How do I measure my roof crossbar width?
Steps
- With the crossbars clamped in their normal position, measure between the two towers along one bar, from the inside face of the front tower to the inside face of the rear tower, in millimetres. That is your usable exposed length.
- Measure the bar cross-section: width and height in millimetres, and note whether the profile is round, square or aero-shaped like an aerofoil.
- Look along the top of the bar. If there is a continuous slot running its length, that is a T-slot and the carrier will bolt into it. If not, the carrier will need a clamp that wraps around the bar.
- Open the owner's handbook to the loading section and read the dynamic roof rating in kilograms. That figure includes the weight of the crossbars themselves and the carrier and the load.
- Subtract the crossbar weight and the empty carrier weight from the dynamic rating. What is left is what the carrier can hold on the move.
What you need
- A tape measure with a lock, at least 2 metres.
- A pair of vernier calipers or a ruler for the bar cross-section.
- The owner's handbook, for the dynamic roof rating and the recommended tower spacing.
- A note of the crossbar make and model, if fitted aftermarket, so bar-specific carrier fit kits can be matched.
Common mistakes
- Measuring the whole bar including the ends that sit inside the towers. Only the exposed span between towers is usable.
- Ignoring the dynamic roof rating. The static rating on the handbook can be several times higher and applies only to a stationary vehicle, not to motorway driving.
- Sliding the towers to make the exposed span longer. Towers must sit at the manufacturer-specified positions on the roof rails, and moving them outboard changes the load distribution the roof structure was engineered for.
Frequently asked questions
Do I measure the whole bar or the exposed section between towers?
The exposed section between the two towers. A cargo box or bike carrier clamps onto the crossbar in the free span between the mounting feet, and that span is what has to be long enough for the carrier's clamp spacing. The full bar length including the bits inside the towers is irrelevant.
Where do I find the dynamic roof rating?
In the owner's handbook, usually in the loading or accessory chapter, and sometimes on a plate inside the driver's door frame. Dynamic rating is the moving-vehicle limit for the roof, including the weight of the crossbars themselves. It is always lower than the static rating that applies when the car is parked, for example under a roof tent.
What is a T-slot and does my bar have one?
A T-slot is a channel running along the top of an aero-profile crossbar that accepts T-shaped bolt heads. Modern aluminium aero bars usually have one and it lets carriers bolt in directly without a round-bar clamp. Older round steel bars, square steel bars and some plastic-coated bars do not, and need clamp-style attachments.
The crossbars are already fitted, do I still need to check the vehicle rating?
Yes. Aftermarket crossbars are rated for their own load capacity, but the vehicle's dynamic roof rating is a separate ceiling set by the car maker for the roof structure itself. The lower of the two applies. Loading a 100 kg cargo box on a bar rated for 100 kg does not help if the vehicle roof rating is 75 kg dynamic.