Size Matters

Squeezing a wide tyre onto a narrow rim can be a big mistake. Here’s why.

Those low-profile 190-series tyres 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 152mm rim, and cramming it onto anything smaller severely changes its profile.

As an experiment, we mounted a 190-series Sports Tyre onto a 140mm rear wheel and took some measurements. Compared to the correctly sized tyre 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 tyre 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, we slipped on a 190/50 rear and rode a portion of the test loop for a practical comparison. With no changes to suspension or geometry, the bike felt substantially different with the wider tyre. With the bike straight up and down, steering was slightly sluggish in comparison, but just off vertical, the bike was quite tippy and darted into corners. The light, neutral steering of the tyre was completely changed and the bike lost its balanced feel. The sensation was very much like riding on a tyre squared off from too much highway riding. At larger lean angles, performance was also affected, while making transitions from side to side unpredictable. And, contrary to the popular myth that the wider tyre puts down a bigger footprint and gives more traction, we felt no improvement in that department from the properly sized tyre.

We’ve experienced similar changes with a 180-series tyre on a 127mm rim meant for a 170-series tyre. Tyre 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 tyre’s performance for appearance’s sake.

Hein Jonker

Founder of the Motorcycle Safety Institute of South Africa
Editor in Chief of Bike Talk South Africa
Chief Instructor of Bike Talk Motorcycle Rider Academy
Motorcycle Safety & Skills Expert for Arrive Alive South Africa

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