BSA: Don’t throw your 20-inch rims
away. They still fit on a BSA.
WHAT HAPPENED TO THE 20-INCHER? Dear MXA, Whatever happened to the 20-inch front wheel? About 10 years ago it was touted as the next big thing and was even offered as an optional front wheel by several manufacturers. Then, it disappeared. What happened to it and all of its alleged benefits? The 20-inch front tire was not new when it was considered to be the next big thing. It had been available several decades earlier on BSAs. Over the last four decades, motocrossers have raced with 19-, 20-, 21- and 23-inch front tires. Rear tires have also run the gambit of diameters. Many of today’s racers can remember when 18-inch rear wheels were the industry standard (and in the eyes of many racers, they never should have been replaced with 19-inchers), but there was also a 17-inch rear-wheel fad on the GP and National circuit back in 1977. Why? Footprint. The 17-inch rear wheel was stronger, had more air volume, and offered a bigger footprint. That should sound familiar, because that was what the 20-inch front-tire movement was all about. In the summer of 1999, Dunlop and Bridgestone unveiled 20-inch front tires on the bikes of select AMA National riders. The 20-inch front tire had a rather simple and short genesis. Honda engineers felt that a tire with more air volume might help take the spike out of the ultra-stiff Honda CR250 aluminum frame. They asked Dunlop and Bridgestone to build them prototype 20-inch front tires to test. The tests went well enough that several National riders, not all on CR250s, wanted to race with the new tires. The original idea of the 20-inch front tire was for it to have the same outer diameter as the 21-inch tire it replaced. The difference would be made up by more width and a taller sidewall (that is where the shock absorption would come from). In production, however, the 20-inch front tire’s outer diameter was considerably smaller than that of the 21-inch tire it was supposed to replace. How much smaller? The ride height of the front end could be as much as 7mm lower with the 20- incher installed. This difference was taken up by sliding the forks down in the triple clamps by 7mm—and Honda even lengthened its forks in production by 7mm to allow buyers to switch between 21- and 20-inch front wheels. The 20-inch front tire put a bigger footprint on the ground and had larger air volume to absorb more energy. It worked well on hardpack, well on prepped dirt, average in loam, and poor in sand, ruts and watered hardpack. The 20-inch front tire took more effort to turn; climbed up the sides of berms; lowered the front of the bike, which was good for some bikes and bad for others; hated water; squirmed in mud; and thudded through sand whoops. It was best suited to hard dirt—dry, hard dirt. The 20-inch front tire was soon considered a failure and disappeared. Ultimately, Honda engineers elected to redesign their frames instead of using the big front tire to soften impacts. But, the 20-inch idea didn’t die completely. Instead, the tire manu- facturers went back to the drawing board and built a wider 21-inch front tire. Instead of the traditional 80/100-21, they designed a 90/100- 21. The bigger and wider 90/100 is approximately 1/4-inch taller and 1/4-inch wider than the 80/100. Of course, it suffers from the same pluses and minuses as the 20-inch front tire, but to a lesser degree on both counts. MXA test riders have tested the 90/100-21 and 80/100-21 front tires back to back and always choose to run the narrower front tire, which is probably why the 90/100 fronts are only available in selected models of each tire brand.