ASKTHE MXPERTS: 18-INCH WHEELS IN SUPERCROSS
My understanding is that 19” rear wheels came to motocross in response to the typical hard surface and tight turns of Supercross. Do any National level riders switch to 18” rear wheels for the outdoor motocross races? It seems like the taller sidewalls on an 18-inch rear tire might be beneficial on an outdoor track.
First, no AMA National-level motocross or Supercross riders run 18-inch rear wheels, but there is evidence that the taller sidewalls of an 18-inch tire could be beneficial on an outdoor track. MXA test riders have always believed in the charms of 18-inch rear wheels, largely because we were around during the introduction of the 19-inch rear wheels (and the 17-inch rear wheel before it). It is obvious that the volume,footprint and flex of an 18-inch wheel would be better for Vet riders on local tracks.
In the early 1990s, AMA Pros were looking for a rim and tire configuration that wouldn’t bounce in the whoops or suffer from sidewall roll-over in hard-pack Supercross corners. The tire manufacturers came up with the 19-inch tire and rim. It is important to note that an 18-inch tire and a 19-inch tire have approximately the same overall diameter; otherwise, they would change the frame geometry of the bikes they were put on. An 18-inch rear wheel gets its overall dimensions with less rim diameter and more sidewall height, while a 19-incher has a larger rim and less sidewall height.
For the average motocross Novice, Vet or off-road racer, the 18-incher offers a bigger footprint, plusher sidewalls, added cushion and better hookup. Those are attributes that pay big dividends for off-road riders—and why virtually every off-road, enduro and cross-country bike comes with an 18-inch rear wheel. The 19-inch has a lower and stiffer sidewall, which translates into less carcass roll-over under side loads and less bounce in consecutive whoops.
Every motocross bike comes with a 19-inch rear wheel because it is the Supercross configuration of choice, and that status made every local racer want to switch from their OEM 18-inch tire to a 19-inch tire, even if he wasn’t hammering through Supercross whoops or turning on groomed, banked, hard-packed bowl turns. The manufacturers complied with their customers’ wishes. All that said, since all motocross bikes come with 19-inch tires, it would be expensive to switch to an 18-inch tire, largely because you would need to have the your 19-inch wheel re-laced to a 18-inch rim.