Rim widths, tubes and rim locks
Off-road rim widths barely vary
Every tire has a range of rim widths it can be mounted on, and the rim width is stamped on the wheel in inches. Production off-road bikes are built from a fairly uniform set of wheels, so the usual figures are these.
| Wheel | Usual rim width | Tires that go on it |
|---|---|---|
| Front, 21 inch (full size) | 1.60 | 80/100-21, 90/90-21, 3.00-21 |
| Rear, 19 inch (MX) | 1.85–2.15 | 100/90-19 (250 class), 110/90-19 (450 class) |
| Rear, 18 inch (trail and enduro) | 1.85–2.15 | 100/90-18 to 120/80-18, 4.60-18 |
| Kids and mini, front | around 1.40 | 60/100-14, 70/100-17, 70/100-19 |
| Kids and mini, rear | 1.60–1.85 | 80/100-12, 90/100-14, 90/100-16 |
Even at one tire size there is usually a step or two either side of rim width that will work. For the exact answer, read the stamp on the wheel, the bike's spec sheet, and the fitment table for the tire you intend to use (every tire maker publishes one). Mount a tire outside its rim width range and you risk the bead unseating or the profile going wrong.
Choosing a tube
One tube covers several nearby tire sizes. A tube for a 21-inch front, for instance, will normally cover 2.75 and 3.00 in inch sizes together with 80/100 and 90/90 in metric. The packaging states the range it fits, and if your tire size falls inside that range, you are fine.
Four things to check:
- 1. Rim diameter — a 21-inch tire needs a 21-inch tube. This is the one thing that is never shared.
- 2. Whether your tire size is inside the stated range — a marking like "2.75:3.00 * 80:90/100-21" is shorthand for "fits 2.75 through 3.00 and 80/100 through 90/100 in 21 inch."
- 3. Valve type — as well as the straight standard valve there are L-shaped and offset types, and the valve has to match the diameter and the angle of the hole in your rim.
- 4. Thickness — besides standard there are heavy-duty tubes for motocross and enduro, and thicker ones again. Heavy duty is the default if you run low pressures. It costs you weight.
Radial tires take their own tubes. Do not substitute one across, even at a close size. Every tube maker publishes a fitment table, so searching for "tube" plus your tire size finds the right part quickly.
Rim lock (bead stopper) sizes
Riding off-road at low pressure, a rim lock to stop the tire creeping around the rim is close to essential. Rim locks are chosen by rim width, not by tire size. The nominal size is the rim width itself, and 1.40, 1.60, 1.85 and 2.15 (2.50 on some bikes) are what you will find.
| Rim lock size | Where it goes |
|---|---|
| 1.40 | Kids and mini fronts |
| 1.60 | Full-size off-road fronts (21 inch), kids' rears |
| 1.85 | Mini rears, 250-class MX rears |
| 2.15 | Full-size off-road rears (18 and 19 inch) |
So a full-size off-road bike is 1.60 front and 2.15 rear as a matter of course. Easiest way to remember it: pick the same number as in the rim width table above. Fitting one requires a rim with a rim lock hole, and be aware that adding one changes the wheel balance.
How to read this
- This page is an estimate, put together by us from several published sources and from how production bikes are generally built.
- Even with the rim width inside range, the bike's own legal requirements (fender contact and so on), the speed rating and the load index still have to be checked.
- Tires and wheels run tubeless (TL) have their own separate rules about whether a tube may or must be used. Follow what both the tire and the wheel specify.
- For the final check, use the fitment table published by the maker of the tire you are fitting.