Polygon Siskiu D6 review

The Polygon Siskiu D6 is the brand’s budget-friendly, 120mm-travel cross-country mountain bike.

Boasting a relatively slack 67-degree head tube angle and fairly steep 75.5-degree seat tube angle, it sits between the trail bike and XC bike categories.

From the moment I jumped on the bike, it provided great all-day climbing comfort, thanks to the 572.7mm effective top tube. Combined with the head and seat tube angles, this meant climbing was neither stretched out nor cramped.

On the descents, it was fun to ride thanks to its modest wheelbase and nippy head angle.

However, the Suntour Raidon LO AIR shock had virtually no internal rebound damping, topping out every time it skipped over bumps. When contacted, the brand said this issue was isolated to this shock and the fault would be covered under the warranty.

While this issue was distracting, it didn’t detract from the bike’s overall descending performance.

Polygon Siskiu D6 frame and suspension

The clean lines of the Siskiu’s sleek hydroformed aluminium tubes make for a good-looking modern mountain bike. It has a linkage-driven single-pivot design, offering 120mm of travel.

Standover height is generous, with the low, downward sweep of the slim top tube.

Polygon’s Wheel Fit system sees sizes small and medium equipped with 27.5in wheels and medium to XL with 29in wheels to tailor the bike to the rider.

The front wheel is secured by a Suntour Q-Loc thru-axle, while the rear has Boost 148mm spacing.

The robust rubber chainstay protector and internally routed cables, complete with routing grommets, are a welcome addition at this price point.

A pair of bottle bosses are located on the upper aspect of the down tube for a bottle cage, but the shock’s horizontal position and down tube mounting point limit the available space for water bottles.

Polygon Siskiu D6 geometry

Polygon’s claims of modern geometry raise expectations, with the days of slack seat tube and steep head angles appearing to be on their way out.

The Siskiu D6 has a relatively climb-friendly seat tube angle of 75.5 degrees (size small) and a 67-degree head angle.

It has a 425mm reach and 1,133mm wheelbase, which should create a nimble and playful ride characteristic.

Polygon Siskiu D6 specifications

Shimano takes care of the stoppers and gears, with MT201 hydraulic disc brakes and 180/160mm Centre Lock rotors front and rear respectively. The 10-speed Shimano Deore drivetrain sports a 10-42 tooth cassette and 32-tooth chainring with KMC chain.

The second-tier Siskiu D6 is fitted with an SR Suntour XCR 32 Coil boost fork up front and a Raidon Air shock with a threshold lever, but no rebound adjustment.

Its double-walled alloy rims are wrapped in Deli Spiderbait 27.5x2.25in tyres front and rear.

Entity Components supplies the saddle, 760mm wide, 31.8mm handlebar (with zero rise, up and back sweep), and a 45mm zero-rise stem.

The bike is fitted with a TranzX 150mm dropper post.

The Siskiu D6 weighs 15.2kg for the small. While not light, it’s in touch with the competition.

Polygon Siskiu D6 ride impressions

I tested the Siskiu on a variety of terrain at Glentress trail centre, including rooty, technical off-piste trails and bike-park style trails with jumps and berms.

Testing in winter, the conditions were mixed, ranging from dry, hard frost in places to snow and mud.

Polygon Siskiu D6 climbing performance

With the saddle pushed forwards on the rails, the seated position was both comfortable and allowed for powerful and efficient pedal strokes.

However, the chunky headline weight sapped some leg power when accelerating up steep climbs.

The Shimano gears were reliable and responsive, with a reassuringly positive click at the shifter.

The 42-tooth cassette and 32-tooth chainring, while perfect for green, blue and red climbs, was a tough pairing for steep sections of the black trail centre climbs.

The ratios forced me to drive at a high pace to maintain good cadence and clear technical rock steps or steep sections.

However, pedalling up gentle fire roads and trail centre climbs at a moderate pace, the Siskiu was adequately equipped.

Having a 1x setup also opens up a much easier 12-speed upgrade than if it were fitted with a front derailleur and corresponding cassette.

Opting to run 20mm of stem spacers under the bars created a more relaxed, upright riding position given the reasonably low 571mm stack height.

Equally, riders who prefer a lower, more aggressive race position for climbing can slam the front end.

The short 45mm stem, which is unusually stubby for an XC bike, helps keep steering sharp.

The bar would benefit from a few degrees of back sweep at the very least, to increase comfort in the position of the wrists, elbows and shoulders.

Despite this, the length of the bike when seated felt spot on and easy to transfer upper-body weight over the front when climbing steps and trail features.

Polygon Siskiu D6 descending performance

With a short yet comfortable seated riding position that’s neither stretched-out nor cramped, the Siskiu has a nimble and playful nature, while hinting towards the stability and control of a more aggressive trail bike.

The rear shock’s lack of compression or rebound-damping adjustment wouldn’t be a problem on its own.

But, when combined with virtually non-existent in-built rebound damping, any time the bike went light over roots, drops and jumps, I could feel a harsh knock as the shock topped out.

This reverberated into my hands and feet, and was very distracting until I got used to it.

Although it’s a disappointing issue that lets this otherwise fun and hugely capable bike down, it didn’t affect the performance of the bike, nor my confidence to ride with speed over technical sections.

On the flipside, this lack of damping means it soaked up mild, rumbly trail chatter well.

Polygon says this appears to be an isolated problem with my test bike, and if it happened to a consumer, any repairs would be covered under warranty.

The Suntour coil fork was a more positive performer, absorbing bumps well without diving too deep into its travel or getting bogged down on multiple hits.

On blue and red-graded trails, littered with berms and jumps, the Siskiu was in its element. It felt composed through bermed turns and had plenty of pop on jumps.

The short wheelbase and small wheels gave it sprightly handling, adding to the sense of fun.

In good, grippy conditions, the Spiderbait tyres maintained traction well.

However, they were on the brink of letting go in wet mud when riding flat turns and across the off-cambers. More aggressive side knobbles would improve confidence.

The power and urgency of the Shimano stoppers was impressive on trails with a relaxed gradient, however their ability to kill speed on steep sections wasn’t quite as good as rival bikes with Tektro HD-M285 brakes.

The 150mm dropper seatpost insertion depth is limited by the bend in the seat tube.

This left only 100mm of the dropper’s travel to play with because so much of the post’s body was above the seat clamp.

Riders with shorter legs might struggle to get the seat low enough on the descents.

That being said, the presence of a dropper at this price point is a welcome addition and meant the whole ride flowed without the stop-start of manually adjusting a fixed post.

Polygon Siskiu D6 bottom line

The Siskiu D6 is a comfortable, well-priced and specced entry-level full-suspension bike that will serve you well on mellow forest trails and smooth green, blue and red bike-park style trail centres.

Some immediate upgrades, namely to a set of the best mountain bike tyres and a new handlebar, would improve riding comfort and performance, but the Siskiu D6 represents an overall sound purchase.