GT Sensor 9er Pro

These days there’s no shortage of 29er bikes on the market, however the majority of them are pegged at the cross-country crowd, with steep-ish geometry and travel that tops out around 100mm. That’s all starting to change however, with the brave few pushing big-wheeled bikes into the hands of more aggressive riders by offering slacker geometry and more travel than previously thought viable for a 29er.

The Sensor 9er Pro from GT is one of these bikes, offering 120mm of travel front and rear and a reasonably slack (for a 29er) 69.6-degree head angle; its full intention is to tempt riders away from their 130-150mm travel 26-inch bikes and into the world of big wheels. One of the strongest temptations is certainly value for money, with Fox suspension at both ends and a full XT drivetrain, combined with Formula’s RX brakes, seeming almost too much bike for the $3,699 asking price. There’s also a cheaper ‘Elite’ version available with X-Fusion and Marzocchi suspension, a basic FSA/Shimano Deore drivetrain and Avid Elixir 1 brakes for $2,399. Our large-sized test bike weighed 12.68kg without pedals, of which 3,016g was comprised of the frame and shock; not bad at all for a well-priced longish longish-travel 29er.

The Sensor 9er’s ‘Speed Metal’ (aka 6061) aluminium frame uses a combination of post-dressed and standard welds and there’s nothing wrong with the overall construction quality. GT have made a conscious effort to keep things colour-coded with plenty of blue anodised bits to match the frame decals and this works to an extent, although depending on your tastes it could be a little over the top. Definitely a case of form over function is the seatpost quick release, which wraps so far around the post that it’s difficult to both open and close; there’s a reason why the majority of clamps use a relatively straight blade, and that’s because it works. The stem top cap is meant to be cool-looking, but in truth it’s reminiscent of a miniature circular saw blade, and almost as sharp. Do what we did and get this thing off the bike before you ride, lest part of your anatomy suffers the unpleasant consequences.

The Sensor 9er’s cable routing looks relatively clean and it’s good to see full length outer casing on all cables to keep things shifting smoothly in adverse conditions. Unfortunately all three of the cables rubbed heavily into the seat tube of our test bike, and without disassembling the bike it’d be difficult to put frame patches where they need to be in order to adequately protect the frame (you’d be replacing them frequently, too). Without some effective frame protection, we could easily imagine the seat tube incurring some serious wear within 12 months.

I-Drive, I-Like

GT have been using their patented I-Drive suspension system, in various forms, for over a decade. It’s been constantly refined to the point where it performs as well as any other suspension system on the market, and better than many. The high main pivot gives the back wheel a very pronounced rearward axle path, which provides the Sensor 9er with about the best square-edged bump absorption you’ll find. Combine this with the excellent roll-over performance of a 29-inch wheel and the big GT is virtually unstoppable when it comes to rough terrain.

The other side of the GT’s suspension is that, because the bottom bracket ‘floats’ between the front and rear triangle, there’s minimal chain growth and pedal feedback, so even under power the back wheel keeps on sucking up the bumps without unduly interrupting your pedalling rhythm. Plenty of people in forum-land bag the I-Drive system without having ever ridden it; take one for a spin and, understand it or not, you’ll become a firm believer in the quality of this design. On buff trails it doesn’t really have the chance to show its full potential, but in the rough it truly shines.

Unconventional Design

It’s safe to say that 29-inch wheeled mountain bikes are still in their developmental phase. They’ve changed a lot from the early generation bikes that were built before designers and manufacturers had figured out what works and what doesn’t. This is even more the case for bikes like the Sensor 9er, which are forging into new territory in terms of 29er wheel travel and geometry.

Whilst all bikes and designers will have a slightly different set of priorities, there are a few things that seem universally agreed upon when it comes to bikes with big wheels. Firstly, handlebar height and head tube length needs to be kept down in order to compensate for the longer axle-crown length of 29er forks. Secondly, oversized frame tubes and tube junctions are your friend when it comes to countering the higher lateral and torsional forces applied by bigger wheels. And thirdly, nothing boosts rear end stiffness like a thru-axle.

It’s curious then, that the Sensor 9er sports none of these things; the quick release drop-outs and spindly rear triangle seem quite out of place, and no doubt contribute to the significant twist and flex you can feel both off the bike and once you get it on the dirt. It’s at the front end, however, where things really buck the trend. The head tube measures 140mm, which makes it 20-30mm taller than most other 29ers of this size. Compounding this, GT has spec’d a headset with a 15mm top cap rather than the lower 5mm option, and instead of using a tapered head tube and fork to increase stiffness at both the head tube/down tube junction and the crown/steerer interface, they’ve chosen to go with a straight 1⅛-inch steerer and 44mm inner diameter head tube. The icing on the cake is the 90mm long high-rise stem.

Of course the stem can be flipped to create a super-steep ‘negative riser’. Whilst this helps to lower the bars somewhat, we can’t help thinking that the need to take such measures could have been alleviated through the use of commonly adopted 29er design wisdom. The overall result is one of the flexiest current-day frames we’ve come across, which isn’t helped any further by the narrow (18mm internal) rims which don’t offer much in the way of lateral support to tyre sidewalls under cornering loads.

Flexible Fun

If all this is sounding a bit dire we want to assure you that, flex issues aside, the Sensor 9er is still very much a fun bike to ride. The relaxed geometry combined with its ability to absorb all manner of bumps whether uphill, downhill or on the flat means that ripping around on the GT is still more enjoyable than on many other 29ers. The Sensor 9er was never intended to win World Cup XC races, nor to be the bike to out-Strava your friends on the local hill climb, but instead it’s about simply going for a ride, feeling the flow of the trail, tackling some challenging terrain both with and against gravity, and doing it all with confidence and comfort.

The Maxxis Aspen tyres roll quickly on hardpack and grip surprisingly well on loose trails too, and their cornering grip, especially when combined with the longer contact patch of the big wheels, far exceeds their relatively benign appearance. The RX brakes may be the base model in Formula’s range, but they’ve got no shortage of power and once you get used to the ‘Formula feel’ they’re easy to modulate, and ours didn’t require any adjusting throughout the test. As you’d expect the Shimano drivetrain worked flawlessly, and the 26/38 dual ring combined with the 11-36T cassette provided a sufficient gear spread (although some may desire the slightly broader gear range offered by a triple).

Technical climbs don’t generally trouble the Sensor 9er, especially if you can come into them with a bit of speed; as mentioned the big wheels paired with the supple, rearward arching I-Drive suspension make it a cinch to maintain momentum over the roughest of climbs, and there’s every chance the GT will have you cleaning climbs you once thought impossible. The only problems arise if you have to apply sharp, ratcheting power strokes where you’ll feel the frame bending and sapping your power as well as making it difficult to change direction mid-climb.

The steep seat angle helps to keep your weight forward and the front end tracking on grinding ascents, although it does make the cockpit a little shorter, so if you’ve got a long torso you may feel a little cramped on really steep climbs. There’s little need to apply high levels of ProPedal on the rear shock because the I-Drive suspension generally pedals very well, although using one of the lighter settings is appropriate if you like a firmer platform to push against or to keep the bike from squatting too much when climbing.

The cornering traction was great, so there was plenty of temptation to lay the Sensor 9er over, but due to the flex issues mentioned (as well as tyre roll) it doesn’t hold a line through corners as well as it could, and the same applies for off-camber sections and rock gardens. In these situations you just have to let the bike have its head and accept that it’ll follow your general directions but without a great deal of precision; thankfully the better roll over of the 29-inch wheels provides a bit of extra tolerance for less than ideal line choices.

Where the Sensor 9er really comes into its own is on descents; I spent a few hours one afternoon chasing 26-inch DH bikes down one of our local runs, and they simply couldn’t pull away from the GT. Admittedly a full-blown downhill bike is overkill for this trail, but nevertheless the Sensor 9er showed that big wheels do have an advantage in maintaining rolling momentum on rough descents (of course a World Cup-level rock garden would be another matter!). The relaxed front end feels confident and controlled on steep chutes and the front and rear suspension balance is excellent, with no sense of wallowing in the mid-stroke and enough progression to use all the travel on bigger impacts without any harsh bottom out; well, mostly. Because the Sensor 9er descends so confidently, it’s easy to forget you’ve only got 120mm of travel available. Whilst bigger wheels help to smooth out trail chatter and small bumps they don’t make any difference when it comes to absorbing drops and heavy landings. Lesson learned; if you wouldn’t do it on a 26-inch wheeled bike with 120mm of travel you shouldn’t treat a 29er any differently!

When all is said and done, the GT Sensor 9er Pro is a bit of a love/hate beast. Love? The outstanding rear suspension, highly capable descending ability afforded by the relaxed front end, and excellent value for money. Hate? The amount of flex inherent in the frame design, poor cable routing, and questionable wheel, stem and headset choice. As it stands the Sensor 9er is a fun bike to ride and is a welcome break from the cross-country race 29ers that we’re becoming familiar with, but it needs some tweaking of the frame design, cabling and components to really unleash its full potential.

Thumbs Up

Excellent suspension

Stable geometry

Value for money

Thumbs Down

Tall handlebar position and front end

Frame flex

Specifications

Frame: Hydroformed Speed Metal Alloy

Shock: Fox RP23 120mm travel

Fork: Fox 32 Float 29 RL FIT 120mm travel

Headset: FSA Integrated 1 1/8

Handlebars: Crank Brothers Cobalt 2, 700mm

Stem: Syncros AM, 3D Forged

Shifters: Shimano XT

Front Derailleur: Shimano XT

Rear Derailleur: Shimano XT

Cassette: Shimano HG81 11/36 10-speed

Chain: KMC X10

Cranks: Shimano XT 26/38

Bottom Bracket: Shimano XT

Pedals: N/A

Brakes: Formula RX

Rims: Alex XD-Lite

Spokes: DT Swiss 1.8/1.6 butted w/alloy nipples

Hubs: All Terra Alloy Sealed Bearing

Tyres: Maxxis Aspen 2.1

Saddle: WTB Silverado Race SL

Seatpost: Crank Brothers Cobalt 1

Weight: 12.68kg without pedals (Large frame 3,016g)

Available Sizes: M, L (tested), XL

Price: $3,699

Distributor: Cycling Sports Group (02) 8595 4444 / www.gtbicycles.com