Review: Trek Introduces New Top Fuel Gen 4 Trail Bike

Today Trek is releasing their latest Top Fuel into the world and it looks, like, well, a Top Fuel. But closer examination reveals some key changes that help cement the new bike’s intentions as a lightweight trail crusher. We’ll go over those changes, and some ride impressions, since I managed to grab a ride with the Trek team on a complete Top Fuel 9.9 XX AXS, and then I’ve spent the last week rallying around a Top Fuel frame built up with some decidedly non-standard parts.

First impressions though? Dear lord, this bike is a hoot, and I really appreciate the direction the Trek team has decided to take here.

Trek Top Fuel Gen 4 in a Nutshell

• Travel: 120 mm (rear) 130 mm (front)
• Wheel Size: 29” or 29/27.5” with a built-in flip chip
• Frame Material: Carbon or aluminum
• Frame Sizes: S-XL
• Head Tube Angle: 65.5°

What’s new?

Trek is up front about the fact that the new Top Fuel is not meant to be a trail-ified XC bike. This is no Epic Evo with its World Cup XC frame bolted to a longer fork and burlier build. No flex stays, no build that comes with a SID fork, none of that nonsense. Instead, it’s a light, efficient bike for people who ride real trails, and (mostly) don’t plan on strapping number plates to their bike. Trek’s World Cup teams are still primarily racing the Supercaliber and would only reach for the Top Fuel in very rare occasions. That stays consistent with the character of the old Top Fuel, and Trek says the new bike is aimed squarely at the same sort of rider, with some refinements to elevate the experience.

Those refinements include: no more Knock Block; a lighter frame (220 g less than the previous version); barely tweaked geometry (the head tube angle is .5° slacker at 65.5°); size-specific chainstays; refined in-frame storage; a new geometry-and kinematic-adjusting chip; and finally, compatibility with 27.5” rear wheels, coil shocks, and long-stroked shocks to eke 130 mm of travel out of the rear end. The sum total? An easier to live with, more adaptable bike that adds a sprinkle more down to its country.

Geometry and Kinematic Adjustments

The heart of the Top Fuel’s adaptability is the new four-position Mino Link at the lower shock mount. Effectively, this set of flip chips allows riders to choose between permutations of high and low geometry, and more linear or more progressive suspension curves.

The Top Fuel ships in its default “low, less progressive” setting. Flip that chip and you steepen the head tube angle by .4° and raise the BB by 6 mm.

If you want to run a 27.5” rear wheel (I do!) Trek says you need to put that link in the “high” position and run a 140 mm fork. In that configuration, the head tube slackens out to 64.6° and the BB is still a very reasonable 342 mm off the ground.

The Top Fuel is a good reminder that no part of a bike’s geometry adjustment system is a closed cell - an adjustment to any one part affects the rest of the geo and kinematics. The Top Fuel has two mount holes. In the stock, forward one, it has 14% progression through its travel. Bump back to the rear for 19% progression. That means that the Top Fuel is a fine candidate for a coil shock, provided that it clears the frame, and that you really want to bolt that onto this whippet.

In the stock less progressive position the Top Fuel’s leverage curve looks very similar to the old version which was 13.5% progressive. It gets a touch more anti-squat to help deliver a snappy feel under power, and then the “more progressive” setting gives riders room to experiment if they’re after a slightly different ride experience.

I’m on the record as a big chip fan , and the Top Fuel’s adjustments make a ton of sense.

Frame Details

First, frames get size-specific chainstays, with small and medium bikes sharing 435 mm chainstays, M/L and large bikes get 440 mm stays, and XL bikes bumping up to 445 mm. The Top Fuel also features Trek’s latest in-frame storage system which debuted on the Slash and is meant to be easier to use, more weatherproof, and has a bigger opening.

Size small bikes come with 27.5” wheels front and reear, but all other sizes come full 29”. Stock builds all come with 130 mm forks, but customers are welcome to run up to 140 mm forks, or bump down to 120 mm if they want to get racy. Similarly, the stock 185x50 mm rear shock serves up 120 mm of travel, but you’re welcome to run a 55 mm stroke shock for 130 mm of travel.

All frame sizes can fit a water bottle inside the main triangle, although if you’re looking for capacity for two, you’ll have to shop elsewhere.

Finally, I guess it’s worth mentioning since this comes only a couple weeks after another high profile bike launch, that the Top Fuel has internal routing for all the cables. You’re welcome to run a cabled derailleur with this bike, and it sure looks like you could cross cables at the rear triangle to run your brakes the wrong way ‘round if you so desire.

Build Options

Trek offers seven complete builds for the Top Fuel. The Top Fuel 5, 8, and 9 all come with aluminum frames, while the 9.8 and 9.9 are full carbon. The Top Fuel 5 retails for $2,699 (only $250 more than the aluminum frame only) and comes with a Rockshox Recon Silver fork, X-Fusion shock, and Shimano Deore drivetrain. Bump up to the 8 to get a Fox Rhythm 34 fork and Performance Float DPS shock, and Shimano Deore/SLX/XT build for $3,999. Trek said they prioritized a “nicer” aluminum build over a more affordable carbon option, and the 9 comes with the same suspension as the 8, but with a GX Transmission drivetrain and Level brakes for $4,799.

Jump to carbon and you can get a 9.8 XT ($6,249) or GX Transmission ($6,999) build, both of which feature Rockshox Select+ suspension and Shimano XT or SRAM Level 4 piston brakes. Upgrade to the big guns and you can get an 9.9 XO Transmission build for $8,499 or an XX SL build for $10,499, both of which feature Ultimate level suspension, and all the high-end goodies you’d expect.

Looking for just a frame? Aluminum will run you $2,499, or carbon costs $3,899. For my money, I’d be awfully tempted to snag the Top Fuel 5, upgrade the suspension and wheels, and party on! For just $250 more than a frame-only the rest of that build is just fine.

Ride Impressions

Initially, I got out on the top-tier XX SL 9.9 build for a quick rip. We rode Slash trails, not Top Fuel trails, and it was a hoot. I immediately felt at home on the bike, and was motivated to do my best to get it in over its head on the descents. But it handled everything I threw at it remarkably well.

Then, about a week ago, my review frame showed up, and I built it up with a very different build that I’ll get into later in its own piece. Right now it’s got a longer dropper (213 mm) and more powerful brakes than any of the stock builds, and holy crap do I enjoy it. It’s been a great motivation to push harder both up and down on familiar trails. It’s so snappy under power, and doesn’t suffer the “I feel like too much man on too light of a bike” sensation I occasionally had on my Transition Spur. I keep finding myself on top of bigger moves than Trek would probably recommend for the Top Fuel, and I keep bouncing out the bottom more in love with the bike.

Building up the Top Fuel was straightforward. My one word of warning (which applies to all Treks) is to check the torque on that non-drive side rear axle pivot in the comfort of your home, where you have a cassette tool handy to tighten it, not out on the trail, where you probably won’t.

Two notes on stock spec: I don’t jive with the Level brakes specced on the SRAM builds of this bike. I think Specialized made the right call putting Codes on the new Epic Evo, and I’d take the Shimano options over the SRAM ones here every time. But I’m also dead set on pointing this bike down steep trails, so for many folks those Levels are probably fine.

Second: Trek recently refreshed their tire line. I rode the stock Bontrager Gunnison/Montrose RSL tire and Line Pro 30 wheel combo on the first test bike, and really got along with those tires and wheels. They felt like a great fit for this bike’s intentions. I don’t have a set on hand for my frame-up build, so I’m running a mix of other brands’ tires, but, unlike the tires on the Slash I reviewed , I’d happily leave these on the Top Fuel until I wore them out.

I’m really excited to dive into this in my full review, but so far, the new Top Fuel feels like a bike that knows what it is. You can go faster and lighter for sure, or you can go beefier and more forgiving, but the Top Fuel strikes a really nice balance for folks who either aren’t sure which end of that spectrum they want to exist on, or who want the option to play across it.

I’ll be running the Top Fuel with a 120 mm fork and super low-profile tires, and I’ll try it with a 140 mm fork and 27.5” rear wheel to get a handle on how it deals with both ends of the component spectrum, and I’m excited to check back.

For Now

Call it the honeymoon period, but so far I’m pretty in love with the new Top Fuel. Trek has made some subtle changes that help solidify its place in their lineup, and make it an attractive option to a wide swath of riders.