Top Speed - Replacing Rear Sproket

[Dygear]

KTM Store :: KTM Street Powerparts :: 390 DUKE :: Chain and Sprokets :: KTM 390 REAR SPROKET ORANGE

Right now everything on my bike is stock. At full throttle in 6th gear when I'm not tucked in I get 103-4ish MPH top speed. When I tuck in, I'm at 111 MPH top speed but I bounce off the rev limiter. The bike clearly has more in it, it just can't get there. I was looking at this part and I was wondering what each of the tooth settings would do to my top speed?

What is the stock tooth count on the rear sprocket right now? 42?
If I move up to a 44 or a 46 does any one have any data on what the top speed would be then? Can some one show me the math for that?

 

[Deleted member 452]

45T is stock and you can do gearing calculations online here: Gearing Commander: Motorcycle Speed, RPM, Chain & Sprockets Calculator

 

[Ryanthegreat1]

I did +1 on the front. Got me to 114-116 depending on which way the wind is blowing.

Don't think it has much more in it. That is right at the peak of the torque range.

 

[isaac_]

My question is: why are you chasing top speed? Does it serve a function for you or just because you want to say it goes XYZ miles per hour? There's no wrong answer. I don't street ride my RC. So, stop speed is not critical for me around my race track. Getting a good drive out of corners... making sure I don't bounce off the limiter... is more important.

Also,... are the 114-116 speedometer readings or GPS? My QStarz will only read 104 at best. Most of the time 100 mph down the front straight.

 

[Ryanthegreat1]

I went to taller gearing for two reasons,

1. Lower RPMs on the street. Really nice for 300 mile days.
2. The Ridge Motorsports Park has a LONG front straight. Was bouncing the rev limiter in 6th for at least half the length of the straight.

Speedo indicated speeds. I could dig out my GPS logs and see what it was actually doing. Worth noting I have a PCV installed.

 

[CDN Duke]

I went to taller gearing for two reasons,

1. Lower RPMs on the street. Really nice for 300 mile days.
2. The Ridge Motorsports Park has a LONG front straight. Was bouncing the rev limiter in 6th for at least half the length of the straight.

Speedo indicated speeds. I could dig out my GPS logs and see what it was actually doing. Worth noting I have a PCV installed.

I also went +1 on front to 16T. Equivalent to between 42-43T rear sprocket (with 15T front).

Primary concern for me was #1 above, better cruising rpm on highway for better fuel economy etc. Bike is geared low enough otherwise.

For racing purposes, I expect gearing will be selected per-track depending on configuration and need for drive on particular portions of course.

 

[Andy]

45T is stock and you can do gearing calculations online here: Gearing Commander: Motorcycle Speed, RPM, Chain & Sprockets Calculator

Anyone tried that 18/42 combination for 130.2 mph!!!

 

[CDN Duke]

Anyonetried that 18/42 combination for 130.2 mph!!!

Doesn't fit, sorry

16T is biggest without removing guard.

 

[isaac_]

Anyone tried that 18/42 combination for 130.2 mph!!!

Can the bike even pull the gear or is this theoretical ?

 

[Michelle]

My last bike was geared up +1 on the front ,, ( not by me ) , ZX7R ,, it was fast , always put a smile on my face when I went out on it..
I do miss it and wished I had never sold it ,, but my husband did not like me on it..... It was a bit heavy for me though I have to admit ,, I would still go out on it and thrash the sh*t out of it ,, then I would out and thrash it some more ,, poor bike....

 

[parillaguy]

my track only RC390 uses the stock engine sprocket and either 43 or 41 tooth rear sprockets depending on the track. I suggest getting a 41 for track use. try the 43 for street. parillaguy

 

[isaac_]

my track only RC390 uses the stock engine sprocket and either 43 or 41 tooth rear sprockets depending on the track. I suggest getting a 41 for track use. try the 43 for street. parillaguy

I don't think you can make a blanket statement like that. Every track is different. To say use 41 rear sprocket for track use doesn't make sense. At my track I run 15/44 after trial and error. Another guy runs 15/45 and another run 14/42. Granted sometimes some gearings might be fitting for most tracks.

 

[Ryanthegreat1]

Can the bike even pull the gear or is this theoretical ?

I know mine won't pull all the way to the end of the 16/45 combo.

 

[CDN Duke]

my track only RC390 uses the stock engine sprocket and either 43 or 41 tooth rear sprockets depending on the track. I suggest getting a 41 for track use. try the 43 for street. parillaguy

I don't think you can make a blanket statement like that. Every track is different. To say use 41 rear sprocket for track use doesn't make sense. At my track I run 15/44 after trial and error. Another guy runs 15/45 and another run 14/42. Granted sometimes some gearings might be fitting for most tracks.

isaac_, he did qualify by adding 'depending on the track', my emphasis added...

Then made the general recommendation as starting point I'd say. But yes, your point is well taken. Will depend on rider and track for sure. For myself, only having had my bike at the track once, and only for a limited number of laps, I'm nowhere near being at the limit nor consistent enough to determine if I'm being held back by the gearing in any particular corner yet...

 

[isaac_]

My bad! Completely missed that part.


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[Maddog Reynolds]

I went from 15:45 to 15:43 on my road bike and the difference is obvious. First gear feels more useful and top gear gives me a few extra mph on fast bits of road (I live and work in Cumbria, UK and there aren't many straights here, so when I get to one I tend to tuck in and see what she'll do ). I prefer the taller gearing, but I don't think the engine will pull any more usefully--I'd end up using 5th gear too much, except with a following wind or downhill.

By the way, for a road bike quite a few chain experts, including Gearing Commander linked above, recommend avoiding an even number of teeth, as it can cause faster chain wear. On a track bike, nobody seems to worry

It's because a full chain link is two pitches, so the chain length is always even and if the same tooth meets the same pitches each time around, you get uneven wear that keeps getting worse over time. Also avoid combinations that divide exactly, such as 16T with 112 pitches of chain (112/16=7.0).