While XC bikes can easily sport 120mm of rear-wheel travel in 2024, that's also the figure given to Trek’s shorter-travel trail bike, the Top Fuel.
I’ve been testing this recently released trail ripper, in its 9.8 GX AXS build, which costs £6,500 or $6,999.99.
The latest iteration of the Top Fuel sees a slightly refined shape, with the usual longer, lower and slacker treatment given to the geometry, along with a two-position lower shock mount to offer more or less progressive suspension. There's also an updated frame storage door – among other tweaks.
The model I’ve been riding isn’t the top-spec bike, featuring alloy hoops and a SRAM GX AXS Transmission groupset.
However, there are some fancier parts bolted to the bike, such as the Bontrager RSL one-piece bar and stem combo, and Gunnison RSL tyres.
If you like your trail bikes short in travel and whippy in nature, you’re going to love the Top Fuel.
While it's not a quiver killer, with a few tweaks it could line up in the XC start gates, or tackle some seriously technical trails.
It’ll be a hard one to give back, that’s for sure.
Trek Top Fuel 9.8 GX AXS Gen 4 frame and suspension details
Trek’s OCLV carbon is, as you’d expect, used here (any Trek mountain bike with a ‘9.something’ name is made from carbon).
With smaller profiles and ‘softer edges’, the brand claims to have taken 220g off the weight of the previous-generation Top Fuel, with a slightly flexier frame also contributing to this weight loss.
The added flex is said to aid ride quality, too, with a more forgiving feel than the previous-generation bike, according to Trek.
The frame has an updated down tube storage access door – it’s larger and has better sealing from the elements.
Inside the carbon bike, you’ll find a padded BITS bag for tools and spares and an un-padded bag for a spare inner tube.
Internally, hose and cable runs are said to be less likely to snag on your bags.
There’s space for moderately large water bottles and longer dropper posts, and there’s no KnockBlock, which is great for those who love an X-Up.
The 120mm of travel is controlled by Trek’s signature ABP linkage. It’s a linkage-driven single pivot, in that there’s a single span from the main pivot to the rear axle.
There’s then a chainstay/seatstay pivot concentric to the rear wheel’s axle.
The idea is to blend the best pedalling and braking characteristics of a Horst-link and a linkage-actuated single-pivot bike.
The shock is mounted vertically, with the lower shock mounting to a four-position flip chip. This offers up more (19 per cent), or less (14 per cent) progression for the air shock, as well as doubling up as the High or Low geometry-setting chip – this is the Mino Link.
In the more progressive setting, the early stroke is softer and more supple. The less progressive setting is stiffer under pedalling.
Both offer the same leverage rate at the end of the travel, so big hits in both settings should provide the same amount of travel.
A longer-stroke shock can be fitted, boosting the travel to 130mm. The frame can also be run as a mullet (with small-sized bikes shipping as such), while it can fit a 120-140mm fork.
Trek Top Fuel 9.8 GX AXS Gen 4 geometry
The Top Fuel’s head angle is either 65.5 or 65.9 degrees, depending on whether the Mino Link is in its high or low position, while the seat tube angle sits at 75.9 degrees in the low setting – or 76.3 degrees in the high – on the size-Large.
Reach figures across the five sizes (small to extra-large) start at 417mm and stretch to 507mm.
The chainstays grow from 435mm to 444mm across the sizes.
| | S | M | M-L | L | XL |
|---|---|---|---|---|---|
| Seat tube angle (degrees) | 76 | 76.9 | 76.2 | 75.9 | 75.2 |
| Head tube angle (degrees) | 65.5 | 65.5 | 65.5 | 65.5 | 65.5 |
| Chainstay (mm) | 435 | 435 | 440 | 440 | 445 |
| Seat tube (mm) | 360 | 410 | 420 | 435 | 470 |
| Top tube (mm) | 559 | 588 | 612 | 631 | 673 |
| Head tube (mm) | 105 | 105 | 110 | 110 | 115 |
| Bottom bracket drop (mm) | 16 | 32 | 32 | 32 | 32 |
| Wheelbase (mm) | 1151 | 1188 | 1210 | 1227 | 1271 |
| Standover (mm) | 700 | 750 | 752 | 753 | 758 |
| Stack (mm) | 570 | 603 | 607 | 612 | 630 |
| Reach (mm) | 417 | 447 | 462 | 477 | 507 |
Trek Top Fuel 9.8 GX AXS Gen 4 specification
This is the third-tier model in the Top Fuel range.
It has Select+ suspension from RockShox – a 130mm Pike fork and a Deluxe shock. The fork features the latest Charger 3.1 damper and the shock has adjustability to its Open mode.
The SRAM GX AXS Transmission is joined by four-piston Bronze-level brakes.
Bontrager's Line Comp 30 alloy wheels are shod in 2.4in-wide Bontrager tyres, while the one-piece Bontrager RSL bar and stem adds a little flair to the build.
Trek Top Fuel 9.8 GX AXS ride impressions
I tested the Top Fuel in its natural habitat – tight and twisty woodland trails, fast and flowing trail centres, and a few chunky lines chucked in for fun.
Much of my testing was done trying out the two suspension positions, often repeating tracks and climbs in both settings to get a feel for it.
Setup
I settled on 28 per cent sag in the rear shock, in the more progressive setting. In the less progressive setting, I found sag levels weren’t hugely impacted, with around 2 per cent less sag given for the same pressure.
Given the two distinct feelings the shock progression settings offer up, I feel 26 per cent sag is a nice level for the more sprightly, less progressive, setting. If I was to own this bike, I doubt I would alter the sag levels often, while I might be tempted to change the progression more frequently.
I played around with the geometry settings a little, however, I settled as usual on the lower, slacker position, in both suspension progression settings.
I altered the tyre pressures depending on the conditions and location of my riding.
With the Gunnison RSL tyres relatively lightweight, I ran 24/25psi (front/rear, respectively) in rockier or drier conditions, dropping them by 1-2psi in more mellow terrain.
Up-front, the new RockShox Pike, with its Charger 3.1 fork, is a stellar option. No longer is a fork ‘fit, twiddle the dials to as open as possible, and ride’ – there’s adjustment range in there, which is worth playing around with.
I ended up with my low-speed compression roughly in the middle of the adjustment, high-speed compression a couple of clicks from fully open, rebound fairly fast – but not as fast as it’ll go – and one token in the fork.
How you set yours up will, of course, depend on your preferences.
Trek Top Fuel 9.8 GX AXS Gen 4 climbing impressions
It’s on the climbs and traverses that the most noticeable differences in the suspension-progression settings become obvious.
If it’s not clear, using the less progressive setting, the suspension is firmer in the early part of the stroke, meaning there’s more resistance to pedal inputs and less pedal bob.
This leads to an efficient, peppy feeling under power.
There’s a slight hint of pedal bob in the lower gears, although in the middle of the cassette this is almost completely tuned out – unless you’re up and out of the saddle, mashing at the cranks.
There’s still enough freedom of movement in the rear wheel to enable the suspension to bump up and over roots and rocks. However, if you want to give the Gunnison tyres the best chance of hooking up in technical terrain, it’s the more progressive setting you’ll want.
Here, the suspension feels noticeably softer, with the shock cycling gently through a few millimetres of its stroke as you turn the cranks.
The ride is more comfortable, though, with small bumps and square edges softened with ease.
However, if you’re looking for the plushest of rides, this sprightly 120mm-travel machine isn’t going to be your bag.
It has an energetic drive about it, and with that comes a direct feel that means you’ll still feel trail imperfections as you cross the ground.
In both suspension modes, should you wish for even more pep, there’s an easily reachable lockout lever on the shock.
I haven't come across the Gunnison tyres before, but they generally performed well in testing.
They have moderately deep, moderately spaced tread blocks. On smooth surfaces, they roll with little complaint – just a subtle hum on tarmac. And, on dirt, they hook up well, considering their fast-rolling nature.
In this respect, they’re similar to Maxxis’ new Forekaster tyres.
The Bontrager Line Comp 30 wheels, with their Rapid Drive 108 hubs, have a pleasingly quick pick-up, which I feel helps drive the bike well when picking your way up technical ascents.
The seated position is reasonable, although those looking for the most modern, aggressive shape might be left a touch cold by the 76.2-degree seat angle in its low setting (steepening by only 0.1 degrees in the high position).
Despite this, it’s a comfortable place to be. In the less progressive setting, where feasibly you’d want the most on-point climbing performance, it doesn’t seem to hold you back.
Trek Top Fuel 9.8 GX AXS Gen 4 descending impressions
Although not specifically a performance asset, one of the first things Rob Weaver commented on when he jumped on the Top Fuel was that it's incredibly quiet.
There are no rattles, twangs or clangs as you fire the bike down the trail. Even the high-engagement hub’s ratchet doesn’t emit undue noise pollution, leaving you with only the sound of dirt under the tyres.
Those tyres formed part of an impressive package when descending.
The Top Fuel is a trail bike built at the lighter end of the scale, so it can be forgiven for not having some of the most burly, puncture-proof, grippy rubber fitted.
The Gunnison tyres fit the brief well. The carcass is adorned with mid-depth, sharp blocks, spaced closely enough to give the bike quick acceleration, but far enough apart to ensure there’s an acceptable level of grip on all but the most scrabbly or slimy trails.
They’re flatter in profile than Specialized’s equivalent Fast Trak tyres, in my experience, but a little more rounded than Maxxis’ Forekaster, for example.
There’s no awkward transition to the shoulder tread as you initiate a turn, either, thanks to block coverage that goes from the centre to the edge.
They don’t feel particularly harsh, and I’ve pinged them off a number of rocks and roots that might have had me wincing with the most lightweight XC tyres around.
If you’re a real shredder, or want to take the bike to tracks that might be beyond the design brief (or, if you’ve fitted a longer shock to boost travel to 130mm), sturdier tyres might not be a bad idea.
Likewise, fit a 120mm fork and line up at an XC race and you might want to pop something skinnier on.
However, the Gunnisons suit the bike to a tee.
The suspension has a significant impact on the Top Fuel’s performance.
The RockShox Pike’s chassis is stiff, the early stroke forgiving and comfortable, and there’s tons of quality support in the mid and late stroke. It’s a cracking mountain bike fork.
When thinking about the rear suspension, as per the climbing, there are two distinct feelings.
The least progressive setting gives the Top Fuel its traditional XC-racer-like feel. It’s sprightly, reactive under pedalling and aggressive.
If you like your bikes to feed back, you like to ride them with plenty of input, and you don’t like any energy to be wasted, the early part of the suspension will suit you.
It’s not super-comfortable, but it will drive you forward off the backside of a roller, or pop you into the air if you drive your heels into the front side of a jump.
As a package, the bike works great in this setup. The suspension, tyres and lighter-weight brakes complement each other, giving the bike an exciting, fast-feeling ride quality.
Pop the lower shock mount into the more progressive setting and you can turn the Top Fuel into more of a true trail bike.
The early part of the stroke is softer and more comfortable, taking the edges off the hits.
Pumping through terrain results in a slightly more muted ride feel, and while the support is there, it’s deeper into its travel.
Hit a jump line and the bike will certainly do the job – remember, this is still a 120mm-travel bike, and so the suspension only has so much give before the support builds, but the difference is noticeable.
With its softer early stroke, the braking performance is improved. The rear wheel tracks the ground a touch better, helping the Gunnison dig in and deliver its braking traction.
However, if you have the bike in this softer setup, where you might expect more tech-focused riders will want that ground-hugging capability, the skinnier rubber, lightweight brakes and one-piece bar and stem start to become noticeable.
The brakes struggle on prolonged steep descents, the tyres are – ultimately – lightweight, and the bar is harsher than I’d opt for (as well as being unadjustable in terms of shape).
It’s worth noting that on bigger hits, the two leverage curves of the shock setups converge.
The Top Fuel deals well with big hits, regardless of which setting you’re in.
It is a 120mm bike – there’s no getting away from that fact – but there’s still a subtlety to its bottom-out performance that belies its travel figures.
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