Bike models › Kids
Kent Abyss 18 BMX: specs, upgrades and what fits
An 18-inch BMX-style kids' bike with a steel frame and fork, steel rims, a rear coaster brake plus front and rear caliper hand brakes, front pegs and removable training wheels. Best for: Kids about 5 to 8 learning to ride.
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My take, from the specs
A starter 18 in. bike for kids about 5 to 8, with removable training wheels, a coaster brake and two hand brakes. Having both brake types lets a child learn the levers while the back-pedal brake still works. The steel frame and steel rims make it heavy at about 12.7 kg, and the weight limit is low, so it suits lighter riders.
Sizes: 18in: 4′0″–4′4″ (122–132 cm)
Stock specs
| Type | Kids |
|---|---|
| Year | 2025 |
| Price when new | $124 |
| Sold at | Walmart |
| Wheels | 20" × 54 mm tires |
| Gears | Single speed · 28/16 |
| Brakes | Generic Side-pull caliper |
| Fork | Kent Hi-tensile steel |
| Crank | One-piece (Ashtabula) crank |
| Axles | Bolt-on |
| Weight | 28.0 lb (12.7 kg) |
| Sizes | 18in |
| Stock colors | Blue, Grey, Red |
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) | 54 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. |
| Brake pads (rim) | Short threaded road pads (side-pull caliper) | 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 | Single speed | One cog and no derailleur. To change the gearing, swap the sprocket or the rear driver/cog (more teeth on the cog = easier pedaling). |
| Cranks & bottom bracket | — | Match crank spindle to BB type. |
| Pedal thread | 1/2" × 20 | Every adult multi-piece crank uses 9/16". One-piece kids/cruiser cranks use 1/2". |
| Stems & bars | ? steerer · ? mm bar clamp | Any 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 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 · No 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) | 71 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 Kent Abyss 18 BMX 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 Continental black pads, short threaded post (2 pairs), $. side-pull pads ↔ your rim brakes
- Jagwire Mountain Pro brake cable kit (Kevlar KEB-SL housing), $$. Made for flat bars ↔ your flat bar
- Kool-Stop Continental salmon pads, short threaded post (2 pairs), $. side-pull pads ↔ your rim brakes
Comfort
- Ergon GP1 ergonomic grips, $$. Made for flat bars ↔ your flat bar
- Budget ergonomic lock-on grips, $. Made for flat bars ↔ your flat bar
- 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…
- Ergon GA3 lock-on grips, $$. Made for flat bars ↔ your flat bar
- Selle Royal Respiro Moderate saddle, $$$. 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.
- Ergon GP5 ergonomic grips with built-in bar ends (S or L), $$$. Made for flat bars ↔ your flat bar
Utility
- USB rechargeable front + rear light set, $$. 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.
Tubes
- Slime self-sealing tubes 20 x 1.75–2.3 Schrader (pair), $$. 20" wheel size matches
- Schwalbe AV7 20 x 1.5–2.4 Schrader tubes (pair), $. 20" wheel size matches · Schrader valve ↔ your Schrader rims
Control
- Wellgo LU-313 alloy platform pedals 1/2", $. 1/2" thread only fits one-piece (Ashtabula) cranks. Adult multi-piece cranks need 9/16". Loose-ball bearings: a drop of grease now and then.
- Odyssey Twisted PC nylon pedals 1/2", $. This is the 1/2" version made for one-piece cranks; the 9/16" version will not thread in. Nylon body shrugs off crashes.
Tires
- Kenda Small Block Eight 20x1.75 (pair), $$. 50 mm tire ↔ your ~54 mm clearance
- Schwalbe Big Apple 20x2.0 (50-406) (pair), $$$. 50 mm tire ↔ your ~54 mm clearance
- Schwalbe Marathon Plus 20x1.75 (47-406) (pair), $$$. 47 mm tire ↔ your ~54 mm clearance
Drivetrain
- KMC Z410 1/8" single-speed chain (112 links), $. 1-speed chain ↔ your 1-speed drivetrain
- KMC Z1 Wide EPT 1/8" anti-rust single-speed chain, $. 1-speed chain ↔ your 1-speed drivetrain
- KMC K710 Kool Chain 1/8" (BMX / single-speed), $$. 1-speed chain ↔ your 1-speed drivetrain
Fits with small changes (3)
- 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).
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Buying one used? Fair prices
Worked out from the price new ($124), 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 | $60–$75 | $40–$55 | $30–$40 |
| Good | $50–$70 | $35–$50 | $25–$35 |
| Fair | $40–$55 | $30–$40 | $20–$30 |
| Needs work | $30–$40 | $25–$30 | $15–$20 |
See what these really sold for on eBay · Other used bike prices
Compare with
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- Specialized Hotrock 24 · compare frames side by side
- woom ORIGINAL 4 · compare frames side by side
Sources: Kent: 18in Abyss (age 5-8, fit 4'0"-4'4", inseam 21-24in, 28 lb, brakes) · Walmart: Abyss 18 (2,400+ reviews, steel rims, coaster + calipers)