Changing from a 180 mm tyre to a 190 mm tyre can produce some interesting results.
Especially if you change brands.
In the past, I have used Pirelli Diablo Super Corsa SP. Dunlop Qualifiers and now I use Michelin power one.
All 190/50 on 17-inch rims. Because of the triangular construction profile, the Dunlop’s were about 5mm narrower. (185mm) The Pirelli and Michelin were both about 5 mm wider before fitting.
The difference in width between a 180 mm and a 190 mm tyre can be from 0 mm to 20 mm depending on the manufacturer.
A 180 mm tyre can measure 170 mm or even 190 mm.
For example, a Pirelli 190/55 may have a different width to a Michelin 190/55 of anywhere from a couple of millimetres to up to 20 millimetres. But you can be almost positive neither tyre will be 195 mm.
The recommended street tyre size for 5.5 inch rims specified by the tyre manufacturers is 180/55 and for 6.0 inch rims it is 190/50.
The bead diameter of both 17 inch tyres is the same. The important difference is that the
180/55 has a 55 aspect ratio, which means that its height is 55% of the width cross-section. The 190/50 is 50% of its width. This means that the 55 aspect ratio tyre has a steeper profile, it's taller giving a larger wheel diameter. If you do the Calculations there is only 4mm difference in height between the two tyres when mounted to the recommended 17 inch rims, width size.
When you mount a 190/50 tyre onto a 5.5 inch rim it's profile becomes slightly incorrect. The narrow rim forces the tyre's outer edges inward into a tighter curve and the center section to slightly raise that results in a slightly smaller contact patch when vertical and a more triangular shape so that you can't use the very edge of the tyre effectively because you would be at an extreme lean angle.
The main influence on contact patch size is the tyres pressure. Too high pressure and the tyre will hold its profile under braking and cornering loads. Therefore, the contact patch remains fairly constant if the tyre radius profile is constant from the center of the tyre to the edge.
If you reduce the pressure significantly, the tyre will have a larger contact patch as it can deform much more under brakign and exceleration.
For example, I run Michelin power one B tyres and I reduce the rear pressure to 22 psi for the rear and to 32 psi on the front for track use only.
The grip from the tyres at this pressure is way better than at the road pressure of 38 front, and 42 psi on the rear. I have tried these pressures on a track and the rear tyre just spins up out of corners as if I was riding a wet track.
The 190/50 has more contact patch in the center of the tyre and as you get out to the edge they are all more or less equal.
If you mounted these same tyres to a 5.5 inch rim. The 190/50 and 190/55 would be higher in the center. The 180/55 would be lower in the center.
A correct tyre profile with the correct tyre pressure for the type of riding creates a correctly shaped contact patch and better handling. The blow graph show you the three tyres mounted to a 6 inch rim.
