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Disc brakes: What is the ideal rotor size?

Sam Challis
28 Sep 2020

Should disc brake rotors be 140mm, 160mm or some other size altogether? Cyclist consults the experts

Above: Shimano Dura-Ace Ice Tech Freeza 140mm/160mm / Weight: 94g/106g / £69.99 / freewheel.co.uk

Photography: Rob Milton

Disc brakes. First endurance bikes had them, which was easy to justify. Their more consistent performance was a natural fit on rough roads in often changeable weather conditions.

Then aero race bikes got them, which again made sense. Weight isn’t such an issue with aero bikes, and disc brakes opened up opportunities for aerodynamic development.

Yet now even lightweight race bikes have them. Disc brakes are no longer the braking system of the future – they are the braking system of the present. Discs are the new normal, and with that maturation has come a settlement on certain standards. 

For example, flat-mount, where the brake calliper sits directly on a chainstay or fork blade, as opposed to post mount, has been universally adopted. However there is an area where a certain amount of debate still exists, and that is around the size of disc rotors.

Should all disc bikes use a pair of 160mm rotors? Why not 140mm, 180mm or even a mixed pair? While several factors enter the mix when it comes to design decisions that account for the current discrepancies, most importantly calls are made based on safety.

‘In my opinion a pair of 140mm rotors look nicest, but as many riders are over 80kg there is a chance braking performance can be affected in certain conditions,’ says Giacomo Sartore, groupset product manager at Campagnolo.

‘This is why we recommend either a pair of 160mm rotors or 160mm front, 140mm rear. With those options a rider can drag their brakes all the way down the Stelvio and not suffer any dip in performance.’

Sram’s road product manager, Brad Menna, agrees: ‘We recommend 160mm for road applications. That’s what provides the most power and best system performance for the widest range of riders and uses.’ 

Shimano’s Ben Hillsdon also agrees, and explains why 160mm rotors might be better able to cope in certain situations.

‘When brake calliper pistons are applied to larger rotors, due to the fact that they are further away from the rotating axle, they provide greater leverage and torque to stop the rotation.’

Menna adds that bigger rotors also have a larger braking surface to dissipate heat: ‘The better you manage heat, the more consistent the brakes work under various loads.’

Given that evidence, it would be reasonable to assume that the case for a pair of 160mm rotors would be cut and dried, but a mixed setup – with 160mm at the front and 140mm at the rear – is just as popular.

Above: Sram Centreline XR 160mm / Weight: 131g / £97 / zyrofisher.co.uk Sram Paceline 140mm / Weight: 94g / £40 / zyrofisher.co.uk

‘There is the belief that it balances out the power, given the weight distribution on the bike,’ says Menna. Under deceleration, a rider’s weight is shifted forward, meaning the requirement for the same level of braking power at the rear of the bike just isn’t necessary.

'It is even potentially undesirable given that there’s a greater chance of locking the rear wheel and skidding under such conditions. There are other factors that lend even more credence to the 160/140 setup.

‘It depends on the rotors, but there can be a 30-40g difference between a pair of 160mm rotors and a pair of 140mm ones,’ says Menna. Shimano’s Hillsdon quotes a similar figure, saying the difference between a 160mm Ultegra rotor and its 140mm counterpart is 20g.

Given that disc brake systems already incur a weight penalty on bikes, it’s understandable that brands would seek ways to offset this by using the smallest possible combination of rotors while still remaining safe.

Campagnolo’s Sartore even suggests there is a cost difference too, with smaller rotors being cheaper for OE manufacturers to buy in bulk, but he admits appearance is an equally persuasive factor. A mix has its advantages but the asymmetry of differing rotor sizes could count against their inclusion.

For once, aero isn’t everything

In an age when all race bikes are being optimised for aerodynamics, it might seem obvious to opt for the smallest rotor size possible to reduce drag. However, as Hillsdon points out, ‘The difference in surface area at a head-on angle is very marginal.’

In which case, if there is no aero penalty involved, why not go the other way and make disc rotors even bigger? After all, the performance advantages 160mm rotors have over 140mm would only be increased in a move to 180mm rotors.

‘If gravel riding and gravel bike design becomes more extreme then there is always potential for riders needing greater braking force,’ says Hillsdon. But Menna doubts that 180mm will be necessary even for gravel: ‘The speed and weights involved don’t exceed what’s on the road.’

Campagnolo’s Sartore hammers home the final nail in the coffin of 180mm rotors for road by saying that he only sees scope for 180mm in the e-bike realm.

That just leaves the continuing debate over whether a pair of 160mm rotors or a combination of 160mm/140mm is best. Each of the groupset manufacturers we talked to confirmed that either setup is safe and offers similarly polished performance.

So, until the industry settles on a single preference, you might as well just choose the setup you like the look of most.

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