1 or two teeth smaller rear sprocket consequences

John390

New Member
Hey, thinking about putting a slightly smaller rear sprocket on at one time. I know Formula390 knows this stuff inside and out.

Iknow there will be slightly less acceleration, slightly greater fuel economy, and slightly less revs at the same speed.

What are the quantitative measurements on this?

how many revs less at highway speed?

how much less acceleration?

Any other issues I should be aware of?

I have a Rapidbike Evo on the way, so, along with airbox mods and a bit of exhaust work, I can gain the acceleration back, but I want to know if taller gearing will pay off

thanks
 

=maz=

Member
Country flag
I race with a 43T sprocket, the gearing suits the particular track...Still out launches the competition and pulls hard out of the corners.

I have no idea what effect it would have in a real world setting for cruising the highway though, sorry.
 

John390

New Member
thanks.

@mattv, I have no idea of the numbers to plug in on that. Any readily available references for all that?
 

=maz=

Member
Country flag
@mattv, I have no idea of the numbers to plug in on that. Any readily available references for all that?

You select the bike you have at the top of the chart.
Then you can trial 2 different gearing combinations in the second section by changing the front and rear sprocket numbers...in the third section you can see what those different gearing combinations will give you at different RPM in different gears.
 

Tom

New Member
I think the 43T might be the sweet spot for a completly factory bike. I went with a 41T rear with zero other mods. It's great for commuting, 1st through 3rd are actually useful. Also, the bike is much more forgiving on throttle control in 1st, it doesnt buck like a wild horse with minor changes. Highway cruising at 70mph is also great, but at stock hp levels, you cant accelerate for shyte in 6th once you hit 80. Effectively the bike becomes a 5 speed, with 6th there for insane gas mileage.

I'm still working on modding the motor and will be installing my PCV along with some extra goodies once I get the bike back from the 600 mile maint. at the dealer. Afterwards, I'll decide whether or not to jump up to the 43T to get more of the fun back on the higher gears.
 

Tom

New Member
Just wanted to report back, after getting the bike back from the 600 mile service. I think the gas I had in my tank was old, it was sitting in a 5 gallon can I had in the garage for a few months (100 octane non-leaded). I rode around on the stuff after I installed the 41T sprocket, so that's probably why it had such a hard time fighting drag when accelerating in 6th. I had a fresh tank of 93 octane on the hour-long ride back from the dealer, and the bike definitely had lots of pull in the higher gears. I didn't wanna be too much of a squid, but I did hit 100 in 6th on the highway and the bike felt like it would have really kept going if I hadn't let off.

I pretty much stayed in 5th once I got off the highway, and the bike responds really well in that gear from 45 to 80-something. You'll top out in the blink of an eye in that gear on full throttle. At this point I am very happy with the 41T rear, and will keep that ratio after I start installing the power mods I have. I think with 40 to 45 hp at the wheel, the 41T sprocket would be the best all around ratio (commuting/spirited riding).
 

Formula390

Supporting Vendor
Vendor
Country flag
Just wanted to report back, after getting the bike back from the 600 mile service. I think the gas I had in my tank was old, it was sitting in a 5 gallon can I had in the garage for a few months (100 octane non-leaded). I rode around on the stuff after I installed the 41T sprocket, so that's probably why it had such a hard time fighting drag when accelerating in 6th. I had a fresh tank of 93 octane on the hour-long ride back from the dealer, and the bike definitely had lots of pull in the higher gears. I didn't wanna be too much of a squid, but I did hit 100 in 6th on the highway and the bike felt like it would have really kept going if I hadn't let off.

I pretty much stayed in 5th once I got off the highway, and the bike responds really well in that gear from 45 to 80-something. You'll top out in the blink of an eye in that gear on full throttle. At this point I am very happy with the 41T rear, and will keep that ratio after I start installing the power mods I have. I think with 40 to 45 hp at the wheel, the 41T sprocket would be the best all around ratio (commuting/spirited riding).


Running a 520 15/41 combo will (theoretically) top out in 6th at 120mph. This is all going to depend on the state of tune of the motor, rear tire circumference, rider weight, wind, clutch slip, body position, blah blah blah but that's what the gearing would take it up to. In 5th gear you would top out at 105mph so you've got quite a bit of room over that 80-something point. A lot of what one would consider to be "best" is going to be very subjective, and depend highly on the particular roads and the riders riding style. Spirited riding will also be highly dependent on the roads. The test road I frequently use here by the ranch is mostly very tight with almost zero visibility through the turn 2nd/3rd gear turns, and there's only one spot I might (briefly) get into 5th... maybe.

If I were instead riding on say Lolo Pass (on the RC) between ID and MT, you'd pretty much be in fast sweepers and in 6th most of the time and reaching for a non-existing 7th gear! :)

lolo.jpg
 

Tom

New Member
Regardless of which rear sprocket size you choose, you will be very happy with the amount of unsprung rotational mass you save. The factory sprockets weigh a ton!
 

John390

New Member
thats a good point too!

I believe I will go 2 teeth smaller once I start making more power. I mostly use the bike in town and in twisties, so I don't need the extra speed, and I don't want to loose acceleration yet.

I do have a Rapidbike Evo now, and plan on intake piping and filter stuff, along with a link pipe for the exhaust. Once my tune for that is stable, I will do the sprocket.
 

Tom

New Member
Honestly, I think a rear sprocket should be one of the first things anyone should do to our bikes. Its such a cheap and easy way to make the bike perform the way you want, and it’s better to do it on a fresh chain.

I did mine with just a basic ratchet set, 2 jackstands (one one each side of the swing arm) and a jack that I used on the center of the engine block by removing the under fairing.
 
I would add that the stock chain that came on my 2015 RC390 was very heavy. I swapped it out for a DID gold 520 chain that weighted what felt like half that of the stocker. parillaguy
 

mantis_13

New Member
Country flag
I dropped all the way to 40t in the rear, stock front.
Downside is that the chain runs against the guide now.
Upside is a significantly lower cruising RPM, and probably higher top speed (though i can't yet attest to that).
There's no sign of wear after a few hundred miles, so I think it's a good trade.

For comparison I show a stock sprocket, 56t, stock, and 40t asking side.
 

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