Diono Radian 3RXT vs Cybex Sirona S
Two premium convertibles with opposite superpowers — the 3RXT is a 17-inch three-across specialist that boosts to 120 lb, while the Sirona S swivels 360° so you never twist to load your child. Here's which trade fits your car and your back.
Last reviewed: 2026-08-27. Specifications verified against Diono's official support knowledge base and CYBEX's official US product page (retrieved 2026-08-23); recall databases re-checked this date (sources below).
The short answer: Diono Radian 3RXT or Cybex Sirona S?
The Diono Radian 3RXT is the space-efficiency pick: a 17-inch-wide shell built for three-across installs, rear-facing from 5 to 50 lb, then harnessing to 65 lb and boosting as a highback up to 120 lb. The Cybex Sirona S with SensorSafe is the convenience-and-tech pick: one-hand 360° rotation, rear-facing from 4 to 50 lb, a SensorSafe chest clip that alerts your phone, and an L.S.P. side-impact system — but it stops at the harness (no booster mode) and weighs over 30 lb.
Side by side
| Spec | Diono Radian 3RXT | Cybex Sirona S w/ SensorSafe |
|---|---|---|
| Type | Convertible car seat (harness + booster) | Rotating convertible car seat (harness only) |
| Rotation | No | One-hand 360° swivel |
| Rear-facing limit | 5–50 lb | 4–50 lb |
| Forward-facing limit | 22–65 lb (harness until 65 lb) | 25–65 lb (harness to 65 lb total, max 49 in; LATCH limits RF 30 / FF 35) |
| Booster mode | Highback booster up to 120 lb | None |
| Seat width | 17 in (H28.5 × W17 × D16) — three-across friendly | 19 in (L24.5 × W19 × H25.5) |
| Seat weight | 25.9 lb | 30.4 lb |
| Extras | Booster mode extends usable life to 120 lb | SensorSafe Bluetooth chest-clip alerts; L.S.P. side-impact protection |
| List price | Not verified at publish time — Diono's US store was in maintenance and CYBEX's price renders via script (both checked 2026-08-23). Neither seat had a verified Amazon US buy box; check current pricing directly. | |
| Recall status | No active recalls for either model as of 2026-08-26. Transport Canada Feb 2025 action covers Radian SafePlus variants only; 2022 CYBEX action covered Sirona M only. Full details above. | |
Radian 3RXT figures per Diono's official support knowledge base (max height/weight and booster-mode articles). Sirona S figures per CYBEX's official US product page. Prices were not verifiable from either channel at check time — we don't print numbers we can't source.
Where the Diono Radian 3RXT wins: slim fit and lifespan
Seventeen inches wide changes what's possible: two other seats beside it, or a fit in the center position of a small car that 19-inch shells can't manage. The 3RXT also outlives the Sirona S by years — its highback booster mode runs to 120 lb, covering roughly ages 4 through 10+ that a harness-only seat can't. And at 25.9 lb it's nearly five pounds lighter, noticeable if you ever travel with or transfer the seat.
Where the Cybex Sirona S wins: swivel rotation and tech
The 360° swivel is the feature parents miss most once they've had it: turn the seat to face the door, buckle, spin forward — no hunching into the backseat, no twisting your spine with a sleeping toddler. SensorSafe adds a phone alert if the chest clip unbuckles mid-drive, if the cabin gets hot, or if a child is left behind. Rear-facing starts a pound lower (4 lb), and CYBEX's L.S.P. system adds structured side-impact protection. What you give up: booster years and four inches of width.
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Our verdict
Buy the Diono Radian 3RXT if you need three across, a slim center fit, or one seat that genuinely lasts to 120 lb in booster mode. It's the pragmatic long-haul choice.
Buy the Cybex Sirona S if you load your child multiple times a day and value your back — the swivel plus SensorSafe make it the most convenient seat in this pairing, and rear-facing starts at 4 lb for newborns.
Tie-breaker: three kids, small car, or budget-per-year math → 3RXT. Newborn start, bad back, tech-forward household → Sirona S.
Where to buy: Neither seat had a verified Amazon US listing at our last check (fail-closed — we won't link substitutes). Both brands sell direct.
Model identity
| Model | Brand | Manufacturer model number |
|---|---|---|
| Radian 3RXT | Diono | UNKNOWN — not stated in Diono's official support documentation we verified |
| Sirona S (SensorSafe) | Cybex | UNKNOWN — not stated on CYBEX's official US product page we verified |
Sources
- Diono — official support KB: maximum height/weight for Radian 3RXT (rear-facing max 50 lb, harness to 65 lb): diono.zendesk.com/hc/en-us/articles/18545341212819 (retrieved 2026-08-23; Cloudflare-gated, figures via search-indexed snippets)
- Diono — official support KB: Radian 3RXT booster mode (highback booster until 120 lb; dims H28.5 × W17 × D16, seat 25.9 lb): diono.zendesk.com/hc/en-us/articles/18550207825811 (retrieved 2026-08-23)
- CYBEX — official US Sirona S with SensorSafe product page: rear-facing 4–50 lb, harness total 4–65 lb max 49 in, LATCH RF 30 / FF 35, dimensions L24.5 × W19 × H25.5, 30.4 lb, 360° rotation, SensorSafe, L.S.P.: cybex-online.com/en/us/p/cs-go-sirona-s-sensorsafe-3.html (retrieved 2026-08-23)
- Transport Canada — recall notice: Radian 3QX/3QXT/3R SafePlus/3RXT SafePlus built Dec 2019–Nov 2024 (plain 3RXT not listed): tc.canada.ca/en/road-transportation/defects-recalls-vehicles-tires-child-car-seats/diono-radian-3qx-car-seats-non-compliance-canada-motor-vehicle-safety-standards (retrieved 2026-08-23)
- SafetyBeltSafe USA — third-party recall list: Sirona S shown with no recalls/warnings: carseat.safetybeltsafeusa.org (retrieved 2026-08-23)
- Recall sweeps: full CPSC SaferProducts.gov pull + NHTSA campaign set searched 2026-08-24 ('radian', '3rxt', 'sirona' tokens); site sweep re-run 2026-08-26 via builds/lane-recall pipeline — no matches for either model beyond the variant-specific actions above.
- Pricing: Diono US store in maintenance and CYBEX price JS-rendered at check time 2026-08-23; Amazon Creators API searches returned no verified 3RXT/Sirona S buy boxes (fail-closed) — prices intentionally not printed.
- Guidance context (not product data): AAP recommends children ride rear-facing to the maximum height or weight their seat allows (healthychildren.org).