240 gearing





#1
i put 240 tire on my bike wich is a 240 40 18 . according to the gear calculator my tire is 80.3 inches in diameter as compared to 76.9 for the stock tire. my question is i was told that the taller tire messed up my gearing ratio like putting a 36 tooth sprocket on the rear wich will slow my acceleration and hurt top end. can anyone clear this up for me please. also if it did change it what gearing do i need to put it back to stock or as close as possible. thank you
 

ibified

Registered
#2
Big: yeah, it DOES throw your final drive off. your acceleration wil DECREASE, but your top end should (theoretically) INCREASE. If stock tire diameter is 76.9, then it has a radius of 38.45, giving you a circumference of approximately 120.8 inches in diameter. The 240 will give you a radius of 40.15, which gives you a circumference of 126.13. What this means is that for every revolution of your back wheel, the bike moves forward 120.8 inches and 126.13 inches, respectively. The bikes speedometer is measuring the revolution of some part (not 100% sure and dont feel like looking because its irrelevant) which is turning in relation to the rear wheel. The bike uses this to calculate speed. If the speedo is BASED on 120.8 inches per revolution of the rear wheel and it is ACTUALLY moving 126.13 inches, then you are going further in one revolution. This will give you a higher top speed, because your final drive ratio has been lowered. Also, your speedometer will not read the same as it did before. You will be going faster, given the same speed reading off the speedo, with the larger diameter tire. Notice I didnt say that your speedo is now off, because they aren't terribly accurate from the factory.

Now, for the acceleration part, and this gets a bit trickier. I'm writing this for you and anyone else reading this, and im making the assumption that SOMEONE reading this didnt take college physics...

Take on faith (notice that "leaving the physics out" part here) that a larger diameter wheel takes more "energy" (the physics thing again) to rotate than a smaller diameter wheel. Also, take on faith that the further out the weight is to the edge of the wheel, the harder an engine has to work to rotate it. What this means for you, rider, is that the acceleration will be slower because more energy is being expended into overcoming the "resistance" of the wheel spinning faster. Also, I'm willing to bet that a 240 wheel and tire weighs a LOT more than the stock wheel does. And all a majority of that weight is at, you guessed it, the OUTSIDE OF THE WHEEL! Heaver wheel means more "resistance" to change of speed as well. honestly, weight is going to play a bigger factor than wheel diameter, but neither of these are working in your favor in going to the 240 wheel and tire combo.



Yeah, that explanation sucks. Try this instead: Put your bike up on stands. Take a little kids bicycle and flip it over. Spin the front tire on your busa. Now, spin the front tire on the kids bike. See which one takes less effort on your part to get spinning quickly. Now, try to stop them with your bare hands. See which one stops easier.

Hope that helps you underststand this a little better.

Now, for the gearing issue: Your overall size has gone up about 4.2 percent. Personally, since our speedos usually are about 5% fast as it is, id leave it alone, and you should still be okay.
 
#3
thanks ibified that helps me understand that my final gearing has been slowed down and i need to go larger on rear sprocket to get back to stock gearing. do you or anyone know what that gearing would be? thank you
 

ibified

Registered
#4
If you want the speedo to be accurate, Id leave the gearing alone, honestly. You're gonna be as close now as you were before you did the wheel change. If you want to be closer to stock gearing, id go up a tooth or two on the back sprocket and you should be close.

This isnt an exact science on these bikes, becaus you're gear ratios are going to be limited by the number of teeth. going up one tooth in the back is gonna change your gearing roughly 2.5% However, it goes back to throwing off your speedo, losing top end, and gaining acceleration again. If you need more detail, PM me and ill get back to you as soon as I can.
 

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