Dunlop Mousse Sizing Guide: Find the Right Fit

Dunlop mousse inserts are labeled with codes like FM21S or FM18C, and matching the wrong one to your tire means a harsh ride at best and a blown mousse at worst. Here's how to read the codes and pick the right size for your bike.

What Dunlop's mousse codes actually mean

Every Dunlop mousse is stamped with a code built from three parts: the rim size, a letter for firmness or profile, and sometimes a suffix for the intended use. FM stands for foam mousse. The number after it is the rim diameter in inches. The letter that follows tells you the compound or width class, S for standard and L for a wider profile that fills more of the tire's internal volume.

So FM21S is a mousse for a 21 inch front rim in the standard profile, while FM18L is built for an 18 inch rear rim in the wider profile. Enduro and rally versions get their own suffix (C for Enduro, L for Rally in some size runs) because those disciplines put different heat and flex demands on the foam.

Dunlop mousse size chart

Always cross-check the code against the tire's own sidewall marking before you buy. Mousse sizing is built around the tire's internal air chamber, not just the rim number, so a tire that's a size up or down in width needs a different mousse even on the same rim.

Matching mousse size to your tire, step by step

Start with your rim diameter. That narrows the choice to one number, 18, 19, or 21. From there, check your tire's width and profile against the fitment column above. If your tire sits between two mousse sizes, go with the mousse rated for the wider tire. An undersized mousse compresses too much and transmits every rock straight to the rim, while an oversized one can overheat and degrade faster than it should.

If you're moving between a stock tube setup and a mousse for the first time, the switch also changes how your suspension feels, since a mousse doesn't compress the same way air does. Riders coming from tubes often size down half a stiffness grade in their forks to compensate. If you're weighing tube, mousse, or Tubliss as your setup, that decision affects sizing too, since each system has its own fitment logic.

How long a properly sized mousse should last

A correctly sized Dunlop mousse typically holds up for one to two full race seasons of regular motocross riding, depending on terrain and rider weight. According to Dunlop's own Geomax mousse documentation, fitment is matched by rim size and tire profile rather than tire brand, so a correctly sized Dunlop mousse will also work under most other 18, 19, or 21 inch off-road tires.

Undersized mousses wear out fast because they're working harder than they're built for. Oversized ones can develop flat spots from uneven compression. Getting the size right the first time is the difference between one set of tire changes and three.

Frequently asked questions

Can I use a Dunlop mousse on a non-Dunlop tire? 

Yes. Mousse sizing is based on rim diameter and tire profile, not tire brand, so a correctly sized Dunlop mousse fits comparable tires from other manufacturers on the same rim.

How do I know if my mousse is the wrong size? 

A harsh, unpredictable ride, unusual heat buildup after a session, or visible flat spots on the foam after removal all point to a sizing mismatch.

Is bib mousse the same as standard mousse? 

No. Bib mousse includes an outer sleeve that adds a layer between the foam and the tire casing, which changes both the fitment and the ride feel compared to a standard mousse insert.

Do mousse sizes differ between front and rear? 

Yes. Front and rear mousses are sized independently based on each wheel's rim diameter and the tire mounted there, so you'll typically buy two different codes for a full set.

Once you know your code, the full Dunlop tire lineup has the matching tires in stock, and you can grab the Dunlop mousse itself here. If you're shopping other brands, our complete mousse selection covers the rest.