Sprocket changes

I'd like to pick your brains for a minute:

Can you explain to me how the changing of the sprockets, both fron t and rear affect the bike in terms of revs, accuracy of the speedo and gearing?

Revs change in proportion to the gearing.
On a GPX250 it has no effect on the speedo because the speedo is driven by the front wheel. Other bikes are driven by the gearbox so the speedo changes in proportion to the gearing (relative to real world speed).
Rear teeth / front teeth = reduction ratio
Going to smaller sprockets on the front is harsher on the chain than larger ones.

Do you need me to elaborate on anything in particular?

VERY basic understanding I have is that if you go for a bigger sprocket at the rear (more teeth) will give you better take off but lesser top speed, this can be compensated by going down a tooth at the front whch some say one tooth down at front is similar to 3 teeth smaller at the back, makes sense given that the front sprocket only has 15-16 teeth, am sure you'll get a veritable shitload of more techo replies soonish

basically what they said.

It can start getting technical, and web sites can explain it a lot better than I can.

Such as this one:

http://www.sportrider.com/tech/146_0008_gearing/

As for speedo, if you have an older bike with a cable-driven speedo (usually off the front wheel) you won't need to worry - your bike will always show the right speed unless you change to a different profile front tyre. Most new bikes take their speedo reading off the output shaft of the gearbox (x revolutions for standard gearing = y km/h). This of course means that changing the gearing causes the speedo to be out of whack as gearing changes your RPM. You can get speedo healers to combat this sort of thing. You usually won't be out too much, but it'll be enough to get you pinged if you're not aware of it. Some newer bikes have a sensor running off the front or back wheel which detects how often the rim goes past. With these setups you also don't need to worry.

VERY basic understanding I have is that if you go for a bigger sprocket at the rear (more teeth) will give you better take off but lesser top speed, this can be compensated by going down a tooth at the front w

ummm....my memory sez that going larger on the rear
is the SAME as going down a tooth on the front
so
a smaller front cog won't compensate for a larger rear
it will exaggerate the enhanced acceleration

Think Smitty has a point

VERY basic understanding I have is that if you go for a bigger sprocket at the rear (more teeth) will give you better take off but lesser top speed, this can be compensated by going down a tooth at the front w

ummm....my memory sez that going larger on the rear
is the SAME as going down a tooth on the front
so
a smaller front cog won't compensate for a larger rear
it will exaggerate the enhanced acceleration

exactly right

My RSV had 15640 as the standard gearing. I went to 15/42. 1 down on the front is approximately 2.5 up on the rear.

Definitely made a nice difference!

depends on what your target is.1tooth in front is about 3 rear.stock zx12 is 18x46-17x46 gives you alot more power in the bottom.17x49 is common for dragstrips.unless your making big power it don't pay to gear to tall 18x45 for more top end if you bypassed your restrictions.

to explain it all mathmatically yet as simply as I can...

Gearbox turns output shaft with front sprocket on it. Each sprocket has a set number of teeth on it, which pulls that number of links passed the sprocket for each turn. Each link can be measured in distance. So if the front sprocket has 17 teeth and each chain link is 10mm, one turn of the front sprocket (17*10) pulls 170mm of chain. If we put a 16 tooth sprocket on the front, one trun will pull 160mm (16*10) of chain past. Also, each link pulled past the front sprocket pulls a link past the rear sprocket. 1 turn at the front (17 teeth) pulls 17 links past the rear sproket, pulling that many teeth with it. As the rear sprocket has more teeth it doesn't manage a full turn. Each turn of the rear sprocket is 1 turn of the rear wheel, and so a distance travelled can be measured. Changing the ratio of sprockets changes the number of times the front turns to turn the rear once, and so changes the distance travelled for each turn of the front sprocket.


My standard setup on the cbr is
17 tooth front sprocket
44 tooth rear sprocket

This gives a ratio of 2.59:1 (44/17) Or it takes 2.59 turns of the front sprocket to get 1 turn of the rear sprocket (and so 1 turn of the rear wheel).

If I put a 16 tooth front sprocket on, the ratio changes to 2.75:1 (44/16) so now the engine has to turn the front sprocket 2.75 time for 1 turn of the rear wheel. Each turn of the front sprocket has moved the bike a shorter distance. The engine has an easier time to move the bike (force = mass * distance) same mass, less distance, therefore less force required (or same force greater accelleration )

Changing the rear sprocket has a similar effect, but a can be more refined. 1 tooth at the front changes the ratio quite a bit (it's 1 out of 17), where changing at the rear has a lesser effect (1 out of 44).

If I were to change up 1 tooth at the rear it has the same effect of taking one off the front, but to a lesser degree

Standard ratio
17/44 = 2.59:1

down 1 at front
16/44 = 2.75:1

up one at rear
17/45 = 2.65:1

Going the other way can be done, but beware that the engine only has so much power, and it won't always make the bike faster or more fuel economic.

I tried
17/42 = 2.47:1

and the bike was hard to get off the line, wouldn't reach top speed in 6th gear and drunk way too much fuel.