[eric nyc]

It came up in an unrelated thread and I thought I'd ask for some advice. My chain is just about shot so it's time to replace it and the sprockets. I'm thinking of going up to a 19-tooth front sprocket. I'm looking to improve some of my mid-high speed cruising smoothness. From what I've read, going up one tooth in the front is the equivalent of going up 2 in the rear. I know I'll sacrifice some dead stop acceleration, but these bikes have more than enough get-up for my taste and it's not like I'm drag racing.

So how much of a difference will I notice going up one tooth in the front? Should I maybe start just going up 1 in the rear and seeing if I like the direction that's moving in? Front sprockets are pretty cheap so I could try a couple configurations without dropping too much cash.

 

[Jayell]

Going up on the rear will have the opposite effect of going up on the front.

 

[eric nyc]

Going up on the rear will have the opposite effect of going up on the front.

Ah...Yeah that makes sense. They weren't very clear on that on the BC site. So I guess the question is just how much is the extra tooth on the front sprocket going to effect the ride?

 

[Jayell]

Try this handy link, Eric: www.gearingcommander.com.

 

[eric nyc]

Try this handy link, Eric: www.gearingcommander.com.

That's a pretty slick tool. Crazy how much more top end speed you can get out of the bike with a $25 sprocket. Have to wonder how good of an idea it is to push it TOO far. All things being equal, the suspension and wheels aren't necessarily designed to take those kinds of speeds. I can imagine with other pretty standard modifications - exhaust, air box removal, carb jetting - you could push the max horsepower up and get that thing REALLY flying. That being said, I doubt I'm ever getting this thing over 100 anyway, so I'm a little more interested in what effect I should expect on the smoothness of the ride.

 

[TX1911fan]

I just want to lower the RPMs a little while cruising on the highway. I'm thinking about going to a 19 tooth too.

 

[DeanRider]

I use this table produced by JT Sprockets. It is invaluable when working out gearing ratios when changing sprockets

A PDF of the above table can be downloaded here: http://tiny.cc/9bkxlw

 

[DeanRider]

That's a pretty slick tool. Crazy how much more top end speed you can get out of the bike with a $25 sprocket. Have to wonder how good of an idea it is to push it TOO far.

The top end speeds quoted are in most cases hypothetical. It all depends whether the engine has the top end power to pull such speeds. Gearing alone will not produce the goods.

 

[DUG]

Hi
I put a 19 tooth on, 3750 rpm = 100 kpr, still plenty of pick up.Still a little busy tho.
What's the next size up? or will that make hillstarts problematic?
DUG

 

[beemerrich]

Dug,

The front spocket diameter gets too big if you go bigger than 19...you have to go smaller in back if'n you want taller gearing than that.

Cheers,

--Rich

 

[jkmolt]

I went with a 19 tooth front sprocket and would not go back to the 18. I've seen no problem starting up even on steep inclines. On the open highway, I now cruise comfortably at 70 to 75 mph with engine speed around 4000 to 4200. Who knows how accurate the speedometer or the tach is? With the 18 tooth I was running 4500 rpm or more all the time. I'll admit that the engine doesn't seem to be working at higher revs but common sense tells me it will probably last a little longer by lowering the revs. Milage went up on the highway by about two mpg. Now I get a consistent 50 to 52 mpg at 70 to 75 mph.

By the way, I'm riding an '09 T100.

Karl

 

[SpeedyPete]

Engine Speed, not Road Speed

If memory serves, the stock sprocket on my 04 bonnie was 17T. Going up a tooth to 18T was one of the first mods I did way back when.

Like jkmolt says, the difference was lower engine speed at a given road speed. I never felt that the bike got faster nor did I notice any decrease in accelleration. I did definitely feel that I was no longer searching for the phantom 6th gear and that was a great benefit. I've stayed 18T ever since.

I wouldn't think that gearing would have a big impact on fuel economy since the engine still needs to do the same amount of work for a given speed (eg. mass remains unchanged) but I can sometimes get up to 50mpg and I'm running 135 mains with basically open pipes but stock airbox. The bigger sprocket may have something to do with that.

So bottom line, I guess I would be pro-tooth.

P

 

[DeanRider]

With a larger front sprocket the drive chain should also last marginally longer.