The latest endurance focused, disc-equipped model from Decathlon’s in-house brand works well as a reliable winter bike or a sportive buster.
Built with 6061 aluminium of “variable thickness” in Decathlon’s words (ie butted tubes) the B’Twin won’t ever worry the UCI weight limit of 6.8kg but it’s certainly no porker with our sized medium test model tipping the scales at 9kg with pedals – a respectable enough weight for an alu frame with a mid-range build.
The inclusion of 12mm thru-axles and flat mount calipers mean it’s bang on trend. The geometry of the Ultra 520 is designed with comfort over long distances in mind so places riders in a more upright, maintainable position.
The 12mm threaded axles usually do a great job at stopping disc rub in our experience but we did need to do some tweaking on our first couple of rides to eliminate unwelcome squeaky noises.
Overall this is a good solid frame with the angular and square tube shapes adding lots of stiffness where it’s needed. The 27.2mm seatpost that comes with the Ultra 520 does a good job of adding comfort.
Stand out stoppers
This being the only disc equipped road bike in B’Twin’s road bike range there’s only one build option – but it’s a good one.
SRAM Rival Disc is a decent mid range option which is becoming ever more common in the adventure/gravel/cyclo-cross scene. The chainset feels rock solid even under high-power efforts, steep climbs and sprints.
Our one issue with the Rival groupset is the large shifters. Being designed for use with hydraulic disc brakes means that the addition of a reservoir for brake fluid turns what could be a nice ergonomic shifter into a big, solid box.
Watch: B’twin buyer’s guide
We struggled to grip the shifters when climbing out of saddle and found them digging into our hands after several hours of riding in thin gloves. The low pivot point on the brake lever means stopping power on the hoods is compromised slightly – fortunately not so much of an issue with powerful disc brakes. There is reach adjustment for those with shorter fingers but this only brings the levers further in and closer to the bars, not higher up towards the hoods.
Far and away the best feature of this bike is the disc brakes. If this is not intended to be a best bike but a good all-rounder, then the discs really add value. You can throw this bike around and into corners with confidence thanks to the impressive stopping power.
The masses of modulation and feel that come from discs removes all fear of feathering the brakes while leaning over, which on grabby rim brakes can quickly lead to a sticky situation.
The 50/34 compact chainset with a wide-range cassette is the perfect choice for this build. We were never left searching for more gears in our hours spent testing this bike. It would get you around all but the most extreme of terrains without bother.
Another fantastic addition to this bike is the Fizik Antares R3 Endurance saddle. While saddle choice is a hugely personal endeavour, we found the Antares R3 to be very comfortable.
Confidence inspiring ride
The ride was smooth and responsive, the B’Twin was stiff enough climbing out of the saddle and assured in the corners. However on steep sections of road and sharp accelerations out of corners it did feel a little sluggish compared to lighter bikes – unsurprisingly.
While the comfort level of an aluminium frame will rarely be on a par with a good carbon one, the B’Twin was very rideable and not harsh at all.
The bike handled well with the Deda RHM02 bar giving a good, aggressive position in the drops and of course the disc brakes are great.
The Mavic Kysrium wheels and Yksion tires are a reliable combination. The wheels rolled nicely and the new breed of Mavic tyres were assured and grippy on the leafy wet corners of the autumnal South Downs. All in all, a confidence-inspiring combination.
Classic Decathlon value
As expected from Decathlon you get great value from the Ultra 520 AF GF. It’s hard to find a place on this bike where money has been saved.
Its carbon seatpost and fork, mid-range groupset and solid wheelset combine to make a very competitively priced bike, even compared with all the online-only, direct-to-consumer brands.
A good solid winter ride that looks great but won’t wow like your summer race bike. It rolls well once it’s up to speed and certainly won’t slow you down. Disc brakes are fast becoming a must for any winter ride on the slippery UK roads and this bike does a great job of ticking that box while offering plenty of fun through the summer months and on those longer rides.