The Takeaway: The fourth-generation Top Fuel completes this model’s evolution from an XC race bike to a true light trail mountain bike. It is a well-mannered and high-performance bike that, more than many in the category, offers a balanced mix of climbing speed and descending capability. It has no weaknesses, but its greatest strength is its balance, which makes it feel somewhat staid and, perhaps, as though it is still searching for its personality. Even so, the Top Fuel never puts a tire wrong and is up for tackling almost any climb or descent.
The Top Fuel was Trek’s premier full-suspension XC race bike several years ago. But starting in 2019, Trek began to transform the Top Fuel into a lightweight trail bike, bumping up the travel and relaxing the geometry but keeping some XC-like features like a remote lockout.
The third generation Top Fuel was launched in 2021 with more travel—120mm rear travel and a 120mm fork—even more relaxed geometry, in-frame storage, and Trek eliminated the remote lockouts. But 2021 was also when some brands began to debut XC race bikes with 120mm travel and slacker geometry in response to rowdier courses. (Scott’s Spark is a good example of this trend, as is the recently launched Specialized Epic 8).
Trek, however, went a different direction and positioned its 120mm bike as a light trail bike and debuted a new, shorter-travel full suspension Supercaliber for XC racing. And the third-generation Top Fuel quickly found itself in a sort of no-bike land. It was intended, designed, and equipped as not an XC race bike while XC race bikes from competitors arrived with travel and geometry similar to the Top Fuel.
In our 2021 review of the Top Fuel, my colleague Dan Chabanov, in addition to dinging the bike for being a bit heavy (compared to XC race bikes like the Spark), said, “Trek does say that the Top Fuel is compatible with a 130mm fork, and I can’t help but think the new Top Fuel would have made a lot more sense with the increased travel straight from the factory.”
Trek agreed, apparently, and after one model year, Trek began shipping the Top Fuel with a 130mm fork. This pushed it more into the trail bike category and helped place daylight between it and the new-school XC race bikes.
For the fourth-generation Top Fuel, Trek maintains the fast trail target. Trek retains the Top Fuel’s 120mm rear/130mm front travel while building in refinements to make the bike more dialed and broadly versatile.
Features and Details
The list of updates to the Top Fuel includes a significant frame-weight reduction. The aluminum and carbon frames are about 220 grams (almost a half-pound) lighter.
A weight reduction is always appreciated, and, built like for like, the new Gen 4 Top Fuel would be lighter than a Gen 3.
However, compared to the Gen 3 initially launched in 2021, Trek pushed the Top Fuel more into the trail category and added heavier components like longer travel, stiffer forks, and more aggressive tires. There’s also the added weight of SRAM’s Transmission compared to SRAM’s standard-mount-derailleur drivetrains.
As a result, even with the lighter frame, the Gen 4 Top Fuel’s overall bike weights are up compared to the Gen 3 Top Fuel that launched in 2021. According to Trek’s claimed weights, the lightest Gen 4 Top Fuel, the top-of-the-line 9.9 XX AXS model, weighs 28.3 pounds in a size medium: The Top Fuel we reviewed in 2021 in an extra-large weighed 26.8 pounds on our scale.
While it has gained weight due to its added capability, it is noteworthy that a Gen. 4 Top Fuel is three-plus pounds lighter than Trek’s longer-travel (140mm rear, 150mm front) Fuel EX. That alone should make it a more compelling trail bike option for many riders.
A striking detail buried in the tech information shared with the press was that the Trek product team made the new Top Fuel’s frame less stiff. Reducing stiffness from one generation to the next is not a typical move. However, it is possible to make a bike too stiff. And that has many drawbacks. In this case, Trek says that reducing stiffness makes the new frame “more balanced and forgiving” than the previous generation and helps reduce frame weight.
For many years, Trek’s mountain bikes have featured a geometry-adjusting flip chip called Mino Link, and some of its more recent mountain bikes have featured a flip chip that makes the suspension more or less progressive.
In the new Top Fuel, Trek combined the two flip chips into one four-position chip that alters geometry and progression (14 or 19 percent progression in this bike’s case). It is a feature likely to make its way into more Trek mountain bikes in the future.
On the theme of adjustments, Trek pitches several approved variations of the stock setup.
If 120mm rear travel isn’t enough for your needs, this Top Fuel will accept a 185x55mm shock (stock is 185x50mm), bumping wheel travel up 10mm to 130mm rear. To round out what would be a more gravity-oriented Top Fuel build, Trek officially sanctions the use of a 140mm travel fork and a 27.5-inch rear wheel for all sizes except the small. Small bikes run 27.5” wheels front and rear to give their riders “a more proportional fit and easier handling” and are not compatible with a 29-inch rear wheel.
But if you're more XC-oriented, the new Top Fuel is also officially approved for use with a 120mm travel fork. The brand says its top XC racers may use the Top Fuel with a 120mm fork and lightweight build instead of the shorter travel Supercaliber as courses and conditions merit.
Trek does not sell the Top Fuel as a complete bike in the 130/140mm gravity(ish) build or the 120/120mm XC build. Unfortunately, altering a stock 120mm rear, 130mm Top Fuel into either variant is not simple.
While the stock forks can be bumped up or down in travel with an air shaft, the swap requires tearing down and rebuilding the forks. In addition, the gravity build requires a new shock (the stock shock stroke cannot be increased), a new 27.5-inch rear wheel, and a new 27.5 tire. Based on my testing, a gravity-built Fuel EX will also need more powerful brakes. Riders who want the more XC-oriented Top Fuel will likely wish for lighter parts, particularly wheels and tires.
Because of these hurdles, I suspect that riders who definitively know they want either the XC or gravity-flavored Top Fuel are likely to buy a frame and build the bike from the ground up exactly how they want it. And here is a good place to mention that the Top Fuel is compatible with a wide range of shocks, including RockShox’s Flight Attendant, Fox’s Live Valve Neo automatic electronically controlled shocks, and even coil-over shocks.
But for all the riders who buy a complete Top Fuel, the option to morph it into a more XC or gravity-oriented bike is there, though it will be costly.
Rounding out the suspension updates, the new Top Fuel features a touch more anti-squat, which should make it feel slightly crisper when the rider pedals.
The in-frame storage gets a polish, with improved sealing, a larger opening in the down tube, and some refinements to the routing tubes so the storage bags slide in and out more easily.
Trek is also debuting improved bags for its in-frame storage. Previously, they had one neoprene bag for tube and tools. That one bag is replaced with two: One unpadded bag for a tube and one padded bag for tools (the padding reduces the chance of tools rattling against the frame). These new bags come with all 2025 Trek bikes with in-frame storage (carbon frames ship with both bags, aluminum frames only get the tool bag) and are also available for purchase.
And finally, Trek relegated the Gen 3 Top Fuel’s Knock Block steering stop system to the dustbin of history.
Geometry
You’ll find frame geometry pasted here for your enjoyment. Trek sent me five different geometry tables, but I’m only pasting the one that details the complete bikes—120mm rear, 130mm front, flip chip in the low position, 29” x 29” wheels (size small has 27.5” x 27.5” wheels)— as they come out of the box.
Trek’s site will have all the variations, but essentially, changing the flip chip to the high position steepens the angles and raises the BB; swapping in a 140mm travel fork slackens the angles and raises the BB; and with a 27.5 rear wheel and 140mm fork, the bike has a 64.6-degree head angle, 342mm BB height, and effective seat tube angles that are about a degree slacker than the out-of-the-box geometry.
Adding a fifth frame size is the most significant geometry change from Gen 3 to Gen 4. Like some other Trek mountain bikes, the Top Fuel now has an ML frame size that fits between the medium and large.
Another notable change is the adoption of size-specific seat stay lengths. Sizes S and M have 435mm stays, ML and L run 440mm stays, and the XL gets 445mm stays (all sizes of the previous generation Top Fuel ran 435mm stays).
Many sizes get a seat tube angle adjustment: the small is half a degree steeper, the medium is 1.4 steeper, the large is 0.3 steeper, and the XL is 0.4 slacker.
Finally, the seat tubes are shorter—the XLs by a whopping 30mm—and there is more dropper post-insertion depth.
Builds, Prices, Claimed Weights
While Specialized tends to introduce its new bikes in high-end carbon versions, with lower-priced aluminum versions coming later, Trek, in its usual practice, is introducing its full line of Top Fuel models all at once.
The full range of seven models starts at $2,700 for the aluminum-framed Top Fuel 5 and ends at the $10k-plus 9.9 XX AXS with a carbon frame.
Regardless of price or frame material, all models have internal frame storage, fully guided hose-and-housing routing, a 12-speed drivetrain, tubeless-ready wheels and tires, lock-on grips, and a dropper post. All except the least expensive model have four-piston brakes (the Top Fuel 5 has two-piston hydraulic calipers).
Ride Impressions
Due to the late arrival of my test bike, I don’t yet have as much time on the new Top Fuel as I like before writing a review. But I’ve gotten in some good rides on some of my most familiar trails, so I feel I have a good sense of its performance and character. Even so, I will continue to ride it and update this review if my impressions change.
In the Notes From The Field section below, I’ve outlined my journey to a good fork setup, plus my feelings on the stock brakes. My impressions here are based on how the bike rides with the fork dialed in and a larger front rotor.
The 120 rear/130 front (ish) light trail category this Top Fuel resides in is lousy with amazing bikes. A quick list off the top of my head and in no particular order: Evil Following, Ibis Ripley, Pivot 429 Trail, Specialized Epic 8 Evo, Spot Ryve, Yeti SB120, Giant Trance Advanced, and the Santa Cruz Tallboy.
That is some stiff competition, but I think Trek made a bike that competes well against this strong field.
The expectation for this style of bike is that it as fast on the climbs and flats as an XC bike and allows the rider to go full send on the descents like a trail bike. That is impossible. So, while everyone is seeking this holy grail of speed and capability in one, many bikes lean one way or the other. For example, the Epic Evo is more XC, while the Tallboy is more Trail.
The Top Fuel, however, feels more equalized. No, it doesn’t climb like an XC bike and descend like a trail bike—again, impossible—but it doesn’t seem to be lean one way or the other, like many of its competitors. It feels balanced: equal parts quick and capable.
That results in a handy bike, no matter the terrain or direction of the slope.
On smoother climbs, the suspension is quiet and efficient. There’s little unwanted motion, and upping the cadence is rewarded with eager thrusts. It is not as quick-feeling or firm at the pedals as some four-bar systems—dw-Link, most notably—but the Top Fuel is far from sluggish.
I will note that I ran the shock with 30 percent sag, the maximum Trek recommends. Traction is a challenge on many of my trails, so I usually prefer my suspension softer off the top. But the low end of the advisable sag range for this bike is 20 percent, and with less sag, the bike will feel more zippy.
But even with 30 percent sag, the rear end is supportive and holds the rider in a good position when the climb gets steeper. And when the climb gets chunky and technical, the rear end is sensitive and offers great traction.
On descents, the Top Fuel is composed and surprisingly confidence-inspiring, and I experienced little bucking or jarring deflections off my chosen line. Chunder, drops, jumps, gaps, and sketchy loose chutes: The Top Fuel telegraphed it was game for all of it. The rear suspension has a lovely tune that provides access to its full travel. It has plenty of bottom-out control and good sensitivity, too.
Nothing particularly stood out about the Top Fuel’s handling as I climbed and descended. It felt…normal, I guess? I wasn’t fighting the bike and didn’t feel I needed to adjust to work around any quirks.
It worked its way through my tightest, steepest climbing trails well. The Top Fuel handled downhill corners of all varieties intuitively. It flew true as an arrow on the fastest straightaways. I could change its direction at will and with little resistance. The bike felt like I could always position it exactly where I wanted and hit my lines accurately.
But for all of its impressively well-rounded performance, I didn’t sense an extra-strong “fun” vibe from the Top Fuel like I get from the Evil Following. In that way, Top Fuel perhaps lacks a little. It is so balanced that it is—while very far from boring—a rather staid bike.
I wonder if some of this is due to the bike’s weight. My ML size, XO AXS model weighs 29 pounds on my scale, which is only 1.2 pounds lighter than Specialized’s Stumpjumper 15 I recently reviewed. The SJ15 has more travel, more adjustments, a unique and fabulous rear shock, sticker tires, more powerful brakes, and is much better on descents and rough trails.
The Top Fuel does feel more lively, rolls faster, and is a bit quicker on climbs than the SJ. But some fast trail bikes like the Trek and the 29.4 lb. Yeti SB120 presents a conundrum because they’re not much lighter than bikes with the next jump up in travel.
To me, the deciding factor is feeling. Do you want a bike that feels snappier and climbs with a bit more pep, or do you emphasize descending speed and confidence? Personally, I prefer shorter-travel bikes. I like the snap and pep of less travel and to feel the trail under my tires. However, another large part of that preference is due to the shape of my trails. I can easily understand how a rider in a different locale would go for more travel.
Choices are good, but they can also be confusing. My best advice: if you're not sure what you want, borrow and demo as many different bikes and different travels as possible.
My time on the Top Fuel so far has left me with the impression that this Top Fuel is a superb bike that isn’t extraordinary. A bike I know will perform brilliantly on most trails, and I am happy to ride it, but it also leaves me wishing for something more from it, even though I can’t pinpoint what more I want.
Non-specific whinging aside, the fourth-generation Top Fuel is an excellent light trail bike that can compete with the best on the market.
Notes From The Field
Random observations and reports from my time testing the bike.
• I appreciate that Trek gave the Top Fuel internal storage AND a cargo mount under the top tube. Having both offers the rider more options for their preferred tube/tool/cargo setup. In my case, I put a (butyl) tube and flat repair kit with CO2 and Dynaplug Racer Pro inside the frame and used the cargo mount for an inline OneUp EDC pump mount, which I fitted with the 70cc pump with an EDC tool inside.
• This Top Fuel offered my first chance to ride the Trek’s updated tire offerings. And they’re pretty good. The Gunnison front and Montrose rear tires offered predictable traction and seemed less flat-prone than the brand’s previous attempts. My trails are littered with tire-eating square-edged rocks, and I heard the familiar sound of a rim out several times while testing the Top Fuel. And though that sound made me mentally scramble to remember where I stashed Dynaplug, the flats, so far, haven’t happened. However, the compound does seem a bit biased towards fast rolling and does feel slightly slippery and bouncy on rock slabs and the hardest hardpack. But they seemed like a solid choice for this style of bike, and I didn’t want to tear them off after the first ride and throw on some of my favored Maxxis or Vittoria treads.
• When I pulled this bike out of the box to build it, I discovered one of the SRAM AXS pods had a dead coin cell battery. I’ve had a run of bikes with AXS pods that required a new coin cell after one or two rides. I don’t know if SRAM has a bunch of old batteries or a run of bad ones, but it’s annoying to discover your brand-new and nearly five-figure bike immediately needs a new battery.
• The SRAM Level Silver four-piston brakes, with 180mm HS2 rotors front and rear, are barely powerful enough for this bike. I realize that weight is a big deal in this bike category and that I am biased toward powerful brakes. But on the steeper trails, this bike is otherwise capable of riding my hands were aching from pulling on the levers so hard. I bumped up to a 200mm front rotor, which helped a lot (and I may yet go up to a 200mm rear). But if you’re considering this bike’s compatibility with longer stroke shock, 140mm fork, and 27.5 in. rear wheel, the stock brakes won’t cut it. You’ll want stoppers like Codes, TRP’s DHR Evo, or the Hayes Dominion.
• I usually don’t detail my suspension settings because I believe suspension settings result from terrain, trail surface conditions, riding style, and personal preference, so unless you are me and riding my trails, how I tune my suspension is irrelevant to you. In this case, however, I will detail a few of my fork settings because it took me a while to dial in this fork properly. The RockShox Pike on this bike has the brand’s recently revised air spring—increased negative spring volume, which softens the initial travel—and the revised Charger 3.1 damper, which has a greater damping adjustment range. Trek also ships this fork with no bottomless air tokens in the air chamber. With recommended pressure and zero tokens, the fork was an overly soft and unsupportive mess. I eventually wound up with two tokens in the fork and 95 psi in the spring, 15 over RockShox’s recommended pressure for my weight. Once the spring felt right, I found I liked the low-speed compression at -2 and the high-speed compression set at +1. This allowed me to add or subtract compression damping as trail conditions demanded.
A gear editor for his entire career, Matt’s journey to becoming a leading cycling tech journalist started in 1995, and he’s been at it ever since; likely riding more cycling equipment than anyone on the planet along the way. Previous to his time with Bicycling, Matt worked in bike shops as a service manager, mechanic, and sales person. Based in Durango, Colorado, he enjoys riding and testing any and all kinds of bikes, so you’re just as likely to see him on a road bike dressed in Lycra at a Tuesday night worlds ride as you are to find him dressed in a full face helmet and pads riding a bike park on an enduro bike. He doesn’t race often, but he’s game for anything; having entered road races, criteriums, trials competitions, dual slalom, downhill races, enduros, stage races, short track, time trials, and gran fondos. Next up on his to-do list: a multi day bikepacking trip, and an e-bike race.
