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Road Bike Disc Brakes- Does bigger mean better?

Disc Brakes
Stu Bowers
9 Jun 2015

The different shapes and sizes of disc brake rotors are not just to make them look pretty.

Size matters. When it comes to the rotors on disc brake systems, the larger the diameter the greater the potential braking force (just ask Isaac Newton). What’s more, a bigger diameter usually means a greater surface area, which helps with heat dissipation. Why, then, aren’t we using dinner plate-sized rotors on our shiny new disc brake road bikes? The main reasons are weight and aesthetics.

‘This was a real problem initially for the road disc brake market,’ says Alan Weatherill of UK-based component manufacturer Hope. ‘People wanted light and neat [eg, 140mm rotors], so big steel rotors didn’t sit well, but you have to be safe, and the potential heat build-up on long road descents is a bigger problem than for mountain biking, and it proved the biggest challenge to overcome.’

A balance needs to be struck, which explains why most disc brake road bikes come fitted with 160mm rotors – not too big, not too small. There’s also a reason why they are not simply solid steel. The cut-out shapes and drillings you see on the rotors aren’t just for aesthetics (although some do look very pretty) – they perform a crucial role for the braking system and are a big part of why disc brakes work so well in wet and filthy conditions.

‘A solid disc would simply trap water or dirt between its surface and the pad,’ Weatherill says. ‘The holes provide spaces for the dirt and water to escape through, and also their tiny sharp edges score the surface of the pad to prevent glazing [when the pad surface gets highly polished by the disc, which leads to poor braking]. Ideally, at some point, every bit of the pad should come into contact with a hole [or edge], or you’d get glazed stripes forming. Reducing weight is just a positive by-product.’

Disc Brakes are becoming more and more popular on the road cycling scene. Here are a few reviews cyclist have done of bikes with them on:

Cielo Road Racer Disc Review

Giant Defy Advanced SL 0

Cannondale Synapse Hi-Mod Review

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