[Alcatraz1]

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

 

[dogger]

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.

 

[Sieg]

This will help you: http://www.gearingcommander.com/

Remember - garbage in, garbage out applies.

 

[Alcatraz1]

So what do you guys think, drop 1 tooth or 2?

 

[Alcatraz1]

This will help you: http://www.gearingcommander.com/

Remember - garbage in, garbage out applies.

:bowdown: excellent, thanks for the link :thumbup:

 

[Z06jerry]

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:

 

[Z06jerry]

My manual says my current gear ratio is 17:42.

Are you sure? My '08 690 SMR is 16:40. Did KTM change this for '08?

 

[Sieg]

17/42 = 2.47
16/40 = 2.50

.03 taller final drive ratio.

 

[Alcatraz1]

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

 

[Z06jerry]

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

I would like to do this too. Did you use this KTM 15t front sprocket part # 58033029015?

https://www.ktm-parts.com/mm5/merch...duct_Code=58033029015&Category_Code=KTMHESPKT

 

[Alcatraz1]

I would like to do this too. Did you use this KTM 15t front sprocket part # 58033029015?

https://www.ktm-parts.com/mm5/merch...duct_Code=58033029015&Category_Code=KTMHESPKT

Sorry, didnt get a parts number. Its the same sprocket that they use on the 640 from what I understand.