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Sprocket Question

ixoiiiiviviiivi

ixoiiiiviviiivi

New member
Joined
Aug 11, 2011
Location
San Francisco
Moto(s)
2009 KLX250SF
Name
Ivan
How is sprocket/chain sizing and ratio determined?

My OEM Sprocket sizing is 39t-13t so the ratio is 3-1.

I wanted to get a bigger rear sprocket. Does this mean I have to get a sprocket that will have to be an even multiple of my front sprocket?
For example: a 52t Sprocket to even ratio to 4-1?

Or could I just get any size spocket greater than 39t?
Another option was using a 16t front with a 54t rear. Would that make any difference than a 13t-42t? Both ratios would be 4-1.
 
Your math is correct. A 13 tooth front sprocket has a hell of a tight radius for the chain to make. If you want the same ratio you could use 14/42 or 15/45.
 
You could, if the sprocket will fit. It might not.

You don't need to stick to given ratios, 13/42 would be fine. You would likely need to add a couple links to the chain (really meaning replace it with a longer one).
 
13t front sprockets are nice for extra torque on small displacement dirt bikes and reduce top speeds but are tough on chains with their small/tight radius.

14t or 15t front sprockets are much nicer to the chain.
 
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If I up the front sprocket to reduce the stress on the chain, would it add more stress to the front sprocket shaft?

Right now I have 250cc Single. Tops off at about 85mph on the freeway in 6th with the 13t:39t setup.
I wanted to try out different setups. A 16t:39t front for better freeway. And a 13t:54t for easier wheelies.
Would a 54t rear sprocket be a bit overkill?
 
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ixoiiiiviviiivi said:
If I up the front sprocket to reduce the stress on the chain, would it add more stress to the front sprocket shaft?
Click to expand...

Non-issue. No worries here.

Right now I have 250cc Single. Tops off at about 85mph on the freeway in 6th with the 13t:39t setup.
Click to expand...

Small steps. Try a 14t and 15t front sprocket. They're fairly cheap. 16t way too much, might not even fit.
 
mototireguy said:
A 54t rear sprocket would be great, for going up 45deg hillsides in 6th gear at a top speed of 35mph.
Click to expand...

That pretty much says it. A jump from a 3:1 ratio to a 4:1 ratio is HUGE. Most changes for the street are only 1 or 2 teeth. Maybe 3.
 
So what is your goal? Why do you want a bigger rear sprocket, just for looks? The factory picks the gearing to be best all around. If you want to improve the gearing in one area (ie, hill climbing) you're going to take away from it's ability in another area (ie, freeway cruising).

You can change the sprockets freely to get the gearing you want. There's no need to keep any ratio. Most of the time people want mild changes and will just change one sprocket. And are you sure you have 13/39? The Kawi data on your bike says 14/39 was OEM. And if you increase the gears too much, you'll have to get a longer chain.
 
ixoiiiiviviiivi said:
Would a 54t rear sprocket be a bit overkill? My OEM is 39t.
Click to expand...

i had a 16t front and 54t rear on my cbr600 f4i..

the stock sprockets f4i has are 16-46

with 54 rear it was a lot torque it sucked being on the freeway.. although i could o 110ish mph but i would be close to the read line..

54 for on a 250.. where stock is 39, it might be an overkill unless it strictly for stunting in the parkinglots.. and yes itll make wheelies easier.. or you could just get a wheelie button on your hand bars.. just press and go 12!!!!!!!!!!!! :twofinger
 
go up 4 or 5 and see how you like it, that will be a huge difference. Typically 1 down 2 up is standard change but you cant go any lower on the front. going to 54 is ridiculous, probably wouldnt be able to reach freeway speeds on a 250.
 
ixoiiiiviviiivi said:
It supposed to make the bike easier to wheelie right. Want to learn to wheelie.
Click to expand...

Ok, enjoy your wheelies. Keep in mind that this change will make your bike useless for most everything else.

Stunt-Sprockets-l.jpg
 
Aluminum is lighter but doesn't last anywhere near as long. For the street go with steel - Superlite or Ironman sprockets aren't cheap, but are very light and last a long time.
 
Op: Go with a 17/46. All the low end you need depending on your bike.
O-Ring. & Alum.
 
For higher top speed less torque you need larger front, smaller back

For lower top speed more torque you need smaller front larger back

A one tooth change on the front is roughly equal to 4-5 on the back.
I.e. You want higher top speed. You can go +1 on the front, or -4/5 on the back and have the same effect
(mentioned below it is closer to 3, point being, changes to front mean more, that why you see +1 front -2 back, for a smaller adjustment)

For wheels you want more torque. Your bike will come up easier, but you will run out of RPM faster

For recommendations: I say steel sprockets and o-ring chain. For a 250, brand doesn't matter that much, you won't make enough power to snap a chain.
 
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