Monster Brompton Challenge: Turning a Brompton in to a folding mountain bike

Brompton bikes are an icon of British cycling, and the compact folding design has made them a staple of the city commuter. They are, however, definitely built for smooth tarmac and tame environments - something that Alex Paton feels needs to change. And so he thought, why not convert a Brompton into a mountain bike?

We might not have the same vision for the humble Brompton that Alex does, but the idea of a mountain bike Brompton intrigued us all. In the first part of this two-part challenge, Alex is taking on the first step of his mission: working out how to convert a folding bike into one capable of tackling the trails.

What is being upgraded?

It might actually be easier to list the things that are not being changed on the Brompton, rather than what is. Alex certainly isn’t going about doing things by halves. The bike is getting completely overhauled, including new wheels and knobbly tyres, disc brakes, new gearing, a new rear triangle, and new forks. These changes retain the DNA of the original bike but with a far more ruggedised flavour.

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The classic Brompton is as far removed from an off road machine as bikes come

For those of you wondering why exactly so much of the bike needs to be swapped out, it comes down to compatibility. The super-wide knobbly mountain bike tyres simply do not fit in the existing fork or rear triangle. Equally, the bike comes equipped as standard with rim brakes, but these wouldn’t work with the new tyres, so to fit disc brakes Alex needed to fit a compatible fork and rear end.

The upgrade to the wheels provides the perfect opportunity to fit a wider spread of gears to make riding off-road that little bit more manageable. One of the unique features of Brompton bikes is that they use internal hub gears rather than a traditional external cassette. As hub gears are an integral part of the wheel, changing them out typically requires a complete wheel strip and rebuild with the new hub or buying a prebuilt wheel with the hub gears that you want. In this case, Alex has upgraded the MTBrompton from the conventional 3-6 speed offerings to a rather on-trend 11-speed affair.

• Read more: Restoring the Trek Madone 5.9 – Alex Paton's dream bike

An added advantage of the new rear triangle is that it increases the wheelbase of the bike. Bromptons have a very short rear end, partly because of the size of the wheels but also to allow it to fold down into the smallest footprint. What this means is that the riding position is not conducive to control on steep gradients. The modification to lengthen the wheelbase puts the rider in a more neutral riding position between the wheels.

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The finished product still resembles the original bike but almost all the major components have been upgraded

After all of these modifications only one thing needed to be tested to ensure the bike still held the credentials essential of any bike bearing the Brompton name. Does it still fold? The overall footprint has grown somewhat, mostly due to the 20-inch wheels and 2.4-inch wide tyres that have been upped from 16-inch wheels that come as standard on a Brompton, so you'll have to watch the video to see if it can still be packed down.

If you want to see Si and Alex put the monster Brompton through its paces keep your eyes on the GCN YouTube channel.