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Discussion Starter #1
Ok, so I need some advice on gear ratio’s :hmmm:

My manual says my current gear ratio is 17:42. The other day I hit 187km/h with another 1000rpm till I hit the rev limiter in 6th. Now, as fun as that was, I am really not that into high speed, and would prefer to have a little bit more snap/ bottom end on my bike, so am considering changing the gear ratio. I figure the easiest thing for me to do is to drop a tooth or two off the front sprocket. Now what I need you help with is determining what sort of top speed loss I can expect by loosing 1 tooth up front, and what loss by loosing 2 teeth up front. I have already ordered a 15 tooth front sprocket, but a mate told me that 1 tooth down will result in 20% off the top end (sounds too much to me?) so by installing a 15 tooth sprocket, I will loose 40% top end?

So, how much top speed will two teeth down result in, and has anyone dropped teeth in the front and what was the result?

The way I worked it out was to take the top speed (187km/h) divide that by the number of teeth on the front sprocket (17) and times that by the new front sprocket teeth:

The results look like this:

(187/17 = 11) x 16 = 176km/h
(187/17 = 11) x 15 = 165km/h

Is this valid?


Thanks
Al
 

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I'm sorry but your buddies theroy doesn't work because he's not factory in other factors like wind restance. my 625 smc had I think a 17t front sprocket and toped out at 105mph I wanted more bottom end so I dropped to 16t and gained some however I actually gained top end as the bike now ops out at 11mp. it is whining a lot higher but it didn't affect top end at all but I gained nice some nice snap.
 

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So what do you guys think, drop 1 tooth or 2?
One tooth equates to about a 6% increase in rpm for a given speed. dropping 2 would be about 11.5%. The difference between 5 & 6th is only about 10% now. Therefor dropping 2 teeth will make 6th gear feel like 5th does now. My 690 SMR with stock gearing is 25kph/1K rpm in 6th gear. i.e 4K rpm = 100kph, 8K= 200kph (theoretical). Also FWIW according to GPS the speedo reads about 3% high at most speeds. 100 indicated is actually 97. Dropping 2 in the front would be too much for my liking, 1 should work out nice though. :bike:
 

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17/42 = 2.47
16/40 = 2.50

.03 taller final drive ratio.
 

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Discussion Starter #9
Turns out my manual was wrong, had a 16 on the front and not a 17 as stated. Anyway, installed the 15 tooth, much nicer power delivery, definetly worth the cash
 

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