will have more aggressive braking power and modulation becomes more difficult.”
This means that if you pulled on the brake levers with the same force from your hands, the larger rotor would lock up the wheels earlier than with a smaller rotor. In practice, you can mitigate against this just by being more gentle with your hands. SRAM’s Danie Lategan suggests that this reduction in hand effort is actually a benefit of larger rotors.
While you might not end up feeling the difference in hand effort, it is worth casting your mind back to when cantilever brakes were commonplace in the world of cyclocross. They lacked power even in the best conditions and the end of a wet race would leave your hands almost as tired as your legs. They were good for teaching you correct line choice, however.
But is bigger actually better when it comes to rotor size? There is no one answer for every rider, but each of the brands that we spoke to was in agreement that if you’re a road rider running a 140mm rotor on the front of your bike, you’ll probably see benefits when going up to a 160mm rotor.
When disc brakes first made their way onto road bikes, 140mm was seen as the size that offered a feel that was most similar to rim brakes, and the smaller size kept the weight down as much as possible. As with many things in cycling, this has stuck around, but there are plenty of bike brands and rotor manufacturers that are choosing to make their standard offering a 160mm, at least on the front wheel.
Some bike brands, such as Open, have also made forks native to 160mm rotors, meaning that you don’t need an adaptor that many road frames require you to use.
Components manufacturer, Hope, meanwhile, says that it has always gone for 160mm rotors over 140mm. Hope’s Robin Godden explains“no matter on road or off road you still have the same moving mass to stop.”
In addition to this, he says that “on the road you could be descending at higher speeds for longer so you would want consistent braking all the time.”
SwissStop’s Christian Heule also pointed to longer and faster road descents, saying that 160mm rotors have better heat management "due to the increased amount of material on the rotor surface and increased time until the brake pad touches the rotor again in the same place."
He continues: "With more efficient heat management, the rotor and pad can remain under their maximum operating temperatures and the whole braking system can perform as designed.”
In essence, under the same braking conditions, SwissStop are saying that a 160mm rotor will continue to work properly for a longer period of time than a 140mm rotor.
Yes, but there isn’t much in it at all. SRAM and Shimano both claim a weight gain of just 20g per rotor when switching from a 140 to a 160mm size, and if your frame is designed to take 160mm rotors natively, you can throw away the adaptor that many setups use.
Hope’s Robin Godden says that “the better braking is worth that extra 20 grams or so” anyway.
Increasing your rotor size from 140mm to 160mm is relatively easy, just check that your frame will fit larger rotors before you spend any money. You will have to remove the rotor that is already attached to your wheel, but for centre-lock rotors that can be done with a cassette lockring tool for rotors such as Shimano and SRAM’s road models, or a 16-notch 44mm lockring tool for systems like Campagnolo.
6-bolt rotors are even simpler to remove with a Torx 25 key all that is required. Once you’ve popped the new rotor on, you will need to adjust the position of the calipers, but that is a simple job.
SwissStop’s Christian Heule suggests that one of those downsides is that they “tend to be less straight and bend easier after use which can result in the rotors rubbing against the pads causing drag.”
To guard against this, Christian says that SwissStop actually makes its rotors thicker as they increase in size. This, he says, “has a secondary effect of aiding the cooling of the rotor.”
You will also need to check that your frame and fork can accommodate larger rotors. This is best done by contacting the manufacturer.
As discussed previously, with the larger rotor providing greater leverage over the wheel, you will need to learn how to control that power. This finer control of your hands might take a little bit of getting used to, but you should be familiar with your new found power within the space of a few rides.
SwissStop’s advice is as follows: “On drop bar bikes, the general recommendation is to use 160 front at all times to optimise braking performance. Almost all UCI Teams use the combination of 160mm and 140mm even though they are usually lighter than we are and far more experienced so we’d at least recommend this setup to start for road bikes and maybe look at increasing the rear rotor for gravel applications.”
Shimano says: “In general bike manufacturers are responsible to make sure their bikes are according to EN/ISO norms, which also includes brake performance. Therefore, bikes are equipped with rotor sizes which should be suitable for the type of bike and the usage. If a rider has different riding needs or characteristics, factors such as weight reduction or more stopping power can determine if they need a bigger or smaller rotor.
SRAM, meanwhile, have made a handy graph. This suggests that for most road riding, 140mm rotors are sufficient for riders up to 73kg. If you are heavier than this, or you ride lots of long, steep descents as found in hilly or mountainous terrain, the graph suggests moving to a 160mm front/140mm rear setup.
Interestingly, cyclocross riders up to 82kg are recommended to use just 140mm rotors. This is an example of where a larger rotor might too easily lock up the wheels due to the lack of traction from the combination of muddy surfaces and comparatively narrow 33mm tyres.
So do the downsides of going bigger outweigh the positives? From what we can see that's a no, and aesthetics are generally the reason that riders cite for sticking with 140mm rotors front and rear. If you can look past the visuals, there are very few downsides.