ASK THE MXPERTS: THE UNINTENDED CONSEQUENCE OF GOING TO A BIG SNEAKER

On MXA’s project KX250F we switched to a 2.15 rim and ran a 110 rear tire. It is a good mod if you have the power to pull it and the knowledge to compensate for it.

THE UNINTENDED CONSEQUENCE OF GOING BIG

Dear MXA,

I read the article on “How to Make the 2018 KX250F Better.” You mention using a set of KX450F rims, but you didn’t address the rear tire sizing issue. My KX250F came with a 100 rear tire on a 1.85 rim. I get the impression that you ran a 2.15 rim and 110 rear tire. Is it okay to run the 110 tire on the narrow 1.85 rim?

It can be done, but it is much better to do what we did, which was switch to a 2.15 rim. Running a 110 tire on your stock KX250F’s 1.85 rim will work, but the typical 110/90-19 tire is designed for the 2.15-wide rim. Wedging a 110/90-19 onto a 1.85 rim will squeeze the larger tire where the beads meet the rim, producing not only a much rounder tire profile but also a much taller rear tire. If you could find a 110/80-19 rear tire, the rounding and height effects would be reduced, but you probably will not be able to find a 110/80-19 rear tire in a premium brand.

Lots of riders do run 110 tires on 1.85 rims and claim that there is an increase in traction from the wider tire, but that is most likely an illusion created by the significant changes to gearing and frame geometry brought on by the larger-diameter rear tire. The increase in tire height will raise the gear ratios, much like taking teeth off the rear sprocket. It will also raise the rear of the bike, which will steepen the head angle. These unintended consequences can be worked out, if recognized, by gearing corrections and sliding the forks up and down in the clamps.

Why would the 110/80 work better than the 110/90? It comes down to the tire’s aspect ratio. The first number on the tire is its width. Width is measured in a straight line from the furthest point on one sidewall, across the tread, to the furthest point on the opposite sidewall. In our example, that would mean 110mm wide. The second number is the height of the tire—not in millimeters, but as a percentage of the first number. Thus, a 110/90 tire is 110mm wide by 90 percent of 110mm tall. The math works out to 110mm wide by 99mm tall. For comparison, a 110/80 tire is 110mm wide by 88mm tall. More significantly, your KX250F’s stock 100/90-19 rear tire is 100mm wide and 90mm tall. Making it a little more confusing is that putting a 110 tire on a 1.85 rim exaggerates the profile because of the ballooning effect of pinching the beads together. This makes the tire taller than its listed percentage.

If it were us and it was on this project bike, we would switch to a 2.15 and run a 110/90 tire, lower the gearing and move the forks to re-establish the proper frame geometry.