Yes, the Polygon Siskiu D5 frame is top-notch but the parts spec is lacking. If it was £750 it’d be a steal but it’s outclassed by other £1k full sussers.

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Squint and it’s hard to tell the Calibre Bossnut and Polygon apart. Look more closely however and it’s evident that the frame finish on the Polygon, especially the front end, is one step ahead of the Bossnut.

Polygon Siskiu D5 review

Curvy hydroformed tubing and a low-slung top tube give the Polygon a much lees boxy, more modern profile than the Calibre. But it’s not just about aesthetics; the low profile top tube with its extended seat mast improves standover clearance while making it much easier to chuck the bike around out on the trail.

Given there similarity it’s hardly surprising that both bikes have similar geometry too, but the Polygon actually has as steeper seat tube angle, which places the rider’s weight more forward when seated, putting you in a much better position for climbing. Factor in the lower BB height for railing turns and the Polygon frame clearly has the upper hand.

Travel on the Siskiu D5 is balanced at 120mm front and rear so 10mm shy of the Calibre, but it’s right in line with the Jamis Dakar A2. But it’s here that the wheels start to fall off the cart.

Slick, modern frame has curves in all the right places

Suspension

It’s the only bike on test to come with a coil-sprung suspension fork. So if you are not of average weight you are going to need to find different strength springs to achieve the correct sag. Sounds simple, but at this price-point replacement parts for the 120mm travel Suntour XCM HLO fork can be nigh on impossible to find; so more often that not, you need to upgrade the fork.

Thankfully the Suntour Raidon LO shock is air-sprung, so you can adjust how firm the suspension is with nothing more than a shock pump. It also has a lock out lever for making light work of fire road climbs or extended sections of black top.

Strangely the shock doesn’t have an external rebound adjuster, so you can’t control how fast it extends after absorbing a bump. The fixed level of damping on the Raidon shock is very light, so heavier riders running higher air pressures to support their weight will feel like they are being catapulted from one turn or bump to the next. Which is a real shame as the Polygon is the only brand in this test to actually deliver the claimed amount of rear suspension travel.

Lanky 80mm stem contributes to unwieldy handling

Components

Hard-compound Schwalbe Smart Sam tyres roll fast and if you’re covering lots of miles they won’t wear out quickly. Get them on moist roots and rocks however and they make it incredibly difficult to keep the Polygon on track. The distinct lack of control compounded by the overly long 80mm stem.

Brands love to boast about how many gears a bike has, but when it comes to mountain bikes, fewer gears are actually better. Not only do 1x drivetrains reduce clutter and save weight, the narrow-wide tooth profile of the single ring helps keep the chain on. So while the 27-speed drivetrain on the Polygon sounds impressive, it’s sporting dated technology that’s unlikely to make a comeback.

Triple chainset is noisy, heavy and antiquated

Performance

By far the biggest frustration with the build kit on the Polygon was the triple chainset. The chain would constantly clatter on the frame and derail on all but the smoothest trails. Even then you where still at risk of dropping the chain if you inadvertently let the cranks spin backwards when you stopped.

Frustrating as that was, it’s the suspension performance, or more specifically the distinct lack of damping, that makes the Polygon ride and feel like a much cheaper bike.

The fork and shock are both sensitive and smooth, but the lack the damping control that is necessary to calm pedal induced bob and counter rider movement just isn’t present. As such, the Polygon feels like a hobbyhorse rocking forwards and back as the rider tries to remain balanced and composed. As such the Polygon cannot be ridden anything like as hard as the three other bikes in test. It’s also the heaviest bike in test by quite some margin, mostly due to the additional weight of the coil sprung fork and triple chainset.