Does Size Matter?
Squeezing a wide tire onto a narrow rim can be a big mistake. Here's why.
By Andrew Trevitt, Sport Rider/June 2002

Those low-profile 190-series tires sure look gnarly on the back of a sportbike, and we've seen them pinched onto all sizes of rims. But in reality, a 190/50-17 fits properly only on a 6.0-inch rim, and cramming it onto anything smaller severely changes its profile.

This scaled line drawing shows how the 190-section tire retains its low profile in the center of the tread, but pinches in to match the 180's profile at the edges. Steering characteristics are substantially changed.

As an experiment, we mounted a 190-series Metzeler Rennsport onto our F4i's 5.5-inch rear wheel and took some measurements. Compared to the correctly sized tire on the same rim, the 190's profile closely matches the 180's near the edges of the tread, but is much lower in the center area-equivalent to about a 6mm change in ride height. Effectively, the wider tire will give more rake and trail when the bike is vertical, while keeping close to the original geometry when the bike is leaned over. Accounting for one (by changing ride height) will unduly affect the other.

Following our test with the Metzeler Sportecs, we slipped a 190/50 rear Sportec onto the F4i and rode a portion of the test loop for a practical comparison. With no changes to suspension or geometry, the F4i felt substantially different with the wider tire. With the bike straight up and down, steering was slightly sluggish in comparison, but just off vertical, the F4i was quite tippy and darted into corners. The light, neutral steering of the Sportecs was completely changed and the bike lost its balanced feel. The sensation was very much like riding on a tire squared off from too many freeway miles. At higher lean angles, performance was less affected, although making transitions from side to side was unpredictable. And, contrary to the popular myth that the wider tire puts down a bigger footprint and gives more traction, we felt no improvement in that department from the properly sized tire.

Cramming a 190-section rear tire onto a 5.5-inch rim results in a multi-radius profile as shown here.
a 180/55-17 rear tire properly mounted on a 5.5-inch rim.	A 190-50/17 tire on the same rim.

We've experienced similar changes with a 180-series tire on a 5.0-inch rim meant for a 170-series bun. Tire engineers work hard to design and match front and rear profiles for characteristics that we sometimes take for granted. Upsetting that balance is surprisingly easy and you should think twice before sacrificing your tire's performance for appearance's sake.