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39-tooth, or 37-tooth?

  • Thread starter Thread starter Ric
  • Start date Start date
R

Ric

Guest
So I know I want to swap to a smaller rear sprocket on the 300. But I don't want to overdo it, either. It's stock 15/42 right now, and as far as I know the front doesn't get any bigger. Here's what I'm looking at:

MPH in1st gear @9500rpm - 24 stock, 25.88 w/39T, 27 w/37T

RPM in 5th @ 60mph - 6785 stock, 6301 w/39T, 5977 w/37T

Max MPH in 5th - 84 stock, 90 w/39T, 95 w/37T

Not that I plan to peg the speedo on the twisty backroads(too much to live for), but does anyone have experience with a swap like this on a small twin? With pipes/pods/jets?
 
I don't have much experience changing sprocket ratios (all of my currently-running bikes are shafties) and even less with "small twins", but I am not sure you will gain much, if anything, by changing your ratio.

Yeah, you will lower the engine speed for any given road speed, but you might also lower it just below the useable power range, and will have to downshift to get back to the useable range.

I'm not sure you have enough power to pull redline in top gear now. Change the ratio and you DEFINITELY won't be able to do that. The engine needs to spin that fast for a reason.

.
 
That's what I'm wrestling with. Unless I'm laying the coal to it heavy, 1st is nearly useless - in and out before I've gone 20 feet, usually. I already have to downshift on some hills around here too, so that's no real worry.

If Suzuki had put the 6-speed in the 300(and the front disc brake), I'd likely not be posting this. They put them on the 250, even!
 
1st gear is always "useless" until you are on a hill starting from stop, with a plump passenger... Then, it allows you to get going very easily without taxing the clutch where a faster ratio might be a bit of a "burn" before you get going?

Me, I've got a 400 and I'm happy with everything it does as stock, but why not try it and tell us how it goes? if you could find a cheap 39T rear it would likely be fine and you may not have to shorten your chain either (just in case it's nogo and you want to goback to stock 42)
 
Some quick math will get you some interesting results.

Dropping from a 42-tooth sprocket to a 38-tooth sprocket will drop your RPM by 10%. Your choices of 37- and 39- are just a little more or a little less. Since your RPM will be 10% less, you should be able to go 10% faster at redline in each gear.

You did not mention your engine speed at the end of that 20-foot distance (do you even have a tach on that bike?), but with the new ratio, you might make it about 22-25 feet before having to shift.

Is all the effort really worth it? How about just starting off in second gear?

.
 
No tach, but as is it'll hit 24mph in 1st before having to upshift. With a 39T it'll hit just shy of 26.

I'm the heaviest load that bike will ever see, so no dramas with taking off. :)

I like the point of being able to adjust out the slack instead of having to remove links. 39 it is!
 
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