Guessing Doesn’t Cut It
Before you order new wheels, swap tires, or start a wide front conversion, there’s one number you can’t afford to get wrong—rim size.
At SMT Wheels, every wheel is custom-cut to order, which means precision matters. Whether you’re going 21”, 26”, or all the way up to 34”, you need to know exactly what you’re working with.
This guide shows you how to measure your motorcycle rim size the right way—no guesswork, no wasted money, just accurate fitment.
What “Rim Size” Actually Means
Rim size includes two critical measurements:
- Diameter – Measured across the wheel’s center from bead seat to bead seat (not edge to edge). This determines overall size.
- Width – Measured between the inner flanges where the tire mounts. This impacts what tires you can safely use.
Example: A 21 x 3.5 rim is 21 inches in diameter and 3.5 inches wide.
At SMT, we offer wheels in diameters from 16” to 34”, and widths from standard 3.5” up to our aggressive 5.5” Bulldog front wheels and beyond.
Step-by-Step: How to Measure Rim Size
If your rim isn’t stamped—or if you’re unsure what you’ve got—here’s how to measure it manually.
1. Look for Size Stamping
Start by checking the inner side of the rim or one of the spokes. You might find a stamp like:
18 x 5.5 DOT
If the numbers are there, you’re good. But many custom or older wheels aren’t marked, which means it’s time to measure.
2. Measure Rim Diameter
- Remove the tire if possible, or work around it carefully.
- Measure from bead seat to bead seat across the center.
- Do not measure from the outer edge—this gives a false reading.
3. Measure Rim Width
- Measure the inside width of the rim, between the inner lips (where the tire bead sits).<
- This measurement determines what tire sizes will fit correctly.
- Do not measure outer edge to outer edge.
🔧 Example Widths:
- 3.5” – Stock front wheel
- 5.5” – SMT Bulldog wide front
- 10.5”+ – Custom rear wheels or trike applications
4. Match the Tire to the Rim
Getting this part wrong leads to bad handling and uneven wear.
Here’s a rough tire sizing guide by rim width:
- 3.5” rim → 120 to 130 tire
- 4.25” rim → 140 to 150 tire
- 5.5” rim → 180 to 200 tire
Common Fitment Mistakes to Avoid
- ❌ Measuring the rim’s outer lip instead of the bead seat
- ❌ Confusing tire size with rim size
- ❌ Skipping width measurement when upgrading tires
- ❌ Guessing based on your bike’s year (many bikes are already modified)
Why Fitment Matters at SMT
We don’t make one-size-fits-all wheels. Every SMT wheel is:
- Manufactured in the USA
- Cut from 6061-T6 billet aluminum
- Engineered for your exact bike and build goals
- Matched to your custom tire size and spacing
- Finished in chrome, black, contrast cut, or 6,500+ powder coat colors
Whether you’re building a stretched bagger, trike, or performance street bike, rim size is the foundation of your setup.
Not Sure? Ask Our Experienced Team
Still unsure about what size rim you’re running? Contact us and our fitment team will confirm everything before you order.
We’ve helped thousands of riders get it right—on the first try.
Shop Custom Wheels from SMT
Looking for 18″, 21″, 30″, or a full custom 34″? We’ve got it—and we’ll cut it to fit your exact bike. With fast shipping on in-stock wheels and parts, financing options, and a lifetime structural guarantee, SMT makes sure your build turns heads and handles like it should.