Changing rear motorcycle sprocket

UrBaN963

UrBaN963

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Hey guys,

So as some of you know, I ride a 250cc motorbike.

It's a 6 speed, but fairly short gearing sees it top out at about 145. However, with the redline starting at 10,000RPM, this means that at 120 I'm sitting at 8,000RPM. Obviously this is pretty harsh on the engine, as well as the fuel consumption.

So, I have been thinking about playing with the gearing to alleviate this.
The current ratio is 2.846(37/13).

So, was thinking to go maybe 35/13 (2,692).
According to http://www.gearingcommander.com/ this would increase theoretical top speed from 158.5km/h to 167.9km/h, which should drop my revs at 120km/h from about 7,500 to about 7,100.

My question is, have any of you done something like this, and were there any tangible differences, or anything you would recommend watching out for?
 
You can go 1 tooth down on the back sprocket, can give you 200-1000 rpm less. Would suggest you do not go more than 2 teeth down. After that you just loose pulling power
 
Changing Sprockets can be tricky when it comes to performance.

But changing them is easy if you have the theoretical all worked out. I had a bigger sprocket for my 1000rr Honda years ago which I put on before going on rallies. It made wheeling so much easier and controllable. But I only changed the back one and not the front. There are different combinations, best you speak to someone who knows more about it. Try speaking to someone at a workshop. I know the bigger one gave me more bottom end torque.
 
Thanks guys.

From the research I have done, changing the rear sprocket to a smaller one will give me a lower ratio, thus a higher top speed. This will affect pulling power, but the bike currently has far more acceleration than I really need. I'm typically in 6th by the time I hit 80, just to keep revs down as it loves to rev. So longer gearing is definitely what I want. In terms of changing it, I can do that easily, have done that a few times when I was having rear-wheel issues (now resolved).
 
UrBaN963 said:
Thanks guys.

From the research I have done, changing the rear sprocket to a smaller one will give me a lower ratio, thus a higher top speed. This will affect pulling power, but the bike currently has far more acceleration than I really need. I'm typically in 6th by the time I hit 80, just to keep revs down as it loves to rev. So longer gearing is definitely what I want. In terms of changing it, I can do that easily, have done that a few times when I was having rear-wheel issues (now resolved).
Click to expand...

Yea, should be easy and fine then if you did the research. Either way, you will always still have the old sprocket to change back to if you not happy :p
 
Lol yea true. Then I can use the spare one for whacking cars that turn in front of me. :D
 
Changing the gearing will change the characteristics on your bike as you have stated.

1. Larger rear sprocket will give you more pulling power, but will rev higher at 120.
2. Smaller rear sprocket will give you less pulling power but will rev lower and be more fuel efficient at 120.

here is a great article which helps explain...

https://lifeatlean.com/the-effects-of-motorcycle-gearing-changes/
 
Thanks man :)

Yea that pretty much ties in with what I have been reading. Only thing now will be to figure out how to adjust the speedo to reflect true speed.
 
And that's why I don't recommend 250's to people...

Anyway back on topic I don't think slightly lower revs are going to be any less rapey to the engine and you'll most certainly lose your in-gear overtaking acceleration which you already have very little of...and that's the stuff that keeps you safe.

Also doubt it would make all that much difference to fuel economy.

Now if you said you wanted to make it even shorter that would have made more sense to me.

I rode a 600cc for a few years and that thing lived at 16000rpm most of the time and never blinked an eye about it.
 
SauRoNZA said:
And that's why I don't recommend 250's to people...

Anyway back on topic I don't think slightly lower revs are going to be any less rapey to the engine and you'll most certainly lose your in-gear overtaking acceleration which you already have very little of...and that's the stuff that keeps you safe.

Also doubt it would make all that much difference to fuel economy.

Now if you said you wanted to make it even shorter that would have made more sense to me.

I rode a 600cc for a few years and that thing lived at 16000rpm most of the time and never blinked an eye about it.
Click to expand...

A typical Sauron response :p

Actually it has decent acceleration, partly due to it's light weight and the fact I weigh nothing, and the fact the gears are so short.

Fact is, it's not a big brand engine so I would prefer to be a little gentler on it, prevention being better than cure and all that.
 
Most fuel consumption comes from aero drag, not from engine friction, so lowering revs isn't really going to lower your consumption that much.
 
Sinbad said:
Most fuel consumption comes from aero drag, not from engine friction, so lowering revs isn't really going to lower your consumption that much.
Click to expand...
Well lower RPM means fuel being pumped into the cylinder less often, resulting in less fuel being used, right?
 
UrBaN963 said:
Well lower RPM means fuel being pumped into the cylinder less often, resulting in less fuel being used, right?
Click to expand...

Nope - because you'll have to pump more fuel in per cycle to produce the increased torque required to produce the same power at lower rpm ... ;)
 
Hmmm...I guess we shall see how it all turns out.
 
UrBaN963 said:
Thanks guys.

From the research I have done, changing the rear sprocket to a smaller one will give me a lower ratio, thus a higher top speed. This will affect pulling power, but the bike currently has far more acceleration than I really need. I'm typically in 6th by the time I hit 80, just to keep revs down as it loves to rev. So longer gearing is definitely what I want. In terms of changing it, I can do that easily, have done that a few times when I was having rear-wheel issues (now resolved).
Click to expand...

Sounds like the 250cc Twister that I used to own. Very quick through the gears
 
UrBaN963 said:
Fact is, it's not a big brand engine so I would prefer to be a little gentler on it, prevention being better than cure and all that.
Click to expand...

Then you might as well not use it at all.

Lowering the revs might result in a paper value different in longevity, but doubt it will mean much in the real world.

If it's going to explode it's going to explode.
 
Btw my Honda changes to 6th at 66km/h ;)
 
SauRoNZA said:
Then you might as well not use it at all.

Lowering the revs might result in a paper value different in longevity, but doubt it will mean much in the real world.

If it's going to explode it's going to explode.
Click to expand...

Umm, no this is not a digital issue with a digital solution. I'm surprised to hear you say such things actually. Wanting to look after and extend the life of an engine is not something I thought you'd scoff at. Either way, no biggie.
 
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