Front tyre choice might not get as much attention as the rear, but get it wrong and you'll feel it in every corner. The 90/100-21 and 90/90-21 are the two most common front tyre sizes for off-road 21-inch wheels — and the difference matters more than you might think.
Breaking Down the Numbers
Both tyres fit a 21-inch rim. The width is the same at 90mm. The difference is in the aspect ratio — the sidewall height as a percentage of the width.
- 90/100-21: Sidewall height = 90mm × 100% = 90mm. Taller, more cushioning.
- 90/90-21: Sidewall height = 90mm × 90% = 81mm. Lower profile, sharper feel.
90/100-21 — The Enduro Standard
The 90/100-21 is the most widely used front tyre size in enduro and hard enduro. The taller sidewall gives you more air volume, better absorption over rough terrain, and improved rim protection on rocky ground. If you're riding technical trails, river crossings, or anything unpredictable, this is your size.
90/90-21 — The MX-Influenced Choice
The 90/90-21 has a lower profile that brings it closer to motocross geometry. It offers a quicker, more direct steering response and suits riders who prioritise speed and precision over comfort. You'll find it more common on fast enduro and cross-country setups.
Which One Is Right for You?
- Technical enduro, hard enduro, or trail riding → 90/100-21
- Fast enduro, cross-country, or MX-influenced riding → 90/90-21
Not sure which suits your bike and riding style? Get in touch — we ride this stuff and we'll give you a straight answer.