Bike models › Kids
woom GO 4: specs, upgrades and what fits
A very light 20-inch kids' bike with a butted aluminum frame and fork, a microSHIFT 7-speed twist-shift drivetrain (28T ring, 11-28 cassette), two kid-size mini V-brakes, a quick-release front wheel and a bolted rear wheel. It replaced the ORIGINAL 4 in the US with BMX-style riser bars, a gear window on the shifter, ABC fit marks and a pre-installed LEGGIE kickstand. Best for: Kids about 6 to 8 years old riding paths, parks and easy hills.
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My take, from the specs
For kids around 6 to 8 who ride paths, parks and neighborhood hills. At about 17 lb with a 7-speed twist shifter and two kid-size V-brakes, it is easy to handle and a good bike for learning gears. The weak spots are rim brakes that lose bite in the wet and an upright, short cockpit that taller or rougher riders outgrow quickly.
Sizes: 20in: 3′9″–4′3″ (114–130 cm)
Stock specs
| Type | Kids |
|---|---|
| Year | 2026 |
| Price when new | $639 |
| Sold at | woom.com / woom dealers |
| Wheels | 20" × 35 mm tires |
| Gears | 1×7 · 28T · 11–28T |
| Brakes | woom Mini V-brake, Jagwire cable |
| Fork | woom Lightweight aluminum unicrown, 1in steerer, QR |
| Rear derailleur | microSHIFT M21SS, 7-speed, short cage |
| Shifter | microSHIFT Ds45 twist shifter, 7-speed, gear window |
| Crank | woom Forged aluminum, 110mm, narrow Q-factor |
| Bottom bracket | Generic Sealed cartridge |
| Axles | Bolt-on |
| Weight | 17.2 lb (7.8 kg) |
| Sizes | 20in |
| Stock colors | woom red, hot pink, vibrant yellow, neon lime (limited edition), powder pink, metallic turquoise, metallic blue |
Upgrade limits
How far each part can go on this frame, and why. Measure your own bike to confirm the estimated ones.
| Part | Limit | Why |
|---|---|---|
| Widest tire (current wheel size) | 35 mm | Unknown until measured. Your 21 mm internal rims suit roughly 31–55 mm tires. |
| Tubeless | Not recommended | Tubeless needs a rim built to seal (bead hook + channel) and tubeless-ready tires. |
| Valve type | Schrader | Decides which tubes and pump heads fit. Presta tubes fit Schrader rims with a grommet; not the other way. |
| Replacement wheels | Bolt-on · ?/? mm | Any wheel matching axle type, spacing and brake mount fits. Rear also needs the right freehub. |
| Biggest rear cog | 28T (now 28T) | Set by the rear derailleur (microSHIFT M21SS 7-speed): max low sprocket 28T, total capacity ?T. 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. |
| Brake pads (rim) | Threaded-post pads (V-brake / U-brake) | Rim brake pads are sorted by how they mount, which depends on the brake: threaded post with a nut (V-brakes, U-brakes), smooth post in an eye bolt (cantilevers), road cartridges (dual-pivot calipers) or short threaded pads (side-pull calipers). Any compound… |
| Front gears | 1x (28T) | 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 size | Unknown (now 28T) | 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 upgrades | 7 → 9 or 10 speed | 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 bracket | Square taper · 68 mm shell · ? mm spindle | Any square-taper crankset fits with the right spindle length. Upgrading to Hollowtech II needs a new BB (same threaded shell). |
| Pedal thread | 9/16" × 20 | Every adult multi-piece crank uses 9/16". One-piece kids/cruiser cranks use 1/2". |
| Stems & bars | 1 steerer · ? mm bar clamp | Any stem for a 1 steerer with a 31.8 mm clamp fits. You can cut a bar narrower but not wider. |
| Drop-bar conversion | Possible with mods | Needs 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+. |
| Seatposts | ? mm | Buy posts in exactly this diameter. |
| Racks, fenders, bottles | No rack · Fender ✓ · 0 bottle | Threaded eyelets at the dropouts and seat stays take a rack and fenders. Fork leg mounts take cargo cages. |
| Max load (rider + cargo) | 154 lb | Wheels are usually the weak point under heavy loads. Stay under this with rider plus luggage. |
Best upgrades by area
Three options that fit the woom GO 4 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.
Upgrades that fit as-is
Brakes
- Kool-Stop Salmon V-brake pads, threaded post (2 pairs), $$. threaded post pads ↔ your rim brakes
- Shimano Deore cartridge V-brake pads (2 pairs), $$. threaded post pads ↔ your rim brakes
- Jagwire Mountain Pro brake cable kit (Kevlar KEB-SL housing), $$. Made for flat bars ↔ your flat bar
- Shimano Deore BR-T610 V-brakes (front + rear), $$. threaded post pads ↔ your rim brakes · Long-pull caliper ↔ your flat-bar (long-pull) levers
- Shimano S65T V-brake pads, threaded post (2 pairs), $. threaded post pads ↔ your rim brakes
- Kool-Stop threaded V-brake cartridge holders with salmon inserts (2 pairs), $$. threaded post pads ↔ your rim brakes
- Shimano Alivio BR-T4000 V-brakes (front + rear), $$. threaded post pads ↔ your rim brakes · Long-pull caliper ↔ your flat-bar (long-pull) levers
Comfort
- WTB Volt saddle, $$. Medium-width shape that suits most riders. Measure sit bones if comfort is the goal.
- Brooks C17 All Weather saddle, $$$. Vulcanized rubber top that flexes like a hammock, and this All Weather version is fully waterproof. No break-in like leather. Firm, not padded: made for long…
- Bikeroo oversized comfort saddle, $. Wide, soft and sprung for short upright rides on cruisers and hybrids. On rides over an hour wide padding can chafe; a firmer saddle that fits your sit bones…
- Selle Royal Respiro Moderate saddle, $$$. Made for a 60° "moderate" back angle (most flat-bar hybrids). Very upright cruisers suit the wider Respiro Relaxed. Standard round steel rails.
- Fizik Terra Argo X5 saddle (150 or 160 mm), $$$. Pick 150 mm for narrower sit bones, 160 mm for wider. Round 7 mm alloy rails fit every standard clamp.
- Ergon SR Allroad Core Comp saddle (men's or women's), $$$. Comes in S/M and M/L widths (measure your sit bones) and men's and women's shapes. Round CroMo rails; 120 kg rider limit.
Control
- Race Face Chester composite flat pedals, $$. Big platform with pins. Feet stay put on bumps and in rain. Install with an 8 mm hex key (no wrench flats).
- ROCKBROS aluminum flat pedals, $$. Budget metal platform with pins. A big step up from stock plastic.
- Shimano PD-M520 SPD clipless pedals (cleats included), $$. Nearly indestructible two-sided SPD pedals. You also need SPD (2-bolt) shoes, about $60–100 more. Set the release tension low and practice unclipping before…
- Crankbrothers Stamp 2 aluminum flat pedals (small or large), $$$. Sized by shoe: small for US men's 5–10, large for 10 and up. Grease the threads and tighten to about 35 Nm.
- OneUp Components Aluminum flat pedals, $$$. Bearings and bushings can be serviced with a cheap kit instead of replacing the pedal. Grease the threads and tighten to about 35 Nm.
- Shimano PD-EH500 dual-sided SPD / flat pedals (cleats included), $$$. Comes with SM-SH56 multi-release cleats, which are easier to get out of in a panic. You still need 2-bolt SPD shoes to clip in.
- Shimano Deore XT PD-M8100 SPD pedals (cleats included), $$$. Cup-and-cone bearings are fully serviceable and last for years. Shoes with a 2-bolt SPD cleat sold separately.
Utility
- USB rechargeable front + rear light set, $$. Daytime flashing lights cut collisions. Required by law at night.
- Pletscher adjustable kickstand, $$. Handy for errands. Check it clears the rotor and chainstay.
- Lisen clamp-style handlebar phone mount, $. Cheap and tool-free for maps on paved rides. On rough gravel or trails a locking mount (Quad Lock or Peak Design, about $45–70 plus a case) is much safer.
- Topeak TopLoader top tube bag (0.75 L), $$. Keeps snacks, keys and a multi-tool within reach. Straps on, with a rain cover. Check your knees do not rub it when you stand up.
- Knog Oi Classic bell, $. Slim ring bell that wraps the bar. Large fits 23.8–31.8 mm bars; Small fits 22.2 mm kids and BMX bars. Required by law in some places.
- Wald 137 front basket, $$. Made for flat bars ↔ your flat bar
- Planet Bike Beamer 200 + Blinky Squared USB light set, $$. USB rechargeable, tool-free mounts. Use flash mode in daylight. Rear light is 30 lumens: good for town, modest on fast roads.
- Cygolite Metro Plus 800 + Hotshot Pro 150 USB light set, $$$. Nine headlight modes including a daytime flash. Aim the beam slightly down so you do not blind oncoming riders.
- Trek (Bontrager) Ion Pro RT + Flare RT light set, $$$. Daytime-running flash modes on both lights. ANT+ lets a Garmin or Wahoo show battery and switch them. Full power runs about 1.5 hours; medium lasts much longer.
Drivetrain
- KMC X8 8-speed chain with Missing Link, $. 6/7/8-speed chain ↔ your 7-speed drivetrain
- KMC Z7 6/7-speed chain (116 links), $. 6/7-speed chain ↔ your 7-speed drivetrain
- KMC Z8.3 6/7/8-speed chain (116 links), $. 6/7/8-speed chain ↔ your 7-speed drivetrain
- KMC X8 EPT 6/7/8-speed anti-rust chain with Missing Link, $$. 6/7/8-speed chain ↔ your 7-speed drivetrain
Gearing
- Shimano CS-HG200-7 7-speed cassette 12–28, $. Cassette ↔ your Shimano HG freehub · 7-speed ↔ your 7-speed drivetrain
- SunRace CSM63 7-speed cassette 11–28, $. Cassette ↔ your Shimano HG freehub · 7-speed ↔ your 7-speed drivetrain
- Shimano Acera RD-M3020-8 SGS 7/8-speed rear derailleur (40T max), $$. 7-speed ↔ your 7-speed drivetrain · Shimano cable pull ↔ your shifter
Fits with small changes (12)
- microSHIFT Advent 1x9 kit (shifter, clutch RD, 11–46 cassette) + 9-speed chain, $$$. Installs as a set: shifter, derailleur, cassette and chain together.
- Ergon GP1 ergonomic grips, $$. You have twist shifters: buy the short (Gripshift) version.
- Budget ergonomic lock-on grips, $. You have twist shifters: buy the short (Gripshift) version.
- Shimano CUES U6000 1x11 kit (SL-U6000-11R shifter, RD-U6000 clutch RD, CS-LG400-11 11–50 LinkGlide cassette, chain), $$$$. Installs as a set: shifter, derailleur, cassette and chain together.
- Ergon GA3 lock-on grips, $$. You have twist shifters: buy the short (Gripshift) version.
- Ergon GP5 ergonomic grips with built-in bar ends (S or L), $$$. You have twist shifters: buy the short (Gripshift) version.
- Stan's NoTubes tire sealant 16 oz, $. In tubes: needs tubes with removable valve cores (Presta or Schrader to match your rims).
- Orange Seal Endurance tubeless sealant 16 oz, $. In tubes: needs tubes with removable valve cores (Presta or Schrader to match your rims).
- SILCA Ultimate Tubeless Sealant 16 oz, $. In tubes: needs tubes with removable valve cores (Presta or Schrader to match your rims).
- Magura HS11 hydraulic rim brakes with levers (front + rear), $$$. Replaces your brake levers. Your shifter stays.
- Shimano CUES U4000 1x9 kit (SL-U4000-9R shifter, RD-U4000, CS-LG300-9 11–46, CN-LG500 chain), $$$. Installs as a set: shifter, derailleur, cassette and chain together.
- Shimano Deore M5100 1x11 kit (SL-M5100 shifter, RD-M5100, CS-M5100 11–51, CN-HG601 chain), $$$. Installs as a set: shifter, derailleur, cassette and chain together.
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Buying one used? Fair prices
Worked out from the price new ($639), age and condition, not live market data. If it needs tires, a chain or brakes, take off what fixing them would cost.
| Condition | 1 year old | 3 years old | 5 years old |
|---|---|---|---|
| Like new | $390–$515 | $330–$435 | $280–$370 |
| Good | $345–$455 | $290–$385 | $245–$325 |
| Fair | $280–$370 | $240–$315 | $200–$265 |
| Needs work | $215–$285 | $180–$240 | $155–$205 |
See what these really sold for on eBay · Other used bike prices
Geometry (size 20in)
| Head angle | 68° |
|---|---|
| Seat angle | 72° |
| Wheelbase | 840 mm |
| Standover | 490° |
Compare with
- Huffy Rock It 20" · compare frames side by side
- Trek Precaliber 20 · compare frames side by side
- Specialized Hotrock 24 · compare frames side by side
- woom ORIGINAL 4 · compare frames side by side
Sources: woom US: GO 4 (price $639, colours, 17.2 lb, 70 kg limit, 45-51 in rider height, microSHIFT Ds45 / M21SS, 28T + 11-28, 110mm cranks, QR front / 5mm hex rear) · woom: GO 4 technical data and geometry in metric (wheelbase 840 mm, standover 490 mm, saddle 568-729 mm) · woom US: size guide (GO 4 inseam 22.1-27.9 in, height 45-51 in) · Two Wheeling Tots: woom GO 4 / woom 4 review (measured 17.2 lb, lever reach screw, gear window, fit notes)