Boat trailer rollers are sized by two key measurements: the roller's overall size (its outside diameter, or for keel rollers its length) and the bore, the hole through the centre that slides onto the shaft or spindle. Diameters typically run from around 2" on bow rollers and end caps up to 12" on large bilge and concave rollers, while keel rollers commonly come in 4", 4.5", 6" and 8" lengths. The most common bores across the range are 17mm and 21mm, so matching a replacement comes down to measuring the old roller and the shaft it sits on.
How are boat trailer rollers sized?
Three dimensions matter when you order a replacement: the diameter or length (how big the roller body is), the bore (the internal hole diameter, in millimetres), and the width or overall length that governs how the roller sits between the yoke or bracket arms. Get the bore wrong and the roller either won't fit the shaft or will spin loose and wear out fast. Get the diameter or length wrong and it can foul the frame or fail to support the hull properly.
Roller type also dictates size. Wobble rollers, keel rollers, bow rollers, Sydney rollers, concave rollers, self-centring rollers, flat bilge rollers and V blocks each cover a particular position on the trailer, and each tends to come in its own size and bore range. You can browse the full spread of shapes and sizes in the boat trailer rollers collection before you commit to a size.
What roller diameters are available (2" to 12")?
Diameter is the headline number for round rollers such as bow rollers, wobble rollers and bilge rollers. Small 2" rollers and end caps handle the bow and the very ends of a roller bank, while the largest 12" concave and bilge rollers spread the load under wider, heavier hulls. As a rule, the further out toward the chine and the heavier the boat, the larger the diameter you want carrying the load. On a multi-roller bank it is common to mix diameters, running smaller rollers at the ends and larger ones under the heaviest part of the hull.
What length keel rollers do I need (4", 4.5", 6", 8")?
Keel rollers sit directly under the keel line and take a big share of the hull's weight during launch and retrieval. They are described by length rather than diameter, and commonly come in 4", 4.5", 6" and 8". Longer 6" and 8" keel rollers give a broader contact patch for heavier boats and smoother tracking up a steep boat ramp, while 4" and 4.5" suit lighter tinnies and tighter roller spacings. Match the bore to the shaft the keel roller runs on before you order.
What bore size fits my trailer shaft (13mm to 26mm)?
The bore is the make-or-break measurement. Boat trailer rollers are commonly bored 13mm, 17mm, 20mm, 21mm, 25mm and 26mm, and the two you will meet most often are 17mm and 21mm, which cover the bulk of keel, Sydney, concave and self-centring rollers. Wobble rollers often run a larger bore. The bore must match the shaft or spindle diameter, so if you are also renewing the mounting hardware, check the shafts in the roller mounts, yokes and assemblies range at the same time so the bore and shaft are a matched pair.
| Roller type | Typical size | Common bore | Usual position |
|---|---|---|---|
| Bow roller / end cap | From 2" | Smaller bores | Front and roller-bank ends |
| Keel roller | 4", 4.5", 6", 8" | 17mm or 21mm | Under the keel line |
| Wobble roller | Mid-range diameters | Larger bore (e.g. 20mm or 26mm) | Along the roller banks |
| Sydney / concave / self-centring | Up to 12" (concave and bilge) | 17mm or 21mm | Bilge, hull sides and centring |
How do I measure and match a replacement roller?
Matching a worn roller is straightforward if you take three measurements before you order:
- Roller size - measure the outside diameter for round rollers, or the length for keel rollers, and note whether it is 4", 6", 8" and so on.
- Bore - pull the roller off and measure the internal hole in millimetres, or measure the shaft it runs on. Match it to 13, 17, 20, 21, 25 or 26mm.
- Width and shape - confirm the overall length between the yoke arms and the profile (flat, concave, V, wobble) so the new roller cradles the hull the same way the old one did.
If the shaft, yoke or bracket is worn, corroded or bent, replace it alongside the roller rather than fitting a fresh roller to tired hardware. Rollers, bunk skids and side guides all share the job of supporting and aligning the hull, so if you are overhauling a roller bank it is worth checking any skids and guides at the same time. If you are unsure which profile suits your setup, the roller selection guide walks through positions and shapes in more detail.
Which roller material should I match to my hull?
Once size and bore are settled, material is the last decision, and it depends on your hull. Hard blue nylon is the toughest, lowest-friction option and suits aluminium and alloy boats under heavy loads and frequent launching. Soft red poly (polyurethane) is marking-resistant and suits fibreglass, gel-coat and painted hulls. Black rubber is the economical general-purpose all-rounder. The same diameter and bore are usually available across all three, so you can keep the fit and simply choose the compound your hull needs. When you replace one roller on an existing bank, match the material to the rollers already fitted so the hull loads evenly.
Request a quote for boat trailer rollers
Viking Rollers is an Australian-owned trade and wholesale supplier based in Coopers Plains, Brisbane, dispatching rollers, brackets, spindles and full trailer running gear Australia-wide. Tell us your roller sizes, bores and quantities and we'll match the right parts for your build or repair. Request a quote or call 07 3274 6967 to talk sizes with the team.