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Bike Check: Fairwheel Bikes’ 5.1kg Factor O2 disc brake road bike

Crazy lightweight disc-equipped race bike weighs just over 5kg

We’ve reported on some extremely lightweight road bikes over the years, but this 5.1kg (11lb) Factor O2 from US bike shop Fairwheel Bikes is definitely one of the lightest disc-equipped bikes we’ve ever seen.

But it could have been much lighter. With a different crankset, carbon disc rotors and a change of tyres, the weight could have been 4.76kg according to Fairwheel. Erring on the side of caution and durability, the bike shop instead wanted to build a bike that was as light as possible whilst still being robust and race-worthy.

- 6 of the lightest road bikes — bike makers challenge the scales with exotic materials


Fairwheel Bikes is no stranger to building one-off custom road bikes that barely get the scales moving, but this latest project was all about trying to build the lightest disc brake road racer. We think it’s fair to say they certainly achieved that.

“The goal for this isn’t necessarily to have the worlds lightest, but rather something that is very light but still race worthy.  Actually, it might be both.”

Obviously, if you’re going to build a lightweight bike you need to start with a light frame. The Factor O2 was chosen in a small 52cm size and it weighed 853.2g on the scales. That’s a good starting point.


The fork with a cut steerer tube added 352.1g, and further weight was saved with a Cane Creek AER headset while Tune Magnesium spacers replaced the stock carbon spacers because they come in at just 8.9g apiece.

SRAM’s eTap HRD wireless groupset was chosen and matched with a THM Clavicula SE 300g crankset with an HSC ceramic bottom bracket, which added just 68.1g. The standard SRAM chainrings were whipped off and replaced with a Carbon-ti 50t big ring and Fibre Lyte 34t inner chainring.

The chain and cassette are far from stock as well. A YBN Titanium chain was chosen for its 215.6g weight while the Recon AL 11-25t cassette with its 150.4g weight saves on the mass - though a SRAM Red 11-25t cassette has a claimed 151g weight so it’s a small saving. But at this level, it’s all about the marginal gains…

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The disc brakes came in for some weight saving attention too, though I’m not sure if I’d want to save weight on the brakes personally. Ashima Ai2 rotors weighing just 114.3g and even the rotor bolts, Custom Rainbow Ti, saved weight compared to the stock setup.

You can save a load of weight with the right wheel choice, and here Fairwheel really went to town. Extralight SPD hubs (64.9g front, 145.4g rear) combined with FSE prototype rims (442g/pair) and Pillar Xtra Titanium spokes (150.4 for both wheels) and 17.7g worth of Pillar nipples ensure a very lightweight wheelset. A pair of Vittoria Corsa Speed tyres added 377.2g.

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Onto the contact points and a 146.6g Schmolke TLO handlebar combined with an 85.1g THM Tibia stem, Lizard Skins DSP tape, Schmolke 78.3g seatpost and Gelu 46.5g saddle. All very lightweight indeed.

You can view the full list of components and weights here. 

What’s the bike like to ride? We’ll never know, partly because it’s an expensive trip to the US to find out but also because at 52cm the bike will never fit anyone on the tech team. It does show how crazy attention to detail, and deep pockets, can really shave away loads of weight.

What do you think? Is it a step too far or should more bike manufacturers be aiming for this sort of weight? 

David worked on the tech team from 2012-2020. Previously he was editor of and before that staff writer at RCUK. He's a seasoned cyclist of all disciplines, from road to mountain biking, touring to cyclo-cross, he only wishes he had time to ride them all. He's mildly competitive, though he'll never admit it, and is a frequent road racer but is too lazy to do really well. He currently resides in the Cotswolds, and you can now find him over on his own YouTube channel David Arthur - Just Ride Bikes

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