ninjette.org - View Single Post

greg737 · September 12th, 2011, 07:14 PM

In the interest of "full disclosure" the following is a public service announcement:

The Sport Demon is a 130/90 16 tire and is larger than the recommended EX-250 tire (which was originally sold with 120/80 and later with a 130/80).

The weight of a larger tire (in this case the Demon) affects a vehicle (motorcycle) in several ways: Unsprung weight, Rotational weight, Total weight, and (unrelated to weight) Overall gearing.

First is "unsprung" weight, which affects suspension performance and is the most complex issue. With no other changes to suspension geometry and damping, more unsprung wieght will negatively affect the suspension's "bump following" ability because the increased mass has more inertia. This is why the designers of racing cars and motorcycles strive for lightness in suspension parts, wheels and tires. A good example of the complexity of this situation is to note the effect of unsprung weight in the interrelations between tires (their weight and bump absorbing ability) and the mass (weight) of the other suspension parts (in total), because it actually creates a "suspension-within-a-suspension" phenomena. This applies to the heavier Sport Demon rear tire because it's extra weight won't accelerate as quickly because of it's greater mass (more mass equals more inertia), but this larger diameter tire itself may have more bump-absorbing ability than the original equipment sized tire, so it's very hard to say what the overall effect it will have on the bike's feel.

Second is "rotating mass" which is easier to understand than unsprung weight because it is simple to see that "more is bad." Any weight added to the bike obviously means less performance, but weight added to any rotating part of the drivetrain not only has to be carried along for the ride, it also has to be accelerated by the engine (i.e. rotated as a means of power transfer) which uses horsepower that could otherwise (in the absense of the extra added rotational weight of the larger Sport Demon rear tire) have been put toward accelerating the vehicle (your EX-250).

Third, which I've already mentioned, is "total vehicle weight." The Sport Demon rear weighs more because it's a larger tire, so it adds to the bike's all-up weight. For any given horsepower, more weight equals less acceleration (damn the inertia, Sir Issac Newton sux! or, did you buy special inertia-free Sport Demons?).

And Forth, an effect unrelated to weight, is that the larger diameter of the Sport Demon affects the overall gearing of the bike's drivetrain. So assuming the bike's horsepower is the same as it was with the old (smaller diameter) tires, the resulting longer gearing will cause slower acceleration. It's the same as changing to a larger front or rear sprocket which lowers RPM at any cruising speed but reduces the bikes ability to accelerate.

September 12th, 2011, 07:14 PM	#83
greg737 - Name: - Location: - Join Date: May 2009 Motorcycle(s): - Posts: A lot.	In the interest of "full disclosure" the following is a public service announcement: The Sport Demon is a 130/90 16 tire and is larger than the recommended EX-250 tire (which was originally sold with 120/80 and later with a 130/80). The weight of a larger tire (in this case the Demon) affects a vehicle (motorcycle) in several ways: Unsprung weight, Rotational weight, Total weight, and (unrelated to weight) Overall gearing. First is "unsprung" weight, which affects suspension performance and is the most complex issue. With no other changes to suspension geometry and damping, more unsprung wieght will negatively affect the suspension's "bump following" ability because the increased mass has more inertia. This is why the designers of racing cars and motorcycles strive for lightness in suspension parts, wheels and tires. A good example of the complexity of this situation is to note the effect of unsprung weight in the interrelations between tires (their weight and bump absorbing ability) and the mass (weight) of the other suspension parts (in total), because it actually creates a "suspension-within-a-suspension" phenomena. This applies to the heavier Sport Demon rear tire because it's extra weight won't accelerate as quickly because of it's greater mass (more mass equals more inertia), but this larger diameter tire itself may have more bump-absorbing ability than the original equipment sized tire, so it's very hard to say what the overall effect it will have on the bike's feel. Second is "rotating mass" which is easier to understand than unsprung weight because it is simple to see that "more is bad." Any weight added to the bike obviously means less performance, but weight added to any rotating part of the drivetrain not only has to be carried along for the ride, it also has to be accelerated by the engine (i.e. rotated as a means of power transfer) which uses horsepower that could otherwise (in the absense of the extra added rotational weight of the larger Sport Demon rear tire) have been put toward accelerating the vehicle (your EX-250). Third, which I've already mentioned, is "total vehicle weight." The Sport Demon rear weighs more because it's a larger tire, so it adds to the bike's all-up weight. For any given horsepower, more weight equals less acceleration (damn the inertia, Sir Issac Newton sux! or, did you buy special inertia-free Sport Demons?). And Forth, an effect unrelated to weight, is that the larger diameter of the Sport Demon affects the overall gearing of the bike's drivetrain. So assuming the bike's horsepower is the same as it was with the old (smaller diameter) tires, the resulting longer gearing will cause slower acceleration. It's the same as changing to a larger front or rear sprocket which lowers RPM at any cruising speed but reduces the bikes ability to accelerate.