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Trek FX 3 Gen 4 (FX 3 Disc): specs, upgrades and what fits

A light aluminum fitness hybrid with a carbon fork, a 1x10 Shimano CUES drivetrain with an 11-48 cassette, Shimano hydraulic disc brakes and tubeless-ready rims. Best for: Fast fitness rides and commuting.

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My take, from the specs

A light, quick fitness hybrid at about 11.5 kg, with a carbon fork, Shimano hydraulic discs and tubeless-ready rims. The 1x10 Shimano CUES drivetrain with an 11-48 cassette gives a wide range with one shifter. Tire clearance tops out at 40mm, so it suits pavement and smooth gravel more than rough trails.

Sizes: XS: 4′10″–5′1″ (147–155 cm) · S: 5′1″–5′5″ (155–165 cm) · M: 5′5″–5′9″ (165–175 cm) · L: 5′9″–6′1″ (175–186 cm) · XL: 6′1″–6′6″ (186–197 cm) · XXL: 6′6″–6′8″ (197–203 cm)

Stock specs

TypeHybrid
Year2025
Price when new$1050
Sold atTrek dealers / trekbikes.com
Wheels700c × 35 mm tires
Gears1×10 · 40T · 11–48T
BrakesShimano UR300 caliper, flat mount
ForkTrek FX Carbon, flat-mount disc, rack mounts, 410mm axle-to-crown, 5mm ThruSkew QR
Rear derailleurShimano CUES RD-U6000 GS, 10-speed
ShifterShimano CUES U6000, 10-speed, optical gear display
CrankProWheel Alloy, 170mm
Bottom bracketGeneric Threaded 68mm shell (cartridge, model not listed)
AxlesQuick release
Seatpost27.2 mm
Weight25.4 lb (11.5 kg)
SizesXS, S, M, L, XL, XXL
Stock colorsDark Carmine, Galactic Grey, Black, Hex Blue

Upgrade limits

How far each part can go on this frame, and why. Measure your own bike to confirm the estimated ones.

PartLimitWhy
Widest tire (current wheel size)40 mmNo measurement yet. Estimate from owner reports and frame design. Your 19 mm internal rims suit roughly 28–49 mm tires.
Other wheel sizes650b up to ~47 mmDisc brakes don't care where the rim sits, so a 650b wheel with a fatter tire fits. A 650b × 47 is about the outer size of a 700 × 28, so the bottom bracket sits about 12 mm lower than on 700 × 40 (watch pedal strikes).
TubelessYes (tubeless-ready)Tubeless needs a rim built to seal (bead hook + channel) and tubeless-ready tires.
Valve typePrestaDecides which tubes and pump heads fit. Presta tubes fit Schrader rims with a grommet; not the other way.
Replacement wheelsQuick release · 100/135 mm · 6-bolt discAny wheel matching axle type, spacing and brake mount fits. Rear also needs the right freehub.
Biggest rear cog48T (now 48T)Set by the rear derailleur (Shimano CUES RD-U6000 GS 10-speed): max low sprocket 48T, total capacity 37T. A hanger extender can add a few teeth (hit-or-miss on non-Shadow derailleurs); a new derailleur is the reliable way to get more.
Front gears1x (40T)One ring, no front derailleur. Adding a front derailleur needs a mount on the seat tube and a 2x crankset, so most riders change the ring size or cassette instead.
Chainring sizeUp to 44T (now 40T)Estimated from similar frames. Measure the chainring-to-chainstay gap to confirm. Check whether your ring is bolted or riveted. Spindle length (? mm) sets chainline; a big change in ring size may need a different BB spindle.
Gear count upgrades10-speed (HG body)Shimano HG 8/9/10-speed freehub: takes 8, 9 or 10-speed cassettes, Shimano 11-speed MTB and CUES/LinkGlide cassettes (7-speed needs a 4.5 mm spacer; most 11/12-speed road cassettes need an 11-speed road freehub, though the 11-34 HG700/HG800 fit). Shifter,…
Cranks & bottom bracketSquare taper · 68 mm shell · ? mm spindleAny square-taper crankset fits with the right spindle length. Upgrading to Hollowtech II needs a new BB (same threaded shell).
Pedal thread9/16" × 20Every adult multi-piece crank uses 9/16". One-piece kids/cruiser cranks use 1/2".
Brake padsSHIMANO B01SDisc pads only fit calipers with the matching shape.
Rotor size160/160 mm now · front 160 (flat mount: check fork)6-bolt rotors. Going bigger needs a caliper adapter: flat-mount adapter, and many flat-mount forks are rated 160 mm max. Most steel forks allow 180 mm; check the fork if a max rotor is printed on it.
Caliper upgradesHydraulic systemHydraulic: match the brand of lever and caliper.
Stems & bars1-1/8 steerer · 31.8 mm bar clampAny stem for a 1-1/8 steerer with a 31.8 mm clamp fits. You can cut a bar narrower but not wider.
Drop-bar conversionPossible with modsNeeds road brake/shift levers (microSHIFT R8 makes 8-speed ones that pull Shimano 8-speed cable correctly), a drop bar, tape, and road-pull brake calipers because road levers pull less cable. Usually $250+.
Seatposts27.2 mm27.2 mm is the most common size: suspension posts and external-cable dropper posts are easy to find. Internal droppers need internal routing the frame does not have.
Racks, fenders, bottlesRack ✓ · Fender ✓ · 2 bottle · Fork cargo ✓Threaded eyelets at the dropouts and seat stays take a rack and fenders. Fork leg mounts take cargo cages.
Max load (rider + cargo)300 lbWheels are usually the weak point under heavy loads. Stay under this with rider plus luggage.

Best upgrades by area

Three options that fit the Trek FX 3 Gen 4 (FX 3 Disc) as-is in each area, from budget to best. ★ marks the one most worth the money. How we pick. Typical US prices, checked October 2026. They change often: check the price at the store.

AreaGoodBetterBest
TiresContinental Contact Urban 700x40 (40-622) wire (pair)
$$$ ★ Our pick
WTB Byway 700x40 (pair)
$$$
Schwalbe Marathon Almotion 700x38 (40-622) V-Guard Addix (pair)
$$$
Brake padsTektro E10.11 resin disc pads (2 sets)
$ ★ Our pick
Kool-Stop D620S sintered disc pads, Shimano B01S / Tektro E10.11 shape (2 pairs)
$$
SwissStop Disc 15 RS organic disc pads, Shimano B-type / Tektro E10.11 shape (2 pairs)
$$
Grips & tapeBudget ergonomic lock-on grips
$
Ergon GP1 ergonomic grips
$$ ★ Our pick
Ergon GP5 ergonomic grips with built-in bar ends (S or L)
$$$
SaddleWTB Volt saddle
$$
Fizik Terra Argo X5 saddle (150 or 160 mm)
$$$
Ergon SR Allroad Core Comp saddle (men's or women's)
$$$ ★ Our pick
PedalsRace Face Chester composite flat pedals
$$ ★ Our pick
Crankbrothers Stamp 2 aluminum flat pedals (small or large)
$$$
OneUp Components Aluminum flat pedals
$$$
Tubes & tubelessRideNow TPU road tubes 700x18–32 Presta 65 mm (pair)
$
Thorn-resistant tubes 700x35-43 (pair)
$
Tubolito Tubo-CX/Gravel-All 700x30–50 Presta 42 mm (pair)
$$ ★ Our pick
ChainShimano CN-HG601-11 11-speed chain (fits CUES 9/10/11, 116 links)
$$
Shimano CN-LG500 LinkGlide chain (CUES 9/10/11, 126 links)
$$ ★ Our pick
Shimano CN-HG901-11 11-speed chain (XTR/Dura-Ace, SIL-TEC, 116 links)
$$
RotorsShimano SM-RT56 160 mm 6-bolt rotor (1 rotor)
$ ★ Our pick
Galfer Wave 160 mm 6-bolt rotor, 1.8 mm (1 rotor)
$$
Shimano SM-RT86 Ice-Tech 160 mm 6-bolt rotor (1 rotor)
$$
Chainring & crankSquare-taper 1x narrow-wide crankset 44T 175 mm
$$ ★ Our pick
Shimano CUES FC-U4000-1 square-taper 1x crankset 40T
$$
Velo Orange Single Ring crankset with narrow-wide 42T (110 BCD, square taper)
$$$
SeatpostSuspension seatpost 27.2 mm (budget)
$$ ★ Our pick
Redshift ShockStop suspension seatpost 27.2
$$$$
Cirrus Kinekt 2.1 Aluminum suspension seatpost 27.2 mm (LR or XR spring)
$$$$
Bar & stem35° rise 60 mm stem
$ ★ Our pick
Redshift ShockStop suspension stem 31.8 (±6° or +30°)
$$$
Redshift ShockStop PRO suspension stem 31.8 (±6°)
$$$$
LightsPlanet Bike Beamer 200 + Blinky Squared USB light set
$$ ★ Our pick
Cygolite Metro Plus 800 + Hotshot Pro 150 USB light set
$$$
Trek (Bontrager) Ion Pro RT + Flare RT light set
$$$
Racks, bags & fendersTopeak Explorer rear rack (disc mount)
$$ ★ Our pick
Topeak Super Tourist DX Tubular rear rack (disc)
$$$
Tubus Disco steel rear rack for disc brakes (26" or 28"/700c)
$$$
WheelsWeinmann Zac19 700c 6-bolt disc rear wheel, QR 135, 36H, HG 8–10 speed
$$$ ★ Our pick
Shimano / DT Swiss R500 700c disc wheelset, QR 100/135, Center Lock, HG + 2 Shimano SM-RT54 160 mm Center Lock rotors
$$$$
HUNT 4 Season Gravel Disc X-Wide wheelset (QR 100/135, HG, Center Lock) + 2 Shimano SM-RT64 160 mm Center Lock rotors
$$$$

Upgrades that fit as-is

Tires

Tubes

Gearing

Brakes

Comfort

Control

Utility

Drivetrain

Wheels

Fits with small changes (22)

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Buying one used? Fair prices

Worked out from the price new ($1050), age and condition, not live market data. If it needs tires, a chain or brakes, take off what fixing them would cost.

Condition1 year old3 years old5 years old
Like new$595–$780$480–$635$390–$510
Good$520–$685$420–$555$340–$450
Fair$425–$560$345–$455$280–$370
Needs work$325–$430$265–$350$215–$280

See what these really sold for on eBay · Other used bike prices

Geometry (size M)

Stack594°
Reach393 mm
Head angle71°
Seat angle74°
Chainstay447°
BB drop65 mm
Wheelbase1070 mm
Head tube165 mm
Seat tube420 mm
Standover754°

Compare with

Sources: Comor Sports: Trek FX 3 Gen 4 full spec list (weight, max tire, max chainring) · Bike Insights: Trek FX 3 Gen 4 M geometry (Trek data) · Gear West: Trek FX 3 Disc Gen 4 (US price, sizes) · Trek FAQ: FX 3 Gen 4 bottom bracket (threaded 68mm)