We descended Mount Etna twice: on the first occasion, riding a BMC Granfondo GF02 aluminium road bike equipped with Shimano’s R785 hydraulic disc brake with 160mm rotors, and on the second, aboard a Giant TCR Advanced fitted with an Ultegra 6800 rim brake. There is no question that the hydraulic disc offered better braking, but it’s important to describe what we mean by ‘better’.
Control was Shimano’s watchword during the three-day press camp and the extra control offered by a disc brake when compared to a rim brake is undeniable. Power is a side issue: it’s already possible to lock a rim brake, and Shimano readily admit that to have designed an alternative simply to apply more stopping force would have been a pointless exercise.
The R785 disc brake offered 50 shades of speed reducing grey; by comparison, the Ultegra rim brake delivered little more than black or white. The modulation of the disc brake, what might be described as ‘feel’ and thus the additional control given to the rider at the lever, was far in advance of the rim brake. Progressive braking is at the heart of it: the ability to squeeze harder through a braking phase and increase stopping power, rather than applying all or nothing. If the least sophisticated rim brakes (and we certainly wouldn’t describe the Ultegra 6800 as such) behave like a conventional light switch – on or off – the function of a hydraulic road disc brake is more accurately compared to a dimmer switch.
The R785 disc brake offered 50 shades of speed reducing grey; by comparison, the Ultegra rim brake delivered little more than black or white
The 17km descent from the south side of Mount Etna has its fair share of long, straight sections of road that terminate in tight hairpins. Here, the progressive quality of disc braking was of greatest value: reducing speed by gently applying the front brake on the approach, before feeding in more stopping power by squeezing harder, and with both levers, as the hairpin tightened. On the faster, ‘open’ corners, when only subtle application of stopping power was required, the disc brake again held the edge, thanks to its superior modulation. At this juncture, it’s important to mention that both of our descents, with disc brake and rim brake, were conducted in dry conditions. We’d expect the value of the disc brake to increase exponentially in wet weather, where a rim brake places a rubber pad against a wet strip of aluminium or carbon. The typical solution – to ‘scrub’ the water off the rim in a pre-braking phase – is hardly satisfactory, and entirely obviated by the disc brake.
It’s also important to highlight that using the disc braking wasn’t an entirely unalloyed joy. They began to screech after about the fifth hairpin; a noise that became progressively louder until we decided to stop to complete a quick roadside inspection. Other riders at the press camp reported the same issue. Then there’s the weight penalty. Shimano’s men in Sicily were unable to place a precise figure on the additional burden carried by installing the R785 disc brake rather than the 6800 rim brake, but estimates varied between 300 and 400 grams. Our test bikes rolled on Shimano’s new, disc specific RX31 wheelset: a 30mm deep, aluminium hoop, with 24 spokes front and rear, and an estimated weight of 1800 grams. Heavier riders, those most likely to benefit from disc braking, particularly if using full carbon clincher wheels, conversely are also those most likely to benefit from lighter equipment.