Is changing the rear sprocket on a motorcycle a bad idea?

vash · vash's Garage

My bike. triumph 800xc

In sixth gear, it feels like I need a seventh gear. Motoring along at 70-75, it revs up there. (Embarrassed I forget the actual RPM; I’ll note it tomorrow)

Fuel mileage is also fairly bad on this thing. 30?

I was just Revving my happy butt home when I tried to switch into 7th. Got me thinking; do people tailor their gear ratios?

Just wondering.

dlockhart · 10-17-2018, 04:35 PM

Quote:

Originally Posted by vash

My bike. triumph 800xc

In sixth gear, it feels like I need a seventh gear. Motoring along at 70-75, it revs up there. (Embarrassed I forget the actual RPM; I’ll note it tomorrow)

Fuel mileage is also fairly bad on this thing. 30?

I was just Revving my happy butt home when I tried to switch into 7th. Got me thinking; do people tailor their gear ratios?

Just wondering.

Yes. Set up for the task at hand. On my atm 640 adv I usually take a tooth off the front for off road adv fun.

DerkPerk · 10-17-2018, 04:50 PM

Like dlockhart said. Not a big deal to “re gear”. A tooth on the front sprocket will make a bigger change than just a single tooth in the rear.

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FPB111 · 10-17-2018, 04:56 PM

I loosely use “One tooth on the front equals three on the rear”

ckelly78z · 10-17-2018, 05:02 PM

Changing 1 tooth on the front sprocket will generally be the same as a 6 tooth change on the rear....usually a 6-1 ratio. Going smaller on the rear sprocket will give you slightly worse acceleration, but a better top speed (lower RPM at 60) while going 1 tooth bigger has the opposite effect.

Eric Coffey · 05:12 PM

Quote:

Originally Posted by vash

My bike. triumph 800xc

In sixth gear, it feels like I need a seventh gear. Motoring along at 70-75, it revs up there. (Embarrassed I forget the actual RPM; I’ll note it tomorrow)

Fuel mileage is also fairly bad on this thing. 30?

I was just Revving my happy butt home when I tried to switch into 7th. Got me thinking; do people tailor their gear ratios?

Just wondering.

Most modern bikes have a top end that is WELL above legal limits, so it's a pretty common mod to go up a few in the rear (and/or down 1 or 2 in front) to swing the balance towards better acceleration vs. higher top-end.
Better for hooning in many cases too.

If you bought the bike used there is a good chance the PO could have done so already. Just count the teeth (or find the "T" number stamp) and compare to stock sprocket specs.

vash · vash's Garage

I’m staring at mtn-bike now....

Go bigger six in the rear. Or smaller one in the front. Got it.

Eric Coffey · 10-17-2018, 05:24 PM

Quote:

Originally Posted by vash

I’m staring at mtn-bike now....

Go bigger six in the rear. Or smaller one in the front. Got it.

If you want taller gearing for better top-end/mpg, then you would go bigger (more teeth) in front and/or smaller (less teeth) in the rear.

scotricker · 10-17-2018, 05:35 PM

Quote:

Originally Posted by vash

I’m staring at mtn-bike now....

Go bigger six in the rear. Or smaller one in the front. Got it.

No, no, no... wait...
What you have just described will make your bike feel like it accelerates faster, but will produce even MORE revs at cruising speeds.

In order to get your cruise RPMs down, you will want to go one bigger front, and/or a few teeth less on the back. Look at a 10 speed bicycle, and you'll see the idea.

id10t · 05:42 PM

In the halcyon days of 356 and early 911/912 racing, there were gearing set ups known as "air port gears"

airport gears

motley911 · 10-17-2018, 05:41 PM

Quote:

Originally Posted by Eric Coffey

If you want taller gearing for better top-end/mpg, then you would go bigger (more teeth) in front and/or smaller (less teeth) in the rear.

^^^ this.....
Another thing you need to consider is chain length. Removing ~6 teeth from the rear sprocket will result in the rear wheel needing to be moved out enough to take up the resulting chain slack - and you may run out of adjustment. This is why most folks just go up or down one tooth in the front, as it doesn't change the chain length significantly.

vash · vash's Garage

Quote:

Originally Posted by scotricker

No, no, no... wait...
What you have just described will make your bike feel like it accelerates faster, but will produce even MORE revs at cruising speeds.

In order to get your cruise RPMs down, you will want to go one bigger front, and/or a few teeth less on the back. Look at a 10 speed bicycle, and you'll see the idea.

Duh! I got flipped around. Haha.

71T Targa · 10-17-2018, 06:57 PM

https://advrider.com/f/threads/tiger-800xc-17t-front-sprocket-love-it.1023024/

A ringing endorsement to going one up in the front.

Jeff Higgins · 09:23 PM

A lot of bikes don't have clearance for bigger countershaft sprockets. 15 teeth is a pretty standard size for these, but going to 16 might have the chain rubbing the cases.

A lot of bikes are geared 15/45, or forty something anyway. So, yes, one tooth on the front (countershaft) sprocket equals about three teeth on the rear sprocket. That makes it really tempting to change the front for an easy, no need to adjust chain length change in gearing. Just make sure you check clearance between the chain and cases.

herr_oberst · 10-17-2018, 07:00 PM

Quote:

Originally Posted by Jeff Higgins

..So, yes, one tooth on the front (countershaft) sprocket equals about one tooth on the rear sprocket. That makes it really tempting to change the front for an easy, no need to adjust chain length change in gearing. Just make sure you check clearance between the chain and cases.

I always thought it was one front, 3 (or 4) rear? Maybe I had bad info, but that stuck with me since I was a kid...

Eric Coffey · 10-17-2018, 07:12 PM

Quote:

Originally Posted by 71T Targa

https://advrider.com/f/threads/tiger-800xc-17t-front-sprocket-love-it.1023024/

A ringing endorsement to going one up in the front.

There you go Vash. Swap in a 17T in the front and you should be GTG for what you are looking to accomplish (or at least further in that direction).
Also, a side benefit of going bigger in the front is that increasing the radius will reduce chain wear a bit.

javadog · 10-17-2018, 07:22 PM

It's fairly simple math to calculate the effective gearing change. Figure out what rpm's you're turning at your cruising speed, decide what you'd rather that be and work from there. Let us know what the numbers we're talking about and we can give you pointers on a few things.

I don't know what kind of mileage you have on your bike, but if it's enough miles that you've got noticeable wear on either the chain or the sprockets, You should probably change all three items at once.

Evans, Marv · 10-17-2018, 08:28 PM

Start out with one more tooth on the front sprocket. You should be able to calculate the difference in RPMs at cruising speed.

Jeff Higgins · 10-17-2018, 08:49 PM

Quote:

Originally Posted by herr_oberst

I always thought it was one front, 3 (or 4) rear? Maybe I had bad info, but that stuck with me since I was a kid...

Oops - of course you are right. My 15/45 ratio = 1/3. Total brain fart on my part, I meant to say "equals about three teeth". Unless, of course, you are from Arkansas, where no one has three teeth...

vash · vash's Garage

thanks!!

DIY!!

of course it will be a $1000 DIY job after i buy the rear wheel stand, new ratchet set, the 36mm socket, air tools..

pay the ER co-pay........etc.