2025 Trek Top Fuel Gen 4 targets refinement rather than revolution – starting at £2,500

Geometry updates, suspension kinematic tweaks and mixed-wheel compatibility extend Top Fuel’s remit

By Alex Evans

Published: Thursday, 11 July 2024 at 15:00 PM

Trek has extended the on-trail behaviour of its popular all-rounder the Top Fuel, with subtle geometry and suspension updates for 2025.

Also refined – rather than redesigned – are its internal frame storage compartment and door, while shock compatibility is broadened for a wider (120mm to 130mm) travel band.

All stock models run 29in wheels front and rear, except for the small, which uses 27.5in hoops. But Trek’s Mino Link means fans of mixed wheels (29in front, 27.5in rear) can give the Top Fuel a more shred-ready setup.

Standout geometry figures include a 65.5-degree head tube angle, a 76-degree seat tube angle and size-specific chainstays, from 435mm to 445mm.

The seven-bike range spans both alloy and carbon frame constructions, with the most affordable Top Fuel 5 costing £2,500 / $2,699.99 and the top-end Top Fuel 9.9 XX AXS going for a whopping £10,000 / $10,499.99.

Better storage and a lighter frame

The frame storage door and aperture have been updated. – Trek

Trek claims both the alloy and carbon fibre versions of the new Top Fuel Gen 4 have lost 220g compared to their outgoing versions.

Claimed full-bike weights start at 15.9kg for the medium Top Fuel 5 and drop more than 2kg to 12.85kg for the medium range-topping 9.9.

We tested the previous-generation Top Fuel 8 as part of 2022’s Bike of the Year and it tipped our scales at 14.8kg (size large, without pedals).

The 2025 Top Fuel 8, in a medium, is claimed to weigh 14.58kg, a saving of – and there are no prizes for guessing – 220g!

Trek says it has saved weight by redesigning its tubes with “smaller profiles and softer edges” and by reducing stiffness.

According to the brand, this reduces mass and “improves the ride quality of the bike”, making it “more balanced and forgiving than the Gen 3”.

The integrated down tube storage and its access door have also been redesigned.

The door now has a tighter seal and a bigger opening, so water is less likely to get in, and sliding stashed items in and out will be easier.

Trek also says gear, dropper and brake cables – which route via the down tube – are better protected to avoid snagging with the supplied BITS bags.

And those bags have also been overhauled.

Carbon fibre frames get two bags; a padded tool bag and a second, unpadded bag for tubes, both made from a ripstop material. Alloy frames only get one redesigned BITS bag.

Details matter

Trek Top Fuel Gen 4 downcountry mountain bike
It’s a downcountry bike with intent. – Trek

Gear, dropper and brake cables are routed via ports on the side of the down tube – instead of through the headset – which no longer gets Trek’s signature Knock Block design.

The 2025 Top Fuel’s bars can be rotated through 360 degrees, if that’s your bag.

Dropper post insertion has also been improved, so longer-travel posts can be fitted.

SRAM’s Universal Derailleur Hanger/Full Mount dropout is of course fitted and specced.

All frame sizes have a single water bottle mount on the frame’s internal storage door. The size small can fit a 620ml bottle, while the medium and above have space for a 760ml bottle.

Upping travel and progression

Trek Top Fuel Gen 4 downcountry mountain bike
The Top Fuel’s Mino Link now has four adjustment positions. – Trek

Thanks to two additional positions on the lower shock mount Mino Link flip chip, the Top Fuel’s suspension kinematics can be tuned.

According to Trek, the forward position is 14 per cent progressive, while the rear position is 19 per cent progressive.

In the more progressive position, the leverage rate starts higher than the less progressive setting, but at full travel both settings terminate at roughly the same ratio.

This means the more progressive position will have a softer, more supple starting stroke compared to the less progressive setting, but both ramp up towards the end of their travel to provide similar bottom-out resistance.

The less progressive setting will ramp up more gradually, because the leverage rate doesn’t change as quickly or as much.

Anti-squat – how much the bike resists pedal bob – has been increased slightly compared to the old model, now sitting just above 100 per cent at full travel.

The Gen 4’s frame travel remains unchanged at 120mm, but the latest Top Fuel is compatible with a longer-stroke shock. Up the stock 180x50mm shock to a 180x55mm version and travel is increased to 130mm.

It’s the same story for the fork. The Stock 130mm fork can be switched to a 140mm model for a more shred-ready frame.

The mixed-wheel setup requires a 140mm-travel fork and for the Mino Link to be set in its high position.

You can also down-fork the Top Fuel to a 120mm version, potentially making it ready for XC racing.

Geometry tweaks

Trek Top Fuel Gen 4 downcountry mountain bike
You can change the fork and switch to a mullet setup for increased downhill performance. – Trek

Although the longer, slacker, lower trend is still ongoing, the rate of progress has been abated as geometry figures converge on an increasingly unified sweet spot.

But this means brands are focusing their efforts on other areas to refine rather than revolutionise handling.

The 2025 Top Fuel Gen 4 is a great example of this.

Its head angle is either 65 or 65.9 degrees, depending on the Mino Link’s high or low positioning, while the seat tube angle is around 76 degrees for both settings.

Reach figures in the five-size line-up (small to extra-large) start at 417mm and rise to 507mm.

Fitting a 27.5in rear wheel and 140mm fork slackens the head angle to 64.6 degrees and the seat tube to 75 degrees.

SizeSmallMediumMedium-longLargeExtra-large
Reach (mm)417447462477407
Stack (mm)570603607612630
Bottom bracket height low/high (mm)342/348345/351345/351345/351345/351
Seat tube angle low/high (degrees)76/76.576.9/77.376.2/76.675.9/76.375.2/75.6
Head tube angle low/high (degrees)65.5/65.965.5/65.965.5/65.965.5/65.965.5/65.9
Wheelbase (mm)11511188121012271271
Chainstay length (mm)435435440440445
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Trek has made the biggest improvements to the chainstays, which are now size-specific.

The small and medium bikes get 435mm stays, the medium-long and large are 440mm, while the extra-large lifts that figure to 445mm.

Interestingly, each size shares chainstays; it’s the front ends that differ, with bottom-bracket placement moving in relation to the rear axle and main pivot to give the extra length.

2025 Trek Top Fuel Gen 4 models and pricing

With no fewer than seven bikes in its latest Top Fuel line-up, Trek is hoping to cater for all price points.

Starting at £2,500 / $2,699.99 for the Top Fuel 5, this alloy-framed bike features a RockShox Recon fork and X-Fusion shock, along with Shimano’s Deore M6100 drivetrain.

At the other end of the spectrum is the Top Fuel 9.9 XX AXS, boasting a full carbon fibre construction, RockShox Pike Ultimate and Deluxe Ultimate shock, along with SRAM’s XX Transmission. However, it costs a chunky £10,000 / $10,499.99.

Trek Top Fuel 5

Trek Top Fuel 8

Trek Top Fuel 9

Trek Top Fuel 9.8 XT

Trek Top Fuel 9.8 Gen 4 XT
Trek Top Fuel 9.8 XT. – Trek

Trek Top Fuel 9.8 GX AXS

Trek Top Fuel 9.8 Gen 4 GX AXS
Trek Top Fuel 9.8 GX AXS. – Trek

Trek Top Fuel 9.9 X0 AXS

Trek Top Fuel 9.9 Gen 4 X0 AXS
Trek Top Fuel 9.9 Gen 4 X0 AXS. – Trek

Trek Top Fuel 9.9 X0 AXS

Trek Top Fuel 9.9 Gen 4 XX AXS
Trek Top Fuel 9.9 Gen 4 XX AXS. – Trek