I've got a 42/16 that's about 70 inches, and a 48/18 that's about 71. Given they produce nearly the same gear inches, is there any advantage to running one vs the other? I'm running single speed, not fixed. I'd like to pick up some freewheel cogs with one and two less teeth to get in the neighborhood of 75 and 80 inches respectively, but don't know which chain ring I should stick with. Thanks.

DanO

Sounds like you're good to go at 48/16.

If you skip or skid, refer to the skid patch table before you switch to 48x16. It's in many threads. Edit--Ignore me. I missed the SS part.

The 48 will give you more of a range for changing your gearing up in the future since the smallest track cog available is a 12t from suntour. As for freewheels, I'm not sure what the smallest is, but I'm guessing it's probably pretty hard to make anything smaller than a 14t...

the smallest freewheel is 13t but it only threads onto the small side of a bmx flip-flop hub. the smallest conventially threaded one is 14t.

i know that when running really compact gears, like 25-9 on a bmx the chain is under a great deal of stress and will break more often.

go for the bigger chainring.

Yea, that's the classic gear, isn't it? Problem is I have a few grades I climb on a regular basis that I can't quite lug the 48/16 up - at least not yet. The 42/16 gets me up and over alright, but then I spin it out pretty quickly. For a while I was running 48 and 38 rings up front with the 16 in back and a tensioner in between. But the bike looks much cooler without all that going on.

DanO

I presently ride 52X17 for 81".

Some time back, after an injury, I went to 47X18, which, if I recall correctly, gave me 70".

I like the big ring and cogs for intuitive reasons; meaning, I can't prove what I suspect intuitively.

It seems to me big rings and cogs, with their larger radius and all-around largeness in general, work more efficiently and with lower stresses than do tiny rings and cogs.
A few extra teeth and an extra inch or two of chain adds a little weight to the bike.
I can only say that the same inch ratio with big rings and cogs seems more efficient to me than the same inch ratio with small rings and cogs, and I have a sense of less wear and tear.
The difference could exist only in my mind.

Anyway, I can go up any hill with 81" that I could go up with 70".
The big difference happens on the downhill side.
With 70" I have much, much more control than I do with 81".
With 81" I can go faster with less monkey-motion, and in some ways I feel safer at speed with 81", except for the braking thing.
I actually thought about going back to about 72" for the all around control and acceleration, but now having lived at 81" for awhile I have picked up some downhill braking skills I didn't have before and I might just stay here.

Anyway, I worked up from 70" to 81" in small steps by using a 47t, 48t and 52t ring in different combinations with 16t, 17t and 18t cogs.
I had a lot of fun experimenting and I might still go back to 72".

The guys who are doing the Furnace Creek 508 (see this thread ) basically said the same thing as Ken. More engagement, happier roll.

Add in the freewheel factor, and you are pretty much forced to stay with the 48t. 17t freewheels are available too, so you can step up in increments, if you must.

Of course all of this is pretty minute to begin with, but wouldn't a smaller ring/cog combo, while yeilding less engagement, be more efficient because of the smaller amount of friction from that smaller engagement? If you're only pulling on two teeth instead of four, you'd have half the surface friction.

Bigger is better. Less backlash, lower chain tension for a given pedal force, longer chain and sproket life. Plus it makes you look like an animal with that big a$$ed front ring.

I think that the friction difference will be essentially a neglible factor assuming that your chain is properly lubed. I think the main reason to run bigger is to try and reduce the stress that the chain has to deal with.

The ladies swoon for the big chainrings.

I run a 38.

Who makes the 14T? just curious....
the smallest one I know of is a 15T

Is there a chart or formula to figure out gear ratios, etc...I feel like I'm in trig...or was it calc??? - where's my abacus?

you're right. sorry.

It will be easier to ride 48/18. But the difference is really very slim. 5 watts?

Do a search for 'Sheldon Brown's Bicycle Gear Calculator'. He's got a page you can enter wheel diameter, tire size, crank length, chain ring and sprocket size and it computes gear inches.

DanO

If you're not lazy, here's how it goes:

ratio = ring teeth / cog teeth

gear inches = ratio * wheel diameter = ring teeth * wheel diameter / cog teeth

gain ratio (some statistic that SB made up) = gear inches / crank length = ring teeth * wheel diameter *.5/ cog teeth / crank length

development = gear inches * pi = ring teeth* wheel diameter * pi / cog teeth

There's your formulas... FOREVER!