Gearing Question

Yes, but that would be a pretty big jump. You might be better off trying a 13 first. Also remember that if the front is that bad, it has been wearing down the chain, and the chain will be affecting the rear. It is always best to change all three at the same time.

changing the front to a smaller gear changes the power curve. All of the power will be in the first little bit of the rpm range. It is best to change the rear. This will not change the power curve but push the power down a few hundred rpm.

if you think of a 13t front and a 52t rear there is a 4:1 ratio. changing one on the front is the same as 4 on the rear, but the spinning ratio of the mechanism is different. think of a 18 speed bicycle. only shift the front sprocket and don't change the rear. Shift from the larger sprocket to the smaller. you will have to spin the front much faster to go the same speed. this works the same way on your bike. You will be turning faster rpm to go the same speed, with twice the effort. but changing the rear the same ratio 4:1 isn't as much of an effort change. give it a try you will be amazed the difference.

my yz250f winds threw the gears really quick i was thinking of upping the front and rear sprockets
front: to 14
rear: add 3 teeth
what do u think i should do?

seeing that the yzf has alot of torque you should just add 1 to the front and see what that does to your performance. changing both will probably give you an adverse effect. here is a sprocket ratio chart this should help everyone with questions. just remember the smaller the front sprocket the faster the motor will rev, which changes the power curve.

http://www.dropbears.com/u/utilities/sprocket.htm

Bruno, I have read all of your posts on this thread, and I must say WOW. The fact that a smaller front sprocket is lighter is not in question, however, being that the front sprocket is connected to the transmission, it will not effect the torque curve, and although the reduced rotating mass will typically make an engine rev up quicker, any gains will immediately be removed as soon as you put a larger rear sprocket in place (it will increase the rotating mass on the rear wheel). This is where aluminum rear sprockets come into play, they are lighter and thus make it easier to turn the wheel, which is the ultimate goal here. Any effort to make the engine rev more or less quickly should be done by adjusting the weight of the flywheel. Changing to a thinner transmission would probably have a greater effect of the rate of acceleration. If I were to argue your case, I would either go with the tight radius that the chain has to redirect itself on, or I would note that each tooth on the gear will be contacted more frequently and thus wear out faster.
Sorry, I have to disagree with your logic unless you can show dyno charts that prove a significant difference in the torque curve based solely on which gear is changed while maintaining the same overall final ratio.

I have a 96 CR250 that I recently purchased. Stock is 13 front and 49 rear. It has a 13 front and a 51 rear. I dont like the way I need to slip the clutch in 1st gear to get rolling. I wanted to change to a 48 rear. The parts dude told me I need to go up on the rear and that going down would give it more top end. Looking at the chart from the link above that can not be correct right?

going with a smaller rear WILL give more speed and slower take off speed. Adding to the rear sprocket will lower the ratio. (The larger the number on the chart the slower you will go and the more power it will feel that it has.)