Bike models › BMX

Mongoose Legion L20: specs, upgrades and what fits

An entry-level freestyle BMX with a hi-ten steel frame, three-piece chromoly cranks, a 9-tooth cassette rear hub and a rear U-brake. Best for: Learning street and park tricks.

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My take, from the specs

An entry freestyle BMX for learning street and park tricks, with three-piece chromoly cranks and a 9T cassette rear hub where it counts. The hi-ten steel frame and fork make it about 12.6 kg, and there is only a rear U-brake. The loose-ball American bottom bracket is the first thing to check if the cranks start to creak or wobble.

Sizes: 20in: 4′8″–5′8″ (142–173 cm)

Stock specs

Upgrade limits

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

Best upgrades by area

Three options that fit the Mongoose Legion L20 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
• 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

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), $$$. Comes in S/M and M/L widths (measure your sit bones) and men's and women's shapes. Round CroMo rails; 120 kg rider limit.
• Odyssey Keyboard v2 flangeless BMX grips, $. Made for bmx bars ↔ your bmx bar
• ODI Longneck SLX flangeless BMX grips (160 mm), $. Made for bmx bars ↔ your bmx bar
• Kalloy Uno SP-602 alloy seatpost 25.4 x 350 mm, $. 25.4 mm post ↔ your 25.4 mm seat tube
• Thomson Elite seatpost 25.4 x 330 mm, $$$. 25.4 mm post ↔ your 25.4 mm seat tube
• Sunlite adjustable suspension seatpost 25.4 mm, $$. 25.4 mm post ↔ your 25.4 mm seat tube

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), $$$. Cup-and-cone bearings are fully serviceable and last for years. Shoes with a 2-bolt SPD cleat sold separately.
• Salt Pro 2-piece BMX bars 22.2, 9" rise, $$. Made for bmx bars ↔ your bmx bar
• Cult Crew BMX bars 22.2, 9.35" or 9.65" rise (heat-treated chromoly), $$$. Made for bmx bars ↔ your bmx bar
• Odyssey Broc BMX bars 22.2, 9.8" rise (multi-butted, heat-treated chromoly), $$$. Made for bmx bars ↔ your bmx 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), $$. Keeps snacks, keys and a multi-tool within reach. Straps on, with a rain cover. Check your knees do not rub it when you stand up.
• Knog Oi Classic bell, $. Slim ring bell that wraps the bar. Large fits 23.8–31.8 mm bars; Small fits 22.2 mm kids and BMX bars. Required by law in some places.
• Planet Bike Beamer 200 + Blinky Squared USB light set, $$. 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

Tires

• Salt Tracer 20x2.35 BMX (pair), $$. 60 mm tire ↔ your ~60 mm clearance
• Kenda Small Block Eight 20x1.75 (pair), $$. 50 mm tire ↔ your ~60 mm clearance
• Schwalbe Big Apple 20x2.0 (50-406) (pair), $$$. 50 mm tire ↔ your ~60 mm clearance
• Schwalbe Marathon Plus 20x1.75 (47-406) (pair), $$$. 47 mm tire ↔ your ~60 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

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Buying one used? Fair prices

Worked out from the price new ($300), age and condition, not live market data. If it needs tires, a chain or brakes, take off what fixing them would cost.

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

Geometry (size 20in)

Compare with

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• Kink Curb 20" · compare frames side by side

Sources: Mongoose (intl): Legion L20 2026 specs · Mongoose (US): Legion L20 price + specs · Kunstform: Legion L20 geometry + weight (earlier year)