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Old March 28th, 2009, 06:09 PM   #15
wyckedflesh
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Name: Mikel
Location: Valley of the Sun
Join Date: Mar 2009

Motorcycle(s): '09 Blue Kawasaki KLE650 Versys, '95 Ducati 900SS/CR (undergoing track conversion)

Posts: 287
Quote:
Originally Posted by Alex View Post
Nobody but you will notice the difference in width. 110 to 120 will not look different to an observer, and a 140 or even 150 in back still looks like a skinny tire and in no way makes the back of the bike look like a literbike with a 190 or a cruiser with a 250 or even wider.



Motorcycle tires aren't like car tires. A wider car tire puts a wider footprint on the ground, no question about it. A motorcycle tire is rounded, and the actual footprint on the ground is a very small oval that changes as the bike leans over left and right. Putting a wider tire on the same size rim can actually cause the profile to put a smaller footprint on the ground at certain lean angles, and a larger footprint at other times. The bike's geometry & suspension was designed for 110/130, and playing with that will cause changes to the handling of the bike. Much more often than not, negative changes. Only way to confirm it is to play around with different size tires and realize how they change the feel, and/or take them out to the track and run them at their limits to see how the changes affect lap times. Richard (racebikerentals) has run some very wide tires on the back of his trackbike with reasonably good results, but as Kkim said, I believe it is because the tires he put on were incredibly sticky and competent tires, that would have behaved even better if they were available in the correct sizes. In other words it was the compound that caused the greater performance, while the increased size didn't hurt as much as it would otherwise.
Alex has a good point, but there is even more to this conversation then just compounds.

Motorcycle brands come in different profiles as well as compounds. Pirelli is notorious for its very triangular profile. Meaning the center is very high and the sides fall away sharply, making the bike tend to fall into the corners quickly. Continental on the other hand tends to have a very flat, rounded profile. This makes the bike lazy to corner but stable all the way round. Michilin Pilots tend to be similar to the Pirellis, but Michilin's sport touring tires are half way between the Conti's and the Pirelli's. Bridgestone also has a varied profile depending on the use of the tires. Rounded for touring/sport touring or just high mileage tires, then going to the triangular profile as they get closer to their track day/race day tires.

So if you take a 120/70/17 Pirelli, with its very triangular profile and squeeze it onto an undersized rim, you could end up with a very tall tire that is very twitchy as you have made the fall on the sides sharper and more pronounced.

If you took a 120/70/17 from Continental and squeezed it onto an undersized rim, you could end up with a very mushroom top looking tire. Making it very slow to respond to input.

The other thing is a 120/70/17 was made for a heavier bike. Its stiffer carcass would need a different tire pressure to work with our lighter bikes. An SV650 for instance weighs 375lbs dry and ours weighs in the neighborhood of 333lbs. That 40lbs makes a difference. Or take my old Buell for example, 460lbs wet, running a 120/70/17...the same tire your thinking of using on a 333+lb bike...Running a tire pressure that is not right for the bike your riding is going to make that tire wear in very bad ways. In this gambit you can not go by what Kawasaki states for tire pressures, and if you call up the tire manufacturer, they will tell you not to run it since the tire was not designed for the rim size.

Doing this with the rear is a little less drastic. There are tires in larger sizes which do match the rear rim, so the profile of the tire is not affected as much.

I have made these mistakes in the past, I did it for the spread of tire types available at other sizes, and I can say that I have seen tires that should have lasted 3000-5000 miles tore to hell, and cupped so bad you had headshake just riding around normal in less then 1000 miles. On the flip side, I have taken touring tires and put them on a light weight bike and found they tended to slip and slide all the time, even running up and down through and back some canyons in the HOT Arizona desert, not a greasy feeling, but a hard, lack of traction feeling. They just never built enough heat in them to get traction because the carcass was made and designed for a bike that weighed 2-300 pounds more.

Just some food for thought.

verbal diarrhea over
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