39 tooth rear sprocket- unreasonable?

thematoosh

thematoosh

new rider
Joined
Jan 14, 2025
Messages
4
Location
Boston, MA
I’m looking to do a gearing change to get a higher top speed out of my 23’ Vitpilen 401. I’ve been referencing the Formula 390 gearing sheet and it seems like a 39 tooth rear sprocket would be the first step to a higher top speed. Most gearing changes I only see an increase/reduction in a few teeth. I’m not looking to wheelie this thing, it already has an insane amount of torque that I am willing to sacrifice some of. Is there any negatives to dropping down 6 teeth from stock at the rear?
 
I went to a 44tooth rear (aluminum superlite) mainly to drop the weight of the stock unit and then a 16t up front. Probably about the same gearing change and I like it.

You will definitely need a new chain, which is fine since the stock one is made of pasta.
 
I have gone down to 42 on the rear with a standard 15 on the front. This calms the acceleration down quite a bit and makes the bike more civilised and manageable, well for a little 400. On the standard 15/45 gearing with a few gentle (air box, fuel X, and End can) mods my smart was a bit mad. Great fun but trying to pick up the front wheel a lot and going through the lower gears really fast was getting tiring, I could not find a 16 tooth front sprocket so I had to go with a 42 rear sprocket. I have tried it but in all honesty I will be going back to the standard 15/45 in the spring. Going down 3 teeth on the rear made the bike easier in traffic ( in live in the country so not required) and less revvy out on twisty roads but more…err…boring! I fell in love with the loony attitude from the moment I first test rode the bike and as I only use my bike for pleasure I have to go with the gearing that gives the best bang for me.
I am glad I tried but will be going back.
You could try a smaller front sprocket first to see if you like the difference a fronts are 1/3 the price of rears. 1 tooth front equals 3 on the rear. You should look up a motorcycle gearing calculator before changing any sprockets as going up and down may require a longer or shorter chain. Just be aware smaller front sprockets will wear your swing arm mounted chain guide really fast and make more noise. Should be fine as an experiment though and you should be able to try without needing a longer chain. You can also flip the rear axle blocks to get more rearward slide from them.
Hope some of this rambling helps, Jes.
 
Jester1971 said:
I have gone down to 42 on the rear with a standard 15 on the front. This calms the acceleration down quite a bit and makes the bike more civilised and manageable, well for a little 400. On the standard 15/45 gearing with a few gentle (air box, fuel X, and End can) mods my smart was a bit mad. Great fun but trying to pick up the front wheel a lot and going through the lower gears really fast was getting tiring, I could not find a 16 tooth front sprocket so I had to go with a 42 rear sprocket. I have tried it but in all honesty I will be going back to the standard 15/45 in the spring. Going down 3 teeth on the rear made the bike easier in traffic ( in live in the country so not required) and less revvy out on twisty roads but more…err…boring! I fell in love with the loony attitude from the moment I first test rode the bike and as I only use my bike for pleasure I have to go with the gearing that gives the best bang for me.
I am glad I tried but will be going back.
You could try a smaller front sprocket first to see if you like the difference a fronts are 1/3 the price of rears. 1 tooth front equals 3 on the rear. You should look up a motorcycle gearing calculator before changing any sprockets as going up and down may require a longer or shorter chain. Just be aware smaller front sprockets will wear your swing arm mounted chain guide really fast and make more noise. Should be fine as an experiment though and you should be able to try without needing a longer chain. You can also flip the rear axle blocks to get more rearward slide from them.
Hope some of this rambling helps, Jes.
Click to expand...
And that’s kind of where I’m at right now. I have every mod you described above. It has so much get up, it will flip me off even in 2nd gear, the acceleration is excellent for a sub 400cc. I guess what my main concern is how much acceleration I will be losing. Are we talking full seconds added to my 0-60, or fractions of a second?
 
You must have the greatest 401 ever built or you are 6'5", 100 lbs and ride all the way back on the seat. With the same mods as you this bike doesn't have enough nuts to power wheelie in 2nd unless I ride off a cliff.
 
Maybe haha. 5’9” 200lbs. Flip might be an exaggeration for 2nd, but definitely in first. Mashing the throttle in 2nd will at least get the wheel off the ground pretty reliably.
 
Never measured times, I think 0-60 will not be any different, it might actually be smoother as you will or have to frantically go through the gears as fast. what I can say is I used to overtake cars in between corners with a quick twist and I would not even have to think about it but on the 15/42 gearing I have to consider my overtakes more. Sometimes even waiting for a larger gap or not squirting past at all. Also I really do not used 6th gear in the higher gearing on the twisty back roads of rural UK but sometimes snicked in 6th on the 15/45. On main roads it will sit at 70-75 without feeling revvy but would definitely require a downshift into 5th for any significant speed increase. It really depends on how you use your bike? I would say a try a 16 front if you can first, then if you feel you need for more speed you could look into a smaller rear sprocket. I think going down 6 on the rear you might be disappointed with the new performance.
 
Mine would definitely bring the front up in second off the throttle in standard gearing. I do ride aggressively though and you are more over the front on a Vit. It will not lift with 15/42 though, off the throttle anyway. I am not a fan of wheelies, I never have been, here in the uk that will definitely get you the wrong type of attention.
 
Gotcha, that makes a lot more sense. So rolling acceleration is more heavily impacted rather than from a standstill. For the amount of highway riding I do that’s definitely a factor I should be considering. 16t sounds like the better move. May keep rear gearing stock but swap to aluminum. Any recommendations? I was looking at Vortex sprockets, none listed for Husqys but plenty of options for 390 Duke, RC, would assume those work just fine?
 
My Afam 42 was listed for Duke 200! I risked it as 45 was the only option for svart or vit, they are the same and I believe they are all on a 520 chain? Even the 125s. Afam were one of the only manufacturers in Europe that make black (42) instead of orange.
 
I have heard 7% for one tooth in the front but that depends on the original gearing.

If you know the max speed you can use an online gear calculator.
 
I went 16T front and 42T rear (cant remember now) and I like this setup for my commute which involves

- mostly speeds higher than 60mph
- 90-95mph on the few open stretches I get
- the final 3 mile stretch lane filtering between stopped cars at sub 20mph, mostly in 1st gear.

The bike runs better with this gearing. You get used to revving out the engine more.

I would like to experiment a little more on the next sprocket change.
The 16T is not cusioned and creates a rattling noise, unlike the stock 15T, which is cusioned. I'll go back to 15T front and may be try out a 39 or 40 rear.
 
You must log in or register to reply here.
Back
Top Bottom