What is the front sprocket size for a 2008 Yamaha yz250f?

Answer 1

13 tooth is the OEM size. If you put a larger front sprocket on it will raise the gearing so that you don't have to shift gears as often. But if you install a smaller front sprocket, you will get quicker acceleration off the line but you will have to shift gears more often.

Think of it like a 10-speed bicycle. The large front chainring allows you to go faster topspeed but it is more difficult to accelerate quickly.

Opposite is true for the rear sprocket. A larger rear sprocket does the same thing as a smaller front sprocket. More teeth on the rear sprocket lowers your gearing so that you get quicker acceleration but you have to shift gears more often.

It all depends on what kind of track you are riding and how important frequent gear changes are to you. Or if you notice you are "in between" gears on a track, meaning you are over-revving one gear, but bogging the next gear to clear most jumps on a particular track, you should try changing the sprocket to either the next lower or next higher number of teeth.

If you ride in very deep or sandy soil, you should raise your gearing (either larger front sprocket or smaller rear sprocket) so that you get better traction in the soil, rather than spinning you tires so much with too low a gearing.

PeeWee racers like on KTM 50 or Cobra 50 should also consider proper gearing depending on the track and soil you are racing on.

Same concept applies as on the bigger bikes. Keep from over-revving you 50cc racing bike by putting a smaller rear sprocket (or larger front sprocket.) This will make the power more controllable for your peewee rider. And it will keep from overheating / over-revving your 50cc racing bike.

If your 50cc racer is getting outrun on long straightaways, raise the gearing with the smaller rear sprocket and you will have higher topspeed on long straights.