From Larry Kelly on the Ducati.net List
TIRE SIZE CONSIDERATIONS
Rim Size Versus Tire Width
Here's a guide to tire width (mm) vs. wheel rim width (inch):
Tire ... Rim (Nominal) ... Range (Pirelli) ..... Range (Dunlop SLICKS)
160 ........ 4.5
170 ........ 5.0
180 .........5.5 ............. 5.5 - 6.0 ...................... 6.0 - 6.25
190 ........ 6.0 ............. 5.5 - 6.0 ...................... 6.0 - 6.25
200 .........6.25 ........... 6.0 - 6.5
However, it's hard to be definitive here. The difference in width between a labeled 180 mm and a labeled 190 mm tire can be as much as 20 mm or as little as 0 mm depending on the manufacturer. So, because there's an allowable tolerance on the nominal width, a 180 labeled tire can measure 170 mm or 190 mm. The manufacturers are given quite a bit of wiggle room in their labeling versus their measurements. So, don't get hung up on the 180 vs. 190 label. When you change brands the widths can be different.
Some tire profiles are more sensitive to rim width than others. Dunlop, for example, says that their 180 slick works fine with up to a 6.25 inch wide rim. In the past, both Michelin and Dunlop have stated that a 5.5 inch wheel is suitable for 160 through 180 tire widths.
The shape of the tire profile for a given section ratio is decided by each tire manufacturer as they balance wear, traction, tread design, stability and handling design objectives for a generic bike type. Often they will design a tire variation intended for use only on a particular make and model bike.
The size of the tire is decided on by each bike manufacturer in development testing, so the recommended street tire sizes will not necessarily be optimum for the race track. Or vice versa.
It's also important to remember that each bike manufacturer develops their suspension system components in combination with the tires that they ship with the bike, so be cautious when you consider switching to tires that test well on a different brand of motorcycle. When you bought your bike, you paid for a lot of development and testing costs, so think carefully about whose advice to take when making changes. A lot will come down to individual riding styles and rider preferences. Be critically honest about your own riding capabilities and needs.
Finally, if you choose to modify your bike you may also need to reconsider your tire needs. More horsepower usually means a larger tire is needed. Changes to suspension settings will also be beneficial so be prepared for some design development of your own.
With that said, here are some specifics.
Ducati 748/916's come with 5.5 inch rims standard, although some 916's did come with 6.0 inch rims. In order to get the correct tire profile, the recommended street tire size for 5.5 inch rims specified by the tire manufacturers is 180/55 and for 6.0 inch rims it's 190/50.
The outside diameter of both size tires is the same so a switch won't require a rear ride height adjustment. The important difference is that the
180 is a 55 section meaning that it's height is 55% of the width cross-section. The 190 is 50% of it's width. This means that the 55-section
tire has a steeper profile, it's taller.
When you mount a 190 tire onto a 5.5 inch rim it's profile becomes slightly incorrect. The too-narrow rim forces the tire's outer edges inward into a tighter curve so that you can't use this part of the tire effectively. A correct tire profile creates a correctly-shaped road contact patch essential to optimum handling, better sidewall stability with less tire flex and, and better overall tire wear.
When developing the suspension for the 916, Ducati had World Superbike racing in mind so when they sold models for the street they decided to mount 190/50 tires to 5.5 inch rims, a good combination for stable handling. It's been pointed out that WSB Ducati's used 19/67 race tires, roughly equivalent to a 190/60 road tire. So, we got the wide tire look without the quicker turn-in handling characteristics of the 60 section race tire.
In the 1995 916 owner's manual, Ducati specified the 180/55 as an "alternative" to the 190/50 and the bike's under-seat specification sticker
also listed both sizes as recommended.
It wasn't too long before buyers figured out that switching from the 190/50 to the 180/55 gave a very noticeable change in cornering feel. The 180's, mainly because of their taller, steeper profile, turn-in much quicker and easier. So eventually the word spread, and everyone who has changed to the 180's has praised its positive effects on handling.
Keep in mind that, as any street tire wears, the center section wears down more rapidly than the sides, so a 55 section tire will drop to an equivalent 50 section over the life of the tire. Consequently, the turn-in handling gets sluggish as the tire looses profile. This partially explains the rejuvenating effect that a new set of tires will have on a bike's handling,
and will give you an idea as to the magnitude of the effect of switching to a taller section tire.
A 180 tire is also slightly lighter. This will account for part of the subjective handling improvement experienced when moving from a 190 section tire. The weight difference between brands is greater, especially for the front tire. For example, 120/70 front Pirelli Supercorsa's (8 lbs. 6 oz.) Dunlop D2O7RR (10 lbs 7 oz.) A 2 pound lighter tire will, for example, reduce rotational inertia by the same order of magnitude that you get when switching from an aluminum to a magnesium wheel.
The 190 size is stiffer because of the shorter sidewall. This results in increased grip and reduced the tire carcass flex, making accelerating out of turns hard much less scary. Also, if you reduce tire size, with the same horsepower you're going to stress the tire carcass more. This however, hasn't been a problem even with the most powerful street bike models.
With all that in mind, I recently purchased a used Dymag with a 6 inch rim so I'm running a 190/50. After previously running a 180/55 on a stock 5.5 inch rim, I'm pleased with the combination.