The arithmetic Thule publishes

Thule publishes the arithmetic on its own roof load page. The dynamic limit — the one that applies while driving — is the figure Thule says to use. Its formula is roof load weight limit minus the weight of the roof racks minus the weight of the roof box equals available cargo weight, and its worked example runs 165 lb of roof load, 11 lb of racks and a 44 lb empty box, leaving 110 lb for gear.

Two things follow from that. The first is that in that worked example the box’s rating never comes into play: Thule rates the Motion 3 XL, the Force 3 XL and the Pulse 2 L at 165 lb each, the roof load figure there is also 165 lb, and 110 lb is left after the rack and box come off. The second is that the inputs you need are your manual’s dynamic limit, the rack’s weight and the box’s empty weight.

What the subtraction looks like across these carriers

Four carriers here publish an empty weight on a manufacturer page: the Pulse 2 L at 35.3 lb, the Force 3 XL at 49 lb, the Motion 3 XL at 51 lb and the SkyBox NX 18 at 53 lb. Using Thule’s own example figures — a 165 lb roof load and 11 lb of racks, neither of which will match your vehicle — those weights leave 118.7, 105, 103 and 101 lb of cargo respectively. The spread between the lightest and heaviest box is 17.7 lb, which is real luggage rather than a rounding difference.

That is the trade-off this category hides. If your manual’s dynamic figure is generous, buy the volume. If it is not, the smaller box may carry more.

Volume does not buy capacity

Within Thule’s published ratings, size and rated capacity are unrelated. The Force 3 M, L and XL all state 165 lb, as do the Motion 3 XL and Pulse 2 L. Going up a size buys space for bulk — sleeping bags, tents, duffels — and not a pound more rated weight.

So read a rated capacity as a ceiling, not a target. An 18 cu ft shell filled with dense gear can pass both the box’s own rating and the figure your subtraction left you, and those are different tests with different answers.

Crossbar spread is the hard compatibility number

Yakima’s published spread for the SkyBox NX 18, in the comparison table above, is the only manufacturer figure here you can measure your bars against for a yes or no before buying.

If you have no rack at all, you are shopping in a different category rather than a different size. The Rightline Gear Range 2 and Range Jr strap to an existing roof rack or attach by Car Clips to the door-frame weather molding, and Rightline states both work with or without a rack. Nothing else here mounts without crossbars.

Height, length and width cause the problems you meet on the trip

Published heights run from 15 in (Range Jr) and 15.6 in (Pulse 2 L) through 16 in (Carbonite 18), 16.5 in (SkyBox NX 18), 16.9 in (Force 3 XL) and 17 in (Motion 3 XL) to 18 in (Range 2), with the Vista XL the tallest here. Measure your garage opening, then add the carrier height to your vehicle’s own roof height.

Length and width decide whether the box fits the roofline and the crossbars. The hard shells cluster long: 84 in for the SkyBox NX 18, 84.7 in for the Motion 3 XL, 84.8 in for the Force 3 XL, against 76.7 in for the Pulse 2 L. The Vista XL is the outlier, packing the same 18 cu ft into 63 x 38 x 19 in — 21 in shorter than the next-shortest 18 cu ft box, at the cost of being the widest and tallest carrier here.

Rear clearance is worth checking separately, because it is where manufacturers make claims rather than publish figures. Yakima lists optimized hatch clearance for the SkyBox NX 18 and a tapered design to reduce hatch interference on the Carbonite line; Thule lists full trunk access for the Motion 3 XL. None of those is a measurement, so treat them as the vendor’s framing and verify against your own hatch.

Where loading access stops being a detail

On a one-time garage install, opening side hardly matters. Over a multi-stop trip it sets your parking and packing order. The Motion 3 XL, Force 3 XL, Pulse 2 L, SkyBox NX 18 and Carbonite 18 all publish dual-side opening. The Motion 3 XL adds lid pull straps and a wide handle Thule states is one-hand operable; the SkyBox NX 18 uses a full metal handle Yakima states locks both the gear and the box to the vehicle. The Pulse 2 L’s central lock confirms closure, and Thule’s SlideLock on the Motion 3 and Force 3 separates locking from opening.

The Vista XL opens from the rear only, which SportRack frames as loading away from traffic. That is a genuine packing constraint: the area behind the vehicle has to be clear each time you want anything, and that is the end of the car families tend to park against a wall or a fence at a campsite.

The figures that let you check fit before buying

Two boxes here publish internal as well as external dimensions: the Force 3 XL at 81.5 x 29.5 x 14.7 in inside an 84.8 x 34.1 x 16.9 in shell, and the Pulse 2 L at 73.5 x 28.5 x 14.5 in inside 76.7 x 33.2 x 15.6 in. In both cases the usable length is roughly 3 in shorter than the outside. For the Motion 3 XL, SkyBox NX 18, Carbonite 18, Vista XL and both Rightline carriers, no internal dimensions appear in the material retrieved, so a long item has to be judged against the ski-length figures instead: 215 cm for the Carbonite 18 per its retailer listing, 200 cm for the Motion 3 XL and Force 3 XL, 195 cm for the SkyBox NX 18 and 175 cm for the Pulse 2 L.

One inconsistency to know about: Thule’s Motion 3 XL page gives 18 cu ft (500 L) in the specification list and overview text, while the page subtitle says 490 L — a 10 L difference in the liter figure. The Force 3 XL states 18 cu ft (500 L) without that conflict.

When the soft carrier is genuinely enough

For a family with no rack and bulky soft gear, the Range 2 is a complete answer, and it packs away afterward. The Range Jr does the same job at 10 cu ft (283 L) in a 36 x 32 x 15 in footprint.

Where it stops being enough is specific. Neither Rightline carrier publishes a weight rating or an empty weight, so you cannot run the subtraction above on manufacturer figures. Both are top-loading soft bags with no rigid shell over the contents, unlike the five hard boxes here. And 10 cu ft is below the smallest hard shell in this set, which is the gap that shows up when a weekend trip becomes a week of camping. On warranty terms the picture is mixed rather than one-sided: Yakima publishes a Limited Lifetime Warranty for the SkyBox NX 18 and on the current 16 cu ft Carbonite page, while no warranty term appears in the Thule or SportRack material retrieved.