Sprockets

[TripleKing]

I want to lower the overall gearing on my 71 T250. After rebuilding the engine with some mild porting and a little boost to the compression I make good power from 5k to the 8K redline (it feels like it would pull through that). My 5th and 6th gears are basically unusable a my rpms are a bit low. The question is do I change the front or rear sprocket and to what? Both are in good shape so neither needs replacing.

 

[Titan performance]

Sounds way over geared if your best speed is in fourth, and you have 2 more gears the engine wont pull. Not pulling top is common on these old bikes, but with some tuning, I might have expected to pull top too. I think it's a trial and error thing...though obviously it's cheaper to go for a smaller front sprocket to start with. Sounds like you may need to drop 2 teeth.

 

[TripleKing]

It will pull in fifth, but the way it is geared, 5000 in fifth is about 60 MPH. I still have another gear. I would like to be at bewteen 5500 and 6000 in fifth at about 60 mph. I can cruise at 50 mph 4th at around 5,000. I am (if I remember correctly) just breaking 6000 at 60 in 4th.

 

[Redbird]

You would need to up the number of teeth on the rear, or drop the number of teeth on the front.
The basic formula for it is: 2 teeth up on the rear = 1 tooth down on the front.
(This "formula" is not an exact scientific rule, since primary and final drive sprockets are rarely on a 2-1 ratio. And while Yes, simple division will give you the exact ratio of both sprockets, if you aren't good at math/don't have a calculator handy/simply dont give a rats ass about the exact ratio, the aforementioned "formula" can be used as a basic reference point)

We used to do this on our liter class sportbikes to make them accelerate faster from the lower rpms (with a small sacrifice to top speed), but it'll make it pull harder to redline in the upper gears.

 

[TripleKing]

I never ride the little bugger above 70 MPH so two teeth down will probably be great. First I have to find what is on there now. It is the OEM sprocket.

 

[xb33bsa]

You would need to up the number of teeth on the rear, or drop the number of teeth on the front.
The basic formula for it is: 2 teeth up on the rear = 1 tooth down on the front.

We used to do this on our liter class sportbikes to make them accelerate faster from the lower rpms (with a small sacrifice to top speed), but it'll make it pull harder to redline in the upper gears.

what formula is that ? that would only be the case in one situation where the teeth on the rear sprocket are double that of the front
simple division gives the ratio of any sprocket set

 

[Redbird]

There... fixed it for ya